Copper catalyzed oxidation of benzylic alcohols in water with H 2O 2

Abstract

A straightforward, efficient and sustainable copper catalyzed method was developed for oxidation of benzylic alcohols with 30% H 2O 2 in water. The reaction proceeded with CuSO 4 catalyst (1 mol%) at 100 °C without additional base or ligand. Primary benzylic alcohols were converted almost quantitatively to aldehydes with 70–90% selectivity, corresponding acids being the major side products. Also secondary benzylic alcohols afforded the corresponding ketones in high conversion with selectivities greater than 90%. It was demonstrated that the CuSO 4 catalyst can be recycled and reused at least for three runs, even though with some loss of catalytic activity. Selectivity of the CuSO 4 based catalyst system could be further increased by using 2- N-( p-fluorophenyl)-pyrrolecarbaldimine ( 1) as a ligand in combination with TEMPO in K 2CO 3 solution. The catalyst system was individually optimized (1 mol% CuSO 4, 2 mol% 1, 0.1 M K 2CO 3 and 5 mol% TEMPO) for a wide range of benzylic and allylic alcohols, which were quantitatively and selectively converted into the corresponding aldehydes with 3 eq. of H 2O 2 in 1 h.