Fulvenes structure

Fulvene seems to have become a benchmark molecule for the study of conical intersections. " In recent work we have been able to optimize five geometries on an extended conical intersection. But this is a 6 orbitals with 6 electrons problem, so one might expect that the photochemistry and the conical intersections could be deduced from the previous discussion. In fact it can The two fulvene structures are A (as for benzene Figure 3.5) for the excited state and a structure not too dissimilar to prefulvene for the excited state shown in Figure 3.20 along with the branching-space vectors, which are similar to a combination of the vectors shown in Figure 3.18 for benzene itself... 

The strongly colored system of double conjugated bonds from the fulvenic structure can be hydrogenated, but the color reappears over time. That is why it is preferred that the initial unsaturation in indene is diminished. 

Other HR modifying methods and agents are known. Thus, aromatic HRs can react with aldehydes due to the reactive CHj groups they contain. Colored resins with fulvenic structure are formed [189). Aldehyde treated inden-couma-ronic fractions, in the presence of acid or basic catalysts, can form mixed resins wherein inden-coumaronic structures coexist with sequences specific for hydrocarbon-formaldehydic resins [253]. 

In a photochemical experiment, irradiation of benzene leads to Sj, which connects to the ground-state surface via the conical intersection shown. Benzene, the much more stable species, is expected to be recovered preferentially, but the prebenzvalene structure which hansfomis to benzvalene is also fomied. Another possible route from the prebenzvalene, along a different coordinate, will lead to fulvene [90, p.357] after a hydrogen-atom transfer from... 

Julg, A., and Pullman, A., J. Chim. Phys. 50, 459, Structure 61ectronique du fulvene Introduction de l interaction des configurations."... 

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. 

The results of our calculations based on both the static and dynamic theories show that most of the nonbenzenoid cyclic conjugated systems examined exhibit in a greater or lesser degree a marked double-bond fixation. The static theory indicates that even in benzene there exists a hidden tendency to distort into a skewed structure and that such a tendency is actually realized in [4n-f-2] annulenes larger than a certain critical size. In nonalternant hydrocarbons bond distortion is a rather common phenomenon. Fulvenes, fulvalenes and certain peri-condensed nonalternant hydrocarbons undergo a first-order bond distortion, and... 

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]. 

The ferrocenyldiphynylpropargyl cation, 77, has an intrinsic delocalization nature exhibiting a valence tautomerization band at 856 nm, and its nucleophilic trapping reactions give rise to the formation of ferrocenyldiphyenylallenes (173). The bis(acetylide) mixed-valence complexes of ferrocene and the Ru complex moiety, 78, also behave as a fulvene-cumulene structure, 79, showing a u(M=C = C—C) band at 1985 cm-1 (174). Related alleylidene and cumulenylidene complexes of transition metals have been reviewed by Bruce (175). 

When 4,4-diacyl triafulvenes are reacted, in addition to the deeply colored fulvenes, colorless isomers were also obtained to which structure 540 was assigned. 

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]. 

The parent fulvene, 5-methylene-1,3-cyclopentadiene, was the subject of numerous calculations and conformational considerations. Both structures derived from microwave... 

The r2 for this line is 0.9995 with a standard deviation of ca 0.9 kJmol 1 (Figure 1). The deviation for R1 = R2 = Ph (129) is some 43 k,I mol 1 below the line. If the exo-methylene/ring bond is quite polar and resonance structures 128 are significant (130 is easily ignorable), then AHf (127, R1, R2) and A//f(0=CR1R2) would be more likely to be linearly related. Another nearly perfect line, that of carbonyls vs fulvenes, can be drawn through the (H,H), (H,Me) and (Me,Me) points (equation 36). 

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... 

The timescale of a microwave observation is ca 10 12s so that an average of the properties of the species in equilibrium (35) is obtained if the equilibrium occurs in a time shorter than this. The X-ray photoelectron spectra of intramolecularly hydrogen-bonded species in the gas phase have been studied in an attempt to obtain an instantaneous picture of the structure of these molecules. In this technique the ionisation of core electrons which occurs within 10 16s is observed. For malondialdehyde, 6-hydroxy-2-formyl-fulvene, 2-hydroxy-1,1,1,5,5,5-hexafluoropent-2-ene-4-one, 9-hydroxyphen-alenone [19], and tropolone [20], two peaks are observed in the Ou region of the photoelectron spectrum (Brown et al., 1979). If these molecules existed in the C2v form with a symmetrical hydrogen bond and equivalent oxygen... 

Alkenyl Fischer carbene complexes can serve as three-carbon components in the [6 + 3]-reactions of vinylchro-mium carbenes and fulvenes (Equations (23)—(25)), providing rapid access to indanone and indene structures.132 This reaction tolerates substitution of the fulvene, but the carbene complex requires extended conjugation to a carbonyl or aromatic ring. This reaction is proposed to be initiated by 1,2-addition of the electron-rich fulvene to the chromium carbene followed by a 1,2-shift of the chromium with simultaneous ring closure. Reductive elimination of the chromium metal and elimination/isomerization gives the products (Scheme 41). 

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). 

Hence, the anion-radical of diphenyl fulvene acquires a spin-charge distribution dictated by steric shielding at Ph2C node and six-n-electron delocalization in the C5H5 ring. Anion-radicals of sterically congested stilbenes represent examples that are quite different, but at the same time are similar in principle. Let us compare the E structures of stilbene and its congested derivative, namely, a,p-di(tert-butyl)stilbene in their neutral and anion-radical forms (Scheme 3.18). 

Dilithium compound 210, obtained from the lithiation of hexasilylfulvene 209 in THF at room temperature (Scheme 72), represents the first X-ray structural analysis of a dUithi-ated fulvene derivative. In the solid state, one lithium centre is capping the central five-membered ring, while the second lithium centre is located at the exocyclic carbon atom of the fulvene unit (Figure 27) . 

The single-crystal structure of 31 clearly reflects the presence of an integral fulvene-type Jt-system on the spherical surface. The average bond length for the [5,6]-bond between C1-C2 and C3-C4 is 1.375 A, which is considerably shorter than a typical [5,6]-bond in CgQ. In contrast, the bond between C2 and C3 (1.488 A) is notably... 

Cyclopentyl isoxazolidine cycloadduct 324 was prepared by intramolecular nitrone cycloaddition by Baldwin et al. (280,281,352,353) as part of studies toward a total synthesis of pretazettine (Scheme 1.69). Related adducts have been prepared elsewhere (354—356) including fluorine-substituted carbocycles (357) and the adducts prepared by lOAC by Shipman and co-workers (333,334) who demonstrated their potential as a route to aminocyclopentitols (Scheme 1.66, Section 1.11.2). Such bicyclic structures have been prepared in rather unique intermolecular fashion by Chandrasekhar and co-workers (357a) from the cycloaddition of C,N-diphenyl nitrone to fulvene (325). 

The substituted cyclopentadienylmagnesium compounds 62a and 62b have been prepared from the corresponding fulvenes (equation 8) and were structurahy characterized in the solid state by X-ray crystallography . The structures are, as expected, (tj -bonded cyclopentadienyl groups) for bis(cyclopentadienyl)magnesium compounds. 

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