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The isolation and structure elucidation of rac-dioncophyllacine A from the leaves of Triphyophyllun peltatum, is described. Unlike all other naphthylisoquinoline alkaloids, this fully dehydrogenated representative has an additional methoxy group at C-4, the position of which is deduced from NOE results. Dioncophyllacine A has a 7,1' site of the biaryl axis, as in dioncophylline A. Its constitution is confirmed by an X-ray structure analysis, which shows that the crystalline form of this new alkaloid is racemic.
In the molecular structure of the title compound, C34H58B2N2, each B atom of the diborane(4) is connected to one dimethylamino group and one Tip ligand (Tip = 2,4,6-triisopropylphenyl). These findings indicate that the increased steric demand of the Tip groups exerts influence solely on the B—B separation but not on the overall geometry of the title compound.
The 1,3-bis(tricyanoborane)imidazolate anion 1 was obtained in high yield from lithium imidazolate and B(CN)\(_3\)−pyridine adduct. Anion 1 is chemically very robust and thus allowed the isolation of the corresponding H\(_5\)O\(_2\)\(^+\) salt. Furthermore, monoanion 1 served as starting species for the novel dianionic N-heterocyclic carbene (NHC), 1,3-bis(tricyanoborane)imidazoline-2-ylidenate anion 3 that acts as ditopic ligand via the carbene center and the cyano groups at boron. First reactions of this new NHC 3 with methyl iodide, elemental selenium, and [Ni(CO)\(_4\)] led to the methylated imidazolate ion 4, the dianionic selenium adduct 5, and the dianionic nickel tricarbonyl complex 6. These NHC derivatives provide a first insight into the electronic and steric properties of the dianionic NHC 3. Especially the combination of properties, such as double negative charge, different coordination sites, large buried volume and good σ-donor and π-acceptor ability, make NHC 3 a unique and promising ligand and building block.
Chapter 1
Thermally activated delayed fluorescence (TADF) materials provide a strategy to improve external quantum efficiencies of organic light emitting diodes (OLEDs). Because of spin-statistics, 25% singlet and 75% triplet excitons are generated in an electronic device. Conventional organic emitters cannot harvest the triplet excitons, due to low spin orbit coupling, and exhibit low external quantum efficiencies. TADF materials have to be designed in such a way, that the energy gap between the lowest singlet and triplet states (ΔES-T) is sufficiently small to allow reverse intersystem crossing (rISC) in organic systems. An established structure property relationship for the generation of TADF materials is the spatial separation of HOMO and LUMO via an orthogonal arrangement of donor and acceptor in donor-π-acceptor (D-π-A) compounds. This is achieved by increasing the steric bulk of the π-bridge. However, this is not always the most efficient method and electronic parameters have to be considered. In a combined experimental and theoretical study, a computational protocol to predict the excited states in D-π-A compounds containing the B(FXyl)2 (FXyl = 2,6-bis(trifluoromethyl)phenyl) acceptor group for the design of new TADF emitters is presented. To this end, the effect of different donor and π-bridge moieties on the energy gaps between local and charge-transfer singlet and triplet states was examined. To prove the computationally aided design concept, the D-π-B(FXyl)2 compounds Cbz-π (1), Cbz-Meπ (2), Phox-Meπ (3), Phox-MeOπ (4), and MeO₃Ph-FMeπ (5) were synthesized and fully characterized. The photophysical properties of these compounds in various solvents, polymeric film and in a frozen matrix were investigated in detail and show excellent agreement with the computationally obtained data (Figure 5.1). A simple structure-property relationship based on the molecular fragment orbitals of the donor and the π-bridge which minimize the relevant singlet-triplet gaps to achieve efficient TADF emitters is presented.
Chapter 2
Three-coordinate boron is widely used as an acceptor in conjugated materials. In recent years the employment of trifluoromethylated aryls was shown to improve the acceptor properties of such boranes. Astonishingly, the use of ortho-trifluoromethylated aryls in boron containing systems also improves the stability of those systems in regard to their inherent reactivity towards nucleophiles. Borafluorenes are stronger acceptors than their non-annulated triarylborane derivatives. In previous studies, the effect of trifluoromethylated aryls as the exo-aryl moieties in borafluorenes, as well as the effect of fluorination on the backbone, were examined. As the latter suffers from a very low stability, systems using trifluoromethyl groups, both on the exo-aryl as well as the borafluorene backbone were designed in order to maximize both the stability as well as the acceptor strength.
Three different perfluoroalkylated borafluorenes were prepared and their electronic and photophysical properties were investigated. The systems have four trifluoromethyl moieties on the borafluorene moiety as well as two trifluoromethyl groups at the ortho positions of their exo-aryl moieties. They differ with regard to the para-substituents on their exo-aryl moieties, being a proton (FXylFBf), a trifluoromethyl group (FMesFBf) or a dimethylamino group (p NMe2-FXylFBf), respectively. Furthermore, an acetonitrile adduct of FMesFBf was obtained and characterized. All derivatives exhibit extraordinarily low reduction potentials, comparable to those of perylenediimides. The most electron deficient derivative FMesFBf was also chemically reduced and its radical anion isolated and characterized. Furthermore, the photophysical properties of all compounds were investigated. All compounds exhibit weakly allowed lowest energy absorptions and very long fluorescent lifetimes of ca. 250 ns up to 1.6 μs; however, the underlying mechanisms differ. The donor substituted derivative p-NMe2-FXylFBf exhibits thermally activated delayed fluorescence from a charge transfer (CT) state, while the FMesFBf and FXylFBf borafluorenes exhibit only weakly allowed locally excited (LE) transitions due to their symmetry and low transition dipole moments, as suggested by DFT and TD-DFT calculations.
Chapter 3
Conjugated dendrimers find wide application in various fields, such as charge transport/storage or emitter materials in organic solar cells or OLEDs. Previous studies on boron containing conjugated dendrimers are scarce and mostly employ a convergent synthesis approach, lacking a simple, generally applicable synthetic access. A new divergent approach was designed and conjugated triarylborane dendrimers were synthesized up to the 2nd generation. The synthetic strategy consists of three steps:
1) functionalization, via iridium catalyzed C–H borylation;
2) activation, via fluorination of the generated boronate ester with K[HF2] or [N(nBu)4][HF2]; and
3) expansion, via reaction of the trifluoroborate salts with aryl Grignard reagents.
The concept was also shown to be viable for a convergent approach. All but one of the conjugated borane dendrimers exhibit multiple, distinct and reversible reduction potentials, making them potentially interesting materials for applications in molecular accumulators (Figure 5.7).
Based on their photophysical properties, the 1st generation dendrimers exhibit good conjugation over the whole system. The conjugation does not further increase upon expansion to the 2nd generation, but the molar extinction coefficients increase linearly with the number of triarylborane sub-units, suggesting a potential application as photonic antennas.
Chapter 4
A surprisingly high electronically-driven regioselectivity for the iridium-catalyzed C–H borylation using [Ir(COD)OMe]2 (COD = 1,5-cyclooctadiene) as the precatalytic species, bis(pinacolato)diboron (B2pin2) as the boron source and 4,4’-ditertbutyl-2,2’-bipyridin (dtbpy) as the ligand of D-π-A systems with diphenylamino (1) or carbazolyl (2) moieties as the donor, bis(2,6-bis(trifluoromethyl)phenyl)boryl (B(FXyl)2) as the acceptor, and 1,4-phenylene as the π-bridge was observed. Under these conditions, borylation was observed only at the sterically least encumbered para-positions of the acceptor groups. As boronate esters are versatile building blocks for organic synthesis (C–C coupling, functional group transformations), the C–H borylation represents a simple potential method for post-functionalization by which electronic or other properties of D-π-A systems can be fine-tuned for specific applications. The photophysical and electrochemical properties of the borylated (1-(Bpin)2) and unborylated (1) diphenylamino-substituted D-π-A systems were investigated. Interestingly, the borylated derivative exhibits coordination of THF to the boronate ester moieties, influencing the photophysical properties and exemplifying the non-innocence of boronate esters.
2,2′-Bipyridyl is shown to spontaneously abstract a borylene fragment (R–B:) from various hypovalent boron compounds. This process is a redox reaction in which the bipyridine is reduced and becomes a dianionic substituent bound to boron through its two nitrogen atoms. Various transition metal–borylene complexes and diboranes, as a well as a diborene, take part in this reaction. In the latter case, our results show an intriguing example of the homolytic cleavage of a B═B double bond.
Two N-methylpyridinium compounds and analogous N-protonated salts of 2- and 2,7-substituted 4-pyridyl-pyrene compounds were synthesised and their crystal structures, photophysical properties both in solution and in the solid state, electrochemical and spectroelectrochemical properties were studied. Upon methylation or protonation, the emission maxima are significantly bathochromically shifted compared to the neutral compounds, although the absorption maxima remain almost unchanged. As a result, the cationic compounds show very large apparent Stokes shifts of up to 7200 cm\(^{-1}\). The N-methylpyridinium compounds have a single reduction at ca. −1.5 V vs. Fc/Fc\(^+\) in MeCN. While the reduction process was reversible for the 2,7-disubstituted compound, it was irreversible for the mono-substituted one. Experimental findings are complemented by DFT and TD-DFT calculations. Furthermore, the N-methylpyridinium compounds show strong interactions with calf thymus (ct)-DNA, presumably by intercalation, which paves the way for further applications of these multi-functional compounds as potential DNA-bioactive agents.
We present herein an in‐depth study of complexes in which a molecule containing a boron‐boron triple bond is bound to tellurate cations. The analysis allows the description of these salts as true π complexes between the B−B triple bond and the tellurium center. These complexes thus extend the well‐known Dewar‐Chatt‐Duncanson model of bonding to compounds made up solely of p block elements. Structural, spectroscopic and computational evidence is offered to argue that a set of recently reported heterocycles consisting of phenyltellurium cations complexed to diborynes bear all the hallmarks of \(\pi\)‐complexes in the \(\pi\)‐complex/metallacycle continuum envisioned by Joseph Chatt. Described as such, these compounds are unique in representing the extreme of a metal‐free continuum with conventional unsaturated three‐membered rings (cyclopropenes, azirenes, borirenes) occupying the opposite end.
A cyclic (alkyl)(amino)carbene (CAAC) has been shown to react with a covalent azide similar to the Staudinger reaction. The reaction of \(^{Me}\)CAAC with trimethylsilyl azide afforded the N‐silylated 2‐iminopyrrolidine (\(^{Me}\)CAAC=NSiMe\(_{3}\)), which was fully characterized. This compound undergoes hydrolysis to afford the 2‐iminopyrrolidine and trimethylsiloxane which co‐crystallize as a hydrogen‐bonded adduct. The N‐silylated 2‐iminopyrrolidine was used to transfer the novel pyrrolidine‐2‐iminato ligand onto both main‐group and transition‐metal centers. The reaction of the tetrabromodiborane bis(dimethyl sulfide) adduct with two equivalents of \(^{Me}\)CAAC=NSiMe\(_{3}\) afforded the disubstituted diborane. The reaction of \(^{Me}\)CAAC=NSiMe\(_{3}\) with TiCl\(_{4}\) and CpTiCl\(_{3}\) afforded \(^{Me}\)CAAC=NTiCl\(_{3}\) and \(^{Me}\)CAAC=NTiCl\(_{2}\)Cp, respectively.
A General Synthetic Route to NHC‐Phosphinidenes: NHC‐mediated Dehydrogenation of Primary Phosphines
(2021)
The dehydrocoupling of primary phosphines with N-heterocyclic carbenes (NHCs) to yield NHC-phosphinidenes is reported. The reaction of two equivalents of the NHCs Me\(_2\)Im (1,3-dimethylimidazolin-2-ylidene), Me\(_4\)Im (1,3,4,5-tetramethylimidazolin-2-ylidene), iPr\(_2\)Im (1,3-di-iso-propylimidazolin-2-ylidene) and Mes\(_2\)Im (2,4,6-trimethylphenylimidazolin-2-ylidene) with PhPH\(_2\) and MesPH\(_2\) led to the NHC stabilized phosphinidenes (NHC)PAr: (iPr\(_2\)Im)PPh (1), (Mes\(_2\)Im)PPh (2), (Me\(_4\)Im)PPh (3), (Mes\(_2\)Im)PMes (4), (Me\(_2\)Im)PMes (5), (Me\(_4\)Im)PMes (6) and (iPr\(_2\)Im)PMes (7). The reaction of tBuPH\(_2\) with two equivalents of the NHCs afforded the corresponding NHC stabilized parent phosphinidenes (NHC)PH: (iPr\(_2\)Im)PH (8), (Mes\(_2\)Im)PH (9) and (Me\(_4\)Im)PH (10). Reaction of 1 with oxygen and sulfur led to isolation of iPr\(_2\)Im-P(O)\(_2\)Ph (11) and iPr\(_2\)Im-P(S)\(_2\)Ph (12), whereas the reaction with elemental selenium and tellurium gave (NHC)PPh cleavage with formation of (iPr\(_2\)Im)Se (13), iPr\(_2\)ImTe (14) and different cyclo-oligophosphines. Furthermore, the complexes [{(iPr\(_2\)Im)PPh}W(CO)\(_5\)] (15), [Co(CO)\(_2\)(NO){(iPr\(_2\)Im)PPh}] (16) and [(η\(^5\)-C\(_5\)Me\(_2\))Co(η\(^2\)-C\(_2\)H\(_4\)){(iPr\(_2\)Im)PPh}] (17) have been prepared starting from 1 and a suitable transition metal complex precursor. The complexes 16 and 17 decompose in solution upon heating to ca. 80 °C to yield the NHC complexes [Co(iPr\(_2\)Im)(CO)\(_2\)(NO)] and [(η\(^5\)-C\(_5\)Me\(_5\))Co(iPr\(_2\)Im)(η\(^2\)-C\(_2\)H\(_4\))] with formation of cyclo-oligophosphines. The reaction of 1 with [Ni(COD)\(_2\)] afforded the diphosphene complex [Ni(iPr\(_2\)Im)\(_2\)(trans-PhP=PPh)] 18.
The reduction of a cyclic alkyl(amino)carbene (CAAC)-stabilized organoberyllium chloride yields the first neutral beryllium radical, which was characterized by EPR, IR, UV/Vis spectroscopy and X-ray crystallography. DFT calculations show significant spin density at beryllium and confirm donor–acceptor bonding between an alkylberyllium radical fragment and a neutral CAAC ligand.
A water‐soluble tetracationic quadrupolar bis‐triarylborane chromophore showed strong binding to ds‐DNA, ds‐RNA, ss‐RNA, as well as to the naturally most abundant protein, BSA. The novel dye can distinguish between DNA/RNA and BSA by fluorescence emission separated by Δv =3600 cm\(^{-1}\), allowing for the simultaneous quantification of DNA/RNA and protein (BSA) in a mixture. The applicability of such fluorimetric differentiation in vitro was demonstrated, strongly supporting a protein‐like target as a dominant binding site of 1 in cells. Moreover, our dye also bound strongly to ss‐RNA, with the unusual rod‐like structure of the dye, decorated by four positive charges at its termini and having a hydrophobic core, acting as a spindle for wrapping A, C and U ss‐RNAs, but not poly G, the latter preserving its secondary structure. To the best of our knowledge, such unmatched, multifaceted binding activity of a small molecule toward DNA, RNA, and proteins and the selectivity of its fluorimetric and chirooptic response makes the quadrupolar bis‐triarylborane a novel chromophore/fluorophore moiety for biochemical applications.
The Fischer carbene synthesis, involving the conversion of a transition metal (TM)-bound CO ligand to a carbene ligand of the form [=C(OR’)R] (R, R’ = organyl groups), is one of the seminal reactions in the history of organometallic chemistry. Carbonyl complexes of p-block elements, of the form [E(CO)n] (E = main-group fragment), are much less abundant than their TM cousins; this scarcity and the general instability of low-valent p-block species means that replicating the historical reactions of TM carbonyls is often very difficult. Here we present a step-for-step replica of the Fischer carbene synthesis at a borylene carbonyl involving nucleophilic attack at the carbonyl carbon followed by electrophilic quenching at the resultant acylate oxygen atom. These reactions provide borylene acylates and alkoxy-/silyloxy-substituted alkylideneboranes, main-group analogues of the archetypal transition metal acylate and Fischer carbene families, respectively. When either the incoming electrophile or the boron center has a modest steric profile, the electrophile instead attacks at the boron atom, leading to carbene-stabilized acylboranes – boron analogues of the well-known transition metal acyl complexes. These results constitute faithful main-group replicas of a number of historical organometallic processes and pave the way to further advances in the field of main-group metallomimetics.
Herein, we report the facile synthesis of a three-dimensional (3D) inorganic analogue of 9,10-diazido-9,10-dihydrodiboraantracene, which turned out to be a monomer in both the solid and solution state, and thermally stable up to 230 °C, representing a rare example of azido borane with boosted Lewis acidity and stability in one. Apart from the classical acid-base and Staudinger reactions, E−H bond activation (E=B, Si, Ge) was investigated. While the reaction with B−H (9-borabicyclo[3.3.1]nonane) led directly to the 1,1-addition on N\(_{α}\) upon N\(_{2}\) elimination, the Si−H (Et\(_{3}\)SiH, PhMe\(_{2}\)SiH) activation proceeded stepwise via 1,2-addition, with the key intermediates 5\(_{int}\) and 6\(_{int}\) being isolated and characterized. In contrast, the cooperative Ge−H was reversible and stayed at the 1,2-addition step.
A convenient route for the synthesis of the cAAC\(^{Me}\) (cAAC=cyclic (alkyl)(amino)carbene, cAAC\(^{Me}\)=1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene) and cAAC\(^{Cy}\) (cAAC\(^{Cy}\)=2-azaspiro[4.5]dec-2-(2,6-diisopropylphenyl)-3,3-dimethyl-1-ylidene) stabilized stibinidenes cAAC\(^{Me}\)⋅SbMes (2a) (Mes=2,4,6-trimethylphenyl) and cAAC\(^{Cy}\)⋅SbMes (2b) is reported. A mechanism for the formation of [cAAC\(^{R}\)Cl][SbCl\(_{3}\)Mes] 1 and cAAC\(^{R}\)⋅SbMes 2 from the reaction of cAAC with the antimony(III) precursor SbCl\(_{2}\)Mes, which proceeds via the isolable intermediate [cAAC\(^{R}\)SbClMes][SbCl\(_{3}\)Mes] (3), is proposed.
Efficient quadrupolar chromophores (A–pi–A) with triarylborane moieties as acceptors have been studied by the Marder group regarding their non‐linear optical properties and two‐photon absorption ability for many years. Within the present work, this class of dyes found applications in live‐cell imaging. Therefore, the dyes need to be water‐soluble and water‐stable in diluted aqueous solutions, which was examined in Chapter 2. Furthermore, the influence of the pi‐bridge on absorption and emission maxima, fluorescence quantum yields and especially the two-photon absorption properties of the chromophores was investigated in Chapter 3. In Chapter 4, a different strategy for the design of efficient two‐photon excited fluorescence imaging dyes was explored using dipoles (D–A) and octupoles (DA3). Finding the optimum balance between water‐stability and pi‐conjugation and, therefore, red‐shifted absorption and emission and high fluorescence quantum yields, was investigated in Chapter 5
The desymmetrization of the cyclic (alkyl)(amino)carbene-supported diboracumulene, B\(_2\)(cAAC\(^{Me}\))\(_2\) (cAAC\(^{Me}\) = 1- (2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) by mono-adduct formation with IMe\(^{Me}\) (1,3-dimethylimidazol-2-ylidene) yields the zerovalent sp-sp\(^2\) diboron compound B\(_2\)(cAAC\(^{Me}\))\(_2\)(IMe\(^{Me}\)), which provides a versatile platform for the synthesis of novel symmetrical and unsymmetrical zerovalent sp\(^2\)-sp\(^2\) diboron compounds by adduct formation with IMe\(^{Me}\) and CO, respectively. Furthermore, B\(_2\)(cAAC\(^{Me}\))\(_2\)(IMe\(^{Me}\)) displays enhanced reactivity compared to its symmetrical precursor, undergoing spontaneous intramolecular C-H activation and facile twofold hydrogenation, the latter resulting in B-B bond cleavage and the formation of the mixed-base parent borylene, (cAAC\(^{Me}\))(IMe\(^{Me}\))BH.
Activation of Ge−H and Sn−H Bonds with N‐Heterocyclic Carbenes and a Cyclic (Alkyl)(amino)carbene
(2023)
A study of the reactivity of several N‐heterocyclic carbenes (NHCs) and the cyclic (alkyl)(amino)carbene 1‐(2,6‐di‐iso‐propylphenyl)‐3,3,5,5‐tetramethyl‐pyrrolidin‐2‐ylidene (cAAC\(^{Me}\)) with the group 14 hydrides GeH2Mes2 and SnH2Me2 (Me=CH\(_{3}\), Mes=1,3,5‐(CH\(_{3}\))\(_{3}\)C\(_{6}\)H\(_{2}\)) is presented. The reaction of GeH\(_{2}\)Mes\(_{2}\) with cAAC\(^{Me}\) led to the insertion of cAAC\(^{Me}\) into one Ge−H bond to give cAAC\(^{Me}\)H−GeHMes\(_{2}\) (1). If 1,3,4,5‐tetramethyl‐imidazolin‐2‐ylidene (Me\(_{2}\)Im\(^{Me}\)) was used as the carbene, NHC‐mediated dehydrogenative coupling occurred, which led to the NHC‐stabilized germylene Me\(_{2}\)Im\(^{Me}\)⋅GeMes\(_{2}\) (2). The reaction of SnH\(_{2}\)Me\(_{2}\) with cAAC\(^{Me}\) also afforded the insertion product cAAC\(^{Me}\)H−SnHMe\(_{2}\) (3), and reaction of two equivalents Me\(_{2}\)Im\(^{Me}\) with SnH\(_{2}\)Me\(_{2}\) gave the NHC‐stabilized stannylene Me\(_{2}\)Im\(^{Me}\)⋅SnMe\(_{2}\) (4). If the sterically more demanding NHCs Me\(_{2}\)Im\(^{Me}\), 1,3‐di‐isopropyl‐4,5‐dimethyl‐imidazolin‐2‐ylidene (iPr\(_{2}\)Im\(^{Me}\)) and 1,3‐bis‐(2,6‐di‐isopropylphenyl)‐imidazolin‐2‐ylidene (Dipp\(_{2}\)Im) were employed, selective formation of cyclic oligomers (SnMe\(_{2}\))\(_{n}\) (5; n=5–8) in high yield was observed. These cyclic oligomers were also obtained from the controlled decomposition of cAAC\(^{Me}\)H−SnHMe\(_{2}\) (3).
Adducts of the parent boraphosphaketene H\(_2\)BPCO and their decarbonylative insertion chemistry
(2021)
The first examples of Lewis base adducts of the parent boraphosphaketene (H\(_2\)B-PCO) and their cyclodimers are prepared. One of these adducts is shown to undergo mild decarbonylation and phosphinidene insertion into a B−C bond of a borole, forming very rare examples of 1,2-phosphaborinines, B/P isosteres of benzene. The strong donor properties of these 1,2-phosphaborinines are confirmed by the synthesis of their π complexes with the Group 6 metals.
In an attempt to assess the structural requirements of hexahydro-sila-difenidol for potency and selectivity, a series of analogues modified in the amino group and the phenyl ring were investigated for their affinity to muscarinic M1- (rabbit vas deferens), Mr (guinea-pig atria) and Mr (guinea-pig ileum) receptors. All compounds were competitive antagonists in the three tissues. Their affinities to the three muscarinic receptor subtypes differed by more than two orders of magnitude and the observed receptor selectivities were not associated with high affinity. The pyrrolidino and hexamethyleneimino analogues, compounds substituted in the phenylring with a methoxy group or a chlorine atom as weil as p-fluoro-hexahydro-difenidol displayed the same affinity profile as the parent compound, hexahydro-sila-difenidol: M1 = M3 > M2 • A different selectivity patternwas observed for p-fluoro-hexahydro-sila-difenidol: M3 > M1 > M2 • This compound exhibited its highest affinity for M3-receptors in guinea-pig ileum (pA 2 = 7.84), intermediate affinity for M1-receptors in rabbit vas deferens (pA 2 = 6.68) and lowest affinity for the Mrreceptors in guinea-pig atria (pA 2 = 6.01). This receptor selectivity profile of p-fluoro-hexahydro-sila-difenidol was confirmed in ganglia (M1), atria (M2 ) and ileum (M 3 ) of the rat. Furthermore, dose ratios obtained with either pirenzepine (Mt) or hexahydrosila- difenidol (M2 and M3) and the p-fluoro analogue used in combination suggested that the antagonism was additive, implying mutual competition with a single population of muscarinic receptor subtypes. These results indicate that p-fluoro-hexahydro-sila-difenidol represents a valuable tool for characterization of muscarinic receptor subtypes.
Designing highly efficient purely organic phosphors at room temperature remains a challenge because of fast non-radiative processes and slow intersystem crossing (ISC) rates. The majority of them emit only single component phosphorescence. Herein, we have prepared 3 isomers (o, m, p-bromophenyl)-bis(2,6-dimethylphenyl)boranes. Among the 3 isomers (o-, m- and p-BrTAB) synthesized, the ortho-one is the only one which shows dual phosphorescence, with a short lifetime of 0.8 ms and a long lifetime of 234 ms in the crystalline state at room temperature. Based on theoretical calculations and crystal structure analysis of o-BrTAB, the short lifetime component is ascribed to the T\(^M_1\) state of the monomer which emits the higher energy phosphorescence. The long-lived, lower energy phosphorescence emission is attributed to the T\(^A_1\) state of an aggregate, with multiple intermolecular interactions existing in crystalline o-BrTAB inhibiting nonradiative decay and stabilizing the triplet states efficiently.
Amber Light Control of Peptide Secondary Structure by a Perfluoroaromatic Azobenzene Photoswitch
(2023)
The incorporation of photoswitches into the molecular structure of peptides and proteins enables their dynamic photocontrol in complex biological systems. Here, a perfluorinated azobenzene derivative triggered by amber light was site‐specifically conjugated to cysteines in a helical peptide by perfluoroarylation chemistry. In response to the photoisomerization (trans→cis) of the conjugated azobenzene with amber light, the secondary structure of the peptide was modulated from a disorganized into an amphiphilic helical structure.
Aminotroponiminate (ATI) ligands are a versatile class of redox-active and potentially cooperative ligands with a rich coordination chemistry that have consequently found a wide range of applications in synthesis and catalysis. While backbone substitution of these ligands has been investigated in some detail, the impact of electron-withdrawing groups on the coordination chemistry and reactivity of ATIs has been little investigated. We report here Li, Na, and K salts of an ATI ligand with a nitro-substituent in the backbone. It is demonstrated that the NO2 group actively contributes to the coordination chemistry of these complexes, effectively competing with the N,N-binding pocket as a coordination site. This results in an unprecedented E/Z isomerisation of an ATI imino group and culminates in the isolation of the first “naked” (i. e., without directional bonding to a metal atom) ATI anion. Reactions of sodium ATIs with silver(I) and tritylium salts gave the first N,N-coordinated silver ATI complexes and unprecedented backbone substitution reactions. Analytical techniques applied in this work include multinuclear (VT-)NMR spectroscopy, single-crystal X-ray diffraction analysis, and DFT calculations.
To understand basic principles about the interaction of electromagnetic radiation with matter is often a challenge in chemical education due to the difficult theoretical background of this topic. The present contribution therefore offers an experimental based introduction into the basic principles of UV/Vis spectroscopy following a three-step strategy. The starting point is to construct a simple self-built spectrometer working within the visible range of light. Learners can explore the most important components of such a device and understand their functions without previous knowledge. In a second step, emission spectra of different common light sources are investigated and compared. Finally, spectroscopic experiments are suggested for chemical education such as the qualitative detection of cations and the quantitative analysis of the dye carmine in food. This context-based introduction links chemical applications with the everyday life. It can be presumed that this way, learners are provided an easier access to radiation-matter interaction.
Using a new divergent approach, conjugated triarylborane dendrimers were synthesized up to the 2nd generation. The synthetic strategy consists of three steps: 1) functionalization, via iridium catalyzed C−H borylation; 2) activation, via fluorination of the generated boronate ester with K[HF\(_{2}\)] or [N(nBu\(_{4}\))][HF\(_{2}\)]; and 3) expansion, via reaction of the trifluoroborate salts with aryl Grignard reagents. The concept was also shown to be viable for a convergent approach. All but one of the conjugated borane dendrimers exhibit multiple, distinct and reversible reduction potentials, making them potentially interesting materials for applications in molecular accumulators. Based on their photophysical properties, the 1st generation dendrimers exhibit good conjugation over the whole system. However, the conjugation does not increase further upon expansion to the 2nd generation, but the molar extinction coefficients increase linearly with the number of triarylborane subunits, suggesting a potential application as photonic antennas.
A one-pot synthesis of a CAAC-stabilized, unsymmetrical, cyclic diborene was achieved via consecutive two-electron reduction steps from an adduct of CAAC and B\(_2\)Br\(_4\)(SMe\(_2\))\(_2\). Theoretical studies revealed that this diborene has a considerably smaller HOMO–LUMO gap than those of reported NHC- and phosphine-supported diborenes. Complexation of the diborene with [AuCl(PCy\(_3\))] afforded two diborene–Au\(^I\) π complexes, while reaction with DurBH\(_2\), P\(_4\) and a terminal acetylene led to the cleavage of B−H, P−P, and C−C π bonds, respectively. Thermal rearrangement of the diborene gave an electron-rich cyclic alkylideneborane, which readily coordinated to Ag\(^I\) via its B=C double bond.
Anionic Adducts
Sp2-sp3 tetraalkoxy diboron compounds have gained attention due to the development of new, synthetically useful catalytic reactions either with or without transition-metals. Lewis-base adducts of the diboron(4) compounds were suggested as possible intermediates in Cu catalyzed borylation reactions some time ago. However, intermolecular adducts of tetraalkoxy diboron compounds have not been studied yet in great detail. In preliminary studies, we have synthesized a series of anionic sp2-sp3 adducts of B2pin2 with alkoxy-groups (L = [OMe]–, [OtBu]–), a phenoxy-group (L = [4-tBuC6H4O]–) and fluoride (L = [F]–, with [nBu4N]+ as the counter ion) as Lewis-bases.
Neutral Adducts
Since their isolation and characterization, applications of N-heterocyclic carbenes (NHCs) and related molecules, e.g., cyclic alkylaminocarbenes (CAACs) and acyclic diaminocarbenes (aDCs), have grown rapidly. Their use as ligands in homogeneous catalysis and directly in organocatalysis, including recently developed borylation reactions, is now well established. Recently, several examples of ring expansion reactions (RER) involving NHCs were reported to take place at elevated temperatures, involving Be, B, and Si.
Furthermore, preliminary studies in the group of Marder et al. showed the presence of neutral sp2-sp3 diboron compounds with B2pin2 and the NHC Cy2Im. In this work, we focused on the synthesis and characterization of further neutral sp2-sp3 as well as sp3-sp3 diboron adducts with B2cat2 and B2neop2 and different NHCs. Whereas the mono-NHC adduct is stable for several hours at temperatures up to 60 °C, the bis-NHC adducts undergo thermally induced rearrangement to form the ring expanded products compound 26 and 27. B2neop2 is much more reactive than B2cat2 giving ring expanded product 29 at room temperature in quantitative yields, demonstrating that NHC ring expansion and B–B bond cleavage can be very facile processes.
Whereas the mono-NHC adduct is stable for several hours at temperatures up to 60 °C, the bis-NHC adducts undergo thermally induced rearrangement to form the ring expanded products compound 26 and 27. B2neop2 is much more reactive than B2cat2 giving ring expanded product 29 at room temperature in quantitative yields, demonstrating that NHC ring expansion and B–B bond cleavage can be very facile processes.
A variety of muscarinic antagonists are currently used as tools to pharmacologically subclassify muscarinic receptors into M\(_1\), M\(_2\) and M\(_3\) subtypes. ln the present study I we have determined the affinity proflies of several of these antagonists at five cloned human muscarinic receptors (m1-m5) stably expressed in Chinesehamster ovary cells (CHO-K1). At all five receptorsl the (R)-enantiomers of trihexyphenidyl and hexbutinol displayed considerably higher affinities (up to 525-fold) than their corresponding (S)-isomers. The stereoselectivity ratios [inhibition constant( S)/inhibition constant(R)] for both pairs of enantiomers were lowest at m2 receptors, suggesting that less stringent configurational demands are made by this receptor subtype. The "M\(_1\)-selective" antagonist (R)-trihexyphenidyl displayed high affinities for m1 and m4 receptors. The "M\(_2\)-selective" antagonists himbacinel (±}-5, 11-dihydro-11-1[(2-[(dipropylamino)methyl]-1- piperidinyllethyl)amino]carbonyii-6H-pyrido(213-b)(1 ~4)benzodiazepine- 6-one (AF-DX 384)1 11-(14-[4-(diethylamino)butyl)-1-piperidinyll acetyl)-5~ 11-dihydro-6H-pyrido(2~3-b) (1~4)benzodiazepine-6-one (AQ-RA 741) and (+K11-(12-[(diethylamino)methyl]-1-piperidinyll acetyl)-5~ 11-di-hydro-6H-pyrido(2~3-b)(1,4)benzodiazepine-6-one (AF-OX 250; the (+)-enantiomer of AF-DX 116] exhibited high affinities for m2 and m41 intermediate affinities for m1 and m3 and low affinities for m5 receptors. This selectivity profile was most prominent for AQ-RA 7 41 I which displayed 195- and 129-fold higher affinities for m2 and m4 receptors than for mS receptors. The "M\(_3\)-selective" antagonist (±)-p-fluoro-hexahydro-sila-difenidol hydrochloride (pFHHsiD) exhibited high affinity for m1 I m3 and m4 receptors. 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) bound with up to 7 -fold higher affinities to m1 I m31 m4 and m5 receptors than to m2 receptors. Although none of the tested antagonists showed more than 2-fold selectivity for one subtype over all other subtypes, each receptor displayed a unique antagonist binding profile.
Antimicrobial resistance is a growing global concern in human and veterinary medicine, with an ever-increasing void in the arsenal of clinicians. Novel classes of compounds including carbon monoxoide-releasing molecules (CORMs), for example the light-activated metal complex [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br, could be used as alternatives/to supplement traditional antibacterials. Avian pathogenic \(Escherichia\) \(coli\) (APEC) represent a large reservoir of antibiotic resistance and can cause serious clinical disease in poultry, with potential as zoonotic pathogens, due to shared serotypes and virulence factors with human pathogenic \(E.\) \(coli\). The \(in\) \(vitro\) activity of [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br against multidrug-resistant APECs was assessed via broth microtitre dilution assays and synergy testing with colistin performed using checkerboard and time-kill assays. \(In\) \(vivo\) antibacterial activity of [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br alone and in combination with colistin was determined using the \(Galleria\) \(mellonella\) wax moth larvae model. Animals were monitored for life/death, melanisation and bacterial numbers enumerated from larval haemolymph. \(In\) \(vitro\) testing produced relatively high [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br minimum inhibitory concentrations (MICs) of 1024 mg/L. However, its activity was significantly increased with the addition of colistin, bringing MICs down to \(\geq\)32 mg/L. This synergy was confirmed in time-kill assays. \(In\) \(vivo\) assays showed that the combination of [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br with colistin produced superior bacterial killing and significantly increased larval survival. In both \(in\) \(vitro\) and \(in\) \(vivo\) assays light activation was not required for antibacterial activity. This data supports further evaluation of [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br as a potential agent for treatment of systemic infections in humans and animals, when used with permeabilising agents such as colistin.
Aims: We set out to investigate the antibacterial activity of a new Mn-based photoactivated carbon monoxide-releasing molecule (PhotoCORM, [Mn(CO)\(_3\)(tpa-kappa\(^3\)N)]\(^+\)) against an antibiotic-resistant uropathogenic strain (EC958) of Escherichia coli. Results: Activated PhotoCORM inhibits growth and decreases viability of E. coli EC958, but non-illuminated carbon monoxide-releasing molecule (CORM) is without effect. NADH-supported respiration rates are significantly decreased by activated PhotoCORM, mimicking the effect of dissolved CO gas. CO from the PhotoCORM binds to intracellular targets, namely respiratory oxidases in strain EC958 and a bacterial globin heterologously expressed in strain K-12. However, unlike previously characterized CORMs, the PhotoCORM is not significantly accumulated in cells, as deduced from the cellular manganese content. Activated PhotoCORM reacts avidly with hydrogen peroxide producing hydroxyl radicals; the observed peroxide-enhanced toxicity of the PhotoCORM is ameliorated by thiourea. The PhotoCORM also potentiates the effect of the antibiotic, doxycycline. Innovation: The present work investigates for the first time the antimicrobial activity of a light-activated PhotoCORM against an antibiotic-resistant pathogen. A comprehensive study of the effects of the PhotoCORM and its derivative molecules upon illumination is performed and mechanisms of toxicity of the activated PhotoCORM are investigated. Conclusion: The PhotoCORM allows a site-specific and time-controlled release of CO in bacterial cultures and has the potential to provide much needed information on the generality of CORM activities in biology. Understanding the mechanism(s) of activated PhotoCORM toxicity will be key in exploring the potential of this and similar compounds as antimicrobial agents, perhaps in combinatorial therapies with other agents.
Fluorinated compounds are an important motif, particularly in pharmaceuticals, as one-third of the top performing drugs have fluorine in their structures. Fluorinated biaryls also have numerous applications in areas such as material science, agriculture, crystal engineering, supramolecular chemistry, etc. Thus, the development of new synthetic routes to fluorinated chemical compounds is an important area of current research. One promising method is the borylation of suitable precursors to generate fluorinated aryl boronates as versatile building blocks for organic synthesis.
Chapter 1
In this chapter, the latest developments in the synthesis, stability issues, and applications of fluorinated aryl boronates in organic synthesis are reviewed. The catalytic synthesis of fluorinated aryl boronates using different methods, such as C–H, C–F, and C–X (X = Cl, Br, I, OTf) borylations are discussed. Further studies covering instability issues of the fluorinated boronate derivatives, which are accelerated by ortho-fluorine, have been reported, and the applications of these substrates, therefore, need special treatment.
Numerous groups have reported methods to employ highly fluorinated aryl boronates that anticipate the protodeboronation issue; thus, polyfluorinated aryl boronates, especially those containing ortho-fluorine substituents, can be converted into chloride, bromide, iodide, phenol, carboxylic acid, nitro, cyano, methyl esters, and aldehyde analogues. These substrates can be applied in many cross-coupling reactions, such as the Suzuki-Miyaura reaction with aryl halides, the Chan-Evans-Lam C–N reaction with aryl amines or nitrosoarenes, C–C(O) reactions with N-(aryl-carbonyloxy)phthalamides or thiol esters (Liebskind-Srogl cross-coupling), and oxidative coupling reactions with terminal alkynes. Furthermore, the difficult reductive elimination from the highly stable complex [PdL2(2,6-C6F2+nH3-n)2] was the next challenge to be targeted in the homocoupling of 2,6-di-fluoro aryl pinacol boronates, and it has been solved by conducting the reaction in arene solvents that reduce the energy barrier in this step as long as no coordinating solvent or ancillary ligand is employed.
Chapter 2
In this chapter, phenanthroline-ligated copper complexes proved to be efficient catalysts for the Suzuki-Miyaura cross-coupling of highly fluorinated aryl boronate esters (ArF–Bpin) with aryl iodides or bromides. This newly developed method is an attractive alternative to the traditional methods as copper is an Earth-abundant metal, less toxic, and cheaper compared to the traditional methods which commonly required palladium catalysts, and silver oxide that is also often required in stoichiometric amounts. A combination of 10 mol% copper iodide and 10 mol% phenanthroline, with CsF as a base, in DMF, at 130 ˚C, for 18 hours is efficient to cross-couple fluorinated aryl pinacol boronates with aryl iodides to generate cross-coupled products in good to excellent yields. This method is also viable for polyfluorophenyl borate salts such as pentafluorophenyl-BF3K. Notably, employing aryl bromides instead of aryl iodides for the coupling with fluorinated aryl–Bpin compounds is also possible; however, increased amounts of CuI/phenanthroline catalyst is necessary, in a mixture of DMF and toluene (1:1).
A diverse range of π···π stacking interactions is observed in the cross-coupling products partly perfluorinated biaryl crystals. They range from arene–perfluoroarene interactions (2-(perfluorophenyl)naphthalene and 2,3,4-trifluorobiphenyl) to arene–arene (9-perfluorophenyl)anthracene) and perfluoroarene–perfluoroarene (2,3,4,5,6-pentafluoro-2’methylbiphenyl) interactions.
Chapter 3
In this chapter, the efficient Pd-catalyzed homocoupling reaction of aryl pinacol pinacol boronates (ArF–Bpin) that contain two ortho-fluorines is presented. The reaction must be conducted in a “noncoordinating” solvent such as toluene, benzene, or m-xylene and, notably, stronger coordinating solvents or ancillary ligands have to be avoided. Thus, the Pd center becomes more electron deficient and the reductive elimination becomes more favorable. The Pd-catalyzed homocoupling reaction of di-ortho-fluorinated aryl boronate derivatives is difficult in strongly coordinating solvents or in the presence of strong ancillary ligands, as the reaction stops at the [PdL2(2,6-C6F2+nH3-n)2] stage after the transmetalations without the reductive elimination taking place. It is known that the rate of reductive elimination of Ar–Ar from [ML2(Ar)(Ar)] complexes containing group-10 metals decreases in the order Arrich–Arpoor > Arrich–Arrich > Arpoor–Arpoor. Furthermore, reductive elimination of the most electron-poor diaryls, such as C6F5–C6F5, from [PdL2(C6F5)2] complexes is difficult and has been a challenge for 50 years, due to their high stability as the Pd–Caryl bond is strong. Thus, the Pd-catalyzed homocoupling of perfluoro phenyl boronates is found to be rather difficult.
Further investigation showed that stoichiometric reactions of C6F5Bpin, 2,4,6-trifluorophenyl–Bpin, or 2,6-difluorophenyl–Bpin with palladium acetate in MeCN stops at the double transmetalation step, as demonstrated by the isolation of cis-[Pd(MeCN)2(C6F5)2], cis-[Pd(MeCN)2(2,4,6-C6F3H2)2], and cis-[Pd(MeCN)2(2,6-C6F2H3)2] in quantitative yields. Thus, it can be concluded that the reductive elimination from diaryl-palladium complexes containing two ortho-fluorines in both aryl rings, is difficult even in a weakly coordinating solvent such as MeCN. Therefore, even less coordinating solvents are needed to make the Pd center more electron deficient. Reactions using “noncoordinating” arene solvents such as toluene, benzene, or m-xylene were conducted and found to be effective for the catalytic homocoupling of 2,6-C6F2+nH3-nBpin. The scope of the reactions was expanded. Using toluene as the solvent, the palladium-catalyzed homocoupling of ArF–Bpin derivatives containing one, two or no ortho-fluorines gave the coupled products in excellent yields without any difficulties.
DFT calculations at the B3LYP-D3/def2-TZVP/6-311+g(2d,p)/IEFPCM // B3LYP-D3/SDD/6-31g**/IEFPCM level of theory predicted an exergonic process and lower barrier (< 21 kcal/mol) for the reductive elimination of Pd(C6F5)2 complexes bearing arene ligands, compared to stronger coordinating solvents (acetonitrile, THF, SMe2, and PMe3), which have high barriers ( > 33.7 kcal/mol). Reductive elimination from [Pd(ηn-Ar)(C6F5)2] complexes have low barriers due to: (i) ring slippage of the arene ligand as a hapticity change from η6 in the reactant to ηn (n ≤ 3) in the transition state and the product, which led to less σ-repulsion; and (ii) more favorable π-back-bonding from Pd(ArF)2 to the arene fragment in the transition state.
Chapter 4
In this chapter, the efficient Pd-catalyzed C–Cl borylation of aryl chlorides containing two ortho-fluorines is presented. The reactions are conducted under base-free conditions to prevent the decomposition of the di-ortho-fluorinated aryl boronates, which are unstable in the presence of base. A combination of Pd(dba)2 (dba = dibenzylideneacetone) with SPhos (2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl) as a ligand is efficient to catalyze the C–Cl borylation of aryl chlorides containing two ortho-fluorine substituents without base, and the products were isolated in excellent yields. The substrate scope can be expanded to aryl chloride containing one or no ortho-fluorines and the borylated products were isolated in good to very good yield. This method provides a nice alternative to traditional methodologies using lithium or Grignard reagents.
The 2‐aryl‐3,4,5,6‐tetraphenyl‐1,2‐azaborinines 1‐EMe\(_{3}\) and 2‐EMe\(_{3}\) (E=Si, Sn; aryl=Ph (1), Mes (=2,4,6‐trimethylphenyl, 2)) were synthesized by ring‐expansion of borole precursors with N\(_{3}\)EMe\(_{3}\)‐derived nitrenes. Desilylative hydrolysis of 1‐ and 2‐SiMe\(_{3}\) yielded the corresponding N‐protonated azaborinines, which were deprotonated with nBuLi or MN(SiMe\(_{3}\))\(_{2}\) (M=Na, K) to the corresponding group 1 salts, 1‐M and 2‐M. While the lithium salts crystallized as monomeric Lewis base adducts, the potassium salts formed coordination polymers or oligomers via intramolecular K⋅⋅⋅aryl π interactions. The reaction of 1‐M or 2‐M with CO\(_{2}\) yielded N‐carboxylate salts, which were derivatized by salt metathesis to methyl and silyl esters. Salt metathesis of 1‐M or 2‐M with methyl triflate, [Cp*BeCl] (Cp*=C\(_{5}\)Me\(_{5}\)), BBr\(_{2}\)Ar (Ar=Ph, Mes, 2‐thienyl), ECl\(_{3}\) (E=B, Al, Ga) and PX\(_{3}\) (X=Cl, Br) afforded the respective group 2, 13 and 15 1,2‐azaborinin‐2‐yl complexes. Salt metathesis of 1‐K with BBr\(_{3}\) resulted not only in N‐borylation but also Ph‐Br exchange between the endocyclic and exocyclic boron atoms. Solution \(^{11}\)B NMR data suggest that the 1,2‐azaborinin‐2‐yl ligand is similarly electron‐withdrawing to a bromide. In the solid state the endocyclic bond length alternation and the twisting of the C\(_{4}\)BN ring increase with the sterics of the substituents at the boron and nitrogen atoms, respectively. Regression analyses revealed that the downfield shift of the endocyclic \(^{11}\)B NMR resonances is linearly correlated to both the degree of twisting of the C\(_{4}\)BN ring and the tilt angle of the N‐substituent. Calculations indicate that the 1,2‐azaborinin‐1‐yl ligand has no sizeable π‐donor ability and that the aromaticity of the ring can be subtly tuned by the electronics of the N‐substituent.
The aim of this work was to synthesize and functionalize different bio-relevant nanomaterials like silica-coated superparamagnetic iron oxide nanoparticles (SPIONs) as contrast agents for T2 magnetic resonance imaging (MRI) and detonation nanodiamond (DND) with the neurohormone peptide allatostatin 1 (ALST1) and a fluorescent dye. Analytical techniques for the determination and quantification of surface functional groups like amines, azides, and peptides were also developed and established.
Thus, in the first part of the work, a TGF-1 binding peptide and allatostatin 1 (ALST1), both supposed to act as active tumour targeting vectors, were synthesized by solid-phase peptide synthesis (SPPS) and characterized by high pressure liquid chromatography (HPLC) and mass spectrometry. Then, azide-functionalized silica nanoparticles were synthesized by the Stöber process and characterized by transmission electron microscopy (TEM) and infrared spectroscopy (IR). The surface loading of amine and azide groups was determined by a new protocol. The azide groups were reduced with sodium boronhydride to amine and then functionalized with Fmoc-Rink Amide linker according to a standard SPPS protocol. Upon cleavage of Fmoc by piperidine, the resulting dibenzofulvene and its piperidine adduct were quantified by UV/Vis spectroscopy and used to determine the amount of amine groups on the nanoparticle surface. Then, ALST1 and related tyrosine- and phenylalanine substituted model peptides were conjugated to the azide-functionalized silica nanoparticles by copper(I)-catalyzed azide-alkyne dipolar cycloaddition (CuAAC). The successful peptide conjugation was demonstrated by the Pauly reaction, which however is only sensitive to histidine- and tyrosine-containing peptides. As a more general alternative, the acid hydrolysis of the peptides to their individual amino acid building blocks followed by derivatization with phenyl isothiocyanate (PITC) allowed the separation, determination, and quantification of the constituent amino acids by HPLC.
In the second part of the work, amine- and azide-functionalized silica-coated superparamagnetic iron oxide nanoparticles (SPIONs) were synthesized by co-precipitation and subsequent silica-coated based on the Stöber process and characterized by TEM and IR. The amine surface loading was determined by the method already established for the pure silica systems. The azide surface loading could also be quantified by reduction with sodium boronhydride to amine groups and then conjugation to Fmoc-Rink amide linker. Upon cleavage of Fmoc with piperidine, the total amine surface loading was obtained. The amount of azide surface groups was then determined from the difference of the total amine surface loading and the amine surface loading. Thus, it was possible to quantify both amine and azide surface groups on a single nanoparticle system. Superparamagnetic iron oxide nanoparticles (SPIONs) are potent T2 contrast agents for magnetic resonance imaging (MRI). Due to their natural metabolism after injection into the blood stream, SPIONs mostly end up inside macrophages, liver, spleen or kidneys. To generate a potential target-specific SPION-based T2 contrast agent for MRI, the neurohormone peptide ALST1 was conjugated by CuAAC to the azide- and amine functionalized superparamagnetic iron oxide nanoparticles, since ALST1 is supposed to target difficult-to-treat neuroendocrinic tumours due to its analogy to galanin and somastatin receptor ligands. The organic fluorescent dye cyanine 5 (Cy5) was also conjugated to the silica-coated superparamagnetic iron oxide nanoparticles (SPIONs) via a NHS-ester to the amines to enable cell uptake studies by fluorescence microscopy. These constructs were characterized by TEM, dynamic light scattering (DLS), and IR. The amino acids of the conjugated ALST1 were determined by the HPLC method as described before for peptide-modified silica nanoparticle surfaces. Then, the relaxivity r2 was measured at 7 T. However, a r2 value of 27 L/mmolFe·s for the dual ALST1-/Cy5-functionalized silica-coated SPIONs was not comparable to T2 contrast agents in clinical use, since their relaxivity is commonly determined at 1.5 T, and no such instrument was available. However, it can be assumed that the synthesized dual
ALST1-/Cy5-functionalized silica-coated SPION would show a lower r2 at 1.5 T than at 7T. Commercial T2 MRI contrast agents like VSOP-C184 from Ferropharm show at r2 values of about 30 L/mmolFe·s at 1.5 T. Still, the relaxivity of the new material has some potential for application as a T2 contrast agent. Then, the material was used in cell uptake studies by fluorescence microscopy with the conjugated Cy5 dye as a probe. The dual
ALST1-/Cy5-functionalized silica-coated SPION showed a high degree of agglomeration with no cellular uptake unlike described for ALST1-functionalized nanoparticles in literature. It is assumed that upon agglomeration of the particles, constructs form which are unable to be internalized by the cellular endocytotic pathways anymore. As a future perspective, the tendency of the particle to agglomerate should be reduced by changing the coating material to polyethylene glycol (PEG) or chitosan, which are known to be bio-compatible, bio-degradable and prevent agglomeration.
In the third part of the work, the rhenium compound [ReBr(CO)3(L)] with L = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline and its manganese analogue were synthesized by heating the ligand and rhenium pentacarbonyl bromide or and manganese pentacarbonyl bromide respectively, in toluene. However, [MnBr(CO)3(L)] was unstable upon illumination by UV light at 365 nm. Thus, it was dismissed for further application. The photophysical properties of [ReBr(CO)3(L)] were explored, by determination of the excited-state life time by the time-correlated single-photon counting (TCSPC) method and the quantum yield by a fluorescence spectrometer equipped with an integration sphere. A value of = 455 ns, a Stokes shift of 197 nm and a rather low quantum yield =were found. Metal complexes are supposed to have superior properties compared to organic dyes due to their large Stokes shifts, long excited-state life times, and high quantum yields. Thus, amine- and azide-functionalized detonation nanodiamond (DND) as an alternative biological inert carrier system was functionalized with ALST1 to enhance its cell uptake properties. A luminescent probe for cell uptake studies using fluorescence microscopy was also attached, either based on the new rhenium complex or the commercially available organic dye Cy5, respectively. The aldehyde-functionalized rhenium complex was conjugated to the DND via oxime ligation, which is known to be a mild and catalyst-free conjugation method. The amount of peptide ALST1 on the DND was analyzed and quantified after acid hydrolysis and PITC derivatization by HPLC as described before. Then, the ALST1-/luminescent probe-functionalized DND was investigated for its photophysical properties by fluorescence spectroscopy. The Cy5-functionalized material showed a slightly lower fluorescence performance in aqueous solution than reported in literature and commercial suppliers with a life time < 0.4 ns and quantum yields not determinable by integration sphere due to the week signal intensity. The rhenium complex-functionalized material had a very low signal intensity in only aqueous medium, and thus determination of life times and quantum yield by fluorescence spectroscopy was not possible. After incubation with MDA-MB 231 cells, the Cy5-functionalized DND could easily be detected due to its red fluorescence. However, it was not possible to visualize the rhenium complex-functionalized DND with fluorescence microscopy due to the low fluorescence intensity of the complex in aqueous medium and the lack of proper filters for the fluorescence microscope. Cy5-functionalized DND did not show any cellular uptake in fluorescence microscopy after conjugation with ALST1. Since the nanodiamond surface is known to strongly adsorb peptides and proteins, it is assumed that the peptide chain is oriented perpendicular to the nanoparticle surface and thus not able to interact with cell membrane receptors to promote cell uptake of the particles. As a future perspective, the ALST1-promoted cellular uptake of the DND should be improved by using different linker systems for peptide conjugation to prevent adsorption of the peptide chain on the particle surface.
The new analytical methods for amino-, azide-, and peptide-functionalized nanoparticles have great potential to assist in the quantification of nanoparticle surface modifications by UV/Vis spectroscopy and HPLC. The determination of surface amine and azide groups based on the cleavage of conjugated Fmoc-Rink amide linker and detected by UV/Vis spectroscopy is applicable to all amine-/azide-functionalized nanomaterials. However, particles which form very stable suspension with the cleavage mixture can cause quantification problems due to scattering, making an accurate quantification of dibenzofulvene and its piperidine adduct impossible. The detection of tyrosine- and histidine-containing peptides based on the Pauly reaction is well-suited as a fast and easy-to-perform qualitative demonstration of successful peptide surface conjugation. However, its major drawback as a colourimetric approach is that coloured particles cannot be evaluated by this method. The amino acid analysis based on HPLC after acid hydrolysis of peptides conjugated to nanoparticle surfaces to its individual building blocks and subsequent derivatization with PITC, can be used on all nanomaterials with peptide or protein surface modification. It allows detection of amino acids down to picomolar concentrations and even enables analysis of very small peptide surface loadings. However, the resulting HPLC traces are difficult to analyze.
Three new analytical methods based on UV/Vis and HPLC techniques have been developed and established. They assisted in the characterization of the synthesized DND and SPIONs with dual functionalization by ALST1 and Cy5 or [ReBr(CO)3(L)], respectively. However, the nanomaterials showed no cellular uptake due to a high tendency to agglomerate. The cellular uptake should be improved and the tendency to agglomerate of the SPIONs should be reduced by changing the surface coating from silica to either PEG or chitosan. Furthermore, different linker systems for connecting peptides to DND surfaces should be synthesized and evaluated to reduce potential peptide chain adsorption.
A practical and direct method was developed for the production of versatile alkyl boronate esters via transition metal-free borylation of primary and secondary alkyl sulfones. The key to the success of the strategy is the use of bis(neopentyl glycolato) diboron (B\(_{2}\)neop\(_{2}\)), with a stoichiometric amount of base as a promoter. The practicality and industrial potential of this protocol are highlighted by its wide functional group tolerance, the late-stage modification of complex compounds, no need for further transesterification, and operational simplicity. Radical clock, radical trap experiments, and EPR studies were conducted which show that the borylation process involves radical intermediates.
Catalytic C−X borylation of aryl halides containing two ortho‐fluorines has been found to be challenging, as most previous methods require stoichiometric amounts of base and the polyfluorinated aryl boronates suffer from protodeboronation, which is accelerated by ortho‐fluorine substituents. Herein, we report that a combination of Pd(dba)2 (dba=dibenzylideneacetone) with SPhos (2‐dicyclohexylphosphino‐2’,6’‐dimethoxybiphenyl) as a ligand is efficient to catalyze the C‐Cl borylation of aryl chlorides containing two ortho‐fluorine substituents. This method, conducted under base‐free conditions, is compatible with the resulting di‐ortho‐fluorinated aryl boronate products which are sensitive to base.
Hexahydro-sila-difenidoJ and eight analogues behaved as simple cumpetitive inhibitors of eHJN·methyl·scopoJamine binding to homogenates frorn human neuroblastoma NB-OK 1 cells (M\(_1\) sites), rat heart (M\(_2\) sites), rat pancreas (M\(_3\) sites), and rat striatum 'B' sites (M\(_4\) sites). Pyrrolidino- and hexamethyleneimino analogues showed the same sekctivity profile as the parent compound. Hexahydro-sila-difenidol methiodide and the methiodide of p-fluoro-hexahydro·sila-difenidol had a fügher affinity but a lower selectivity than the tertiary amines. Compounds containing a p·methoxy, p-chJoro or p-fluoro substituent in the phenyl ring of hexahydro-sila-difenidol showed a qualitative)y similar selectivity profile as the parent compound (i.e., M\(_1\)= M\(_3\) = M\(_4\) >M\(_2\) ), but up to 16-fold lower affinities. o-Methoxy-hexahydro-sila-difenidol has a lower affinity than hexahydro-sila-difeni.:!o! at the four binding sites. lts selectivity profile (M\(_4\) > M\(_1\), M\(_3\) > M\(_2\) ) was different from hexahydro-sila-difenidol. Replacement of the centrat silicon atom of hexahydro-sila-difenidol, p-fluoro-hexahydro-sila-difenidol and thdr quatemary (N-methylated) analogues by a carbon atom did not change their binding affinities significantly. The iour muscarinic receptors showed a higher affinity for the (R)- than for the (S)-enantiomers of hexahydro-difenidol, p-fluorohexahydro-difenidol and their methiodides. The stereoselectivity varied depending on the receptor subtype and drug considered.
l In an attempt to assess the structural requirements for the musearlnie receptor selectivity of hexahydro-diphenidol (hexahydro-difenidol) and hexahydro-sila-diphenidol (hexahydro-sila-difenidol), a serles of structurally related C/Si pairs were investigated, along with atropine, pirenzepine and methoctramine, for their binding affinities in NB-OK 1 cells as well as in rat heart and pancreas. 2 The action of these antagonists at musearlnie receptors mediating negative inotropic responses in guinea-pig atrla and ileal contractions has also been assessed. 3 Antagonist binding data indicated that NB-OK 1 cells (M\(_1\) type) as weil as rat heart (cardiac type) and pancreas (glandularjsmooth muscle type) possess different muscarinic receptor subtypes. 4 A highly significant correlation was found between the binding affinities of the antagonists to muscarinic receptors in rat heart and pancreas, respectively, and the affinities to muscarinic receptors in guinea-pig atria and ileum. This implies that the musearlnie binding sites in rat heart and the receptors in guinea-pig atrla are essentially similar, but different from those in pancreas and ileum. 5 The antimuscarinic potency of hexahydro-diphenidol and hexahydro-sila-diphenidol at the three subtypes was inftuenced differently by structural modifications (e.g. quaternization). Different selectivity profiles for the antagonists were obtained, which makes these compounds useful tools to investigate further muscarinic receptor heterogeneity. lndeed, the tertiary analogues hexahydrodiphenidol (HHD) and hexahydro-sila-diphenidol (HHSiD) bad an M\(_1\) = glandularjsmooth muscle > cardiac selectivity profile, whereas the quaternary analogues HHD methiodide and HHSiD methiodide were M\(_1\) preferring (M\(_1\) > glandularjsmooth muscle, cardiac).
1 We have compared the binding properties of several hexocyclium and sila-hexocyclium derivatives to muscarinic Ml receptors (in rat brain, human neuroblastoma (NB-OK I) cells and calf superior cervical ganglia), rat heart M2 receptors, rat pancreas M3 receptors and M4 receptors in rat striatum, with their functional antimuscarinic properties in rabbit vas deferens (Ml/M4-like), guinea-pig atria (M2), and guinea-pig ileum (M3) muscarinic receptors.
2 Si la-substitution (C/Si exchange) of hexocyclium (~ sila-hexocyclium) and demethyl-hexocyclium (~demethyl-sila-hexocyclium) did not significantly affect their affinities for muscarinic receptors. By contrast, sila-substitution of demethoxy-hexocyclium increased its affinity 2 to 3 fold for all the muscarinic receptor subtypes studied.
3 The p-fluoro- and p-chloro-derivatives of sila-hexocyclium had lower affinities than the parent
compound at the four receptor subtypes, in binding and pharmacological studies.
4 In binding studies, o-methoxy-sila-hexocyclium (Ml = M4 ~ M3 ~ M2) had a much lower affinity than sila-hexocyclium for the four receptor subtypes, and discriminated the receptor subtypes more poorly than sila-hexocyclium (Ml = M3> M4> M2)' This is in marked contrast with the very clear selectivity of demethoxy-sila-hexocyclium for the prejunctional MtlM4-like heteroreceptors in rabbit vas deferens.
5 The tertiary amines demethyl-hexocyclium, demethyl-sila-hexocyclium and demethyl-o-methoxy-silahexocyclium had 10 to 30 fold lower affinities than the corresponding quaternary ammonium derivatives.
Bis( 4-fluorophenyl)methyl(l H-1,2,4-triazol-1-yl-methyl)germane (2), a germanium analogue of the agricultural fungicide flusilazole (1), has been synthesized from Cl\(_3\)GeCH\(_2\)CI (3) by both a three-step and a four-step synthesis (3-> (p-F-C\(_6\)H\(_4\))\(_2\)Ge(CH\(_2\)Cl)Br (4)-> (p-F-C\(_6\)H\(_4\))\(_2\)Ge(CH\(_2\)CI)CH\(_3\) (S)-> 2; S ~ (p-F-C\(_6\)H\(_4\))\(_2\)Ge(CH\(_2\)I)CH\(_3\) (6)-> l). The fungicidal properties of l have been compared with those of the parent silicon compound 1 (studies on Si/Ge bioisosterism). In various test systems, the SijGe analogues 1 and 2 showed comparable fungicidal properlies (in activity against plant pathogenic fungi: in agar plate diffusion tests and greenhause evaluations; in activity against human pathogenic fungi: in serial dilution tests). In addition, 1 and 2 displayed comparable potencies in respect of sterol biosynthesis inhibition in Sacclulromycopsis üpolytica and Pyricularia oryzae, the mode of action being primarily an inhtbition of oxidative C14-demethylation.
We report four new luminescent tetracationic bis-triarylborane DNA and RNA sensors that show high binding affinities, in several cases even in the nanomolar range. Three of the compounds contain substituted, highly emissive and structurally flexible bis(2,6-dimethylphenyl-4-ethynyl)arene linkers (3: arene=5,5′-2,2′-bithiophene; 4: arene=1,4-benzene; 5: arene=9,10-anthracene) between the two boryl moieties and serve as efficient dual Raman and fluorescence chromophores. The shorter analogue 6 employs 9,10-anthracene as the linker and demonstrates the importance of an adequate linker length with a certain level of flexibility by exhibiting generally lower binding affinities than 3–5. Pronounced aggregation–deaggregation processes are observed in fluorimetric titration experiments with DNA for compounds 3 and 5. Molecular modelling of complexes of 5 with AT-DNA, suggest the minor groove as the dominant binding site for monomeric 5, but demonstrate that dimers of 5 can also be accommodated. Strong SERS responses for 3–5 versus a very weak response for 6, particularly the strong signals from anthracene itself observed for 5 but not for 6, demonstrate the importance of triple bonds for strong Raman activity in molecules of this compound class. The energy of the characteristic stretching vibration of the C≡C bonds is significantly dependent on the aromatic moiety between the triple bonds. The insertion of aromatic moieties between two C≡C bonds thus offers an alternative design for dual Raman and fluorescence chromophores, applicable in multiplex biological Raman imaging.
Bis(μ-diisopropyl-phosphanido-\(κ^2\)P:P)bis-[hydrido(triisopropyl-phosphane-κP)platinum(II)]
(2012)
In the centrosymmetric molecular structure of the title compound \([Pt_2(C_6H_{14}P)_2H_2)(C_9H_{21}P)_2]\), each \(Pt^{II}\) atom is bound on one side to a phosphane ligand \((PiPr_3)\) and a hydrido ligand. On the other side, it is bound to two phosphanide ligands \((μ-PiPr_2)\), which engage a bridging position between the two \(Pt^{II}\) atoms, forming a distorted square-planar structure motif. The PtPt distance is 3.6755(2)Å. A comparable molecular structure was observed for bis-(μ-di-tert-butyl-phosphanido)bis-[hydrido(triethyl-phosphane)platinum(II)] [Itazaki et al. (2004 ). Organometallics, 23, 1610-1621].
The zwitterionic spirocyclic \(\lambda_5\)-germanate bis(2,3-naphthalenediolato( 2-)](pyrrolidiniomethyl)germanate (8) was synthesized and the crystal structure of its tetartoacetonitrile solvate 8 · 1/4 CH\(_3\)CN studied by single-crystal X-ray diffraction. Compound 8 was prepared by reaction of (MeO)\(_3\)GeCH\(_2\)NC\(_4\)H\(_8\) (11; NC\(_4\)H\(_8\) = pyrrolidino) with two equivalents of 2,3-naphthalenediol (isolated as 8 · 1/4 CH\(_3\)CN; yield 92%). The coordination polyhedron around the pentacoordi- naphthalenediolatonate germanium atom of 8 · 1/4 CH\(_3\)CN can be described as a strongly distorted trigonal bipyramid (the structure is displaced by 38.9% from the ideal trigonal bipyrarnid towards the ideal square pyramid), the carbon atom occupying an equatorial position. In the crystal lattice of 8 · 1/4 CH\(_3\)CN, the zwitterions form intermolecular N-H ... o hydrogen bonds leading to the formation of dimers. 1H- and \(^{13}\C-NMR studies revealed that 8 also exists in solution ([D\(_6\)]DMSO).
Bismuth Amides Mediate Facile and Highly Selective Pn–Pn Radical‐Coupling Reactions (Pn=N, P, As)
(2021)
The controlled release of well‐defined radical species under mild conditions for subsequent use in selective reactions is an important and challenging task in synthetic chemistry. We show here that simple bismuth amide species [Bi(NAr\(_2\))\(_3\)] readily release aminyl radicals [NAr\(_2\)]. at ambient temperature in solution. These reactions yield the corresponding hydrazines, Ar\(_2\)N−NAr\(_2\), as a result of highly selective N−N coupling. The exploitation of facile homolytic Bi−Pn bond cleavage for Pn−Pn bond formation was extended to higher homologues of the pnictogens (Pn=N–As): homoleptic bismuth amides mediate the highly selective dehydrocoupling of HPnR\(_2\) to give R\(_2\)Pn−PnR\(_2\). Analyses by NMR and EPR spectroscopy, single‐crystal X‐ray diffraction, and DFT calculations reveal low Bi−N homolytic bond‐dissociation energies, suggest radical coupling in the coordination sphere of bismuth, and reveal electronic and steric parameters as effective tools to control these reactions.
Bis‐NHC Aluminium and Gallium Dihydride Cations [(NHC)\(_{2}\)EH\(_{2}\)]\(^{+}\) (E = Al, Ga)
(2020)
The NHC alane and gallane adducts (NHC)·AlH\(_{2}\)I (NHC = Me\(_{2}\)Im\(^{Me}\) 7, iPr\(_{2}\)Im 8, iPr\(_{2}\)Im\(^{Me}\) 9) and (NHC)·GaH\(_{2}\)I (NHC = Me\(_{2}\)Im\(^{Me}\) 10, iPr\(_{2}\)Im\(^{Me}\) 11, Dipp\(_{2}\)Im 12; R\(_{2}\)Im = 1,3‐di‐organyl‐imidazolin‐2‐ylidene; Dipp = 2,6‐diisopropylphenyl; iPr = isopropyl; Me\(_{2}\)Im\(^{Me}\) = 1,3,4,5‐tetra‐methyl‐imidazolin‐2‐ylidene) were prepared either by the simple yet efficient reaction of the NHC adduct (NHC)·AlH\(_{3}\) with elemental iodine or by the treatment of (NHC)·GaH\(_{3}\) with an excess of methyl iodide at room temperature. The reaction of one equivalent of the group 13 NHC complexes with an additional equivalent of the corresponding NHC afforded cationic aluminium and gallium hydrides [(NHC)\(_{2}\)·AlH\(_{2}\)]\(^{+}\)I− (NHC = Me\(_{2}\)Im\(^{Me}\) 13, iPr\(_{2}\)Im 14, iPr\(_{2}\)Im\(^{Me}\) 15) and [(NHC)\(_{2}\)·GaH\(_{2}\)]\(^{+}\)I− (NHC = Me\(_{2}\)Im\(^{Me}\) 16, iPr\(_{2}\)Im\(^{Me}\) 17) and the normal and abnormal NHC coordinated compound [(Dipp\(_{2}\)Im)·GaH\(_{2}\)(aDipp\(_{2}\)Im)]+I− 18. Compounds 7–18 were isolated and characterized by means of elemental analysis, IR and multinuclear NMR spectroscopy and by X‐ray diffraction of the compounds 7, 9, 10, 15, 16 and 18.
The synthesis, photophysical, and electrochemical properties of selectively mono-, bis- and tris-dimethylamino- and trimethylammonium-substituted bis-triarylborane bithiophene chromophores are presented along with the water solubility and singlet oxygen sensitizing efficiency of the cationic compounds Cat\(^{1+}\), Cat\(^{2+}\), Cat(i)\(^{2+}\), and Cat\(^{3+}\). Comparison with the mono-triarylboranes reveals the large influence of the bridging unit on the properties of the bis-triarylboranes, especially those of the cationic compounds. Based on these preliminary investigations, the interactions of Cat\(^{1+}\), Cat\(^{2+}\), Cat(i)\(^{2+}\), and Cat\(^{3+}\) with DNA, RNA, and DNApore were investigated in buffered solutions. The same compounds were investigated for their ability to enter and localize within organelles of human lung carcinoma (A549) and normal lung (WI38) cells showing that not only the number of charges but also their distribution over the chromophore influences interactions and staining properties.
Blending different low molecular weight gelators (LMWGs) provides a convenient route to tune the properties of a gel and incorporate functionalities such as fluorescence. Blending a series of gelators having a common bis-urea motif, and functionalised with different amino acid-derived end-groups and differing length alkylene spacers is reported. Fluorescent gelators incorporating 1- and 2-pyrenyl moieties provide a probe of the mixed systems alongside structural and morphological data from powder diffraction and electron microscopy. Characterisation of the individual gelators reveals that although the expected α-urea tape motif is preserved, there is considerable variation in the gelation properties, molecular packing, fibre morphology and rheological behaviour. Mixing of the gelators revealed examples in which: 1) the gels formed separate, orthogonal networks maintaining their own packing and morphology, 2) the gels blended together into a single network, either adopting the packing and morphology of one gelator, or 3) a new structure not seen for either of the gelators individually was created. The strong binding of the urea functionalities to anions was exploited as a means of breaking down the gel structure, and the use of fluorescent gel blends provides new insights into anion-mediated gel dissolution.
While polysulfones constitute a class of well‐established, highly valuable applied materials, knowledge about polymers based on the related sulfoximine group is very limited. We have employed functionalized diaryl sulfoximines and a p ‐phenylene bisborane as building blocks for unprecedented BN‐ and BO‐doped alternating inorganic–organic hybrid copolymers. While the former were accessed by a facile silicon/boron exchange protocol, the synthesis of polymers with main‐chain B–O linkages was achieved by salt elimination.
Within this thesis, the analysis and hence the better comprehension of the chemical bond within metal–element compounds is the central topic. By use of various DFT methods a selection of M–E interactions have been modeled and analyzed via Bader’s QTAIM, the ELF and NBO techniques. Special focus was set on a series of transition metal borylene and carbene complexes, and the Li–C bonds as representatives for main group organometallics. Therefore, this thesis is split into three parts:(I) An introduction reviewing the quantum chemical machinery as well as the analysis tools applied for the evaluation of chemical bonds. (II) Within the second part the chemical interactions taking place in transition metal complexes are studied focusing on borylenes and cognate carbenes. (III) In Part III, a broad overview of the appropriate modeling and nature of the Li–C bond as well as intermolecular interactions in methyllithium is provided.
Among the parent borirane, benzoborirene and ortho‐dicarbadodecaborane‐fused borirane, the latter possesses the highest ring strain and the highest Lewis acidity according to our density functional theory (DFT) studies. The synthesis of this class of compounds is thus considerably challenging. The existing examples require either a strong π‐donating group or an extra ligand for B‐coordination, which nevertheless suppresses or completely turns off the Lewis acidity. The title compound, which possesses both features, not only allows the 1,2‐insertion of P=O, C=O or C≡N to proceed under milder conditions, but also enables the heretofore unknown dearomative 1,4‐insertion of Ar−(C=O)− into a B−C bond. The fusion of strained molecular systems to an o‐carborane cage shows great promise for boosting both the ring strain and acidity.
Diplatinum A‐frame complexes with a bridging (di)boron unit in the apex position were synthesized in a single step by the double oxidative addition of dihalo(di)borane precursors at a bis(diphosphine)‐bridged Pt\(^{0}\)\(_{2}\) complex. While structurally analogous to well‐known μ‐borylene complexes, in which delocalized dative three‐center‐two‐electron M‐B‐M bonding prevails, theoretical investigations into the nature of Pt−B bonding in these A‐frame complexes show them to be rare dimetalla(di)boranes displaying two electron‐sharing Pt−B σ‐bonds. This is experimentally reflected in the low kinetic stability of these compounds, which are prone to loss of the (di)boron bridgehead unit.
Cyclic alkyl(amino)carbene-stabilized (cyano)hydroboryl anions were synthesized by deprotonation of (cyano)dihydroborane precursors. While they display boron-centered nucleophilic reactivity towards organohalides, generating fully unsymmetrically substituted cyano(hydro)organoboranes, they show cyano-nitrogen-centered nucleophilic reactivity towards haloboranes, resulting in the formation of hitherto unknown linear 2-aza-1,4-diborabutatrienes.
Project Borylene
A new borylene ligand ({BN(SiMe\(_3\))(t-Bu)}) has been successfully synthesized bound in a terminal manner to base metal scaffolds of the type [M(CO)\(_5\)] (M = Cr, Mo, and W), yielding complexes [(OC)\(_5\)Cr{BN(SiMe\(_3\))(t-Bu)}] (19), [(OC)\(_5\)Mo{BN(SiMe\(_3\))(t- Bu)}] (20), and [(OC)\(_5\)W{BN(SiMe\(_3\))(t-Bu)}] (21) (Figure 5-1). Synthesis of complexes 19, 20, and 21 was accomplished by double salt elimination reactions of Na\(_2\)[M(CO)\(_5\)] (M = Cr (11), Mo (1), and W (12)) with the dihaloborane Br\(_2\)BN(SiMe\(_3\))(t-Bu) (18). This new “first generation” unsymmetrical borylene ligand is closely akin to the bis(trimethylsilyl)aminoborylene ligand and has been shown to display similar structural characteristics and reactivity. The unsymmetrical borylene ligand {BN((SiMe\(_3\))(t-Bu)} does display some individual characteristics of note and has experimentally been shown to undergo photolytic transfer to transition metal scaffolds in a more rapid manner, and appears to be a more reactive borylene ligand, than the previously published symmetrical {BN(SiMe\(_3\))\(_2\)} ligand, based on NMR and IR spectroscopic evidence.
Photolytic transfer reactions with this new borylene ligand ({BN((SiMe\(_3\))(t-Bu)}) were conducted with other metal scaffolds, resulting in either complete borylene transfer or partial transfer to form bridging borylene ligand interactions between the two transition metals. The unsymmetrical ligand’s coordination to early transition metals (up to Group 6) indicates a preference for a terminal coordination motif while bound to these highly Lewis acidic species. The ligand appears to form more energetically stable bridging coordination modes when bound to transition metals with high Lewis basicity (beyond Group 9) and has been witnessed to transfer to transition metal scaffolds in a terminal manner and subsequently rearrange in order to achieve a more energetically stable bridging final state.
Figure 5-2 lists the four different transfer reactions conducted between the chromium borylene species [(OC)\(_5\)Cr{BN(SiMe\(_3\))(t-Bu)}] (19) and the transition metal complexes [(η\(^5\)-C\(_5\)H\(_5\))V(CO)\(_4\)] (51), [(η\(^5\)-C\(_5\)Me\(_5\))Ir(CO)\(_2\)] (56), [(η\(^5\)-C\(_5\)H\(_4\)Me)Co(CO)\(_2\)] (59), and [{(η\(^5\)-C\(_5\)H\(_5\))Ni}\(_2\){μ-(CO)\(_2\)}] (53). These reactions successfully yielded the new “second generation” borylene complexes [(η\(^5\)-C\(_5\)H\(_5\))(OC)\(_3\)V{BN(SiMe\(_3\))(t-Bu)}] (55), [(η\(^5\)-C\(_5\)Me\(_5\))Ir{BN(SiMe\(_3\))(t-Bu)}\(_2\)] (58), [{(η\(^5\)-C\(_5\)H\(_4\)Me)Co}\(_2\)(μ-CO)\(_2\){μ- BN(SiMe\(_3\))(t-Bu)}] (61), and [{(η\(^5\)-C\(_5\)H\(_5\))Ni}\(_2\)(μ-CO){μ-BN(SiMe\(_3\))(t-Bu)}] (62), respectively.
Analysis of the accumulated data for all of the terminal borylene species discussed in this section, particularly bond distances, infrared spectroscopy, and \(^{11}\)B{\(^1\)H} NMR spectroscopic data, has been performed, and a trend in the data has led to the following conclusions:
[1] NMR spectroscopic data for the \(^{11}\)B{\(^1\)H} boron and \(^{13}\)C{\(^1\)H} carbonyl environments of the first generation borylene species ([(OC)\(_5\)M{BN(SiMe\(_3\))(t-Bu)}] (M = Cr (19), Mo (20), and W (21))) all show progressive up-field shifting as the Group 6 metal becomes heavier (Cr (19) to Mo (20) to W (21)), indicating maximum deshielding for these nuclei in the [(OC)\(_5\)Cr{BN(SiMe\(_3\))(t-Bu)}] (19) complex.
[2] The boron-metal-trans-carbon (B-M-C\(_{trans}\)) axes of the first generation borylene complexes [(OC)\(_5\)M{BN(SiMe\(_3\))(t-Bu)}] (M = Mo (20), and W (21)) are not completely linear, preventing direct IR spectroscopic comparison. The chromium analog [(OC)\(_5\)Cr{BN(SiMe\(_3\))(t-Bu)}] (19), however, is essentially linear and displays the expected three carbonyl IR stretching frequencies, all at higher energy than those of the chromium bis(trimethylsilyl)aminoborylene complex [(OC)\(_5\)Cr{BN(SiMe\(_3\))\(_2\)}] (13), indicating that the ({BN(SiMe\(_3\))(t-Bu)}) ligand is either a stronger σ-donor or a poorer π-acceptor compared to the chromium metal center.
[3] In transfer reactions, the {BN(SiMe\(_3\))(t-Bu)} fragment appears to be more stable as a terminal ligand when bound to more Lewis acidic first row transition metals and appears to prefer coordination in a bridging motif when coordinated to more Lewis basic first row transition metals.
Project Borirene
The synthesis of the first platinum bis(borirene) complexes are presented along with findings from structural and electronic examination of the role of platinum in allowing increased coplanarity and conjugation of twin borirene systems. This series of trans-platinum-linked bis(borirene) complexes (119/120, 122/123, and 125/126) all show coplanarity in the twin ring systems and stand as the first verified structural representations of two coplanar borirene systems across a linking unit. The role of a platinum atom in mediating communication between chromophoric ligands can be generalized by an expected bathochromic (red) shift in the absorption spectrum due to an increase in the electronic delocalization between the formerly independent aromatic systems when compared to the platinum mono-σ-borirenyl systems. The trans-platinum bis(borirene) scaffold serves as a simplified monomeric system that allows not only study of the effects of transition metals in mitigating electronic conjugation, but also the tunability of the overall photophysical profile of the system by exocyclic augmentation of the three-membered aromatic ring.
A series of trans-platinum bis(alkynyl) complexes were prepared (Figure 5-3) to serve as stable platforms to transfer terminal borylene ligands {BN(SiMe\(_3\))\(_2\)} onto 95, 102, 106, and 63. Mixing of cis-[PtCl\(_2\)(PEt\(_3\))\(_2\)] (93) with two equivalents of corresponding alkynes in diethylamine solutions successfully yielded trans-[Pt(C≡C-Ph)\(_2\)(PEt\(_3\))\(_2\)] (95), trans-[Pt(C≡C-p-C\(_6\)H\(_4\)OMe)\(_2\)(PEt\(_3\))\(_2\)] (102), trans-[Pt(C≡C-p-C\(_6\)H\(_4\)CF\(_3\))\(_2\)(PEt\(_3\))\(_2\)](106), and trans-[Pt(C≡C-9-C\(_{14}\)H\(_9\))\(_2\)(PEt\(_3\))\(_2\)] (63) through salt elimination reactions.
Three of the trans-platinum bis(alkynyl) complexes (95, 102, and 106) successfully yielded trans-platinum bis(borirenyl) complexes 119/120, 122/123, and 125/126 through photolytic transfer of two equivalents of the terminal borylene ligand {BN(SiMe\(_3\))\(_2\)} from [(OC)\(_5\)Cr{BN(SiMe\(_3\))\(_2\)}] (13) (Figure 5-4). Attempted borylene transfer reactions to the trans-platinum bis(alkynyl) complex trans-[Pt(C≡C-9-C\(_{14}\)H\(_9\))\(_2\)(PEt\(_3\))\(_2\)] (63) failed due to the complex’s photoinstability. Although a host of other variants of platinum alkynyl species were prepared and attempted, these three were the only ones that successfully yielded trans-platinum bis(borirenyl) units. Attempts were also made to create a cis variant for direct UV-vis comparison to the trans-platinum bis(borirenyl) variants, however, these attempts were also not successful. Gladysz-type platinum end-capped alkynyl species were also synthesized to serve as transfer platforms for borirene synthesis in sequential order, however, these species were also shown to not be photolytically stable.
A host of new monoborirenes: Ph-(μ-{BN(SiMe\(_3\))(t-Bu)}C=C)-Ph (148), trans- [PtCl{(μ-{BN(SiMe\(_3\))(t-Bu)}C=C)-Ph}(PEt\(_3\))\(_2\)] (149), and [(η\(^5\)-C\(_5\)Me\(_5\))(OC)\(_2\)Fe(μ- {BN(SiMe\(_3\))(t-Bu)}C=C)Ph] (150) were synthesized by photo- and thermolytic transfer of the unsymmetrical {BN(SiMe\(_3\))(t-Bu)} ligand from the complexes [(OC)\(_5\)M{BN(SiMe\(_3\))(t-Bu)}] (M = Cr (19), Mo (20), and W (21)) to organic and organometallic alkynyl species to verify that the borylene complexes all display similar reactivity to the symmetrical terminal borylenes of the type [(OC)\(_5\)M{BN(SiMe\(_3\))\(_2\)}] (M = Cr (13), Mo (14), and W (15)). These monoborirenes are all found to be oils when in their pure states and X-ray structural determination was impossible for these species.
Project Boratabenzene
The bis(boratabenzene) complex [{(η\(^5\)-C\(_5\)H\(_5\))Co}\(_2\){μ:η\(^6\),η\(^6\)-(BC\(_5\)H\(_5\))\(_2\)}] (189) was successfully prepared by treatment of tetrabromodiborane (65) with six equivalents of cobaltocene (176) in a unique reaction that utilized cobaltocene as both a reagent and reductant (Figure 5-5). The bimetallic transition metal complex features a new bridging bis(boratabenzene) ligand linked through a boron-boron single bond that can manifest delocalization of electron density by providing an accessible LUMO orbital for π-communication between the cobalt centers and heteroaromatic rings.
This dianionic diboron ligand was shown to facilitate electronic coupling between the cobalt metal sites, as evidenced by the potential separations between successive single-electron redox events in the cyclic voltammogram. Four formal redox potentials for complex 189 were found: E\(_{1/2}\)(1) = −0.84 V, E\(_{1/2}\)(2) = −0.94 V, E\(_{1/2}\)(3) = −2.09 V, and E\(_{1/2}\)(4) = −2.36 V (relative to the Fc/Fc+ couple) (Figure 5-6). These potentials correlate to two closely-spaced oxidation waves and two well-resolved reduction waves ([(189)]\(^{0/+1}\), [(189)]\(^{+1/+2}\), [(189)]\(^{0/–1}\), and [(189)]\(^{–1/–2}\) redox couples, respectively). The extent of metal-metal communication was found to be relative to the charge of the metal atoms, with the negative charge being more efficiently delocalized across the bis(boratabenzene) unit (class II Robin-Day system). Magnetic studies indicate that the Co(II) ions are weakly antiferromagnetically coupled across the B-B bridge.
While reduction of the bis(boratabenzene) system resulted in decomposition of the complex, oxidation of the system by one- and two-electron steps resulted in isolable stable monocationic (194) and dicationic (195) forms of the bis(boratabenzene) complex (Figure 5-7). Study of these systems verified the results of the cyclic voltammetry studies performed on the neutral species. These species are unfortunately not stable in acetonitrile or nitromethane solutions, which until this point are the only solvents that have been observed to dissolve the cationic species. Unfortunately, this instability in solution complicates reactivity studies of these cationic complexes.
Finally, reactivity studies were performed on the neutral bis(boratabenzene) complex 189 in which the compound was tested for: (A) cleavage of the boratabenzene (cyclo-BC\(_5\)H\(_5\)) ring from the cobalt center, and (B) oxidative addition of the B-B bond to a transition metal scaffold to attempt synthesis of the first ever L\(_x\)M-η\(^1\)-(BC\(_5\)H\(_5\)) complex. Both of these reactivity studies, however, proved unsuccessful and typically witnessed decomposition of the bis(boratabenzene) complex or no reactivity. After repeated attempts of these reactions, no oxidative addition of the bis(boratabenzene) system could be confirmed.
Boron's unique position in the Periodic Table, that is, at the apex of the line separating metals and nonmetals, makes it highly versatile in chemical reactions and applications. Contemporary demand for renewable and clean energy as well as energy‐efficient products has seen boron playing key roles in energy‐related research, such as 1) activating and synthesizing energy‐rich small molecules, 2) storing chemical and electrical energy, and 3) converting electrical energy into light. These applications are fundamentally associated with boron's unique characteristics, such as its electron‐deficiency and the availability of an unoccupied p orbital, which allow the formation of a myriad of compounds with a wide range of chemical and physical properties. For example, boron's ability to achieve a full octet of electrons with four covalent bonds and a negative charge has led to the synthesis of a wide variety of borate anions of high chemical and electrochemical stability—in particular, weakly coordinating anions. This Review summarizes recent advances in the study of boron compounds for energy‐related processes and applications.
The potassium salt of the boron-centred nucleophile B(CN)\(_{3}\)\(^{2-}\)(1) readily reacts with perfluorinated arenes, such as hexafluorobenzene, decafluorobiphenyl, octafluoronaphthalene and pentafluoropyridine, which results in KF and the K\(^{+}\) salts of the respective borate anions with one {B(CN)\(_{3}\)} unit bonded to the (hetero)arene. An excess of K\(_{2}\)1 leads to the successive reaction of two or, in the case of perfluoropyridine, even three C–F moieties and the formation of di- and trianions, respectively. Moreover, all of the 11 partially fluorinated benzene derivatives, C\(_{6}\)F\(_{6-n}\)H\(_{n}\) (n = 1–5), generally react with K\(_{2}\)1 to give new tricyano(phenyl)borate anions with high chemo- and regioselectivity. A decreasing number of fluorine substituents on benzene results in a decrease in the reaction rate. In the cases of partially fluorinated benzenes, the addition of LiCl is advantageous or even necessary to facilitate the reaction. Also, pentafluorobenzenes R–C\(_{6}\)F\(_{5}\) (R = –CN, –OMe, –Me, or –CF\(_{3}\)) react via C–F/C–B exchange that mostly occurs in the para position and to a lesser extent in the meta or ortho positions. Most of the reactions proceed via an S\(_{N}\)Ar mechanism. The reaction of 1,4-F\(_{2}\)C\(_{6}\)H\(_{4}\) with K\(_{2}\)1 shows that an aryne mechanism has to be considered in some cases as well. In summary, a wealth of new stable tricyano(aryl)borates have been synthesised and fully characterized using multi-NMR spectroscopy and most of them were characterised using single-crystal X-ray diffraction.
The self-stabilizing, tetrameric cyanoborylene [(cAAC)B(CN)]4 (I, cAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) and its diborene relative, [(cAAC)(CN)B=B(CN)(cAAC)] (II), both react with disulfides and diselenides to yield the corresponding cAAC-supported cyanoboron bis(chalcogenides). Furthermore, reactions of I or II with elemental sulfur and selenium in various stoichiometries provided access to a variety of cAAC- stabilized cyanoboron-chalcogen heterocycles, including a unique dithiaborirane, a diboraselenirane, 1,3-dichalcogena-2,4-diboretanes, 1,3,4-trichalcogena- 2,5-diborolanes and a rare six-membered 1,2,4,5-tetrathia-3,6-diborinane. Stepwise addition reactions and solution stability studies provided insights into the mechanism of these reactions and the subtle differences in reactivity observed between I and II.
A 1,4,2,3‐diazadiborinine derivative was found to form Lewis adducts with strong two‐electron donors such as N‐heterocyclic and cyclic (alkyl)(amino)carbenes. Depending on the donor, some of these Lewis pairs are thermally unstable, converting to sole B,N‐embedded products upon gentle heating. The products of these reactions, which have been fully characterized by NMR spectroscopy, elemental analysis, and single‐crystal X‐ray diffraction, were identified as B,N‐heterocycles with fused 1,5,2,4‐diazadiborepine and 1,4,2‐diazaborinine rings. Computational modelling of the reaction mechanism provides insight into the formation of these unique structures, suggesting that a series of B−H, C−N, and B−B bond activation steps are responsible for these “intercalation” reactions between the 1,4,2,3‐diazadiborinine and NHCs.
Pyrene is a polycyclic aromatic hydrocarbon (PAH) that has very interesting photophysical properties which make it suitable for a broad range of applications. The 2,7-positions of pyrene are situated on nodal planes in both the HOMO and LUMO. Hence, electrophilic reactions take place at the 1-, 3-, 6-, and 8-positions. The goal of this project was to develop novel pyrene derivatives substituted at the 2- and 2,7-positions, with very strong donors or/and acceptors, to achieve unprecedented properties and to provide a deeper understanding of how to control the excited states and redox properties. For that reason, a julolidine-type moiety was chosen as a very strong donor, giving D-π and D-π-D systems and, with Bmes2 as a very strong acceptor, D-π-A system. These compounds exhibit unusual photophysical properties such as emission in the green region of the electromagnetic spectrum in hexane, whereas all other previously reported pyrene derivatives substituted at the 2,7-positions show blue luminescence. Furthermore, spectroelectrochemical measurements suggest very strong coupling between the substituents at the 2,7-positions of pyrene in the D-π-D system. Theoretical studies show that these properties result from the very strong julolidine-type donor and Bmes2 acceptor coupling efficiently to the pyrene HOMO-1 and LUMO+1, respectively. Destabilization of the former and stabilization of the latter lead to an orbital shuffle between HOMO and HOMO 1, and LUMO and LUMO+1 of pyrene. Consequently, the S1 state changes its nature sufficiently enough to gain higher oscillator strength, and the photophysical and electrochemical properties are then greatly influenced by the substituents.
In another project, further derivatives were synthesized with additional acceptor moieties at the K-region of pyrene. These target derivatives exhibit strong bathochromically shifted absorption maxima (519-658 nm), which is a result of the outstanding charge transfer character introduced into the D-π-D pyrene system through the additional acceptor moiety at the K-region. Moreover, emission in the red to NIR region with an emission maximum at 700 nm in CH2Cl2 is detected. The excited state lives unusual long for K-region substituted pyrenes; however, such a lifetime is rather typical for 2,7-substituted pyrene derivatives.
The polycyclic aromatic hydrocarbon perylene, especially perylene diimide, has received considerable attention in recent years and has found use in numerous applications such as dyes, pigments and semiconductors. Nevertheless, it is of fundamental importance to understand how to modulate the electronic and photophysical properties of perylene depending on the specific desired application. Perylenes without carboxyimide groups at the peri positions are much less well studied due to the difficulties in functionalizing the perylene core directly. In particular, only ortho heteroatom substituted perylenes have not been reported thus far (exception: (Bpin)4-Per was already reported by Marder and co-workers). Thus, the effect of substituents on the ortho positions of the perylene core has not been investigated.
Two perylene derivatives were synthesized that bear four strong diphenylamine donor or strong Bmes2 acceptor moieties at the ortho positions. These compounds represent the first examples of perylenes substituted only at the ortho positions with donors or acceptors.
The investigations show that the photophysical and electronic properties of these derivatives are unique and different compared to the well-studied perylene diimides. Thus, up to four reversible reductions or oxidations are possible, which is unprecedented for monomeric perylenes. Furthermore, the photophysical properties of these two ortho-substituted derivatives are unusual compared to reported perylenes on many regards. Thus, large Stokes shifts are obtained, and the singlet excited state of these derivatives lives remarkably long with intrinsic lifetimes of up to 94 ns.
In a cooperation with Dr. Gerard P. McGlacken at University College Cork in Ireland, different quinolones were borylated using an iridium catalyst system to study the electronic and steric effect of the substrates. It was possible to demonstrate that the Ir-catalyzed borylation with the dtbpy ligand allows the direct borylation of various 4-quinolones at the 6- and 7-positions. Thus, later stage functionalization is possible with this method and more highly functionalized quinolones are also compatible with this mild reaction conditions.
Geringe HOMO-LUMO-Abstände und eine hohe Ladungsträgermobilität prädestinieren die höheren Acene für Anwendungen im Bereich der Organoelektronik. Die Leistungsfähigkeit derartiger Verbindungen steigt hierbei dramatisch mit der Anzahl anellierter Benzolringe. Größere Acenmengen sind synthetisch bisher jedoch nur für Acene bis Heptacen verlässlich zugänglich. Theoretischen Studien zufolge besitzen (Oligo)acene offenschalige Singulettbiradikal- und (Poly)acene polyradikalische Grundzustände. Eindeutige experimentelle Belege für diese Vorhersagen sind hingegen äußerst selten. Durch den Einbau von zwei Boratomen in das Anthracengrundgerüst konnten wir den HOMO-LUMO-Abstand von Acenen dramatisch verringern und zwar ohne die Notwendigkeit einer Ausweitung des konjugierten π-Systems. Stabilisierung der Borzentren durch cyclische (Alkyl)(amino)carbene lieferte hierbei neutrale 9,10-Diboraanthracene mit disjunkten, offenschaligen Singulettbiradikal-Grundzuständen.
cAAC‐Stabilized 9,10‐diboraanthracenes—Acenes with Open‐Shell Singlet Biradical Ground States
(2020)
Narrow HOMO–LUMO gaps and high charge‐carrier mobilities make larger acenes potentially high‐efficient materials for organic electronic applications. The performance of such molecules was shown to significantly increase with increasing number of fused benzene rings. Bulk quantities, however, can only be obtained reliably for acenes up to heptacene. Theoretically, (oligo)acenes and (poly)acenes are predicted to have open‐shell singlet biradical and polyradical ground states, respectively, for which experimental evidence is still scarce. We have now been able to dramatically lower the HOMO–LUMO gap of acenes without the necessity of unfavorable elongation of their conjugated π system, by incorporating two boron atoms into the anthracene skeleton. Stabilizing the boron centers with cyclic (alkyl)(amino)carbenes gives neutral 9,10‐diboraanthracenes, which are shown to feature disjointed, open‐shell singlet biradical ground states.
Major advances in the chemistry of 5th and 6th row heavy p-block element compounds have recently uncovered intriguing reactivity patterns towards small molecules such as H\(_2\), CO\(_2\), and ethylene. However, well-defined, homogeneous insertion reactions with carbon monoxide, one of the benchmark substrates in this field, have not been reported to date. We demonstrate here, that a cationic bismuth amide undergoes facile insertion of CO into the Bi–N bond under mild conditions. This approach grants direct access to the first cationic bismuth carbamoyl species. Its characterization by NMR, IR, and UV/vis spectroscopy, elemental analysis, single-crystal X-ray analysis, cyclic voltammetry, and DFT calculations revealed intriguing properties, such as a reversible electron transfer at the bismuth center and an absorption feature at 353 nm ascribed to a transition involving σ- and π-type orbitals of the bismuth-carbamoyl functionality. A combined experimental and theoretical approach provided insight into the mechanism of CO insertion. The substrate scope could be extended to isonitriles.
A case study on the effect of the employment of two different NHC ligands in complexes [Ni(NHC)\(_{2}\)] (NHC=\(^{i}\)Pr\(_{2}\)Im\(^{Me}\) 1\(^{Me}\), Mes\(_{2}\)Im 2) and their behavior towards alkynes is reported. The reaction of a mixture of [Ni\(_{2}\)(\(^{i}\)Pr\(_{2}\)Im\(^{Me}\))\(_{4}\)(μ-(η\(^{2}\) : η\(^{2}\))-COD)] B/ [Ni(\(^{i}\)Pr\(_{2}\)Im\(^{Me}\))\(_{2}\)(η\(^{4}\)-COD)] B’ or [Ni(Mes\(_{2}\)Im)\(_{2}\)] 2, respectively, with alkynes afforded complexes [Ni(NHC)\(_{2}\)(η\(^{2}\)-alkyne)] (NHC=\(^{i}\)Pr\(_{2}\)Im\(^{Me}\): alkyne=MeC≡CMe 3, H\(_{7}\)C\(_{3}\)C≡CC\(_{3}\)H\(_{7}\) 4, PhC≡CPh 5, MeOOCC≡CCOOMe 6, Me\(_{3}\)SiC≡CSiMe\(_{3}\) 7, PhC≡CMe 8, HC≡CC\(_{3}\)H\(_{7}\) 9, HC≡CPh 10, HC≡C(p-Tol) 11, HC≡C(4-\(^{t}\)Bu-C\(_{6}\)H\(_{4}\)) 12, HC≡CCOOMe 13; NHC=Mes\(_{2}\)Im: alkyne=MeC≡CMe 14, MeOOCC≡CCOOMe 15, PhC≡CMe 16, HC≡C(4-\(^{t}\)Bu-C\(_{6}\)H\(_{4}\)) 17, HC≡CCOOMe 18). Unusual rearrangement products 11 a and 12 a were identified for the complexes of the terminal alkynes HC≡C(p-Tol) and HC≡C(4-\(^{t}\)Bu-C\(_{6}\)H\(_{4}\)), 11 and 12, which were formed by addition of a C−H bond of one of the NHC N-\(^{i}\)Pr methyl groups to the C≡C triple bond of the coordinated alkyne. Complex 2 catalyzes the cyclotrimerization of 2-butyne, 4-octyne, diphenylacetylene, dimethyl acetylendicarboxylate, 1-pentyne, phenylacetylene and methyl propiolate at ambient conditions, whereas 1\(^{Me}\) is not a good catalyst. The reaction of 2 with 2-butyne was monitored in some detail, which led to a mechanistic proposal for the cyclotrimerization at [Ni(NHC)\(_{2}\)]. DFT calculations reveal that the differences between 1\(^{Me}\) and 2 for alkyne cyclotrimerization lie in the energy profile of the initiation steps, which is very shallow for 2, and each step is associated with only a moderate energy change. The higher stability of 3 compared to 14 is attributed to a better electron transfer from the NHC to the metal to the alkyne ligand for the N-alkyl substituted NHC, to enhanced Ni-alkyne backbonding due to a smaller C\(_{NHC}\)−Ni−C\(_{NHC}\) bite angle, and to less steric repulsion of the smaller NHC \(^{i}\)Pr\(_{2}\)Im\(^{Me}\).
Chapter two reports the catalytic triboration of terminal alkynes with B2pin2 using readily available Cu(OAc)2 and PnBu3. Various 1,1,2-triborylalkenes, a class of compounds which have been demonstrated to be potential Matrix Metalloproteinase-2 (MMP-2) inhibitors, are obtained directly in moderate to good yields. The process features mild reaction conditions, broad substrate scope, and good functional group tolerance were observed. This Cu-catalyzed reaction can be conducted on a gram scale to produce the corresponding 1,1,2-triborylalkenes in modest yields. The utility of these products is demonstrated by further transformation of the C-B bonds to prepare gem-dihaloborylalkenes (F, Cl, Br), monohalodiborylalkenes (Cl, Br), and trans-diaryldiborylalkenes, which serve as important synthons and have previously been challenging to prepare.
A convenient and efficient one step synthesis of 1,1,1-triborylalkanes was achieved via sequential dehydrogenative borylation and double hydroboration of terminal alkynes with HBpin (HBpin = pinacolborane) catalyzed by inexpensive and readily available Cu(OAc)2. This protocol proceeded under mild conditions, furnishing 1,1,1-tris(boronates) with wide substrate scope, excellent selectivity and good functional group tolerance, and is applicable to gram-scale synthesis without loss of yield. The 1,1,1-triborylalkanes can be used in the preparation of α-vinylboronates and borylated cyclic compounds, which are valuable but previously rare compounds. Different alkyl groups can be introduced stepwise via base-mediated deborylative alkylation to produce racemic tertiary alkyl boronates, which can be readily transformed into useful tertiary alcohols.
Chapter 4 reported a NaOtBu-catalyzed mixed 1,1-diboration of terminal alkynes with an unsymmetrical diboron reagent BpinBdan. This Brønsted base-catalyzed reaction proceeds in a regio- and stereoselective fashion affording 1,1-diborylalkenes with two different boryl moieties in moderate to high yields, and is applicable to gram-scale synthesis without loss of yield or selectivity. Hydrogen bonding between the Bdan group and tBuOH is proposed to be responsible for the observed stereoselectivity. The mixed 1,1-diborylalkenes can be utilized in stereoselective Suzuki-Miyaura cross-coupling reactions.
The behavior of the redox‐active aminotroponiminate (ATI) ligand in the coordination sphere of bismuth has been investigated in neutral and cationic compounds, [Bi(ATI)\(_{3}\)] and [Bi(ATI)\(_{2}\)L\(_{n}\)][A] (L=neutral ligand; n=0, 1; A=counteranion). Their coordination chemistry in solution and in the solid state has been analyzed through (variable‐temperature) NMR spectroscopy, line‐shape analysis, and single‐crystal X‐ray diffraction analyses, and their Lewis acidity has been evaluated by using the Gutmann–Beckett method (and modifications thereof). Cyclic voltammetry, in combination with DFT calculations, indicates that switching between ligand‐ and metal‐centered redox events is possible by altering the charge of the compounds from 0 in neutral species to +1 in cationic compounds. This adds important facets to the rich redox chemistry of ATIs and to the redox chemistry of bismuth compounds, which is, so far, largely unexplored.
A series of five new homoleptic, linear nickel d\(^{9}\)‐complexes of the type [Ni\(^{I}\)(NHC)\(_{2}\)]\(^{+}\) is reported. Starting from the literature known Ni(0) complexes [Ni(Mes\(_{2}\)Im)\(_{2}\)] 1, [Ni(Mes\(_{2}\)Im\(^{H2}\))2] 2, [Ni(Dipp\(_{2}\)Im)\(_{2}\)] 3, [Ni(Dipp\(_{2}\)Im\(^{H2}\))\(_{2}\)] 4 and [Ni(cAAC\(^{Me}\))\(_{2}\)] 5 (Mes\(_{2}\)Im=1,3‐bis(2,4,6‐trimethylphenyl)‐imidazolin‐2‐ylidene, Mes\(_{2}\)Im\(^{H2}\)=1,3‐bis(2,4,6‐trimethylphenyl)‐imidazolidin‐2‐ylidene, Dipp\(_{2}\)Im=1,3‐bis(2,6‐diisopropylphenyl)‐imidazolin‐2‐ylidene, Dipp\(_{2}\)Im\(^{H2}\)=1,3‐bis(2,6‐diisopropylphenyl)‐imidazolidin‐2‐ylidene, cAAC\(^{Me}\)=1‐(2,6‐diisopropylphenyl)‐3,3,5,5‐tetramethylpyrrolidin‐2‐yliden), their oxidized Ni(I) analogues [Ni\(^{I}\)(Mes\(_{2}\)Im)\(_{2}\)][BPh\(_{4}\)] 1\(^{+}\), [Ni\(^{I}\)(Mes\(_{2}\)Im\(^{H2}\))\(_{2}\)][BPh\(_{4}\)] 2\(^{+}\), [Ni\(^{I}\)(Dipp\(_{2}\)Im)\(_{2}\)][BPh\(_{4}\)] 3\(^{+}\), [Ni\(^{I}\)(Dipp\(_{2}\)Im\(^{H2}\))\(_{2}\)][BPh\(_{4}\)] 4\(^{+}\) and [Ni\(^{I}\)(cAAC\(^{Me}\))\(_{2}\)][BPh\(_{4}\)] 5\(^{+}\) were synthesized by one‐electron oxidation with ferrocenium tetraphenyl‐borate. The complexes 1\(^{+}\)–5\(^{+}\) were fully characterized including X‐ray structure analysis. The complex cations reveal linear geometries in the solid state and NMR spectra with extremely broad, paramagnetically shifted resonances. DFT calculations predicted an orbitally degenerate ground state leading to large magnetic anisotropy, which was verified by EPR measurements in solution and on solid samples. The magnetic anisotropy of the complexes is highly dependent from the steric protection of the metal atom, which results in a noticeable decrease of the g‐tensor anisotropy for the N‐Mes substituted complexes 1\(^{+}\) and 2\(^{+}\) in solution due to the formation of T‐shaped THF adducts.
Muscarinic receptors of rcsistance vessels (submucosal artcrioles, outside diametcr 50-75 J,Lm) from the guinea-pig small intestinc were invcstigatcd in vitro using a computcr-assisted vidcomicroscopy system (Diamtrak <~t ). The muscarinic receptor which mediates vasodilation of prccontractcd [U-46619 (300 nM) or (- )-noradrcnaline (1 0 J.L M)] artcriolcs was characterized with scveral muscarinic agonists and subtypc-sclectivc antagonists. Thc following agonists all produccd cquivalent maximum vasodilation (given in rank ordcr of potency): acctylcholinc = arccaidinc propargyl cstcr (APE) > oxotremorine = ( ± )-muscarinc = ( ± )-mcthacholinc > carbachol > 4-[[N-{4-chlorophenyl)carbamoyl]oxy]-2-hutynyltrimcthylammonium iodide (4-CI-McN-A- 343). 4-([N-(3-ChlorophcnyD-carbamoyl)oxy]-2-butynyltrimcthylammonium chloride (McN-A-343) and N-ethyl-guvacinc propargyl ester (NEN-APE) produccd minimal or no artcriolar vasodilation. Thc muscarinic antagonists pircnzcpinc, ( ± )-5,11-dihydro-11- [[[2-[2-((dipropylamino)methyl}-1-pipcridinyl]ethyl]amino ]-carbonyi]-6H-pyrido(2,3-h)( 1 ,4)-benzodiazcpin-6-onc (AF-DX 384 ), 11- [[ 4-[4-(dicthylamino)butyl]-1-piperidinyl]acetyl]-5, ll-dihydro-6H-pyrido(2.3-h)( 1,4 )-bcnzodiazepin-6-onc (AQ-RA 741 ), p-fluorohexahydro- sila-difcnidol (p-F-HHSiD), 4-diphcnylacetoxy-N-methylpipcridine mcthiodidc (4-DAMP) and (R)- and (S)hexahydro- difcnidol [(R)-HHD, (S)-HHD] shifted thc muscarinc, mcthacholinc or carbachol dosc-rcsponsc curve to the right in a compctitive manner. Schildanalysis of the data yicldcd pA\(_2\) valucs for pircnzcpinc (6.74/6.9), AF-DX 384 (6.72), AQ-RA 741 (6.58), p-F-HHSiD (7.53/7.57), 4-DAMP (9.06), (R)-HHD (7.88/8.32) and (S)-HHD (5.52/5.88). Thus, it can he concluded that submucosal arteriolcs posscss only the M\(_3\) functional muscarinic reccptor, the activation of which causcs hlood vcsscl dilation. The preparation dcscribcd is considcrcd to be a valuable now bioassay for pharmacological investigations of drug actions at muscarinic receptors in the peripheral vascular system.
The muscarinic receptor mediating vasodilation of resistance vessels in the rat isolated, constant-pressure perfused kidney (preconstriction by w- 7 M cirazoline) was characterized by subtype-preferring agonists and se]ective antagonists. The agonists produced vasodi1ation with the fol1owing rank order of potency: arecaidine propargy] ester (APE) > 5-methylfurtrethonium = methacholine = oxotremorine > (S)-aceclidine > arecaidine 2-butyne-1,4-diyl bisester > 4-Cl-McN-A-343 = (R)-nipecotic acid ethyl ester = N-ethyl-guvacine propargyl ester- (R)-aceclidine = (S)-nipecotic acid ethyl ester > McN-A-343. Agonist-induced vasodilation disappeared after destruction of the endothelium with detergent. Highly significant correlations of agonist potencies for vasodilation were found between rat kidney and guinea-pig ileum submucosal arterioles as weH as agonist potencies at smooth muscle muscarinic M\(_3\) receptors of the guinea-pig ileum. The rank order of antagonist potencies (4-diphenylacetoxy-Nmethylpiperidine methiodide (4-DAMP) > (R)-hexahydro-difenidol - hexahydro-sila-difenidol > pirenzepine - p-fluorohexahydro- sila-difenidol- himbacine- AF-DX 384- AQ-RA 741 > (S)-hexahydro-difenidol) to attenuate vasodilation to APE in rat kidney, correlated significantly with affinities at M\(_3\) receptors in submucosal arterioles and in smooth muscle of the guinea-pig ileum, but differed from those at M\(_1\) and M\(_2\) receptors in rabbit vas deferens. The agonist and antagonist potencies suggest that vasodilation elicited by muscarinic stimuli in endothelium-intact rat renal vasculature is mediated by functional muscarinic M\(_3\) receptors.
A doubly base-stabilized diborane based on a benzylphosphine linker was prepared by a salt elimination reaction between 2-LiC\(_6\)H\(_4\)CH\(_2\)PCy\(_2\).Et\(_2\)O and B\(_2\)Br\(_4\). This compound was reduced with KC8 to its corresponding diborene, with the benzylphosphine forming a five-membered chelate. The diborene reacts with butadiene, 2-trimethylsiloxy-1,3-butadiene and isoprene to form 4,5-diboracyclohexenes, which interconvert between their 1,1- (geminal) and 1,2- (vicinal) chelated isomers. The 1,1-chelated diborene undergoes a halide-catalysed isomerisation into its thermodynamically favoured 1,2-isomer, which undergoes Diels-Alder reactions more slowly than the kinetic product.
Chiral 2-alkylbranched acids, esters and alcohols. Preparation and stereospecific flavour evaluation
(1991)
Racemic 2-alkylbranched acids are transformed to diastereomeric derivatives with (S)-2-hydroxy-3-phenylpropionic acid-N-methylamide or (S)-(-)-l-phenylethylamine and separated by liquid chromatography to pure diastereoisomers, which are subsequently hydrolyzed to yield optically pure acids. Enantiomeric alcohols are generated by LiAlH4-reduction of the corresponding acids, esters are synthesized by different methods. The odour impression of the enantiomeric compounds is investigated.
CO\(_2\) is found to undergo room-temperature, ambient- pressure reactions with two species containing boron-boron multiple bonds, leading to incorporation of either one or two CO\(_2\) molecules. In one case, a thermally-unstable intermediate was structurally characterized, indicating the operation of an initial 2+2 cycloaddition mechanism in the reaction.
A series of methylene-bridged bis(triarylboranes) has been synthesized via two complementary routes using metal-free catalytic Si/B exchange condensation under mild conditions. The title compounds comprise two borane moieties that show effective internal π-conjugation involving the respective boron centers and the adjacent hetaryl groups. Conjugation between both borane units, however, is disrupted by the aliphatic linker. Cyclic voltammetry revealed minimal electronic communication between the boron centers, as evidenced by two closely spaced reduction processes. The UV-vis spectra showed bathochromic shifted absorption bands compared to related monoboranes, which is attributed to the methylene bridge. A further red-shift results upon introduction of methyl or SiMe\(_3\) groups at the terminal thiophene rings.
In recent years <mi>PI</mi>-conjugated organoboron polymers and BN-doped polycyclic aromatic hydrocarbons have attracted a lot of interest due to their great potential in organic electronics. However, there are only few known examples of conjugated polymers with BN units in their main chain. Within this work silazane cleavage with silicon-boron (Si/B) exchange for the synthesis of a novel class of inorganic-organic hybrid polymers is demonstrated. These polymers consist of alternating NBN and para-phenylene units in the main chain. Photophysical studies and TD-DFT calculations for the polymer and molecular model systems were carried out, revealing a low extent of <mi>PI</mi>-conjugation across the NBN units. The new polymers can be used as macromolecular polyligands by a cross-linking reaction with a ZrIV compound. In the next chapter the synthesis and characterization of the first poly(p-phenylene iminoborane) is presented. This novel inorganic–organic hybrid polymer can be described as a BN analogue of the well-known poly(p-phenylene vinylene) (PPV) and is also accessible using the previously described Si/B exchange as synthetic strategy. Photophysical investigations and TD-DFT calculations on the polymer and corresponding model oligomers provide clear evidence for <mi>PI</mi>-conjugation across the B=N units and extension of the conjugation path with increasing chain length. Furthermore, a possible application of Si/B exchange for the synthesis of polysulfoximines was explored. Herein, diaryl sulfoximines and a p-phenylene bisborane serve as building blocks for new BN- and BO-doped alternating inorganic–organic hybrid copolymers. While the BN-linked polymers were accessible by a facile silicon/boron exchange protocol, the synthesis of polymers with B–O linkages in the main chain is achieved by salt elimination. In the last chapter the concept of Si/B exchange was investigated for the synthesis of BP-linked oligomers. Herein oligomers with sterically less demanding substituents (substituents: 2,4,6-trimethylphenyl or 2,4,6-tri-iso-propylphenyl) at the phosphorus are accessible using Si/B exchange, but the oligomer with Mes* (2,4,6-tri-tert-butylphenyl) as substituent needed a salt elimination pathway to give the desired product. Experimental data and theoretical investigations indicate, that the P-substituent has a high influence on the geometry of the phosphorus center and therefore on the possible conjugation over the BP units.
Sterically unencumbered diborenes based on a benzylphosphine chelate undergo diboration reactions with bis(catecholato)diboron in the absence of a catalyst to yield tetraboranes. The symmetrical diborenes studied undergo 1,2- diborations, whereas an unsymmetrical derivative was found to yield a triborylborane-phosphine adduct as the result of a formal 1,1-diboration. A related borylborylene compound also underwent a 1,2-diboration to produce a borylene-borane adduct.
This thesis contributes to the field of silicon chemistry, with a special emphasis on the chemistry of penta- and hexacoordinate silicon.The spirocyclic zwitterionic Lambda5Si-silicates 1–6 with a (2,2,6,6-tetramethylpiperidinio)- methyl group and two identical bidentate chelate ligands derived from glycine, (S)-alanine, (S)-phenylalanine, (S)-valine, (S)-tert-leucine, or (S)-proline bound to the silicon(IV) coordination center were synthesized and structurally characterized for the first time.The hitherto unknown spirocyclic zwitterionic Lambda5Si-silicates 7–12 with an (ammonio)- methyl group and two identical bidentate chelate ligands derived from (S)-lactic acid, (S)-3- phenyllactic acid, or (S)-mandelic acid were synthesized and structurally characterized in the solid state (elemental analyses (C, H, N), crystal structure analyses, 15N and 29Si VACP/MAS solid-state NMR experiments) and in solution (except 10; 1H, 13C, and 29Si NMR experiments)The spirocyclic zwitterionic Lambda5Si-silicates 13, 15, and 16 with an (ammonio)methyl group and two bidentate meso-oxolane-3,4-diolato(2–) ligands bound to the silicon(IV) coordination center were synthesized for the first time. The already existent compound 14 was resynthesized in order to perform a crystal structure analysis. All compounds were characterized by elemental analyses (C, H, N), 29Si VACP/MAS solid-state NMR experiments, and solution NMR studies (1H, 13C, 15N, and 29Si NMR experiments), and compounds 14–16 were additionally studied by single-crystal X-ray diffraction.The already existent zwitterionic Lambda5Si-silicate 17 was synthesized by new methods, including a remarkable Si–C cleavage reaction with benzoin. To investigate the dynamic behavior of the known zwitterionic Lambda5Si-silicate 18 in solution, VT 1H NMR experiments in CD2Cl2 were performed in the temperature range –100 °C to 23 °C.The hexacoordinate silicon compounds 19–22 containing multidentate ligands derived from citric acid or (S)-malic acid were synthesized for the first time. The anionic Lambda6Si-silicates 19–22 were structurally characterized in the solid state by single-crystal X-ray diffraction and VACP/MAS NMR spectroscopy (13C, 15N, 29Si). Upon dissolution in water at 20 °C, spontaneous hydrolysis of the Lambda6Si-silicate anions was observed.
The main aim of this thesis was the synthesis and structural characterization of penta and hexacoordinate silicon(IV) complexes. In the course of these studies, the neutral pentacoordinate silicon(IV) complexes 38, 39, 43−48, 54 and 55 were prepared. Furthermore, the neutral hexacoordinate silicon(IV) complexes 33−36, 49, 50, 52, 53, 56−62, 63, 64 and 65 were synthesized. All compounds were characterized by elemental analyses, NMR spectroscopy in solution (1H, 13C, 15N, 29Si) and in the solid-state (13C, 15N, 29Si VACP/MAS NMR), as well as single-crystal X-ray diffraction (except 45, 47−49, 52, 53 and 63).
A number of novel alkynyl-functionalized diarylbis(dimethylamino)diboranes(4) are prepared by salt metathesis, and the appended alkynyl groups are subjected to hydroboration. Their reactions with monohydroboranes lead to discrete boryl-appended diborane(4) species, while dihydroboranes induce their catenation to oligomeric species, the first known examples of well-characterized macromolecular species with B−B bonds. The oligomeric species were found to comprise up to ten repeat units and are soluble in common organic solvents. Some of the oligomeric species have good air stability and all were characterized by NMR and vibrational spectroscopy and size-exclusion chromatography techniques.
The present thesis comprises synthesis and stoichiometric model reactions of well-defined NHC-stabilized copper(I) complexes (NHC = N-heterocyclic carbene) in order to understand their basic reactivity in borylation and cross-coupling reactions. This also includes the investigations of the reactivity of the ligands used (NHCs and CaaCs = cyclic alkyl(amino)carbenes) with the substrates, i.e. diboron(4) esters and arylboronates, which are addressed in the second part of the thesis.
A combination of copper iodide and phenanthroline as the ligand is an efficient catalyst for Suzuki‐Miyaura cross‐coupling of highly fluorinated boronate esters (aryl−Bpin) with aryl iodides and bromides to generate fluorinated biaryls in good to excellent yields. This method represents a nice alternative to traditional cross‐coupling methods which require palladium catalysts and stoichiometric amounts of silver oxide. We note that π⋅⋅⋅π stacking interactions dominate the molecular packing in the partly fluorinated biaryl crystals investigated herein. They are present either between the arene and perfluoroarene, or solely between arenes or perfluoroarenes, respectively.
Herein, the copper-catalyzed borylation of readily available acyl chlorides with bis(pinacolato)diboron, (B\(_{2}\)pin\(_{2}\)) or bis(neopentane glycolato)diboron (B\(_{2}\)neop\(_{2}\)) is reported, which provides stable potassium acyltrifluoroborates (KATs) in good yields from the acylboronate esters. A variety of functional groups are tolerated under the mild reaction conditions (room temperature) and substrates containing different carbon-skeletons, such as aryl, heteroaryl and primary, secondary, tertiary alkyl are applicable. Acyl N-methyliminodiacetic acid (MIDA) boronates can also been accessed by modification of the workup procedures. This process is scalable and also amenable to the late-stage conversion of carboxylic acid-containing drugs into their acylboron analogues, which have been challenging to prepare previously. A catalytic mechanism is proposed based on in situ monitoring of the reaction between p-toluoyl chloride and an NHC-copper(I) boryl complex as well as the isolation of an unusual lithium acylBpinOBpin compound as a key intermediate.
We report herein the catalytic triboration of terminal alkynes with B\(_2\)pin\(_2\) (bis(pinacolato)diboron) using readily available Cu(OAc)\(_2\) and P\(^n\)Bu\(_3\). Various 1,1,2‐triborylalkenes, a class of compounds that have been demonstrated to be potential matrix metalloproteinase (MMP‐2) inhibitors, were obtained directly in moderate to good yields. The process features mild reaction conditions, a broad substrate scope, and good functional group tolerance. This copper‐catalyzed reaction can be conducted on a gram scale to produce the corresponding 1,1,2‐triborylalkenes in modest yields. The utility of these products was demonstrated by further transformations of the C−B bonds to prepare gem ‐dihaloborylalkenes (F, Cl, Br), monohaloborylalkenes (Cl, Br), and trans ‐diaryldiborylalkenes, which serve as important synthons and have previously been challenging to prepare.
A convenient and efficient one‐step synthesis of 1,1,1‐triborylalkanes was achieved via sequential dehydrogenative borylation and double hydroborations of terminal alkynes with HBpin (HBpin=pinacolborane) catalyzed by inexpensive and readily available Cu(OAc)\(_2\). This process proceeds under mild conditions, furnishing 1,1,1‐tris(boronates) with wide substrate scope, excellent selectivity, and good functional‐group tolerance, and is applicable to gram‐scale synthesis without loss of yield. The 1,1,1‐triborylalkanes can be used in the preparation of α‐vinylboronates and borylated cyclic compounds, which are valuable but previously rare compounds. Different alkyl groups can be introduced stepwise via base‐mediated deborylative alkylation to produce racemic tertiary alkyl boronates, which can be readily transformed into useful tertiary alcohols.
We report herein a mild procedure for the copper‐catalyzed oxidative cross‐coupling of electron‐deficient polyfluorophenylboronate esters with terminal alkynes. This method displays good functional group tolerance and broad substrate scope, generating cross‐coupled alkynyl(fluoro)arene products in moderate to excellent yields. Thus, it represents a simple alternative to the conventional Sonogashira reaction.
The crystal structures of the (R)-enantiomer (2b) and the racemate (1 b) of (cyclohexyl)phenyl[2- (pyrrolidin-1-yl)ethyl]silanol (sila--procyclidine) have been determined by X -ray structural analysis. The absolute configuration of (2b) was established. (2b) crystallizes in the orthorhombic space group P2\(_1\)2\(_1\)2\(_1\), with a = 15.221 (1 ), b = 17.967(1 ), c = 6.463(1) A, and Z = 4. (1 b) crystallizes in the monoclinic space group P2\(_1\)/c, with a = 6.441 (1 ), b = 17.1 82(7), c = 16.707(4) A, ß = 1 03.86(2r, and Z = 4. The structures were refined to respective R factors of 0.044 and 0.058. The molecular conformation of sila-procyclidine is identical in the two different structures. lntermolecular 0-H • • • N hydrogen bonding is observed in both crystallattices.ln (1 b) (R)- and (S)-configurated molecules form centrosymmetric dimers, in (2b) the (R)-configurated molecules are linked into infinite chains parallel to the c axis. The (R)-configurated sila--procyclidine (2b) has higher affinity for ileal and atrial muscarinic receptors of the guinea pig than the (S)-configurated enantiomer (3b).
In the molecular structure of the dinuclear title compound \([η^5-(C_5(CH_3)_5)(CO)Fe{(μ-BCl)(μ-CO)}PtCl(P(C_6H_{11})_3)]·C_6H_6\), the two metal atoms, iron(II) and platinum(II), are bridged by one carbonyl (μ-CO) and one chloridoborylene ligand (μ-BCl). The \(Pt^{II}\) atom is additionally bound to a chloride ligand situated trans to the bridging borylene, and a tricyclohexylphosphane ligand \((PCy_3)\) trans to the carbonyl ligand, forming a distorted square-planar structural motif at the \(Pt^{II}\) atom. The \(Fe_{II}\) atom is bound to a pentamethylcyclopentadienyl ligand \([η^5-C_5(CH_3)_5]\) and one carbonyl ligand (CO), forming a piano-stool structure. Additionally, one benzene solvent molecule is incorporated into the crystal structure, positioned staggered relative to the pentamethylcyclopentadienyl ligand at the \(Fe^{II}\) atom, with a centroid–centroid separation of 3.630 (2) Å.
Upon complexation to CuOTf, a PMe\(_3\)-stabilized bis(9-anthryl) diborene slowly undergoes an intramolecular hydroarylation reaction at room temperature. Subsequent triflation of the B–H bond with CuOTf, followed by a PMe\(_3\) transfer, finally yields a cyclic sp\(^2\)-sp\(^3\) boryl-substituted boronium triflate salt.
The title compound, [Zr(3)(C(14)H(20)Si(2))(3)O(3)], consists of three disila-bridged zirconocene units, which are connected via an oxide ligand, forming a nearly planar six-membered ring with a maximum displacement of 0.0191 (8) A. The compound was isolated as a by-product from a mixture of [(C(5)H(4)SiMe(2))(2)ZrCl(2)] and Li[AlH(4)] in Et(2)O.
no abstract available
Four different syntheses of the potent and selective muscanruc antagonist cyclohexyl( 4- fluorophenyl)(3-piperidinopropyl)silanol ( p-fluoro-hexahydro-sila-difenidol, p-F-HHSiD (2b); isolated as hydrochloride 2b· HCl) are described (starting materials: (CH\(_3\)O)\(_2\)SiCH\(_2\)CH\(_2\)CH\(_2\)Cl and Si(OCH\(_3\))\(_4\) ). In addition, the synthesis of the corresponding carbon analogue p-fluoro-hexahydro-difenidol ( p-F-HHD (2a); isolated as 2a· HCI) and the syntheses of three p-F-HHSiD derivatives (3-5), with a modified cyclic amino group, are reported (3: piperidinojpyrrolidino exchange, isolated as 3· HCI; 4: piperidinoj hexamethylenimino exchange, isolated as 4 · HCl; 5: quaternization of 2b with methyl iodide). The chiral compounds 2a, 2b, 3, 4 and 5 were prepared as racemates. In functional pharmacological studies, 3-5 behaved as simple competitive antagonists at musearlnie Ml receptors in rabbit vas deferens, M2 receptors in guinea-pig atria, and M3 receptors in guinea-pig ileal smooth rnuscle. The pyrrolidino (3) and hexamethylenimino (4) analogues of the parent drug p-F-HHSiD (2b) displayed the highest affinity for Ml and M3 receptors (pA\(_2\) values: 7.0-7.4) but exhibited lower affinity for cardiac M2 receptors (pA\(_2\) : 5.9 and 6.0). Their affinity profile (Ml- M3 > M2) is different from that of p-F-HHSiD (2b) (M3 > Ml > M2), but qualitatively very similar tothat of p-F-HHD (2a). The methiodide 5 exhibited the highest affinity for Ml receptors (pA\(_2\) : 8.5) but lower affinity for M2 and M3 receptors by factors of 5.6 and 3.6, respectively.
Alkylboronates play an important role in synthetic chemistry, materials science and drug discovery. They are easy to handle due to their good air and moisture stability, and can be readily employed to form carbon–carbon and carbon–heteroatom bonds and can be converted to various functional groups under mild reaction conditions. Compared with conventional groups, such as aryl (pseudo)halides or alcohols, organosulfur compounds represent an alternative and complimentary substitute in coupling reactions. The construction of C–B bond from C–SO bond of aryl sulfoxide is presented in Chapter 2. The selective cleavage of either alkyl(C)-sulfonyl or aryl(C)-sulfonyl bonds of an aryl alkyl sulfone via Cu-free or Cu-mediated processes generates the corresponding boronate esters, which are presented in Chapter 3 and Chapter 4. 1,2-Bis(boronate esters) are emerging as important synthetic intermediates for preparing 1,2-difunctional compounds. In addition, the boryl moieties in different environments in a 1,2-bis(boronate ester) can be differentiated and converted selectively, allowing the synthesis of a wide variety of complex molecules. A direct and selective diboration of C–X and C–O bonds for the preparation of 1,2-bis(boronate esters) is presented in Chapter 5.
A cyclic alkyl(amino)carbene (CAAC)‐stabilized dicoordinate aminoborylene is synthesized by the twofold reduction of a [(CAAC)BCl\(_{2}\)(TMP)] (TMP=2,6‐tetramethylpiperidyl) precursor. NMR‐spectroscopic, X‐ray crystallographic and computational analyses confirm the cumulenic nature of the central C=B=N moiety. Irradiation of [(CAAC)B(TMP)] (2) resulted in an intramolecular C−C bond activation, leading to a doubly‐fused C\(_{10}\)BN heterocycle, while the reaction with acetonitrile resulted in an aryl migration from the CAAC to the acetonitrile nitrogen atom, concomitant with tautomerization of the latter to a boron‐bound allylamino ligand. One‐electron oxidation of 2 with CuX (X=Cl, Br) afforded the corresponding amino(halo)boryl radicals, which were characterized by EPR spectroscopy and DFT calculations. Placing 2 under an atmosphere of CO afforded the tricoordinate (CAAC,CO)‐stabilized aminoborylene. Finally, the twofold oxidation of 2 with chalcogens led, in the case of N\(_{2}\)O and sulfur, to the splitting of the B−C\(_{CAAC}\) bond and formation of the 2,4‐diamino‐1,3,2,4‐dichalcogenadiboretanes and CAAC‐chalcogen adducts, whereas with selenium a monomeric boraselenone was isolated, which showed some degree of B−Se multiple bonding.
In this work we utilized Density Functional Theory to calculate EPR parameters and spin-density distributions of several transition metal complexes. To demonstrate the performance of our theoretical approach several validation studies were performed (Chapters 3-5). In contrast, the last three chapters of the thesis deal with specific chemical problems regarding several classes of biologically relevant transition metal complexes.
This Ph.D. thesis has addressed several main issues in current ASSB research within four studies. Ceramic ASSBs are meant to enable the implementation of Li-metal anodes and high voltage cathode materials, which would increase energy density, power density, life time as well as safety aspects in comparison with commercially available liquid electrolyte LiBs. In this thesis, several scientific questions arising on the cathode side of ASSBs have been focused on. With respect to the target system of a ternary composite bulk cathode consisting of ceramic active material, ceramic SSE and an electrically conductive component, studies about the thermal stabilities of these components and their impact on the electrochemical performance have been conducted. Particulate bulk cathode composites have to fulfil electrochemical, chemical, mechanical and structural requirements in order to compete with commercial LiBs. Particularly, the production process requires high-temperature sintering to obtain firmly bonded contacts in order to maximize the electrochemically active area, charge transfer and ionic conduction. However, interdiffusion, intermixing and decomposition of the initial components during sintering result in low-performing ASSBs so far.
These side reactions during high-temperature treatment have been investigated in order to gain a better understanding of these mechanisms and to enable a better controlling of the manufacturing process as well as to simplify the choice of material combinations. The first two parts of this thesis deal with the thermal stability of the ceramic SSE LATP in combination with various active materials and with the validation of a probable improvement of the sintering process due to liquid phase sintering of LATP by adding Li3PO4. In the third and fourth parts, the impact of interdiffusion, intermixing and decomposition on the electrochemical performance of TF-SSBs based on the active material LMO and the ceramic SSE Ga-LLZO has been investigated.
The 1st chapter provides a detailed review of the development of synthetic approaches to triarylboranes from their first report nearly 135 years ago to the present. In the 2nd chapter, a novel and convenient methodology is reported for the one-pot synthesis of sterically-congested triarylboranes, using bench-stable aryltrifluoroborates as the boron source. The new procedure gives access to symmetrically- and unsymmetrically-substituted triarylboranes. The borylated triarylboranes are suggested as building blocks for the design of functional materials. In the 3rd chapter, four luminescent tetracationic bis-triarylborane DNA and RNA sensors that show high binding affinities, in several cases even in the nM range, are investigated. The molecular structures of two of the neutral precursors reveal some structural flexibility for these compounds in the solid state. The compounds were found to be highly emissive even in water and DNA and RNA binding affinities were found to be dependent on linker length and flexibility. Strong SERS responses for three of the four compounds demonstrate the importance of triple bonds for strong Raman activity in molecules of this compound class. In chapter 4, the compound class of water-soluble tetracationic bis-triarylborane chromophores is extended by EDOT-linked compounds and those are compared to their thiophene-containing analogs. Absorption and emission are significantly red-shifted in these compounds, compared to their thiophene-containing analogs and, due to a large Stokes shift, one of the reported compounds exhibits the most bathochromically shifted emission, observable well into the near infrared region, of all tetracationic water-soluble bis-triarylborane chromophores reported to date. Long-lived excited states, completely quenched by oxygen, were observed for the water-stable compounds of this study via transient absorption spectroscopy and a quantum yield for singlet oxygen formation of 0.6 was determined for one of them.
In this work we have developed the method of back-transfoprmation within the Douglas-Kroll-Hess (DKH) framework, which has simplified the picture-change consistent transformation of first-order property operators in the DKH approach, making the implementation feasible. This has enabled us to implement the first all-electron scalar relativistic calculations of hyperfine coupling tensors at DKH2 level. Furthemore we have presented a general, relativistic two-component DFT approach for the unrestricted calculations of electronic g-tensors, based on DKH Hamiltonian. Additionally we have derived the expressions for the evaluation of hyperfine structurs and two-component unrestricted treatment of g-tensor within the Resolution of Identity Dirac Kohn Sham method developed by Stanoslav Komorovsky and Michal Repisky in collaboration with other members of the group of V. G. Malkin. All these approaches have been extensively validated.
The design of ligands is one of the most important and simultaneously challenging fields of research in modern inorganic chemistry. The aim is to synthesise ligands that can serve as coordination units for a broad variety of metal fragments and different purposes. The ligands have to be very flexible concerning their donating behaviour and geometrical prerequisites in order to correspond to the required metal fragments.