Anodic Oxidation Potentials of Substituted Pyrroles: Derivation and Analysis of Substituent Partial Potentials

With cyclic voltammetry, anodic oxidation potentials for a series of 117 substituted pyrroles have been measured and analyzed. The pyrroles studied are useful intermediates for the synthesis of polypyrrole pigments, and the information gained is related to reactivity in pyrrole systems and should be applicable to synthetic endeavors. Substituent partial potentials for the various common peripheral groups are derived by using a least-squares analysis and are interpreted. With a few exceptions, on the basis of the derived substituent contributions it is possible to deduce accurate calculated oxidation potentials for virtually any common substituted pyrrole. Calculated and measured oxidation potentials differ in two major circumstances: (a) when two strongly electron-with drawing substituents are located at the 2-and 5-positions and (b) when an unsubstituted 2-position occurs concomitantly with a strongly electron-with drawing 5-position substituent. Though correlations between electrochemical substituent partial potentials and standard σp and σm coefficients are not apparent, a good correlation was obtained between pKa and oxidation potential for a series of pyrrole-5-carboxylic acids. © 1984, American Chemical Society. All rights reserved