 Solution

HMPA coordinates Li${}^{+}$ through its phosphoryl oxygen and makes the anionic oxygen sites more available for acylation. Strong cation solvation changes contact 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.^[ref] The useful conclusion here is altered lithium coordination and enhanced oxygen accessibility; the exact aggregate of this bifunctional substrate is not supplied.