Facile Synthesis of α-N-Heterocyclic Carbene-Boryl Ketones from N-Heterocyclic Carbene-Boranes and Alkenyl Triflates

Reactions of readily available alkenyl triflates with N-heterocyclic carbene (NHC)-boranes in the presence of diisopropyl ethyl amine provided about three dozen stable alpha-NHC-boryl ketones. Isolated yields were typically 40-56% for B-unsubstituted NHC-boranes (NHC-BH3), and somewhat lower for NHC-boranes with B-substituents (NHC-BH2R). The requisite alkenyl triflates can be made separately or prepared in situ from either ketones or alkynes. The experimental evidence supports a radical chain mechanism that involves the following: (1) addition of an NHC-boryl radical to the alkenyl triflate, (2) fragmentation to give the alpha-NHC-boryl ketone, SO2, and trifluoromethyl radical, and (3) hydrogen abstraction by trifluoromethyl radical from the starting NHC-borane to return the NHC-boryl radical along with trifluoromethane. Reactions 1 and 3 are both new and evidently rather fast.