Reductive N-Heterocyclic Carbene Catalysis via Hydride Transfer: Generating Homoenolates from Unsaturated Esters

In N-heterocyclic carbene (NHC) catalysis, the reduction of NHC-bound acyl azolium species is achieved with progress limited to the generation of ketyl radical intermediates. Here, we report a novel NHC-catalyzed reductive mode for acyl azolium intermediates, which accept a hydride to generate the Breslow intermediate. Then, the reaction of the Breslow intermediate with chalcone results in the formation of cyclopentene, demonstrating a classic umpolung transformation. We expect that this catalytic reductive mode will open new avenues for synthetic transformations.