Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States.
J Am Chem Soc. 2015 Jun 17;137(23):7347-56. doi: 10.1021/jacs.5b01906. Epub 2015 May 15.
We report the crystal structure of a substoichiometric, HMPA-trisolvated lithium pinacolone enolate tetramer (LiOPin)4·HMPA3 abbreviated as T3. In this tetramer one HMPA binds to lithium more strongly than the other two causing a reduction in spatial symmetry with corresponding loss of C3 symmetry. A variety of NMR experiments, including HMPA titration, diffusion coefficient-formula weight (D-FW) analysis, and other multinuclear one- and two-dimensional NMR techniques reveal that T3 is the major species in hydrocarbon solution when more than 0.6 equiv of HMPA is present. Due to a small amount of moisture from HMPA or air leaking into the solution, a minor complex was identified and confirmed by X-ray diffraction analysis as a mixed aggregate containing enolate, lithium hydroxide, and HMPA in a 4:2:4 ratio, [(LiOPin)4·(LiOH)2·HMPA4], that we refer to as pseudo-T4. A tetra-HMPA-solvated lithium cyclopentanone enolate tetramer was also prepared and characterized by X-ray diffraction, leading to the conclusion that steric effects dominate the formation and solvation of the pinacolone aggregates. An unusual mixed aggregate consisting of pinacolone enolate, lithium diisopropyl amide, lithium oxide, and HMPA in the ratio 5:1:1:2 is also described.
我们报告了一个亚化学计量的 HMPA-三溶剂合的锂频哪醇酮烯醇化物四聚体（LiOPin）4·HMPA3 的晶体结构，简称为 T3。在这个四聚体中，一个 HMPA 与锂的结合比另外两个更强，导致空间对称性降低，并相应地失去了 C3 对称性。各种 NMR 实验，包括 HMPA 滴定、扩散系数-分子量（D-FW）分析以及其他多核一维和二维 NMR 技术表明，当存在超过 0.6 当量的 HMPA 时，T3 是烃溶液中的主要物种。由于 HMPA 或空气泄漏到溶液中的少量水分，鉴定并通过 X 射线衍射分析确认了一个少量的配合物，该配合物为含有烯醇化物、氢氧化锂和 HMPA 的摩尔比为 4:2:4 的混合聚集体，我们称之为伪-T4。还通过 X 射线衍射制备和表征了一个四-HMPA-溶剂合的锂环戊酮烯醇化物四聚体，得出结论认为空间位阻效应主导了频哪醇酮聚合体的形成和溶剂化。还描述了一个由频哪醇酮烯醇化物、二异丙基胺锂、氧化锂和 HMPA 组成的摩尔比为 5:1:1:2 的不寻常的混合聚集体。
