Correlation of chemical shifts with substituent parameters in N-benzyl derivatives of pyrrole, 3a, 7a-dihydroindole, and indole esters

Series of m- and p-substituted benzyl derivatives of pyrrole, tetramethyl 1-benzyl-3a, 7a-dihydroindole-2,3,3a,4-tetracarboxylate, and trimethyl 1-benzylindole-2,3,4-tricarboxylate were prepared and their 13C NMR spectra were obtained in 0.1 M solutions of chloroform-d. Both single substituent parameter and dual substituent parameter analyses were carried out to correlate the substituent chemical shifts. The β carbon of the indole series showed the most profound substituent effect dependence as well as the best correlation. The results are explained by the hyperconjugation of the benzyl methylene group