Fullerene radical addition

The new absorption band at 435 ran in the C60 spectrum has been attributed to the 1,2 addition to the fullerene cage to the fatty acid chains either across to the double bonds by a Diels-Alder addition or, more simply, by radical addition (Cataldo and Braun, 2007). Thus, fatty acid esters are able to not only dissolve C60, but also react with this molecule causing the addition of the fatty chain to the fullerene cage. In fact, the bands at 435 ran shown in Fig. 13.3 appear only when C60 is stirred at 75°C for a couple of hours in the esters of fatty acids. Only for olive oil the new band appears much weaker than in the other cases and displaced at 450 ran (Fig. 13.3B). Since this oil contains chlorophyll, the displacement may be probably due also to a charge-transfer interaction between C60 and chlorophyll or with other impurities. 

A water-soluble Cj-symmetrical trisadduct of Cjq showed excellent radical scavenging properties in vitro and in vivo and exhibits remarkable neuro-pro tective properties [7,8]. It is a drug candidate for the prevention of ALS and Parldnsoris disease. Concerning the reaction mechanism, nucleophilic additions and radical additions are closely related and in some cases it is difficult to decide which mechanism actually operates [92]. For example, the first step in the reaction of f-eo with amines is a single electron transfer (SET) from the amine to the fullerene. The resulting amines are finally formed via a complex sequence of radical recombinations, deprotonations and redox reactions [36]. 

Mattay and coworkers extended the investigations of photoinduced electron tansfer from C6o to excited sensitizers and cosensitizers (Scheme 4) [173-175], They used dicyanoanthracene (DCA), dicyanonaphthaline (DCN), A-methylacri-dinium hexafluorophosphate (MA+), and triphenylpyrylium tetrafluoroborate (Tpp+) as sensitizers. In the case of DCA, DCN, and MA+, the addition of a cosensitizer (biphenyl) was necessary to produce the fullerene radical cation in sufficiently high yields [175], Otherwise, fast back-electron transfer seems to be predominant. However, by using TPP+ the formation of Q,o could be detected by EPR measurements even in the absence of a cosensitizer. This can be explained by (1) the high reduction potential (Ejed = 2.53 V vs SCE) and (2) the neutral form of the reduced sensitizer (electron shift) [174-176], Nevertheless, no influence of the cosensitizer on the EPR signal was observed under irradiation [175],... 

The radical anion Cw, can also be easily obtained by photoinduced electron transfer from various strong electron donors such as tertiary amines, fer-rocenes, tetrathiafulvalenes, thiophenes, etc. In homogeneous systems back-electron transfer to the reactant pair plays a dominant role resulting in a extremely short lifetime of Qo. In these cases no net formation of Qo is observed. These problems were circumvented by Fukuzumi et al. by using NADH analogues as electron donors [154,155], In these cases selective one-electron reduction of C6o to Qo takes place by the irradiation of C6o with a Xe lamp (X > 540 nm) in a deaerated benzonitrile solution upon the addition of 1-benzyl-1,4-dihydronicoti-namide (BNAH) or the corresponding dimer [(BNA)2] (Scheme 15) [154], The formation of C60 is confirmed by the observation of the absorption band at 1080 nm in the near infrared (NIR) spectrum assigned to the fullerene radical cation. 

Fig. 65 Copper-catalyzed radical addition/cyclization sequences with fullerenes 245...

Fullerenes are excellent electron acceptors. The early examples for the high electron affinity of fullerenes include efficient nucleophilic addition reactions of fullerenes with electron donors such as primary and secondary amines. Since then, there have been many studies of electron transfer interactions and reactions involving fullerene molecules. It is now well established that both ground and excited state fullerene molecules can form charge transfer complexes with electron donors. The photochemically generated fullerene radical anions as a result of excited state electron transfers serve as precursors for a wide range of functionalizations and other reactions. 

Radiolytic experiments of Ceo/Y CD under conditions that generate carbon-centered radicals, such as CH3, were in line with a radical addition mechanism. It is interesting to note that a reaction of [60]fullerene even with the strongly oxidizing Clj radicals (oxidation potential of +2.3 V versus SCE) give an absorption that suggests a (Ceo-Cl) adduct. This is in contrast to a prediction that is purely based on the... 

Similar to the [60]fullerene case, addition of [76]fullerene and [78]fullerene in the 10 M concentration range resulted in an accelerated decay of the arene radical cation s UV-VIS absorption, with rates linearly depending on the fullerene concentration. At the same time, formation of the fullerene radical cations became observable in the NIR... 

In a very elegant approach, Okamura et al. attached Cgo to a range of styrene polymers with different molecular weights (from 1000 to 10,000) [32]. The attachment was provided by 1,4-radical addition to Cgg, producing narrow-dispersity polystyryl adducts (Scheme 3). The electrochemical and ground state absorption properties of the polymers were identical to those of a low-mass, fully characterized model (Scheme 3). This indicates that the reported procedure is useful for the preparation of high-mass polymers, with retained fullerene properties. 

Fullerenes Early studies on FRP of vinyl monomers in the presence of C50 reported very low yields of polymer formed in solution, or even complete inhibition [123]. From these studies, it has been suggested that, in the case of St, the free radicals are trapped by fullerene, and the resulting fullerene radicals do not propagate but can terminate instead. Nevertheless, there are pieces of evidence suggesting that if the polymerization is carried out in the presence of a large excess of initiator, the radicals undergo multiple additions on the fullerene surface, changing their nature sufficiently not to inhibit polymerization [124]. Thus,... 

In CMRP, all chains are - ideally - terminated by a cobalt-carbon bond, and can be reactivated at a moderate temperature so as to allow the release of macroradicals. When carried out in the presence of radical traps, reactivation of the protected chains can be used not only for the end-functionalization of polymers but also for removing the cobalt complex that originally was attached to the chains. As a rule, the treatment of polymers prepared by CMRP with nitroxides and thiols produced cobalt-free polymers that were terminated by an alkoxyamine and hydrogen, respectively [44]. Both, fullerenes [45, 46] and nanotubes [47], which are prone to radical addition, were similarly used as radical traps, leading to carbon nanoobjects grafted with polymers. 

Addition reactions, electron transfer reactions, and reactions involving the opening of the fullerene cage (chemical surgery) have been thoroughly studied on fullerenes. Other reactions such as nucleophilic additions, cycloaddition reactions, free-radical additions, halogenations, hydroxylation, redox reactions, and metal transition complexations have been reported for Cgo as well. Furthermore, fullerenes are easily reduced by electron-rich chemical reagents as well as electrochemically. Their oxidation, however, is considerably more difficult to achieve [17]. Thus, electrochemical measurements showed the formation from the monoanion to the hexaanion [18]. 

During the past 15 years since Qo became available in macroscopic quantities in 1990 [1], a wide variety of its derivatives have been synthesized as part of the explosive development of the study of its chemistry [2). Various organic reactions have been reported, most of which are cycloadditions, nucleophilic additions, and radical additions. Fullerenes, as represented by Qo, are now commonly accepted to behave as electron-deficient olefins, hence there have been numerous studies on their anions. This has led to a situation where the other equally important species, the fullerene cations, have been left unexplored for nearly a decade in spite of their significance in both fundamental and application studies. Clearly, a systematic study of this class of species is needed. 

This paper is concerned with the structures of the simplest possible adducts of the Ceo and C70 fullerenes, namely the monohydrides, CmH and C H. These open shell species or radicals may be considered as the product of the addition of one atom of hydrogen or one of its isotopes, among which we include specifically the light pseudoisotope of hydrogen known as muonium. Mu = pfe. Although Ceo//has been observed [1], the stimulus for these calculations arose from the experiments on muon implantation in solid [2,3] and C70 [4]. 

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