EFFECTS OF PYRIDINE AND ITS DERIVATIVES ON THE EQUILIBRIA AND KINETICS PERTAINING TO EPOXIDATION REACTIONS CATALYZED BY METHYLTRIOXORHENIUM

The coordination of substituted pyridines to MTO (methyltrioxorhenium) is governed by both electronic and steric effects. For example, the b inding constant of pyridine to MTO is 200 L mol(-1), whereas that of t he better donor 4-picoline is 730 L mol(-1) and that of the sterically encumbered 2,6-di-tert-butyl-4-methylpyridine is <1 L mol(-1). A Hamm ett reaction constant rho = -2.6, derived from meta- and para-substitu ted pyridines, applies to this equilibrium. Pyridine stabilizes the MT O/H2O2 system and accelerates the epoxidation of alpha-methylstyrene. The steady-state concentration of MTO is decreased during the catalyti c epoxidation reaction by coordinating a pyridine derivative, thus sta bilizing the MTO/H2O2 system against irreversible decomposition. Pyrid ine as a Lewis base accelerates the generation of the peroxorhenium ca talysts, whereas coordination of pyridine to the diperoxorhenium compl ex appears responsible for the acceleration of epoxidation. Ultimately , however, it is the Bronsted basicity of pyridine that lowers the act ivity of hydronium ion, reducing the rate of epoxide ring opening.