Aggregation Studies of Complexes Containing a Chiral Lithium Amide and n-Butyllithium.

A system consisting of a chiral lithium amide and n-BuLi in tol-d8 solution was investigated with 1H and 13C INEPT DOSY, 6Li and 15N 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. 1H and 13C 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 Drel vs log FW are linear (r > 0.9900). 6Li and 15N 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. [ABSTRACT FROM AUTHOR]