Photocatalytic redox-neutral reaction of γ-indolyl α-keto esters

Abstract

The direct γ-C(sp³)–H activation of saturated α-keto esters has long been an elusive transformation. We found that photoredox catalysis involving Ir^III in combination with DABCO as a dual hydrogen-bonding donor and organic base can lead to the transient generation of the multi-π-electron γ-enolyl radical from γ-indolyl α-keto ester via enolization, oxidation and γ-deprotonation. In the spin delocalized system, the γ-enolyl radical can be easily reduced by Ir^II to form the γ-enolyl anion and subsequently couple with a proton at the α-position of the ester. This mode of activation is suitable for the synthesis of various 4-indolyl substituted 2-hydroxy-3-enoic acid esters (in up to 90% yield and with up to Z/E >96 : 4) and is amenable for enantioselective catalysis.