ON THE STRUCTURE OF LITHIUM 2,2,6,6-TETRAMETHYLPIPERIDIDE (LITMP) AND LITHIUM DIISOPROPYLAMIDE (LDA) IN THE PRESENCE OF HEXAMETHYLPHOSPHORAMIDE (HMPA) - STRUCTURE-DEPENDENT DISTRIBUTION OF CYCLIC AND OPEN DIMERS, ION TRIPLETS, AND MONOMERS

Isotopically labeled samples of lithium diisopropylamide (LDA) and lithium 2,2,6,6-tetramethylpiperidide (LiTMP) in THF/pentane containing varying quantities of hexamethylphosphoramide (HMPA) have been subjected to Li-6, N-15, and P-31 NMR spectroscopy at low temperatures. The LDA dimer serially solvates through a monosolvated and disolvated form as evidenced by both Li-6-N-15 and Li-6-P-31 coupling patterns. No evidence of further solvation or deaggregation is observed. LiTMP is shown in previous work to exist as a 90:10 dimer-monomer equilibrium. In the presence of HMPA, the monomer solvates in two discrete steps, although evidence of the specific solvation states is indirect. The dimer undergoes parallel solvation via a mono- and disolvate with 0.0-1.0 equiv of added HMPA. A disolvated open dimer is also observed. At elevated HMPA concentrations, anionic ion triplets of general structure [R2NLiNR2]-//+LiS(n) bearing both 3 and 4 HMPA ligands on the Li+ gegenion are readily observed and fully characterized. In addition, indirect evidence of a tetrasolvated closed dimer is also reported. Arguments refuting HMPA as a deaggregating agent are briefly presented. Further evidence against the existence of solvated lithium dialkylamide trimers and a discussion of the possible roles of anionic ion triplets in organolithium chemistry are included.