Model systems for cytochrome P450 dependent mono-oxygenases. Part 7. Alkene epoxidation by iodosylbenzene catalysed by ionic iron(III) tetraarylporphyrins supported on ion-exchange resins

Abstract

Ionic iron(III) tetraarylporphyrins are readily adsorbed onto cross-linked polystyrene ion-exchange resins. These supported metalloporphyrins can act as heterogeneous catalysts for the oxidation of organic compounds by iodosylbenzene in methanolic solution. Their catalytic activity for the epoxidation and allylic oxidation of cyclohexene and he epoxidation of (Z)-cyclooctene has been studied. With cyclohexene, epoxidation is favoured by increased cross-linking of the resin support, and allylic oxidation is shown to arise largely from an autoxidation which can be minimised by carrying out the reactions under nitrogen. (Z)-Cyclooctene gives only epoxidation which is also favoured by increased cross-linking and is unaffected by changes in the surface area of the support. The influence of the support cross-linking on catalyst performance is discussed. Supported anionic iron(III) porphyrins are poor catalysts unless they are sterically hindered with ortho-groups on the aromatic substituents. The fate of the oxidant in the heterogeneous oxidations is examined and comparison with the analogous homogeneous reactions suggests that some of the oxidant is consumed in oxidising the polymer support. A brief study of the oxidation of cyclohexene by iodosylbenzene catalysed by a cation exchange resin in its acidic form, in the absence of metalloporphyrins, is reported.