Solution Structures of Lithium Monoalkylamides (RNHLi)

Abstract

6Li, 13C, and 15N NMR spectroscopic studies of lithiated monoalkylamides (RNHLi) reveal a range of structural types. Lithium tert-butylamide (t-BuNHLi) is a single oligomer, assigned as either a cyclic trimer or prismatic oligomer (tetramer, hexamer, or octamer). Lithium N,N-dimethylethylenediamine (Me2NCH2CH2NHLi, LiDMEDA) is a prismatic higher oligomer (hexamer or octamer) of ill-defined stereochemistry. Variable-temperature NMR spectroscopic 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 D2-symmetric tetramer whose symmetry properties allow for a complete structural and stereochemical assignment. The tetramer is converted to a cyclic dimer at elevated THF concentrations. 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.