Organophotocatalytic Reduction of Benzenes to Cyclohexenes

The reduction of abundant benzene rings to scarce C(sp3)-rich motifs is invaluable for drug design as C(sp3) content is known to correlate with clinical success. Cyclohexenes are attractive targets as they can be rapidly elaborated into large product libraries and are stable against rearomatization. However, partial reduction reactions of benzenes to cyclohexenes are rare and have a very narrow scope. Herein, we report a broadly applicable method that converts electron-poor benzenes to cyclohexenes and tolerates Lewis-basic functional groups like triazoles and thioethers, as well as reducible groups like cyanides, alkynes, and sulfones. The reaction utilizes an organic donor that induces mild arene reduction by pre-association to a photo-excitable electron-donor-acceptor (EDA) complex and mild isomerization of redox-inert 1,4-cyclohexadienes to reducible 1,3-cyclohexadienes without a strong base in its oxidized thioquinone methide form.