标题：Theoretical investigation on the chiral diamine-catalyzed epoxidation of cyclic enones: Mechanism and effects of cocatalyst
作者：Lv, P.-L.;Zhu, R.-X.;Zhang, D.-J.;Duan, C.-G.;Liu, C.-B.
作者机构：[Lv, P.-L] Key Lab of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China;[ Zhu, R.-X] Key Lab of Colloid 更多
通讯作者地址：[Zhu, RX]Shandong Univ, Key Lab Colloid & Interface Chem, Minist Educ, Sch Chem & Chem Engn, Jinan 250100, Peoples R China.
来源：The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory
摘要：The asymmetric epoxidation of 2-cyclohexen-1-one with aqueous H _2O _2 as oxidant, 1,2-diaminocyclohexane as catalyst, and a Br?nsted acid trifluoroacetic acid (TFA) as cocatalyst has been studied by performing density functional theory calculations. It is confirmed that the catalyzed epoxidation proceeds via sequential nucleophilic addition and ring-closure processes involving a ketiminium intermediate. Four possible pathways associated with two Z isomers and two E isomers of ketiminium have been explored in detail. Our calculation indicates that these four pathways have high barriers and a small energy gap between two more favorable R and S pathways. We have analyzed the effects of the TFA anion and H _2O on the activity and enantioselectivity of catalytic epoxidation. It is found that the TFA anion acts as a counterion to stabilize the transition states of the catalytic epoxidation by hydrogen-bond acceptance, leading to decreases in the barriers of the nucleophilic addition and ring-closure processes. The most significant decrease occurred in the ring-closure step of the Z-R-pathway, resulting in H-bond-induced enantioselectivity. Our calculations also show that water cooperates with TFA to further increase the reaction rate significantly.