Kinetics and mechanism of the epoxidation of alkyl-substituted alkenes by hydrogen peroxide, catalyzed by methylrhenium trioxide

Epoxidations of alkyl-substituted alkenes, with hydrogen peroxide as the oxygen source, are catalyzed by CH3ReO3 (MTO). The kinetics of 28 such reactions were studied in 1:1 CH3CN-H2O at pH 1 and in methanol. To accommodate the different requirements of these reactions, H-1-NMR, spectrophotometric, and thermometric techniques were used to acquire kinetic data. High concentrations of hydrogen peroxide were used, so that diperoxorhenium complex CH3Re(O)(eta(2)-O-2)(2)(H2O), B, was the only predominant and reactive form of the catalyst. The reactions between B and the alkenes are about 1 order of magnitude more rapid in the semiaqueous solvent than in methanol. The various trends in reactivity are medium-independent. The rate constants for B with the aliphatic alkenes correlate closely with the number of alkyl groups on the olefinic carbons. The reactions become markedly slower when electron-attracting groups, such as halo, hydroxy, cyano, and carbonyl, are present. The rate constants for catalytic epoxidations with B and those reported for the stoichiometric reactions of dimethyldioxirane show very similar trends in reactivity. These findings suggest a concerted mechanism in which the electron-rich double bond of the alkene attacks a peroxidic oxygen of B. These data, combined with those reported for the epoxidation of styrene (a term intended to include related molecules with ring and/or aliphatic substituents) by B and by the monoperoxo derivative of MTO, suggest that all of the rhenium-catalyzed epoxidations occur by a common mechanism. The geometry of the system at the transition state can be inferred from these data, which suggest a spiro arrangement.