“Lithium 2,2,6,6-tetramethylpiperidide and lithium 2,2,4,6,6-pentamethylpiperid

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 (C4h, D2h, C2v, and Cs) and o­ne isomeric cyclic trimer (C3h). 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.

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