1,4-Dihydropyrrolo[3,2-b]pyrrole and its π-expanded analogues

Various approaches to the synthesis of 1,4-dihydropyrrolo[3,2-b]pyrroles are summarized. Many two- and three-step reaction sequences have been developed, and have allowed access to a broad variety of structures, including not only the parent 1,4-dihydropyrrolo[3,2-b]pyrroles, but also their π-expanded analogues. The newest approaches are critically compared with older strategies. The reactivity of these compounds is also reviewed, with special emphasis on electrophilic aromatic substitution. The synthesis of indolo[3,2-b]indole derivatives has been the subject of intense investigation. Overall, a few interesting and ingenious approaches toward these ladder-type heteroacenes have been proposed, reaching total yields in the region of 30%. Finally, the optical, electrochemical, and other physicochemical properties are presented in the broader perspective of heteropentalenes. The parent 1,4-dihydro-pyrrolo[3,2-b]pyrroles constitute the most electron-rich, simple, aromatic heterocycles, and their simple derivatives and π-expanded analogues possess strong violet, blue, or green fluorescence both in solution and in the solid state.