Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, China.
Chemistry. 2010 Apr 12;16(14):4359-67. doi: 10.1002/chem.200902488.
The mechanism of the hetero-Diels-Alder reactions of Brassard's diene and 1,3-butadiene catalyzed by a titanium(IV) complex of a tridentate Schiff base was investigated by DFT and ONIOM methods. The calculations indicate that the mechanism of the reaction is closely related to the nucleophilicity-electrophilicity between diene and carbonyl substrates. A stepwise pathway is adopted for Brassard's diene, and the step corresponding to the formation of the C--C bond is predicted to be the rate-determining step with a free-energy barrier of 8.4 kcal mol(-1). For 1,3-butadiene, the reaction takes place along a one-step, two-stage pathway with a free-energy barrier of 14.9 kcal mol(-1). For Brassard's diene as substrate, the OCH(3) and OSi(CH(3))(3) substituents may play a key role in the formation of the transition state and zwitterionic intermediate by participating in charge transfer from Brassard's diene to formaldehyde. The combination of the phenyl groups at the amino alcohol moiety and the ortho-tert-butyl group of the salicylaldehyde moiety in the chiral tridentate Schiff base ligand plays an important role in the control of the stereoselectivity, which is in agreement with experimental observations.
通过密度泛函理论（DFT）和 ONIOM 方法研究了 Brassard 二烯与 1,3-丁二烯的杂 Diels-Alder 反应的机理，该反应由钛（IV）配合的三齿席夫碱催化。计算表明，反应机理与二烯和羰基底物之间的亲核性-亲电性密切相关。Brassard 二烯采用逐步途径，形成 C-C 键的步骤被预测为速率决定步骤，自由能垒为 8.4 kcal mol(-1)。对于 1,3-丁二烯，反应沿一步、两阶段途径进行，自由能垒为 14.9 kcal mol(-1)。对于 Brassard 二烯作为底物，OCH(3)和 OSi(CH(3))(3)取代基可能通过参与从 Brassard 二烯到甲醛的电荷转移，在过渡态和两性离子中间体的形成中起关键作用。手性三齿席夫碱配体中氨基醇部分的苯基基团和水杨醛部分的邻叔丁基的组合对立体选择性的控制起着重要作用，这与实验观察结果一致。
