Unnatural Thiamine Radical Enzymes for Photobiocatalytic Asymmetric Alkylation of Benzaldehydes and Α-Ketoacids

Despite substantial progress made toward elucidating the natural radical enzymology with thiamine pyrophosphate (TPP)-dependent pyruvate:ferredoxin oxidoreductases (PFORs) and pyruvate oxidases (POXs), repurposing naturally occurring two-electron TPP-dependent enzymes to catalyze single-electron transformations with significant synthetic value remains a daunting task. Enabled by the synergistic use of visible-light photocatalyst fluorescein and a set of engineered TPP-dependent enzymes derived from benzoylformate decarboxylase (BFD) and benzaldehyde lyase (BAL), we developed an asymmetric photobiocatalytic decarboxylative alkylation of benzaldehydes and alpha-keto acids to produce highly enantioenriched alpha-branched ketones. Mechanistically, we propose that this dual catalytic radical alkylation involves single-electron oxidation of the enzyme-bound Breslow intermediate and subsequent interception of the photoredox-generated transient alkyl radical. In conjunction with visible light photoredox catalysis, thiamine radical biocatalysis represents an emerging platform to discover and optimize asymmetric radical transformations that are unknown to biological systems and not amenable to small-molecule catalysis.