“Metalation of Imines by Lithium Diisopropylamide Solvated by N,N,N’,N’-Tetramet

Metalation of Imines by Lithium Diisopropylamide Solvated by N,N,N',N'-Tetramethylethylenediamine: Evidence for Solvent-Free Open Dimer Reactive Intermediates Max P. Bernstein and David B. Collum* Baker Laboratory, Department of Chemistry Cornell University, Ithaca, New York 14853- I301 Received September 14, I992 Previous rate studies of N,N-dimethylhydrazone metalations by LDA in THF/hexane or TMEDA/hexane (TMEDA = N,- N,N',N'-tetramethylethylenediamine) implicated mechanisms involving deaggregation of LDA dimers without participation by additional donor solvent (eq 1, R1 = Me, R2 = NMe2).*J We concluded that TMEDA functions as a monodentate rather than bidentate ligand in both the ground state (e.g., 1) and the transition state.2 Furthermore, the surprisingly low affinity of TMEDA for LDA led us to question the mode of action of TMEDA as a ligand for lithium in a more general sense.3 In this communication we will provide evidence that LDA/TMEDA-mediated metalations of simple N-isopropyl imine 2 proceed by a mechanism involving a dimer-monomer pre-equilibrium akin to that observed for the isostructural N,N-dimethylhydrazones. In contrast, metalation of imine 3 bearing a pendant NMe2 moiety is extremely rapid and proceeds by a mechanism involving facile dissociation of both TMEDA ligands from LDA dimer 1 followed by direct reaction of the LDA dimer. We ascribe the change in the rate equation to the Me2N moiety. The kinetics provide clear evidence of a ligand-assisted metalation (complex-induced proximity effect; CIPE)4 and gives experimental support to speculations that lithium amide open dimers may be important reactive intermediates5-'

For full article Click Here