Development of new and efficient copper(ii) complexes of hexyl bis(pyrazolyl)acetate ligands as catalysts for allylic oxidation

Abstract

In this study, two new hexyl bis(pyrazol-1-yl)acetate ligands and related copper(II) complexes were prepared and fully characterized in the solid state and in solution. Their electronic and molecular structures were investigated by X-ray photoelectron spectroscopy and near edge X-ray absorption; their ligand molecular structural stability upon coordination to copper was also investigated. The Cu(II) complexes were studied as new catalysts in copper-catalyzed C–H oxidation for allylic functionalization (the Kharasch–Sosnovsky reaction) avoiding the use of any external reducing agents. Using 5 mol% of these catalysts and tert-butylperoxybenzoate as the oxidant, allylic benzoates were obtained in up to 90% yield: the general reaction time was decreased to 6 h and a 5 to 1 ratio of the alkene and tert-butylperoxybenzoate was used to overcome the two most important limitations on their use in chemistry.