Aromatic compounds fullerenes

Now and then, projectiles from outer space cause excitement and surprises, as in January 2000, when a meteorite impacted the frozen surface of Lake Targish in Canada. It was a new type of C-chondrite with a carbon concentration of 4-5%, and probably came from a D-type asteroid (Hiroi et al., 2001). More exact analysis of the Targish meteorite showed the presence of a series of mono- and dicarboxylic acids as well as aliphatic and aromatic hydrocarbons (Pizzarello et al., 2001). Aromatic compounds and fullerenes were detected in the insoluble fraction from the extraction this contained planetary helium and argon, i.e., the 3He/36Ar ratio was... 

Cycloaddition with nitrile oxides occur with compounds of practically any type with a C=C bond alkenes and cycloalkenes, their functional derivatives, dienes and trienes with isolated, conjugated or cumulated double bonds, some aromatic compounds, unsaturated and aromatic heterocycles, and fullerenes. The content of this subsection is classified according to the mentioned types of dipolarophiles. Problems of relative reactivities of dienophiles and dipoles, regio- and stereoselectivity of nitrile oxide cycloadditions were considered in detail by Jaeger and... 

Corannulene is the simplest example within the class of polynuclear aromatic compounds with curved surfaces that are related by carbon framework to the i fullerenes. These aromatic hydrocarbons possess bowl-shaped geometries, consist... 

Fullerenes, among which the representative and most abundant is the 4 symmetrical Cgg with 30 double bonds and 60 single bonds, are known to behave as electron-deficient polyenes rather than aromatic compounds [7]. The energy level of the triply degenerate LUMO of Cgg is almost as low as those of p-benzoquinone or tetracyanoethylene. Thus, a wide variety of reactions have been reported for Cgg such as nucleophilic addition, [4-1-2] cycloaddition, 1,3-dipolar addition, radical and carbene additions, metal complexation, and so on [7]. Fullerene Cgg also undergoes supramolecular complexation with various host molecules having electron-donating ability and an adequate cavity size [8]. 

Several organofullerene donor-acceptor molecular material hybrid systems have been synthesized via 1,3-dipolar cycloaddition reactions of azomethine ylides, via Bingel cyclopropanation and methanofullerene formation intermediates as well as via cycloaddition reactions, that have already been discussed in previous sections. The majority of such hybrid systems possess always as acceptor unit the fullerene core and as donor moieties porphyrins, tetrathiafulvalenes, ferrocenes, quinones, or electron-rich aromatic compounds that absorb visible light [190-193]. The most active research topic in this particularly technological field relies (i) on the arrangement of several redox-active building blocks in... 

From the perspective of structural chemistry, the modes of bonding, coordination, and the bond parameters of a particular element in its allotropic modifications may be further extended to its compounds. Thus organic compounds can be conveniently divided into three families that originate from their prototypes aliphatic compounds from diamond, aromatic compounds from graphite, and fullerenic compounds from fullerenes. 

This method has been applied to the synthesis of various polycyclic aromatic compounds [91]. Among the interesting applications are lactone formation in the preparation of natural products having biaryl skeletons (Eq. 42) [87] and double cyclization leading to spirocyclic compounds (Eq. 43) [92] and a fullerene fragment (Eq.44) [93]. 

The thermodynamic characteristics of adsorption of some organic compounds on fullerene crystals Ceo and C70 are presented on Table 1 and those on Carbopack on Table 2. Adsorption equilibrium constant values of n-alkane and aromatic compounds on fullerenes... 

Davydov et al. [46] used IGC to determine several adsorption thermodynamic properties (equilibrium constants and adsorption heats) for the adsorption of organic compounds on C q crystals, and compared them with those obtained for graphitized carbon black. The adsorption potential of the surface of fiillerene crystals was much lower than that of a carbon black surface. The dispersive interaction of organic molecules with C q is much weaker than with carbon black. The adsorption equilibrium constant for alkanes and aromatic compounds is therefore lower in the case of fullerenes. Aliphatic and aromatic alcohols as well as electron-donor compounds such as ketones, nitriles and amines were adsorbed more efficiently on the surface of fiillerene crystals. This was taken as proof that fiillerene molecules have electron-donor and electron-acceptor properties, which is in agreement with the results of Abraham et al. [44]... 

Studies of fullerenes as adsorbents from an aqueous solution in relation with the removal of pollutants are relatively scarce. Thus, Berezkin et al. [100, 101] investigated the adsorptive activity of fullerenes for organic pollutants in water and compared them with activated carbon and soots with and without fullerenes. They studied the purification of natural river water and waste from a pharmaceutical plant. The latter liquid contained various aliphatic, cyclic and aromatic compounds, the overwhelming majority of organic impurities being chlorinated compounds. The adsorption behavior of soot was found to be similar... 

Beside the practical importance of aromatic compounds, there has always been an interest in more or less theoretical problems like the scope, limitation and effects of electron delocalization in aromatic compounds (the aromaticity problem ). These investigations were strongly encouraged by the discovery of fullerene formation in a carbon plasma [18], in fuel-rich flames [24] or by the pyrolytic transformation of PAHs [25] together with a variety of the as yet potential application of these aromatic carbon cage compounds [18]. New selective C-C bond formation reactions as well as mechanisms of the rearrangement in carbon skeletons have been studied. 

This contribution will focus on new developments in the key step for the synthesis of PAHs the C-C bond formation as one of the most fundamental and challenging processes in organic and organometallic chemistry. In addition, these new synthetic approaches often give access to substituted aromatic structures. The authors attempt to cover the literature until the end of 1996. Important contributions in the synthesis of PAHs during the work on this review in the beginning of 1997 will also be considered whenever possible. The functionalization of aromatic compounds [10] and the chemistry of fullerenes [18] are beyond the limits of this contribution. As mentioned above, this review will concentrate on modern routes to preferably large condensed PAHs. Classical approaches towards aromatic hydrocarbons like the Pschorr, Elbs, Hayworth,... 

Numerous aromatic compounds including several bowl-shaped fullerene fragments have been prepared by this method, e.g. cyclopenta[z/]fluoranthene (22, Scheme8, [54d,56a]), cyclopenta[bc]corannulene (23, [55a]) and diace-naphtho[3,2,l,8-cdejg 3, 2, l, 8 -Zmnop]chrysene (24, see Scheme 9, [55b,c]). Of special importance is this approach for the synthesis of cyclopenta-annelated PAHs, e.g. and the three isomeric dicyclopentapyrenes (25-27, Scheme 9, [54e, 56b]). Using these reference samples, several cyclopenta-annelated PAHs could be identified as byproducts formed in the incomplete oxidation of hydrocarbons in fuel rich flames [57]. 

When hydrocarbons are not completely incinerated, carbon black is obtained. Under suitable conditions this may contain not only particles of classical soot, but also fullerenes. The trace existence of fullerenes in lamp black was first proven by mass spectrometry, but after working out appropriate protocols for the reaction, smoking flames can by now be used for the production of weighable amounts. Benzene is the most common source of carbon for this process. It is mixed with oxygen and argon and burned in a laminar flame (Figure 2.18). The resulting mixture contains soot, polycychc aromatic compounds and a certain fraction of fullerenes that make up 0.003-9.0% of the soot s total mass. Other hydrocarbons like, for example, toluene or methane may be employed as well. 

A striking observation is the high solubility of fullerenes in higher aromatic compounds (Table 2.3). Obviously the k-k interactions causing the solubilization of individual Cso-molecules are most effective here. A chlorinated aromatic solvent positively affects the solubility of C, too. In aliphatic solvents only minor amounts... 

Similar arguments like for the peapods hold for C ) surrounded by a belt-hke aromatic compounds (Figure 2.72a). Depending on their side chains the latter may confer some solubility in polar solvents. The interactions between belt and central fullerene are mainly of the ir-ji-type. They are exceptionally favorable due to the bent surface of both bonding partners. Inherently curved compounds tike perchlo-rotriquinacene can as well coordinate to by n-n interactions. 

Fullerenes behave like electron-deficient polyolefins and not like aromatic compounds. 

An interesting idea would thus consist in a functionalization with aromatic compounds bent around the z-axis. These should interact especially well with the 7t-system of the nanotubes, possibly even featuring certain selectivity for nanotubes with different diameters. A first attempt of this kind has been presented just recently. A belt-shaped aromatic molecule has been put over a fullerene here (refer to Section 2.5.6). Such a procedure should in principle be applicable to nanotubes as well. A success in performing this reaction would mean a big step toward a size-selective functionalization and possibly even toward a directed separation of individual species. Here an interaction with aromatic compounds immobilized within tubular templates, for example, within zeolites, would be another conceivable strategy. 

A general difficulty in fullerene chemistry resides in the structural characterization of new compounds. Contrary to most aliphatic or aromatic compounds where connectivities can be deduced readily from H-H coupling data, carbon-carbon connectivities in fullerene frameworks cannot be directly ascertained on a routine basis from C NMR experiments unless recourse is made to... 

Silicon alkoxide functionalized fullerenes can be profitably used in the preparation of HPLC stationary phases [152]. The silicon alkoxide group, in fact, guarantees the chemical grafting to silica (Scheme 9). Standard chromatographic tests with simple aromatic compounds revealed that the new phase works with high efficiency both in organic and water-rich media. Specific inter-... 

When laser energies are low, measurements of absorbates or thin films of molecules on surfaces could be made without the complications described, and this was demonstrated for polycyclic aromatic hydrocarbons (PAHs). ° An elegant design for laser desorption was used to measure mobility coefficients of 19 aromatic compounds, ranging from benzene to Cgo fullerene, all without an ion shutter. ... 

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