Heptafulvenes 8-substituted

Krygowski, T. M., Ciesielski, A., and Cyrahski, M. 1995. Aromatic character and energy of the five- and seven-membered rings in derivatives of penta- and heptafulvene substituted in exocyclic position. Chem. Pap. 49 128-132. 

Furthermore, the methyl-substituted triafulvenes 222 underwent Diels-Alder addition to 2-pyrone55 giving rise to heptafulvenes (510) by elimination of carbon dioxide. Extension of this reaction to other triafulvenes was unseccessful. 

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,... 

Suitably substituted tropylium salts offer an alternative to condensation with active methylene compounds (cf. Section IV,A,4,c) even if the application of tropones fails. Thus, 6-chloro derivative 266c (82CB3756) or thioether 605 (Scheme 163 91KGS1432) were used to get, respectively, sesquifulvalenes and heptafulvene 606. O-Functionalized compounds are likewise applicable (Scheme 110 cf. 73CRV293, p. 308). 

Heptafulvenes assume a push-pull character when the exocyclic carbon atom (C8) is substituted by acceptor groups90. Daub and coworkers91 have studied this type of compound with dimethylamino groups and other donor groups on C8. NMR spectra of the bis(dimethylamino) compound 41a show that the compound is boat-shaped with alternating single and double bonds, and a barrier to rotation about the exocyclic C=C bond >27 kcal mol-1 is reported for the 8-dimethylamino-8-triethylsilyloxy compound 41b. No low-temperature spectra are reported, but the C—N barrier is likely to be low. 

The trithione -methides (3-methylene-1,2-dithioles) are iso-77-electronic with the heptafulvenes,71 and possess a basicity comparable to that of the latter. Some true 3-alkylatSd 1,2-dithiolium salts can be obtained by protonation of compounds 52. This method of preparation is rather narrowly limited, however, by the fact that the simple derivatives of 52 (as in the hydrocarbon series) are evidently very unstable and have not as yet been described. These dithiafulvenes become stable, easily handled compounds only when they contain aryl or typical acceptor residues (R and/or R = CN, C02R) in the 6-position.53, 72 This substitution, however, again as in the hydrocarbon series,73 lowers the basicity to such an extent that the dinitrile (52) (Rj = R2 = H, R = R = CN), for example, is not appreciably protonated even in pure trifluoroacetic acid.52... 

Daub and colleagues studied the [8 + 2] cycloaddition reactions of electron-rich 8-substituted heptafulvenes with a wide variety of acceptor substituted alkenes. 8-Methoxyheptafulvene (534) proved to give the best results, the more electron-rich heptafulvenes being less reactive toward [8 -b 2] cycloaddition reactions and more prone to oxidative dimerization . The reactions of 8-methoxyheptafulvene with acceptor substituted polyenophiles 535 can in principle produce up to 8 diastereomers. The reactions proved, however, highly regioselective, the exo and site selectivities being moderate to good, and afforded mixtures of 536, 537 and 538 (equation 155, Table 31). ... 

The attack of the acid (154) on the readily polarizable 1,2-dithiafulvene (155) corresponds to the extremely ready addition of electrophilic reagents to the simple and vinylogous heptafulvene derivatives, which are iso-n-electronic with 155. The opening of the dithiole rings in 156 and 158 under the pressure of the carbanionoid electron pair liberated by the proton abstraction and of the free electron pair on the sulfur, as well as the elimination of elementary sulfur and the intramolecular electrophilic attack of the mercaptide ion (157) on the 5-position to form 158, are simply the typical reactions of 1,2-dithioles that have already been discussed (Section II, B, 3). The reactivity of the 3-methyl group in 154 finds many parallels in the ease of condensation of the methyl-substituted pyridinium, pyrylium, thiopyrylium, and tropylium salts, and particularly... 

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. ... 

Because the five-membered ring is a substituted cyclopentadienide anion in some dipolar resonance structures, it might be expected that exocyclic groups that could strongly stabilize a positive charge might lead to a large contribution from dipolar structures and enhanced stability. The heptafulvene (8) and triafulvene (9)... 

Substituted heptafulvenes react with DDQ to give 3-substituted 1,2-dicyano-azulenes. 7-Oxoheptafulvene oligomerizes to give the substituted phenyl azulene (181) in solution at room temperature and undergoes [2 + 2] cycloaddition with olefins,e.p. (182) is obtained using ds-cyclo-octene. ... 

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. 

A series of 7-substituted norbornadienes have been synthesized by quenching the 7-norbornadienyl cation with various nucleophiles of sulphur (SMOj), arsenic (AsPhj), and antimony (SbPhg). 7-t-Butylnorbornadiene, prepared by reaction of t-butyl-lithium with 7-t-butoxynorbornadiene, has been transformed to, inter alia, (128) and (129). By a similar route, (20) and related compounds, required for study of their photoelectron spectra, were prepared (Scheme 5). Propenylidene-norbornadienes were accessible by the routes shown in Scheme 6. They provide a novel photochemical synthesis of dihydro-azulenes, azulenes, and of vinylogous heptafulvenes. 

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