MECHANISMS FOR THE OXIDATION OF SECONDARY ALCOHOLS BY DIOXORUTHENIUM(VI) COMPLEXES

Possible mechanisms for the oxidation of alcohols by dioxoruthenium(VI ) complexes are critically evaluated. Rate constants for the reduction of trans-[(TMC)Ru-VI(O)(2)](++) (TMC = ,11-tetramethyl-1,4,8,11-tetra azacyclotetradecane) by substituted benzhydrols are correlated more sa tisfactorily with Hammett sigma substituent constants (rho = -1.44 +/- 0.08, r(2) = 0.98) than with sigma(+) substituent constants (p = -0.7 2 +/- 0.11, r(2) = 0.83). Similar observations for the oxidation of su bstituted benzyl alcohols have recently been reported, confirming that the transition state for these reactions is not carbocation-like. Pri mary deuterium isotope effects indicate that cleavage of the alpha-C-H bond is rate-limiting. The lack of an observable O-D isotope effect a nd the ease of oxidation of ethers indicates that the presence of a hy droxyl is not essential. The previously reported observation that cycl obutanol is quantitatively converted into cyclobutanone by dioxoruthen ium(VI) complexes eliminates free-radical intermediates from considera tion as part of the mechanism, and negative entropies of activation (- Delta S double dagger = 96-137 J mol(-1) K-1) suggest a structured tra nsition state. Only two of eight possible reaction mechanisms consider ed were found to be consistent with the available data. A critical ana lysis of the available data indicates that a 2 + 2 (C-H + Ru=O) additi on and a reaction initiated by ligand formation through the interactio n of the reductant's HOMO with the oxidant's LUMO are the most likely reaction mechanisms.