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Aggregation studies of complexes containing a chiral lithium amide and n-Butyllithium.

The Journal of organic chemistry (2008-02-26)
Deyu Li, Chengzao Sun, Jia Liu, Russell Hopson, Weibin Li, Paul G Williard
ABSTRACT

A system consisting of a chiral lithium amide and n-BuLi in tol-d(8) solution was investigated with (1)H and (13)C INEPT DOSY, (6)Li and (15)N NMR, and other 2D NMR techniques. A mixed 2:1 trimeric complex was identified as the major species as the stoichiometry approached 1.5 equiv of n-BuLi to 1 equiv of amine compound. (1)H and (13)C INEPT DOSY spectra confirmed this lithium aggregate in the solution. The formula weight of the aggregate, correlated with diffusion coefficients of internal references, indicated the aggregation number of this complex. Plots of log D(rel) vs log FW are linear (r > 0.9900). (6)Li and (15)N NMR titration experiments also corroborated these results. These NMR experiments indicate that this mixed aggregate is the species that is responsible for asymmetric addition of n-BuLi to aldehydes.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
n-Butyllithium solution, 2.5 M in hexanes
Sigma-Aldrich
n-Butyllithium solution, 1.6 M in hexanes
Sigma-Aldrich
n-Butyllithium solution, 2.0 M in cyclohexane
Sigma-Aldrich
n-Butyllithium solution, 2.7 M in heptane
Sigma-Aldrich
n-Butyllithium solution, 11.0 M in hexanes
Sigma-Aldrich
n-Butyllithium solution, 1.4 M in toluene