Synthesis of Heterocycles from Tetracyanoethylene

The literature survey about the title subject since 1985 is presented. The new heterocycles are classified by the tetracyanoethylene (TCNE) reaction type. Accordingly, four classes of TCNE-derivative heterocycles can be identified: (i) reaction products obtained by nucleophilic attack on tetracyanocyclopropanes, which in turn are formed from the reaction of TCNE with aliphatic monobromoderivatives; (ii) products resulting from the thermal decomposition of unstable charge-transfer compounds of TCNE and an electron-donor; (iii) Diels-Alder cycloadducts between TCNE and a diene; (iv) products derived from previously unknown reaction of TCNE. The most interesting new heterocyclic compounds belong to the last (iv) class of TCNE-derivatives, where the 2-(5-amino-3,4-dicyano-2H-pyrrol-2-ylidene)-1,1,2-tricyanoethanide (L'), synthesized for the first time by the author after the reductive autocondensation of TCNE promoted by Lewis acids, occurs too. Further, we report on the properties of L' both as metal-chelating ligand and material for sensor applications. L' undergoes an intramolecular cyclization to the 1,2,6,7-tetracyano-3,5-diimino-3,5-dihydropyrrolizinide (L) on coordination to transition metal cations. A number of metal-complexes of L, either mono- (ML) or bispyrrolizinates, M(L)2, have been isolated and structurally characterized. The most interesting property of these species is that they are metal-phthalocyanine-like dyes and thus they may be useful for the interpretation of the electronic structure of porphyrinic systems. The protonation of L' furnishes the neutral 5-amino-3-imino-1,2,6,7- tetracyano-3H-pyrrolizine (HL). Like other similar conjugated heteropolycyclic molecules and their conjugated anions and complexes, L' and all its derivatives show suitable properties for application in the construction of optochemical sensors, potentially useful for detecting species of environmental interest.