Search

Borane sind weit verbreitete Lewis-Sauren, und N-heterocyclische Carbene (NHCs) sind beliebte Lewis-Basen. Dennoch war bis in die jungste Zeit erstaunlich wenig uber Komplexe dieser beiden Verbindungsklassen miteinander bekannt. NHC-Borane sind leicht herzustellen, und teilweise sind sie so stabil, dass sie weniger als Komplex, sondern vielmehr als eigenstandige organische Verbindung betrachtet werden konnen. Sie verhalten sich nicht wie klassische Borane, und sie konnen ein breites Spektrum an funktionellen Gruppen enthalten. Als vielseitige Reaktionspartner, Reagentien und Katalysatoren verfugen NHC-Borane uber ein immenses Potenzial fur die organische Synthese und die Polymerchemie. Viele Reaktionen von NHC-Boranen laufen uber neue reaktive Intermediate wie Boreniumkationen, Borylradikale und sogar Borylanionen ab. Hier geben wir eine umfassende Ubersicht uber die Synthese, Charakterisierung und Reaktionen von NHC-Boranen.

Eight N-heterocyclic carbene-boranes (NHC-boranes) are proposed as new efficient co-initiators for acrylate photopolymerization reactions. They are particularly interesting in aerated conditions, where they help overcome the classical oxygen inhibition. The carbene boryl radicals that are the initiating species have been characterized by their transient absorption spectra obtained in laser flash photolysis (LFP) experiments. Rate constants for the generation of the carbene boryl radicals by hydrogen abstraction with t-butoxyl radical and triplet benzophenone as well as the reactions with oxygen, electron rich and electron poor alkenes, two alkyl halides (CHCl3 and iodopropane) and diphenyliodonium hexafluorophosphate have been measured. The reactivity of N-heterocyclic carbene borane radicals is clearly affected by the NHC substituent.

Newly discovered N-heterocyclic carbene-boryl radicals (NHC-BH2•) derived from readily available N-heterocyclic carbene−boranes are found to be efficient initiators for acrylate photopolymerization. Laser flash photolysis (LFP) experiments were used to generate three carbene-boryl radicals, which were characterized by their transient absorption spectra with the aid of DFT calculations. Rate constants were measured for the generation of the carbene-boryl radicals by hydrogen abstraction with tert-butoxyl radical, a ketophosphonyl radical, and triplet benzophenone. Rate constants were also measured for the reactions of the carbene-boryl radicals with oxygen, three alkenes, two alkyl chlorides, and diphenyliodonium hexafluorophosphate. The observed trends were interpreted with the aid of measured oxidation potentials of the carbene−boranes and calculated ionization potentials of the carbene-boryls. N-Heterocyclic carbene−boranes show excellent potential as both photopolymerization co-initiators and mediators...

Complexes of triaryl- and trialkylboranes with N-heterocyclic carbenes (NHCs) participate in Suzuki-Miyaura cross-coupling reactions and provide coupled products in good yields under base-free conditions. The reaction can be applied to Csp(2)-Csp(2) and Csp(2)-Csp(3) carbon-carbon bond formation with triflates, iodides, bromides, and chlorides. These results enrich the utility of NHC-borane complexes, which can be added to the toolkit of Suzuki-Miyaura cross-couplings, along with boronic acids and organotrifluoroborates.

A versatile new method for the preparation of NHC boranes starting from two stable, readily available reactants-an heterocyclic salt and an amine or phosphine-borane-is reported. It uses a Lewis base exchange at boron and provides easy access to new NHC boranes, in particular B-substituted borane ones.

International audience; Substituents on the boron atom of NHC-boranes direct the reactivity of the ligated boreniums obtained through hydride abstraction. Depending on the electronics of the substituent, the reaction is selectively steered toward either B-substitution or Lewis base exchange.

By using the Telospot assay, 27 different extracts of cyanobacteria were evaluated for telomerase inhibition. All extracts showed varying, but significant activity. We selected Microcystis aeruguinosa PCC 7806 to identify the active compound and a bioassay guided fractionation led us to isolate mixtures of sulfoquinovosyl diacylglycerols (SQDGs), which were identified by 2D NMR and MS/MS experiments. Pure SQDG derivatives were then synthesized. The IC(50) values of pure synthetic sulfoquinovosyl dipalmitoylglycerol and the monopalmitoylated derivative against telomerase were determined to be 17 and 40 μM, respectively. A structure-activity relationship study allowed the identification of compounds with modified lipophilic acyl groups that display improved activity.

Boranes are widely used Lewis acids and N-heterocyclic carbenes (NHCs) are popular Lewis bases, so it is remarkable how little was known about their derived complexes until recently. NHC-boranes are typically readily accessible and many are so stable that they can be treated like organic compounds rather than complexes. They do not exhibit “borane chemistry”, but instead are proving to have a rich chemistry of their own as reactants, as reagents, as initiators, and as catalysts. They have significant potential for use in organic synthesis and in polymer chemistry. They can be used to easily make unusual complexes with a broad spectrum of functional groups not usually seen in organoboron chemistry. Many of their reactions occur through new classes of reactive intermediates including borenium cations, boryl radicals, and even boryl anions. This Review provides comprehensive coverage of the synthesis, characterization, and reactions of NHC-boranes.

International audience; Fifteen second-generation NHC-ligated boranes with aryl and alkyl substituents on boron were prepared, and their radical chemistry was explored by electron paramagnetic resonance (EPR) spectroscopy and calculations. Hydrogen atom abstraction from NHC-BH(2)Ar groups produced boryl radicals akin to diphenylmethyl with spin extensively delocalized across the NHC, BH, and aryl units. All of the NHC-B*HAr radicals studied abstracted Br-atoms from alkyl bromides. Radicals with bulky N,N'-dipp substituents underwent dimerization about 2 orders of magnitude more slowly than first-generation NHC-ligated trihydroborates. The evidence favored head-to-head coupling yielding ligated diboranes. The first ligated diboranyl radical, with a structure intermediate between that of ligated diboranes and diborenes, was spectroscopically characterized during photolysis of di-t-butyl peroxide with N,N'-di-t-butyl-imidazol-2-ylidene phenylborane. The reactive site of B-alkyl-substituted NHC-boranes switched from the boron center to the alkyl substituent for both linear and branched alkyl groups. The β-borylalkyl radicals obtained from N,N'-dipp-substituted boranes underwent exothermic β-scissions with production of dipp-Imd-BH(2)* radicals and alkenes. The reverse additions of NHC-boryl radicals to alkenes are probably endothermic for alkyl-substituted alkenes, but exothermic for conjugated alkenes (addition of an NHC-boryl radical to 1,1-diphenylethene was observed). A cyclopropylboryl radical was observed, but, unlike other α-cyclopropyl-substituted radicals, this showed no propensity for ring-opening.

Boranes are widely used Lewis acids and N-heterocyclic carbenes (NHCs) are popular Lewis bases, so it is remarkable how little was known about their derived complexes until recently. NHC-boranes are typically readily accessible and many are so stable that they can be treated like organic compounds rather than complexes. They do not exhibit “borane chemistry”, but instead are proving to have a rich chemistry of their own as reactants, as reagents, as initiators, and as catalysts. They have significant potential for use in organic synthesis and in polymer chemistry. They can be used to easily make unusual complexes with a broad spectrum of functional groups not usually seen in organoboron chemistry. Many of their reactions occur through new classes of reactive intermediates including borenium cations, boryl radicals, and even boryl anions. This Review provides comprehensive coverage of the synthesis, characterization, and reactions of NHC-boranes.

Complexes of triaryl- and trialkylboranes with N-heterocyclic carbenes (NHCs) participate in Suzuki−Miyaura cross-coupling reactions and provide coupled products in good yields under base-free conditions. The reaction can be applied to Csp2−Csp2 and Csp2−Csp3 carbon−carbon bond formation with triflates, iodides, bromides, and chlorides. These results enrich the utility of NHC−borane complexes, which can be added to the toolkit of Suzuki−Miyaura cross-couplings, along with boronic acids and organotrifluoroborates.

Surgical reduction: N-Heterocyclic carbene-borane complexes such as depicted are neutral, organic soluble analogues of borohydride anions with a weak hydridic character, compatible with organometallic catalysis. They are applicable for surgical reductions in complex, multifunctional molecules. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Calculations suggest that complexes of borane with N-heterocyclic carbenes (NHC) have B-H bond dissocation energies more then 20 kcal/mol less than free borane, diborane, borane-THF, and related complexes. Values are in the range of popular radical hydrogen atom donors like tin hydrides (70-80 kcal/mol). The resulting prediction that NHC borane complexes could be used as radical hydrogen atom donors was verified by radical deoxygenations of xanthates by using either AIBN or triethylborane as initiator.

Calculations suggest that complexes of borane with N-heterocyclic carbenes (NHC) have B-H bond dissocation energies more then 20 kcal/mol less than free borane, diborane, borane-THF, and related complexes. Values are in the range of popular radical hydrogen atom donors like tin hydrides (70-80 kcal/mol). The resulting prediction that NHC borane complexes could be used as radical hydrogen atom donors was verified by radical deoxygenations of xanthates by using either AIBN or triethylborane as initiator.