First success of catalytic epoxidation of olefins by an electron-rich iron(III) porphyrin complex and H2O2: imidazole effect on the activation of H2O2 by iron porphyrin complexes in aprotic solvent

An electron-rich iron(III) porphyrin complex (meso-tetramesitylporphinato)i ron(III) chloride [Fe(TMP)CI], was found to catalyze the epoxidation of ole fins by aqueous 30% H2O2 when the reaction was carried out in the presence of 5-chloro-1-methylimidazole (5-Cl-1-MeIm) in aprotic solvent. Epoxides we re the predominant products with trace amounts of allylic oxidation product s, indicating that Fenton-type oxidation reactions were not involved in the olefin epoxidation reactions. cis-Stilbene was stereospecifically oxidized to cis-stilbene oxide without giving isomerized trans-stilbene oxide produ ct, demonstrating that neither hydroperoxy radical (HOO .) nor oxoiron(IV) porphyrin [(TMP)Fe-IV=O] was responsible for the olefin epoxidations. We al so found that the reactivities of other iron(III) porphyrin complexes such as (meso-tetrakis(2,6-dichlorophenyl)porphinato)iron chloride [Fe(TDCPP)Cl] , (meso-tetrakis(2,6-difluorophenyl)porphinato)iron(III) chloride [Fe(TDFPP )Cl], and (meso tetrakis(pentafluorophenyl)porphinato)iron(III) chloride [F e(TPFPP)Cl] were significantly affected by the presence of the imidazole in the epoxidation of olefins by H2O2. These iron porphyrin complexes did not yield cyclohexene oxide in the epoxidation of cyclohexene by H2O2 in the a bsence of 5-Cl-1-MeIm in aprotic solvent; however, addition of 5-Cl-1-MeIm to the reaction solutions gave high yields of cyclohexene oxide with the fo rmation of trace amounts of allylic oxidation products. We proposed, on the basis of the results of mechanistic studies, that the role of the imidazol e is to decelerate the O-O bond cleavage of an iron(III) hydroperoxide porp hyrin (or H2O2-iron(III) porphyrin adduct) and that the intermediate transf ers its oxygen to olefins prior to the O-O bond cleavage. (C) 2000 Elsevier Science S.A. All rights reserved.