Shape selective oxidation using titanium silicates: epoxidation of dihydromyrcene and the model compounds 2-methylpent-2-ene and 3-methylpent-1-ene

Abstract

The regioselective epoxidation of dihydromyrcene has been studied in the presence of the titanium-containing silicates TS-1 and TiAlβ using aqueous hydrogen peroxide, tert-butyl hydroperoxide and urea–hydrogen peroxide as oxidants. Epoxides were observed with TS-1 and aqueous hydrogen peroxide, and with TiAlβ when used in conjunction with the urea–hydrogen peroxide complex and tert-butyl hydroperoxide. Epoxidation occurs exclusively at the more electron-rich double bond in the presence of both catalysts. The epoxidation of dihydromyrcene has also been studied under triphasic conditions (two immiscible liquid phases and one catalyst phase) rather than biphasic conditions (one liquid phase and one catalyst phase). The alcoholysis reaction of the resulting epoxide was found to proceed via the more stabilised cation intermediate under biphasic conditions. In contrast, alcoholysis under triphasic conditions proceeded to form both the favoured (major) and unfavoured (minor) ether alcohols in ratios up to 2 ∶ 1. Model compounds, (2-methylpent-2-ene and 3-methylpent-1-ene) which simulate the electronic environment around each of the double bonds in dihydromyrcene, have been used to study the degree of epoxidation of each double bond separately and under competitive conditions. When the model substrates are studied separately, the rate of epoxidation of the two double bonds are comparable. When the model substrates are epoxidised in a competitive manner, the electron-deficient double bond is oxidised in preference which is different to that observed for dihydromyrcene.