Visible-Light-Mediated [4+2] Annulation of N-Cyclobutylanilines with Alkynes Catalyzed by Self-Doped Ti3+ @TiO2.

We herein report a visible-light-mediated heterogeneous [4+2] annulation of N-cyclobutylanilines with alkynes catalyzed by self-doped Ti3+ @TiO2 . The self-doped Ti3+ @TiO2 is stable under photooxidation conditions, easy to recycle, and can be used multiple times without appreciable loss of activity. Extensive mechanistic studies suggest that the annulation reaction is mediated by singlet oxygen, which is generated through the photosensitization of oxygen in the air by the self-doped Ti3+ @TiO2 . In contrast, the homogeneous variant catalyzed by a far more expensive iridium complex proceeds under an inert atmosphere, which indicates a different mechanism. The substrate scopes of the two processes are comparable.