Electronic substituent effects on nitrogen-15 shieldings of N-(arylmethylidene)cyclohexanamine hydrofluoroacetates

Abstract

An earlier ^(15)NMR study of several series of para-substituted N-(arylmethylidene)amines 1 and 2 has shown that the nitrogen shifts are sensitive to the electronic influence of the ring substituents (X), as reflected by the Hammett o substituent parameters. Reasonable linear correlations could also be obtained for the nitrogen shifts of compounds in each series and the β-carbon shieldings of 4-substituted ethenylbenzenes 3, with the changes in the ^(15)N shifts generally being about twice that of the ^(13)C shifts with respect to the substituent effects. To determine whether the greater sensitivity of the nitrogen shifts arises predominantly from a substituent induced perturbation of the nitrogen unshared pair of electrons, we have measured ^(15)N chemical shifts of a series of para-substituted N-(arylmethylidene)cyclohexanamine hydrotrifluoroacetates.