Acid-Catalyzed Cyclization of Terpenes Under Homogeneous and Heterogeneous Conditions as Probed Through Stereoisotopic Studies: A Concerted Process with Competing Preorganized Chair and Boat Transition States
作者:Christos Raptis、Ioannis N. Lykakis、Constantinos Tsangarakis、Manolis Stratakis
DOI:10.1002/chem.200901563
日期:2009.11.9
concerted dicyclization occurs with a preorganized boat–chair transition state competing with the chair–chair transition state. Under zeolite confinement conditions, the chair–chairlike dicyclization transition state is highly favorable. The preference of chairlike transition states within the cavities of zeolite Y is attributed to a transition state shape selectivity effect.
基于立体同位素研究和β-次级同位素效应,我们提出乙酸香叶酯乙酸的酸催化环化是通过协同机制进行的。在非均质条件下(Y型沸石限制),一个预先组织的椅子状过渡态占主导,而在均匀条件下,船形和椅子状过渡态几乎是等能量的。对于乙酸法呢酯,我们建议在均质条件下协同二环化发生,其中预先组织的船椅过渡态与椅椅过渡态竞争。在沸石限制条件下,椅-椅状二环化过渡态非常有利。沸石Y的腔内的椅子状过渡态的偏爱归因于过渡态形状选择性效应。