Homolytic reactions of ligated boranes. Part 18. The scope of enantioselective hydrogen-atom abstraction by chiral amine–boryl radicals for kinetic resolution under conditions of polarity reversal catalysis
作者:Hai-Shan Dang、Valérie Diart、Brian P. Roberts
DOI:10.1039/p19940001033
日期:——
carbonyl-containing compounds. The key step involves enantioselective abstraction of hydrogen from a C–H bond α to the carbonyl function by optically active amine–boryl radicals derived from the catalyst by hydrogen-atom transfer to tert-butoxyl radicals generated by photolysis of di-tert-butyl peroxide. Chiral discrimination is generally not large, although enantioselectivity factors up to 8.8 were obtained at
多种新的和先前已知的旋光胺-硼烷络合物已被用作极性反转催化剂,用于动力学拆分代表性的外消旋含羰基化合物。的关键的步骤涉及由氢原子转移源自催化剂的光学活性胺的氧硼基团从一个C-H键α的羰基官能团的氢的对映选择性抽象叔由二的光解产生的自由基-butoxyl叔正丁基过氧化物。尽管在–74°C的环氧乙烷溶液中,对映体选择性高达8.8,但手性分辨力通常并不大。通常可以通过考虑以非对映异构过渡态连接到硼原子和α-碳原子上的取代基之间的空间相互作用来预测反应性更高的底物对映异构体。但是,氢键和偶极-偶极相互作用以及立体电子效应,也可能在确定对映选择性中起作用,特别是在反应中心周围没有明显的空间不对称性的情况下。