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.