Lithium Diisopropylamide-Mediated Enolizations:  Solvent-Independent Rates, Solvent-Dependent Mechanisms

Rate studies of a lithium diisopropylamide-mediated ester enolization reveal solvation effects that are not in accord with expectation. Metalations in THF, t-BuOMe, HMPA/THF, and DMPU/THF (HMPA = hexamethylphosphoramide, DMPU = 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidone) occur at nearly the same rates, yet proceed via four fundamentally distinct scenarios:  (1) disolvated monomer in THF, (2) monosolvated dimer in t-BuOMe, (3) both monosolvated monomer and tetrasolvated dimer in HMPA/THF, and (4) mono- and disolvated monomers in DMPU/THF. The implication of reactive open dimers in t-BuOMe and triple ions in HMPA/THF underscores the importance of aggregate-based reactions.