Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095, United States.
J Org Chem. 2018 Mar 2;83(5):2611-2616. doi: 10.1021/acs.joc.7b03016. Epub 2018 Feb 15.
We report a DFT computational study (M06-2X) of π-facial selectivity in the Diels-Alder reactions of thiophene 1-oxide. The preference for the syn cycloaddition arises because the ground state geometry of thiophene 1-oxide is predistorted into an envelope conformation that resembles the syn transition state geometry. The syn distortion occurs to minimize the effect of hyperconjugative antiaromaticity in the thiophene 1-oxide, arising from overlap of the σ* with the π-system. The syn selectivity follows through to the product structure that is stabilized by a π-σ* interaction, related to the 7-norbornenyl ion stability.
我们报告了噻吩 1-氧化物的 Diels-Alder 反应中π-面选择性的密度泛函理论（DFT）计算研究（M06-2X）。顺式环加成的偏好是因为噻吩 1-氧化物的基态几何形状被预先扭曲成类似于顺式过渡态几何形状的信封构象。顺式扭曲的发生是为了最小化噻吩 1-氧化物中超共轭反芳香性的影响，这是由于 σ与π系统的重叠引起的。顺式选择性一直延续到产物结构，产物结构通过π-σ相互作用得到稳定，这与 7-降冰片烯基离子的稳定性有关。
