Rational design of efficient chiral lithium amides for enantioselective deprotonations demands understanding of the origin of the selectivity. The mechanism of deprotonation of cyclohexene oxide 1 by lithium (1R,2S)-N-methyl-1-phenyl-2-pyrrolidinylpropanamide 3, which yields (S)-cyclohex-2-en-1-ol (S)-5 in 93% enantiomeric excess in tetrahydrofuran (THF), has been investigated. Kinetics have been used to show that the reaction is first order with respect to the reagents 1 and 3, respectively. NMR investigations of a 6Li and 15N labelled isotopologue of 3 have previously shown that 3 is mainly
a dimer of the lithium amide monomer in THF in the initial state. On the basis of these results it is concluded that the rate-limiting activated complexes for the epoxide deprotonation are composed of two molecules of monomer of lithium amide 3 and one molecule of epoxide. Structures and energies of unsolvated and specific THF-solvated reagents and activated complexes have been calculated using PM3 and B3LYP/6-31+G(d). The results are currently being explored for the rational design of chiral lithium amides with improved stereoselectivities.
高效手性
锂胺的合理设计以实现对称选择性去质子化,要求理解选择性的来源。研究了
环己烯氧化物1与
锂(1R,2S)-
N-甲基-1-苯基-2-
吡咯烷基丙酰胺3的去质子化机制,该反应在
四氢呋喃(THF)中产生了93%对映体过量的(S)-环己-2-烯-1-醇(S)-5。动力学研究表明,该反应对反应物1和3均为一阶反应。对标记有6Li和15N的3的同位素分子进行了NMR研究,结果表明在THF中,3主要以
锂胺单体的二聚体形式存在于初始状态。基于这些结果,可以得出结论:
环氧化物去质子化的速率限制性激活复合物由两个
锂胺单体3的分子和一个
环氧化物分子组成。未溶剂化和特定THF溶剂化反应物和激活复合物的结构和能量使用PM3和B3LYP/6-31+G(d)计算。当前结果正在被探索,以合理设计具有更好立体选择性的手性
锂胺。