Lithium 2,2,6,6-Tetramethylpiperidide and Lithium 2,2,4,6,6-Pentamethylpiperidide:  Influence of TMEDA and Related Chelating Ligands on the Solution Structures. Characterization of Higher Cyclic Oligomers, Cyclic Dimers, Open Dimers, and Monomers

• Published in: Journal of organic chemistry 1997-08, Vol.62 (17), p.5748-5754
• Main Authors: Remenar, Julius F, Lucht, Brett L, Kruglyak, Dmitriy, Romesberg, Floyd E, Collum, David B
• Format: Article
• Language: English
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo970520l

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. 6Li and 15N NMR spectroscopic studies of [6Li]LiPMP and [6Li,15N]LiPMP in hydrocarbon solution reveal a mixture comprised of four isomeric cyclic tetramers (C 4h , D 2h , C 2v , and Cs ) and one isomeric cyclic trimer (C 3h ). These results are compared with the aggregation numbers and conformational 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), [6Li,15N]LiPMP affords monomer and open dimer to the exclusion of solvated cyclic dimer. The open dimer undergoes a degenerate intramolecular rearrangement. Spectroscopic studies in TMEDA/THF mixtures reveal an unexpected competitive solvation and implicate the observable THF-solvated cyclic dimer to possess one THF per Li. Dimethylethylamine (a monodentate analog of TMEDA) and N,N,N‘,N‘-tetramethylpropylenediamine (a six-membered ring chelating analog of TMEDA) are inferior as ligands for LiPMP. Investigations of LiPMP in the presence of other bidentate ligands reveal related monomers and open dimers whose proportions depend upon ligand structure and concentration. The relative binding constants and binding energies to the LiPMP monomer are reported. Ligands studied include the following:  N,N,N‘,N‘-tetraethylethylenediamine, N,N-diethyl-N',N'-dimethylethylenediamine, N,N'-diethyl-N,N'-dimethylethylenediamine, N-ethyl-N,N',N'-trimethylethylenediamine, 1,2-dipyrrolidinoethane, 1,2-dipiperidinoethane, 1-(dimethylamino)-2-pyrrolidinoethane, 1-(dimethylamino)-2-methoxyethane, 1-(dimethylamino)-2-ethoxyethane, 1-azetidino-2-methoxyethane, 1-methoxy-2-pyrrolidinoethane, 1-methoxy-2-piperidinoethane, 1-ethoxy-2-pyrrolidinoethane, trans-N,N,N‘,N‘-tetramethylcyclohexanediamine, trans-1-(dimethylamino)-2-methoxycyclohexane, and sparteine.