FORMATION OF BOTH PRIMARY AND SECONDARY N-ALKYLHEMINS DURING HEMIN-CATALYZED EPOXIDATION OF TERMINAL ALKENES

The N-alkylhemin formation during iron(III) porphyrin-catalyzed epoxid ation of alkenes is studied in a homogeneous system using pentafluoroi odosobenzene (PFIB) as the oxidant. The system is a model for the enzy me cytochrome P-450. A second N-alkyl species other than the well-docu mented primary N-alkylhemin is observed during the oxidation of termin al alkenes. It is unstable and decomposes under the reaction condition s. It is also a catalyst for alkene epoxidation by PFIB. Its character istics are consistent with a secondary N-alkylhemin. The kinetics were partially resolved, and rate constants-were estimated for the formati on of both N-alkylhemins, for the decomposition of the secondary N-alk ylhemin, and for the epoxidation by the hemin and the N-alkylhemins. T he partition ratios were measured by a kinetic method, which provides better insight for the mechanisms of both N-alkylation and epoxidation than the measurements by product analysis using heterogeneous systems . We suggest that alkenes reversibly inhibit cytochrome P-450 through the formation of the unstable N-alkylporphyrin species.