Collect. Czech. Chem. Commun. 1999, 64, 1274-1294

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


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.