The chiral lithium amide (S)-2-(1-pyrrolidinylmethyl)pyrrolidide, Li-1, which is extensively used in stereoselective synthesis, yields (Li-1)2/1 chelates in the presence of the corresponding diamine 1 as shown by dynamic NMR spectroscopy. The lithium amide Li-1 has low solubility in DEE, but the chelate (Li-1)2/1 is highly soluble. The diamine TMEDA also forms a chelate with dimeric Li-1, (Li-1)2/TMEDA. Uncomplexed diamine 1 exhibits ligand exchange with complexed 1 in (Li-1)2/1 by a dissociative mechanism ( = 7.8 kcal mol-1). The (Li-1)2/1 chelate undergoes a fast intra-complex diamine−amide interconversion via degenerate proton transfer between diamine and amide (Δ = 10.9 kcal mol-1). Computational studies, using semiempirical (PM3) and density functional (B3LYP) methods, show the stabilities of the complexes in agreement with experiments.
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Arvidsson et al. (1999) studied this question.