= Solution
\b[<HMPA> coordinates Li$^+$ through its phosphoryl <oxygen> and makes the anionic <oxygen> sites more available for acylation.] Strong cation solvation changes contact <ion pair>[ion pairing], chelation and aggregate structure. It can therefore accelerate trapping of both the <enolate> and <phenoxide> by the <chloroformate>, helping realize the observed O-acylation pathway.
It is too strong to assert that <HMPA> necessarily produces completely free, monomeric anions. In a primary study of a <lithium> <enolate> in <THF>, low-temperature multinuclear <NMR> and reaction kinetics identified an <HMPA>-coordinated dimer as the reactive form, even with excess <HMPA>.<Effects of HMPA on the Structure and Reactivity of the Lithium Enolate of Cyclopentanone in THF: The Dimer is Responsible for Alkylation and Proton Exchange Reactions by Masaaki Suzuki and others>[2004, pp. 259–268, HMPA titration, aggregate assignment and alkylation kinetics]{ref} The useful conclusion here is altered <lithium> coordination and enhanced <oxygen> accessibility; the exact aggregate of this bifunctional substrate is not supplied.
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