Synthesis and Reactions of Heterosubstituted N-Heterocyclic Carbene-Boranes

Carbene-boranes 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene borane (dipp-Imd-BH3) and 1,3-dimethylimidazol-2-ylidene borane (diMe-Imd-BH3) were treated with various electrophiles to compare reactivity of carbene-borane with nucleophilicity values of carbene-borane (dipp-Imd-BH3 = 9.55 N, diMe-Imd-BH3 = 11.88 N). Compounds with electrophilicity values greater –15 E reacted rapidly with carbene-borane, but compounds with electrophilicity values less than –15 E reacted slowly or not at all with carbene-borane. Hydroboration complexes were isolated from the reactions of diMe-Imd-BH3 with benzylidenemalononitrile and 2-ethylidenemalononitrile. Overall the reactivity of carbene-borane matched predictions based on nucleophilicity and electrophilicity values. \ud Carbene-boranes were also treated with acid halides and halogens to form halogenated carbene-boranes. Iodination occurs most rapidly followed by bromination and then chlorination. Dipp-Imd-BH3 can only be iodinated once likely due to the steric bulk around the boron atom while diMe-Imd-BH3 can be iodinated once (0.5 equiv of I2 added) or twice (1.0 equiv of I2 added). Tribromination and trichlorination was possible for both carbene-boranes. Disproportionation of the halides bonded to the boron atoms occurred rapidly with iodide and slowly with bromide. These halogenated carbene-boranes also were reduced by lithium aluminum deuteride to give selectively deuterated carbene-borane species, and also underwent nucleophilic substitutions to give carbene-boryl-azide. \ud [3+2]-Dipolar cycloaddition reactions were performed on carbene-boryl-azide with electron deficient alkynes, alkenes, and nitriles to give new carbene-boryl-triazoles, -triazolidines, and -tetrazoles. Electron deficient alkynes and alkenes such as propiolate and acrylate were reactive with carbene-boryl-azide even at room temperature, but less electron deficient alkynes, nitriles, and alkenes required heating or microwave radiation to form cycloaddition borane products. \ud Carbene-boryl-triazoles are very robust and can withstand reductive and acidic conditions. The triazole can even be methylated by methyl iodide or methyl triflate to give a carbene-boryl-triazole salt. The triazole could be detached from the boron atom by reacting the boron atom with halogen, and decomplexing the carbene-borane in methanol. \u