Transmission of substituent effects through the methyleneamino group NCH: 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 σ_C^– parameters (mixture of polar-inductive and mesomeric effects), and by biparametric relationships in terms both of the σ_1B parameter (representative of the polar inductive effect) and of the σ_R^– 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.