Krenske Elizabeth H, Houk K N, Harmata Michael
School of Chemistry and Molecular Biosciences, University of Queensland , Brisbane, QLD 4072, Australia.
J Org Chem. 2015 Jan 16;80(2):744-50. doi: 10.1021/jo501906m. Epub 2014 Dec 19.
Computations show why the catalytic, asymmetric (4 + 3)-cycloaddition reaction developed in the Harmata laboratories proceeds with facial selectivity opposite to that for models proposed for related catalyzed Diels-Alder reactions. Computations with M06-2X/6-311+G(d,p)//B3LYP/6-31G(d) show that iminium ions derived from MacMillan's chiral 2-tert-butyl-5-benzylimidazolidinone and siloxypentadienals undergo (4 + 3)-cycloadditions with furans preferentially on the more crowded face. Conformational reorganization of the benzyl group, to avoid intramolecular interaction with the silyl group, is responsible for differentiating the activation barriers of top- and bottom-face attack.
计算结果表明了为什么在哈马塔实验室开发的催化不对称(4 + 3)环加成反应的面选择性与相关催化狄尔斯-阿尔德反应所提出的模型相反。使用M06 - 2X/6 - 311+G(d,p)//B3LYP/6 - 31G(d)进行的计算表明，源自麦克米伦手性2 - 叔丁基 - 5 - 苄基咪唑啉酮和硅氧基戊二烯醛的亚胺离子与呋喃优先在更拥挤的面上进行(4 + 3)环加成反应。苄基的构象重排以避免与硅烷基发生分子内相互作用，这是区分顶面和底面进攻的活化能垒的原因。
