Kinetic role of t-amines in the ligand inhibited osmium tetraoxide catalyzed trimethylamine-N-oxide dihydroxylation of alkenes

Kinetic studies on the t-amine inhibited catalytic OsO₄ dihydroxylation of cyclooctene, octene, styrene and 4-methyl-, 4-methoxy-, 4-chloro-, 4-(chloromethyl), 4-(trifluoromethyl) and 3-chloro-derivatives of styrene with Me₃NO in t-BuOH have been carried out at 50 °C. The reactions follow identical kinetics: first order in total osmium species, first order in Me₃NO and zero order in alkene. All t-amines have been found to retard the catalysis and the reaction order in t-amine changes from inverse first order to zero. The involvement of dioxomonoglycolataosmium(VI) esters and their monoamine adducts in the rate-determining oxidation step was established by the linear plots of 1/k₂versus. 1/[L] where k₂ is the decrease in second order rate constant in the presence of [L] concentration of t-amine ligand. Beyond a definite concentration of t-amine, the rate reaches a minimum and remains constant. Activation parameters were evaluated for catalytic OsO₄ dihydroxylation of 4-(chloromethyl)styrene at different t-amine concentrations. The values of H and S are consistent with the proposed mechanism.