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

The structure and reactivity of the lithium enolate of cyclopentanone are strongly influenced by coexisting HMPA molecules. Detailed low-temperature I-7.i, P-31, and C-13 NMR studies of the 0.04-0.2 M THF solutions indicate that excess quantities of HMPA act primarily to form a bis-HMPA coordinated dimer. Tetrameric, trimeric, and monomeric enolates were not detected by HMPA titration. The convergency on the dimeric aggregate has been monitored by the successive displacement of THF with HMPA molecules. Even at a high concentration of HMPA, the formation of monomeric enolate was not observed. Kinetic experiments, combined with the structural information, showed that alkylation of the enolate by methyl iodide proceeds via the dimer with the assistance of HMPA. Proton exchange between the enolate and 2-methyl-cyclopentanone also occurs via the dimer, but without the participation of additional HMPA. These findings explain the role of HMPA in the selective monoalkylation of lithium enolate.