Fulvenes heptafulvenes

Due to the high conformational demands which are imposed on higher-order cycloadditions, fulvenes, heptafulvenes and tropones have been mostly applied in uncatalyzed [6 + 4] cycloadditions. The scope of metal-promoted cycloadditions, however, is much broader due to the preorganized orientation of the reactants which are both co-ordinated to the metal center. 

There are four classical nonaltemant jr-electron hydrocarbons which have long been the subject of many studies of their aromatic character fulvene, heptafulvene, azulene, and pentalene. " Eulvene, heptafulvene, and pentalene have been considered for a long time as nonaromatic (or even antiaromatic), both for their chemical Instability and the negative REPE values. More recent studies strongly support this hypothesis. The X-ray determined geometry of heptafulvene" allowed us to estimate the aromaticity the HOMA value for the ring is equal to 0.257. The case of fulvene is more complicated—only geometries for the substituted species are known. The mean HOMA value calculated for 11 exocyclically substituted fulvenes is 0.001. ... 

Of the fundamental nonalternant hydrocarbons, only two prototypes were known about fifteen years ago azulene (XI, Fig. 5), the molecular structure of which was determined by Pfau and Plattner and fulvene (XIX) synthesized by Thiec and Wiemann. Early in the 1960 s many other interesting prototypes have come to be synthesized. Doering succeeded in synthesizing heptafulvene (XX) fulvalene (XXI) and heptafulvalene (XXIII). Prinzbach and Rosswog reported the synthesis of sesquifulvalene (XXII). Preparation of a condensed bicyclic nonalternant hydrocarbon, heptalene (VII), was reported by Dauben and Bertelli . On the other hand, its 5-membered analogue, pentalene (I), has remained, up to the present, unvanquished to many attempts made by synthetic chemists. Very recently, de Mayo and his associates have succeeded in synthesizing its closest derivative, 1-methylpentalene. It is added in this connection that dimethyl derivatives of condensed tricyclic nonaltemant hydrocarbons composed of 5- and 7-membered rings (XIV and XV), known as Hafner s hydrocarbons, were synthesized by Hafner and Schneider already in 1958. 

In contrast to the usual (4 + 2) diene reactivity of fulvenes 3-amino fulvenes 539 in some cases are capable of expanding the five-membered ring to heptafulvenes 541 by addition of acetylene dicarboxylate in a (2 + 2) fashion299. ... 

Liu and colleagues295,296 studied the cycloaddition reactions between electron-deficient 8,8-disubstituted heptafulvenes 466 and electron-rich 6,6-disubstituted fulvenes. The substituted heptafulvene reacted as the trienophile in this case. Only when 6,6-dimethylfulvene (465) and heptafulvenes 466a-b were used as the triene and trienophiles, respectively,... 

Figure 7. Illustration of the Huckel rule for the fulvene and heptafulvene derivatives. (Reprinted with permission from ref 65. Copyright 1995 Slovak Academy of Sciences.)...

Heptafulvenes (XLII) are analogues of fulvenes, but in this case the exocyclic group would carry a negative charge if there were any contribution from a dipolar structure such as (XLIII) consequent upon development of tropylium ion character in the seven-membered ring. 

The intramolecular 8 -I- 2-cycloaddition of heptafulvene-fulvene (199) produced the intermediate adduct (200) which converted into the product (201) by 1,5-sigmatropic shifts in 46% yield (Scheme 78). 2-Oxo-27/-cyclohepta[ ]furan derivatives (202) undergo 8 -I- 2-cycloaddition with dienes to yield cycloadducts (203) in high yields (Scheme 79) ° Ab initio calculations on the cycloaddition of tropone with maleic anhydride favour the 4 - - 2-endo pathway while calculations of tropothione with maleic anhydride favour the 8 -I- 2-endo pathway. ... 

Stepien, B. T., Cyranski, M. K., Krygowski, T. M. (2001). Aromaticily strongly affected by substituents in fulvene and heptafulvene as a new method of estimating the resonance effect. Chem. Phys. Lett. 350,537-542. 

Figure 6.1 Pentafulvene (fulvene) 1, triafulvene 2, heptafulvene 3 and their polar structures.

Fulvenes, cyclic molecules with odd number of carbon atoms in the ring, represent one of the fundamental categories of cross-conjugated systems (see Chapters 1 and 6). According to the ring size of these molecules, the compounds are classified astriafulvenes 1, pentafulvenes 2, heptafulvenes 3, and so on (Scheme 7.1). Owing to their dipole moments, fulvenes possess unique reactivity, different from that of open-chain polyolefins and aromatic compounds. In 1989, Neuenschwander... 

In addition, the same group reported on a synthesis of heptafulvenes and related seven-membered ring systems (e.g., 363) from triafulvene 357 (Scheme 7.80) [91]. A series of donor-substituted 1,3-dienes, when reacted with tria-fulvenes by a Diels-Alder addition, yielded bicyclic methylene cyclopropanes, which were subsequently transformed to 1,6-diphenyl-substituted heptafulvenes or seven-membered ring derivatives. 

Although pentafulvenes are the by far most common fulvenes, there are some publications describing the synthesis of heptafulvene complexes. The reaction of (cycloheptatrienyl-7-methyl) phenylsulfonate (92) with sodium [cyclopentadi-enyldicarbonylferrate(II)] afforded complex 93 in 45% yield, from which hydride was abstracted quantitatively by treatment with triphenylcarbenium ions. The heptafulvene complex formed was described by resonance formulas 94 and 95, the latter with an intact aromatic cycloheptatrienylium substructure (Scheme 10.32) [84, 85]. 

---