Cyclo-octa[def]carbazole, a new heterocyclic paratropic ring system

Abstract

Photolysis of the 1-(1-naphthyl)benzotriazoles (5a,b,g,h) was found to give the deep red cyclo-octa[def]carbazoles (6a,b,g,h), a new heterocyclic ring system formed by an unusual cyclisation onto the naphthalene ring junction (Scheme 1). The ¹H n.m.r. spectra and high chemical reactivity of these cyclo-octacarbazoles (6) are consistent with antiaromatic paratropic character, associated with the 16π-electron periphery (25), comparable with the isoelectronic fluorenyl anion (4). The strained butadiene portion of the cyclo-octacarbazoles (6) is reactive towards addition and cycloaddition reactions, forming a tricarbonyliron complex (27) and Diels–Alder adducts (28). Naphthylbenzotriazoles (5c,d,e) with lone pair bearing substituents (MeO, Cl, Br) adjacent to the triazole ring do not give cyclo-octacarbazoles, but only products derived by cyclisation onto the naphthalene 2-position. On similar photolysis the quinolinyltriazoles (39) give the stable ylides (40) and the acridinylbenzotriazole (45) gives the quinoacridine (46). Mechanisms are proposed for all of these reactions.