π‐Amphoteric Heterocycles

Abstract

The formation of the Wheland or Meisenheimer cr-complexes the wellknown phenomena in chemistry of aromatics and heteroaromatics [1, 2] is a key step in the both classical SE2Ar and SN2Ar substitution. The ability of arenes and hetarenes to form predominantly the cationic or the anionic type of s-complexes is, in general, determined by the p-excessive or p-deficient character of aromatic nucleus (cf. known C-protonation of azulenes, indoles, donor-substituted benzenes or vice versa addition of nucleophiles to nitroazines and polynitroarenes). To our knowledge the simple aromatics, capable of undergoing easy transformation in the both directions into the anionic, as well as the cationic s-complexes are still unknown. The simplest possible aromatics of this type would be expected to contain two condensed rings together with a high asymmetry of the p-electron density. With the indolizines la,b, we found the first example of the bicyclic arenes, which easily form cationic and anionic s-complexes on carbon atoms with a simple variation in the pH of the solvent system.