Abstract
The kinetics of oxygen-atom transfer from the peroxo complexes of methyltrioxorhenium (MTO) to alkenes in ionic liquids have been investigated. Noncatalytic conversions of alkenes to epoxide were monitored by UV/Vis at 360 nm, where the monoperoxorhenium (mpRe) and diperoxorhenium (dpRe) complexes absorb. Water- and peroxide-free dpRe was prepared in situ by the reaction of MTO and urea hydrogen peroxide (UHP) in dry THF. The observed biexponential time profiles in conjunction with kinetic modeling allow the assignment of the fast step to the reaction of olefin with dpRe (k4) and the slow step to the analogous reaction with mpRe (k3). In most of the tudied ionic liquids, k4 approximately 5 x k3. 2H NMR experiments conducted with [D3]dpRe under non-steady-state conditions confirm the speciation of the catalytic system in ionic liquids and assert the validity of the UV/Vis kinetics. Deuteriated alkenes were used to study the catalytic epoxidation and dihydroxylation of alkenes by 2H NMR spectroscopy. The values of k4 for alpha-methylstyrene in several ionic liquids exceed what is observed in acetonitrile by an order of magnitude. While the rate of olefin epoxidation is unaffected by the nature of the ionic liquid cation, a discernible kinetic effect is observed with coordinating anions such as nitrate.
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