Aerobic oxidation of benzylic alcohols with bis(3,5-di- tert-butylsalicylaldimine)copper(II) complexes

Abstract

A series of bis(3,5-di- tert-butylsalicylaldimine)copper(II) complexes were synthesized and employed in the catalytic oxidation of alcohols using molecular oxygen as a terminal oxidant. These complexes, when combined with TEMPO form a highly efficient catalytic system for the aerobic oxidation of primary benzylic and allylic alcohols under mild reaction conditions. For example at 60 °C and 1 bar of O 2 pressure benzyl alcohol is quantitatively converted to benzyl aldehyde in 2 h with low catalyst loading. Attractive features of this catalytic system include the efficient use of molecular oxygen, high selectivity, low loading of both TEMPO (2 mol%) and copper catalyst (0.66 mol%) and mild reaction conditions (atmospheric oxygen and 60 °C temperature). In addition, the crystal structure of the complex was determined by X-ray diffraction analysis. The structure presents the Cu ion in a monomeric distorted square-planar geometry involving the unusual and sterically unfavorable cis-N 2O 2 coordination sphere.