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

Several manganese(III) porphyrins showed significantly increased turnover-n umbers for the catalytic epoxidation of styrene upon addition of bulky Lewi s acids such as ZnTPP and MgTPP (TPP = tetraphenylporphyrin). Catalytic enh ancement of two- to four-fold, depending on the additive, was observed with a manganese porphyrin bearing bis-pyridyl groups. The enhancement is prima rily attributed to the formation of a coordination trimer, which provides s teric protection of the catalyst from bimolecular decomposition. In the abs ence of coordinating pyridyl moieties on the ligand framework, less signifi cant enhancement is observed; the residual enhancement is speculatively asc ribed to the protection of the active manganese-ore intermediate through di rect catalytic site coordination. Finally, in several cases, the addition o f Lewis acids also led to the regeneration of deactivated catalysts and ful l recovery of the original catalytic activity. (C) 2000 Elsevier Science B. V. All rights reserved.