Synthesis of Boriranes by Double Hydroboration Reactions of N-Heterocyclic Carbene Boranes and Dimethyl Acetylenedicarboxylate.

Reaction of bis-(2,6-diisopropylphenyl)-imidazol-2-ylidene borane with dimethyl acetylenedicarboxylate gives 80% yield of a stable borirane (boracyclopropane) formed by formal double hydroboration along with 5% of the (E)-alkenylborane. DFT calculations suggest a mechanism where divergence to the two products occurs after a common initial stage of hydride transfer from the NHC-borane to the acetylenedicarboxylate.

[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.

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.