Visible-Light-Driven Reductive (Cyclo)Dimerization of Chalcones over Heterogeneous Carbon Nitride Photocatalyst

Single-electron reduction of chalcones to the respective radical anions is a useful technique to activate these molecules toward subsequent transformations. Herein, a metal free photocatalytic version of chalcones reduction in the presence of triethanolamine as a convenient electron donor and using heterogeneous carbon nitride visible-light photocatalyst is presented. The reaction proceeds via a long-lived radical species of the heterogeneous organic semiconductor. The scope of the reaction was studied, and regioselectivity of the chalcone radicals coupling was investigated. (1) Ten chalcones gave selectively polysubstituted cyclopentanoles with 31-73% isolated yield; (2) Two chalcones bearing electron-donor groups, 4-MeOC6H4 and 2-thienyl, gave selectively the beta-ketodienes in 42% and 53% isolated yield, respectively; (3) Pentafluorophenyl substituted chalcone gave exclusively the product of the radicals coupling followed by hydrogen transfer from triethanolamine hexane-1,6-dione in 65% isolated yield. Reductive cross cyclodimerization of a mixture of two different chalcones proceeded regioselectively with the formation of one product out of four possible. The mechanism was investigated by cyclic voltammetry and linear sweep voltammetry and suggests that the reaction proceeds through proton-coupled electron transfer.