Structure and Reactivity of Lithium Diisopropylamide Solvated by Polyamines:  Evidence of Monomer- and Dimer-Based Dehydrohalogenations

⁶Li and ¹⁵N NMR spectroscopic studies show that hexane solutions of LDA containing <1.0 equiv of N,N,N‘,N‘‘,N‘‘-pentamethyldiethylenetriamine (PMDTA) per lithium contain a mixture of unsolvated LDA oligomers, monosolvated open dimer, and monosolvated monomer. At >1.0 equiv of PMDTA per lithium, monomer is the dominant species. Addition of PMDTA to LDA in toluene affords open dimer at low [PMDTA] and a mixture of LDA monomer and benzyllithium (resulting from toluene deprotonation) at high [PMDTA]. The results are compared and contrasted with previous investigations of LDA solvated by N,N,N‘,N‘-tetramethylethylenediamine (TMEDA) and (±)-trans-N,N,N‘,N‘-tetramethylcyclohexanediamine (TMCDA). The reactivities of LDA solvated by TMEDA, TMCDA, and PMDTA were probed by investigating the dehydrohalogenation of (±)-2-exo-bromonorbornane. All three ligands afford qualitatively similar behavior:  (1) a maximum reactivity at low ligand concentrations ascribed to monosolvated LDA dimers and (2) ligand-concentration-independent rates at high ligand concentrations ascribed to monosolvated LDA monomers. Structure and rate differences in hexane and toluene solutions are noted.