The investigation of π polarization effects in stilbazoles, stilbazole methiodides, and styrylfurans by 13C NMR spectroscopy

Abstract

The ¹³C NMR chemical shifts of the heterocyclic ring carbons of a number of 2-, 3-, and 4-(2-arylvinyl)pyridines, their methiodides, and 2- and 3-(2-arylvinyl)furans have been measured. The values obtained are compared with those for stilbenes, and show that ring size has a far greater influence on field-induced π polarization than the presence of a highly electronegative hetero atom. There is clear evidence for localization of the π bonds in the 5-membered, but none in the six-membered rings.