Transmission of substituent effects through the methyleneamino group NCH: aldehyde phenylhydrazones and conjugate anions

Abstract

The para13C resonance of the phenyl ring of a series of phenylhydrazones PhNHNCHX (XII) and of their conjugate anions (XII) is taken as a monitor of the effect exerted by the substituent X: the chemical shift variations are treated by monoparametric relationships in terms of the previously defined, blended σC– parameters (mixture of polar-inductive and mesomeric effects), and by biparametric relationships in terms both of the σ1B parameter (representative of the polar inductive effect) and of the σR– parameter (representative of the mesomeric effect exerted by the substituent X). Relative to the PhNHX systems, interposition of the methyleneamino group NCH in phenylhydrazones (XII) induces a decrease to one-third of the transmission of substituent effects. Substituents X = 2-pyridyl, NO2, CN, COMe, COPh do not correlate in the neutral species (XII) with the previously proposed σ constants. It is proposed that the methyleneamino group exerts a short-range electrostic effect that alters the response of the monitor to the polar-inductive effect exerted by polar substituents X. In the phenylhydrazone anions (XIII) a sizeable amount of the negative charge is empirically calculated to be present on the aldehydic methine carbon, in line with the reported nucleophilic character of this centre.