Synthesis of Metal–(Pentadentate‐Salen) Complexes: Asymmetric Epoxidation with Aqueous Hydrogen Peroxide and Asymmetric Cyclopropanation (salenH₂: <i>N</i>,<i>N′</i>‐bis(salicylidene)ethylene‐1,2‐diamine)

Abstract

It is known that the rates and stereochemical outcomes of epoxidations and cyclopropanations using a metallosalen (salenH(2): N,N'-bis(salicylidene)ethylene-1,2-diamine) complex as catalyst are affected by a trans effect of the apical ligand of the complex. By taking into consideration this trans effect, we have synthesized optically active pentadentate salen ligands bearing an imidazole or pyridine derivative as the fifth coordinating group, and have prepared the corresponding manganese(III) and cobalt(II) complexes, in which the fifth ligand is expected to intramolecularly coordinate to the metal center and exert a trans effect. Indeed, high enantioselectivity has been achieved in epoxidations using aqueous hydrogen peroxide as the terminal oxidant and in cyclopropanations with these complexes as catalysts. In general, metallosalen-catalyzed reactions have been carried out in the presence of an excess of a donor ligand; however, the present reactions do not need the addition of any extra donor ligand.