Cationic fullerene intermediate

Bowl-shaped carbocations (33) were obtained by alkylation of the parent corannulene. ° The cations were obtained as stable salts and characterized by X-ray analysis. Phenalenyl cations (e.g. (34)) were obtained as air-stable BPh4 salts. Observations made in a selective synthesis of l,2-di(organo)fullerenes led to a mechanism whereby a cationic fullerene intermediate is generated by Cu(ll) oxidation of a fullerene radical (or anion). ... 

A powerful technique to detect paramagnetic radical species, such as the one-electron reduced 7t-radical anions (Ceo ) and oxidized 7t-radical cation (Ceo ) is electron spin resonance (ESR) (9,10). Studies, regarding the characterization of fullerene intermediates via employing the ESR technique are, however, still somewhat controversial. Absorption spectrophotometry, on the other hand, is been successfully employed in matrix irradiation, silver mirror reduction in tetrahydrofuran (THF), and detection of transients in time-resolved laser photolysis and pulse radiolysis (10,11). 

Irradiation of 2,3-diphenyl-2//-azitine in the presence of Cgo fullerene leads to the formation of mono- and ohgo adducts (98,99). A monoadduct, l,9-(3,4-dihydro-2,5-diphenyl-2//-pytTolo)fullerene-60 was isolated and characterized. Mechanistic studies showed that under conditions of direct irradiation it was formed by a classic nitrile yhde cycloaddition but in the presence of 1,4-napthalenedicarbonitrile (DCA) it resulted from reaction of the radical cation intermediate 108. Cycloaddition reactions have also been carried out with diaza-phospholes and diazaarsoles (100) to give adducts of the type 189 (A=As,P) and with cyanogen to give 190 and with atyldiazocyanides where addition to both the azo moiety and the cyano group were observed (101). 

Results of Fourier-transform mass spectrometry (FT-MS) studies with the carbon oxides 3-5 conclusively demonstrate that one reaction pathway to the formation of fullerenes is the coalescence of large cyclocarbon ions [21, 23], In the negative-ion mass spectra of the carbon oxides, the expected successive losses of CO molecules from the precursor anions to give the cyclocarbon ions Cii, and were observed. In the positive-ion mode, gas-phase coalescence reactions of the cyclocarbon ions produced fullerene ions. Ion-molecule reactions starting from the cyclocarbon cations Cl s (formed by laser desorption of 3), and 2 (formed from 4) led through distinct intermediates Cjg -... 

Derivatized fullerene cations, such as RCso and RC7o, are formally produced by adding a cationic species R+ to an sp hybridized carbon of a fullerene core. Cations with additional attached groups, such as R Cgo n odd number) are also conceivable. Unlike radical cations, these cations are carbenium ions (trivalent carbo-cations) and have no unpaired electrons. Besides the addition of R to C o (method (a)), four other methods ((b)-(e)), shown in Scheme 9.2, are currently known for the generation of RCeo. As described in the following sections, these approaches have been successfully employed to produce the derivatized Ceo and C70 cations, either as long-lived species or as reactive intermediates. 

The involvement of a fullerene cation as an intermediate has been proposed in some reactions. In the examples shown below, the cationic intermediates are generated either by an electrophilic addition to C o (Scheme 9.2(a)), or by the ionization of a C o-Cl bond (Scheme 9.2(c)). 

Treatment of chlorinated fullerenes, such as 12a-d, 14, and 19, with AICI3 or FeCh in the presence of an aromatic compound gave the substitution products, as a result of the abstraction of a chloride ion and the subsequent electrophilic attack of the intermediate cation to the aromatic ring (Scheme 9.13) [21, 31]. The chloride... 

Treatment of (Cs9N)2 with an electron-rich aromatic compound in the presence of an excess of p-toluenesulfonic acid at high temperature ( 150 °C) under air provides an arylated aza[60]fullerene 21 (Scheme 9.19) [40-46]. The intermediate cation C59N+, which is formed in situ from (C5gN)2, undergoes electrophilic aromatic substitution. ODCB is inert to electrophilic attack by C59N+ and thus can be used as the solvent. Only para-substitution products 21a-d are obtained from monosub-... 

An ether, such as 24 can be produced, when the intermediate cation C59N+ is trapped by an alcohol (Scheme 9.21) [42]. This product contains a carbon atom attached to an oxygen and a nitrogen atom, providing a potential precursor of open-cage fullerenes [42]. 

FuUerenes are such active compounds that they form derivatives easily. Radical anions and cations are also intermediates for the formation reaction of the fullerene derivatives. Some examples are shown below. 

The radical cation of fullerene is also an active intermediate which generates fullerene adducts. Siedschlag et al. reported that the Qo -adduct generated via the radical cation of Cgo which was formed by the cosensitizer under photoirradiation [69]. The Qo radical cation abstracts a-proton from the alcohol to generate CggH cation and alkyl radical, which forms alkylated Qg (Scheme 7). 

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