Mechanism of alcohol oxidation by trans-dioxoruthenium(VI): the effect of driving force on reactivity

Abstract

The effect of driving force on the rate of oxidation of alcohols by trans-[Ru^VILO₂]²⁺{L¹=(2,2′-bipyridine)₂; L²=N,N′-dimethyl-6,7,8,9,10,11,17,18-octahydro-5H-dibenzo[en][1,4,8,12]dioxadiaza-cyclopentadecine; 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 E°(Ru^VI–Ru^IV) ranging from 0.66 to 1.01 V vs. saturated calomel electrode has been investigated. In most cases the complexes behave as two-electron oxidants being reduced to trans-[Ru^IVL(O)(H₂O)]²⁺. The rate constants (k₂) for alcohol oxidation increase with E° of the ruthenium oxidant. The slopes of the linear free-energy plots of log k₂vs. E° 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 isotope effects and negative ΔS^‡ values, suggesting association of RuO and the α-C–H bond in the transition state. For trans-[Ru^VIL²O₂]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 Hammett plots for the oxidation of substituted benzyl alcohols indicate a charge-transfer mechanism. A common mechanism involving either a hydride or hydrogen atom abstraction is proposed.