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Investigation of the mechanism of the NHC-initiated hydrosilylation of styryl alcohols in the presence of a dihydrosilane suggests a general base catalysis mechanism and not the activation of the dihydrosilane by the NHC. © 2013 The Royal Society of Chemistry.

Reaction of the triflate group in dipp-Imd-BH2OTf with nucleophiles generally occurs by direct substitution to give products dipp-Imd-BH2Nu. In contrast, reaction of this boryl triflate with aryloxides (ArO-) in THF resulted in insertion of THF with ring opening in between the NHC-boryl electrophile and the aryloxide to give products dipp-Imd-BH2O(CH2)4OAr. The incorporation of THF was observed with other nucleophiles such as ethylthiolate and trimethylsilanolate whose conjugate acids have pKa values similar to that of phenol. The cleavage of ethyl acetate and oxetane is reported as well. A possible mechanism is proposed. © 2013 The Royal Society of Chemistry.

Treatment of alkenes such as 3-hexene, 3-octene, and 1-cyclohexyl-1-butene with the N-heterocyclic carbene (NHC)-derived borane 2 and catalytic HNTf 2 (Tf = trifluoromethanesulfonyl (CF3SO2)) effects hydroboration at room temperature. With 3-hexene, surprisingly facile migration of the boron atom from C(3) of the hexyl group to C(2) was observed over a time scale of minutes to hours. Oxidative workup gave a mixture of alcohols containing 2-hexanol as the major product. A similar preference for the C(2) alcohol was observed after oxidative workup of the 3-octene and 1-cyclohexyl-1-butene hydroborations. NHC-borenium cations (or functional equivalents) are postulated as the species that accomplish the hydroborations, and the C(2) selective migrations are attributed to the four-center interconversion of borenium cations with cationic NHC-borane-olefin π-complexes. © 2012 American Chemical Society.

Reactions of NHC-BH 2X (where X = Cl, OTf and NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with 1 equiv of triflic acid provide the stable products of acid/base reactions NHC-BH(OTf) 2 and NHC-BH(OTf)Cl. Further reactions of these compounds with additional triflic acid (or direct reactions of NHCBH 3 with excess triflic acid) produce the isolable dihydroxyborenium triflate [NHCB(OH) 2] +TfO -. This first-in-class ligated borenium ion has been characterized by NMR spectroscopy and X-ray crystallography. © 2011 American Chemical Society.

Still elusive: The reduction of dimethylimidazol-2-ylidene dichloroborane by sodium naphthalenide has been suggested to provide a borylene that cycloadds to naphthalene to make a borirane. Evidence has been provided that this borirane instead arises from coupling of a boryl radical and sodium naphthalenide. In contrast, reduction of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene dichloroborane and the related dimesityl derivative provide novel C-H insertion products that may have arisen from a borylene. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Otherwise sluggish or completely ineffective radical reductions of alkyl and aryl halides by N-heterocyclic carbene boranes (NHC-boranes) are catalyzed by thiols. Reductions and reductive cyclizations with readily available 1,3-dimethylimidazol-2-ylidene borane and a water-soluble triazole relative are catalyzed by thiophenol and tert-dodecanethiol [C 9H 19C(CH 3) 2SH]. Rate constants for reaction of the phenylthiyl (PhS•) radical with two NHC-boranes have been measured to be ∼10 8 M -1 s -1 by laser flash photolysis experiments. An analysis of the available evidence suggests the operation of polarity reversal catalysis. © 2012 American Chemical Society.

The hydrosilylation of carbonyl derivatives has been explored by the activation of diphenylsilane in the presence of a catalytic amount of an N-heterocyclic carbene (NHC). Presumably, a hypervalent silicon intermediate featuring strong Lewis acid character allows dual activation of both the carbonyl moiety and the hydride at the silicon center. Reduction under mild conditions could be accomplished using this organocatalytic process. Some interesting selectivities have been encountered. © Georg Thieme Verlag Stuttgart · New York.

The nucleophilicity parameters (N) of 1,3-bis(2,6-diisopropylphenyl) imidazol-2-ylidene borane and 1,3-dimethylimidazol-2-ylidene borane are 9.55 and 11.88. This places N-heterocyclic carbene boranes (NHC-boranes) among the most nucleophilic classes of neutral hydride donors. Reductions of highly electron-poor C=N and C=C bonds provide hydrogenation products along with new, stable borylated products. The results suggest that NHC-boranes have considerable untapped potential as neutral organic reductants. © 2011 American Chemical Society.

N-Hetereocyclic carbene (NHC) complexes of boranes are stable compounds that can be used as coinitiators for the type II photopolymerization of acrylates. The present account summarizes this new development for polymerization and boron chemistry. In particular, NHC-boranes are air- and water-tolerant, which enhances the practicality of the photopolymerizations. © Schweizerische Chemische Gesellschaft.

Rate constants for important elementary reactions of 2,4-dimethyl-1,2,4- triazol- 3-ylidene borane (1) and its derived boryl radical have been measured in the presence and absence of water. Reactions including H-abstraction by tBuO, and onward reactions of the derived radical with methyl acrylate (addition), iodopropane (halogen abstraction), and an iodonium salt (oxidation) are unaffected by the presence of a high concentration (1.5 M) of water. Extending these results, borane (1) has been found to be an efficient new co-initiator for the visible light photopolymerization of monomers in the presence of both air and water. © 2011 American Chemical Society.

To protect and serve: The efficiency of TypeI radical photoinitiators for acrylate polymerization is greatly increased by the presence of NHC-boranes. Conversion of the initially photogenerated radicals, R . (see scheme), into strongly nucleophilic NHC-boryl radicals, through hydrogen-atom transfer, gives a better matched radical for subsequent reaction with monomer, ensuring better initiation. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Thermal 1,3-dipolar cycloaddition reactions of 1,3-bis(2,6- diisopropylphenyl)imidazol-2-ylidene dihydridoboron azide occur smoothly with alkynes, nitriles, and alkenes bearing electron-withdrawing groups. New, stable NHC-boryl-substituted triazoles, tetrazoles, and triazolidines are formed in good to excellent yields. © 2012 American Chemical Society.

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. © The Royal Society of Chemistry 2011.

1,3-Dimethylimidazol-2-ylidene borane and 2,4-dimethyl-1,2,4-triazol-3- ylidene borane are found to be useful reagents for the reduction of alkyl iodides and bromides bearing nearby electron withdrawing substituents. Signatures of radical chain reactions are seen in many cases, but ionic reductions may also be occurring with some substrates. The reagents are attractive because of their low molecular weight, their availability from inexpensive precursors, and their stability. Separation of the borane products from the target products is readily accomplished either with or without prior regeneration of the borane for later reuse. 2,4-Dimethyl-1,2,4-triazol-3-ylidene borane is versatile because both starting borane and its derived products can be removed by extraction with water. © 2011 The Royal Society of Chemistry.

Reducing alkenes to tears: Addition of structurally diverse N-heterocyclic carbenes (NHCs) to silicon allows the reduction of propargylic and styryl alcohols through an organocatalyzed silylation/direct hydride transfer tandem reaction (see scheme). Catalytic turnover is enabled by the switch to and from hypervalent silicon. This provides a new synthetic application of NHC-main group element complexes. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

N,N′-Dialkyl and N,N′-diaryl imidazol-2-ylidene-boranes and trifluoroboranes are rapidly lithiated at C4 of the imidazole ring, and the resulting intermediates have been quenched with an assortment of electrophiles to provide ring-functionalized imidazol-2-ylidene-boranes. Further deprotonation and functionalization of C5 have been demonstrated. Deboronation of the products by treatment with triflic acid or iodine and then methanol opens a route to C4/C5 functionalized imidazolium salts and imidazol-2-ylidenes. © 2011 American Chemical Society.

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. Forget everything you know about boron chemistry ... NHC-boranes (see picture) are different. Functionalities that are not usually present in organoboron compounds can be easily introduced. New classes of rare boron-based reactive intermediates (cations, radicals, and anions) have emerged. And NHC-boranes are promising as reagents and catalysts in organic synthesis and as co-initiators in radical polymerization. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

N-Heterocyclic carbene boranes (NHC-boranes) are a new "clean" class of reagents suitable for reductive radical chain transformations. Their structures are well suited for their reactivity to be tuned by inclusion of different NHC ring units and by appropriate placement of diverse substituents. EPR spectra were obtained for the boron-centered radicals generated on removal of one of the BH3 · hydrogen atoms. This spectroscopic data, coupled with DFT computations, demonstrated that the NHC-BH2radicals are planar Π-delocalized species. tert-Butoxyl radicals abstracted hydrogen atoms from NHC-boranes more than 3 orders of magnitude faster than did C-centered radicals, although the rate decreased markedly for sterically shielded NHC-BH3 centers. Combinations of two NHC-boryl radicals afforded 1,2-bisNHC-diboranes at rates which also depended strongly on steric shielding. The termination rate increased to the diffusion-controlled limit for sterically unhindered NHC-boryls. Bromine atoms were rapidly transferred to imidazole-based NHC-boryl radicals from alkyl, allyl, and benzyl bromides. Chlorine-atom abstraction was, however, much less efficient and only observed for sterically unhindered NHC-boryls reacting with allylic and benzylic chlorides. For an NHC-borane containing a bulky thexyl substituent at boron, the tertiary H atom of the thexyl group was selectively removed. The resulting ß-boron- containing alkyl radical rapidly underwent ß scission of the B-C bond with production of an NHC-boryl radical and an alkene. © 2010 American Chemical Society.

Newly discovered N-heterocyclic carbene-boryl radicals (NHC-BH 2•) 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 carbeneboranes show excellent potential as both photopolymerization co-initiators and mediators of small molecule radical reactions, and these results will help guide further development in both fields. © 2010 American Chemical Society.

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. © 2010 American Chemical Society.

Boryl halide, carboxylate and sulfonate complexes of 1,3-bis(2,6- diisopropylphenyl)imidazol-2-ylidene (dipp-Imd-BH2X, X = halide or sulfonate) have been prepared from the parent borane dipp-Imd-BH3 by (1) substitution reactions with R-X (X = halide or sulfonate), (2) reactions with electrophiles (like I2 or NIS), or (3) acid/base reactions with HX (provided that HX has a pKa of about 2 or less). Dipp-Imd-BH 2I is most conveniently prepared by reaction with diiodine while dipp-Imd-BH2OTf is best prepared by reaction with triflic acid. These and other less reactive complexes behave as electrophiles and can be substituted by a wide range of heteroatom nucleophiles including halides, thiolates and other sulfur-based nucleophiles, isocyanate, azide, nitrite, and cyanide. The resulting products are remarkably stable, and many have been characterized by X-ray crystallography. Several are members of very rare classes of functionalized boron compounds (boron azide, nitro compound, nitrous ester, etc.). © 2010 American Chemical Society.

Reaction of lithium 1,3-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2- diazaborol-2-ide with borane·THF provides the first boryl-substituted borohydride: lithium [1,3-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2- diazaborol-2-yl]trihydroborate. The compound is fully characterized by 11B, 1H, and 7Li NMR spectra and other means, and these data are compared to neutral and anionic benchmark compounds. The compound crystallizes as a dimer complexed to four THF molecules. The dimer lacks the bridging B-H bonds seen in neutral boranes and is instead held together by ionic Li - -HB interactions. A preliminary scan of reactions with several iodides shows that the compound participates in an ionic reduction (with a primary-alkyl iodide), an organometallic reduction (Pd-catalyzed with an aryl iodide), and a radical reduction (AIBN-initiated with a sugar-derived iodide). Accordingly the new borylborohydride class may share properties of both traditional borohydrides and isoelectronic N-heterocyclic carbene boranes. © 2010 American Chemical Society.

Rate constants for hydrogen abstraction by a nonyl radical from 20 complexes of N-heterocyclic carbenes and boranes (NHC-boranes) have been determined by the pyridine-2-thioneoxycarbonyl (PTOC) competition kinetic method at a single concentration point. The rate constants range from <1 × 104 to 8 × 104 M-1 s-1. They show little dependence on the electronic properties of the carbene core, but there is a trend for increasing rate constants with decreasing size of the carbene N-substituents. Two promising new reagents with small N-substituents (R = Me) have been identified. © 2010 American Chemical Society.

Minimalist N-heterocyclic carbene boranes 1,3-dimethylimidazol-2- ylideneborane and 2,4-dimethyl-1,2,4-triazol-3-ylideneborane are readily available and have low molecular weights. They exhibit superior performance to first-generation NHC-boranes, providing improved yields in reductions of xanthates and related functional groups. © 2010 American Chemical Society.

Lying low: A lithiated unsubstituted N-heterocyclic carbene (NHC) boryl anion can be generated by reduction, and trapped by electrophiles (see scheme; dipp=2,6-diisopropylphenyl) to provide new substituted NHC boranes. It is yet another example of a low-valent boron compound or boron-containing reactive intermediate stabilized by an NHC, thereby extending the scope of NHC borane chemistry. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Reduction of xanthates by N-heterocyclic carbene boranes (NHC-boranes) has been suggested to occur by a radical chain mechanism involving heretofore unknown NHC-boryl radicals. In support of this suggestion, both the expected borane dithiocarbonate product and an unexpected borane xanthate product have now been isolated. These are the first NHC-boranes with boron-sulfur bonds, and their structures have been secured by spectroscopic and crystallographic means. The first rate constants for H-atom transfer from an NHC borane complex were determined by using the ring opening of a substituted cyclobutylcarbinyl radical as a clock reaction. The rate constant for reaction of the NHC-borane with a secondary alkyl radical at ambient temperature is 4 × 104 M-1 s-1, and the Arrhenius function displayed an entropic term (log A term) that was typical for a bimolecular reaction. The B-H bond dissociation energy of an NHC-borane complex has been estimated at 88 kcal/mol. The putative NHC-boryl radical in these transformations has been detected by EPR spectroscopy. Spectral analysis suggests that it is a π-radical, analogous to the benzyl radical. © 2009 American Chemical Society.

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.

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 Suzukl-Miyaura cross-couplings, along with boronlc acids and organotrlfluoroborates. © 2009 American Chemical Society.

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. Copyright © 2008 American Chemical Society.