Ring lithiation and functionalization of imidazol-2-ylidene-boranes

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

NHC-boranes: Air- and water-tolerant coinitiators for type II photopolymerizations

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

Tetrahydrofuran ring opening and related reactions with an N-heterocyclic carbene-boryl trifluoromethanesulfonate

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

Reactions of boron-substituted N-heterocyclic carbene boranes with triflic acid. Isolation of a new dihydroxyborenium cation

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

[3+2]-Dipolar cycloaddition reactions of an N-heterocyclic carbene boryl azide

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

Radical reductions of alkyl halides bearing electron withdrawing groups with N-heterocyclic carbene boranes

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

Complexes of borane and N-heterocyclic carbenes: A new class of radical hydrogen atom donor

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

NHC-catalyzed chemo- and regioselective hydrosilylation of carbonyl derivatives

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

N-heterocyclic carbene boranes are good hydride donors

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

Substitution reactions at tetracoordinate boron: Synthesis of N-heterocyclic carbene boranes with boron-heteroatom bonds

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