Epoxidation with Ti(OSiMe<sub>3</sub>)<sub>4</sub>: a homogeneous model mimicking titania–silica mixed oxides?
作者:Carsten Beck、Tamas Mallat、Alfons Baiker
DOI:10.1039/b300809f
日期:——
Ti(OSiMe3)4 is an active homogeneous catalyst for the epoxidation of allylic alcohols under mild conditions using tert-butyl hydroperoxide as oxidant in an apolar solvent. It affords an average turnover frequency of up to 1470 hâ1 and 100% regioselectivity (geraniol) or 100%
cis-selectivity (cyclohexenol). UV-Vis measurements indicate that the ability of substrates to coordinate to the catalyst determines the reaction rate and stereoselectivity. This is the likely explanation for the poor or completely missing reactivity of substrates possessing no OH function (cyclohexene, methoxycyclohexene) and also for the substrate-dependent effect of water in the system. Ti(OSiMe3)4 is suggested to be a good soluble homogeneous model catalyst mimicking the highly active isolated tetrahedral Ti sites in (silylated) titaniaâsilica mixed oxides.
Ti(OSiMe3)4是一种活性均相催化剂,可在温和条件下,以叔丁基过氧化氢为氧化剂,在无极性溶剂中进行烯丙基醇的环氧化反应。它的平均转化率高达 1470 hâ1 ,具有 100% 的区域选择性(香叶醇)或 100% 的顺式选择性(环己烯醇)。紫外可见光测量结果表明,底物与催化剂配位的能力决定了反应速率和立体选择性。这很可能解释了不含羟基官能团的底物(环己烯、甲氧基环己烯)反应性差或完全丧失反应性的原因,同时也解释了体系中水对底物的影响。Ti(OSiMe3)4被认为是一种良好的可溶性均相模型催化剂,它模仿了(硅烷基化)二氧化钛二氧化硅混合氧化物中的高活性孤立四面体 Ti 位点。