Imidazole-linked porphyrin-based conjugated microporous polymers for metal-free photocatalytic oxidative dehydrogenation of N-heterocycles

Catalytic oxidative dehydrogenation (ODH) of N-heterocycles has been considered to be an important transformation to access the corresponding nitrogen-containing heteroarenes. Herein, we developed porphyrin-based and imidazole-linked conjugated microporous polymers by metal-free catalytic condensation of a carboxyl porphyrin and phenylic diamine in polyphosphoric acid medium. The two synthesized polymers, TCPP-TAB and TCPP-HATP, exhibit a broad visible light response, high surface area and suitable redox potentials that are tunable. As expected, TCPP-TAB and TCPP-HATP as metal-free photocatalysts exhibit excellent photocatalytic performance and good substitution tolerance in ODH reactions of various N-heterocycles including tetrahydroisoquinolines, tetrahydroquinolines and indolines under base- and additive-free conditions with ambient air at room temperature. More importantly, heterogeneous TCPP-TAB and TCPP-HATP can be reused at least five times and ten times without obvious loss of catalytic activity, respectively, which is possibly attributed to their ultrastable cyclic imidazole joints. The current work provides a metal-free, efficient, green, and reproducible approach to perform ODH reactions of N-heterocycles under mild conditions.