6Li NMR Study of the Equilibrium between Methyllithium and n-Butyllithium in THF. Influence of a Third Partner, a Chiral Lithium Amide

A 6 Li NMR structural study of mixtures of labeled methyllithium, n -butyllithium, and 3-aminopyrrolidine lithium amide has been achieved in THF at −78 °C. In the first part, the study was focused on the mixed aggregates formed between methyllithium and n -butyllithium. The spectral data suggest that the two reactants form a series of tetramers (MeLi) 4 ( n -BuLi) 4− n, of which abundance depends on the proportion of the two alkyllithiums following a purely statistical distribution. The labeled chiral lithium amide was next introduced in the sample, and the competitive formation of the amide−methyllithium and amide−butyllithium mixed dimers was monitored in situ . The NMR spectra show a juxtaposition of signals typical of the entities characterized separately, suggesting that there is no preferential formation of one aggregate over the others.