Stereoselective diamine chelates of a chiral lithium amide dimer: New insights into the coordination chemistry of chiral lithium amides

The chiral lithium amide (S)-2-(1-pyrrolidinylmethyl)pyrrolidide, Li-l, 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-l 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-l, (Li-1)(2)/TMEDA. Uncomplexed diamine 1 exhibits ligand exchange with complexed 1 in (Li-1)(2)/1 by a dissociative mechanism (Delta G(175K)double dagger = 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 (Delta G(268K)double dagger = 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.