Shape selective oxidation using titanium silicates: epoxidation of dihydromyrcene and the model compounds 2-methylpent-2-ene and 3-methylpent-1-ene
作者:Lee J. Schofield、Owain J. Kerton、Paul McMorn、Donald Bethell、Simon Ellwood、Graham J. Hutchings
DOI:10.1039/b207425g
日期:2002.12.6
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.
二氢美克烯的选择性环氧化在含钛的硅酸盐TS-1和TiAlβ的存在下进行了研究,使用的氧化剂包括水合过氧化氢、叔丁基过氧化氢和尿素-过氧化氢复合物。在TS-1和水合过氧化氢的体系中观察到了环氧化物,在TiAlβ的体系中则需与尿素-过氧化氢复合物和叔丁基过氧化氢共同使用。环氧化反应仅在更富电子的双键上进行,在两种催化剂的存在下均为如此。二氢美克烯的环氧化反应还在三相条件下(两个不混溶的液相和一个催化剂相)进行了研究,而非在两相条件下(一个液相和一个催化剂相)。结果发现,在两相条件下生成的环氧化物的醇解反应通过更稳定的阳离子中间体进行。相比之下,在三相条件下的醇解反应则同时形成了主要( favored)和次要(unfavored)的醚醇,其比例可达2:1。为了研究每个双键的环氧化程度,使用了模拟二氢美克烯中每个双键周围电子环境的模型化合物(2-甲基戊-2-烯和3-甲基戊-1-烯),在竞争条件下分别进行了单独研究。当单独研究这些模型底物时,两个双键的环氧化速率相当。然而,在竞争性环氧化中,电子不足的双键优先被氧化,这与二氢美克烯的观察结果有所不同。