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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(1 H )-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.
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Sun et al. (2000) studied this question.
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