Lithium Pinacolone Enolate Solvated by Hexamethylphosphoramide

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.