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

6 Li and 15 N 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.