LITHIUM 2,2,6,6-TETRAMETHYLPIPERIDIDE AND LITHIUM 2,2,4,6,6-PENTAMETHYLPIPERIDIDE - INFLUENCE OF TMEDA AND RELATED CHELATING LIGANDS ON THESOLUTION STRUCTURES - CHARACTERIZATION OF HIGHER CYCLIC OLIGOMERS, CYCLIC DIMERS, OPEN DIMERS, AND MONOMERS

Spectroscopic investigations of 2,2,6,6-tetramethylpiperidide (LiTMP) and the conformationally locked (but otherwise isostructural) lithium 2,2,4,6,6-pentamethylpiperidide (LiPMP) are described. Li-6 and N-15 N MR spectroscopic studies of [Li-6]LiPMP and [Li-6,N-15]LiPMP in hydroc arbon solution reveal a mixture comprised of four isomeric cyclic tetr amers (C-4h, D-2h, C-2v, and C-s) and one isomeric cyclic trimer(C-3h) . These results are compared with the aggregation numbers and conforma tional preferences of lithium 2,2,6,6-tetramethylpiperidide (LiTMP) in the solid state studied by Lappert and co-workers. In the presence of N,N,N',N'-tetramethylethylenediamine (TMEDA), [Li-6,N-15]LiPMP afford s monomer and open dimer to the exclusion of solvated cyclic dimer. Th e open dimer undergoes a degenerate intramolecular rearrangement. Spec troscopic studies in TMEDA/THF mixtures reveal an unexpected competiti ve solvation and implicate the observable THF-solvated cyclic dimer to possess one THF per Li. Dimethylethylamine (a monodentate analog of T MEDA) and N,N,N',N'-tetramethylpropylenediamine (a six-membered ring c helating analog of TMEDA) are inferior as ligands for LiPMP. Investiga tions of LiPMP in the presence of other bidentate ligands reveal relat ed monomers and open dimers whose proportions depend upon ligand struc ture and concentration. The relative binding constants and binding ene rgies to the LiPMP monomer are reported. Ligands studied include the f ollowing: N,N,N',N'-tetraethylethylenediamine, N,N-diethyl-N',N'-dimet hylethylenediamine, N,N'-diethyl-N,N'-dimethylethylenediamine, N-ethyl -N,N',N'-trimethylethylenediamine, 1,2-dipyrrolidinoethane, 1,2-dipipe ridinoethane, 1-(dimethylamino)-2-pyrrolidinoethane, 1-(dimethylamino) -2-methoxyethane, 1-(dimethylamino)-2-ethoxyethane, 1-azetidino-2-meth oxyethane, 1-methoxy-2-pyrrolidinoethane, 1-methoxy-2-piperidinoethane , 1-ethoxy-2-pyrrolidinoethane, trans-N,N,N',N'-tetramethylcyclohexane diamine, trans-1-(dimethylamino)-2-methoxycyclohexane, and sparteine.