EPOXIDATION OF STYRENES BY HYDROGEN-PEROXIDE AS CATALYZED BY METHYLRHENIUM TRIOXIDE

Methylrhenium trioxide, CH3ReO3, catalyzes the oxidation of styrenes by hydrogen peroxide. Kinetic studies by three methods were carried out in acidic CH3CN/H2O (1:1 v/v) solutions. The catalytically-active species are the mono-peroxide, CH3Re(O)(2)(O-2), A, and the bis-peroxide, CH3Re(O)(O-2)2, B, which epoxidize a given styrene at a similar rate. The rate constants are relatively insensitive to steric hindrance, but increase with the nucleophilicity of the styrene, electron-donating groups on the olefinic carbons or on the aromatic ring enhancing the rate. The rate constants for meta- and para-substituted styrenes follow a linear Hammett relationship; correlation with sigma(+) gave rho = -0.93 + 0.05. In CD3CN, epoxides were observed by H-1 NMR spectroscopy. cis-beta-Methylstyrene and trans-beta-methylstyrene led to the cis epoxide and the trans epoxide, respectively. In acidic CH3CN/H2O, the major products were 1,2-diols. In some cases C-C bond cleavage products were also observed, the extreme case being beta-methoxystyrene where the C-C bond was completely cleaved to yield benzaldehyde, formaldehyde, and methanol.