Su Zhishan, Kim Chan Kyung
Department of Chemistry and Chemical Engineering and Center for Design and Applications of Molecular Catalysts, Inha University, 100 Inha-ro, Incheon, 402-751, Korea.
Org Biomol Chem. 2015 Jun 14;13(22):6313-24. doi: 10.1039/c5ob00797f.
The mechanism and origin of the selectivity of asymmetric Diels-Alder reactions of 2,4-dienones (R1) by trienamine catalysis is investigated at the M06-2X/6-311++G**(SMD, toluene)//B3LYP/6-311G** level. The acidic additive (salicylic acid or trifluoroacetic acid) promotes the isomerization of the ketoiminium ion into the key reactive cis-trienamine catalytic species with a preferred conformation by constructing a suitable hydrogen-bond bridge for a H shift. One-step cycloaddition was adopted for N-phenylmaleimide (R2), while a stepwise process was used for benzylidenecyanoacetate (R3) dienophile. The -CN and -CO2Et substituents in R3 may play an important role in forming a zwitterionic intermediate by participating in charge transfer by trienamine catalysis. The combination of a hydrogen bond from the protonated N atom of the tertiary amine and steric repulsion from the α-aryl group of trienamine makes the dienophile substrates approach the trienamine from one preferred face. The orbital factors define the most favored alignment of the trienamine and dienophile to realize the endo-selectivity of the product.
在M06 - 2X/6 - 311++G**(SMD, 甲苯)//B3LYP/6 - 311G**水平上研究了通过烯胺催化2,4 - 二烯酮（R1）的不对称狄尔斯 - 阿尔德反应选择性的机理和起源。酸性添加剂（水杨酸或三氟乙酸）通过构建合适的氢键桥用于氢转移，促进酮亚胺离子异构化为具有优选构象的关键反应性顺式烯胺催化物种。对于N - 苯基马来酰亚胺（R2）采用一步环加成，而对于亚苄基氰基乙酸酯（R3）亲双烯体则采用逐步过程。R3中的 - CN和 - CO2Et取代基可能通过参与烯胺催化的电荷转移在形成两性离子中间体中起重要作用。叔胺质子化N原子的氢键与烯胺α - 芳基的空间排斥作用相结合，使亲双烯体底物从一个优选面接近烯胺。轨道因素决定了烯胺和亲双烯体最有利的排列方式，以实现产物的内型选择性。
