Aromatic metamorphosis of thiophenes: replacing endocyclic sulfur with a different atom or group of atoms

This account deals with our research about the development of synthetic approaches to replace the endocyclic sulfur in thiophenes with a different atom or group of atoms, that is, an aromatic metamorphosis of thiophenes, as a game-changer in organic synthesis. The aromatic metamorphosis has allowed thiophenes and their benzo-analogs to be converted to a variety of cyclic compounds such as triphenylenes, carbazoles, dibenzophospholes, fluorenes, dibenzoazepines, siloles, germoles, and boroles, some of which are difficult to synthesize by conventional methods. The transformations have been achieved by using one of the following dearomatization tricks for enhancing the reactivity of the stable thiophene cores: (1) alkylation at the sulfur atom to yield sulfonium salts; (2) oxidation into sulfones; (3) reduction into 1,4-dianions.