High-Valent Salen Complexes as a Key to Investigate Mechanistic Aspects of Asymmetric Oxidation Catalysts and Metalloenzymes

摘要

In order to garner in-depth understanding of oxidation catalysis by salen complexes, we investigated the conformation, electronic structure and reactivity of high-valent salen complexes. We herein show that the conformation of a chiral manganese salen complex (Jacobsen's complex) is critically dependent on the oxidation state of the manganese ion and the external axial ligands that coordinate to the manganese ion, in relevance to asymmetric reactions. We also show that the reaction of a manganese(III) salen complex with m-chloroperoxybenzoic acid, an oxidant utilized for enantioselective epoxidation, generates a mixture of manganese(IV)-oxo and manganese(IV)-hydroxo species at –80°C. In contrast, the reaction of an iron(III) salen complex with m-chloroperoxybenzoic acid at –80°C generates a ligand radical complex with an iron(III) ion. The difference of electronic structures might be related to the fact that a manganese salen complex is a superior epoxidation catalyst than an iron salen complex. We also describe a model study for protocatechuate 3,4-dioxygenase using an iron(III) salen complex with sterically-hindered mesityl groups.