Perturbation of Conjugation in Internally Solvated Allylic Lithium Compounds: Variation of Ligand Structure. NMR and X-ray Crystallography

Several allyic lithium compounds were prepared with different potential ligands tethered at C2.
These are with CH3OCH2CH2NCH3CH2-, 5 and 1-TMS 6, with (CH3)2NCH2CH2NCH3CH2-, 1-TMS 7, and
with ((CH3)2NCH2CH2)2NCH2-, 8 and 1-TMS 9. In all these compounds Li is fully coordinated to the pendant
ligand and is sited off the axis perpendicular to the allyl plane at one of the allyl termini as indicated by a
combination of X-ray crystallography and NMR spectra. Compounds 5 and 8 are Li-bridged dimers as
shown by X-ray crystallography and also dimeric in benzene solution as determined from freezing point
determinations. Compounds 6, 7, and 9 are monomeric in THF-d8 or diethyl ether-d10 solution and exhibit
one bond 13C1,6Li scalar coupling at low temperature. Taken together the crystallographic and NMR data
indicate that all of these compounds incorporate partially delocalized allylic moieties. Compounds 5 and 8
undergo fast 1,3-Li-sigmatropic shifts that are proposed to take place within low concentrations of monomers
in fast equilibrium with prevalent dimers. Averaging with increasing temperature of the one-bond 13C,6Li
coupling constant in 6, 7, and 13 provided the dynamics of bimolecular C−Li exchange with ΔH⧧ values of
6.7, 12, and 13 kcal·mol-1, respectively. Averaging of the diastereotopic N(CH3)2 13C resonances of 7 is
indicative of fast transfer of coordinated ligand between faces of the allyl plane ΔH⧧ = 5.3 kcal·mol-1
combined with slower inversion at nitrogen. Compound 8 exhibits similar effects. It is concluded that variation
of the ligand structure changes dynamic behavior of the compounds but has little influence of their degrees
of delocalization.