Well-controlled radical-based epoxidation catalyzed by copper complex immobilized on bipyridine-periodic mesoporous organosilica

The development of synthetic methods and reaction systems for safe radical reactions is of extremely industrial importance. Here we proposed new concept for a safe radical reaction system based on combined use of a mesoporous catalyst and an insoluble solid scavenger. We selected Mukaiyama epoxidation of olefin as a model radical reaction and investigated the catalysis of Cu-bipyridine complexes immobilized on trimethylsilylated bipyridine-periodic mesoporous organosilica as a solid support. The immobilized Cu complex exhibited high catalytic activity and reusability for Mukaiyama epoxidation at low substrate concentration (1 mmol) but free radical auto-oxidation also occurred at high substrate concentration (7 mmol). Although both epoxidation reactions outside and inside the mesochannels were almost completely quenched by addition of molecular scavenger, addition of solid scavenger allowed quenching the reaction outside the mesopores but not inside the mesopores because the solid scavenger could not access the interior of the mesochannels. Thus, the combined use of a mesoporous catalyst and a solid radical scavenger would offer new reaction system for safe radical reactions.