Activation of the C-H bond by electrophilic attack: theoretical study of the reaction mechanism of the aerobic oxidation of alcohols to aldehydes by the Cu(bipy) 2+ /2,2,6,6-tetramethylpiperidinyl-1-oxy cocatalyst system

We have investigated the reaction mechanism of the selective aerobic oxidation of primary alcohols into aldehydes using a bipy-copper complex and the 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) radical as cocatalysts (Gamez et al. Chem. Commun. 2003, 2412-2415) and compared it to the well-known oxidation by the TEMPO+ ion. Our theoretical investigation shows that (a) the oxidation of alcohols to aldehydes by uncoordinated TEMPO+ takes place by electrophilic attack on the C-HR bond of the alcohol; (b) the Cu(bipy)2+ complex has two functions, namely, (1) it acts as a template that brings TEMPO and the (deprotonated) alcohol in proximity by coordinating these moieties in adjacent coordination sites, and (2) it oxidizes the TEMPO radical to (coordinated) TEMPO+ ion. The H abstraction from alcohol by TEMPO+ then proceeds as an intramolecular reaction, very much analogous to one of the reaction pathways with free TEMPO+ and with a remarkably low barrier. We stress that compared to other A-H bonds (A=C, N, O, F), the relatively high-lying C-H bonds are particularly susceptible to electrophilic attack, and notably the C-HR bond next to the O in an alcohol is so because it is pushed up by an O lone pair. Electrophilic attack, being common to the particular catalytic system studied in this paper and the well-known biotic and abiotic oxidation catalysis by heme and non-heme complexes of the ferryl (FeIVO)2+ ion, appears to be a unifying electronic structure principle of C-HR hydroxylation and oxidation reactions.