Alcohol oxidation by dioxygen and aldehydes catalysed by square-planar cobalt(III) complexes of disubstituted oxamides and related ligands

The square-planar cobalt(III) complexes of o-phenylenebis(N ' -methyloxamid ate) (Me(2)opba) and related oxamate (Meopba) and bis(oxamate) (opba) ligan ds catalyse the selective oxidation, by dioxygen and pivalaldehyde, of a wi de range of secondary alcohols to the corresponding ketones, in good yields and under mild conditions in acetonitrile at room temperature. Thus, the o xidation of the series of alpha -alkylbenzyl alcohols PhCH(OH)R (R = Me, Et , iPr, tBu) results in the exclusive formation of ketones as a product of C -H bond cleavage, and no C-C bond cleavage products are observed in any cas e. The modulation of catalytic activity by ligand substituents among this s eries of cobalt catalysts highlights the role of oxocobalt(IV) species as t he putative inter mediates in these aerobic alcohol oxidation reactions. Re lative reactivities for the oxidation of 1-phenylethanol increase in the or der Me(2)opba < Meopba < opba(,) which correlates with the oxidising power of the postulated Co-IV=O intermediate. In contrast, competitive reaction s tudies on the oxidation of the series of para-substituted 1-phenylethanol d erivatives XPhCH(OH)CH3 (X = H, OMe, Br, CF3, NO2) show little variation in relative rate (k(X)/k(H)) with the para substituent and no correlation wit h Hammett sigma parameters. Overall, these results are more in line with a mechanism involving concerted hydride transfer in the oxidation of alcohols by high-valent oxocobalt species (rather than a hydrogen atom transfer or an electron transfer mechanism).