An NMR study of the aggregation and alkylation reactions of some lithiated imines.

Abstract

The solution structure of lithiated N-iso-propylacetaldimine, 1, was determined by a variety of NMR methods. $\sp $N-$\sp6$Li Doubly labelled samples of 1 allowed the determination of lithum-nitrogen connectivities on the basis of $\sp $N-$\sp6$Li NMR coupling patterns. The lithiated imine was found to exist as a rapidly exchanging mixture of two isomeric dimers and a triple ion, whose relative proportions were sensitive to THF concentration, and which exhibited discrete $\sp $C and $\sp $N NMR signals at $-$90$\sp\circ$C. The NMR signals for each of these nuclei were found to coalesce at higher temperatures. Under conditions of increasing THF concentration the triple ion was favoured. Addition of HMPA or PMDETA resulted in solutions containing only triple ions and monomers, but no dimers. The triple ion was found to be solvated by only one more molecule of THF than was the dimer, and HMPA and PMDETA were demonstrated to have the effect of displacing THF from coordination to the lithiated imine. The choice of solvent, and by inference, the nature of the dominant solution structures, was found not to alter the predominant selectivity for alkylation to yield $>$90% syn products. A structurally similar compound, lithiated N-iso-propylpropionaldimine, 3, was found to exhibit coalescence behaviour in the $\sp $C spectrum similar to that observed for 1. Upon addition of HMPA, however, an apparently new species was observed. $\sp1$H NMR evidence suggested that this new species had anti stereochemistry. Alkylation of solutions of 3 containing HMPA were found to give anti products in the same proportion as the new species introduced upon addition of HMPA, while in the absence of HMPA there was 93% syn selectivity.