Blue Light-Promoted Synthesis of Thiochromenopyrroledione Derivatives via Titanium Dioxide-Catalyzed Dual Carbon–Carbon Bond Formation with Thioanisole and Maleimide Derivatives

Abstract

Under anaerobic conditions, thioanisole derivatives bearing a thiomethyl group participated in dual carbon-carbon bond-forming reactions with maleimide derivatives in the presence of titanium dioxide. Blue light irradiation facilitated these reactions, producing thiochromenopyrroledione derivatives and nearly equimolar quantities of succinimide derivatives. Specifically, a thiochromenopyrroledione derivative formed from thioanisole and N-benzylmaleimide was achieved with a 43% yield after 12 hours of blue light exposure (>420 nm), approaching its theoretical maximum yield of 50%. Without titanium dioxide, the reaction remained dormant. Our investigations with five varieties of 4-substituted thioanisoles and four N-substituted maleimides as precursors resulted in the synthesis of 20 distinct thiochromenopyrroledione derivatives with moderate to high yields. From observed substituent effects, we postulate that this reaction proceeds through a charge transfer from thioanisole to the conduction band of titanium dioxide during blue light irradiation, initiating the one-electron oxidation of thioanisole and subsequent generation of α-thioalkyl radicals via deprotonation.