Bioinspired Radical Stetter Reaction: Radical Umpolung Enabled by Ion-Pair Photocatalysis

Abstract

A bioinspired, intermolecular radical Stetter reaction of α-keto acids and aldehydes is disclosed that is contingent on a formal "radical umpolung" concept. Enabled by secondary amine activation, electrostatic recognition ensures that the α-ketocarboxylic acids, which function as latent acyl radicals, are proximal to the in situ generated iminium salts. This photoactive contact ion pair is an electron donor-acceptor (EDA) complex, and undergoes facile single electron transfer (SET) and rapid decarboxylation prior to radical-radical recombination. Importantly, decarbonylation is mitigated by this strategy. The initial computational validation on which the process is predicated matches closely with experiment. Synergising organo- and photocatalysis activation principles finally expands the mechanistic and synthetic scope of the classic Stetter reaction to include α,β-unsaturated aldehydes as acceptors.

Keywords: Stetter reaction; electrostatic interactions; ion pairs; radicals; umpolung.