Biocatalytic Asymmetric Michael Addition of Thiols to α,β‐Unsaturated Aldehydes via Enzyme‐Bound Iminium Ion Intermediates

The thia‐Michael addition is one of the most versatile and practical reactions for CS bond formation in organic synthesis, but remains a continuing challenge for biocatalysis. Here we report engineered cofactor‐independent enzymes, based on the promiscuous enzyme 4‐oxalocrotonate tautomerase, for the enantioselective synthesis of various ( R )‐β‐thioenals via the nucleophilic addition of aliphatic thiols to cinnamaldehyde. These engineered enzymes efficiently promote the formation of the carbon–sulfur bond with good stereocontrol, affording the desired ( R )‐β‐thioenal products with excellent conversions (up to 99%) and with moderate‐to‐high enantiopurity (e.r. up to 95:5). Our results highlight the power of catalytic promiscuity for expanding the biocatalytic repertoire for non‐natural reactions.