Oxidation of Alcohols to Carboxylates with N2O Catalyzed by Ruthenium(II)-CNC Complexes

Air-stable ruthenium(II) complexes based on a picoline-derived CNC pincer ligand, [RuH(CNC)(CO)L]X (L = PPh3, X = Br; L = CO, X = BF4), were found to catalyze under basic conditions the oxidation with N2O of a series of alcohols to carboxylates. Both [RuH(CNC)(CO)L]X complexes react readily with strong bases (tBuOK or KHMDS), giving rise to a Ru(II) complex containing a deprotonated CNC* ligand (when L = PPh3) or a Ru(0)-CNC derivative (for L = CO). Furthermore, the mechanism of the catalytic reaction has been elucidated through density functional theory (DFT) calculations. The catalytic cycle has been shown to proceed through an outer-sphere mechanism comprising four key transformations, which involve Ru(II) intermediates based on the deprotonated CNC* ligand: (i) alkoxide dehydrogenation to yield a Ru(II) hydride complex and an aldehyde molecule, (ii) N2O insertion into the ruthenium–hydride bond to yield a hydroxy ruthenium species and N2, (iii) nucleophilic attack of the hydroxo ligand in the Ru–OH complex to the intermediate aldehyde, and (iv) dehydrogenation of the formed alcoholate to regenerate the catalytically active Ru(II) hydride and produce the carboxylate product.