LIGAND EFFECTS ON THE REDOX CHEMISTRY OF OXO(PHOSPHINE)RUTHENIUM(IV) COMPLEXES - OXIDATION OF ALCOHOLS

Oxoruthenium complexes which contain para-substituted triphenylphosphine ligands cleanly oxidize para-substituted benzyl alcohols to the corresponding benzaldehydes. Linear free energy correlations between the logarithms of the second-order rate constants of benzyl alcohol oxidation and either the potential values of the ruthenium(IV)/ruthenium(III) redox couples of the oxoruthenium complexes or the Hammett a values of the substituents of the para-substituted triphenylphosphine ligands were observed. However, there were no good correlations for plots of log (k(X)/k(H)) versus the Hammett substituent constants sigma, sigma+, or sigma- for the oxidation of the para-substituted benzyl alcohols by [Ru(bpy)2(O)PPh3](ClO4)2. Notably, the plot of log (k(X)/k(H))-sigma. versus sigma-, where sigma. is the free radical stabilization constant, gave an excellent correlation with p = -0.57 and R2 = 0.99 for the oxidation of the parasubstituted benzyl alcohols by [Ru(bpy)2(O)PPh3](ClO4)2 in methylene chloride. From these and other results, we propose a reaction pathway for the oxidation of benzyl alcohol by oxoruthenium(IV) complexes which involves a partial hydrogen atom abstraction from the benzylic carbon in the rate-determining step.