Department of Chemistry and Chemical Biology Baker Laboratory, Cornell University Ithaca, New York 14853-1301, United States.
J Am Chem Soc. 2022 Dec 28;144(51):23379-23395. doi: 10.1021/jacs.2c09341. Epub 2022 Dec 19.
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) 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 serves roles. The stereoselectivity stems from the chirality within the sultam ring and not the camphor skeletal core.
以樟脑磺内酰胺为基础的锂烯醇盐，通常称为 Oppolzer 烯醇盐，通过光谱、晶体、动力学和计算方法进行了检查，以确定烷基化的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烯醇盐的四聚体、甲苯中芳基取代烯醇盐的不对称二聚体、THF 和 THF/甲苯混合物中底物独立的对称二聚体、低 HMPA 浓度下的 HMPA 桥接三溶剂化二聚体以及升高的 HMPA 浓度下芳基取代烯醇盐的单溶剂化单体。包括对聚集立体化学的广泛分析。与烯丙基溴反应的速率研究表明存在 HMPA 溶剂化的离子对，其中 Li(HMPA) 是抗衡离子。对甲苯和 THF 的依赖性归因于仅次级壳（介质）效应。借助密度泛函理论（DFT）计算，提出了一个立体化学模型，其中既没有螯合物，也没有锂反离子起作用。立体选择性源于磺内酰胺环内的手性，而不是莰骨架核心。
