Heptafulvenes

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. 

The cyanosubstituted diphenylcyclopropenyl heptafulvenylium cation 144 (R = CN) was recently synthesized by Kitahara110 from 8-cyano heptafulvene and ethoxy cyclopropenium cation 75 and its structure has been proven by X-ray analysis111. ... 

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. 

A very elegant formation of heptafulvenes from triafulvenes was found by Gompper294 utilizing the push-pull stabilized cyclobutadiene 511 in the case of 1,2-diphenyl-4,4-diacetyl triafulvene (180) the 4-acyl group caused additional ring closure yielding cyclohepta(b)furan 514 ... 

The heptafulvenes 512/513 were believed to originate via dipolar (2 + 2) cycloaddition of a cyclobutadiene to triafulvene CVC2 bond according to the following scheme ... 

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. ... 

FIGURE 5. Representative graphs Q for an independent electron model based on Unear combinations of two-centre ir-orbitals nIL. Examples 1,3-butadiene 2, f3 )-hexa-1,3,5-triene 161, heptafulvene and... 

C1 and C6 chemical shifts were not well modelled by equation 105. The 1 and 6, 2 and 5, and 3 and 4 positions of heptafulvene are equivalent to each other when groups in position 8 are the same. Results for these positions are comparable, as expected in view of the fact that for seven of the twelve compounds in the data set X1 = X2. The cyclic 8,8 substituents CO(CH2)4CO and (CH2)4 were assumed equivalent to two Ac and two Et groups, respectively, in the parameterization of the substituents. 

Hitoshi Takeshita, Akira Mori, and Kanji Kubo 210 SYNTHESIS OF 8,8-DICYANO-HEPTAFULVENE FROM CYCLOHEPTATRIENYLIUM TETRAFLUOROBORATE AND BROMOMALONONITRILE... 

The 13C NMR data for pentafulvene (1) and heptafulvene (2) (Table 3) and for 6,6-dimethylpentafulvene (5) and sesquifulvalene (6), afford evidence of the extent to which polar structures of the types 5a and 6a contribute to the ground state. If the chemical shifts are analyzed on the basis of electron density, these hydrocarbons are to be considered as olefinic systems with only a small contribution (10% at most) from the polar structures 5a and 6a. 

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. 

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,... 

The regioselectivity observed was in agreement with the calculated orbital coefficients for the HOMO of heptafulvene 534 and the LUMOs of the polyenophiles. The largest coefficient in the HOMO of 534 is at C(8). The reactions of nitroethene and (E)-fi-nitrostyrene with 533 (entries 4 and 5) afforded merely exo adducts, the two isomers arising from attack of the polyenophile at the two different sites of 534. 

Stable heptafulvenes (53) with strong acceptor groups in the exocyclic position... 

The heptafulvene 103 seems to deviate from this scheme, since it has R(Ct=C2) = 142.2 pm and probably AG a 25 kcal/mol (cf. 54). It is a planar molecule, and it is also unusual in its response to steric effects. While all other systems in Table 19 avoid undue steric interactions between donors and acceptors... 

Diphenylketene undergoes 2 + 2- or 2 -E 4-cycloaddition reactions with various 1,3-diazabuta-1,3-dienes. The 2 - - 4-, 4 -E 2-, 6 -E 4- and 8 -E 2-cycloaddition reactions of heptafulvenes have been reviewed. ... 

Hunt s group (50, 51) have pioneered the application of the Cl source to organometallics such as the iron tricarbonyl complex of heptafulvene, whose electron impact spectrum shows (M—CO)+ as the heaviest ion, in contrast to the methane Cl spectrum with the ion as base peak. Boron hydrides (52) and borazine (53) have also been studied. The methane Cl spectrum of arenechromium and -molybdenum (54) show protonation at the metal giving a protonated parent or molecular ion. Risby et al. have studied the isobutane Cl mass spectra of lanthanide 2,2,6,6-tetramethylheptane-3,5-dionates[Ln(thd)3] (55) and 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-oetanedione [H(fod)] lanthanide complexes (56). These latter complexes have been suggested as a means of analysis for the lanthanide elements. 

While electrophilic reactions of alkylidenecycloproparenes are generally rapid, the nucleophilic pathways require more vigorous conditions. The only nucleophilic addition of cycloproparenes known is that of /-butoxide across the bridge bond of 237,240 and 241 which leads to the corresponding heptafulvenes 407. ... 

Prompted by W. M. Jones work on the formation of heptafulvene from phenyl-carbene in the gas phase,which implied a seven-membered ring intermediate, and by the implications of the possible reversibility of the process, Baron et al. rediscovered Vander Stouw s cryptic work and showed that all three possible tolyl-carbenes gave benzocyclobutene and styrene, albeit in different ratio from the ortho isomer than from the meta or para species.Baron et al. ° proposed a mechanism in which a methylcycloheptatrienylidene (CH3-7I) was the active seven-membered species, and which additionally feamred the intermediacy of methylbicyclo[4.1.0]-heptatrienes 73 and 73 (Scheme 7.35). 

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