Mechanistic Insight into the Alcohol Oxidation Mediated by an Efficient Green [CuBr2(2,2 '-bipy)]-TEMPO Catalyst by Density Functional Method

Density functional theory (DFT) calculations have been performed to investigate the alcohol oxidation to acetaldehyde catalyzed by [CuBr2(2,2'-bipy)]-TEMPO (TEMPO stands for 2,2,6,6-tetramethylpiperidinyloxy; bipy stands for bipyridine). The total charge for the studied catalytic system is +1. The catalytic cycle consists of two parts, namely, alcohol oxidation and TEMPO regeneration. In alcohol oxidation, the reaction follows the Sheldon's mechanism for the proposed two mechanisms, i.e., Semmelhack's mechanism and Sheldon's mechanism. The water participation plays minor role in the H atom abstraction step. In TEMPO regeneration, the proposed three paths are competitive in energy. By comparing with experimental observation, it is found that the path, in which alcohol provides the proton to TEMPO- to produce TEMPOH followed by the oxidation of TEMPOH directly to TEMPO by O-2, is favored. In TEMPO regeneration, CH3CN acts as the ligand to stabilize the Cu-I species during the catalytic cycle