Bakalova Snezhana M, Santos A Gil
REQUIMTE-CQFB, Dep. Química, FCT, Universidade Nova Lisboa, 2829-516 Caparica, Portugal.
J Org Chem. 2004 Nov 26;69(24):8475-81. doi: 10.1021/jo049298s.
The eight diastereoisomeric transition structures of the Diels-Alder addition of ethyl-S-lactyl acrylate and cyclopentadiene have been investigated in the gas phase and in solution by HF, MP2, and density functional theory (B3LYP and B3PW91) methods with the 6-31G(d,p) basis set. At all levels of theory used, the s-cis transition structures are more stable than the s-trans ones. The contribution of the s-trans transition structures increases in solution and, although still small, has to be taken in consideration for correct prediction of stereoselectivity. Diastereofacial selectivity is interpreted in terms of electrostatic (weak hydrogen bonding) C=O...H(C) interactions between the carbonyl group(s) of the dienophile and cyclopentadiene in the energetically favored transition structures. Endo/exo reaction selectivity is attributed to positive orbital interactions between the diene and the acrylate carbonyl oxygen in the endo s-cis transition structures. Ab initio methods reproduce well the experimentally observed trends in both endo/exo and diastereofacial selectivity. Density functional calculations in the gas phase correctly reproduce the observed trends in diastereofacial selectivity but single-point MP2 calculations are necessary to reproduce the experimental trend in endo/exo selectivity.
采用HF、MP2以及密度泛函理论（B3LYP和B3PW91）方法，在6-31G(d,p)基组下，对S-乳酸乙酯丙烯酸酯与环戊二烯的狄尔斯-阿尔德加成反应的8种非对映异构过渡结构在气相和溶液中进行了研究。在所使用的所有理论水平下，s-顺式过渡结构比s-反式过渡结构更稳定。s-反式过渡结构在溶液中的贡献增加，尽管仍然很小，但在正确预测立体选择性时必须予以考虑。非对映面选择性是根据亲双烯体的羰基与环戊二烯在能量有利的过渡结构中的静电（弱氢键）C=O...H(C)相互作用来解释的。内型/外型反应选择性归因于内型s-顺式过渡结构中二烯与丙烯酸酯羰基氧之间的正轨道相互作用。从头算方法很好地再现了实验观察到的内型/外型和非对映面选择性趋势。气相中的密度泛函计算正确地再现了观察到的非对映面选择性趋势，但需要进行单点MP2计算才能再现内型/外型选择性的实验趋势。
