Anionic ligand effect on the nature of epoxidizing intermediates in iron porphyrin complex-catalyzed epoxidation reactions

Abstract

We have studied an anionic ligand effect in iron porphyrin complex-catalyzed competitive epoxidations of cis- and trans-stilbenes by various terminal oxidants and found that the ratios of cis- to trans-stilbene oxide products formed in competitive epoxidations were markedly dependent on the ligating nature of the anionic ligands. The ratios of cis- to trans-stilbene oxides obtained in the reactions of Fe(TPP)X (TPP = meso-tetraphenylporphinato dianion and X- = anionic ligand) and iodosylbenzene (PhlO) were 14 and 0.9 when the X- of Fe(TPP)X was Cl- and CF3SO3 -, respectively. An anionic ligand effect was also observed in the reactions of an electron-deficient iron(III) porphyrin complex containing a number of different anionic ligands, Fe(TPFPP)X [TPFPP = meso-tetrakis-(pentafluorophenyl)porphinato dianion and X- = anionic ligand], and various terminal oxidants such as PhlO, m-chloroperoxybenzoic acid (m-CPBA), tetrabutylammonium oxone (TBAO), and H2O2. While high ratios of cis- to trans-stilbene oxides were obtained in the reactions of iron porphyrin catalysts containing ligating anionic ligands such as Cl- and OAc-, the ratios of cis- to trans-stilbene oxide were low in the reactions of iron porphyrin complexes containing nonligating or weakly ligating anionic ligands such as SbF6 -, CF3SO3 - and ClO4 -. When the anionic ligand was NO3 -, the product ratios were found to depend on terminal oxidants and olefin concentrations. We suggest that the dependence of the product ratios on the anionic ligands of iron(Ill) porphyrin catalysts is due to the involvement of different reactive species in olefin epoxidation reactions. That is, high-valent iron(IV) oxo porphyrin cation radicals are generated as a reactive species in the reactions of iron porphyrin catalysts containing nonligating or weakly ligating anionic ligands such as SbF6 -, CF3SO3 -, and ClO4 -, whereas oxidant-iron(III) porphyrin complexes are the reactive intermediates in the reactions of iron porphyrin catalysts containing ligating anionic ligands such as Cl- and OAc-.