Alkene epoxidation catalysed by iron(III) and manganese(III) tetraarylporphyrins coordinatively bound to polymer and silica supports

Abstract

Four tetraaryl metalloporphyrin catalysts [Fe^III, Ar = phenyl and pentafluorophenyl; Fe^III and Mn^III, Ar = 2,6-dichlorophenyl (Fe^IIITDCPP and Mn^IIITDCPP)] have been coordinatively bonded to poly(4-vinylpyridine), and imidazole modified polystyrene (PS-Im) and silica (Si-Im). Evidence is presented that suggests that the iron(III) porphyrins are predominantly bis-ligated to the polymer supports whereas with Mn^IIITDCPP mono-ligation is preferred. On Si-Im all the complexes are mono-ligated. A fifth metalloporphyrin, the ionic manganese(III) 5,10,15,20-tetra(N-methyl-4-pyridyl)porphyrin (Mn^IIIT4MPyP); which binds strongly to unmodified silica, does not ligate to Si-lm.

Leaching experiments show that Fe^IIITDCPP is most strongly anchored to the supports. The resulting materials have been used as catalysts for the oxidation of cyclohexene and cyclooctene by iodosylbenzene. The oxidant accountabilities are good and product distributions from reactions under nitrogen are very comparable to those from analogous homogeneous oxidations. However, the rates of the former are markedly lower. Oxidations of cyclohexene, but not of cyclooctene, in air are biphasic. The first phase, where epoxidation predominates, is attributable to an Fe^IIITDCPP-catalysed oxidation by PhIO and the second, which leads to allylic oxidation, to radical autoxidation.

The best catalysts for large turnover epoxidations are Fe^IIITDCPP and Mn^IIITDCPP on Si–lm. The causes of the lower yields with catalysts on the more flexible polymer supports are discussed.

A limited study with H₂O₂ as the oxidant reveals that Mn^IIITDCPP on Si–Im, in contrast to the ionic Mn^IIIT4MPyP on silica, catalyses the epoxidation of cyclooctene.