Triafulvalene

Scheme 11 summarizes the reactions of bis(diisopropylamino)lithiocyclopropenylium perchlorate with a variety of electrophiles. These include acid hydrolysis, silylation, oxidative coupling with iodine to the symmetrical tetraisopropylamino triafulvalene dica-tion, reaction leading to C—C bond formation, with electrophilic allenes to give a zwitterionic dihydrotriafulvenes, with disubstituted cyclopropenylium perchlorate, and... 

While nonannulated triafulvalenes are unknown, tetraaryldibenzotriafulvalenes 10 were synthesized as stable substances by dimerization of diarylbenzocyclopropenylidenes. ... 

Triafulvalene (cyclopropenylidenecyclopropene) has not been isolated. A substantial number of pentafulvalene derivatives have been prepared. The chemical properties of these molecules are those of reactive polyenes. The NMR spectrum of pentafulvalene is characteristic of a localized system. Heptafulvalene (cycloheptatrienylidenecy-cloheptatriene) is a well-characterized compound with the properties expected for a polyene. ... 

Agranat, Radom, and co-workers surveyed the fulvene and fulvalene combinations including three-, five-, and seven-membered rings. Structures and energies were calculated at the BLYP/6-31G and MP2/6-31G levels. A large destabilization was found for triafulvalene on the basis of homodesmotic reactions. The potentially... 

Despite intensive efforts, the isolation and characterization of triafulvalene derivatives 51 was not achieved until quite recently theoretical calculations predict that triafulvalene 51 (R = H) possesses antiaromatic character. In 2009, Bertrand and coworkers synthesized kinetically protected triafulvalenes 52 as thermally stable compounds, as shown in Scheme 6.6 [27]. Selection of the bulky 2,4,6-triisopropylphenyl (Tip) and mesityl (Mes) groups as protective groups formed the basis of success. An X-ray diffraction analysis of an anti-isomer 52a shows that the triafulvalene skeleton is almost perfectly planar, and the observed short bond distance (1.303(5) A) confirms the highly localized nature of the % system. 

Yoshida and his co-workers have continued to contribute greatly to the field of cyclopropenium ion chemistry. In addition to describing a large number of cyclopropenium ions, cyclopropen-ones, and -thiones in the patent literature they have produced the first triafulvalene dication (5). This was obtained, by treatment of (6 X = H) with butyl-lithium and then allowing the resulting carbenoid (6 X = Li) to react with (6 X = C1). The transformation illustrates... 

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