Solution structures of lithium monoalkylamides (RNHLi)

Li-6, C-13, and N-15 NMR spectroscopic studies of lithiated monoalkylamides (RNHLi) reveal a range of structural types. Lithium tert-butylamide (t-BuN HLi) is a single oligomer, assigned as either a cyclic trimer or prismatic oligomer (tetramer, hexamer, or octamer). Lithium N,N-dimethylethylenediami ne (Me2NCH2CH2NHLi, LiDMEDA) is a prismatic higher oligomer (hexamer or oct amer) of ill-defined stereochemistry. Variable-temperature NMR spectroscopi c studies reveal several dynamic processes within the LiDMEDA oligomer. (R) -PhCH(NHLi)CH2N(CH2)(4) both in hydrocarbons and in hydrocarbons containing small concentrations of THF exists as a single D-2-symmetric tetramer whos e symmetry properties allow for a complete structural and stereochemical as signment. The tetramer is converted to a cyclic dimer at elevated THF conce ntrations. A mixed dimer of t-BuNHLi and lithium phenylacetylide is readily characterized. A more complex mixed aggregate of (R)-PhCH(NHLi)CH2N(CH2)(4 ) and lithium phenylacetylide is shown to be a 4:2 mixed hexagonal prism.