First Determination of the Structure of an Ion-Paired Species in Nonpolar Media: 1H, 13C, and 7Li NMR Spectra of Peralkylcyclohexadienyllithium Compounds

Article abstract of DOI:10.1021/ja00409a005

Peralkylcyclohexadienyllithium compounds (2a,b), formed by addition of alkyllithiums to triene 1, are shown to exist as ion-paired aggregates containing conjugated anions bent out of the plane at the saturated ring carbon.Both 13C and 7Li NMR spectra show 2a,b to form two distinctly different ion-paired species - tight peripherally solvated ion-pair aggregates in the presence of mainly tertiary amines and loose separated ion-pair dimers favored with THF, glymes, and HMTP.The latter exhibit two 7Li(+) peaks, one due to Li sandwiched between two anions in a triple ion and the other situated external to it.In the case of 2b with THF both ion-pair species are dimers, and the equlibrium (A(-)Li(+))2*(THF)2 + THF <--> (A2Li)(-) + Li(THF)3(+) is characterized with ΔH = -4 kcal and ΔS = -16 eu.The exchange of ions between ion pairs of 2b with THF is generally slow on the NMR time scale.However, above 0 deg C the lithium exchange rate between the solvated site in the loose ion-pair dimer and the lithium in the tighter ion-pair dimer lies on the NMR time scale.The mean lifetime of lithium in the triple ion is always > 10 s.The slow ion-exchange rates are ascribed to the crowded substitution around 2a,b.