Collect. Czech. Chem. Commun. 1999, 64, 1274-1294
https://doi.org/10.1135/cccc19991274

Sterically Crowded Heterocycles. X. A New Mechanistic Approach to the Ferricyanide Oxidation of 4,6'-Disubstituted 1-(Pyridin-2'-yl)-2,6-diphenylpyridinium Salts

Radek Pohl, Stanislav Böhm and Josef Kuthan*

Department of Organic Chemistry, Prague Institute of Chemical Technology, 166 28 Prague 6, Czech Republic

This article is part of serial Sterically crowded heterocycles.

Abstract

The oxidations of the title perchlorates, bearing the sterically diverse 6'-substituents (H, Me, Et, i-Pr, n-Bu, t-Bu and Ph) in two series with the same 4-substituents (Ph and t-Bu) lead to pairs of isomeric 3',5-disubstituted (Z)-1'-phenyl-3'-(2-phenylimidazo[1,2-a]pyridin-3-yl)prop-2'- en-1'-ones and 3,6'-disubstituted [5-phenyl-1-(6'-pyridin-2'-yl)-1H-pyrrol-2-yl](phenyl)methanones except where the both variable substituents are t-Bu and then only pyrrolic product is formed. Considering steric interactions of the substituents in some intermediate and/or transition states a multistep mechanism for the oxidative transformation is proposed and supported by model PM3-PECI calculations of some radical intermediates.

Keywords: Imidazo[1,2-a]pyridines; Pyrroles; Ferricyanide oxidation; Pyridinium salts; Pyrilium salts; Aminopyridines; PM3 calculations; Semiempirical calculations.