Model systems for cytochrome P450 dependent monooxygenases. Part 8. A study of the epoxidation of (Z)-cyclooctene by iodosylbenzene catalysed by cationic iron(III) tetra(N-methylpyridyl)porphyrins adsorbed on Dowex MSC1

Abstract

Tetra(N-methyl-4-pyridyl)porphyrinatoiron(III) pentachloride and its N-methyl-2-pyridyl isomer have been adsorbed onto the highly crosslinked cation ion-exchange resin Dowex MSC1 and have been used to catalyse the reaction of Z-cyclooctene with iodosylbenzene in methanol. Two oxidation products have been detected, epoxycyclooctane and formaldehyde. The dependence of the initial rate of epoxidation and the yields of the oxidation products on the concentrations of oxidant, alkene and catalyst and on the solvent composition and catalyst loading have been investigated. The results are interpreted in terms of the following competitive reactions of the active oxidant; alkene epoxidation, solvent oxidation and oxidative degradation of catalyst and support. Adsorption of the metalloporphyrins on the polymeric resin surprisingly leads to a decrease in the catalyst efficiency when compared with the same metalloporphyrin in free solution. The cause of this effect is discussed and it is proposed that it arises through the aggregation of the metalloporphyrins on the resin favouring the intermolecular self-oxidation of the catalyst.