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A light-driven enzymatic enantioselective radical acylation
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A light-driven enzymatic enantioselective radical acylation
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Journal Article
A light-driven enzymatic enantioselective radical acylation
2024
Overview
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 catalysts
4
. Thiamine diphosphate (ThDP)-dependent enzymes
5
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 systems
9
. Here we repurpose a ThDP-dependent lyase as a stereoselective radical acyl transferase (RAT) through protein engineering and combination with organophotoredox catalysis
10
. 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.
Enzyme-bound ketyl radicals derived from thiamine diphosphate are selectively generated through single-electron oxidation by a photoexcited organic dye and shown to lead to enantioselective radical acylation reactions.
