Pseudophedrine-Derived Myers Enolates: Structures and Influence of Lithium Chloride on Reactivity and Mechanism

The structures and reactivities of pseudoephedrine-derived dianionic Myers enolates are examined. A combination of NMR and IR spectroscopic, crystallographic, and computational data reveal that the homoaggregated dianions form octalithiated tetramers displaying S₄-symmetric Li₈O₈ cores and overall C₂ symmetry. Computational and isotopic labeling studies reveal strong N-Li contacts in the carboxamide enolate moiety. The method of continuous variations proves deceptive, as octalithiated tetrameric homoaggregates afford hexalithiated trimeric heteroaggregates. A lithium diisopropylamide-lithium enolate mixed aggregate is found to be a C₂-symmetric hexalithiated species incorporating two enolate dianions and two lithium diisopropylamide (LDA) subunits. Structural and rate studies show that lithium chloride has little effect on the dynamics of the enolate homoaggregates but forms adducts of unknown structure. Rate studies of alkylations indicate that the aging of the aggregates can have effects spanning orders of magnitude. The LiCl-enolate adduct dramatically accelerates the reaction but requires superstoichiometric quantities owing to putative autoinhibition. Efforts and progress toward eliminating the requisite large excess of LiCl are discussed.