A CORRELATION-ANALYSIS OF C=N C-13 CHEMICAL-SHIFTS - THE USE OF SUBSTITUTED BENZALDEHYDE (2-HYDROXYCYCLOHEXYL)HYDRAZONES AS PROBES

The C=N C-13 chemical shifts of two series of substituted benzaldehyde (2-hydroxycyclohexyl)hydrazones were measured. The aromatic ring-subs tituent chemical shifts (SCS) of hydrazones were for the first time an alyzed in terms of separate resonance and inductive effects through us e of the dual substituent parameter (DSP) approach. The SCS values sho wed that electron-donating substituents cause downfield and electron a ccepters upfield shifts, thus following the reverse trend. The inducti ve effects of the substituents are predominant, the inductive transmis sion coefficient rho(I) for both para- and meta-substituted benzaldehy de (2-hydroxycyclohexyl)hydrazones having a general value of -6.0 +/- 0.2. This reverse inductive contribution is consistent with the pi-pol arization mechanism. The resonance contribution to SCS varies from cas e to case and as expected is higher in the para series than in the met a series. The C=N C-13 chemical shifts of hydrazones are appreciably m ore susceptible to substituent-induced electronic changes than those o f imines, although both are more sensitive than the a-carbon C-13 chem ical shifts of styrenes, phenylacetylenes, benzonitriles, or the side- chain carbonyl derivatives of benzene to substituent effects.