Acidity measurements in THF. III. Aldimines and ketimines

Abstract

The relative acidities of fiteen aldimines and ketimines have been measured in THF using either 13 C nmr spectroscopy or trapping experiments with methyl iodide. The pK A 's are found to vary over a range of more than four pK units. The effects of structure on acidity indicate the acid-weakening effect of alkyl substitution in ketimines to be due to steric effects. This interpretation is supported by the results of abinitio calculations. The data also permit a quantitative estimation of the preference for the syn configuration of lithiated imines to be at least four kcal/mol. The anti selectivity previously reported for the lithiation of endocyclic imines can be accounted for by angular constraints, as indicated by both abinitio calculations and pK a data on this class of imine.