Epoxidation of an alkene promoted by various nickel(II) multiaza macrocyclic complexes

The epoxidation of trans-β-methylstyrene promoted by various Ni(II) complexes of macrocyclic ligands (cyclam and 1–5) using PhIO as a terminal oxidant has been investigated. In terms of the rate of epoxide formation, the complexes of monocyclic ligands (cyclam, 1 and 2) are better catalysts than those of polycyclic ligands (3–5) and the cyclam complex without pendant arms is better catalyst than those (1 and 2) with pendant arms. However, a series of the complexes show remarkably similar reactivity in the transfer of oxygen from active high-valent intermediate to the alkene and they provide nearly the same final yield in certain reaction conditions. Therefore, the yield of epoxide produced in a given period depends mainly on the rate of reaction of the complex with PhIO, which is greatly affected by the ligand structure. In order to become a better catalyst, the complex should have low Ni(II)/Ni(III) oxidation potential and the macrocyclic ligand should exert less steric hindrance around the Ni(II) center to allow easy axial approach of the oxidant.