Oxidation of the C=C bond

Abstract Metal complexes promote catalytic epoxidation in two different ways: (i) by activation of an oxidant and subsequent migration of an oxygen from the oxidant to olefins; (ii) by oxidation of a metal ion by an oxidant to the high-valent metal oxide and subsequent oxygen transfer to olefins with the reduction of metal ion which can be reoxidized to the metal oxide in situ.The former types of reactions are catalyzed mostly by early transition metal complexes and the latter type of reactions mainly by group VII-X transition metal complexes. For example, t-butyl hydroperoxide is activated by O=V(OR)3 or O2Mo(OR)2 and undergoes epoxidation. During these reactions, the oxidation states of the vanadium and molybdenum ions do not change (Scheme 2.1.1). In general, olefins bearing a precoordinating functional group are good substrates for this type of reaction (see Section 2.3), while epoxidation of isolated olefins is rather slow. On the other hand, stereospecific epoxidation of olefins and C—H hydroxylation of alkanes are realized in living cells with the aid of oxidizing enzymes. Based on the clarification of the catalysis of an iron porphyrin complex at the active site of the representative oxidizing enzyme cytochrome P450, investigations into the enantioselective epoxidation of simple olefins with metalloporphyrin complexes or its equivalents as catalysts were commenced. The P-450-catalysed reactions transfer oxygen atoms to substrates via a high-valent oxo iron porphyrin complex that is produced by the complex redox procedure containing the enzymatic reaction with molecular oxygen. However, the formation of the same oxo species can be chemically effected by treatment of an iron porphyrin complex with stoichiometric oxidants such as iodosylbenzene (Scheme 2.1.2). Besides iron porphyrins, many other metal complexes have been found to catalyse oxygen-transfer reactions via the corresponding high-valent metal oxides.