Of fulvenes

Irradiation of benzene and certain of its derivatives results in bond reorganization and formation of nonaromatic products. Irradiation of liquid benzene with light of 254-nm wavelength results in the accumulation of fulvene and a very small amount of tricy-clo[3.1.0.0 ]hex-3-ene, also known as benzvalene. The maximum conversion to this product in liquid benzene is about 0.05%. The key intermediate is believed to be a biradical formed by 1,3-bonding. 

Tlie name fulvaleiie was first mentioned by R. Brown (49TFS296), who expanded the class of fulvenes for those compounds containing two cyclic poly-enic systems with a central double bond. Tlius, depending on the ring size, cy-clopropylidenecyclopropene 1, fulvaleiie 2, heptafulvalene 3 and the unsymmetrical hybrid molecules triapentafulvalene (calicene) 4, triaheptaful-valene 5, and pentaheptafulvalene (sesquifulvalene) 6 are members of this class of cyclic cross-conjugated systems (Scheme 1). 

The cycloprop anation of fulvenes has been effected with alkynylcarbene complexes (see Sect. 2.1.1). However, this reaction is inhibited in the presence of CO and under these conditions a formal [4S+3C] cycloadduct is formed [15a]... 

Scheme 70). The formation of these products likely involves two key steps (i) 1,2-addition of fulvene to the carbene carbon and (ii) regioselective cycli-sation promoted by [l,2]-W(CO)5 shift. 

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. 

Parallel to this use of relatively simple approximations of the molecular orbital theory to the study of complex molecules Berthier has investigated the possible utilization of more refined molecular orbital procedures in the study of necessarily smaller molecules. We owe him the first application of the SCF method to the study of fulvene and azulene and also a pioneering extension, presented in 1953, of the SCF method to the study of molecules with incomplete electronic shells. 

SCHEME 2.19 Generation of fulvene from the decarbonylation of o-QM, via two different pathways. Enthalpies of formation for the intermediates and activation enthalpies (data above the arrows) are reported in kcal/mol (data have been taken from Ref. [23]). 

Wheeless, C. J. M., Zhou, X., Liu, R., 1995, Density Functional Theory Study of Vibrational Spectra. 2. Assignment of Fundamental Vibrational Frequencies of Fulvene , J. Phys. Chem., 99, 12488. 

Eq. (4.4 a) is satisfied in the position 1 of azulene. Eq. (4.4b) is valid in position 6 of fulvene, position 6 of azulene, position 3 of fluoranthene, and position 5 of acenaphthylene. Even in a few exceptional cases where the previous relations do not hold, a consideration of the coulombic effect of attacking reagents leads to a conclusion favorable to the hypothesis of frontier density growth. An example of such cases is position 3 of... 

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. ... 

Hong and coworkers have investigated the cycloaddition chemistry of fulvenes with a wide variety of alkenes and alkynes in great detail [191]. As one example, the reaction of 6,6-dimethylfulvene with benzoquinone is shown in Scheme 6.92. Under microwave conditions in dimethyl sulfoxide (DMSO) at 120 °C, an unusual hetero-[2+3] adduct was formed in 60% yield, the structure of which was determined by X-ray crystallography. The adduct is a structural analogue of the natural products aplysin and pannellin and differs completely from the reported thermal (benzene, 80 °C) Diels-Alder cycloaddition product of the fulvene and benzoquinone (Scheme 6.92) [191]. 

Scheme 6.92 Thermal versus microwave-assisted Diels-Alder cycloaddition reactions of fulvenes with benzoquinones.

Scheme6.241 Azadiene Diels—Alder cycloaddition of fulvenes.

A general hetero-Diels-Alder cycloaddition of fulvenes with azadienes to furnish tetrahydro-[l]pyrindines has been described by Hong and coworkers (Scheme 6.241 see also Scheme 6.92) [424]. A solution of the azadiene and fulvene (1.2 equivalents) precursors in chlorobenzene was heated under open-vessel microwave irradiation for 30 min at 125 °C to provide the target compounds in excellent yields and with exclusive regio- and diastereoselectivity. Performing the reactions under conventional conditions or under microwave irradiation in different solvents provided significantly reduced yields. 

See, for example, R. S. Hosmane and J. F. Liebman, Tetrahedron Lett., 33, 2303 (1992). We additionally note that in the absence of any conjugative interaction, the difference of the enthalpies of formation of fulvene (vide infra) and benzene would very nearly equal the difference of the enthalpies of formation of methylenecyclopentane and cyclohexene. The former difference is 161 kJ mol-1 while the latter difference is but 17 kJ mol-1. 

The desired enthalpy of formation of fulvene and of its 6-methyl derivative were determined by Roth by measurement of the appropriate enthalpy of hydrogenation. The facile polymerization of this compound precludes conventional bomb calorimetry. 

These phenomena can be illustrated by the cycloaddition reactions of fulvenes with electron-deficient a-pyrones. In general, the Diels-Alder reactions of electron-deficient dienes such as 458 with 6-alkyl substituted fulvenes favor addition across one of the endocyclic... 

FIGURE 8. Relative frontier orbital coefficients of fulvene... 

Niggli and Neuenschwander294 studied the reaction of fulvene (461) with cyclopen-tadiene. The main product fraction consisted of three 1 1 adducts, as illustrated in equation 138. Diels-Alder Adducts 462 and 463 resulted from attack of cyclopentadiene at the endocyclic and exocyclic double bonds of fulvene, respectively. The formation of 464 was rationalized by a [6 + 4] cycloaddition reaction followed by two [1,5] hydrogen shifts. It was stated that due to the absence of electron-donating and electron-withdrawing groups on both triene and diene, fulvene may have reacted via its HOMO as well as its LUMO. 

Kato et al. (119) explored reactions of fulvenes with a variety of mesoionic heterocycles. Unfortunately, reactions of miinchnone 38 with several fulvenes afforded complex mixtures in each case, and no identifiable products were reported, although Friedrichsen and co-workers (120-122) previously reported the reaction between mtinchnones and fulvenes to give cycloadducts. Kato et al. (123) also studied the cycloaddition reactions of tropone with several mesoionic heterocycles. Despite heroic efforts, the reaction of tropone with miinchnone 38 was complex and could not be unraveled. However, as described later, the reaction of tropone with isomtlnchnones was successful. Wu et al. (124) effected the cycloaddition between a miinchnone and fullerene-60 (Ceo) to give the corresponding dihydropyrrole in excellent yield. 

The preparation of fulvene enamines by the reaction of sodium cyclopent-adienide with dimethyl sulfate complexes of amides or lactams has recently been reported by several investigators [77, 78] (Eq. 22). 

Direct reaction of fulvenes with metal atoms appears to afford a useful synthetic route to alkyl-bridged metallocenes (122), e.g.,... 

A less usual method for the generation of starting dienones is thermal rearrangement of fulvene-l,2-dioxetanes 139a,b, leading to 2//-pyrans (140a,b).182... 

The formation of fulvene (450) by sodium borohydride reduction of 2-formyl-4//-pyran (371) in water347 may be explained by the mechanism shown in Scheme 21. 

The photochemical formation of fulvene from benzene in the region 2400-2700 A has been reported previously, particularly in the liquid phase.51... 

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