Structural Consequences of the Addition of Lithium Halides in Enolization and Aldol Reactions

Aggregates consisting of lithium halides with either lithium
amides or lithium enolates have been
characterized by X-ray crystallography. Two structural types,
solvated heterodimers and heterotrimers, have been
found for halide/amide combinations. Two compounds containing both
lithium halide and lithium enolate have also
been identified as heterodimers. Using this information, we
propose a reaction sequence for enolization and subsequent
aldol addition reaction involving halide-containing aggregates. An
ab initio and PM3 theoretical study of model
systems shows that solvated heterodimers between LiBr and either
LiNH2 or LiOC(H)=CH2 are favored over
the
respective homodimers. Calculations reveal a stable eight-membered
ring transition state for the enolization step
between LiCl·LiNH2 and acetaldehyde. Two
independent transition states, a 4,8 and a 4,4,6 ring system,
were
calculated for the model reaction between the heterotrimer
[(LiNH2)2·LiCl] and acetaldehyde.
Dissociation of donor
solvent was computed to require more energy for heterodimers than for
homodimers.