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Facile synthesis of α-NHC-boryl ketones from NHC-boranes and alkenyl triflates.

Reactions of readily available alkenyl triflates with NHC-boranes in the presence of diisopropyl ethyl amine provided about three-dozen stable α-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: (1) addition of an NHC-boryl radical to the alkenyl triflate, (2) fragmentation to give the α-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.