关键词: 氮杂环卡宾催化, 光催化, 协同催化, 对映选择性, 氧酰化反应

Chiral α-aryl carboxylic acids are a common structural motif and serve as the key pharmacophore in many drug molecules. Traditional methods for constructing such chiral structures typically rely on asymmetric transition-metal catalysis. In this communication, an asymmetric benzylic C—H oxyacylation reaction enabled by cooperative photoredox and N-heterocyclic carbene (NHC) catalysis is reported. This reaction affords a series of chiral α-aryl esters in moderate to good yields with high enantioselectivities. These chiral esters could serve as precursors for α-aryl carboxylic acids, providing a more atom-economical synthetic route to this important class of bioactive frameworks. A plausible mechanism was proposed for this reaction: First, the free NHC adds to diethyl pyrocarbonate to form the corresponding acyl azolium intermediate, while the excited-state photosensitizer oxidizes the alkyl arene substrate to generate the aryl radical cation intermediate, which subsequently forms benzyl radical under basic conditions. Then, the acyl azolium intermediate was reduced by the reduced-state photosensitizer to give the corresponding ester radical, which then couples with the benzyl radical to form the zwitterionic intermediate. Finally, the zwitterionic intermediate releases the target product and regenerates the NHC catalyst to complete the catalytic cycle. In this reaction, photocatalysis generates highly active radical species, while NHC catalysis controls the enantioselectivity, with the two catalytic modes working in excellent synergy. A representative procedure is described as follows. To an oven-dried 4-mL vial equipped with a magnetic stirring bar was charged with alkylarene 1 (0.2 mmol, 1 equiv.), diethyl pyrocarbonate 2a (0.8 mmol, 4.0 equiv.) and the NHC F (0.04 mmol, 20 mol%), K₂CO₃ (0.3 mmol, 1.5 equiv.), photocatalyst PC3 (Ir[dFCF₃ppy]₂[bpy]PF₆) (0.004 mmol, 2 mol%). The tube was evacuated and backfilled with N₂ for three times. To this mixture was added dry CH₃CN (3 mL) and stirred at room temperature under blue LED irradiation for 24 h. The reaction mixture was concentrated under reduced pressure and the residue was purified by column chromatography on silica gel to give the desired product 3.

Key words: N-heterocyclic carbene catalysis, photocatalysis, cooperative catalysis, enantioselectivity, oxyacylation reaction