EPR Studies of the Generation, Structure, and Reactivity of N-Heterocyclic Carbene Borane Radicals

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 BH(3) hydrogen atoms. This spectroscopic data, coupled with DFT computations, demonstrated that the NHC-BH(2)* radicals are planar pi-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-BH(3) centers. Combinations of two NHC-boryl radicals afforded 1,2-bis-NHC-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 beta-boron-containing alkyl radical rapidly underwent beta scission of the B-C bond with production of an NHC-boryl radical and an alkene.