Measurements of Lithiated Cyclohexanone Phenylimine Solvated by Tetrahydrofuran

"N, 6Li, and I3C NMR spectroscopic studies of lithiated cyclohexanone phenylimine (1) in tetrahydrofuran/hydrocarbon solutions detected three different species. 6Li and I5N NMR spectroscopic studies of [6Li,15N]-1 showed the two species observed at low THF concentrations to be a pair of stereoisomeric dimers and the single species appearing at high THF concentrations to be a contact ion paired monomer. Colligative measurements demonstrated the dimers to be bis-solvated at 0 OC. Measurements of the dimer-monomer equilibrium at -94 "C provided best fit to an equilibrium between tris-solvated monomers and bis or higher solvated dimers. W e recently isolated a crystalline diisopropylamine solvate of lithiated cyclohexanone phenylimine that was shown to possess a bis-solved dimer structure in the solid state (Figure l ) . ) Spectroscopic analyses showed the existence of a 2: 1 mixture of two rapidly interconverting species in hydrocarbon solution. Although previous studies of lithiated imines had uncovered similar resonance duplications," explanations hinging upon various spatial orientations of the imine carbon skeleton (Figure 2) failed to account for the specific case in question. For example, discrete rotamers around the N-Ci+, bond of lithiated isopropylimines described by Fraser2 seemed highly unlikely for an N-phenylsubstituted imine anion. It seemed equally improbable that both syn and anti isomers would be observable since e ~ p e r i m e n t a l ~ ~ and theoretical evidence5 supported a very large (4-5 kcal/mol) stabilization of the syn orientation. E / Z isomer mixtures observed for acyclic lithiated N-alkyland N-phenylimines would be geometrically impossible for the corresponding endocyclic imine anion^.^ To further understand the solution behavior of such a poorly precedented diisopropylamine solvate,6 we turned to a strategy for determining both aggregation and solvation states that has been little used in organolithium chemistry since its introduction7~*