Lithiated Oppolzer Enolates: Solution Structures, Mechanism of Alkylation, and Origin of Stereoselectivity.
Lithiated Oppolzer Enolates: Solution Structures, Mechanism of Alkylation, and Origin of Stereoselectivity.
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DOI：
10.1021/jacs.2c09341
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发表时间：
2022-12-28
影响因子：
15
通讯作者：
Collum, David B.
中科院分区：
文献类型：
--
作者：
Lui, Nathan M.;MacMillan, Samantha N.;Collum, David B.
基于樟脑磺内酰胺的锂烯醇化物（俗称 Oppolzer 烯醇化物）通过光谱、晶体学、动力学和计算方法进行检查，以确定烷基化机制和立体选择性的起源。溶剂和底物依赖性结构包括甲苯中烷基取代的烯醇化物的四聚体、甲苯中芳基取代的烯醇化物的不对称二聚体、THF和THF/甲苯混合物中的底物无关的对称二聚体、低HMPA浓度下的HMPA桥联三溶剂化二聚体以及高HMPA浓度下芳基取代的烯醇化物的溶解单体。包括对聚集的立体化学的广泛分析。与烯丙基溴反应的速率研究表明 HMPA 溶剂化离子对与 +Li(HMPA)4 抗衡离子。对甲苯和 THF 的依赖性完全归因于二次壳层（介质）效应。在密度泛函理论（DFT）计算的帮助下，提出了一个立体化学模型，其中螯合物和锂双离子都不发挥作用。立体选择性源于磺内酰胺环内的手性，而不是樟脑骨架核心。
Camphorsultam-based lithium enolates referred to colloquially as Oppolzer enolates are examined spectroscopically, crystallographically, kinetically, and computationally to ascertain the mechanism of alkylation and the origin of the stereoselectivity. Solvent- and substrate-dependent structures include tetramers for alkyl-substituted enolates in toluene, unsymmetric dimers for aryl-substituted enolates in toluene, substrate-independent symmetric dimers in THF and THF/toluene mixtures, HMPA-bridged trisolvated dimers at low HMPA concentrations, and disolvated monomers for the aryl-substituted enolates at elevated HMPA concentrations. Extensive analyses of the stereochemistry of aggregation are included. Rate studies for reaction with allyl bromide implicate an HMPA-solvated ion pair with a +Li(HMPA)4 counterion. Dependencies on toluene and THF are attributed to exclusively secondary shell (medium) effects. Aided by density functional theory (DFT) computations, a stereochemical model is presented in which neither chelates nor the lithium gegenion serve roles. The stereoselectivity stems from the chirality within the sultam ring and not the camphor skeletal core.
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