Oxidation of Benzyl Alcohols with Extraordinarily High Kinetic Isotope Effects

Reactions of benzyl alcohol and its derivatives by [Ru-IV(tpy)(dcbpy)(O)](2+) (tpy = 2,2':6',2 ''-terpyridine; dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) leading to the corresponding benzaldehydes in acetonitrile and water have been studied. Kinetic studies show that the reaction is first-order in both alcohol and oxidant, with k = 1.65 (+/- 0.1) M(-1)s(-1) at 20 degrees C, Delta H-double dagger = 4.3 (+/- 0.1) kcal/mol, Delta S-double dagger = -22 (+/- 1) eu, and E-a = 4.9 (+/- 0.1) kcal/mol. High alpha C-H kinetic isotope effects are observed, but O-H solvent isotope effects are negligible. Spectral evidences with the isotope effects suggest that oxidation of benzyl alcohols occurs by a two-electron, hydride transfer. The catalytic cycles of aerobic benzyl alcohol oxidation are employed.