Analysis of an asymmetric addition with a 2:1 mixed lithium amide/n-butyllithium aggregate
(Chemical Equation Presented) A 2:1 lithium amide/n-butyllithium aggregate 1 is investigated as an asymmetric addition template in hydrocarbon solvents. Several different chiral lithium amides were synthesized from L-valine and tested in the asymmetric addition of n-BuLi to various aldehydes. Enantiomeric excesses up to 83% were obtained in the case of the addition of n-BuLi to pivaldehyde at -116°C in pentane. 1H and 13C INEPT DOSY were utilized to characterize a new trimeric complex 12 between 2 equiv of lithium amide and 1 equiv of lithium alkoxide. This mixed aggregate strongly indicates the possibility of product-induced chirality inhibition that is detrimental to the enantioselectivity of asymmetric addition reaction.
Aggregation studies of complexes containing a chiral lithium amide and n-butyllithium
(Graph Presented) 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 Dre1 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.
Li, Deyu,Sun, Chengzao,Liu, Jia,Hopson, Russell,Li, Weibin,Williard, Paul G.
p. 2373 - 2381
(2008/09/19)
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