546 Anorganische Chemie
Filtern
Volltext vorhanden
- ja (26)
Gehört zur Bibliographie
- ja (26)
Erscheinungsjahr
- 2020 (26) (entfernen)
Dokumenttyp
Sprache
- Englisch (26) (entfernen)
Schlagworte
- boron (4)
- Borylene (3)
- Borylierung (3)
- diborenes (3)
- luminescence (3)
- Fluoreszenz (2)
- Triarylborane (2)
- borylation (2)
- cations (2)
- chemistry (2)
Institut
Sonstige beteiligte Institutionen
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.
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.
It is a challenge in chemical education to understand basic principles of chemical reaction kinetics on an experimental basis because of the relatively extensive experimental setup and the often time-consuming measurement series. This contribution offers an introduction into the field of the temperature dependence of reaction rate with easy-to-use experiments. Data logging systems have been used to get sufficient data-sets to evaluate different measurements in reaction kinetics. Several experiments were designed for practical courses in chemistry, which allow students to derive the simple van‘t Hoff rule on the one hand. On the other hand, the Arrhenius equation can only be derived on the basis of experimental data with the help of information from collision theory and Maxwell-Boltzmann distribution.
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
C\(_{19}\)H\(_{16}\)N\(_2\)OS, triclinic, P (1) over bar (no. 2), a= 8.1510(3) angstrom, b = 8.8021(3) angstrom, c =11.3953(5) angstrom, alpha =72.546(2)degrees, beta=84.568(2)degrees, gamma =80.760(2)degrees, V =768.86(5) angstrom(3), Z =2, R\(_{gt}\)(F) = 0.0491, WR\(_{ref}\)(F-2) = 0.1494, T =100 K.
Chapter 1
N-Heterocyclic olefins (NHOs), relatives of N-heterocyclic carbenes (NHCs), exhibit high nucleophilicity and soft Lewis basic character. To investigate their π-electron donating ability, NHOs were attached to triarylborane π-acceptors (A) giving donor(D)-π-A compounds 1-3. In addition, an enamine π-donor analogue (4) was synthesized for comparison.
UV-visible absorption studies show a larger red shift for the NHO-containing boranes than for the enamine analogue, a relative of a CAAC. The red shifted absorption of NHO-containing boranes indicate smaller energy gaps of NHO-containing boranes than CAAC-containing boranes. Solvent-dependent emission studies indicate that 1-4 have moderate intramolecular charge transfer (ICT) behavior.
Electrochemical investigations reveal that the NHO-containing boranes have extremely low reversible oxidation potentials (e.g., for 3, E1/2ox = –0.40 V vs. Fc/Fc+ in THF) which indicate the electron rich property of NHOs.
Furthermore, TD-DFT calculations were carried out on these four D-π-A boranes. The results show that the LUMOs of 1-4 only show a small difference, but the HOMOs of 1-3 are much more destabilized than that of the enamine-containing 4, which is in agreement with the electrochemical investigations and confirms the stronger donating ability of NHOs.
Chapter 2
Since the beginning of this century, the chemistry of (hetero)arene-fused boroles has attracted increasing interest. (Hetero)arene-fused boroles exhibit strong Lewis acidity, distinct fluorescence properties, strong electron accepting abilities, etc. However, their chemistry been only very briefly reviewed either as part of reviews on “free” boroles or on boron-doped polycyclic aromatic hydrocarbons (PAHs). In this chapter, we addressed the chemistry of (hetero)arene-fused boroles from fundamentals to their widely varying applications. It includes:
1) Synthetic methodology Both historical and recently developed strategies for the synthesis of fused boroles.
2) Stabilities A comparison of different kinetic protection strategies.
3) 9-Borafluorenes with a fluorinated backbone Application as Lewis acids, forming ion pairs with Cp2Zr(CH3)2 and applied as activators for polymerization, activators of H2, and other related applications.
4) Donor-acceptor 9-borafluorenes Applications as F– “turn on” sensors, potential applications as electron accepting units for organic (opto)electronics, bipolar transporting materials, TADF materials, and different functionalization strategies.
5) Heteroarene-fused boroles Enhanced antiaromaticity, unique coordination mode and their interesting properties.
6) Intramolecular dative bonding in 9-borafluorenes Bond-cleavage-induced intramolecular charge transfer (BICT), BICT-induced large Stoke shifts and dual emissions, application as a ratiometric sensor.
7) 9-Borafluorene-based main chain polymers Application in polymer chemistry and their distinct properties, e.g., as a sensor for gaseous NH3.
8) Electrochemistry A comparison of electron-accepting ability of different functionalized fused boroles through electrochemical studies.
9) Chemical reduction of fused boroles Stable radical anions and dianions of fused boroles and their properties.
10) Three-coordinate borafluorenium cations Cationic 9-borafluorenes and their interesting properties, e.g., in THF, reversible thermal colour switching properties.
Finally, a conclusion and outlook regarding the chemistry, properties and applications, and suggestions for areas which require further study was provided.
Chapter 3
Interested in fusing electron-poor arene onto boroles, two electron-poor phenylpyridyl-fused boroles, [TipPBB1]4 and TipPBB2 were prepared. [TipPBB1]4 is a white solid adopting a unique coordination mode, which forming a tetramer with a cavity in both the solid state and solution (1H DOSY). The boron center of TipPBB2 is 4-coordinate in the solid state, evidenced by a solid-state 11B{1H} RSHE/MAS NMR study, but the system dissociates in solution, leading to 3-coordinate borole species.
[TipPBB1]4 exhibits two reduction processes which are attributed to the phenylpyridyl cores. TipPBB2 also exhibits two reduction processes with the first half-reduction potential of E1/2red = –1.94 V. The electron accepting ability of TipPBB2 is largely enhanced and comparable to that of FMesBf. This enhanced electron accepting ability is attributed to the electron withdrawing property of the pyridyl group.
TipPBB2 exhibits concentration- and temperature-dependent dual fluorescence in solution. With the temperature is lowered, the emission intensity decreases (Figure 6.4, left). We suggested that the dual fluorescence is caused by an equilibrium between 3-coordinate TipPBB2 and a weak intermolecular adduct of TipPBB2 via a B–N bond. This hypothesis was further supported by lifetime measurements at different concentrations, low temperature excitation spectra low temperature 1H NMR spectra and lifetime measurements upon addition of DMAP to a solution of TipPBB2 to simulate the 4-coordiante TipPBB2 species. Interestingly, the ratio of the relative percentages of the two lifetimes shows a linear relationship with temperature; thus, TipPBB2 could serve as a fluorescent thermometer.
Furthermore, theoretical studies were carried out on TipPBB2, and two models, ((BMe3)TipPBB1(NMe3) and (BMe3)TipPBB2(NMe3)), which utilize a BMe3 group as the Lewis acid coordinated to pyridine and an NMe3 group as the Lewis base coordinated to the boron center of the borole, were used to simulate the [TipPBB1]4 and intermolecular 4-coordinate TipPBB2, respectively. Theoretical studies indicate that the HOMO of TipPBB2 is located at the Tip group, which is in contrast to its borafluorene derivatives for which the HOMOs are located on the borafluorene cores.
Chapter 4
Two derivatives of phenylpyridyl-fused boroles were prepared via functionalization of the pyridyl groups in two different directions, namely an electron-rich dihydropyridine moiety (compound 10) and an electron-deficient N-methylpyridinium cation (compound 11). Both compounds were fully characterized. The 11B NMR signal of compound 10 was observed at 58.8 ppm in CDCl3, which suggests strong conjugation between the boron atom and dihydropyridine moiety. Compound 11 shows a reversible coordination to THF which was confirmed by NMR studies. Compared to other 2,4,6-triisopropylphenyl protected 9-borafluorenes which only coordinate to CH3CN or DMF, the coordination of the weaker and bulkier THF to compound 11 indicates an extremely electron-deficient boron center in compound 11.
The electron-rich property of the dihydropyridine moiety of compound 10 was confirmed by its oxidation potential (Epc = +0.37 V). Due to the strong conjugation of the dihydropyridine moiety with the boron atom, the reduction potential of compound 10 shifts cathodically and is more negative than –2.5 V. Compound 11 exhibits three reduction processes with the first reversible reduction potential at Ered1/2 = –1.23 V, which is significantly anodically shifted compared to that of its precursor (TipPBB2) or its framework 1-methyl-2-phenylpyridin-1-ium triflate (12). This significantly anodically shifted reduction potential confirms an extremely electron-deficient property of compound 11.
Photophysical studies indicate that the lowest energy transition of compound 10 is more likely a locally-excited (LE) transition and compound 11 exhibits a polarized ground state.
Furthermore, we performed theoretical studies for both compounds. The electron cloud distribution of the HOMO of compound 10 supports the strong conjugation between the boron atom and the dihydropyridine moiety in the ground state. An extremely low LUMO energy was determined by theoretical studies which confirmed the extremely electron-deficient property of compound 11.
Chapter 5
Inspired by the enhancement of electron accepting ability with increasing numbers of electron withdrawing groups at boron, we tried to study the properties of a bis(pyridyl)arylboranes. In our attempt to synthesize a bis(pyridyl)arylborane, we obtained a bis(2-pyridyl)methoxyborate Li+ complex which is as a dimer both in solution and the solid state.
In the solid state, compound [16]2 is a dimer containing two bis(2-pyridyl)methoxyborate which are linked by two lithium cations. Each lithium cation coordinates to one methoxy group and two pyridyl groups, one from each of the two bis(2-pyridyl)methoxyborate anions. The parameters of [16]2 were compared with other bis(2-pyridyl)methoxyborate stabilized Pt(IV) complex, bis(2-pyridyl)hydroxylborate stabilized Ru(II) complex and the dimer of EtAl(OMe)(2-pyridyl)2Li.
To confirm the coordination mode in solution, 1H DOSY spectroscopy was carried out in CD2Cl2. The van der Waals radius obtained by 1H DOSY nicely matches with the result from the solid state and thus proves the dimer of 16 is persistent in solution.
Finally, different Lewis acids (e.g., TMSCl, BF3•Et2O, AlCl3, HCl) were used to attempt to detach the methoxy group of [16]2. However, we observed either decomposition or selective cleavage of the Tip group, or no reaction at all, rather than cleavage of the methoxy group from boron.
The reductive coupling of an NHC-stabilized aryldibromoborane yields a mixture of trans- and cis-diborenes in which the aryl groups are coplanar with the diborene core. Under dilute reduction conditions two diastereomers of a borirane-borane intermediate are isolated, which upon further reduction give rise to the aforementioned diborene mixture. DFT calculations suggest a mechanism proceeding via nucleophilic attack of a dicoordinate borylene intermediate on the aryl ring and subsequent intramolecular B-B bond formation.
The reductive coupling of an N-heterocyclic carbene (NHC) stabilized (dibromo)vinylborane yields a 1,2-divinyl- diborene, which, although isoelectronic to a 1,3,5-triene, displays no extended p conjugation because of twisting of the C\(_2\)B\(_2\)C\(_2\) chain. While this divinyldiborene coordinates to copper(I) and platinum(0) in an η\(^2\)-B\(_2\) and η\(^4\)-C\(_2\)B\(_2\) fashion, respectively, it undergoes a complex rearrangement to an η\(^4\)-1,3-diborete upon complexation with nickel(0).
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.
N-Heterocyclic Carbene and Cyclic (Alkyl)(amino)carbene Complexes of Titanium(IV) and Titanium(III)
(2020)
The reaction of one and two equivalents of the N ‐heterocyclic carbene IMes [IMes = 1,3‐bis(2,4,6‐trimethyl‐phenyl)imidazolin‐2‐ylidene] or the cyclic (alkyl)(amino)carbene cAAC\(^{Me}\) [cAAC\(^{Me}\) = 1‐(2,6‐diisopropyl‐phenyl)‐3,3,5,5‐tetra‐methylpyrrolidin‐2‐ylidene] with [TiCl\(_{4}\)] in n ‐hexane results in the formation of mono‐ and bis‐carbene complexes [TiCl\(_{4}\)(IMes)] 1 , [TiCl\(_{4}\)(IMes)2] 2 , [TiCl\(_{4}\)(cAAC\(^{Me}\))] 3 , and [TiCl\(_{4}\)(cAAC\(^{Me}\))\(_{2}\)] 4 , respectively. For comparison, the titanium(IV) NHC complex [TiCl\(_{4}\)(Ii Pr\(^{Me}\))] 5 (Ii Pr\(^{Me}\) = 1,3‐diisopropyl‐4,5‐dimethyl‐imidazolin‐2‐ylidene) has been synthesized and structurally characterized. The reaction of [TiCl\(_{4}\)(IMes)] 1 with PMe\(_{3}\) affords the mixed substituted complex [TiCl\(_{4}\)(IMes)(PMe\(_{3}\))] 6 . The reactions of [TiCl\(_{3}\)(THF)\(_{3}\)] with two equivalents of the carbenes IMes and cAAC\(^{Me}\) in n ‐hexane lead to the clean formation of the titanium(III) complexes [TiCl\(_{3}\)(IMes)\(_{2}\)] 7 and [TiCl\(_{3}\)(cAAC\(^{Me}\))\(_{2}\)] 8 . Compounds 1 –8 have been completely characterized by elemental analysis, IR and multinuclear NMR spectroscopy and for 2 –5 , 7 and 8 by X‐ray diffraction. Magnetometry in solution, EPR and UV/Vis spectroscopy and DFT calculations performed on 7 and 8 are indicative of a predominantly metal‐centered d\(^{1}\)‐radical in both cases.