An electron-rich iron(III) porphyrin complex (meso- tetramesitylporphinato)iron(III) chloride [Fe(TMP)Cl], was found to catalyze the epoxidation of olefins 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 were the predominant products with trace amounts of allylic oxidation products, 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 product, demonstrating that neither hydroperoxy radical (HOO·) nor oxoiron(IV) porphyrin [(TMP)Fe(IV) = O] was responsible for the olefin epoxidations. We also found that the reactivities of other iron(III) porphyrin complexes such as (meso-tetrakis(2,6- dichlorophenyl)porphinato)iron(III) chloride [Fe(TDCPP)Cl], (meso- tetrakis(2,6-difluorophenyl)porphinato)iron(III) chloride [Fe(TDFPP)Cl], and (meso-tetrakis(pentafluorophenyl)porphinato)iron(III) chloride [Fe(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 absence 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 formation of trace amounts of allylic oxidation products. We proposed, on the basis of the results of mechanistic studies, that the role of the imidazole is to decelerate the O-O bond cleavage of an iron(III) hydroperoxide porphyrin (or H2O2-iron(III) porphyrin adduct) and that the intermediate transfers its oxygen to olefins prior to the O-O bond cleavage. (C) 2000 Elsevier Science S.A.
Bibliographical noteFunding Information:
Financial support for this research from the Korea Science and Engineering Foundation (96-0501-01-01-3, 1999-2-122-002-4), Center for Cell Signaling Research at Ewha Womans University, and Non-Directed Research Fund, Korea Research Foundation (1996) is gratefully acknowledged. S.O. is the recipient of a Research Fellowship (Brain Korea 21 Project).
- Biomimetic oxidation
- Hydrogen peroxide
- Iron porphyrins
- O-O activation