Redox-neutral photoredox catalytic Csp-H carboxylation with CO2 using Visible Light

Redox-neutral photoredox catalytic C-H carboxylation with carbon dioxide (CO2) using visible light is an ideal CO2 transformation to chemicals with low consumption of resources and energy by the catalytic, redox-neutral, and direct C-H functionalization process and the use of sustainable visible light source, sunlight, consequently achieving resource conservation and low CO2 emission. Here, we report on a redox-neutral catalytic Csp-H carboxylation of terminal alkynes with CO2 using a nonmetallic cyanoarene-based photocatalyst and visible light under mild and sustainable conditions without heating, high pressure conditions, or precious transition metal catalyst. Addition of secondary amine is crucial for promoting the reaction. A wide range of substrates, aromatic and aliphatic terminal alkynes were applicable, and various synthetically useful propiolic acid derivatives were obtained, incorporating the carbon atom of CO2 into the carbon skeletons. Mechanistic studies were also conducted, and the reaction via generation of anion radical species of alkynes through photoredox process, followed by addition to CO2, proton-transfer with secondary amine, and finally electron transfer, was suggested.