Enhanced activity of manganese III porphyrin epoxidation catalysts through supramolecular complexation

Ž . Several manganese III porphyrins showed significantly increased turnover-numbers for the catalytic epoxidation of Ž . styrene upon addition of bulky Lewis acids such as ZnTPP and MgTPP TPPs tetraphenylporphyrin . Catalytic enhancement of twoto 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-oxo 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. q 2000 Elsevier Science B.V. All rights reserved.