US EPA

7742124 

Journal Article 

STRUCTURE AND REACTIVITY OF LITHIUM DIISOPROPYLAMIDE IN THE PRESENCE OF N,N,N',N'-TETRAMETHYLETHYLENEDIAMINE 

Bernstein, MP; Romesberg, FE; Fuller, DJ; Harrison, AT; Collum, DB; Liu, QY; Williard, PG 

1992 

Yes 

Journal of the American Chemical Society
ISSN: 0002-7863
EISSN: 1520-5126 

114 

13 

5100-5110 

English 

10.1021/ja00039a022 

WOS:A1992JA17100022 

Lithium diisopropylamide (LDA) crystallizes from N,N,N′,N′-tetramethylethylenediamine (TMEDA)/hexane mixtures as an infinite array of dimers linked by bridging (nonchelating) TMEDA ligands. 6Li and 15N NMR spectroscopic studies reveal that LDA in neat TMEDA exists as a cyclic dimer bearing a single η1-coordinated TMEDA ligand on each lithium. The equilibrium of solvent-free LDA and the TMEDA-solvated dimer shows a very strong temperature dependence. TMEDA coordinates readily only at low temperature. High TMEDA concentrations are required to saturate the lithium coordination spheres at ambient temperatures. One equivalent of THF readily displaces TMEDA from the coordination sphere to produce the dimeric LDA/THF solvate characterized previously. Kinetics of metalation of 2-methylcyclohexanone N,N-dimethylhydrazone monitored by Fourier-transform infrared (FT-IR) spectroscopy are consistent with a rapid, spectroscopically undetectable dimer-monomer preequilibrium followed by rate-limiting proton transfer. The reaction rate is independent of the TMEDA concentration. Related spectroscopic and rate studies using dimethylethylamine lacking the capacity to chelate show strong parallels with the TMEDA data. We conclude that TMEDA chelates of LDA are of no measurable consequence throughout the reaction coordinate. MNDO calculations of THF and TMEDA solvates of Me2NLi and LDA monomers and dimers qualitatively support the conclusions derived from the experimental data. © 1992, American Chemical Society. All rights reserved.