Other Fulvenes

Several fulvene derivatives bearing a macrocyclic ring system were synthesized by lyoda s and Ojima s groups in the 1980s [301, 302]. During the past two decades. 

(a) De Meijere, A. (ed) (1998) Carbon Rich Compounds I, Topic in Current Chemistry, vol. 196, Springer, Berlin  

Dumele, O., Zalibera, M., Confortin, D., Cias, P., Jayamurugan, 

(a) Brisset, H., Thobie-Gautier, C., Jubault, M., Gorgues, A., and Roncali, 

Harada, Y, Nakanishi, J., Fujihara, FI., Tobisu, M., Fukumoto, Y., and Chatani, 

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This division is not possible for other hydrocarbons such as fulvene and hepta-fulvene (Figure 7.5). 

The unconventional structure of fulvenes with a unique C=C bond conjugation leads to unusual cycloaddition reactions with other unsaturated systems. For example, alkenylcarbene complexes react with fulvenes leading to indanone or indene derivatives which can be considered as derived from a [6S+3C] cycloaddition process [118] (Scheme 72). The reaction pathway is well explained by an initial 1,2-addition of the fulvene to the carbene carbon followed by [1,2]-Cr(CO)5-promoted cyclisation. 

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. 

On the other hand, in cata-condensed nonalternant hydrocarbons IV, VI, X and XI, peri-condensed nonalternant hydrocarbons XIV — XVIII, fulvenes XIX and XX, and fulvalenes XXI—XXIII, self-consistency was achieved only for the fully-symmetrical nuclear arrangement. All these molecule, except azulene pCl), also show in a greater or lesser degree a pronounced double-bond fixation. 

Due to the pronounced electron donating character of ferrocene, ot-ferrocenyl carbocations 3 possess a remarkable stability and can therefore be isolated as salts [16]. They can also be described by a fulvene-type resonance structure 3 (Fig. 4) in which the Fe center and the ot-center are significantly shifted toward each other as revealed by crystal stmcture analysis, indicating a bonding interaction [17]. 

Cyclopentadienyl radical can combine with methyl radicals to form fulvene, along with a subsequent loss of H atom. Fulvene may also be formed by reactions of the iso isomer of C4H5 with aeetylene. Once fulvene adds an H atom, subsequent rearrangement to benzene has been shown to be thermodynamically favorable using quantum chemieal methods.Cyelopentadienyl radicals may also combine with eaeh other and then rearrange to form naphthalene. 

Two equiv. of 6,6-di(cyclopropyl)fulvene react at 60 °C over a period of a week with Ca[N(SiMe3)2]2-(THF)2 bis in THF to yield the metallocene 170. The heteroleptic amido complex 171 is detected as an intermediate with 111 and 13C 1H NMR spectroscopy. A 1 1 reaction of the calcium amide and 170 also produces 171 in solution, an equilibrium involving these three derivatives exists (Equation (30)). The calcocene 170 crystallizes at — 20 °C from THF as colorless cuboids. The metal center is surrounded by the four ligands in a distorted tetrahedral manner, and the cyclopentadienyl group and the propylidene fragment are coplanar with each other.393... 

Dimethyl anthracene and diphenyl isobenzofuran form remarkably stable233 cyclopropanone derivatives (353/354), whilst with other diene components (butadiene, tetracyclone, and fulvene) the primarily formed Diels-Alder adducts either suffer ketalizing attack of the solvent (356 - 357, 359 - 358/360) or undergo irreversible changes such as decarbonylation to 362 or rearrangement to 355. 

From the enthalpies of formation from Roth for the fulvenes and from Pedley for the other hydrocarbons in equations 37 and 38, we find the former reaction is exothermic by 12 kJmol-1 while the latter is endothermic by 12 kJmol-1. Ionic resonance structures analogous to 128 are expected to be of less importance for the ring alkylated species than for the parent species 103 negatively charged carbon is destabilized by adjacent... 

Stewart clearly stated the dilemma for organic chemists at the turn of the century. Through laboratory experience, they had learned to interpret the chemical bond in different ways. Although they might draw double bonds in formulas for diphenyl-ethylene, ethylene, and fulvene, chemists did not really take the bonds to resemble each other chemically. Chemists "knew" that there is an increase in unsaturation, or reactivity, of the double bond toward bromine or oxygen, from one of these compounds to the next. They "knew" that a bond must be looked at not as a fixed unit but as a sum of an infinite number of small forces or partial valences. This is what Polanyi later called tacit knowl-... 

Similar sequences starting with appropriate 1,2,3,4-tetrahaloarenes (1,3-diaryne equivalents) yield hindered phenanthrenes.9 Cyclopentadienes,7 fulvenes,7 isoindoles,7 anthracenes10 and other dienes11 have also been used as diaryne traps. 

Other structural motifs for designing molecular glasses have been exploited (Fig. 3.16). Some of them are based on five-membered rings as the central part of the molecule, like 1,2,4-triazoles (48) [94] or the electron-transporting material NAPOXA (49) [95]. Pentaphenylcyclopentane (50) exhibits a Tg of 57°C [27]. Braun et al. [96] synthesized some fulvenes with Tg ranging from 74 to 120°C (51). 

Titanium dioxide suspended in an aqueous solution and irradiated with UV light X = 365 nm) converted benzene to carbon dioxide at a significant rate (Matthews, 1986). Irradiation of benzene in an aqueous solution yields mucondialdehyde. Photolysis of benzene vapor at 1849-2000 A yields ethylene, hydrogen, methane, ethane, toluene, and a polymer resembling cuprene. Other photolysis products reported under different conditions include fulvene, acetylene, substituted trienes (Howard, 1990), phenol, 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2,6-dinitro-phenol, nitrobenzene, formic acid, and peroxyacetyl nitrate (Calvert and Pitts, 1966). Under atmospheric conditions, the gas-phase reaction with OH radicals and nitrogen oxides resulted in the formation of phenol and nitrobenzene (Atkinson, 1990). Schwarz and Wasik (1976) reported a fluorescence quantum yield of 5.3 x 10" for benzene in water. 

Two additional synthetic routes to ( )-j8-vetivone (350) have been developed. In one of these a suitably substituted spirocyclic system [cf. (349)] is constructed by addition of Me2CuLi to the fulvene derivative (348)/ Subsequent functional group modification (cf. Scheme 32) provides ( )-j8 -vetivone (350). In the other total synthesis d the well-known intramolecular alkylation of para-substituted phenols has been used to produce a spirocyclic intermediate (353) which can be converted into ( )-/3-vetivone (350) (cf. Scheme 33). 

Kato et al. (151,152) explored the chemistry of 2-ferf-butylMvenes with isomiinchnones, as well as with several other mesoionic compounds, in a novel approach to pseudo-hetero-azulenes. Thus, isomtinchnone 51a, generated as before in situ from A-benzoylphenylglyoxyanilide 253 with triethylphosphite, reacts with 2-ferf-butyl-6-(dimethylamino)fulvene to give the [47i+6ti] adduct diphenylcyclo-penta[c]pyran in low yield. Likewise, reaction of 51a with dimethylfulvene gave a mixture of two adducts, one of which arises from a [47i+2ti] cycloaddition. 

Although valence isomerization reactions of aromatic compounds have found little by the way of practical application, they are a fascinating area for mechanistic and theoretical study. The details are not completely dear, but it seems that, for benzene itself, benzvalene arises from the lowest excited singlet state, perhaps by way of a biradical intermediate (3.32) that could also be a precursor to fulvene bicyclohexadiene is probably produced from the second excited singlet state. For some other aromatic compounds the electronic nature of 5, and S2 may be reversed, or at least the states are much closer in energy, so that the preference for benzvalene or bicyclohexadiene formation under conditions of long-wavelength irradiation can be rationalized. 

Cyclopentadiene triphenylarsorane reacted withp-nitrobenzaldehyde in chloroform to give triphenylarsine (10% yield), triphenylarsine oxide (80% yield), and two other unidentified products (24). The corresponding 2,3,4-triphenyl- or tetraphenylcyclopentadienylide under similar conditions afforded triphenylarsine oxide and the corresponding substituted fulvene (25, 68). 

Various cross-conjugated enediynes undergo Bergman-type cycloaromatizations upon reduction with potassium metal, generating anions of fulvenes and fulvalene derivatives (Scheme l).6 Not all cross-conjugated enediynes yield cyclized dianions upon reduction some give uncyclized, Y-shaped, cross-conjugated dianions, whereas others apparently yield radical anions that either dimerize or persist as monomers. 

In summary, a variety of interesting bifunctional radical cation structures have been established. At least one species (152) features an orthogonal arrangement of spin and charge, whereas another (155) clearly meets the definition of a distonic radical cation. The fulvene radical cations (154), on the other hand, are bifunctional and may deviate to some extent from planarity however, they appear to be ordinary olefin radical cations rather than perpendicular ones. 

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