Enhanced activity of manganese(III) porphyrin epoxidation catalysts through supramolecular complexation

Several manganese(III) porphyrins showed significantly increased turnover-numbers for the catalytic epoxidation of styrene upon addition of bulky Lewis acids such as ZnTPP and MgTPP (TPP = tetraphenylporphyrin). Catalytic enhancement of two- to four-fold, depending on the additive, was observed with a manganese porphyrin bearing bis-pyridyl groups. The enhancement is primarily attributed to the formation of a coordination trimer, which provides steric protection of the catalyst from bimolecular decomposition. In the absence of coordinating pyridyl moieties on the ligand framework, less significant enhancement is observed; the residual enhancement is speculatively ascribed to the protection of the active manganese-ore intermediate through direct catalytic site coordination. Finally, in several cases, the addition of Lewis acids also led to the regeneration of deactivated catalysts and full recovery of the original catalytic activity. (C) 2000 Elsevier Science B.V. All rights reserved.