Fullerene nucleophilic addition

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]. 

Compared with the variety of existing carbon or nitrogen nucleophiles that were subjected to nucleophilic addition to there are few examples for phosphorus nucleophiles. Neutral trialkylphosphines turn out to be to less reactive for an effective addihon to Cjq even at elevated temperatures [114], Trialkylphosphine oxides show an increased reactivity. They form stable fullerene-substituted phosphine oxides [115] it is not yet clear if the reaction proceeds via a nucleophilic mechanism or a cycloaddition mechanism. Phosphine oxide addition takes place in refluxing toluene [115], At room temperature the charge-transfer complexes of with phosphine oxides such as tri-n-octylphosphine oxide or tri-n-butylphosphine oxide are verifiable and stable in soluhon [116],... 

Most of the reactions of silicon or germanium organic compounds proceed photochemically via a radical mechanism or via a cycloaddition mechanism (Chapters 4 and 6). There are few examples of nucleophilic addition of RSi or RGe to Cgo [118,119]. Reaction of silyUithium derivatives RjSili or germyllithium derivatives RjGeLi with different alkyl- and aryl-substituents R yields mainly the 1,2-adduct 28 or the 1,16-adduct 29 1,4-addition and dimerization of two fullerene-units was also found as a minor pathway. One example is given in Scheme 3.15. 

This consideration is corroborated by calculations at different levels of theory [11-13], showing that 1,2-additions to C q occur preferentially at a and (3 bonds between C(l) and C(2) and C(5) and C(6), respectively (Figure 13.2). Also, Mulliken charges on the fullerene core have been used in predicting the regiochemistry of nucleophilic additions to C70 [11]. Finally, 7t-bond orders have also been employed to evaluate the reactivity of different types of bonds [14-16]. 

CgQ is a radical sponge (Scheme 14.4) and readily adds organic as well as inorganic radicals (see Chapter 6). As with nucleophilic additions, multiple additions take place if an access of radicals is allowed to react with the fullerene. 

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]. 

NMA+) and 2,4,6-triphenyl-pyrylium tetrafluoroborate (TPP+) in the presence of biphenyl as cosensitizer were suitable for this reaction [174], The assumed mechanism of formation of do by this cosensitization is shown in Scheme 7. Reaction of do with H-donors such as te/t-butylmethylether, propionaldehyde and alcohols results in the formation of 1 1 adducts, the 1-substituted 1,2-dihydro-[60]fullerenes. Product structure support a H-abstraction process [212,213] rather than nucleophilic addition. In Scheme 8, the general formation of 1-substituted l,2-dihydro-[60]fullerenes is shown. Selected examples of the products obtained by this method are summarized in Table 10. 

Finally, even fullerene ( buckminsterfullerene, Qq) was found to undergo nucleophilic addition of Grignard reagents [42],... 

Apart from the nucleophilic addition, also radical reactions can occur on the surface of the fullerenes. These act as a sponge for radicals which are easily added, yielding dia- or paramagnetic products. Dimers of fullerenes are frequently obtained as well in this kind of reaction. 

Figure 2.54 (a) General mechanism of the nucleophilic addition to a fullerene (b) reaction... 

The fullerene skeleton can not only undergo nucleophilic additions, but it also enters into radical reactions very easily. The fullerene acts as a radical sponge in these reactions, and either dia- or paramagnetic derivatives are obtained depending on the number of transferred electrons. 

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. 

Nucleophilic addition of organozinc reagents is crucial step in Reformatsky reaction which was employed for fullerene derivatization. Ethylbromoacetate reagent (5 equiv.) in conjunction with zinc dust (20 equiv.) reacts with fullerene to obtain product 48 in high yield. In this process, known cyclopropane molecule 49 was formed as a side product, as well as two products unknown in the literature, 50 and 51 (Scheme 7.14) [26]. Formation of these products is rationalized by mechanism which starts with formation of organozinc reagent, which is followed by nucleophilic addition to fullerene. 

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]. 

The bound fullerenes are capable of reacting with other compounds such as aza-crown ethers, which in turn can bind other compounds. Because of their oleflnic and electrophilic characteristics, fullerenes can react in several addition reactions, such as nucleophilic addition of amines, which can be used to achieve permanent fixation of fullerenes to polyamide and wool fibers. 

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. 

Addition reactiorrs are very important from atom economic point of view as all the atoms of reactants are incorporated in the final product and nothing goes as a waste. Some work has been done on nucleophilic addition with a major stress on Michael addition reaction, but little information is available on the other counter part, the electrophilic addition. Oidy some reactions, involving electrophilic addition to fullerenes, have been reported and a lot more is to be investigated in this direction. 

Karfunkel, H.R. and Hirsch, A., Prediction of the nucleophilic addition products of fullerene... 

The direct addition of an end-functionaUzed linear polymer chain onto a 6-6 bond on Ceo is the most commonly employed method to synthesize star-shaped polymers with a fullerene core. This grafting can occur via radicals, nucleophilic addition or insertion. 

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