For reasons of atom economy and waste prevention replacements of high molecular weight stoichiometric oxidants with O2 are most desirable. However, oxidations using O2 are problematic due to a high energy barrier for the reaction between oxygen and the substrate leading to unselective reactions. Here we present aerobic oxidative multistep electron transfer N-heterocyclic carbene (NHC) catalysis, a general oxidation protocol, that mediates selective oxidation of the α,β-unsaturated aldehyde derived homoenolate to the synthetically useful α,β-unsaturated acyl azolium intermediate. The methodology relies on a system of electron transfer mediators that provides a stepwise low energy transfer of electrons from the electron rich homoenolate to O2. Several independent oxidative NHC-catalyzed reactions, such as the enantioselective synthesis of dihydropyranones, previously performed with high molecular weight stoichiometric oxidants, are viable with this strategy and the products can be isolated in high to excellent yields. These findings mediate large-scale preparations involving the important chemistry developed within the field of oxidative NHC catalysis.

Oxidative aerobic NHC catalysis via multistep electron transfer

253rd National Meeting (2017)

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