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

Abstract

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.