Mechanistic insight into alcohol oxidation mediated by an efficient green Cu(II)-bipy catalyst with and without TEMPO by density functional methods.

Density functional theory (DFT) calculations have been performed to investigate the alcohol oxidation to acetaldehyde catalyzed by the Cu-bipy (bipy = 2,2'-bipyridine) catalyst with and without TEMPO (TEMPO stands for 2,2,6,6-tetramethylpiperidinyloxy). In the presence of TEMPO, two mechanisms are proposed, which are (1) TEMPO and the alcohol coordinate to the Cu center and react in the coordination sphere, and (2) the formation of the nitronium cation. On the basis of our calculations, the nitronium cation mechanism can be ruled out. For the case without TEMPO, it is found that the change of the Cu oxidation states (Cu(II) <--> Cu(I)) plays an important role in assisting the catalytic reaction cycle, while the O(2) molecule assists the formation of the product acetaldehyde. In addition, the overall activation barrier with TEMPO is preferred over the pathway without TEMPO by 5.2 kcal mol(-1). This is consistent with the experimental observation that the Cu-bipy catalyst in the absence of TEMPO is less efficient when compared with that in the presence of TEMPO. The difference in mechanism is discussed tentatively based on the molecular orbitals.