MIXED AGGREGATION OF LITHIUM ENOLATES AND LITHIUM HALIDES WITH LITHIUM 2,2,6,6-TETRAMETHYLPIPERIDIDE (LITMP)

Li-6 and N-15 NMR spectroscopic studies of [Li-6]-LiTMP and [Li-6,N-15]-LiTMP support an earlier suggestion that LiTMP exists as a dimer-monomer mixture in THF. In the presence of [Li-6]-lithium cyclohexenolate as a representative enolate, one observes mixed aggregates with 2:1, 1:1, and 2:2 LiTMP/enolate stoichiometries. Evidence of conformational isomerism is observed in the slow-exchange limit. Studies of conformationally mobile [Li-6]-lithium di-tert-butylamide and conformationally locked [Li-6]-lithium 2,2,4,6,6-pentamethylpiperidide shed further light on the spectroscopic consequences of the chair form of the piperidine ring system. The corresponding studies of LiTMP/LiBr mixtures reveal the predominance of a 1:1 mixed aggregate, a lower propensity to form 2:1 mixed aggregates than the analogous lithium enolate case, and no tendency whatsoever to form 2:2 mixed aggregates. LiTMP/LiCl mixtures appear to contain two conformational isomers of the 2:1 stoichiometry analogous to the LiTMP enolate case as well as a 1:1 mixed aggregate in the limit of high LiCl concentration. Severe spectral overlaps and several unassigned resonances render the LiTMP-LiCl mixed aggregate structure assignments the most tentative.