LITHIUM DIISOPROPYLAMIDE SOLVATED BY MONODENTATE AND BIDENTATE LIGANDS - SOLUTION STRUCTURES AND LIGAND-BINDING CONSTANTS

Li-6 and N-15 NMR spectroscopic studies of lithium diisopropylamide ([ Li-6]LDA and [Li-6,N-15]LDA) in toluene/pentane solutions containing a variety of mono- and polydentate ligands are reported. LDA forms excl usively dimers in the presence of n-BuOMe, Et2O, t-BuOMe, THF, 2-methy ltetrahydrofuran, 2,2-dimethyltetrahydrofuran, tetrahydropyran, dimeth oxyethane, N,N,N',N'-tetramethylethylenediamine, and MeOCH2CH2NR2 (NR2 = NMe2, NEt2, pyrrolidino). Addition of 1,2-dipyrrolidinoethane and ( 2-pyrrolidinoethyl)dimethylamine provides monomer-dimer mixtures. Trea tment of LDA with trans-N,N,N',N'-tetramethylcyclohexanediame (TMCDA) or trans-1-(dimethylamino)-2-isopropoxycyclohexane in hydrocarbons aff ord exclusively monomers. Sparteine binds only reluctantly, giving a m ixture of unsolvated oligomers and monomer. Competitions of the ethere al ligands vs TMCDA afford binding constants and associated free energ ies for dimer solvation which are correlated with those obtained previ ously for lithium hexamethyldisilazide.