Department of Chemistry and Biochemistry , University of California , Los Angeles , California 90095-1569 , United States.
Department of Chemistry , Yale University , New Haven , Connecticut 06520 , United States.
J Org Chem. 2019 Jan 4;84(1):273-281. doi: 10.1021/acs.joc.8b02603. Epub 2018 Dec 19.
The origins of π-facial selectivities in the borohydride reduction of endocyclic iminium ions have been elucidated by density functional theory calculations. In reductions of conjugated ("thermodynamic") iminium ions, the π-facial preference of the hydride attack was found to be due to torsional steering. Attack at the favored π-face leads to a lower-energy "half-chair"-like conformation of the tetrahydropyridine product, whereas attack at the other π-face results in an unfavorable "twist-boat" conformation. In reductions of nonconjugated ("kinetic") iminium ions, torsional distinction is small between the top- and bottom-face attacks, and the π-facial selectivity of the hydride approach is primarily due to steric hindrance.
通过密度泛函理论计算阐明了硼氢化物还原中环内亚胺离子时 π-面选择性的起源。在共轭（“热力学”）亚胺离子的还原中，发现氢化物进攻的 π-面选择性归因于扭转导向。在有利的 π-面进攻导致四氢吡啶产物的低能“半椅”构象，而在另一 π-面进攻则导致不利的“扭曲船”构象。在非共轭（“动力学”）亚胺离子的还原中，顶位和底位进攻之间的扭转差异很小，而氢化物接近的 π-面选择性主要归因于空间位阻。
