A light-driven enzymatic enantioselective radical acylation

Abstract

Enzymes are recognized as exceptional catalysts for achieving high stereoselectivities^1-3, but their ability to control the reactivity and stereoinduction of free radicals lags behind that of chemical catalysts⁴. Thiamine diphosphate (ThDP)-dependent enzymes⁵ are well-characterized systems that inspired the development of N-heterocyclic carbenes (NHCs)^6-8 but have not yet been proved viable in asymmetric radical transformations. There is a lack of a biocompatible and general radical-generation mechanism, as nature prefers to avoid radicals that may be harmful to biological systems⁹. Here we repurpose a ThDP-dependent lyase as a stereoselective radical acyl transferase (RAT) through protein engineering and combination with organophotoredox catalysis¹⁰. Enzyme-bound ThDP-derived ketyl radicals are selectively generated through single-electron oxidation by a photoexcited organic dye and then cross-coupled with prochiral alkyl radicals with high enantioselectivity. Diverse chiral ketones are prepared from aldehydes and redox-active esters (35 examples, up to 97% enantiomeric excess (e.e.)) by this method. Mechanistic studies reveal that this previously elusive dual-enzyme catalysis/photocatalysis directs radicals with the unique ThDP cofactor and evolvable active site. This work not only expands the repertoire of biocatalysis but also provides a unique strategy for controlling radicals with enzymes, complementing existing chemical tools.