Substrate-dependent order of catalytic activity for a series of Fe(III) and Mn(III) porphyrins in the oxidation of organic sulfides and olefins with periodate

Abstract

The oxidation of organic sulfides and olefins with tetra-n-butylammonium periodate (TBAP) in the presence of Fe(III) and Mn(III) meso-tetraarylporphyrins bearing phenyl, 2-chlorophenyl, 4-chlorophenyl and 4-methoxyphenyl groups at the meso positions and imidazole (ImH) has been studied and compared. With the exception of the metalloporphyrins with 4-methoxyphenyl substituents, the oxidative stability of the iron porphyrins was considerably greater than that of the manganese ones. The stronger π–π interaction between the porphyrin core and the iron center in comparison with the manganese center was suggested to explain the higher oxidative stability of the iron porphyrins. The metalloporphyrins with poor electron-donating substituents, i.e., 2-chlorophenyl and 4-chlorophenyl often provided much lower catalytic activity compared to the other catalysts. The departure of IO3 −, leading to the formation of high valent oxo metal species, seems to be facilitated by the presence of strong electron-donating groups at the meso positions. In this catalytic system, the sulfoxide/sulfone molar ratio is determined by stereoelectronic properties of the substituents attached to the sulfur atom of the organic sulfide and the meso-aryl groups of the metalloporphyrins. Although the iron porphyrins showed significantly higher catalytic activities compared to the manganese counterparts in the oxidation of methyl phenyl sulfide, the catalytic activities were comparable for the oxidation of diallyl sulfide. On the other hand, the oxidation of olefins with TBAP gave different order of catalytic activities for the metalloporphyrins. However, the manganese porphyrins were generally more efficient catalysts compared to the iron one. Also, the iron or manganese porphyrins with more electron-donating meso substituents were found to be more efficient than the other ones. The competitive oxidation of cis- and trans-stilbene provided evidence for the participation of a high valent oxo metal species and a six-coordinate periodato one as the active oxidants, although in the case of MnT(2-Cl)PPOAc and FeT(2-Cl)PPOAc the results are strongly in favor of a high valent oxo metal species as the dominant active oxidant.