Enediolate–Dilithium Amide Mixed Aggregates in the Enantioselective Alkylation of Arylacetic Acids: Structural Studies and a Stereochemical Model

A combination
of X-ray crystallography, 6Li, 15N, and 13C NMR spectroscopies, and density functional
theory computations affords insight into the structures and reactivities
of intervening aggregates underlying highly selective asymmetric alkylations
of carboxylic acid dianions (enediolates) mediated by the dilithium
salt of a C2-symmetric chiral tetraamine.
Crystallography shows a trilithiated n-butyllithium–dilithiated
amide that has
dimerized to a hexalithiated form. Spectroscopic studies implicate
the non-dimerized trilithiated mixed aggregate. Reaction of the dilithiated
amide with the dilithium enediolate derived from phenylacetic acid
affords a tetralithio aggregate comprised of the two dianions in solution
and the dimerized octalithio form in the solid state. Computational
studies shed light on the details of the solution structures and afford
a highly predictive stereochemical model.