Visible-Light-Catalyzed Regioselective 1,4-Carbon/Carboxylation of 1,3-Enynes with Malonates and CO2 toward 2,3-Allenoic Acids.

Difunctionalization of unsaturated carbon-carbon bonds with carbon dioxide (CO2) provides expedient access to functionalized carboxylic acids. Compared to the well-developed carboxylation of alkenes or alkynes with CO2, the catalytic carboxylation of 1,3-enynes has remained unknown. Herein, we report the first visible-light-promoted 1,4-carbon/carboxylation of 1,3-enynes with malonates and CO2, leading to a series of valuable tetrasubstituted 2,3-allenoic acids with high efficiency. This new methodology features good functional group compatibility, ease of scale-up, and facile transformations. Mechanistic investigations indicate that successive single-electron transfer (SSET) reduction is involved to generate the key allenoic anion intermediate.