Mechanism of Alcohol Oxidation by trans-Dioxoruthenium(VI): The Effect of Driving Force on Reactivity

Article abstract of DOI:10.1039/DT9920001551

The effect of driving force on the rate of oxidation of alcohols by trans-2+(L1 = (2,2'-bipyridine)2; L2 = N,N'-dimethyl-6,7,8,9,10,11,17,18-octahydro-5H-dibenzo<1,4,8,12>dioxadiazacyclopentadecine; L3 = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)propylenediamine; L4 = meso-2,3,7,11,12-pentamethyl-3,7,11,17-tetraazabicyclo<11.3.1>heptadeca-1(17),13,15-triene; L5 = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) with E⁰(Ru(VI)-Ru(IV)) ranging from 0.66 zo 1.01 V vs. saturated calomel electrode has been investigated.In most cases the complexes behave as two-electron oxidants being reduced to trans-2+.The rate constants (k2) for alcohol oxidation increases with E⁰ of the ruthenium oxidant.The slopes of the linear free-energy plots of log k2 vs.E⁰ for benzyl alcohol and propan-2-ol are -14.7 and -16.9 V^-1 respectively.The oxidation is accompanied by large kinetic α-C-H bond in the transition state.For trans-2+ the existence of a linear-free energy relationship between log k2 and the ionization energies of the alcohols and the large negative ρ values in Hammet plots for the oxiation of substituted benzyl alcohols indicate a charge-transfer mechanism.A common mechanism involving either a hydride or hydrogen atom abstraction is proposed.

Full text of DOI:10.1039/DT9920001551

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