7Li- and 31P NMR spectra of cyclopentanone lithium enolate in ethereal solvents: identification of the HMPA-coordinated aggregate structures

The structures of cyclopentanone lithium enolate under HMPA titration in 0.04–0.8 M diethyl ether and dimethyl ether solvents have been investigated using the low-temperature 7Li, 31P, and 13C NMR. The progressive solvation by HMPA occurs for the tetra- and dimeric enolates, and upon addition of >2 equiv. of HMPA, the lithium enolate has been converged on a mixture of tetra-HMPA coordinated tetramer and bis-HMPA coordinated dimer with the ratio of 5:95 and <1:99 in diethyl ether and dimethyl ether, respectively. Neither monomeric nor trimeric enolate is detectable under such HMPA titration.