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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.
A series of tetracationic bis-triarylborane dyes, differing in the aromatic linker connecting two dicationic triarylborane moieties, showed very high submicromolar affinities toward ds-DNA and ds-RNA. The linker strongly influenced the emissive properties of triarylborane cations and controlled the fluorimetric response of dyes. The fluorene-analog shows the most selective fluorescence response between AT-DNA, GC-DNA, and AU-RNA, the pyrene-analog’s emission is non-selectively enhanced by all DNA/RNA, and the dithienyl-diketopyrrolopyrrole analog’s emission is strongly quenched upon DNA/RNA binding. The emission properties of the biphenyl-analog were not applicable, but the compound showed specific induced circular dichroism (ICD) signals only for AT-sequence-containing ds-DNAs, whereas the pyrene-analog ICD signals were specific for AT-DNA with respect to GC-DNA, and also recognized AU-RNA by giving a different ICD pattern from that observed upon interaction with AT-DNA. The fluorene- and dithienyl-diketopyrrolopyrrole analogs were ICD-signal silent. Thus, fine-tuning of the aromatic linker properties connecting two triarylborane dications can be used for the dual sensing (fluorimetric and CD) of various ds-DNA/RNA secondary structures, depending on the steric properties of the DNA/RNA grooves.
The 2- and 2,7- substituted para-N-methylpyridinium pyrene cations show high-affinity intercalation into ds-DNAs, whereas their non-methylated analogues interacted with ds-DNA/RNA only in the protonated form (at pH 5), but not at physiological conditions (pH 7). The fluorescence from non-methylated analogues was strongly dependent on the protonation of the pyridines; consequently, they act as fluorescence ratiometric probes for simultaneous detection of both ds-DNA and BSA at pH 5, relying on the ratio between intensities at 420 nm (BSA specific) and 520 nm (DNA specific), whereby exclusively ds-DNA sensing could be switched-off by adjustment to pH 7. Only methylated, permanently charged pyrenes show photoinduced cleavage of circular DNA, attributed to pyrene-mediated irradiation-induced production of singlet oxygen. Consequently, the moderate toxicity of these cations against human cell lines is strongly increased upon irradiation. Detailed studies revealed increased total ROS production in cells treated by the compounds studied, accompanied by cell swelling and augmentation of cellular complexity. The most photo-active 2-para-N-methylpyridinium pyrene showed significant localization at mitochondria, its photo-bioactivity likely due to mitochondrial DNA damage. Other derivatives were mostly non-selectively distributed between various cytoplasmic organelles, thus being less photoactive.
Ionic liquids-assisted ring opening of three-membered heterocycles with thio- and seleno-silanes
(2022)
Ring opening reactions of strained heterocycles (epoxides, aziridines, thiiranes) by silyl chalcogenides, such as thiosilanes and selenosilanes, can be efficiently performed in a variety of ionic liquids, which can behave as reaction media and in some cases also as catalysts. This protocol enables an alternative access to β-functionalized sulfides and selenides under mild conditions.
The solvatochromic behavior of two donor-π bridge-acceptor (D-π-A) compounds based on the 2-(3-boryl-2-thienyl)thiazole π-linker and indandione acceptor moiety are investigated. DFT/TD-DFT calculations were performed in combination with steady-state absorption and emission measurements, along with electrochemical studies, to elucidate the effect of two different strongly electron-donating hydrazonyl units on the solvatochromic and fluorescence behavior of these compounds. The Lippert–Mataga equation was used to estimate the change in dipole moments (Δµ) between ground and excited states based on the measured spectroscopic properties in solvents of varying polarity with the data being supported by theoretical studies. The two asymmetrical D-π-A molecules feature strong solvatochromic shifts in fluorescence of up to ~4300 cm\(^{−1}\) and a concomitant change of the emission color from yellow to red. These changes were accompanied by an increase in Stokes shift to reach values as large as ~5700–5800 cm\(^{−1}\). Quantum yields of ca. 0.75 could be observed for the N,N-dimethylhydrazonyl derivative in nonpolar solvents, which gradually decreased along with increasing solvent polarity, as opposed to the consistently reduced values obtained for the N,N-diphenylhydrazonyl derivative of up to ca. 0.20 in nonpolar solvents. These two push–pull molecules are contrasted with a structurally similar acceptor-π bridge-acceptor (A-π-A) compound.
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.
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.
Ten thiosemicarbazone ligands obtained by condensation of pyridine-2-carbaldehyde, quinoline-2-carbaldehyde, 2-acetylpyridine, 2-acetylquinoline, or corresponding 2-pyridyl ketones with thiosemicarbazides RNHC(S)NHNH\(_{2}\) and R=CH\(_{3}\), C\(_{6}\)H\(_{5}\) were prepared in good yield. The reaction of [PdCl\(_{2}\)(cod)] with cod=1,5-cyclooctadiene or K\(_{2}\)[PtCl\(_{4}\)] resulted in a total of 17 Pd(II) and Pt(II) complexes isolated in excellent purity, as demonstrated by \(^{1}\)H, \(^{13}\)C, and, where applicable, \(^{195\)Pt NMR spectroscopy combined with CHNS analysis. The cytotoxicity of the title compounds was studied on four human glioblastoma cell lines (GaMG, U87, U138, and U343). The most active compound, with a Pd(II) metal centre, a 2-quinolinyl ring, and methyl groups on both the proximal C and distal N atoms exhibited an EC\(_{50}\) value of 2.1 μM on the GaMG cell lines, thus being slightly more active than cisplatin (EC\(_{50}\) 3.4 μM) and significantly more potent than temozolomide (EC\(_{50}\) 67.1 μM). Surprisingly, the EC\(_{50}\) values were inversely correlated with the lipophilicity, as determined with the “shake-flask method”, and decreased with the length of the alkyl substituents (C\(_{1}\)>C\(_{8}\)>C\(_{10}\)). Correlation with the different structural motifs showed that for the most promising anticancer activity, a maximum of two aromatic rings (either quinolinyl or pyridyl plus phenyl) combined with one methyl group are favoured and the Pd(II) complexes are slightly more potent than their Pt(II) analogues.
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.
A series of unprecedently air-stable (tricyanoboryl)plumbate anions was obtained by the reaction of the boron-centered nucleophile B(CN)\(_{3}\)\(^{2-}\) with triorganyllead halides. Salts of the anions [R\(_{3}\)PbB(CN)\(_{3}\)]\(^{-}\) (R=Ph, Et) were isolated and found to be stable in air at room temperature. In the case of Me\(_{3}\)PbHal (Hal=Cl, Br), a mixture of the anions [Me\(_{4-n}\)Pb{B(CN)\(_{3}\)}\(_{n}\)]\(^{n-}\) (n=1, 2) was obtained. The [Et\(_{3}\)PbB(CN)\(_{3}\)]− ion undergoes stepwise dismutation in aqueous solution to yield the plumbate anions [Et4\(_{4-n}\)Pb{B(CN)\(_{3}\)}\(_{n}\)]\(^{n-}\) (n=1–4) and PbEt\(_{4}\) as by-product. The reaction rate increases with decreasing pH value of the aqueous solution or by bubbling O\(_{2}\) through the reaction mixture. Adjustment of the conditions allowed the selective formation and isolation of salts of all anions of the series [Et\(_{4-n}\)Pb{B(CN)\(_{3}\)}\(_{n}\)]\(^{n-}\) (n=2–4) including the homoleptic tetraanion [Pb{B(CN)\(_{3}\)}\(_{4}\)]\(^{4-}\).