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

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• 作者：Julius F. Remenar and David B. Collum
• 刊名：Journal of the American Chemical Society
• 出版年：1998
• 出版时间：May 6, 1998
• 年：1998
• 卷：120
• 期：17
• 页码：4081 - 4086
• 全文大小：114K
• 年卷期：v.120,no.17(May 6, 1998)
• ISSN：1520-5126

文摘

⁶Li and ¹⁵N NMR spectroscopic studiesshow that hexane solutions of LDA containing <1.0 equivofN,N,N',N'',N''-pentamethyldiethylenetriamine(PMDTA) per lithium contain a mixture of unsolvated LDAoligomers, monosolvated open dimer, and monosolvated monomer. At>1.0 equiv of PMDTA per lithium,monomer is the dominant species. Addition of PMDTA to LDA intoluene affords open dimer at low [PMDTA]and a mixture of LDA monomer and benzyllithium (resulting from toluenedeprotonation) at high [PMDTA].The results are compared and contrasted with previousinvestigations of LDA solvated byN,N,N',N'-tetramethylethylenediamine (TMEDA) and(±)-trans-N,N,N',N'-tetramethylcyclohexanediamine(TMCDA). Thereactivities of LDA solvated by TMEDA, TMCDA, and PMDTA were probed byinvestigating thedehydrohalogenation of (±)-2-exo-bromonorbornane. Allthree ligands afford qualitatively similar behavior:(1) a maximum reactivity at low ligand concentrations ascribed tomonosolvated LDA dimers and (2) ligand-concentration-independent rates at high ligand concentrations ascribedto monosolvated LDA monomers.Structure and rate differences in hexane and toluene solutions arenoted.