TiO₂–Photocatalyzed Cyclization of Azobenzenes with Methanol To Access 1,2,4,5-Tetraaryl 1,2,4,5-Tetrazinanes

A photoredox-catalyzed cyclization of azobenzenes with methanol as the C1 source has been developed. Using anatase TiO2 as the photocatalyst, the reaction proceeded smoothly at room temperature in the absence of any additives, affording 1,2,4,5-tetraaryl 1,2,4,5-tetrazinanes in high yields. The photocatalyst TiO2 can be readily recycled and reused. Mechanistic studies indicated that the reaction proceeds via a photoredox-catalyzed radical pathway and that H2O acts as a hydrogen atom transfer (HAT) reagent.