Sterically crowded heterocycles. X. A new mechanistic approach to the ferricyanide oxidation of 4,6 '-disubstituted 1-(pyridin-2 '-yl)-2,6-diphenylpyridinium salts

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 s ame 4-substituents (Ph and t-Bu) lead to pairs of isomeric 3',5-disubstitut ed (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] (phen yl)methanones except where the both variable substituents are t-Bu and then only pyrrolic product is formed. Considering steric interactions of the su bstituents in some intermediate and/or transition states a multistep mechan ism for the oxidative transformation is proposed and supported by model PM3 -PECI calculations of some radical intermediates.