As part of a continuing study on substituent effects, several series of azulene derivatives were synthesized. Carbon-13 and proton NMR spectra were obtained and this data was analyzed using Taft's Dual-Substituent Parameter (DSP) relationship for the separation of the substituent effect into an inductive and resonance component.
The azulene compounds were synthesized by following literature procedures or by simple variations on these procedures. In a few isolated instances products or product mixtures are obtained that differ significantly from literature claims. Two major pathways were utilized to provide key intermediates with the synthesis of azulene as one pathway and the synthesis of diethyl 2-aminoazulene-l,3-dicarboxylate the prime objective for the other pathway. From these two compounds, a number of 1-, 2- and 6-substituted azulenes were prepared and analysis of these derivatives constituted the major component of this study.
The NMR data served to test the applicability of this analysis procedure to the azulene system with the conclusion being that positions remote to the substituent site(s) are successfully correlated. Furthermore, from comparisons to NMR data and DSP assessments for alternant aromatics (benzene, naphthalene and 1,6-methano[l0]annulene systems) it is evident that the remote conjugative sites of the azulene system are more sensitive to the substituent effect than the analogous sites in these other systems.
Comparisons of the NMR data to group electronegativities yielded relatively inconclusive results. From comparisons of the summation of SCS values in these substituted aromatics it was proposed that these summation values may reflect a nett perturbation of the aromatic system by the substituent with the further conclusion that the azulene system is the most easily perturbed of the aromatic systems considered. A reasonable degree of additivity of the substituent effect is shown for the 13C results from the 1,3-disubstituted azulenes.
Comparisons of the results from the DSP analysis of these aromatic systems with theoretically derived atom-atom polarizabilities led to the conclusion that a qualitative relationship existed between these polarizabilities and the empirically derived ρR values.