MOLYBDENUM-CATALYZED EPOXIDATIONS OF OCT-1-ENE AND CYCLOHEXENE WITH ORGANIC HYDROPEROXIDES - STERIC EFFECTS OF THE ALKYL SUBSTITUENTS OF THE HYDROPEROXIDE ON THE REACTION-RATE

A kinetic study of the epoxidation of oct-1-ene and cyclohexene with a lkyl hydroperoxides is reported. The alkyl hydroperoxides were obtaine d in a moderate to high purity from the corresponding alcohols by acid -catalyzed exchange with hydrogen peroxide. The reaction rates in pseu do first-order experiments of these olefins with various alkyl hydrope roxides strongly depend on the structure of the alkyl group of the alk yl hydroperoxide. When one of the methyl groups in tert-butyl hydroper oxide (TBHP, 4a) is substituted by an alkyl group, R, the reaction rat e decreases in the order Et > Pr > Bu > (t) BuCH(2) > (t)Bu. Substitut ion of two methyl groups of TBHP as in 1-ethyl-1-methylpropyl hydroper oxide (5a) and 1-ethyl-1-methylbutyl hydroperoxide (5b) showed a furth er decrease in reaction rate of epoxidation. When all three methyl gro ups are substituted by, for example, three ethyl groups as in 1,1-diet hylpropyl hydroperoxide (6a) a decrease of approximately 99% in reacti on rate is observed. Introduction of a ring system in the hydroperoxid e such as in cyclohexyl hydroperoxide (3), 1-methyl-cyclohexyl hydrope roxide (2) and pinane hydroperoxide (1) also showed a dramatic decreas e in reaction rate of epoxidation. An investigation of relative rates of epoxidation in competition experiments of cyclohexene and hex-1-ene with 1-tert-butylcyclohexene with different alkyl hydroperoxides also showed them to depend on the structure of the alkyl group of the alky l hydroperoxide. These results are rationalized on the basis of a mech anism involving nucleophilic attack of the olefin on an alkylperoxomol ybdenum(VI) intermediate. Bulky substituents at the alpha-position in the alkyl hydroperoxide can seriously impede the approach of the olefi n to the O-O bond.