Copper catalyzed oxidation of benzylic alcohols in water with H2O2

A straightforward, efficient and sustainable copper catalyzed method was developed for oxidation of benzylic alcohols with 30% H2O2 in water. The reaction proceeded with CuSO4 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 CuSO4 catalyst can be recycled and reused at least for three runs, even though with some loss of catalytic activity. Selectivity of the CuSO4 based catalyst system could be further increased by using 2-N-(p-fluorophenyl)-pyrrolecarbaldimine (1) as a ligand in combination with TEMPO in K2CO3 solution. The catalyst system was individually optimized (1 mol% CuSO4, 2 mol% 1, 0.1 M K2CO3 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 H2O2 in 1 h.

Graphical abstract.Figure optionsDownload full-size imageDownload high-quality image (124 K)Download as PowerPoint slideHighlights► Efficient Cu-catalyzed methods for alcohol oxidation with H2O2 were introduced. ► CuSO4 alone oxidizes selectively secondary alcohols in sole water. ► With a ligand modified catalyst, the oxidation is selective toward primary alcohols.