STRUCTURE AND REACTIVITY OF LITHIUM DIISOPROPYLAMIDE SOLVATED BY POLYAMINES - EVIDENCE OF MONOMER-AND DIMER-BASED DEHYDROHALOGENATIONS

Li-6 and N-15 NMR spectroscopic studies show that hexane solutions of LDA containing <1.0 equiv of N,N,N,N',N'-pentamethyldiethylenetriamin (PMDTA) per lithium contain a mixture of unsolvated LDA oligomers, mon osolvated open dimer, and monosolvated monomer. At >1.0 equiv of PMDTA per lithium, monomer is the dominant species. Addition of PMDTA to LD A in toluene affords open dimer at low [PMDTA] and a mixture of LDA mo nomer and benzyllithium (resulting from toluene deprotonation) at high [PMDTA]. The results are compared and contrasted with previous invest igations of LDA solvated by N,N,N',N'-tetramethylethylenediamine (TMED A) and (+/-)-trans-N,N,N',N'-tetramethylcyclohexane (TMCDA). The react ivities of LDA solvated by TMEDA, TMCDA, and PMDTA were probed by inve stigating;the dehydrohalogenation of (+/-)-2-exo-bromonorbornane. All three Ligands afford qualitatively similar behavior: (1) a maximum rea ctivity at low ligand concentrations ascribed to monosolvated LDA dime rs and (2) ligand-concentration-independent rates at high ligand conce ntrations ascribed to monosolvated LDA monomers. Structure and rate di fferences in hexane and toluene solutions are noted.