Ketone enolization by sodium hexamethyldisilazide (NaHMDS) shows a marked solvent and substrate dependence. Enolization of 2-methyl-3-pentanone reveals E–Z selectivities in Et₃N/toluene (20:1), methyl-t-butyl ether (MTBE, 10:1), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDTA)/toluene (8:1), TMEDA/toluene (4:1), diglyme (1:1), DME (1:22), and tetrahydrofuran (THF) (1:90). Control experiments show slow or nonexistent stereochemical equilibration in all solvents except THF. Enolate trapping with Me₃SiCl/Et₃N requires warming to −40 °C whereas Me₃SiOTf reacts within seconds. In situ enolate trapping at −78 °C using preformed NaHMDS/Me₃SiCl mixtures is effective in Et₃N/toluene yet fails in THF by forming (Me₃Si)₃N. Rate studies show enolization via mono- and disolvated dimers in Et₃N/toluene, disolvated dimers in TMEDA, trisolvated monomers in THF/toluene, and free ions with PMDTA. Density functional theory computations explore the selectivities via the E- and Z-based transition structures. Failures of theory-experiment correlations of ionic fragments were considerable even when isodesmic comparisons could have canceled electron correlation errors. Swapping 2-methyl-3-pentanone with a close isostere, 2-methylcyclohexanone, causes a fundamental change in the mechanism to a trisolvated-monomer-based enolization in THF.

中文翻译：

---

六甲基二硅氨基钠的酮烯醇化：溶剂和底物依赖性 E-Z 选择性和附属机制

六甲基二硅化钠 (NaHMDS) 的酮烯醇化显示出明显的溶剂和底物依赖性。2-甲基-3-戊酮的烯醇化揭示了Et 3 N /甲苯 (20:1)、甲基-叔丁基醚 (MTBE, 10:1)、_N ,N,N',N″, N”-五甲基二亚乙基三胺 (PMDTA)/甲苯 (8:1)、TMEDA/甲苯 (4:1)、二甘醇二甲醚 (1:1)、DME (1:22) 和四氢呋喃 (THF) (1:90)。对照实验表明在除 THF 之外的所有溶剂中立体化学平衡缓慢或不存在。_{用 Me 3} SiCl/Et ₃ N捕获烯醇化物需要升温至 −40 °C，而 Me ₃SiOTf 会在几秒钟内做出反应。_{使用预先形成的 NaHMDS/Me 3} SiCl 混合物在 −78 °C 下原位捕获烯醇化物在 Et ₃ N/甲苯中有效，但在 THF 中通过形成 (Me ₃ Si) ₃ N 失败。速率研究表明通过单聚体和溶解二聚体进行烯醇化Et ₃ N/甲苯，TMEDA 中的溶解二聚体，THF/甲苯中的三溶剂化单体，以及 PMDTA 中的游离离子。密度泛函理论计算通过E - 和Z探索选择性基于过渡结构。离子碎片的理论-实验相关性的失败是相当大的，即使等线比较可以消除电子相关性错误。用紧密等排体 2-甲基环己酮交换 2-甲基-3-戊酮，导致 THF 中基于三溶剂化单体烯醇化的机制发生根本变化。