Radical Reactions of Fullerenes: From Synthetic Organic Chemistry to

Review pubs.acs.org/CR

Radical Reactions of Fullerenes: From Synthetic Organic Chemistry to Materials Science and Biology Manolis D. Tzirakis*,† and Michael Orfanopoulos* Department of Chemistry, University of Crete, 71003 Voutes, Heraklion, Greece 13.3. Dichlorophenyl Derivatives of Insoluble EMFs 13.4. Disilylation of EMFs 14. Radical Reactions of Heterofullerenes 15. Radical Scavenging Activity of Fullerenes: Applications in Polymer Science and Biology 15.1. Antioxidant Activity of Fullerenes and Their Derivatives 15.2. Applications in Polymer Science 15.2.1. Anti-Radical Activity of Fullerenes in Radical Polymerization Reactions 15.2.2. Anti-Radical Activity of Fullerenes in the Thermal/Thermo-oxidative Degradation of Polymers 15.3. Biological Activity and Pharmacological Potential of Fullerenes 15.3.1. Malonyl Carboxyfullerenes 15.3.2. Hexasulfobutylated Fullerene 15.3.3. Dendro[60]fullerene 15.3.4. Fullerenols 15.3.5. Other Water-Soluble C60 Derivatives 16. Summary Author Information Corresponding Author Present Address Notes Biographies Acknowledgments List of Acronyms and Abbreviations References

CONTENTS 1. Introduction 2. Addition of C-Centered Radicals to Fullerenes 2.1. Metal-Mediated Addition of C-Centered Radicals to C60 2.1.1. Radical Reactions of C60 Mediated by Decatungstate (W10O324−) 2.1.2. Radical Reactions of C60 Mediated by Mn(OAc)3, Fe(ClO4)3, and Cu(OAc)2·H2O 2.1.3. Radical Reactions of C60 Mediated by CoCl2dppe 2.2. Addition of Fluoroalkyl Radicals to C60 2.3. Other Reactions of C60 Involving C-Centered Radicals 3. Addition of Si-Centered Radicals 4. Addition of O- and S-Centered Radicals 5. Addition of P-Centered Radicals 6. Addition of N-Centered Radicals 7. Addition of Metal-Centered Radicals 8. Addition of Hydrogen Atom(s) 9. Addition of Halogens 10. Organofullerenyl Radicals RC60• via Oxidation of Organofullerenyl Anions RC60− 11. Fullerene Dimers via Dimerization of RC60• Radicals 12. Ion Radical Reactions of Fullerenes 12.1. Reactions of Fullerene Radical Anion (C60•−) 12.1.1. Addition of Amines to C60 12.1.2. Addition of Alkyl Halides to C60 12.1.3. Addition of Ketene Silyl Acetals to C60 12.1.4. One- and Two-Electron Reduction of C 60 with NADH and NAD Dimer Analogues 12.1.5. [4 + 2] and [2 + 2] Cycloadditions 12.1.6. Reaction of C60 with CyclopropylSubstituted Olefins 12.2. Reactions of Fullerene Radical Cation (C60•+) 13. Radical Reactions of Endohedral Metallofullerenes (EMFs) 13.1. Radical Reactions of Paramagnetic M@C82 13.2. Radical Reactions of Diamagnetic EMFs © XXXX American Chemical Society

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1. INTRODUCTION Since their discovery in 19851 and their production on a preparative scale 5 years later,2 fullerenes have attracted a great deal of attention. Their chemical modification has emerged as a formidable challenge to synthetic organic chemistry. Thus, over the last two decades, a great diversity of reactions have been developed to functionalize C60 or C70 and thus take advantage of, or even enhance, the unique physical and chemical properties of the fullerene core. In this context, free-radical reactions were among the first investigated reactions of C60 and continue to be an important methodology for the functionalization of fullerenes.3−6 The phrase “radical sponge” is one of the most widely used terms that has become synonymous with fullerenes, especially C60.7,8 This term hints at the high radical scavenging activity

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dx.doi.org/10.1021/cr300475r | Chem. Rev. XXXX, XXX, XXX−XXX

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biological systems. Thus, quite apart from the purely synthetic applications mentioned above, the remarkable ability of fullerenes to trap free radicals endows them with broad biological potential and previously unforeseen applications in materials science (especially in polymer science). In biology, fullerenes are powerful antioxidants, reacting readily and at a very fast rate with free radicals, which may often be the cause of oxidative cell damage or death.37−48 For instance, they have been shown to exhibit much stronger antioxidant performance than current leading antioxidants such as vitamin E (i.e., α-tocopherol). This property has stimulated major pharmaceutical companies and research institutions to explore the use of fullerenes in controlling many diseases that result from radical damage. Thus, a series of fullerene-based antioxidant drugs has been developed for applications in neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease, amyotrophic lateral sclerosis (Lou Gehrig’s disease), as well as in more commercially minded applications, such as their use in antiaging skin creams, where prevention of oxidative cell damage or death is desirable. Several other biological applications can be found in the literature (i.e., inhibition of various enzymes, cytotoxicity against tumor cells, and DNAcleaving activity through photoinduced formation of superoxide anion radicals), and a plethora of both in vitro and in vivo biological studies are ongoing in this direction.37,49−52 In materials science the free-radical scavenging properties of fullerenes have been widely exploited for the thermal and thermo-oxidative stabilization of polymeric materials against various free-radical-mediated degradative processes.53,54 On the other hand, radical copolymerizations that are typically used for incorporation of fullerenes into a polymer structure, are selfinhibited too, due to the fullerene’s inherent antioxidative properties.54 The radical scavenging nature of fullerenes has been also used for (i) improving their solubility in conventional polar solvents by grafting macroradicals of hydrophilic poly(ethylene oxide) moieties, and (ii) obtaining fullerenecontaining branched or star-like polymers in which the fullerene core bears a well-controlled number of polymeric arms. Finally, it has been demonstrated that fullerenes can be used as radical scavengers for protection of organic and hybrid organic−inorganic materials from high-energy radiation (Xray), which is a fundamental technological issue for broadening the range of applications of such materials.55 In conclusion, the main objective of this review is to provide a comprehensive and up-to-date review of the radical reactions of fullerenes and their applications within synthetic organic chemistry, biology, and materials science. In this context, another major goal is to highlight the scope and limitations of radical reactions as a general strategy for the functionalization of fullerenes, and thus, special emphasis is given to the most recent and outstanding reactions which have particular synthetic relevance. We believe this review will serve as a handy reference for researchers interested in the past achievements, current challenges, and future perspectives of contemporary fullerene radical chemistry. In essence, this review is also expected to provide an important impulse for further studies in this field.

that fullerenes exhibit by virtue of their high electron affinity (ca. 2.7−2.8 eV) and the large number of conjugated double bonds that can be readily attacked by radical species. The facile addition of up to 11 phenyl groups, 15 benzyl groups, and 34 methyl groups to the C60 core has been previously singled out as the first landmark example of the radical reactivity of fullerenes.7,9 Nowadays, the high affinity of fullerenes to free radicals is a well-established fact that underlies key applications of fullerenes and is the basis of a plethora of original publishing records at the interface between chemistry, biology, and physics. The aim of the present review is to provide a complete and up-to-date evaluation of the radical chemistry of fullerenes from both a f undamental and a practical point of view. From a f undamental perspective, this review will provide valuable insight into the basic characteristics of the radical reactions of fullerenes, based on several chemical and/or physical investigations, especially electron spin resonance (ESR) spectroscopic studies.3,7,9−36 Such characteristics include (i) the reactivity of radicals of various chemical nature (i.e., alkyl, perfluoroalkyl, Si-, O-, S-, P-, and metal-centered radicals) with respect to C60 and C70, (ii) the tendency of free radicals to add multiple times and rapidly to C60, (iii) the allylic or cyclopentadienyl structural pattern that results from multiple addition of free radicals to C60, (iv) the delocalization of the unpaired electron in fullerene radical adducts over certain cage carbon atoms, and (v) the tendency of fullerenyl radicals to dimerize in a “head-to-head”-type and several other related aspects. From a practical point of view, this review will provide a comprehensive survey of the applications that the radical reactivity of fullerenes has found in various fields ranging from synthetic organic chemistry and material sciences to biomedicine. Recent advances in the field of synthetic organic chemistry have enabled the development of appropriate reaction protocols that allow for selective preparation of structurally diverse monoaddition fullerene adducts, or multiadducts, with well-defined addition patterns. Consequently, such developments have renewed interest in the radical chemistry of fullerenes, particularly in the synthesis of novel architectures with potentially important applications. To date, there are several examples of addition of radicals of different chemical nature (e.g., C-, Si-, O-, S-, P-, and metal-centered radicals) to C60 and/or C70. Typically, there are three forms of fullerenes that take part in radical reactions, and they will be discussed individually in this article: (i) neutral fullerenes (i.e., C60) which react directly with free radicals; (ii) fullerene radical anions (i.e., C60•−) generated upon a single electron transfer (SET) from electron donors; and (iii) fullerene radical derivatives (i.e., RC60•) produced by nucleophilic addition followed by oxidation of the primarily formed fullerene anion (i.e., RC60−). Similarly, a detailed review on the progress made so far in the radical reactions of fullerene derivatives, endohedral metallofullerenes (e.g., La@C82, Sc3N@C80), heterofullerenes (i.e., aza[60]fullerene), and higher fullerenes will be also provided. As evidenced by the most recent achievements in contemporary fullerene functionalization chemistry, synthesis of fullerene-based compounds via radical reactions is a topic of considerable ongoing research interest. The past decade has also witnessed, especially in the fields of materials science and biomedicine, an increase in the number of synthetic fullerene derivatives which have been prepared and utilized as stabilizing agents in polymers or as antioxidants in

2. ADDITION OF C-CENTERED RADICALS TO FULLERENES Reactions of photolytically and thermally generated alkyl radicals R• (R = tert-butyl, 1-adamantyl, isopropyl, ethyl, and benzyl) to C60 were among the first radical reactions to be B


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radicals follows the “head-to-head” rather than the “head-totail” type and involves the C3 or C3′ atoms, whereas steric hindrance prevents dimerization at the C1 atom (Figure 1).11,14 The tendency of fullerenyl radicals to dimerize has led to the development of appropriate reaction protocols for synthesis of singly bonded C60 dimers (see section 11).58−61 On the other hand, it has been found that the alkylfullerene dimers have very weak C−C bond energies and can be photolytically cleaved into the corresponding alkylfullerene radicals by excitation at 532 nm.22 Multiple radical addition to C60 is another important characteristic in the chemistry of fullerenes. Pioneering studies in this field have shown that prolonged irradiation of C60 solutions in the presence of excess radical precursors leads to multiple radical additions, as confirmed by line broadening in the ESR spectra and by mass spectroscopy.7,9,56,62 For example, it has been shown that photochemically generated benzyl radicals react with C60, producing radical and nonradical adducts RnC60 (R = C6H5CH2) with n ranging from 1 to at least 15.7 Radical adducts with n = 3 and 5 have been identified by ESR spectroscopy as the allylic radical R3C60• (3) and cyclopentadienyl radical R5C60• (5), respectively, whereas the unpaired electrons were found to be highly localized on the C60 surface (Scheme 1). The extraordinary stability of radicals 3

studied in fullerene chemistry.10 The corresponding radical adducts RC60• were identified by the proton and 13C hyperfine interactions obtained from their ESR spectra. Initially, it was found that the unpaired electron in the resulting RC60• radical adducts is mostly confined to the three carbon atoms ortho (C1, C5, C5′) and the two carbon atoms para (C3, C3′) to the point of attachment of R (Figure 1), whereas extensive

Figure 1. Atom numbering in the pyracylene unit of RC60• radicals.

delocalization of the unpaired electron over the C60 sphere was ruled out.10,11 Further studies led to a reassignment of the density of the unpaired spin; the highest spin density was found for C1 and the second highest for C3 and C3′, whereas almost no spin density was found on C5 and C5′, leading to only two major canonical resonance structures for RC60•.12−14 An important feature of fullerenyl radicals is the hindered rotation about the R−C60• bond, so that the alkyl groups adopt preferred conformations relative to the C60 framework.3,13−17,56 For example, the CH3CH2C60• radical adopts an asymmetric equilibrium conformation in which the terminal methyl group moiety is placed directly above one of the two 6-membered fused rings of C60 (Figure 2a),15 whereas, in contrast, the

Scheme 1. Stepwise Addition of Five Benzyl Radicals R• to C60

Figure 2. Preferred conformation of (a) CH 3 CH2 C 60 •, (b) CF3CH2C60•, and (c) (CH3)3CC60• radicals.

and 5 was attributed to the steric protection lent by the three or five benzyl substituents that shelter the surface’s radical sites.7 The initially formed radical RC60• (1) is probably a short-lived species that is rapidly consumed by reaction with R• that is continuously being generated by photolysis.7 The reaction pathway leading to 3 and 5 (Scheme 1) is the result of a radical addition or coupling in a way that minimizes both steric hindrance and [5,6] double bonds.7 Similarly, it has been found that photochemically generated methyl radicals add readily to C60 to afford (CH3)nC60 with n ranging from 1 to at least 34, as evidenced by mass spectrometric analyses.7 Employment of thermal instead of photochemical methods for radical generation has allowed for more selective addition of benzyl radicals to C60. In particular, Tumanskii and co-workers have shown that the 2-(p-fluorophenyl)hexafluoroisopropyl radical, produced by thermal dissociation of the dimer [(FC6H4)C(CF3)2]2, can abstract a hydrogen atom from the methyl group of toluene and mesitylene to form the corresponding radicals which then add selectively to fullerene C60.62 Thus, it was found that addition of benzyl radicals affords adducts containing from 3 to 5 benzyl groups, whereas in the case of the 3,5-dimethylphenylmethyl radicals, the fifth radical

terminal CF3 group in CF3CH2C60• prefers the position above a pentagon (Figure 2b).13,18,19 Typically, in radicals of the general type XYZCC60• (where X, Y, and Z are CF3, F, H, or CH3) a CH3 group will never gain the pentagon position except when there is no alternative as in (CH3)3CC60 (Figure 2c), whereas, a CF3 group will never gain the hexagon(s) position except when there is no alternative as in (CF3)3CC60.13,19 Another remarkable feature of the RC60• radicals is their tendency to dimerize. In particular, ESR studies for a series of RC60• radicals have shown an increase in intensity of the ESR spectrum with increasing temperature; this behavior has been attributed to an equilibrium between the fullerenyl radicals and their diamagnetic dimers and thermal dissociation of the latter species upon an increase in temperature (eq 1).11,14,20,56,57 RC60−C60R ⇌ 2RC60•

(1)

The bond strength in the fullerenyl dimers RC60−C60R depends strongly on the size of R, which in turn suggests that bonding in the dimer is greatly influenced by steric effects.14 Moreover, molecular mechanic calculations, as well as experimental results suggest that dimerization of the fullerenyl C


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generation of C-centered radicals by activating the methyl C−H bond in a series of substituted toluenes (6a−e), anisoles (8a− d), and thioanisole (10).67 The resulting C-centered radical intermediates could be successfully trapped by C60 to afford selectively the corresponding monofunctionalized fullerene adducts in moderate to good yields (Schemes 2 and 3).

does not add, probably due to the steric hindrance caused by its large size compared to that of the benzyl radical.62 The high affinity of free radicals for C60 has been quantified by determining the rate constants for addition of various carbon-centered radicals to fullerene with the help of laser flash photolysis or the method of competitive addition of free radicals to spin traps.63,64 These rate constants were found to be two orders of magnitude higher than those for addition of radicals to a wide class of monosubstituted unsaturated compounds.63,64 Unlike C60 in which all 60 carbon atoms are equivalent, C70 contains five types of carbon atoms, and thus, five regioisomeric radical monoadducts RC70• are possible (Figure 3).34,65 ESR

Scheme 2. TBADT-Mediated Radical Reaction of ParaSubstituted Toluenes with C60a

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Reprinted with permission from ref 67. Copyright 2008 American Chemical Society.

Typically, this reaction is carried out by irradiating a solution of C60 with a large excess of the organic substrate, in the presence of TBADT in a mixture of chlorobenzene/acetonitrile. The observed single addition of radicals to C60 is of particular interest, given the well-established high affinity of C60a “radical sponge”with free radicals, which generally results in multiple radical additions. For example, as already mentioned in section 2, up to 15 benzyl radicals have been reported to add to C60.7 Thus, this is the first report that concerns the isolation and characterization of C60 monoadducts via a single benzyl radical addition (Scheme 2).67 Similarly, it has been found that C60 reacts readily with parasubstituted phenoxymethyl or phenylthiomethyl radicals (derived from the corresponding anisoles 8a−d or thioanisole 10, respectively) to give the corresponding ether or thioether fullerene derivatives 9a−d or 11, respectively, in good yield under similar experimental conditions (Scheme 3).67 Apart from the synthetic importance of this reaction, a mechanistic investigation has also been presented (Scheme 4). Intramolecular primary isotope effect studies have shown that hydrogen-atom abstraction from the CH3 or the CH3O group occurs in the rate-determining step of the reaction. In particular, the reaction is initiated by photoexcitation of both the catalyst TBADT and the reagent C60. The singlet excited state (1C60*) of the fullerene C60 almost quantitatively decays via intersystem crossing (ISC) to the energetically lower lying triplet excited state of C60 (3C60*). On the other hand, simultaneous illumination of the TBADT anion (W10O324−) generates a charge transfer excited state (W10O324−*), which reacts exclusively with the organic substrates (RH) to give the corresponding radicals (R•) and the reduced tungstate species (W10O325−). This reduced form of decatungstate is most likely reoxidized by the triplet excited state of C60 (3C60*) leading to the radical anion of fullerene C60•−; electron transfer from W10O325− to the ground state instead of the triplet excited state of C60 could also contribute in this process. Finally, coupling of C60•− radical anion with the organic radical (R•) followed by protonation of the resulting anion (RC60−) yields the observed 1-substituted 1,2-dihydro[60]fullerenes (RC60H).67

Figure 3. Structure of C70 indicating the five possible regioisomeric structures of RC70• (A−E, double bonds have been omitted). Dots represent the carbons with the highest spin density, as calculated for the HC70• isomer (ref 65).

studies on addition of several photochemically generated aryl and fluoroalkyl radicals to fullerene C70 have shown that while simple alkyl radicals afford only three of the five expected RC70• regioisomers, the more reactive aryl and fluoroalkyl radicals give rise to four and the •CF3 and CH3O• radicals yield the ESR spectra of all five expected isomers.21,34 2.1. Metal-Mediated Addition of C-Centered Radicals to C60

In recent years, there has been increasing interest and remarkable progress in understanding the radical reactions of fullerenes catalyzed by metal complexes. Such metal-based catalysts include mostly tetrabutylammonium decatungstate [(n-Bu4N)4W10O32],66−71 manganese(III) acetate dihydrate [Mn(OAc)3·2H2O],72−84 ferric perchlorate [Fe(ClO4)3],74,83 lead(IV) acetate [Pb(OAc)4],76 copper(II) acetate monohydrate [Cu(OAc)2·H2O],81 and CoCl2dppe [dppe = bis(diphenylphosphino)ethane].85,86 2.1.1. Radical Reactions of C60 Mediated by Decatungstate (W10O324−). The use of tetrabutylammonium decatungstate [TBADT, (n-Bu4N)4W10O32] catalysis66 can be regarded as a general and highly efficient strategy for C−C bond formation in fullerenes.67−70 In essence, this method has enabled the otherwise inaccessible chemical modification of fullerene C60 with several classes of organic compounds, including toluenes, anisoles, and thioanisole,67 aldehydes,68 ethers, sulfides,69 and alcohols.70 2.1.1.1. Addition of Benzyl, Phenoxymethyl, and Phenylthiomethyl Radicals. The first report on the use of TBADT as a radical initiator in fullerene chemistry appeared in 2008.67 In this study, it was shown that decatungstate catalyst promoted D


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Scheme 3. TBADT-Mediated Radical Reaction of Para-Substituted Anisoles and Thioanisole with C60a

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Scheme 4. Proposed Mechanism for the TBADT-Catalyzed Reaction of Aromatic Compounds RH with Fullerene C60a

Scheme 5. Direct Acylation of C60 via TBADT-Mediated Radical Addition of Aldehydes

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2.1.1.2. Addition of Acyl Radicals. TBADT catalysis has allowed also for the development of an efficient photochemical methodology for direct acylation of C60.68 This method includes reaction of C60 with a wide variety of acyl radicals derived from the corresponding aldehydes 12a−m through a hydrogen-atom abstraction process mediated by TBADT [(nBu4N)4W10O32].68 Thus, the preparation of a novel class of fullerene-based materials, namely, acylated fullerene monoadducts 13a−m, has been achieved in moderate to good yields (Scheme 5).68 Importantly, this methodology is directly applicable even in the cases of the cyclopropyl-substituted aldehydes 12j−k, where rapid rearrangement of the cyclopropyl acyl radical intermediate can potentially occur. Product analysis for this reaction has shown that decarbonylation and acylation pathways compete when a tertiary or phenylacetyl aldehyde is the starting material. For example, when pivalaldehyde (12l), phenylacetaldehyde (12m), or 2phenylpropionaldehyde (12n) was employed in this reaction, the corresponding RC60H monoadducts (14l−n) were obtained as the major, or sole, addition products (Scheme

6).68 Apparently, these products derive from decarbonylation of the corresponding parent acyl radicals (Scheme 6). To overcome the observed decarbonylations encountered in certain acyl radical addition reactions where the acyl radical intermediates are prone to undergo rapid decarbonylation, the Scheme 6. Radical Additions to C60 upon TBADT-Mediated Decarbonylation of Tertiary or Benzyl Aldehydes

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authors established a simple low-temperature approach.68 This approach was found to be applicable even in the case of phenylacetaldehyde (12m) where the decarbonylation rate constant is high (kd > 106 s−1 at ambient temperature).68 Thus, these studies represent a neat example of thermal control of decarbonylation. Another important finding from these studies concerns the regiochemistry of the tert-butyl radical addition. Thus, it has been observed that when the reaction is performed at ca. 10 °C, both 1,2- and 1,4-isomers are obtained with the thermodynamically more stable 1,2-isomer being the major one. On the other hand, the kinetically favored 1,4-adduct is the major product when the reaction is performed at −40 °C. The mechanism of this reaction was studied by performing a series of isotope-labeling experiments, including D2O trapping experiments and kinetic isotope effect (KIE) studies.68 For example, measurement of the intra- and intermolecular deuterium isotope effect showed a rate-determining C−H(D) bond breaking in the transition state of the first slow carbonyl radical forming step.68 The mechanism that has been proposed for this functionalization is similar to that proposed previously for the TBADT-mediated reaction of C60 with toluenes and anisoles (see Scheme 4). Accordingly, this reaction should be initiated by the photoexcited TBADT (W10O324−*) through a hydrogen-atom transfer (HAT) from the aldehyde. Subsequently, the carbonyl radical is trapped by the C60 radical anion, affording, after protonation, the observed acylated fullerene adducts.68 These acylated fullerenes have been shown to be excellent C60H2 precursors, since they can be quantitatively converted into the simplest [60]fullerene hydride C60H2 upon simple treatment with basic Al2O3.71 2.1.1.3. Addition of α-Hydroxyl- or α-Oxy-Alkyl Radicals. In 2010, our group reported a facile free-radical route toward the direct functionalization of fullerene C60 with ethers and thioethers.69 In this study, the otherwise unreactive α-C−H bond in a series of structurally diverse mono- or polyethers and sulfides was achieved via decatungstate catalysis. Importantly, this method allows for direct access to the simplest [60]fullerene/crown ether conjugates that possess great potential as building blocks in supramolecular materials science (Scheme 7).69

A mechanistic explanation for this new reactivity of C60 has also been provided, based primarily on analysis of captured intermediates and intermolecular deuterium isotope effect studies.69 According to these studies, the mechanism proposed is similar to that observed in the previous TBADT-mediated reactions of C60 with organic substrates (see Scheme 4). In this mechanistic scheme, the initially formed photoexcited state of decatungstate (W10O324−*) abstracts an α-hydrogen atom from the ether, affording the one-electron-reduced form of decatungstate (W10O325−) and the corresponding α-oxy Ccentered radical. The triplet excited state of C60 regenerates W10O324− through a single-electron transfer (SET) process, thus completing the catalytic cycle. Finally, radical coupling followed by protonation of the resulting anion affords the observed fullerene adducts.69 Another important study that investigated the functionalization of C60 with α-oxy C-centered radicals included development of a direct method for the selective monohydroxyalkylation of C60 (Scheme 8).70 This method involves the Scheme 8. TBADT-Mediated Radical Addition of Alcohols to C60

decatungstate-photocatalyzed reaction of C60 with α-hydroxy C-centered radicals derived from commercially available alcohols, including MeOH (17a), primary (17b−c), propargyl (17d), benzyl (17e), and secondary (17f) alcohols. In this way, the otherwise inaccessible monohydroxyalkylated fullerene derivatives can be selectively obtained in moderate yields (Scheme 8).70 Importantly, the use of ethylene glycol as a mechanistic probe revealed that the reaction pathway does not involve the intermediacy of the fullerene radical cation C60•+, a finding which is in full agreement with the mechanism proposed previously for the TBADT-mediated reactions of C60 with organic substrates.67−70 2.1.2. Radical Reactions of C60 Mediated by Mn(OAc)3, Fe(ClO4)3, and Cu(OAc)2·H2O. In 2003, Wang et al. reported the first free-radical reaction of fullerene C60 promoted by manganese(III) acetate dihydrate [Mn(OAc)3·2H2O].72 In this study, free radicals were generated from various active methylene compounds (i.e., dialkyl malonates). In particular, hydrogen-atom abstraction from β-diesters 19a−b by Mn(OAc)3·2H2O in refluxing chlorobenzene afforded C-centered radicals which reacted with fullerene C60 with short reaction times (20 min) to give singly bonded fullerene dimers 20a−b or with longer reaction times (1 h) to give the corresponding 1,4-bisadducts 21a−b (Scheme 9). Control experiments showed that under these latter reaction conditions the initially formed dimer could be converted to the corresponding 1,4adduct and C60. When a bromo-substituted β-diester (22) was used, this reaction afforded the 1,4- and 1,16-bisadducts of C60 (23 and 24, respectively), as shown in Scheme 9.

Scheme 7. TBADT-Mediated Radical Reactions of C60 with Ethers and Sulfides

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unsymmetrical 1,4-adducts of C60 (30a−e), where the benzyl substituent had come from the reaction solvent (Scheme 11).73

Scheme 9. Mn(OAc)3-Mediated Reaction of C60 with Malonate Esters 19a−b or Diethyl Bromomalonate 22a

Scheme 11. Mn(OAc)3-Mediated Reaction of C60 with Substituted Malonates or Ethyl Cyanoacetate in Refluxing Toluenea

a

a

More recently and in order to avoid the participation of toluene in the reaction, Wang et al. studied the Mn(OAc)3mediated reaction of C60 with diethyl methylmalonate (29a) in an alternative solvent, namely, o-dichlorobenzene at 140 °C.74 The major products of this reaction were the corresponding 1,4- and 1,16-adducts of C60 along with a disubstituted C60fused γ-lactone. The presence of dimethylaminopyridine (DMAP) in this reaction decreased the yield of the disubstituted C60-fused γ-lactone. The selectivity of this reaction was, however, too poor to find further applications in fullerene chemistry. The selectivity toward the disubstituted C 60 -fused γ-lactone was greatly improved when ferric perchlorate [Fe(ClO4)3] was used instead of Mn(OAc)3 as the reaction catalyst, in the presence of acetic anhydride. Thus, the Fe(ClO4)3-mediated reaction of substituted malonate esters 22, 29a−c, and 31a−b with C60 altered the reaction pathway and afforded the desired disubstituted C60-fused lactones 32a− f, albeit in moderate yields (Scheme 12).74

Yields shown are based on consumed C60.

On the other hand, when cyano-substituted activated methylene compounds (25a−b) were used, the corresponding cyclopropane-fused C60 derivatives 28a−b were obtained exclusively (Scheme 10). The proposed mechanism for this Scheme 10. Proposed Mechanism and Reaction of C60 with Malononitrile and Ethyl Cyanoacetate via Mn(OAc)3 Catalysisa

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Scheme 12. Fe(ClO4)3-Mediated Reaction of C60 with Malonate Esters for the Construction of Disubstituted C60Fused Lactones


This study was complementary to previous studies reported by Wang et al., where a series of [60]fullerene-fused lactones 36a−j had been prepared by the Mn(OAc)3·2H2O-mediated reaction of C60 with carboxylic acids 33a−j, carboxylic anhydrides 34a−c, or malonic acids 35a−c (Scheme 13).75−77 The majority of these C60-fused γ-lactones were monosubstituted lactones; the only disubstituted C60-fused γlactone that had been reported in these studies was derived from the Mn(OAc)3-mediated reaction of C60 with isobutyric acid.76 Even in this case, however, the desired C60-fused γ-

reaction includes addition of Mn(OAc)3-promoted C-centered radical from malononitrile or ethyl cyanoacetate (26a and 26b, respectively), followed by intramolecular cyclization through release of hydrogen radical (Scheme 10). Later the same research group found that the aforementioned Mn(OAc)3-promoted reaction of C60 with substituted malonate esters (22 and 29a−c) or ethyl cyanoacetate (25b) in refluxing toluene afforded the corresponding benzyl-substituted G


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Scheme 13. Mn(OAc)3-Mediated Reaction of C60 with Carboxylic Acids, Carboxylic Anhydrides, or Malonic Acids for Construction of Monosubstituted C60-Fused Lactones

Scheme 15. Proposed Mechanism for Formation of C60Fused Lactones from C60 and Carboxylic Acids or Carboxylic Anhydridesa

lactone was a minor product since concurrent formation of a fullerenyl ester, originating from facile generation of a secondary radical via decarboxylation of isobutyric acid, predominated. As recently shown by Zhang, Zhu, and co-workers, also fluorinated phenylacetic acids 33k−p can react with C60, under similar conditions to those reported above, to afford the corresponding fullerene-fused fluorine-containing lactones 36k−p in 15−21% yield (Scheme 14).78

a


Scheme 14. Mn(OAc)3-Mediated Reaction of C60 with Fluorinated Phenylacetic Acids

reaction steps with C60 are the same as those shown in Scheme 15. It should be noted that the final C60-fused lactones can undergo further transformations to afford reductive ringopening products,75 fullerene hemiketals and hemiacetals, fullerenols, and C60-fused dihydrofurans.87 More recently, Wang and co-workers reported that addition of aluminum chloride to the manganese acetate-mediated radical reaction of [60]fullerene with malonates and cyanoacetates substituted with an aryl or a benzyl group (31b, 44a−e) can switch the reaction pathway shown in Schemes and 12, thus affording C60-fused tetrahydronaphthalene and indane derivatives 45a−f (Scheme 16).79 The proposed mechanism for this reaction includes once again a Mn(OAc)3-mediated radical addition followed by a Friedel−Crafts-type annulation with the assistance of AlCl3.

The mechanism that has been proposed for formation of these lactones is shown in Scheme 15. According to this mechanism, carboxylic acids (33) react with Mn(OAc)3·2H2O to give manganese(III) carboxylates (37), which are deprotonated at the α-carbon by DMAP, followed by oxidation with another molecule of Mn(OAc)3·2H2O to generate the corresponding C-centered radicals (38). Addition of these radicals to C60 produces fullerenyl radicals 39 which cyclize to radicals 40. Loss of Mn(II) species from 40 then affords the corresponding C60-fused lactones 36. Similarly, carboxylic anhydrides 34 can be deprotonated by DMAP, followed by oxidation with Mn(OAc)3·2H2O to afford radicals 41, which add to C60 to generate fullerenyl radicals 42. Cyclization of the radicals 42 to 43 and subsequent oxidation by a second molecule of Mn(OAc)3·2H2O produces the observed C60-fused lactones 36. As for the malonic acids (35a−c), reactions probably proceed via radicals 38, which are generated from the malonic acids by the action of Mn(OAc)3·2H2O; subsequent

Scheme 16. Reaction of C60 with 2-Benzylmalonates, 2Arylmalonate, and 2-Arylcyanoacetates in the Presence of Mn(OAc)3 and AlCl3a

a


H


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(Scheme 18),80 as has been similarly observed by Wang et al. (i.e., for substituted malonates, see Scheme 11).73 The mechanism that has been proposed for the reaction of C60 with α-unsubstituted β-diketones or β-ketoesters is shown in Scheme 19.80 Initially, the organic substrate (47a−g) reacts

Interestingly, in one of the above studies, Wang et al. showed that the lead(IV) acetate-promoted radical reaction of [60]fullerene with carboxylic acids, including those mentioned above (33a−i) for the construction of C60-fused γ-lactones, yields selectively a different type of fullerene product, namely, a fullerenyl ester (Scheme 17).76

Scheme 19. Proposed Mechanism for the Mn(III) AcetateMediated Reactions of [60]Fullerene with α-Unsubstituted β-Diketones or β-Ketoesters

Scheme 17. Pb(OAc)4-Promoted Radical Reaction of C60 with Carboxylic Acids

Similarly, Gao and co-workers studied systematically the freeradical reactions of some β-dicarbonyl compounds (i.e., malonate esters, β-keto esters, and β-diketones) in chlorobenzene and/or toluene with [60]fullerene in the presence of manganese(III) acetate dihydrate [Mn(OAc)3·2H2O].80 These substrates showed different tendencies to generate dihydrofuran-fused C60 derivatives (48a−g) or 1,4-bisadducts (30a, 50a− d). In particular, dihydrofuran-fused C60 derivatives could be formed by treatment of α-unsubstituted β-diketones (47a−e) or β-ketoesters (47f−g) with C60 in refluxing chlorobenzene (Scheme 18). On the other hand, solvent-participated unsymmetrical 1,4-bisadducts (30a, 50a−d) were obtained through reaction of [60]fullerene with dimethyl malonate (19a) or αsubstituted β-dicarbonyl compounds (29a, 49a−c) in toluene

with Mn(OAc)3·2H2O to produce its Mn(III) enolate salt (51a−g), which may yield the fullerene radical 53a−g via one of two paths. In the first pathway, the enolate 51a−g adds directly to C60 with loss of Mn(II), whereas in the second pathway, radical 52a−g adds to C60. In chlorobenzene, intermediate 53a−g is rapidly enolized again to generate 54a−g, which undergoes cyclization to afford dihydrofuranfused [60]fullerene derivatives 48a−g with loss of Mn(II). When the reaction was performed in toluene [i.e., in the case of dimethyl malonate (19a) or α-substituted β-dicarbonyl compounds (29a and 49a−c)], the corresponding intermediate fullerene radical 53 is coupled with a benzyl radical derived via H-atom abstraction from toluene to afford the final 1,4bisadducts 30a and 50a−d. Similar results regarding the reaction of β-diketones and βketo esters with C60 in the presence of Mn(OAc)3·2H2O have been independently reported by Wang et al. (Scheme 20).81 In this study, addition of 4-dimethylaminopyridine (DMAP) allowed for lower reaction temperatures (80 °C), shorter reaction times, and, usually, higher product yields for the corresponding C60-fused dihydrofuran derivatives.81 Moreover, it was found that copper(II) acetate monohydrate [Cu(OAc)2·H2O] could replace manganese(III) acetate dihydrate [Mn(OAc)3·2H2O] in these radical reactions of C60 (Scheme 20). On the other hand, reaction of C60 with aromatic methyl ketones 55a−c, under similar conditions, gave two different products, namely, once again dihydrofuran-fused C60 derivatives 56a−c and/or methanofullerenes 57a−c (Scheme 21). In this latter reaction, Cu(OAc)2·H2O was found to favor formation of methanofullerenes.81

Scheme 18. Mn(OAc)3-Mediated Free-Radical Reactions of [60]Fullerene with β-Dicarbonyl Compoundsa

a

Yields shown are based on consumed C60. I


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Scheme 22. Radical Reactions of [60]Fullerene with βEnamino Carbonyl Compounds Mediated by Manganese(III) Acetate

Scheme 20. Mn(III)− or Cu(II)−Acetate-Mediated Reactions of [60]Fullerene with β-Diketones or β-Ketoesters in the Presence of DMAP

Scheme 21. Cu(II)−Acetate- and Mn(III)−AcetateMediated Radical Reactions of [60]Fullerene with Aromatic Methyl Ketones Mn(OAc)3·2H2O-mediated reactions of C60 with β-keto esters and β-diketones (Scheme 19).84 Another class of amino compounds that has been reacted with C60 under Mn(III) catalysis includes the tertiary diallylamines 60a−d (Scheme 23).88 The major products of this reaction were the corresponding trans-pyrrolidinofullerene isomers 61a−d accompanied by a small amount of the corresponding cis isomers and, in two cases, traces of cyclopentafullerenes 62a−b (Scheme 23). Exclusion of Mn(OAc)3 from the reaction of C60 with 61a resulted in exclusive formation of 62a in 25% yield (based on consumed C60). The proposed reaction mechanism for the formation of pyrrolidinofullerene isomers 61 is shown in Scheme 24. Mn(OAc)3 adds to diallylamine with loss of acetate ion to form 63, which is subsequently oxidized to amine radical cation 64 with concomitant loss of Mn(OAc)2. Deprotonation of 64 by AcO− affords radical 65, which adds to C60 to give fullerenyl radical 66. Repetition of the first three steps generates diradical 67, which cyclizes to afford isomeric compounds 61. 2.1.3. Radical Reactions of C 60 Mediated by CoCl2dppe. Recently, Jin, Yamamoto, and co-workers developed an efficient cobalt-catalyzed hydroalkylation of C60 with active alkyl bromides (68a−l) that affords, at ambient temperature, monoalkylated hydrofullerenes (69a−l) in moderate to high yields (Scheme 25).85 After screening several sets of reaction conditions, including different transition-metal catalysts (i.e., Pd, Rh, Ni, Co) having different counteranions and ligands and different additives (Mn, Fe, Zn, Cu), it was found that the use of CoCl2dppe catalyst [dppe = bis(diphenylphosphino)ethane] with a Mn additive was the most efficient protocol. Moreover, it was found that without H2O the reaction did not proceed at all, whereas the presence of oxygen resulted in a very low conversion of C60. Therefore, all of these reactions were conducted in a glovebox under argon gas, in wet 1,2-dichlorobenzene (o-DCB), at room temperature. This hydroalkylation method has also been applied to the synthesis of monosubstituted fullerenes bearing Zn−porphyrin (69k) (in this case a Fe-based additive was more effective), a fullerenebound dendrimer (69l), and fullerene dimers (Scheme 25). In contrast to the activated alkyl bromides, unactivated alkyl bromides, such as butyl bromide or bromomethylcyclopropane, did not undergo this catalytic hydroalkylation even at elevated temperatures. It is most likely that this reaction proceeds through the Co-catalyzed generation of an active alkyl radical (R•) followed by its addition to C60.

The mechanism that leads to dihydrofuran-fused C60 derivatives is in the most part similar to that shown in Scheme 19. On the other hand, methanofullerenes derive from the intermediate fullerene radicals RC60• upon cyclization with loss of hydrogen radical from the methylene group and is similar to the case shown in Scheme 10. Dihydrofuran-fused C60 derivatives have been also prepared through solvent-free reactions of 2,4-pentanedione (47c), 1,3cyclohexanedione (47d), 5,5-dimethyl-1,3-cyclohexanedione (47e), and ethyl acetoacetate (47h) with C60 in the presence of Mn(OAc)3·2H2O and ceric ammonium nitrate (CAN) under the high-speed vibration milling (HSVM) conditions.82 CAN was found to be a better oxidant than Mn(OAc)3·2H2O in these mechanochemical reactions of β-dicarbonyl compounds with C60. Finally, employment of Fe(ClO4)3 instead of Mn(OAc)3 may provide another alternative for construction of dihydrofuran-fused C60 derivatives in solution as shown recently for reaction of C60 with ethyl acetoacetate (47h).83 Given the structural similarity between β-enamino carbonyl compounds and the enol forms of β-keto esters or β-diketones that had been successfully reacted with C60, Wang and coworkers chose also to study the Mn(OAc)3·2H2O-catalyzed reaction of [60]fullerene with this class of amino compounds.84 Accordingly, reaction of C60 with β-enamino carbonyl compounds 58a−h in the presence of Mn(OAc)3·2H2O in a molar ratio of 1:2:2.5, in refluxing chlorobenzene, was shown to afford the C60-fused pyrroline derivatives 59a−h in yields of 16−62% (Scheme 22). The mechanism that has been proposed for the formation of 59a−h is similar to that for the J


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Scheme 23. Reaction of C60 with Tertiary Diallylamines Mediated by Mn(OAc)3·2H2Oa

a


Scheme 24. Proposed Reaction Mechanism for the Mn(III)-Mediated Reaction of C60 with Tertiary Allylaminesa

a

Reprinted with permission from ref 88. Copyright 2004, Chinese Chemical Society.

2.2. Addition of Fluoroalkyl Radicals to C60

of metal trifluoroacetates [(CF3CO2)nM; M = Ag, Hg, Cu, Pd, Cr] 100−103,108,111 or pyrolysis of trifluoromethyl iodide.104−107,109,112 Analogous methods were used for the preparation of trifluoromethylated C70,104,111,113−121 higher fullerenes,122−130 and aza[60]fullerene.130 Variations in the conditions of these methods (e.g., the nature of the perfluoroalkyl radical precursor, temperature, pressure, reaction medium, duration) allowed for preparation of variegated trifluoromethylated fullerene adducts. The majority of these reactions were shown to be nonselective, and with some exceptions,105,131 separation of the product mixture into pure isomers is either difficult or impossible. However, separation of several poly(trifluoromethyl)fullerenes has been achieved by means of HPLC. Thus, several individual isomers (C60(CF3)2n, C70(CF3)2n, n = 1−10) have been isolated and structurally characterized by means of X-ray crystallography and 19F NMR spectroscopy. At this point, it should be noted that derivatization, especially trifluoromethylation of fullerenes, provides an expedient route toward structural characterization (i.e., cage connectivity) of higher fullerenes (e.g., C74, C76, C78, C84, C88, C92, C94, C96)122,124−129 because of the easier separation of their various trifluoromethylated derivatives.

Fluoroalkyl groups (RF) are known to add to C60 via a radical mechanism.17−19,31,56 Perfluoroalkylation of C60 is among the most thoroughly studied of the radical reactions of [60]fullerene and may result in a plethora of new compounds, mostly polyadducts, with interesting addition patterns and physicochemical properties. Early studies on this reaction type showed that perfluoroalkylation of C60 could be achieved by thermal or photochemical decomposition of fluoroalkyl iodides or fluorodiacyl peroxides.56,89 Soon after these initial studies, Yoshida, Iyoda, and co-workers reported the preparation of 1-fluoroalkyl-1,2dihydro[60]fullerenes (RFC60H) by reaction of C60 with fluoroalkyl halides (RFX; RF = CF2CO2Et, n-C6F13, CF2Br, nC12F25, (CF2)6I) in the presence of Bu3SnH.90 More recent studies have shown that photolysis of alkylmercury compounds [Hg(RF)2]3,91 or high-temperature reaction of C60 or C70 with RFI92−98 provide other routes toward fluoroalkyl radical addition to fullerenes. Trifluoromethylation of [60]fullerene is the most widely studied reaction of C60 with fluoroalkyl radicals and remains an area of ongoing research interest.99−112 Similar to other perfluoroalkyl radicals mentioned above, trifluoromethyl radicals have been typically prepared by thermal decomposition K


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Scheme 25. Cobalt-Catalyzed Hydroalkylation of [60]Fullerene with Active Alkyl Bromides

Scheme 26. Radical Reaction of C60 with Azo(bisisobutyronitrile) and Azo(bisisobutyrate)

(Scheme 27).137 In particular, Wang and co-workers found that the reaction of C60 with 4-substituted phenylhydrazine Scheme 27. Proposed Mechanism and Reaction of Fullerene C60 with Phenylhydrazine Derivatives

Cycloperfluoroalkyl adducts of fullerenes have also been prepared through addition reaction of biradicals generated thermally from terminal diiodoperfluoroalkanes I(CF2)nI to C70 or C60.132,133 Fluoroalkyl radical addition to C60 has been well studied mainly by ESR spectroscopy.13,17−19,31,56 As with alkyl radical additions, fluoroalkyl monoadducts exhibit hindered rotation about the RF−C60 bond and a tendency to dimerize. For example, photochemical addition of perfluoroalkyl radicals [derived by photolysis of perfluoroalkyl iodides in the presence of (R3Sn)2] to C60 affords the corresponding fullerene dimers in good yields (see section 11, Scheme 62).58

hydrochlorides in refluxing chlorobenzene under aerobic conditions affords 1-aryl-1,2-dihydro[60]fullerenes (79a−c), which could be subsequently oxidized to 1-acetoxyl-4-aryl-1,4dihydro[60]fullerenes (80a−c) using Mn(OAc)3·2H2O in a one-pot procedure (Scheme 27).137 Formation of the final adduct was rationalized through the radical addition to C60 of aryl radicals (77a−c) generated in situ from the phenylhydrazine derivative (75a−c) by oxidation with oxygen to produce the fullerene radical 78a−c, followed by hydrogen abstraction and subsequent oxidation by Mn(OAc)3·2H2O (Scheme 27). Upon addition of sodium nitrite (NaNO2) in the presence of oxygen and moisture, the above reaction affords fullerenols of the type 1,4-C60ArOH (81a−d) (Scheme 28).138 The first step of this reaction involves again addition of Ar• radicals to C60. Besides C-centered monoradicals, biradicals have also been employed in fullerene radical functionalization chemistry. For example, Ohno et al. showed that the cycloaddition reaction of C60 with 3,4-fused pyrrolo-3-sulfolenes (82a−c) affords, after SO2 extrusion, a C60-pyrrole derivative (84a−c, Scheme 29).139 The proposed reaction mechanism involves trapping of an intermediate C-centered biradical (83a−c) by a fullerene double bond (Scheme 29).139 Similarly to the decatungstate-catalyzed reaction of C60 with ethers (see section 2.1.1, Scheme 7), tetrahydrofuranyl radicals have been added to C60 by reaction of C60 with arylzinc halides

2.3. Other Reactions of C60 Involving C-Centered Radicals

Photochemical or thermal decomposition of 2,2′-azo(bisisobutyronitrile) (70a, AIBN) or dimethyl azo(bisisobutyrate) (70b, DMAIB) has also been utilized for the production of alkyl radicals.134,135 For example, Ford et al. showed that thermal decomposition of AIBN in an 1,2dichlorobenzene solution of C60 generates 2-cyano-2-propyl radicals (71a) which add to C60 to afford three regioisomeric fullerene adducts, namely, the 1,2-, 1,4-, and 1,16-isomers (72a, 73a, and 74a, respectively) (Scheme 26).135,136 On the other hand, thermolysis of DMAIB in the presence of C60 produced only the corresponding 1,4- and 1,16-isomers (73b and 74b, respectively) (Scheme 26).135 Another C-centered radical addition to C60 involves the addition of aryl radicals derived from phenylhydrazines 75a−c L


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anyl radical to the radical anion of C60. More specifically, it has been proposed that the first step in this transformation is a single-electron oxidation of the arylzinc reagent by fullerene, which generates the corresponding aryl radical (Ar•) and a fullerene radical anion (C60•−), whereas the second step is a hydrogen-atom abstraction from the α position of THF by the aryl radical. Coupling of the 2-tetrahydrofuranyl radical with the fullerene radical anion produces the mono(2-tetrahydrofuranyl) fullerene anion (C4H7O)C60−, which after protonation affords the final adduct (C4H7O)C60H.140 Construction of fullerene-based polymers is another research field where addition of C-centered radicals to C60 is a key feature. Application of the usual radical polymerization methods for construction of polystyrene-C60 (PS-C60) adducts produce, in most cases, a mixture of poorly defined multiadducts of PS. In order to control radical polymerization with C60, a “living” radical polymerization has been developed which includes reaction of TEMPO-terminated polystyrene (87) with C60 (TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl).141 Living radical polymerization has led to the synthesis of well-defined disubstituted polymer derivatives of C60.141,142 The first example of this polymerization technique was reported in 1997 by Fukuda and co-workers and included thermolysis of TEMPO-PS adducts in the presence of excess C60 in odichlorobenzene to afford the corresponding 1,4-bisadduct derivatives of C60 (88) in good yields (56−78%, Scheme 31).141 Bromo-terminated polymers, including bromo-termi-

Scheme 28. Reaction of C60 with 4-Substituted Phenylhydrazine Hydrochlorides in the Presence of NaNO2, H2O, and O2

Scheme 29. Cycloaddition of C60 with 3,4-Fused Pyrrolo-3sulfolenes

in a mixture of THF and DMF.140 This reaction produces a mono(2-tetrahydrofuranyl) adduct of C60 (15c) in good yield (46−63%). On the other hand, in the presence of a copper(I) complex the aryl group of the zinc reagent (85a−g) adds four times, regioselectively, to the mono(2-tetrahydrofuranyl) adduct to produce a pentaadduct 86a−g (Scheme 30).140

Scheme 31. Synthesis of 1,4Dipolystyryldihydro[60]fullerenesa

Scheme 30. Addition of Tetrahydrofuran to C60 through Arylzinc Halide-Induced C−H Bond Activationa

a

Tetraaryl addition products have been isolated as a mixture of diastereomers; yields shown correspond to this mixture of diastereomers. Reprinted with permission from ref 140. Copyright 2008 American Chemical Society.

a


Electron donating on the aromatic ring of the zinc reagents gave higher chemical yields (i.e., 89−94% for MeO-, iPr-, Ph-, and Me-substituted aryl zinc halides 85b−e) than their electron-withdrawing analogues [i.e., 53% and 28% for 4BrC6H4ZnX (85f) and 4-(EtO2C)C6H4ZnX (85g), respectively]. In these latter two cases, where an arylzinc reagent bearing an electron-withdrawing group was employed, the normal pentaaryl adducts C60Ar5H were also formed in 42% and 59% yields, respectively.140 The proposed reaction mechanism involves, as was the case in the decatungstatecatalyzed reaction (Scheme 7),69 addition of 2-tetrahydrofur-

nated polystyrene, have been also added to C60 through an atom-transfer radical polymerization.143 Several other freeradical-mediated polymerizations with C60 have been reported in the literature.144

3. ADDITION OF Si-CENTERED RADICALS Early studies on the radical reactivity of fullerenes showed that hydrogen-atom abstraction from silanes (HSiR3) by photolytically generated tert-butoxyl radicals generates trialkylsilyl M


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radicals, which can in turn add to C60 to afford R3SiC60• radicals (eq 2).16 The Si−C60 bond is slightly longer than the C−C60 bond in an alkyl fullerenyl radical, thus exhibiting a lower rotation barrier when compared to their alkyl analogues. C60 + R3Si• → R3SiC60•, R = Me, Et, i Pr, tert ‐butyl

Scheme 33. Photochemical Reaction of 3,4-Benzo-1,1,2,2tetraisopropyl-1,2-disilacyclobutene with C60a

(2)

Silylated fullerenes have been prepared by addition of silyl radicals, generated upon photolysis of the corresponding di- or polysilanes.145−150 For example, the 1,16-bissilylated adducts 91a−c were isolated as the major products of the reaction of C60 with tert-butyl-substituted silyl radicals 90a−c (Scheme 32).145−147 On the other hand, silylsilyl radicals (90d)

a

Yield shown is based on unreacted C60.

Scheme 34. Photochemical Addition of Cyclo-(Ar2Si)4 to C60a

Scheme 32. Photochemical Reactions of Phenylsilanes with C60

a

Yields shown are based on unreacted C60.

Scheme 35. Photochemical Cycloaddition of C60 with Disilirane 101 and Digermirane 103

generated upon photolysis of the corresponding disilanes (89d) add to C60 to afford, apart from the 1,16-bisadduct (91d), an unexpected 1,2-adduct where the silyl and phenyl groups are attached on the 1,2-positions of C60 (92d) (Scheme 32).145,146 In some cases, this latter adduct was the only isolated product (92e−f).146 Ando and co-workers found that disilylcyclobutanes 93 (Scheme 33)151,152 and cyclotetrasilanes 96a−b (Scheme 34)151,153 also react with C60 through formation of an intermediate Si-centered biradical (94 and 97, respectively) generated upon photochemically induced cleavage of the fourmembered ring followed by formal cycloaddition to the [6,6] double bond of C60. Similar studies have been conducted for a cyclotetragermane analogue [i.e., (Ph2Ge)4].152,153 Photochemical addition of disiliranes to C60 or C70 has also been studied (Scheme 35).154,155 For example, it has been shown that irradiation of a toluene solution of 1,1,2,2tetramesityl-1,2-disilirane (101) and C60 with a high-pressure mercury-arc lamp (cut off < 300 nm) results in formation of

102 in 82% yield (Scheme 35).154 Similar to this case, photochemical addition of 101 to C70 affords the corresponding cycloadduct in 85% yield.155 Also, similar to the bis-silylation of C60, the photochemical bisgermylation of C60 with 1,1,2,2tetrakis(2,6-diethylphenyl)-1,2-digermirane (103) affords the N


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C60• (RS−C70•) radical adducts, which in turn can be detected by ESR spectroscopy.21,32,157 Typically, alkoxy radicals (RO•) have been generated photolytically, either from the appropriate peroxides ROOR (R = t-Bu, PhCMe2, CF3)32 or in a more convenient and safer way from photolysis of dialkoxy disulfides ROSSOR (R = Me, Et, i-Pr, t-Bu, i-PrCH2, t-BuCH2) (Scheme 37).21,157 Similarly, alkyl disulfides (RSSR) and bis(alkylthio) mercury compounds (RSHgSR) have been used as the source of the alkylthio radicals (RS•) (Scheme 38).21,32

corresponding 1,4-cycloadduct (104) in 61% yield (Scheme 35). In addition to disiliranes, also siliranes (silacyclopropanes) have been studied for photochemical silylation of C60.156 Thus, in their recent study, Akasaka and co-workers showed that photochemical reactions of C60 with siliranes (105, 108, 113a− b) afford various adducts, including carbosilylated and hydrosilylated C60 derivatives (Scheme 36).156 Scheme 36. Photochemical Addition of C60 with Siliranes

Scheme 37. Addition of Alkoxy Radicals to C60

Scheme 38. Addition of Alkylthio Radicals to C60

As discussed in section 2, there is only one regioisomer that is expected from the single addition radical adducts RO−C60• and RS−C60•, whereas for C70 there are five possible regioisomeric radical monoadducts RC70• as a result of the five different types of carbon atoms in C70 *(Figure 3). This has been experimentally evidenced by photolysis of MeOSSOMe in the presence of C70 that afforded all five expected regioisomers of MeO−C70• (detected by ESR spectroscopy), although only three of the five expected regioisomers of MeS−C70• could be obtained upon photolysis of dimethyl disulfide (MeSSMe) in the presence of C70.21 Unlike the spectra of RC60• radicals, which typically become more intense at higher temperatures owing to the dissociation of RC60−C60R dimers present in solution,11 the signal intensities in the ESR spectra of RSC60• decrease rapidly above room temperature. This difference in behavior has been attributed to the reversibility of the alkylthiyl radical addition due to the weaker fullerene−sulfur bond, rather than to failure of the corresponding RSC60−C60SR dimers to dissociate.32 Further strong evidence for this conclusion has been provided by the observation of a change in color, from the light amber color of the solution of phenylthiyl radical adducts (PhSC60•) to the purple color of C60, upon cessation of irradiation.9 This observation indicates that this radical addition, in contrast to the alkoxy fullerenyl derivatives, is highly reversible. In a few cases cleavage of the carbon−sulfur instead of the sulfur−sulfur bond of RSSR has also been observed, particularly if the light is not filtered through the Ni/Co sulfate solution.32 For example, radicals of the type RC60• have been exclusively observed when R = benzyl, whereas with tert-butyl disulfide cleavage of both C−S and C−C has led to both thio [(CH3)3CSRC60•] and alkyl [(CH3)3CC60•] radical adducts, respectively. However, even in this latter case, the alkyl (CH3)3CC60• radical adduct has been exclusively observed when the reactions are conducted at elevated temperatures.32 Molecular mechanics calculations performed for CH3SC60• have shown, in a similar manner to the corresponding ethyl

4. ADDITION OF O- AND S-CENTERED RADICALS Besides addition of C- and Si-centered radicals, addition of Oand S-centered radicals to fullerenes has been well documented in the literature. Thus, several alkoxy (RO•) and alkylthio (RS•) radicals are known to add to the C60 (or C70) sphere, affording the corresponding RO−C60• (RO−C70•) and RS− O


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adduct,15 a preference for an asymmetric conformation for this radical, that is, the methyl group of the CH3S moiety is placed directly above one of the two 6-membered fused rings of C60 (Figure 4).32

butylperoxy groups added around the central part of C70 by consecutive 1,4-addition. Further studies showed that this reaction proceeds in a stepwise radical addition manner to give a mixture of products with a different number of tertbutylperoxy groups attached to the fullerene cage.160−162 Thus, for C60 this reaction affords adducts with general formula C60(O)m(OOtBu)n (m = 0, n = 2, 4, 6; m = 1, n = 0, 2, 4, 6) through a stepwise cyclopentadienyl addition mechanism.159,160 Low concentrations of tert-butylperoxy radicals, long reaction times, and irradiation with visible light favors epoxide formation [i.e., C60(O)(OOtBu)4], whereas high concentrations of tertbutylperoxy radical favors homoperoxy adducts [i.e., C60(OOtBu)6].160 In contrast, no epoxy moiety is present on the corresponding products of C70 [C70(OOtBu)n, n = 2, 4, 6, 8, 10], which are formed through both equatorial and cyclopentadienyl addition modes.161 Other peroxy radicals such as cumyl peroxy radical [Ph(Me2)COO•] react similarly with C60 in the presence of a catalytic amount of Ru(PPh3)3Cl2, but in this case a mixture that is difficult to purify is formed.160 Troshin et al. have shown that reaction of C60 with acyl hypohalogenites CF3COOBr or CF3COOI (formed in situ from CF3COOAg and Br2 or I2, respectively) in the presence of water results to formation of an orthoester-type 1,3dioxolanofullerene 117 in 40−50% yield (Scheme 40).163 This method, however, is limited to a specific range of substrates that are stable toward radical halogenation.163

Figure 4. Preferred asymmetric conformation of CH3SC60• radical.

More recently, the addition of S-centered radicals to fullerenes C50, C60, C70, and C100 has been investigated by means of DFT calculations.158 The radical species that have been studied included •S, •SH, •SCH3, •SCH2CH3, •SC6H5, • SCH2C6H5, and the open-disulfide •SCH2CH2CH2CH2S•. Sulfur was found to be the most reactive S-centered radical with a preference of attachment to a [6,6] bond of C60 rather than to a [5,6] bond. Moreover, the stabilizing effect of the phenyl group on the S-centered radical makes •SC6H5 very unreactive, which is not the case for •SCH2C6H5. It was also shown that addition of a single sulfur-centered radical to C60 results in a weak C−S bond. For instance, the binding energy between C60 and SH was found to be 50% smaller as compared with C60 and OH. However, the binding energy of •SR radicals can be increased by adding two or more radicals to C60; for example, in the case of •SH addition, the C60−SH binding energy can be duplicated if two radicals are added in the ortho or para position, although steric effects may not facilitate ortho addition. Gan and co-workers found that tert-butylperoxy radicals (tBuOO•) generated from tert-butyl hydroperoxide (TBHP) and Ru(PPh3)3Cl2 (or other catalysts) add to fullerenes C60 and C70, selectively, to form stable fullerene peroxides C60(O)(OOtBu)4 (115) and C70(OOtBu)10 (116), respectively (Scheme 39).159 The four tert-butylperoxy groups in the C60 mixed peroxide 115 are located around a pentagon, and the epoxy-O is located at what was the remaining [6,6] double bond connected to the same pentagon, whereas the C70 decaadduct 116 possesses C2 symmetry with the 10 tert-

Scheme 40. Reaction of C60 with CF3COOIa

a

Reprinted with permission from ref 163, Copyright 2006 Elsevier.

More recently, a facile synthesis of a series of C60-fused 1,3dioxolanes 119a−j and 121a−k has been achieved using the Fe(ClO4)3-promoted reaction of C60 with ketones 118a−j and aldehydes 120a−k, respectively (Scheme 41).164 The mechanism that has been proposed for this reaction is shown in Scheme 42. According to this mechanistic rationale, ketones (118) or aldehydes (120) react with the H2O in hydrated Fe(ClO4)3·xH2O to produce the Fe(III) complex 122 upon loss of HClO4. Then, homolytical addition of 122 to C60 with elimination of Fe(ClO4)2·(x − 1)H2O generates fullerenyl radical 123, which coordinates with another molecule of Fe(ClO4)3·xH2O to form Fe(III) complex 124. Loss of a Fe(II) species from 124 affords the final C60-fused 1,3-dioxolanes 119/121.164 Building on this work, it was later shown that similar conditions may be applied for reaction of C60 with various arylboronic acids (125a−i), thus providing access to the corresponding fullerenyl boronic esters 126a−i in 13−38% yield (Scheme 43); on the other hand, no or only trace amounts of fullerenyl boronic esters could be obtained for arylboronic acids bearing electron-donating groups such as 4methylphenylboronic acid and 4-methoxyphenylboronic acid as well as alkyl boronic acids such as isopropyl boronic acid.165 The obtained fullerenyl boronic esters could undergo further

Scheme 39. Metal-Catalyzed Reaction of tert-Butyl Hydroperoxide with Fullerenes C60 and C70a

a

Reprinted with permission from ref 159. Copyright 2002 American Chemical Society. P


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Scheme 41. Ferric Perchlorate-Mediated Reaction of C60 with Aldehydes and Ketones

Scheme 43. Ferric Perchlorate-Mediated Reaction of C60 with Arylboronic Acids

functionalization with diols to afford C60-fused dioxane/ dioxepane derivatives.165 The mechanism that leads to fullerenyl boronic esters is in the most part similar to that shown in Scheme 42. The reaction starts with formation of an intermediate Fe(III) complex 127 (analogous to 122) via reaction of boronic acids with Fe(ClO4)3 followed by elimination of HClO4 (Scheme 44). Addition of this Fe(III) complex to C60 generates the corresponding fullerenyl radical 128, which, in turn, coordinates with another molecule of Fe(ClO4)3 to form a second intermediate Fe(III) complex 129. Finally, intramolecular

cyclization with loss of a Fe(II) species affords the observed boronic esters 126 (Scheme 44). Similarly, ferric perchlorate-mediated reaction of C60 with acid chlorides (130a−f) has been recently reported (Scheme 45).166 This reaction affords monohydroxylated fullerenols 1,2C60(OCOR)(OH) 131a−f in 17−47% yield. In the case of cinnamoyl chloride 130f, a C60-fused lactone was also obtained in 26% yield besides the expected 1,2-fullerenol 131f. On the other hand, reaction of C60 with 4-nitrobenzoyl chloride bearing the strong electron-withdrawing NO2 group afforded mainly some unknown byproducts probably due to the higher reactivity of 4-nitrobenzoyl chloride. The initial step of the proposed reaction mechanism includes again formation of an intermediate Fe(III) complex 132 (analogous to 122, Scheme 42) via reaction of acyl chlorides with Fe(ClO4)3 and H2O (derived either from the hydrated H2O in Fe(ClO4)3·6H2O or the reaction solvent) followed by elimination of HClO4 (Scheme 46). Then, addition of this

Scheme 42. Proposed Reaction Mechanism for the Formation of C60-Fused 1,3-Dioxolanesa

a

Reprinted with permission from ref 164. Copyright 2010 American Chemical Society. Q


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been reported to add to C60 and C70; these examples include mostly phosphoryl radicals.3,14,23−26,167−174 Due to the exceptionally high constants for the hyperfine coupling of the unpaired electron with the phosphorus nucleus, phosphoryl radicals have been utilized as “paramagnetic reporters”.3,4 Thus, radical phosphorylation is considered the most informative method for studying the structure of the radical adducts of fullerenes by ESR studies. Phosphoryl radicals have been prepared either by photolysis of diphosphoryl mercury compounds (Scheme 47)3,24−26,167−169 or by hydrogen abstraction from the appropriate hydrophosphoryl compound.23,170

Scheme 44. Proposed Mechanism for the Fe(ClO4)3Mediated Reaction of C60 with Arylboronic Acidsa

Scheme 47. Addition of •P(O)(OR)2 Radicals (derived from photolysis of diphosphoryl mercury compounds) to C60 and C70 a


Scheme 45. Ferric Perchlorate-Mediated Reaction of C60 with Acid Chlorides

Similar to the alkylfullerenyl radicals,12,13 the unpaired electron in phosphoryl-fullerenyl radicals is delocalized mainly over the two six-membered rings adjacent to the C−P bond.171 Moreover, a rotation barrier of 4.8 kcal mol−1 has been calculated for the radical •C60P(O)(OiPr)2.172 Recently, Wang, Murata, and co-workers reported the radical reaction of C60 with P-centered radicals generated from different phosphonate esters via manganese(III) acetate catalysis in chlorobenzene (Scheme 48).173 The different

Scheme 46. Proposed Mechanism for the Fe(ClO4)3Mediated Reaction of C60 with Acid Chlorides

Scheme 48. Manganese(III) Acetate-Mediated Radical Reaction of [60]Fullerene with Phosphonate Esters or Diphenylphosphine

intermediate Fe(III) complex to C60 followed by nucleophilic addition of H2O to the resulting fullerenyl radical (133) with loss of H+ affords the corresponding radical anion 134, which is then oxidized with another molecule of Fe(ClO4)3 to afford the observed products 131.166 phosphonate esters that were employed in this study included dimethyl phosphonate (135a), diethyl phosphonate (135b), and 5,5-dimethyl-1,3,2-dioxaphosphorinan-2-one (135c). Products of this reaction were the corresponding singly bonded fullerene dimers, of which the individual meso (136a−c) and racemic (137a−c) isomers could be separated. In particular,

5. ADDITION OF P-CENTERED RADICALS On the basis of the coordination number of phosphorus in any given compound, P-centered radicals can be divided into phosphinyl (•PR2), phosphoryl [•P(O)R2], and phosphoranyl radicals (•PR4). Thus far, only a few P-centered radicals have R


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excess of Mn(OAc)3 was employed, the corresponding acetoxylated fullerenes 139a−d were obtained selectively (Scheme 50).

reaction of C60 with dimethyl phosphonate (135a) in the presence of Mn(OAc)3 in a molar ratio of 1:2:2, in refluxing chlorobenzene for 50 min, gave the corresponding singly bonded fullerene dimer as a meso and racemic isomeric mixture in 29% yield (88% based on consumed C60). Under similar conditions, the reaction of 135b and 135c afforded again the corresponding singly bonded fullerene dimers in 28% and 34% isolated yield, respectively.173 Later, Wang and co-workers applied the aforementioned conditions in the reaction of diphenylphosphine oxide (135d) with C60, affording once again the corresponding fullerene dimer in 29% (Scheme 48).174 In contrast to the Mn(OAc)3-mediated reaction of C60 with malonate esters that initially gives the singly bonded fullerene dimers and, subsequently, 1,4-bisadducts of C60 (Scheme 9),72 the corresponding reaction with phosphonate esters did not produce the desired 1,4-bisadducts of C60 (i.e., 1,4-C60[P(O)R2]2) even with prolonged reaction times. In addition, the isolated dimers did not afford C60[P(O)R2]2 upon heating. Thus, it was concluded that coupling of fullerenyl radical • C60P(O)R2 with phosphorus radical •P(O)R2 is extremely difficult and/or the •C60P(O)R2 radical undergoes rapid dissociation to give C60 and •P(O)R2. Furthermore, the Mn(OAc)3-mediated reaction of C60 with the aforementioned phosphonate esters in refluxing toluene did not afford benzylsubstituted fullerenes PhCH2C60P(O)R2, as previously observed for the corresponding reaction with substituted malonate esters (Scheme 11)73 but, instead, the corresponding [R2P(O)C60]2 dimers (meso and racemic isomers). Thus, the reaction behavior of phosphorus radicals is not similar to that observed for C-centered radicals in fullerene chemistry. More importantly, the separation of the individual meso and racemic isomers of [R2P(O)C60]2 dimer could be achieved simply by flash column chromatography on silica gel. Building upon this reaction, Wang and co-workers found recently that the Mn(OAc)3-mediated reaction of C60 with phosphonate esters 135a−c or phosphine oxide 135d affords actually three different types of phosphorylated fullerene derivatives depending on the reaction conditions.174 These three different derivatives are (i) singly bonded fullerene dimers similar to those mentioned above, (ii) hydrofullerenes, and (iii) acetoxylated fullerenes. In particular, it was found that when an excess of the phosphorus compound is used, the corresponding hydrophosphorylated fullerenes 138a−d are obtained selectively (Scheme 49).174 These hydrofullerenes were formed exclusively from reaction of C60 with a phosphorus compound and Mn(OAc)3·2H2O in a molar ratio of 1:10:2 or 1:5:1. On the other hand, when the molar ratio of C60/phosphorus compound/Mn(OAc)3·2H2O was 1:2:10, that is, when an

Scheme 50. Mn(OAc)3-Mediated Reaction of C60 with Phosphonates or Diphenylphosphine Using an Excess of Mn(OAc)3

Wang and co-workers also explored the interconversion between the three types of phosphorylated fullerene derivatives.174 These studies revealed that both singly bonded dimers and hydrofullerenes could be converted efficiently to the corresponding acetoxylated fullerenes. Hydrofullerenes could also be transformed to singly bonded dimers. The reaction mechanism proposed to explain both formation and interconversion of these fullerene products is shown in Scheme 51. Phosphorus radical •P(O)R2 (140), generated from a phosphonate ester or phosphine oxide by Mn(OAc)3, adds to Scheme 51. Proposed Mechanism for the Mn(OAc)3Mediated Reaction of C60 with Phosphorus Compounds

Scheme 49. Mn(OAc)3-Mediated Reaction of C60 with Phosphonates or Diphenylphosphine Using an Excess of the Phosphorus Compound

S


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C60 to give the 1,2-addition-type fullerenyl radical R2(O)PC60• (141), which is in equilibrium with the corresponding 1,4fullerenyl radical (142). Hydrogen abstraction by the former radical species produces the corresponding hydrophosphorylated fullerenes (138). On the other hand, homocoupling of the 1,4-fullerenyl radical (142) affords the corresponding singly bonded dimers (136 and 137), while reaction with an excess of Mn(OAc)3 furnishes the corresponding acetoxylated 1,4adducts of C60 (139).

Scheme 53. Addition of Tributyltin Hydride to C60 in Refluxing Benzene

6. ADDITION OF N-CENTERED RADICALS In contrast to addition of N-centered radical cations to a fullerene radical anion (see below, section 12.1.1), direct addition of N-centered radicals to fullerenes is rather rare in fullerene chemistry. An example of this type of reaction has been reported recently by Wang and co-workers for the synthesis of fullerooxazoles 144a−h. This method includes the ferric perchlorate-mediated free-radical reaction of C60 with various nitriles 143a−h in o-dichlorobenzene under a N2 atmosphere (Scheme 52).175

The same year, 1993, addition of rhenium pentacarbonyl radicals •Re(CO)5, generated upon photodissociation of the Re−Re σ-bond in Re2(CO)10, to C60 was reported (Scheme 54).183 The product observed was assigned as C60[Re(CO)5]2

benzene solution of C60 with a 20-fold excess of Bu3SnH (Scheme 53).182

Scheme 54. Addition of Pentacarbonylrhenium Radicals to C60

Scheme 52. Fe(ClO4)3·6H2O-Mediated Reactions of C60 with Nitrilesa

a

(146), primarily on the basis of a quantitative IR studies. 146 was found to be unstable and decomposed at room temperature. The same adduct could be similarly obtained from the thermal reaction of C60 with (η3-Ph3C)Re(CO)4a known source of •Re(CO)5 radicalsin the presence of CO (Scheme 54).183 Another early example of the addition of metal-centered radicals to fullerenes was the addition of Pt-centered free radicals derived from photolysis of an appropriate precursor containing a platinum−mercury bond.184,185 For example, Tumanskii et al. have shown that photolysis of cis(CF3)2CFHgPt(PPh3)2CHCPh2 (147) generates the Pt-centered free radical •Pt(PPh3)2CHCPh2 (149) which adds to C60 to afford a moderately stable platinumcontaining fullerenyl radical cis-Ph2CCHPt(PPh3)2C60• (150) (Scheme 55).185 EPR studies revealed, in addition to this


7. ADDITION OF METAL-CENTERED RADICALS The organometallic chemistry of C60 has attracted the attention of many researchers ever since the availability of large quantities of fullerenes followed quickly on the heels of their initial discovery. For example, exohedral alkali metal fullerenes (fullerides) containing discrete anionic species C60n− (n = 1− 6) are some of the most important compounds in organometallic fullerene chemistry due to their exceptional properties such as superconductivity or ferromagnetism.176−179 In addition, several studies on the exohedral complexation of C60 with various transition metal clusters have led to the preparation and characterization of a plethora of exohedral metallofullerene complexes with mostly η1-, η2-, η5-, and η6bonding modes.180,181 However, despite the great progress made so far in the organometallic chemistry of fullerenes, the direct addition of metal-centered free radicals to fullerenes has remained relatively unexplored, probably due to the high reactivity of the resulting fullerenyl radicals and/or the low stability of the final adducts. One of the earliest examples of direct addition of metalcentered radicals to fullerenes includes the regioselective 1,2hydrostannylation of [60]fullerene upon thermal treatment of a

Scheme 55. Addition of Platinum-Centered Radicals • Pt(PPh3)2CHCPh2 Generated upon Photolysis of cis(CF3)2CFHgPt(PPh3)2CHCPh2 to C60

radical adduct, the presence of products obtained through multiple addition of carbon-centered fluorinated radicals formed upon homolysis of the (CF3)2CFHg bond.185 The addition of chromium-centered organometallic free radicals to fullerenes C60 and C70 has also been explored by ESR spectroscopy.186 The metal−metal-bonded (cyclopentaT


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dienyl)chromium tricarbonyl dimer [CpCr(CO)3]2 (Cp = η5C5H5 or η5-C5Me5) has been used as a source of Cr-centered free radicals due to the facile and reversible homolytic cleavage of the weak CrCr bond. Quantum-chemical calculations have shown that in the resulting paramagnetic adducts of C60 a chromium metal atom is η2 bonded to the CC bond between two hexagons.186 As similarly observed for the addition of Cr-centered radicals to C60 and C70, Mo-centered radicals [CpMo(CO)3]•, generated photochemically from the corresponding dimer [CpMo(CO)3]2, react with fullerenes C60 and C70 in toluene to give η2-fullerenyl radical-adducts.187 The structures of these adducts have been assigned on the basis of ESR, TGA, Raman, and XPS studies.

upon photolysis of a saturated benzene solution containing 1,2cyclohexadiene.65 Finally, generation of HC60• has also been studied by means of electrochemistry (i.e., protonation of the radical anion of C60).209,210

9. ADDITION OF HALOGENS Halogenation reactions were among the first reactions to be studied in fullerene chemistry.211−218 In general, halogenation of fullerenes is considered a radical process that can be achieved for fluorination, chlorination, and bromination but not iodination.212−220 However, it should be noted that although a radical mechanism has been suggested in several cases, the exact mechanism of halogenation in some other cases is still unclear, and so, it is not unlikely that some of the reactions discussed below do not proceed via a pure radical mechanism. Halogenation of fullerenes is a very important reaction, especially for exploration of the chemistry and connectivity patterns of the carbon atoms in higher fullerenes. Thus, as with perfluoroalkylation (see section 2.2), halogenation of higher fullerene mixtures has enabled confirmation of their cage connectivities by X-ray crystallographic studies.221 In particular, in contrast to C60 and C70, as well as C72 and C74, which have only one IPR (isolated pentagon rule) isomer (with Ih, D5h, D6d, and D3h symmetry, respectively), the cages of other higher fullerenes may be present in two (C76) or more forms (starting from C78). Generally, structural characterization of higher fullerenes has been accomplished by 13C NMR spectroscopy (after chromatographic separation by HPLC), which provides information on the molecular symmetry. However, it does not always confirm the definitive cage isomer because several isomers may exhibit the same molecular symmetry. Additionally, the direct method of structure elucidation by X-ray analysis of single crystals has rather limited application for investigating the structure of nonfunctionalized higher fullerenes due to the difficulty involved in separating individual isomers of higher fullerenes and the orientational disorder of fullerene molecules in the crystal state, even at lower temperatures. On the other hand, derivatization (i.e., halogenation and perfluoroalkylation) can help to elucidate the structure of higher fullerenes because fullerene derivatives are typically much better fixed in crystal packing than the pristine fullerenes, thus facilitating their investigation.221 Fluorination of C60 has been achieved through reaction of C60 with (i) fluorine gas (F2), (ii) metal fluorides (e.g., MnF3, K2PtF6, CoF3, TbF4, CeF4), (iii) noble gas fluorides (e.g., XeF2, KrF2), or, less efficiently, with (iv) halogen fluorides (e.g., BrF5, IF5).217 Direct fluorination of C60 with F2 proceeds in a stepwise manner,222 and except for C60F48,223−226 it results in a mixture of C60Fn with a wide range of compositions.213,214 A more selective fluorination method of C60 involves its treatment with metal fluorides (MnF3, CoF3, K2PtF6, etc.) at high temperatures. For example, MnF3 leads selectively to the formation of C60F36,227 whereas fluorination of C60 with K2PtF6 affords C60F18.228,229 Several other isolated examples can be found in the literature.213,214,230,231 Fluorination of lower polyfluorofullerenes with fluorine also proceeds via a radical mechanism.218 Apart from FC60, Morton and co-workers reported detection by EPR spectroscopy of four of the five possible isomers of FC70 derived from addition of fluorine atoms (from SF5Cl) to C70.232 Moreover, it has been shown that fluorination of C70 by MnF3 affords a mixture of C70F36/38/40, with C70F38 being the major product.227

8. ADDITION OF HYDROGEN ATOM(S) Hydrogenated fullerenes, also known as fullerene hydrides, are the simplest derivatives of the fullerene family of carbon allotropes. Due to their unique properties and their synthetic utility, several direct or indirect71 protocols have been developed for their preparation.188−191 Typically, radical hydrogenation of fullerenes involves addition of hydrogen atoms to the CC double bonds of fullerenes. Among the most important methods for achieving this hydrogenation process is the direct reaction of C60 with hydrogen gas at elevated temperatures and pressures, which presumably proceeds through HH bond cleavage of molecular hydrogen to produce H atoms that subsequently add to fullerenes C60, C70, or fullerite (a mixture of crude C60 and C70).192−203 Alternatively, generation of hydrogen atoms and their direct addition to C60 has been also achieved by photolysis of thiophenol, tri-n-butyltin hydride, or 1,4-cyclohexadiene in a C60-saturated benzene solution.27 Hydrogen atoms have also been reacted with C60 under matrix isolation conditions in cyclohexane at 77 K204 or solid neon at 4 K.27 On the other hand, indirect methods for generation of hydrogenated C60 radicals (HC60•) including (i) photolysis of di-tert-butylperoxide in the presence of various alcohols (i.e., MeOH, EtOH, cyclohexanol, 2-pentanol, 2-butanol)205 and (ii) photoreduction of acetophenone, benzophenone, or acetone again in the presence of various alcohols (i.e., 2-propanol) have been investigated.205 In these two methods, the tert-butoxy radical or the photoexcited carbonyl group of the various ketones, respectively, abstracts a hydrogen atom from the alcohols; subsequent formation of HC60• is the result of a sequential electron/proton transfer reaction (eq 3) •

+

R1R 2COH + 1C60 / 3C60 → [R1R 2COH + C•− 60 ] → R1R 2 •

CO + HC60

(3)

where R1R2Ċ (OH) derives from activated alcohol or photoreduced ketone in the first or second method, respectively. Moreover, similar to the alkyl fullerenyl radical analogues (RC60•),11,14,57,59,206 HC60• tends to dimerize to form (C60H)2.204,207 As with C60, addition of hydrogen atoms to C70 has been achieved by irradiation of C70 in dry benzene solution containing benzophenone and isopropanol. This reaction afforded adducts of 5 different types: four isomers of HC70 and a single isomer of H3C70.208 At the same time, three of the five possible isomers of HC70• were detected by ESR studies U


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tion of C70 in neat bromine, o-dichlorobenzene, or CS2 produces C70Br10.258

Apart from polyfluoro[60]fullerenes, polychloro derivatives of C60 have been also studied.213,214,233 For example, it has been found that chlorination of C60 by chlorine gas at 250 °C gives a light orange mixture of chlorinated fullerenes C60Cln where an average of 24 chlorine atoms have been attached to C60.234 Moreover, reaction of C 60 with an excess of iodine monochloride (ICl) in either benzene or toluene produces mainly C60Cl6, which has been proven to be isostructural with C60Br6.235,236 Since the synthesis of C60Cl6 in toluene, which is a good radical scavenger, is slower than in benzene, it has been suggested that a radical mechanism is in operation for chlorination of C60. Preparation of other highly chlorinated [60]fullerene derivatives, including C60Cl24 (upon treatment of C60 with VCl4),237 C60Cl28 (upon treatment of C60 with ICl),238 and C60Cl30 (upon treatment of C60 with ICl, ICl3, or SbCl5),238,239 has also been reported.240,241 With regard to C70, its chlorination with ICl in benzene leads to the Cs-symmetric compound C70Cl10.242 More recently, preparation of highly chlorinated derivatives of C70 such as C70Cl28 (treatment of C70 with SbCl5, VCl4 or PCl5)243 and C70Cl16 (treatment of C70 with Br2/TiCl4)244 as well as of higher fullerenes such as C76Cl18 (treatment of C76 with Br2/ TiCl4),245 C76Cl24 (treatment of D2-C76 with SbCl5),246 C76Cl34 (treatment of D2-C76 with ICl),247 C80Cl12 (treatment of C80 with Br2/TiCl4),248 C84Cl32 (treatment of C86 with VCl4),249 C88Cl16 and C88Cl22 (treatment of C88 with VCl4 or SbCl5),250 and seven C90Cln compounds (n = 22, 24, 28, 32; treatment of C90 with VCl4 or SbCl5)251 containing six different isolated pentagon rule (IPR) C90 cages has been reported. An interesting phenomenon that has been observed during the chlorination of higher fullerenes is their carbon cage isomerization.252,253 For example, reaction of D2-C76 (the conventional isomer, which obeys the isolated pentagon rule (IPR)) with SbCl5 afforded the non-IPR chlorinated product C76Cl24, which contains five pairs of fused pentagons.246 On the other hand, when the same reaction was carried out with VCl4 (or PCl5), no skeletal rearrangements were observed, thus highlighting the key role of the chlorinating agent in this rearrangement; this latter conclusion was further supported by theoretical investigations.253 Yet another example of an unexpected rearrangement was observed in the chlorination of C86 with VCl4.249 In this case, the structure of the isolated chlorinated adduct C84Cl32 is the result of an unusual carbon cage rearrangement involving C2 loss and formation of a heptagon ring.249 Thus, it can be envisioned that chlorination of higher fullerenes may lead to other unconventional fullerene structures, as observed for C76Cl24 (multiple pentagon adjacency) or C84Cl32 (heptagon formation). Bromination of fullerenes is also considered a radical process. Bromination of C60 can be conducted with bromine either neat or in solution. Treatment of C60 with neat bromine affords the bromofullerene C60Br24, which is a yellow orange crystalline compound.254 This reaction proceeds in a stepwise manner to give successively C60Br8, C60Br14, and, finally, C60Br24.255 On the other hand, bromination of C60 with bromine in benzene or CCl4 affords C60Br6, whereas in CS2 or CHCl3 it affords C60Br8.256 Additionally, it has been reported that reaction of C60 with bromine in various organic solvents (i.e., 1,2- and 1,3dichlorobenzene, 1,2-dibromobenzene, 1,2,4-trichlorobenzene, chlorobenzene, bromobenzene, and CS2) affords bromides C60Br6, C60Br8, or C60Br24, the exact distribution of which depends on the bromine concentration.257 Similarly, bromina-

10. ORGANOFULLERENYL RADICALS RC60• VIA OXIDATION OF ORGANOFULLERENYL ANIONS RC60− A close inspection of fullerene radical chemistry shows that apart from the parent fullerenes (i.e., C60, C70, etc.), another fullerene form also takes part in radical reactions, namely, the fullerene radical derivative RC60• formed via oxidation of RC60− anions. Oxygen is one of the most commonly employed oxidants for this process (Table 1).259−263 Typical examples Table 1. Formation of RC60• Radicals via Oxidation of RC60− Anions by O2

include the reaction of C60 with (a) PhCH2ONa/PhCH2OH (Table 1, entry 1), 259 (b) 1-(4-methoxyphenyl)-1(trimethylsilyloxy)ethylene in the presence of the complex of KF and 18-crown-6 (Table 1, entry 2),260 (c) lithium fluorenide (Table 1, entry 3),261 and (d) active methylene compounds in the presence of bases under HSVM conditions (Table 1, entries 4−6).263 The key step in all these reactions is the O2-mediated oxidation of the anion RC60¯ (152a−k), which is formed through a nucleophilic addition of the appropriate nucleophile (151a−k) to C60. The resulting free-radical intermediates RC60• (153a−k) afford the final products (154a−k) through V


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silylmethylcopper reagent takes place as smoothly as that of arylcopper reagents (Scheme 57).265 In the pentaphenyl fullerene adduct C60Ph5H the five phenyl groups are introduced around one pentagon of the fullerene molecule, which is then converted to a cyclopentadiene. The phenyl groups surround the cyclopentadiene moiety and form an interesting cavity structure. Penta(aryl)[60]fullerenes exhibit many fascinating physical and chemical properties.272−278 Pentaarylation of C60 has been proposed to proceed through a stepwise mechanism as shown in Scheme 58.265 First addition

different reaction pathways including (i) addition of molecular oxygen followed by an intramolecular rearrangement,259 (ii) a second oxidation by O2 followed by intramolecular rearrangement260,263 or radical addition,263 (iii) a second nucleophilic addition/oxidation by O2,261 or (iv) dimerization.261 In addition to the nucleophilic reagents mentioned above, addition of organometallics (especially alkyl, allyl, or aryl Grignard and organolithium reagents) to the highly electrophilic C60 has been extensively studied since shortly after the discovery of fullerenes.264 A very important feature of this reaction is that it stops at the monoaddition stage and does not proceed further to multiaddition stages; the desired multiaddition may however be achieved in the presence of an oxidant (e.g., molecular oxygen) as found for the bis-addition of Grignard reagents to C60.265−268 In this case it has been proposed that the initially formed monoalkyl[60]fullerene anion (C60R−) is oxidized in situ to the monoalkyl[60]fullerene-radical (C60R•), to which the second equivalent of the Grignard reagent then adds (Scheme 56).

Scheme 58. Proposed Mechanism for the Pentaarylation of [60]Fullerenea

Scheme 56. Bis-Addition of Grignard Reagents to C60 in the Presence of Molecular Oxygen

In contrast to Grignard reagents, the multiple additions of organocopper reagents to C60 have been exploited much more frequently, especially by the research group of Nakamura.265 Thus, in the past decade, pentaaddition of aryl or alkyl groups as well as octa- and decaadditions to fullerenes has been well developed, affording a wide variety of fullerene derivatives.265 Initially, multiaddition of organocopper reagents to C60 was discovered serendipitously in 1996 and included the regioselective 5-fold addition of a phenylcopper reagent to C60 to afford the pentaphenyl fullerene adduct, C60Ph5H (155, R = Ph), as an orange powder in almost quantitative yield (Scheme 57);269,270 this reaction could also be conducted in up

a


of one molecule of Ar2Cu− produces a mono(aryl)[60]fullerene anion (156); this is the slowest step in the whole reaction scheme. This anion is oxidized by the copper(I) salt to afford a mono(aryl)[60]fullerene radical (157), which undergoes a further addition reaction to give a diaryl[60]fullerene radical anion (158). Oxidation of this latter species gives a neutral 1,4bisadduct (159). This intermediate is probably more strained and more reactive than [60]fullerene. Subsequently, the same addition/oxidation reactions may occur twice more until addition of the fourth aryl group (160). The fifth aryl group adds to the reactive fulvene moiety of 160 to produce the final cyclopentadienide. Formation of metallic copper in this reaction confirms the role of the copper(I) atom as both an oxidizing agent and the carrier of the aryl group. The cyclopentadienide part of the product donates electrons to the bottom 50π system through endohedral homoconjugation,279 and this must be the reason that the reaction stops at the pentaaddition stage, even when a large excess of the organocopper reagent is used. On the other hand, octa- and decaadditions take place when an excess of pyridine is present in the reaction mixture. Thus, although the addition of more than five organic groups using the copper methodology initially appeared to be impossible,

Scheme 57. General Scheme for the Pentaaddition of Organocopper Reagents to C60

to 100 g scale.265 Similarly, it was later found that an organocopper reagent (RCu, R = 4-CF3−C6H4) adds three times to C70 to afford the tris-arylated fullerene C70R3H in quantitative yield.271 The reaction conditions include treatment of C60 or C70 with an excess amount of the organocopper reagent prepared from RMgBr (16 equiv) and CuBr·SMe2 (16 equiv) at room temperature followed by mildly acidic quenching (NH4Cl).270,271 Addition of a methylcopper and a W


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because the reaction stopped completely at the pentaaddition stage, it was later found that when an excess of pyridine is added to the reaction mixture octa- and decaaddition of arylcopper reagents can be achieved.280−285 Yet another example of fullerenyl radical formation (C60R•) upon one-electron oxidation of fullerene anion (C60R−) includes the solid state dimerization of C60 with KCN under HSVM conditions.286 In this case, the initially formed anion C60(CN)− is oxidized by C60 to give the radical anion C60•−, which could couple with a neutral molecule of C60 to give C120•−, followed by a second electron transfer to another C60 molecule to afford the final [2 + 2] dimer C120.286 Another example of this type of reactivity of fullerenes concerns the reaction of C60 with carbanions generated from aldehydes. In particular, Wang and co-workers have shown that the reaction of C60 with aldehydes 162a−c and MeONa− MeOH in anhydrous chlorobenzene in the presence of air at room temperature affords the corresponding fullerene acetals 163a−c, while the same reaction with acetones 162d−g under the same conditions affords fullerene ketals 163d−g (Scheme 59).287 Also, the corresponding ethoxy-substituted adducts were obtained in comparable yields under similar conditions when MeONa−MeOH was replaced with EtONa−EtOH.

Scheme 60. Proposed Mechanism for the Reaction of [60]Fullerene with Aldehydes/Ketones and Alkoxidesa

Scheme 59. Reaction of C60 with Aldehydes/Ketones and Alkoxides

Scheme 61. Reaction of [60]Fullerene with Sodium Alkoxidesa

a


a

Yields shown are the total isolated yields for the cis and trans isomers of 170b−d.

with other oxidants [i.e., I2, 1O2, N-chlorosuccinimide (NCS)], but this will be discussed further in the next section where the dimerization of RC60• radicals is reported.57,59,206,289

The mechanism that has been proposed for this reaction is shown in Scheme 60.287 Initially, the carbanion 164 generated by deprotonation at the R carbon of an aldehyde or a ketone attacks C60 to give fullerene anion 165, which is subsequently oxidized by oxygen to give radical 166. Addition of the alkoxide to the carbonyl group on the side chain of species 166 leads to the anionic form of an acyclic hemiacetal/hemiketal 167. Ring closure of intermediate 167 by an intramolecular nucleophilic addition to the fullerene skeleton affords radical anion 168, which is oxidized by air to give acetal/ketal 163. Later, the same research group reported a similar transformation involving reaction of C60 with freshly prepared RCH2CH2ONa/RCH2CH2OH (R = H, Me, Et, Ph) in anhydrous toluene under air atmosphere (Scheme 61).288 In accordance with previous observations,287 addition of the corresponding aldehydes RCH2CHO (R = H, Me, Et, Ph) in the reaction mixture afforded again the same products 170a−d in relatively higher yield and shorter reaction time.288 Similar to the mechanism shown in Scheme 60, oxidation of the intermediate RC60− anion by O2 to the corresponding RC60• is again a key step for completion of this reaction.288 Apart from O2 and Cu(I), one-electron oxidation of the monoanion RC60− (Table 1) has also been reported to occur

11. FULLERENE DIMERS VIA DIMERIZATION OF RC60• RADICALS As discussed in section 2, one of the most remarkable characteristics of the fullerene radical derivatives RC60• is their tendency to dimerize. In particular, pioneering studies on the ESR spectra of a series of RC60• radicals have shown an increase in intensity of the spectrum with increasing temperature, which indicates that the fullerenyl radicals are in an equilibrium with their diamagnetic dimers (eq 4).11,14,20,56,57 RC60−C60R ⇌ 2RC60•

(4)

Both theoretical and experimental studies have shown that the fullerene dimers (C60R)2 are usually formed by dimerization of the corresponding RC60• radicals, which, in turn, are generated by various radical reactions. For example, Yoshida and co-workers have shown that photoirradiation of C60 with perfluoroalkyl iodides 171a−c in the presence of (R3Sn)2 affords dimeric C60 derivatives 173a−c/174a−c through addition of fluoroalkyl radicals (172a−c) to C60 followed by X


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recombination of the fluoroalkylated fullerene radical RFC60• (Scheme 62).58

the final fullerene dimers, a highly effective method for dimerization of various monosubstituted hydrofullerenes has very recently been reported by Jin, Yamamoto, and coworkers.60 This method utilizes Cu(OAc)2 as a catalyst in the presence of a small amount of dimethylformamide (DMF) at room temperature under air in 1,2-dichlorobenzene (o-DCB) (Scheme 63). The so-obtained single-bonded fullerene dimers were prepared as a mixture of racemic and meso isomers in excellent yields.

Scheme 62. Photochemical Reactions of C60 with Perfluoroalkyl Iodides in the Presence of Hexaalkylditina

Scheme 63. Cu(OAc)2-Catalyzed Homodimerization of Monosubstituted Hydrofullerenesa

a

Yields based on consumed C60.

Singly bonded fullerene dimersvia dimerization of RC60• radicalshave also been obtained through a Mn(OAc)3mediated radical reaction of C60 with dialkyl malonates (Scheme 9)72 or phosphonate esters (Scheme 48).173 Moreover, similar to the corresponding alkylated fullerene dimers (RC60)2, the (C60H)2 dimer has been prepared by dimerization of the HC60• radical generated by protonation of the radical anion C60•−.207 The desired alkylated fullerene radicals RC60• (176) have also been obtained via a one-electron oxidation of the corresponding monoanion RC60− (175) by various oxidants [i.e., I2, 1O2, NCS] (Table 2).57,59,206,289 In contrast to the majority of the aforementioned methods, which are not very effective in terms of yield and diversity of

a

Reprinted with permission from ref 60. Copyright 2012 Wiley-VCH.

A plausible mechanism for this reaction involves a oneelectron oxidation of RC60H by Cu(OAc)2 followed by elimination of a proton from the resulting RHC60•+ to afford the corresponding fullerenyl radicals RC60•, which, in turn, produces the corresponding singly bonded dimers. It should be noted that the Cu(OAc)2 catalyst, combined with a small amount of DMF under air, is crucial for efficient formation of the corresponding singly bonded fullerene dimers. Notably, this method also allowed for preparation of various cross-dimers, which are otherwise difficult to obtain, through cross-dimerization of two different hydrofullerenes.60 An interesting finding was that the cross-dimers are in equilibrium with the homodimers in solution. In fact, conversion of a pure cross-dimer to the corresponding homodimers could be monitored by NMR spectroscopy, which implies that the single-bonded dimers dissociate to the stable monoradicals in solution, which may then recombine to form various dimers. This observation is in good agreement with previous studies reported by the groups of Komatsu and Wang.59,173 More recently, the same research group reported an alternative method for catalytic homo- or cross-dimerization of monofunctionalized hydrofullerenes that circumvent the use

Table 2. Oxidation of RC60− Anion and Subsequent Dimerization of the Initially Formed RC60• Radicals

Y


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of transition metals.61 This method employs catalytic amounts of NaOH in a mixture of o-DCB/THF (3:1) under air atmosphere. By applying these conditions, dimerization of 178c−k was accomplished with almost equal efficiency with that observed in the corresponding Cu(OAc)2-catalyzed reaction (Scheme 63). The NaOH-catalyzed dimerization of RC60H proceeds again via intermediate formation of RC60•, although in this case the mechanism that leads to RC60• is completely different from that proposed for the Cu(OAc)2catalyzed dimerization. Thus, it has been proposed that the RC60• radical is formed via a one-electron oxidation of the initially formed RC60− monoanion by O2 (or in situ generated O2•−).

mides.319 In addition, C60 has been found to form weak ground state complexes with a variety of amine donors.294,320 The excellent electron-accepting properties of the fullerene moiety together with the exceptionally low reorganization energy of the rigid fullerene core in electron transfer reactions have triggered covalent binding of fullerene to a number of interesting electroactive donor molecules, especially for photovoltaic applications. For example, covalent linkage of fullerene to porphyrins allows for development of molecular dyads in which photoinduced energy and/or electron transfer takes place. In many such architectures C60 accelerates photoinduced charge separation and retards charge recombination in the dark.321−323 As a consequence, long-lived charge-separated states can be generated that can be used for subsequent energy conversion processes. To date a tremendous variety of fullerene-based electron donor−acceptor systems have been established, in most of which the charge transfer evolves from a photoexcited state; in contrast, ground state charge transfer is rather unusual.324−329 The fundamental aspects and applications of photoinduced energy and electron transfer in fullerenebased donor−acceptor systems is, however, out of the scope of the present article. Further information can be found within the given references, as well as in a number of relevant reviews.321−330 This section will focus only on well-established radical anion reactions of parent fullerenes with both fundamental and synthetic relevance. 12.1.1. Addition of Amines to C60. Addition of amines and diamines to C60 was one of the first reactions to be investigated in fullerene chemistry.331−333 With regard to the reaction mechanism, nucleophilic addition and radical addition of amines to C60 are closely related and in some cases it is difficult to decide which mechanism is actually operating.334 For example, the first step in the reaction of C60 with amines is a single electron transfer (SET) from the amine to the fullerene,331,334 whereas, the product amines are finally formed via a complex sequence of radical recombinations, deprotonations, and redox reactions.335−337 Typically, the addition of primary and secondary aliphatic amines331,332,338 or diamines332,339−343 to C60 is initiated by a SET process (from amine to C60) to afford a radical ion pair, namely, the C60 radical anion (C60•−) and the amine radical cation (R2NH•+).337 This process is favored in polar solvents. As a specific example, the reaction between an excess of a secondary amine, such as morpholine (181), and [60]fullerene in the presence of oxygen affords exclusively dehydrogenated amino adducts with a well-defined 1,4-addition pattern with respect to the amino groups (i.e., bismorpholino[60]fullerene 182 and tetrakismorpholino[60]fullerene epoxide 183) as well as morpholino[60]fullerene dimer 177c (Scheme 64).206 The presence of a free-radical mechanism is strongly supported by this addition pattern as first reported for free-radical additions to [60]fullerene by Krusic et al. (i.e., successive 1,4-radical additions to [60]fullerene, Scheme 1).7 Reaction of C60 with piperidine showed similar results.206 More recent studies include the one-step multiple addition of amines 181 and 184a−k to C60 under either photochemical or thermal aerobic conditions for the synthesis of tetra(amino)fullerene epoxides 183 and 185a−k, respectively, in moderate to excellent yields (Scheme 65).344,345 Under photochemical conditions, the tetrakismorpholino[60]fullerene epoxide 183 was prepared in much higher yield compared to that described above for the thermal conditions.344 However, after careful reexamination of this photoreaction it was found that addition

12. ION RADICAL REACTIONS OF FULLERENES Apart from the radical reactions involving neutral fullerenes (i.e., C60 or C70) or fullerene radical derivatives RC60•, a major part of fullerene radical chemistry involves radical ion reactions, that is, reactions of the radical anion (C60•−) or radical cation (C60•+) of C60 generated upon photoinduced or, in some cases, thermally induced electron transfer processes.3,290 Conversion of fullerenes, especially C60, into an ion radical brings about a significant change in its electronic structure and a corresponding alteration in its reactivity. This conversion allows a great variety of otherwise inaccessible fullerene derivatives to be obtained under mild conditions with good yields. The aim of this section is to provide a detailed overview of the progress that has been made to date in the radical ion chemistry of fullerenes. 12.1. Reactions of Fullerene Radical Anion (C60•−)

Fullerenes (C60 and C70) are well-established electron acceptors. Radical anions of fullerenes are produced mostly via a photoinduced or, to a lesser extent, a thermally induced electron transfer mechanism using organic donors. In particular, the photoinduced electron transfer (PET) process yielding photoexcited fullerenes (C60 and C70) and their derivatives has been extensively investigated since shortly after the discovery of fullerenes.291−303 [60]Fullerene can easily be excited into its triplet state (3C60) with a quantum yield nearing unity, elevating the reduction potential from −0.42 V vs SCE of the ground state to close to 1.14 V vs SCE of the triplet state,295,304 and thus making the triplet excited state of C60 (3C60*) propitious to abstract an electron from suitable donors. PET reactions of fullerenes have been successfully investigated by photochemical techniques, such as laser flash spectroscopy.292,295−302 It has been revealed that one of the common routes of the PET processes of fullerenes includes the excited states of fullerenes accepting electrons from the ground states of electron donors,305−307 and the relative contribution of the singlet states and triplet states of fullerenes to the PET process varies with the strength and concentration of electron donors. 308 As electron donors, aliphatic or aromatic amines,291,294−297,303,306,309,310 tetrathiafulvalenes,300,311,312 and porphyrins/phthalocyanines313−315 have been mostly employed; these compounds have been proven to donate the electron efficiently, affording the corresponding fullerene triplet states. Other substrates that have been studied for an electron transfer reaction with fullerenes, either thermally or photochemically, include aromatic thiols,301 tetraphenylborate and triphenylbutylborate,316 stable free radicals such as nitroxides,317 phenothiazine derivatives,318 and phosphorus triaZ


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Scheme 64. Reaction of Morpholine with C60 in the Presence of Oxygen

Scheme 66. Photochemical Reaction of C60 with Glycine and Sarcosine Esters

among the most critical steps involved in the reaction mechanism (Scheme 66).347 Later studies showed that the reaction between glycine- or sarcosine-methyl esters and C60 can be effectively controlled by different iodo reagents, namely, DIB [(diacetoxyiodo)benzene], DIB-I2, or I2.348 More recently, the photochemical addition of glycine methyl or ethyl ester, (189a−b) has been reinvestigated.349 In this latter study, it was shown that apart from the fulleropyrrolidine monoadduct 187a−b, other polyadducts (i.e., bis-, tris-, and tetrakis-fulleropyrollidine adducts) can also be formed in a stepwise addition manner upon prolonged irradiation regardless of the C60/substrate ratio. Moreover, it was corroborated that the presence of oxygen is necessary to initiate the addition of 189a−b to C60, which is in full agreement with previously reported studies on the photochemical reaction of C60 and amines where intervention of singlet oxygen (1O2) has been proposed as a key step of the reaction mechanism (vide infra).353 In addition to the photochemical reactions mentioned above, thermal reactions of C60 with a series of amino acids and amino acid esters under aerobic and dark conditions have also been investigated.350 In particular, it has been found that treatment of C60 with N-unsubstituted (190a−d) or N-substituted amino acids (192a−d) in o-DCB at 180−200 °C affords the corresponding fulleropyrrolidines in 11−37% yield, as shown in Scheme 67.

Scheme 65. Oxygenative Tetraamination of C60

of DMSO to the reaction mixture allowed the reaction to take place without photoirradiation.345 This latter tetraamination reaction of [60]fullerene with secondary aliphatic amines was found to be far superior to the photochemical method in terms of both the range of applications and the product yield.345 Radical photoaddition of mono-N-substituted piperazines to C60 has also been reported.346 In particular, Troshin and coworkers have shown that the reactions of C60 with piperazines bearing bulky electron-withdrawing groups (i.e., 2-pyridyl, 2pyrimidinyl) are the most selective and yield C60(amine)4O as major products accompanied by small amounts of C60(amine)2. On the other hand, the reactions of fullerene with Nmethylpiperazine and N-(tert-butoxycarbonyl) piperazine are not selective due to different side reactions.346 Glycine esters (H2NCH2COOR) 189a−c add directly to C60 upon photolysis to form pyrrolidine ring-fused fullerenes 187a−c (Scheme 66).347 On the other hand, photochemical reactions between C60 and sarcosine esters (CH3NHCH2COOR) 186a−c, which have one more methyl group than their glycine analogues, yield the same pyrrolidine derivatives 187a−c along with another pyrrolidine-type derivative C60−(CH2N(Me)CHCOOR) 188a−c (Scheme 66).347 Initial addition of a C-centered radical (formed with the assistance of 1O2) to C60 and a C−N bond breakage are

Scheme 67. Thermal Reaction of C60 with N-Unsubstituted or N-Substituted Amino Acids

AA


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Scheme 68. Thermal Reaction of C60 with N-Substituted Amino Acid Esters

On the other hand, unlike the reaction with amino acids, the reaction of C60 with amino acid esters 195a−d was found to be much more complex, affording up to four fulleropyrrolidine derivatives in 0.7−31% isolated yields as shown in Scheme 68. The proposed reaction mechanisms involve uncommon C−N bond cleavages for the generation of aldehydes, which then react with amino acids and amino acid esters to provide azomethine ylides, which, in turn, add to [60]fullerene via 1,3dipolar cycloaddition to afford the observed fulleropyrrolidines.350 At this point, it should be noted that fulleropyrrolidines are commonly obtained by the Prato reaction, that is, 1,3-dipolar cycloaddition of C60 with azomethine ylides; these intermediates are generated in situ by decarboxylation of the iminium salts formed by condensation between Nsubstituted glycines and aldehydes.351,352 On the other hand, the aforementioned reactions involve direct thermal reaction of C60 with amino acids or amino acid esters in refluxing o-DCB without the purposely added aldehydes under aerobic and dark conditions.350 Addition of tertiary amines to C60 is also a radical process.353−360 In this case, the reaction takes place at the carbon α to the nitrogen of the amine. Intervention of 1O2 in the radical reaction of C60 with tertiary amines (i.e., triethylamine) has been proposed when the reaction is carried out in the presence of molecular oxygen or in air-saturated solutions.353,354 A plausible reaction mechanism involves primary interaction of 1O2 with the amine, followed by addition of the resulting radical to the fullerene, to afford, after repeating the same sequence of proton and electron transfer steps, the final pyrrolidine adduct of C60 (Scheme 69).353,354 On the other hand, when photochemical addition of triethylamine to C60 is carried out in deoxygenated toluene, the major product obtained is the corresponding adduct RC60H 203 (Scheme 70).354 In a similar manner to that of primary and secondary amines, the mechanism proposed for the photoreactions between C60 and tertiary amines (i.e., Et3N) starts with a PET from the amine to photoexcited [60]fullerene to give a radical ion pair.354−357 This is followed by deprotonation of the amine radical cation 201 by the [60]fullerene radical anion (C60•−) to yield the corresponding radical pair, which then combine to afford the final adducts (Scheme 70). The final

Scheme 69. Proposed Mechanism for the Photocycloaddition of Tertiary Amines to C60

Scheme 70. Proposed Mechanism for the Photochemical Reaction between C60 and Triethylamine

monoadduct 203 can react further through similar electron/ proton transfer and radical pair recombination processes to afford the corresponding cycloadducts.355,356 Similar to the reaction of C60 with glycine- or sarcosinemethyl esters (Scheme 66),347,348 pyrrolidine ring-fused fullerene multicarboxylates have been produced upon photolysis of aminopolycarboxylic esters in the presence of C60.361 For example, irradiation of the tetramethyl ester of ethylenediaminetetraacetic acid (EDTA) 204 with C60 afforded the corresponding EDTA-containing fullerene monoadducts (205 and 206) along with two adducts where either the nitrogen ethylene bond N−CH2CH2 (187a) or the methylene CH2− CH2 bond (207) of EDTA had been broken (Scheme 71). Similar results were observed with pentamethyldimethyleneAB


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Scheme 71. Photochemical Reaction of C60 with Tetramethyl Ethylenediaminetetraacetate

Scheme 72. Reactions of C60 with Et3N in the Presence or Absence of Aldehydes

RR′C60.367 Their studies revealed a mechanism that involves a combination of electron transfer and SN2 reactions from C602− and the alkyl halides (Scheme 73). In particular, it has been Scheme 73. Formation of Organofullerenes RC60R′ through Reaction of C602− with Alkyl Halides

triaminepentaacetate (DTPA).361 Aminocarboxylic acids are also reactive toward C60 under photochemical conditions, affording, upon decarboxylation, the corresponding organodihydrofullerenes.361 A radical mechanism has been proposed for these photochemical reactions.361 A series of similar pyrrolidinofullerenes bearing ester group functionalities has been prepared through the Pb(OAc)4-mediated reaction of [60]fullerene with the methyl esters of nitrilotriacetic, ethylenediaminetetraacetic, and hexamethylenediaminetetraacetic acids.362 Finally, Wang et al. have shown that [60]fullerene reacts with amino acid ester hydrochlorides and triethylamine in refluxing o-dichlorobenzene to afford unexpected pyrrolidinofullerene derivatives that contain a CH3CH fragment derived from Et3N, presumably through C−N bond cleavage.363 Further investigations revealed interesting reactions between C60 and tertiary amines or reactions of C60 with tertiary amines and aldehydes that afforded cyclopentafullerene derivatives with high stereoselectivity. For example, treatment of C60 and Et3N (1:100) in o-DCB, at 220 °C for 1 h, under dark and aerobic conditions, afforded adduct 208 in 52% yield, whereas addition of 4methoxybenzaldehyde (209a) and butyraldehyde (209b) to this reaction mixture resulted in the formation of products 210a and 210b, respectively, in which the CH3CH group in adduct 208 has been replaced with the RCH moiety of the aldehydes (Scheme 72).363 The proposed reaction mechanism calls once again on initiation by electron transfer from the amine to C60 to form the corresponding amine radical cation and C60•−, respectively. 12.1.2. Addition of Alkyl Halides to C60. It is well-known from the literature that reactions of electrochemically and chemically generated C602− with alkyl halides in benzonitrile yields dialkyl (R2C60) and tetraalkyl (R4C60) adducts of C60.364−367 Fukuzumi, Kadish, and co-workers studied systematically the mechanism of this reaction by employing two different alkyl halides (RX or R′X) that react with C602− in benzonitrile to afford organo[60]fullerenes of the type R2C60 or

proposed that the first step in this reaction sequence involves electron transfer from C602− to RX to form an intermediate radical ion pair C60•− and R•X−. The R−X bond is cleaved upon dissociative electron transfer to afford, upon radical coupling with C60•−, the intermediate RC60−. Final alkylation of RC60− with RX or R′X takes place via an SN2 mechanism (Scheme 73). Interestingly, it has been observed that the solvent has a profound effect on the regiochemistry of this reaction. For example, reaction of C602− with PhCH2Br in PhCN affords the 1,4-(PhCH2)2C60 adduct as the exclusive or major product,365−367 whereas when the same reaction is conducted in DMF 1,2-H(PhCH2)C60 is predominantly formed (in 50% yield) while the amount of 1,4-(PhCH2)2C60 is greatly reduced (10% yield).368 This difference in reactivity has been ascribed to the presence of traces of water residue in DMF, which react with the intermediate RC60− anion to afford via proton transfer the observed 1,2-dihydro[60]fullerenes (RC60H). Moreover, the reactivity difference of the water content in DMF and PhCN was ascribed to the miscibility of the solvent with water.368 Also, similar to the results obtained in PhCN, the corresponding 1,4-di(organo)fullerene derivatives (1,4-C60R2) were the predominant products in the reaction of C602− with other organic halides [e.g., X(CH2)6I, X = Cl, I; X−(CH2)5− CO2Et, X = Cl, I; XCH2CO2Et, X = Br, I; BrCH2COPh; BrCH2−C6H4−CH2OH] in CH3CN.369,370 Mixed 1,4-di(organo)fullerene derivatives of the type 1,4-C60RR′ have also been prepared in CH3CN via the same C602− route.371 Finally, in addition to the major 1,4-bisadduct, also the corresponding 1,16-bisadduct was obtained in the reaction of C602− with sterically hindered 2-bromo-2-methylmalonate esters [CH3C(COOR)2Br, R = Et, t-Bu] albeit as a minor product.372 AC


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Apart from the solvent effect, steric effects also influence the regioselectivity of this reaction, as shown for the reaction of C602− with o-, m-, or p-CH3PhCH2Br and m- or p-BrPhCH2Br in DMF.368 In particular, for o-CH3PhCH2Br the 1,4di(organo)fullerene is still the predominant product. On the other hand, for m-CH3PhCH2Br and m-BrPhCH2Br both 1,2dihydrofullerene and 1,4-di(organo)fullerene are formed as major products, whereas for p-CH3PhCH2Br and p-BrPhCH2Br the 1,2-dihydrofullerenes are the exclusive products and only trace amounts of 1,4-di(organo)fullerenes are formed.368 12.1.3. Addition of Ketene Silyl Acetals to C60. As first reported by Nakamura and co-workers373 and later by Fukuzumi, Mikami and co-workers,374,375 ketene silyl acetals (i.e., 211a−f) add to fullerenes via a photoinduced single electron transfer (SET) to the triplet excited state of the C60, finally yielding the corresponding monoaddition adducts (213a−f, Scheme 74). The mechanism of this reaction involves

Scheme 75. Mono- and Pentaaddition of Enol Silyl Ethers to C60a

Scheme 74. Proposed Mechanism and Reaction for Photoinduced C−C Bond Formation of C60 with Ketene Silyl Acetals

a


which, almost quantitatively, decays via intersystem crossing (ISC) to the energetically lower lying triplet excited state (3C60*).295,304 This triplet excited state has a reduction potential of E°red = 1.14 V vs SCE, thus facilitating reduction of 3C60* to C60•− with a variety of organic compounds. In homogeneous systems, however, fast back electron transfer to the reactant pair generally results in an extremely short lifetime for the generated C60•−, and no net formation of C60•− is therefore observed. To overcome these problems, Fukuzumi et al. introduced the use of NADH analogues as electron donors.305 In particular, it was found that the PET from a NADH analogue, 1-benzyl-1,4-dihydronicotinamide (BNAH), and the dimer analogue [(BNA)2] to the triplet excited state of C60 (3C60*) yields stable C60•− in benzonitrile solution with a high quantum yield.305 The reaction mechanism in the latter process is initiated by an electron transfer from (BNA)2 to the triplet excited state (3C60*) followed by fast C−C bond cleavage in the resulting (BNA)2•+ to give BNA• and BNA+. A second electron transfer from BNA• to C60 yields BNA+ and C60•− (Scheme 76).305 When BNAH is replaced by 4-tertbutylated BNAH (t-BuBNAH), the photochemical reaction with C60 yields selectively instead of the C60•− the tert-butylated anion (t-BuC60−).305 Similarly, selective two-electron reduction of C60 to 1,2dihydro[60]fullerene (1,2-C60H2) has been achieved using another NADH analogue, 10-methyl-9,10-dihydroacridine (AcrH2), under visible light irradiation in deaerated benzonitrile solution containing trifluoroacetic acid (TFA) (Scheme 77).305

radical coupling of the initially formed radical ion pair to give a zwitterionic species (212a−f), which, upon water-assisted removal of the silyl group and protonation, affords the final adducts. No free-radical species are involved since the addition of a radical trap (i.e., TEMPO) has no effect on the reaction.373 Also, similar to ketene silyl acetals, allylic stannanes can also efficiently add to C60 via a mechanism including PET to the triplet excited state of C60.376 More recent studies have shown that addition of silyl enol ethers to C60 can be achieved without photoirradiation under an oxygen atmosphere to afford a pentaaddition product or under argon to afford the corresponding monoaddition product.377 For example, treatment of C60 with 20 equiv of ketene silyl acetal 211a and 10 equiv of K2CO3 in 20% DMSO in chlorobenzene under an atmospheric pressure of oxygen at ambient temperature furnished a pentaaddition product 214 in 72% yield (based on HPLC analysis); on the other hand, when the same reaction was conducted under an argon atmosphere, the monoadduct 213a was obtained in 62% yield (Scheme 75). The monoaddition reaction proceeds with both substituted and unsubstituted ketene silyl acetals and 1,2-siloxyalkenes.377 12.1.4. One- and Two-Electron Reduction of C60 with NADH and NAD Dimer Analogues. It is well-known that photoexcitation of C60 leads to its singlet excited state (1C60*),

Scheme 76. Selective One-Electron Reduction of C60 via Photoinduced Electron Transfer from 1-Benzyl-1,4dihydronicotinamide Dimer (BNA)2 to 3C60*

AD


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Scheme 77. Selective Two-Electron Reduction of C60 to C60H2 via Photoinduced Electron Transfer

Scheme 78. Fullerene Diels−Alder Reaction with Danishefsky’s Dienes

This reaction proceeds via PET from AcrH2 to 3C60* followed by proton transfer from AcrH2•+ to C60•− and a second electron transfer from the deprotonated acridinyl radical (AcrH•) to HC60• in the presence of TFA to yield the final products 10methylacridinium ion (AcrH+) and 1,2-C60H2 (Scheme 77). In a subsequent study, Fukuzumi, Ito, and co-workers revealed the enhanced reactivity of C70 as compared to C60 which was ascribed to a more localized unpaired electron and negative charge in C70•− due to the loss of symmetry which facilitates the follow-up reaction in competition with the back electron transfer to the ground state reactant pair. In the case of 4-isopropyl-1-benzyl-1,4-dihydronicotinamide (i-PrBNAH), photochemical reaction with C70 yields not only C70 but also the isopropylated anion (i-PrC70−).378 12.1.5. [4 + 2] and [2 + 2] Cycloadditions. The [4 + 2] and [2 + 2] cycloadditions to C60 formally belong to another class of reactions in fullerene chemistry, namely, cycloadditions, and are rarelyif at allincluded in the radical chemistry of fullerenes. Accordingly, these reactions will not be explicitly covered in this review, but instead the discussion will be restricted to those instances where a radical ion mechanism has been suggested and/or experimentally proven. 12.1.5.1. [4 + 2] Cycloadditions. Diels−Alder reactions of C60 are generally believed to proceed via a thermally allowed concerted (suprafacial) process or a photochemical concerted (antarafacial) process.379 On the other hand, an alternative stepwise (open-shell) mechanism for the Diels−Alder reaction has also been considered in some cases. Thus, there have been some reports on an electron transfer with formation of radical ion pairs as the primary step of the Diels−Alder reactions, followed by stepwise bond formation. In particular, Mikami, Fukuzumi, and co-workers reported a PET in the solid state fullerene Diels−Alder reaction with anthracenes;380 whereas the first evidence for a stepwise bond formation in the solutionphase fullerene Diels−Alder reaction appeared a few years later for both the photochemical and the thermal Diels−Alder reactions of C60 with Danishefsky’s dienes.381 In the case of the photochemical Diels−Alder reaction, a stepwise bond formation via a PET was unequivocally shown (i.e., by detection of the transient absorption spectrum of C60•− in the visible and near-IR region using laser flash photolysis and other measurements). In this study, the fullerene Diels−Alder reaction was examined with a stereochemically defined (1E,3Z)-1,4-disubstituted Danishefsky’s diene (215b) as a stereochemical probe (Scheme 78). Treatment of the primarily formed intermediate silylenolether with SiO2 and triethylamine leaves the methoxy group unaffected, and yields a mixture of the two cis/trans isomers 217b (Scheme 78). This reaction can be carried out either photochemically or thermally. Because the trans product 217b is the major product under both thermal and photo-

chemical conditions, the mechanism of this addition is concluded to proceed in a stepwise manner. The first step is probably an electron transfer from the Danishefsky diene to C60. Addition of o-quinodimethanes (o-xylylenes) to C60 is yet another cycloaddition reaction that is relevant to those reported above. This reaction has attracted much interest in fullerene chemistry, not only as an efficient and versatile method for the functionalization of fullerenes but also due to the utilization of the resulting cycloadducts as C60-based electroactive compounds.382−387 Theoretical studies have established that oquinodimethanes can be seen as either tetraenes (structural type A) or biradicals (structural type B) coupled or decoupled with the aromatic system, as shown in Scheme 79.388 Similar to Scheme 79. Quinoidal (A) and Biradical (B) Resonance Structures of o-Quinodimethane

s-cis dienes, o-quinodimethanes and related compounds have remarkable Diels−Alder reactivity.382−387,389−391 Thus, reactions of C60 with o-quinodimethanes have been classified as [4 + 2] cycloadditions rather than radical or ion radical reactions and will therefore not be further discussed here.379,387,392−394 In general, Diels−Alder reaction of C60 with o-quinodimethanes has been carried out from different precursors which under various conditions generate in situ the reactive diene intermediates, which, in turn, add to C60. Further details can be found in the cited literature.382−387,389−394 12.1.5.2. [2 + 2] Cycloadditions. Although less common than the [4 + 2] and 1,3-dipolar cycloadditions, photochemical [2 + 2] cycloadditions of C60 with unsaturated compounds have also been widely studied. For example, preparation and isolation of well-characterized [2 + 2] monoadducts of C60 with alkynes,395,396 arylalkenes,397,398 dienes,399 cycloenones,400 acyclic enones,401 and diones402 has been reported (Scheme 80). The triplet excited state of C60 (3C60) with a high reduction potential is responsible for most of the [2 + 2] photocycloadditions to C60. As an exception, Schuster and coworkers reported that photochemical [2 + 2] addition of enones to C60 proceeds by addition of the enone triplet excited state to the ground state of the fullerene via a triplet 1,4biradical intermediate.400,402 For all these reactions, however, mechanistic studies have shown that [2 + 2] photoAE


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Scheme 80. Selected Examples of [2 + 2] Cycloadditions to C60

397,398

399

Scheme 82. Proposed Mechanism for the Photochemical Cycloaddition of (E,Z)-220 to C60

Newcomb and co-workers as a hypersensitive molecular mechanistic probe for determining the nature of an intermediate generated in the course of a stepwise reaction mechanism.409,410 Typically, formation of a cyclopropylcarbinyl radical or a cation intermediate results in a cyclopropyl ring opening favoring a benzylic radical or an oxonium ion, respectively. The results of this study suggest that the [2 + 2] photocycloaddition of an alkene to 3C60 proceeds through a photoinduced electron transfer step between the cyclopropylalkene 220 and 3C60, which leads to a biradical (not a dipolar) intermediate 222, as shown in Scheme 82.408 The photocycloaddition of dienyl cyclopropanes 224a−b to C60 has also allowed for stereospecific preparation of five-, seven-, and nine-membered [60]fullerene adducts (Scheme 83).411 For example, photochemical addition of 224b to C60

400,401

cycloaddition of alkenes, dienes, and enones occurs by a two-step mechanism involving the formation of a dipolar/biradical or charge transfer intermediate in the ratedetermining step. Electron transfer from alkenes to 3C60 is most likely the first step of the reaction followed by rapid collapse of the initial open intermediate to furnish the [2 + 2] adducts. Apart from the photochemically initiated [2 + 2] cycloadditions which represent the majority type for this reaction, a few examples of thermal [2 + 2] cycloaddition of fullerenes have also been reported, including reactions with benzyne,403 tetraalkoxyethylenes,404 1,2-dicarbomethoxycyclobutadiene,405 allenamides,406 and, more recently, morpholinocycloalkenes (enamines).407 In this latter report, Oshima and co-workers found that thermal reaction of C60 with five- and six-membered morpholinocycloalkenes 218a−b, in refluxing toluene, affords exclusively the [2 + 2] cycloadducts 219a−b in high yields, although the increasing steric hindrance upon going from a cyclopentene or cyclohexene to a cycloheptene homologue 218c resulted in reduced or negligible reactivity (Scheme 81).

Scheme 83. Photochemical Ring-Opening Cycloaddition of Cyclopropyl-Dienes to C60

Scheme 81. Reaction of Morpholinoenamines with C60

results in a mixture of products (60% based on the recovered C60) including a major cycloproduct with a five-membered ring 225b (60% relative yield) along with seven- and ninemembered cycloproducts (226b and 227b, respectively) (Scheme 83). Again, formation of a biradical intermediate, which is preceded by an electron transfer step between the diene and 3C60, is involved in the reaction mechanism. More recently, the photochemical cycloaddition of biscyclopropyl-substituted alkenes (i.e., 228) with C60 has been investigated.412 Importantly, it has been found that this reaction proceeds in a totally regioselective manner, affording, in good yield, cis-1 fullerene isomers 229 featuring a unique 5− 4−5 tricyclic fused ring system (Scheme 84). All four stereoisomeric adducts that resulted from this reaction have been separated, purified, and structurally characterized. Formation of these adducts has been rationalized by a stepwise reaction mechanism which is initiated by PET from the double

The suggested mechanism for this cycloaddition reaction includes a SET, radical coupling, and subsequent ion cyclization (Scheme 81).407 It should also be noted that this reaction is markedly in contrast with the thermal reaction of morpholine itself (181), since cyclic secondary amines are reported to bring about the tetraamination of C60 via oxygen-mediated ground state SET in DMSO (see Scheme 65).345 12.1.6. Reaction of C60 with Cyclopropyl-Substituted Olefins. The nature of the intermediate of the [2 + 2] cycloaddition between dienes or arylalkenes397−399 and C60 has been investigated by utilizing a mechanistic probe, namely, 2(2-methoxy-3-phenylcyclopropyl)-5-methyl-2,4-hexadiene 220 (Scheme 82).408 This olefin incorporates the 2-methoxy-3phenylcyclopropylcarbinyl moiety which was first developed by AF


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radical cation C60•+ has been assumed on the basis of NMR, ESR, or NIR spectroscopies. The radical cation C60•+ has also been prepared via PET from the triplet state of C60422−424 or upon disproportionation of two 3C60 molecules.425 Oxidation of C60 by treatment with chlorine dioxide leads to instant formation of a brown precipitate, which also shows an ESR signal indicative of the radical cation.426 It should be noted, however, that the first synthesis of an isolated and a sufficiently characterized fullerene radical cation was conducted by Reed and co-workers in 2000.427 In this work, the oxidant was the radical cation of a hexabrominated phenylcarbazole, and the anion was the carborane CB11H6Cl6−. Early studies by Foote and co-workers in 1992 showed generation of the radical cation of C60 via electron transfer to singlet N-methylacridinium hexafluorophosphate (NMA+).428 Higher yield production of C60•+ could be achieved by cosensitization using high concentrations of biphenyl to form the biphenyl radical cation which was the ultimate oxidant for C60.428 Later, Schuster et al. reported the generation of C60 radical cations by photosensitized electron transfer from the electronically excited 1,4-dicyanoanthracene (DCA), which resulted in addition of alcohols or hydrocarbons to C60.423 Mattay et al. also reported generation of C60•+ via PET.429,430 It was later found that when 2,4,6-triphenylpyrylium tetrafluoroborate (TPP+BF4−) is used as a sensitizer an electron transfer from C60 to TPP occurs without application of a cosensitizer.431 The existence of C60•+ has been proved using ESR studies.431 The proposed mechanism for the formation of C60•+ by the aforementioned cosensitization method is depicted in Scheme 86. The radical cation C60•+, generated by these methods, abstracts a hydrogen atom from an appropriate H donor (i.e., tert-butylmethyl ether, propionaldehyde, N,N-dimethylformamide, 1,3-dioxolane, phenylacetaldehyde, methyl formate, tertbutanol, propionic acid, glycol, and methoxyethanol) to give after reduction of HC60+ to HC60• (i.e., from the reduced sensitizer) and recombination with R• the corresponding 1substituted 1,2-dihydro[60]fullerenes (Scheme 87).429,430 In 2004, Komatsu and co-workers showed that the RC60• radicals could be formed from the corresponding fullerenyl

Scheme 84. Photocycloaddition of BiscyclopropylSubstituted Alkene 228 to C60a

a


bond of 228 to 3C60*, thus forming the corresponding geminate radical ion pair. The incipient radical cation 230 undergoes a facile ring opening of the two cyclopropane rings to form 232 before combining with its geminal radical anion C60•−. From this point forward two reaction pathways may be considered. On one hand (pathway A), coupling of distonic radical cation 232 with C60•− produces 1,4-cycloadduct 234 which, in turn, may spontaneously undergo an intramolecular [2 + 2] photocycloaddition reaction to produce the final adducts 229. On the other hand (pathway B), radical coupling of 232 with C60•− may be followed by an intramolecular nucleophilic addition to the proximal double bond, generating in situ the 1,3-dipolar intermediate 236 which, ultimately, may cyclize to adduct 229 via an intramolecular 1,3-dipolar addition to the adjacent [6,6] double bond of C60 (Scheme 85).412 12.2. Reactions of Fullerene Radical Cation (C60•+)

It has been found that C60 may be reversibly reduced, accepting up to six electrons with formation of the radical anions C60•n− or the anions C60n−.413−415 On the other hand, oxidation of fullerene is limited by its ability to transfer only one electron, yielding the radical cation C60•+. Formation of C60•+ in solution has been observed upon treatment of ground state C60 with powerful oxidizing reagents such as SbF5/SO2ClF,416,417 SbCl5,418 concentrated and fuming sulfuric acids,419 magic acid (FSO3H, SbF5),420 or a mixture of fuming sulfuric acid and SO2ClF.421 In these studies, the formation of the fullerene

Scheme 85. Proposed Mechanism for the Photocycloaddition of Biscyclopropyl-Substituted Alkenes to C60a

a

Reprinted with permission from ref 412. Copyright 2011 American Chemical Society. AG


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Scheme 86. Formation of C60•+ by Cosensitizationa

a

1,2- and 1,4-isomers in a ratio of 4:1 (Scheme 88). Under exactly the same conditions, the same cationic species was generated quantitatively from the corresponding singly bonded fullerene dimer 238. The proposed reaction mechanism includes first a one-electron oxidation of (EtO)2P(O)CH2− C60H to give the corresponding radical cation. Then this radical cation releases the proton directly attached to the C60 cage to give radical (EtO)2P(O)CH2−C60•; this radical species should be in equilibrium with its dimer shown in Scheme 88, as similarly reported in other studies.60,61,173 The second oneelectron oxidation of (EtO)2P(O)CH2−C60• can give the corresponding cation. The cation 239 was also formed in CH2Cl2 by one-electron oxidation with an aminium radical cation and further quenched with trifluoroethanol, allyltrimethylsilane, or benzene to afford the corresponding 1,2-addition products of C60 in 60−72% yield.432 Fukuzumi and co-workers reported that a photocatalytic oligomerization of C60 can be realized efficiently via an electron transfer oxidation and reduction of C60 with the electron transfer state of 9-mesityl-10-methylacridinium ion (Acr+− Mes) followed by radical coupling between C60•+ and C60•−.433 Because the photogenerated electron transfer state of Acr+− Mes (Acr•−Mes•+) has both highly oxidizing and reducing abilities, C60 can be oxidized and reduced by Acr•−Mes•+ to produce both the radical cation C60•+ and the radical anion C60•−, which can be coupled to yield the dimer C60−C60.433 Coupling products are fullerene oligomers, C120, C180, and C240.433 Recently, the C60•+ radical cation has also been proposed as a possible intermediate in the FeCl3-mediated conversion of C60 to polyarylated fullerenes containing pentaaryl(chloro)[60]fullerenes.434

Reprinted with permission from ref 429, Copyright 1997 Elsevier.

Scheme 87. Reaction of C60•+ with Organic Hydrogen Donors RH

radical cation (RHC60•+), which, in turn, can be generated from monofunctionalized hydrofullerenes or dimers by one-electron oxidation in sulfuric and sulfonic acids.432 More specifically, this study allowed for the generation of a novel fullerenyl cation (EtO)2P+(OH)CH2−C60+ (239) by simply dissolving the monofunctionalized hydrofullerene RC60−H (237) or the singly bonded dimer RC60−C60R (R = CH2P(O)(OEt)2) (238) in oxidizing acids such as H2SO4 and FSO3H (Scheme 88). For example, when (EtO)2P(O)CH2−C60H was dissolved in H2SO4 at room temperature under air a reddish purple solution was immediately formed, which was shown to be (EtO)2P+(OH)CH2−C60+ (239). Quenching this solution with CF3CH2OH afforded the fullerenyl ether RC60−OCH2CF3 [R = (EtO)2P(O)CH2] (240) in 85% isolated yield as a mixture of Scheme 88. Generation of Fullerenyl Cation (EtO)2P+(OH)CH2−C60+ from RC60−H or RC60−C60R (R = CH2P(O)(OEt)2)

13. RADICAL REACTIONS OF ENDOHEDRAL METALLOFULLERENES (EMFS) Endohedral metallofullerenes (EMFs) are fullerenes encapsulating one or more metal atoms or a cluster in the inner space of their fullerene cage. Since their initial discovery in 1991, EMFs have attracted broad interest because encapsulated metal atoms or clusters show unique electronic and magnetic properties.435−441 The vast majority of EMFs is limited to the group I−III metals. In particular, in the first decade of their discovery (1990s) the research into EMFs concerned mostly monometallofullerenes (mono-EMFs; M@C2n) or dimetallofullerenes (di-EMFs; M2@C2n), whereas in the past decade, many new types of EMFs have been synthesized, including nitride−, carbide−, oxide−, and sulfide−cluster fullerenes.438−441 A close inspection of the chemistry of endohedral metallofullerenes shows that there are three different classes of EMFs that have been involved in radical reactions, namely, paramagnetic EMFs (i.e., La@C82 and Y@C82), diamagnetic EMFs (i.e., Sc3N@C80, Lu3N@C80), as well as insoluble EMFs (i.e., La@C2n, 2n = 72, 74, 80, 82).438 These three different classes will be discussed separately in the following three subsections, whereas the fourth subsection involves the photochemical or thermal disilylation of both paramagnetic and diamagnetic EMFs. It should be noted that paramagnetic EMFs tend generally to form a closed-shell configuration so that an odd number of singly bonded addends are usually attached (although formation of exohedrally functionalized paramagnetic La@C82 with an even number of addends has also been reported, vide infra), whereas diamagnetic EMFs tend to form AH


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an even number of new bonds. As a final general remark, it should be noted that although the [6,6] bonds of empty fullerenes are generally more reactive, endohedral metallofullerenes exhibit the possibility of reacting at either of the [5,6] or [6,6] bonds as has been shown mostly in cycloaddition reactions of EMFs.438,441−444

Scheme 90. Thermal Reaction of La@C82(C2v) with 3Triphenylmethyl-5-oxazolidinone in Toluene

13.1. Radical Reactions of Paramagnetic M@C82

Among the many kinds of EMFs reported to date, M@C82 (M = group 2 and 3 metals) are the most abundantly produced.445−447 In fact, M@C82 is well-known to have electronic states that may be described [M]2+[C82]2− and [M]3+[C82]3− because of electron transfer from the M atom to the C82 cage.436,437,448−452 The latter electronic state engenders a radical character on C82 with an unpaired electron being widely delocalized on the surface of the C82 cage. Some characteristic examples of this class of paramagnetic EMFs include La@C82 and Y@C82. The first reports of radical addition to M@C82 include multiple additions of phosphoryl radicals or phenyl radicals to La@C82 and perfluoroalkylation of La@C82. In particular, in 1998 Tumanskii and co-workers reported the addition of free phosphoryl radical •P(O)(OiPr)2 to La@C82, with the adducts being obtained as a mixture of several isomers.453 Similarly, multiple addition of phenyl radicals to La@C82 leading to La@ C82(Ph)n (n = 1, 2, 3, 4...) was reported by Kalina and coworkers.454 Later, in 2002, Shinohara et al. showed that seven isomers of exohedrally perfluoroalkylated endohedral metallofullerenes La@C82(C8F17)2 could be isolated by photoirradiation of a mixture La@C82 and perfluorooctyl iodide.455 It should be noted, however, that no clear structural information on the monoadducts was presented in any of these studies, partially due to the high reactivity and hence low selectivity of the radical additions involved. Trifluoromethylation of EMFs was first achieved on an Y@ C82-enriched sample (Scheme 89).456 An odd number of CF3

(POAV) values of carbon atoms of the fullerene cage of La@ C82.457 Similar reactions were also observed for other paramagnetic EMFs such as La@C82(Cs) and Ce@C82(C2v), but only pyrrolidino derivatives were found for the diamagnetic La2@C80 and Sc3N@C80,458,459 which suggests the characteristic behavior of toluene to paramagnetic EMFs. The formation of the benzyl adducts results from the addition of benzyl radicals generated under irradiation of toluene. These results confirmed that paramagnetic M@C82 prefers radical coupling reactions to 1,3-dipolar reactions because it tends to form closed-shell electronic configurations whenever possible. Also, this was the first synthesis, isolation, and structural elucidation of monoadducts by the radical reaction of La@C82(C2v).457 Similarly, photoirradiation of La@C82(C2v) in 1,2-dichlorobenzene in the presence of α,α,2,4-tetrachlorotoluene (243) afforded the corresponding monoadducts 244 (Scheme 91). Scheme 91. Photochemical Reaction of La@C82 with α,α,2,4-Tetrachlorotoluene in o-DCB

Scheme 89. Trifluoromethylation of Y@C82

groupsup to fivewere attached, as identified by mass spectroscopy, indicating that paramagnetic Y@C82 tends to form a closed-shell electronic structure upon radical addition. Two Y@C82(CF3)5 isomers were isolated and characterized with 19F NMR. Quantum chemical calculations and NMR spectroscopic data suggested a 1,4-chain addition pattern of the five CF3 groups, as frequently found for empty fullerenes. However, in striking contrast to the empty fullerenes which form complex mixtures with up to 22 CF3 groups, Y@C82 only forms products with one, three, and five CF3 groups. It has been also found that paramagnetic endohedral metallofullerenes are able to undergo radical coupling reactions in toluene or 1,2-dichlorobenzene under photoirradiation. For example, in an attempt to synthesize pyrrolidino adducts of La@C82 using 3-triphenylmethyl-5-oxazolidinone (241), Akasaka, Nagase, and co-workers found that this reaction results unexpectedly in the singly bonded benzyl adducts 242 (Scheme 90).457 The reaction of La@C82 proceeded regioselectively, affording four benzyl monoadducts. This selectivity was rationalized by the combination of a single-crystal X-ray diffraction analysis and theoretical calculations, suggesting the difference among spin densities and π-orbital axis vector

Theoretical calculations showed that the cage carbons having high spin densities are selectively attacked by radical species to form the monoadducts linked by a C−C single bond. However, benzene is not suitable for such radical reactions because no radicals could be generated under photoirradiation. For example, unlike the reaction in toluene, thermal reaction of La@C82(C2v) with 241 in benzene afforded metallofulleropyrrolidine La@C82(C2v)(C2H4NCPh3).457 The high affinity of La@C82 toward radicals has been demonstrated also in a recent study, where an unexpected radical addition of NO2 to La@C82 occurred during cycloaddition of benzyne to this endohedral metallofullerene.460 Among the detected addition products (La@C82(C6H4)nNO2, n = 1−10), the trisadduct La@C82(C6H4)2NO2 was isolated and characterized via X-ray crystallography. Interestingly, the structure of this adduct showed that the three addends prefer a 1,4-addition pattern, which implies a positional directing effect AI


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In 2007, a manganese(III) acetate-catalyzed addition of carbon radicals generated from diethyl malonate to trimetallic nitride EMFs was reported.468 In this study it was found that heating of diethyl malonate in the presence of manganese(III) acetate generates the corresponding carbon-centered radicals which are added to M3N@C80 (M = Sc, Lu) to afford cyclopropanated adducts. Overall, two methano-bridged monoadducts of Sc3N@C80(Ih) were isolated, whereas similar analogues were produced from Lu3N@C80. Moreover, multiadducts with up to 10 methano units were produced from Ih Sc3N@C80 and Lu3N@C80 by controlled reaction conditions. NMR data and UV spectra together with geometry optimization suggested that these are the most thermodynamically stable [6,6] open methanofullerides. Later, the same research group showed that photochemically generated benzyl radicals react with M3N@C80 (M = Sc, Lu) to produce only a dibenzyl adduct 246 in high yield (83% when M = Sc; 63% when M = Lu) and regioselectivity (Scheme 92).469 DFT calculations and NMR and X-ray studies confirmed a 1,4addition pattern for these adducts (246).469

of the NO2 group on subsequent addition of the two benzene moieties. Akasaka and co-workers have also shown that the radical monoadducts of a paramagnetic endohedral metallofullerene, La@C82(C2v), undergo retro-reaction under thermal conditions in the presence of a radical trapping reagent such as TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), affording pristine La@ C82(C2v) in high yield (up to 96%).461 Therefore, such radical reactions are useful for selective isolation of paramagnetic EMFs. 13.2. Radical Reactions of Diamagnetic EMFs

Although no radical reactions of divalent mono-EMFs (M@ C2n, M = alkali earth metal, Sm, Eu, Tm, Yb) or di-EMFs (M2@ C2n, i.e., La2@C80, Y2@C82, Sc2@C84) have been reported to date, a few examples of the radical derivatives of nitride cluster EMFs (MIII3N@C2n, M = Sc, Y, and lanthanides) are available. In 2007, the preparation, isolation, spectroscopic, and electrochemical characterization of the first CF3 derivatives of both isomers of Sc3N@C80 was reported.462 These isomers can be readily transformed into fluoroalkylated derivatives using high-temperature polyfluoroalkyl radical addition methodologies previously developed for fullerenes.123,456,463 Notwithstanding the much lower reactivity of Sc3N@(C80-Ih) isomer compared to that of the D5h cage isomer, both showed essentially the same reaction rate toward CF3 radicals at 520 ± 10 °C. All the trifluoromethylated adducts contained an even number of CF3 groups (from 2 to 12). A bisadduct Sc3N@ C80(CF3)2 was isolated, and NMR results indicated a 1,4addition pattern. Calculations showed that the Sc3N cluster is nearly fixed in the derivative with two Sc atoms coordinating with the para carbons of the addition sites. Moreover, it has been disclosed that addition of two CF3 groups to Sc3N@C80 changes dramatically its molecular orbital levels. With an obvious decrease of HOMO/LUMO band gap, several new redox processes were observed in Sc3N@C80(CF3)2.464 Thus, trifluoromethylation can be considered as an effective method for tuning the electronic structures of EMFs. Recently, two endohedral metallofullerene derivatives Sc3N@ C80(CF3)n (n = 14, 16) were synthesized by heating Sc3N@C80Ih(7) and Ag(CF3CO2) to 350 °C in a sealed tube. Their molecular structures were determined by single-crystal X-ray diffraction, which provided valuable information about the mutual influence between the internal cluster and the exohedral substituents.465 Interestingly, four and eight triple-hexagon junctions (THJs) are substituted for the isomers with n = 14 and 16, respectively, although typically THJs are always less reactive than fullerene carbons of other types such as PHHJs (pentagon−hexagon−hexagon junctions) and PPHJs (pentagon−pentagon−hexagon junctions) because of their low degree of pyramidalization. Detailed structural analyses and theoretical calculations suggested that the formation of isolated aromatic rings (i.e., two negatively charged isolated aromatic C(sp2)5− pentagons) and their coordination with one of the endohedral Sc atoms are the main reasons for such addition patterns. Similar results were later obtained by Yang, Troyanov, and coworkers.466 A recent study regarding the synthesis and characterization of a family of Sc3N@C80(CF3)n (n = 2−16) and their radical anions confirmed further the mutual influence between the geometry of the Sc3N cluster and the multiple addition sites.467 All these results are very informative for the design of multiple additions to EMFs in the future.

Scheme 92. Synthesis of a Dibenzyl Adduct of Trimetallic Nitride-Templated Endohedral Metallofullerenes

13.3. Dichlorophenyl Derivatives of Insoluble EMFs

Typically, investigations of fullerenes and EMFs have largely focused on soluble species, such as C60, C70, and La@C82, although soot contains various fullerene species with cages ranging from C60 to larger than C400. In contrast, little is known about insoluble fullerenes, which are estimated to be more abundant than soluble species in soot.470 Akasaka, Nagase, and co-workers developed a method to extract EMFs from soot using 1,2,4-trichlorobenzene (TCB). This method includes the generation of highly reactive dichlorophenyl radicals from refluxing TCB and their subsequent reaction with some insoluble EMF species, thus allowing for dissolution and hence isolation of several missing cage EMFs La@C2n (2n = 72, 74, 80, 82) in the form of their dichlorophenyl derivatives.471−475 The structure of these EMFs has been established by X-ray analysis showing cage structures different from the corresponding empty fullerenes and di- or cluster-EMFs. The aforementioned method has enabled the isolation of metallofullerene La@C72 as a derivative, La@C72(C6H3Cl2).472 Spectroscopic and X-ray crystallographic analysis showed that the most stable La@C72 isomer has a non-IPR carbon cage with one pair of fused pentagons. The La atom is located close to the fused pentagon junction. The dichlorophenyl group is not added to either of the [5,5] carbons but is singly bonded to an adjacent carbon, confirming that the fused pentagon has been stabilized by the encapsulated metal. As for La@C74, spin density distribution calculations showed that about 50% of the total spin densities on C74 are localized AJ


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tetramesityl-1,2-disilirane (249) both photochemically and thermally, whereas cluster EMFs, resembling empty fullerenes (C60 and C70), undergo only photochemical reactions.477−483 The high reactivity of mono- and di-EMFs results from their high electron affinities as evidenced from their higher redox potential as compared to those of empty fullerenes and cluster EMFs.438 Reactions of the anions and cations of M@C82 (M = Y, La, and Ce) with 1,1,2,2-tetramesityl-1,2-disilirane (249) have also been investigated.485 Importantly, it was shown that the cations react readily even at room temperature in the dark, but the anions do not react either thermally or photochemically, proving that oxidation is an effective method for increasing the reactivity of EMFs. Although disilirane 249 is an effective probe for investigating the chemical reactivity of EMFs, it invariably generates many structural isomers when reacting with the low-symmetric M@ C82, which renders further isolation and structural elucidation of the adducts extremely difficult. Fortunately, reaction of 249 with the highly symmetric Sc3N@C80 and La2@C80 gave rise to a limited number of monoadduct isomers: both 1,2- and 1,4isomers are formed for Sc3N@C80, and only a 1,4-isomer is formed for [email protected],482 Also, it was shown that the 1,2disilylated Sc3N@C80 isomerizes to the thermodynamically more stable 1,4-adduct upon heating. Importantly, it has been also shown that attachment of the disiliranes moieties affects the motional behavior of the internal metals. In particular, X-ray crystallographic studies revealed that the circular motion of the Sc3N cluster in Sc3N@C80 is restricted by the exohedral addition of 249481 and that the three-dimensional random motion of the two La atoms in the parent La2@C80 is lowered down to a two-dimensional horizontal hopping along the equator of the C 80 cage in La 2 @C 80 [(2,6-diethylphenyl)2Si]2CH2 because of perturbation of the electrostatic potentials inside the cage caused by the electropositive silyl groups.482 Similarly, reaction of 249 with Ce2@C78 afforded regioselectively the corresponding 1,4-isomer with the two La atoms being fixed along the equator in the ellipsoidal cage.484 In addition to disiliranes, also siliranes (silacyclopropanes) have been studied for thermal silylation of EMFs (Scheme 94).486 In this study, two diastereomers of the carbosilylated

on the three types of carbons, allowing these carbons to have high radical character.471 In fact, the dichlorophenyl radical which may be produced by reaction of TCB with a reductant, such as lanthanum carbide in the raw soot, adds to one of these carbons to give the stable adduct. From these results, the unconventionally high reactivity of La@C74 was ascribed to the high radical character of the C74 cage. More recent studies showed that the addition of dichlorophenyl radical to La@C74 actually occurs at two particular neighboring cage carbon atoms, both of which are very close to the internal metal atom and highly reactive toward radicals as a result of strong metal− cage interactions.476 Thus, single addition of each of the three dichlorophenyl radical isomers (2,4-, 2,5-, or 3,4-dichlorophenyl group) at each of the two aforementioned carbon atoms resulted in six isomers of La@C74(C6H3Cl2), two of which were identified by X-ray crystallography.476 Similarly, the high reactivity of La@C2v-C80 toward radical species can be ascribed to the radical character and local strain of the corresponding cage carbon atom.473 Addition of a dichlorophenyl radical on the less stable La@C2v-C80 with high radical character affords the stable La@C80(C6H3Cl2) with a closed-shell structure. Moreover, because of its radical character and small ionization potential, La@C2v-C80 might interact strongly and bind with amorphous carbon or other fullerenes in soot and thereby become insoluble in common organic solvents. Similarly, C3v(7)-C82 has been recently isolated as the dichlorophenyl derivative La@C82(C6H3Cl2) by Akasaka, Nagase, and co-workers.474 Importantly, NMR and X-ray studies revealed that the dichlorophenyl group is singly bonded to a triple-hexagon junction (THJ) carbon atom on the C3 axis, so that the high C3v symmetry is maintained in the adducts. Theoretical studies showed that this special THJ carbon atom has pronounced radical character due to strong metal−cage interactions. It is worthy to note that this is the only example of fullerene derivatives in which a single addend is linked to one of the THJ carbon atoms, which are otherwise believed to be the least reactive carbons on a fullerene cage.474 13.4. Disilylation of EMFs

Similar to the disilylation of fullerenes C60 and C70 (see section 3), disilylation of EMFs is among the most effective ways for the exohedral functionalization of EMFs, especially for tuning their electrochemical properties.438 Although this reaction has been classified into the more general type of cycloaddition reactions,438 it can be considered also in the context of ion radical reactions of EMFs, since an electron transfer process is involved in the proposed reaction mechanism. Disilylation with different disiliranes has been studied for several EMFs, such as M@C82 (M = Y, La, Ce, Gd), Ce2@C78, La2@C80, Sc2C2@ C3v(8)-C82, Sc3N@C80, and Lu3N@Ih-C80 (Scheme 93).477−484 The results showed that mono- and di-EMFs react with 1,1,2,2-

Scheme 94. Thermal Carbosilylation of La2@Ih-C80

Scheme 93. General Scheme for Disilylation of EMFs La2@Ih-C80 were isolated and characterized, while single-crystal X-ray crystallographic and 139La NMR spectral analyses revealed that the encaged La atoms move dynamically inside the carbon sphere.

14. RADICAL REACTIONS OF HETEROFULLERENES In heterofullerenes, one or more carbon atoms that form the fullerene carbon cage are replaced by a noncarbon atom, i.e., a heteroatom.487 Consequently, substitution of an odd number of AK


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C atoms by trivalent atoms such as nitrogen or boron leads to radicals which can be stabilized by dimerization, whereas replacement of an even number of C atoms would directly result in closed-shell systems. Thus far, the chemistry of heterofullerenesin terms of synthesis, structural characterization, and isolation in bulk quantitiesis still restricted to nitrogen heterofullerenes, that is, azafullerenes (C59N)2 and (C69N)2.488,489 In 1995, Wudl and co-workers reported the first synthesis of aza[60]fullerene dimer (C59N)2 in bulk quantities using a ketolactam fullerene derivative.488 According to this report, the initially formed azafullerenyl radical •C59N (250), which is isoelectronic to the C60•− radical anion, readily dimerizes to yield bisaza[60]fullerene (C59N)2 (251) (Figure 5). Shortly afterward, the group of Hirsch also succeeded in bulk preparation of (C59N)2 using a bisazafulleroid precursor.489

Scheme 95. Synthesis of Hydroaza[60]fullerene (C59NH) via Treatment of Precursor Ketolactam with p-TsOH and Hydroquinone in o-DCB

Scheme 96. Photochemical Synthesis of Hydroaza[60]fullerene (C59NH) via Trapping of the •C59N Radical Intermediate with Bu3SnH

Figure 5. Aza[60]fullerene radical (•C59N), its dimer (C59N)2, and aza[60]fulleronium ion (C59N+).

regioselective reactions, in which one specific carbon either takes up a hydrogen or binds to a like •C59N radical. The first example of a heterofullerene derivative synthesized according to route B was reported by Wudl and co-workers.492 This study included the thermal treatment of (C59N)2 in oDCB in the presence of excess diphenylmethane (Scheme 97).

The two C59N spheres in the (C59N)2 dimer are connected through their sp3 carbon atoms that are adjacent to the nitrogen atoms. This intradimer bond appears to be very long and relatively weak (ca. 18 kcal/mol),490 and thus, it can be thermally or photochemically homolyzed, giving back the corresponding radical •C59N.491,492 Although the synthetically most valuable intermediate in heterofullerene chemistry so far has been the aza[60]fulleronium ion C59N+ (252, Figure 5), which can be generated in situ by the thermally induced homolytical cleavage of (C 59 N) 2 and subsequent oxidation (i.e., with O 2 or chloranil),489,493−495 the •C59N radical has been also utilized in free-radical reactions for the functionalization of aza[60]fullerene.487 In fact, the first reported azafullerene functionalization methodology is based on trapping of the intermediate aza[60]fullerenyl radical. At this point, two types of synthetic methods toward monomeric aza[60]fullerene derivatives have to be considered, that is, starting from a nonaza[60]fullerene precursor (route A) and starting from the dimer (C59N)2 (route B).487 The first example of a heterofullerene derivative synthesized according to route A was the parent hydroaza[60]fullerene 254 (C59NH), which was obtained by treatment of precursor ketolactam 253 with p-toluenesulfonic acid (pTsOH) and hydroquinone in 1,2-dichlorobenzene (o-DCB) at 170−180 °C, under argon atmosphere (Scheme 95).491 The hydroquinone is assumed to reduce the •C59N radical intermediate. The product HC59N (254) was also obtained via the second route (route B) when (C59N)2 was irradiated in o-DCB in the presence of tributyltin hydride (Bu3SnH) in argon gas at 25−35 °C (Scheme 96).491 This second route (route B) that involves the dimer (C59N)2 as the precursor requires homolytical cleavage of the central dimer bond of (C59N)2. The resulting C59N• has been shown to be a very reactive radical which readily saturates through

Scheme 97. Synthesis of (Ph2CH)C59N (255) from (C59N)2

The resulting formation of (Ph 2 CH)C 59 N (255) is consistent with a free-radical chain mechanism that is initiated with homolysis of the azafullerene dimer to C59N• followed by a propagation (hydrogen-atom abstraction from diphenylmethane) and a termination (radical coupling between •C59N and Ph2CH•) step (Scheme 98).492 In the last step, HC59N (254) undergoes a free-radical reaction to afford C59N•, which in turn dimerizes to afford (C59N)2, and this must be the reason that HC59N was only 2% of the reaction mixture.492 Scheme 98. Proposed Free-Radical Chain Mechanism for the Formation of (Ph2CH)C59N (255)

AL


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chain mechanism similar to that presented in Scheme is again operating in these reactions. The mechanism of reaction of aza[60]fullerene radical •C59N with diphenylmethane and 9-methyl-9H-fluorene (256b) has been studied using primary and secondary kinetic isotope effects (KIEs).498 These studies provided strong evidence for a stepwise radical mechanism, in which a hydrogen-atom abstraction is the rate-determining step, followed by a fast coupling of C59N• with the diphenylmethane or methylfluorene radicals. A free-radical reaction mechanismsubstantially similar to the one proposed for the reaction between (C59N)2 and diphenylmethane (Scheme 98)has been proposed also for the thermal and photochemical reaction between aza[60]fullerene (251) and benzyltrimethylsilane (261) in thoroughly degassed o-DCB under argon atmosphere (Scheme 101).499 In this case, aza[60]fullerene adduct 262 was isolated in very low yield ( C60/ C70 (80/20) > C60/C70 (93/7) > C60. It was also shown for the first time that the fullerene soot not only retards oxidation in the mode of an alkyl radical quencher but also operates as a peroxy radical scavenger. Despite the fact that the rate constant of interaction with peroxy radicals obtained for the fullerene soot is not so high, the double antioxidant function puts it in the front position of effective nanocarbon-based antioxidants.513 For instance, it has been found that addition of fullerene soot significantly hampers peroxide formation and thus increases the oxidation stability of rapeseed vegetable oils.514 15.2. Applications in Polymer Science

In this section the effect and applications of the radical scavenging ability of fullerenes and their derivatives in the field of materials, especially polymer science, will be discussed. Analysis of literature data clearly shows that fullerenes act as efficient radical scavengers in chain radical processes, including polymerization and degradation. These processes involve mostly C- or O-centered radicals which are susceptible to inhibition by compounds capable of trapping such radical species. Indeed, it has been shown that the rate constants of addition of alkyl or benzyl radicals to C60 are in the range of 107−109 M−l s−l,63,515−518 thus confirming their facile addition to fullerenes. Typically, the radical scavenging ability of fullerenes (i.e., C60, C70) and their derivatives has been successfully applied for retarding the thermo-oxidative degradation of polymers, but at the same time it has been shown that this property is also responsible for retarding the radicalinitiated polymerization of monomers.54,519 15.2.1. Anti-Radical Activity of Fullerenes in Radical Polymerization Reactions. The easiest method for incorporation of fullerene into a polymer is the radical copolymerizaAP


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acrylic acid has been found to be inhibited by fullerene C60.545−547 In order to further investigate the influence of fullerenes in the thermal and thermal-oxidative stability of polymers, Ginzburg and co-workers studied the thermal and oxidative degradation of two types of fullerene-containing polymer systems (FCPS): (1) FCPS with covalent interaction of fullerene C60 with polymers and (2) FCPS representing mixtures of C60 with polymers in which interaction of components is realized only by means of van der Waals forces.548 These FCPS included PS and PMMA containing 1− 10% w/w C60. In both types of FCPS the electron acceptor properties of C60 were manifested. In some systems of the first type, a great decrease in thermal stability of a polymer component was observed, which can be explained by formation of weak β-linkages. On the other hand, in systems of the second type the ability of fullerene C60 to trap free radicals was manifested, which resulted in increased thermal stability of the polymers for temperature intervals that lay below the temperature of breaking of β-linkages. Also, Zeynalov and co-workers studied the antioxidant properties of fullerenes C60/C70 and C70 in polystyrene by a two-step investigation.518 In particular, the antioxidative activity of fullerene C60/C70 has been studied by model reaction of the initiated oxidation of styrene and then in accelerated tests of C60/C70 and C70 mixtures with polystyrene. It was established that the initiation and oxidation rates of the model reaction are substantially reduced in the presence of C60/C70. The rate constant for addition of styryl radicals to C60/C70 was determined to be k(333 K) = (9.0 ± 1.5) × 107 M−1 s−1. By differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) it was demonstrated that fullerenes show a stabilizing effect comparable with the influence of the sterically hindered phenol Irganox 1010 and amine Agerite White. It was suggested that the retarding effect of fullerenes is connected with its interaction with macroradicals R•, leading to formation of less active compounds.518 In another study, Zuev et al. studied the effect of fullerene additives on the thermal behavior and thermal degradation of acrylic polymers including poly-n-alkyl acrylates (from butyl to heptyl) and the corresponding polymethacrylates by means of thermogravimetry under dynamical conditions and pyrolysis/ gas chromatography in isothermal conditions at 400−650 °C.549 By virtue of its well-known radical accepting ability, the presence of fullerenes influences the thermal degradation of acrylic polymers, shifting the decomposition process from a radical pathway to a nonradical mechanism. For poly-n-alkyl acrylates, addition of fullerenes leads to the increase in the yields of olefin and alcohol, which are the degradation products coming from nonradical pathways. On the other hand, the yields of the pyrolysis products that derive from the random main-chain scission (i.e., monomer, dimer, saturated diester, trimer, the corresponding acetate, and methacrylate) decrease. The recorded temperatures of maximum weight loss (obtained by TGA experiments) were slightly increased by the presence of fullerene. The effect of fullerene is more noticeable in the thermal behavior of poly-n-alkyl methacrylates; in fact, enhancements of the temperature of maximum weight loss are 19−25 °C. Mixtures containing fullerene give rise to a marked decrease of monomer yield and, at the same time, an increase of olefin and methacrylic acid amounts. The fullerene acts as radical acceptor, suppressing the unzipping process and favoring the nonradical side-chain reactions.549

investigated by monitoring BPO concentration, C60 content, and polymerization time. It was found that C60 acts like a radical absorber which multiply absorbs primary radicals from BPO and propagating radicals. By virtue of this radicalabsorbing effect of C60 chain propagation is restricted at the beginning of the reaction. Therefore, in the presence of C60 both the polymer yield and the molecular weight decrease significantly.533 Similarly, it has been found that fullerene C60 acts as an effective inhibitor of radical polymerization of vinyl acetate with dimethyl 2,2′-azobisisobutyrate (MAIB) in benzene.534 All C60 molecules used were incorporated into poly(vinyl acetate) during polymerization. The relationship between the induction period and the initiation rate reveals that a single C60 molecule can trap 15 radicals formed in the polymerization system. Recently, C60 has been shown to inhibit also homopolymerization of N-vinylpyrrolidone and its copolymerization with (di)methacrylates.535 15.2.2. Anti-Radical Activity of Fullerenes in the Thermal/Thermo-oxidative Degradation of Polymers. By virtue of its free-radical accepting properties, [60]fullerene may act as an antioxidant in polymers, that is, C60 is able to interrupt or at least to slow down the free-radical reactions which lead to polymer degradation. Such reactions on polymer substrates involve the main chain scission of the polymer backbone, thus forming either alkyl radicals orin the presence of airoxygen-containing radicals such as RO•, ROO•, and •OH. As mentioned previously, C60 reacts promptly with free radicals of different nature (see sections 2−9), and thus, a single or multiple addition of free radicals including alkyl (i.e., macroradicals formed by chain scission) and oxygencentered radicals (which are formed during thermo-oxidative degradation of polymers) can occur. Indeed, a literature search indicates that there are a significant number of publications (vide infra) that report on the stabilizing role of fullerenes C60 and C70 in the thermal and thermo-oxidative degradation of many polymers. Specifically, early studies in this field have shown that fullerene C60 inhibits poly(vinylchloride) dehydrochlorination,536 as well as thermo-oxidative degradation of poly(methylmethacrylate),537−543 polydimethylsiloxane rubber,517 and polystyrene.517,542 In the latter study, fullerene exhibited a stabilizing effect comparable with the activity of the known stabilizer Irganox 1076.517 It has also been shown that fullerenes C60 and C70 are new high-temperature antioxidants of polymers, which in some cases are more effective than wellknown inhibitors.544 Also, C60 appears to be very effective in synergic mixtures with Ph3Sb and Neozone-D (N-phenyl-2naphthylamine) in the thermo-oxidative degradation of polystyrene.544 The inhibiting action of fullerene C60 on the thermal and thermo-oxidative degradation of poly(methylmethacrylate) (PMMA) and polystyrene (PS) has been described in many studies.537−543 It has been proposed that in the case of PMMA the inhibiting effect of fullerene is due to its interaction with R• and oxygen-containing radicals with formation of more stable compounds.542 In the case of PS, the retarding effect of fullerene is connected with its interaction mainly with oxygencontaining radicals, but the interaction of C60 with macroradicals R• could not be ruled out. Similarly, the thermal and thermo-oxidative degradation of copolymers of methyl methacrylate with styrene, butyl acrylate, glycidyl methacrylate, hydroxyethyl methacrylate, and methAQ


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of C60−OH molecules. Furthermore, natural rubber is a nonpolar material, thus exhibiting weak interaction with the polar compound of C60−OH. More importantly, it was found that C60−OH has a larger antiaging effect than C60.556 This conclusion was supported by the finding that C60−OH had larger radical scavenging ability and gel forming ability during heat treatment. In particular, the radical concentration decrease rate of DPPH showed that the radical scavenging ability of C60−OH was much higher than that of C60. Moreover, determination of the gel content of the unvulcanized samples after heat treatment showed that (i) the sample without any additive contained no gel, (ii) the sample containing C60 possessed a small amount of gel, whereas (iii) the sample containing C60−OH mixed by the wet method had 4% gel. Therefore, it was concluded that the higher reactivity of C60− OH to the radical may facilitate formation of cross-link by reacting with carbon and/or oxygen radicals in the rubber produced during heat treatment. The small amount of gel in the sample containing C60 could be explained by the slow reaction rate of C60 with radicals. Similarly, Cataldo had previously studied the reaction of C60 with natural rubber and synthetic cis-1,4-polyisoprene.551,552 This was conducted under nitrogen or air flow by simultaneous thermogravimetric analysis and differential thermal analysis (TGA-DTA) on rubber samples containing known quantities of fullerene in comparison to a “blank” of pure rubber. The results showed that fullerene C60 (in the absence of oxygen) is a thermal stabilizer of cis-1,4-polyisoprene because it reacts with the polyisoprene macroradicals formed by the thermally induced chain scission reaction, thus slowing down the degradation reaction. Conversely, under thermo-oxidative degradation conditions (in air flow) fullerene C60 acts as an antioxidant for cis-1,4-polyisoprene, provided that the heating rate of the samples is slow (5 °C/min). At higher heating rates (20 °C/min) C60 does not show any antioxidant effect.

Thermal degradation of poly(2,6-dimethyl-1,4-phenylene oxide) and its blends with 1−4% fullerene C60 or C70 has been studied by mass spectrometric thermal analysis and DSC.550 This study showed that addition of fullerene shifts the onset of the processes of thermal degradation and homolytic decomposition of the polymer with formation of gaseous products to higher temperatures, thereby increasing its thermal stability. Notably, the inhibiting effect of fullerene C70 was found to be stronger compared to that observed for C60. Fullerene-containing natural rubber has also been found to have superior antiaging properties.551−553 In fact, [60]fullerene has been studied as thermal stabilizer and antioxidant of both natural and synthetic rubber (cis-1,4-polyisoprene).551,552 The stabilizing effect is due to a reaction between the rubber chain macroradicals (originated by thermal degradation under nitrogen) and fullerene C60, which is alkylated repeatedly by these chains. In this way the free-radical chain process, which guides rubber decomposition, is slowed down. Conversely, under thermo-oxidative degradation conditions (in air flow) fullerene C60 acts as an antioxidant for cis-1,4-polyisoprene but only at slow heating rates (5 °C/min). The thermal stabilization of isotactic polypropylene (i-PP) in the presence of fullerene C60, as well as its adduct with levopimaric acid, nanocarbon, and carbon black has been investigated by chemiluminescence.554 The selected temperatures were 170, 180, and 190 °C, while the thermal oxidation of i-PP samples was carried out in air. Several kinetic parameters, including oxidation induction time, half time of degradation, oxidation rate, maximum CL intensity, and maximum oxidation times, were calculated from CL measurements. The efficiency of additives places the studied compounds in the order fullerene C60 < nanocarbon < carbon black < fullerene C60 adduct. Chemiluminescence investigation of thermo-oxidative degradation of polyethylenes stabilized with fullerenes has also been reported.555 These studies showed that [60]fullerene can be considered as a weak thermal stabilizer for polyethylene, while methyl [60]fullerepimarate shows better activity in the thermal degradation of all sorts of polyethylenes. Other adducts similar to fullerene did not display antioxidant activity at these concentrations. The same conclusion could be drawn for nanocarbon and carbon black.555 The fine dispersion of fullerene derivatives in polymers (as reinforcing agents or radical scavengers) is sometimes difficult due to their tendency to agglomerate as a result of strong van der Waals or hydrogen-bonding interactions. Recently, however, a rubber composite having homogeneously dispersed C60−OH was obtained by mixing C60−OH aqueous solution with natural rubber latex (wet method).556 The vulcanized sample of this composite exhibited higher modulus, tensile strength, and elongation, indicating that finely dispersed C60− OH had a reinforcing effect. The sample showed smaller damage because of the heat treatment, showing that C60−OH possessed an antiaging effect. On the other hand, when C60− OH was added by mixing C60−OH powder with solid rubber by an open roll mixer (dry method) it formed aggregates and had negligible reinforcing and antiaging effects. At this point, it is worth mentioning that although it has been reported that C60 can be finely dispersed in polymers, such as polystyrene, poly(methyl methacrylate), and natural rubber by dry mixing,517,541,542 in this case it was found that C60−OH aggregate could not be broken by mechanical mixing. This difference could be explained by the strong hydrogen bonding

15.3. Biological Activity and Pharmacological Potential of Fullerenes

Apart from the several studies concerning the potential of fullerenes for applications in polymer science, their biological activity has also been studied in depth. Some of the most important biological activities of fullerenes include anti-HIV and antibacterial activity, DNA photocleavage, enzyme inhibition, tumor therapy, controlled drug delivery, as well as radical scavenging and antioxidant activity. All these and other important biological activities of fullerenes and their derivatives have been surveyed in a number of reviews.37,49−52 This section will focus only on biological applications of fullerenes which are directly related to their radical scavenging activity. It is well-established that fullerenes and their derivatives possess a unique capacity for scavenging reactive oxygen species (ROS).38−48,512,513,557,558 ROS are chemically reactive molecules containing oxygen, such as hydrogen peroxide (H2O2), hydroxyl radical (•OH), and superoxide anion radical (O2•−). In addition to ROS, nitric oxide (NO) is a highly reactive free radical that belongs to both ROS and reactive nitrogen species (RNS). Nitric oxide is a very reactive short-lived gaseous radical that generates a variety of secondary products that together with NO itself have been associated with a variety of cell damaging processes, including lipid oxidation, mitochondria and DNA damage, protein modification, and alteration in enzyme activity, which ultimately lead to apoptotic or necrotic cell death. AR


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logically active fullerenes with high radical scavenging capacity will be discussed individually in the following paragraphs. 15.3.1. Malonyl Carboxyfullerenes. Typically, carboxyfullerenes are highly potent neuroprotective agents, preventing cellular death across a variety of different neuronal types in disease models as diverse as Parkinson’s, Alzheimer’s, amyotrophic lateral sclerosis (ALS), excitotoxicity, macular degeneration, and stroke.566−572 Among the different carboxyfullerenes, fullero-tris-methanodicarboxylic acid (272, Figure 9) represents the most widely studied example of this family of compounds that, thanks to its radical scavenging ability, has been exploited for therapeutic purposes. For example, in vitro experiments using cultures of neocortical cells showed that 272 inhibited neuronal death in a dose-dependent manner, with the C3 regioisomer of 272 being more effective than its D3 symmetric analogue.566 Furthermore, continuous infusion of 272 in a transgenic mouse carrying the human mutant (G93A) superoxide dismutase gene responsible for a form of familial amyotrophic lateral sclerosis delayed both death and functional deterioration.566 In superoxide anion radical (O2•−) dismutation, fullerene 272 exhibits superoxide dismutase-mimetic properties, thereby protecting neuronal cells from oxidative damage.573 In particular, it has been found that 272 is capable of removing the biologically important superoxide radical with a rate constant k272 = 2 × 106 mol−1 s−1.573 Although this rate constant is approximately 100-fold slower than the superoxide dismutases (SOD), a family of enzymes responsible for endogenous dismutation of superoxide, it is well within the range of values reported for several manganese-containing SOD mimetic compounds. Moreover, in vivo studies showed that it can reconstitute mitochondrial superoxide dismutase protection from superoxide radicals in SOD2 genetically deficient mice and increase the lifespan by 300%, which supports the hypothesis that 272 is localized in mitochondria and acts as a biologically effective SOD mimetic.573 272 has been also shown to decrease brain oxidative stress and therefore extend the life span of mice when chronically administered in their drinking water574 while improving their cognitive performance.575 Superoxide radical quenching has been also observed in a bismalonic acid derivative of [60]fullerene.576 Finally, the ROS scavenging capacity of carboxyfullerene 272 has also been exploited for preparation of skin rejuvenation cosmetics, since it has been proven to protect human keratinocytes from apoptosis induced by ultraviolet B (UVB) irradiation.577 Structure−function studies on these carboxyfullerenes indicated that the antioxidant properties of the different compounds depend on redox behavior, charge, size, shape, and hydrophobicity.578 Moreover, the reactivity of carboxyfullerenes toward superoxide radical was found to be sensitive to changes in dipole moment, which in turn is dictated not only by the number of carboxyl groups but also by their distribution on the fullerene surface.579 In agreement with this finding, Dugan,566,570 as well as Hwang580 and co-workers previously reported that the antioxidant activity of the C3-symmetric isomer of tris-malonyl carboxyfullerene 272 is higher than that of the corresponding D3 isomer. In addition, Hirsch and coworkers have shown that the SOD activity of different carboxyfullerenes is dependent on their reduction potentials, namely, the higher the reduction potential (ability to be reduced by O2•−) the higher the SOD dismutation activity.581 On the other hand, the molecular structure of the fullerene derivative plays an important role too, as fullerene derivatives

In living beings ROS are formed as a natural byproduct of the normal metabolism of oxygen and have important roles in cell signaling and homeostasis. However, under certain conditions (e.g., UV or heat exposure) ROS levels can increase dramatically, resulting in significant damage to cell structures, a situation typically known as oxidative stress. All cellular biomacromolecules including lipids, sugars, proteins, and DNA are vulnerable to reaction with ROS, while the secondary generated products can be even more prejudicial than the initially formed ROS. Normally, cells defend themselves against ROS damage either with enzymes (i.e., superoxide dismutase, catalase, lactoperoxidase, glutathione peroxidase, and peroxiredoxin) or with small antioxidant molecules such as ascorbic acid (vitamin C), α-tocopherol (vitamin E), and glutathione. Typically, oxidative stress is believed to play an important role in most neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Alzheimer’s disease, Parkinson’s disease, and Huntington disorders. Because underivatized fullerenes are insoluble in water and biological systems, they have been appropriately functionalized to afford water-soluble fullerene derivatives. Apart from their increased water solubility, derivatized fullerenes have also shown superior membranotropic functions (interactions with cellular and subcellular structures), greater biological affinity to certain nucleic acids and proteins (e.g., interaction with active or allosteric sites of enzymes), and an increase in their ROS scavenging activity to target cells and tissues.37,558,559 Early examples of water-soluble derivatives with good biological distribution, cell penetration, and antioxidant activity include the fullero-tris-methanodicarboxylic acid (272),560,561 hexasulfobutylated fullerenes (273),562−564 dendro[60]fullerene (274),565 and polyhydroxylated fullerenes (fullerenols) (Figure 9). More recent examples of this type of fullerene compounds include various derivatives of C60 with amino acids, peptides, vitamins, sugars, and polyvinylpyrrolidone (PVP), as well as liposome-incorporated fullerenes. All these classes of bio-

Figure 9. Early examples of water-soluble fullerene derivatives with biological activity. AS


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the genotoxic and antigenotoxic potential of fullerenol C60(OH)24 have shown that fullerenol has no genotoxicity in a wide range of concentrations (11−221 μM) and can protect mitomycin C-damaged CHO-K1 cells.589 The antioxidative activity of fullerenols has been evidenced also by in vivo studies. Fullerenol C60(OH)18−20 can preventively and therapeutically scavenge the free radicals that are massively induced during ischemia−reperfusion injury of the small intestine in dogs590 and in intestinal grafts after transplantation.591 Injac and co-workers have shown that fullerenol C60(OH)24 is a potential organoprotector for anticancer therapy, since it prevents doxorubicin (Dox)induced chronic cardio- and hepatotoxicity in rats with colorectal cancer through reduction of oxidative stress.592,593 In a similar study, fullerenol C60(OH)24 was shown to have a preventive effect on Dox-caused acute cardiotoxicity in rats.594 Finally, Mirkov and co-workers demonstrated that C60(OH)24 is able to quench NO and block its biological activity in vivo.42 Highly hydroxylated fullerenes [i.e., C60(OH)32·8H2O] have excellent antioxidative abilities too, as demonstrated by electron spin resonance (ESR) and β-carotene bleaching assay.44 Building on this study, Miwa and co-workers studied the preventive effect of highly hydroxylated fullerenes in obesityassociated metabolic syndrome.595−597 It should be noted that ROS have been suggested to be one of the key factors associated with development of obesity. For example, during spontaneous differentiation of mouse stromal preadipocytes OP9 into adipocytes, intracellular superoxide anion radicals (O2•−) level increases markedly and is accompanied by a significant elevation of intracellular lipid accumulation. This differentiation-dependent increase in intracellular O2•− level is positively correlated with the intracellular augmentation of lipids level. Thus, ROS scavengers and antioxidants are expected to be effective in preventing obesity-associated metabolic syndrome and decreasing the risk of complications related with obesity. In their studies, Miwa and co-workers demonstrated that highly polyhydroxylated fullerenes (C60(OH)n; n = 36 or 44) suppressed lipid accumulation, ROS generation, and macrophage activation in hormoneinduced adipocyte differentiation models and a subcutaneous adipose-tissue equivalent model.595,596 It was also shown that C60(OH)44 exerted a higher antioxidant ability and more significant suppressive effects on lipid accumulation than other hydroxylated fullerenes (C60(OH)n; n = 6−34).595 More recent studies regarding the mechanism underlying the efficacy of this highly hydroxylated fullerene, showed that C 60 (OH) 44 suppresses intracellular lipid accumulation, particularly in lipid droplets, and decreases O2•− level and subsequent PPARγ2 (peroxisome proliferator-activated receptor gamma) upregulation during spontaneous differentiation of OP9 preadipocytes into adipocytes.597 Wang and co-workers found that administration of gadolinium endohedral metallofullerenol ([Gd@C82(OH)22]n nanoparticles) to tumor-bearing mice can restore efficiently their damaged liver and kidney.598 All activities of enzymes and other parameters related to oxidative stress were restored to normal levels after [Gd@C82(OH)22]n administration, which suggests that [Gd@C82(OH)22]n nanoparticle treatment could regulate ROS production in vivo.598 In fact, it has been shown that both fullerenol C60(OH)22 and Gd@C82(OH)22 can scavenge all physiologically relevant ROS, thus protecting cells from hydrogen peroxide-induced oxidative damage.559 Consistent with their cytoprotective abilities, these derivatives

with similar reduction potentials but different structure exhibit different antioxidant activities. More importantly, it was found that the activity of monoadducts (i.e., only one malonyl unit attached to the fullerene surface) is higher as compared to that observed for their trisadduct analogues (i.e., C3 regioisomer of 272).581 Additional advantages of the monoadducts include their increased stability, they are considerably less toxicity, and they can be readily produced in large quantities. 15.3.2. Hexasulfobutylated Fullerene. Hexasulfobutylated fullerenes (273, Figure 9) have shown significant reduction of death and permanent tissue loss associated with severe ischemia/reperfusion injury resulting from complete blockage and subsequent reopening of coronary563 and carotid564 vasculature. In both cases, the observed cytoprotective effects were attributed to the free-radical scavenging activity of hexasulfobutylated fullerenes. 15.3.3. Dendro[60]fullerene. The radical scavenging activity of fullerenes can also find applications in radioprotection. For example, Daroczi and co-workers used zebrafish embryos as an in vivo model to evaluate the radioprotective effect of 274 (Figure 9).582 Zebrafish embryos were exposed to ionizing radiations with consequent dose- and time-dependent alterations of morphology and physiology. Pretreatment with dendrofullerene 274 decreased radiation damage with an efficacy comparable to amifostine, a well-known radioprotector used currently as a cytoprotective adjuvant in cancer radiotherapy.582 Importantly, administration of 274 up to 15 min after irradiation exposure shows effective protection too. The proposed mechanism involves scavenging of radiation-induced ROS.582 15.3.4. Fullerenols. In effect, the interest in hydroxylated fullerenes (fullerenols) was spawned by the study of Chiang and co-workers,38 who first reported the free-radical scavenging activity of water-soluble fullerenols. Since then, and by virtue of their excellent radical scavenging ability and low toxicity, polyhydroxylated fullerenols [C60(OH)n, n = 3−36] have attracted much attention in biomedical research as effective radical scavengers and antioxidants in both in vivo and in vitro studies. For example, two polyhydroxylated C 60 derivatives, C60(OH)12 and C60(OH)nOm (n = 18−20, m = 3−7 hemiketal groups), decreased excitotoxic neuronal death after brief exposure to glutamate receptor agonists and reduced neuronal apoptosis induced by serum deprivation owing to their ROS scavenging activity.583 In agreement with this study, it was later shown that fullerenols C60(OH)18−20 too, exert their neuroprotective functions by blocking glutamate receptors.584 For example, in neuronal cultures fullerenols reduced glutamateinduced neurotoxicity by about 80% at 50 mM. The same fullerenols C60(OH)18−20 prevented again via radical scavenging the hydrogen peroxide- and cumene hydroperoxide-elicited changes in rat hippocampus in vitro.585 Fullerenol C60(OH)22 protected rat brain cerebral microvessel endothelial cells against nitric oxide (NO)-induced apoptosis depolymerization of cytoskeleton and nuclear damage and also accelerated cell repair.586 Also, fullerenol C60(OH)24 has been found to be a strong antioxidant since it reacts with superoxide anion radical, hydroxy radical, and nitrous oxide radical in various chemical and biological systems. 587,588 For example, fullerenol C60(OH)24 has a direct NO quenching activity to prevent NO-induced decrease in catalase, glutathione transferase, and glutathione peroxidase activities in the enucleated fraction of interstitial testicular cells of adult rats.42 In addition, studies on AT


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can scavenge the stable N-centered 2,2-diphenyl-1-picryhydrazyl radical (DPPH) and reactive oxygen species (ROS) such as O2•−, 1O2, and HO• and can also efficiently inhibit lipid peroxidation in vitro.559 15.3.5. Other Water-Soluble C60 Derivatives. Miwa and co-workers have shown that a number of water-soluble fullerene derivatives behave as potent ROS scavengers in cell cultures and can protect human skin keratinocytes from UV irradiation and oxidative damage by tert-butyl hydroperoxide.599 These derivatives included PEG, polyvinylpyrrolidone (PVP), hydroxyl, isostearic acid, and γ-cyclodextrin-modified [60]fullerene (Figure 10).599

Figure 11. Sugar-pendant [60]fullerenes.

Figure 10. Water-soluble fullerene derivatives. Reprinted with permission from ref 599. Copyright 2005 Elsevier.

Aoshima and co-workers showed that PVP/fullerenes are safe for use as antioxidants in cosmetic and pharmaceutical applications, since they did not show cytotoxicity under photoirradiation or pro-oxidant activity in the presence of Fe3+ or Cu2+.600 Also, C60/PVP showed stronger antimelanogenic potential than naturally occurring whitening agents, such as arubutin and L-ascorbic acid.601 This PVP-entrapped C60 (Radical Sponge) is the first fullerene-based cosmetic ingredient that has been put to practical use since 2005 by a Japanese company (Vitamin C60 BioResearch Corp.).599,602,603 Another fullerene-based cosmetic product that has been commercialized under the trademark LipoFullerene by the same company is the vegetable squalane-dissolved fullerene C60.603 LipoFullerene acts as a free-radical scavenger in skin cells and prevents skin cell damage such as mitochondrial injury and DNA fragmentation, and therefore, it has been used as an active ingredient of antiwrinkle cosmetics.604−606 Moreover, Miwa and co-workers have shown that LipoFullerene does not exhibit any significant biological toxic effects such as photocytotoxicity, bacterial reverse mutagenicity, and permeability into the human skin.607 Niki and co-workers studied the reactivity of two watersoluble sugar-pendant C60 derivatives toward peroxyl radicals and their antioxidant activity against lipid peroxidation in human plasma (Figure 11).608 Their study showed that the reactivity of sugar-pendant C60 fullerenes 275 and 276 toward peroxyl radicals is similar to those of many phenolic antioxidant compounds although smaller than those of vitamins E and C and β-carotene. Other water-soluble fullerene derivatives of C60 with amino acids (β-alanine, valine) or folic acid (vitamin B9) (277−279, Figure 12), as well as C60 nanoparticles have been suggested as potent protectors of cell against nitric oxide induced

Figure 12. Water-soluble fullerene derivatives 277−279.

cytotoxicity, which may find potential applications in NOrelated disorders.609,610 Fullerene nanoparticles were prepared by mechanochemically assisted complexation with anionic surfactant sodium dodecyl sulfate, macrocyclic oligosaccharide γ-cyclodextrin, or the copolymer ethylene vinyl acetate− ethylene vinyl versatate.609 In the case of water-soluble fullerene derivatives 277−279, their protective activity against NOinduced cytotoxicity was mainly attributed to their direct NO scavenging activity.610 On the other hand, the protective action of C60 nanoparticles was not exerted via direct interaction with NO but through neutralization of mitochondria produced superoxide radical in NO-treated cells.609 Similarly, Guan and co-workers studied the ROS scavenging activity of two water-soluble amino acid/C60 derivatives, namely, β-alanine/C60 (277) and cystine/C60 (280), as well as of the oxidized glutathione/C60 derivative 281 (Figure 13) in the hydrogen peroxide-induced oxidative stress and apoptotic death in cultured rat pheochromocytoma (PC12) cells.611−613 Importantly, the results of these studies suggested that fullerenes 277 and 280−281 have the potential to efficiently AU


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liposome-incorporated fullerene-C60 does not have significant biological toxicity (such as photocytotoxicity and bacterial reverse mutagenicity)619 and exhibits antioxidant ability against UVA-induced O2•− and cytoprotective effects under UVA irradiation in HaCaT keratinocytes and 3D-human skin tissue models.620,621 Later, the same research group evaluated the antioxidant activity of liposome-incorporated fullerene-C60 against HO• generated in H2O2−UVA/B irradiation system or in Fenton reaction by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) spin trap/ESR method and β-carotene bleaching assay.622 Their results showed that the antioxidant activity of liposome-incorporated fullerene-C60 was significantly superior to C60-deficient liposome. Moreover, liposome-incorporated fullerene-C60 could scavenge HO• persistently in contrast to Lascorbic acid or Trolox and retained β-carotene which is one of the intracellular lipophilic antioxidants in the β-carotene/ linoleic acid system. Recent clinical studies have also shown that fullerenes formulated into a gel can be successfully used to treat acne vulgaris, an inflammatory disease associated with oxidative stress.623 Notwithstanding the vital role of fullerene and its derivatives as excellent scavengers of a number of reactive oxygen species that are known to be the predominant elicitors for oxidative stress within mammals, their toxicity under certain conditions (i.e., exposure to photoirradiation) should be also considered when pharmaceutical applications are envisioned. It has been well-established that the structure of water-soluble fullerene species is a critical factor that has a substantial impact on their in vitro cytotoxicity. For example, it has been shown that for certain fullerene compounds (i.e., fullerenols) the increasing number of surface groups decrease dramatically (over seven orders of magnitude) the toxicity of pristine C60.624 On the other hand, studies of fullerene toxicity within biological systems, especially those concerning the generation of oxidative stress, are still controversial.625−628 Thus, potential hazards to human health from exposure to fullerenes are still uncertain, and further investigations are currently underway to address this important issue that will eventually determine in large part their further use in industrial and biomedical applications.

Figure 13. Cystine and oxidized glutathione C60 derivatives.

prevent oxidative stress-induced cell death without evident toxicity. Another amino acid that has been employed to prepare water-soluble fullerene derivatives is α-alanine; the resulting compound exhibited excellent radical scavenging activity, especially for O2•− and •OH.614 O2•− has been efficiently quenched also by a fulleropyrrolidine dicarboxylic acid 282 (Figure 14).615

Figure 14. Fulleropyrrolidine dicarboxylic acid 282.

Apart from its water-soluble derivatives, C60 itself is a powerful antioxidant that possesses broad applications in biomedicine as a quencher of free radicals. For example, Moussa and co-workers studied the effects of C60 pretreatments on acute carbon tetrachloride intoxication in rats, which is a classical model for studying free-radical-mediated liver injury.616 Their results showed that aqueous C60 suspensions prepared without using any polar organic solvent not only have no acute or subacute toxicity in rodents but also protect their livers in a dose-dependent manner against free-radical damage. Thus, according to histopathological examinations and biological tests, pristine C60 has been considered as a powerful liverprotective agent.616 Another approach to overcome the problem of the poor solubility of fullerenes is their incorporation in artificial lipid membranes (vesicular and micellar membranes) and into liposomes.617 Given that liposomes act as drug carriers that enhance penetration of drugs through the skin, Lens et al. studied the antioxidant capacity of C60 and two fulleropyrrolidine derivatives [N-methyl-(2-quinolyl)fulleropyrrolidine and N-methyl-(2-indolyl)fulleropyrrolidine] incorporated in multilamellar liposomes. This study proved that C60 and its fulleropyrrolidine derivatives exhibit high skin absorption and excellent antioxidant activity when encapsulated in liposomes.618 Also, Miwa and co-workers reported that

16. SUMMARY In this review we outlined the progress made thus far in deconvoluting the radical reactivity of fullerenes from both practical and fundamental points of view. Besides being the very first reference gathering several data, methods, and applications from various fields, this review contributes to bridging the gap between research communities (chemistry, materials science, biology, and physics) that do not always communicate and share their knowledge to one another with enough clarity. From a fundamental perspective, we provided valuable insight into the basic characteristics of radical reactions of fullerenes. ESR and other physical/chemical studies have led to the establishment of key features of the radical reactions of fullerenes, some of which are as follows: (i) the rate constants for addition of various C-centered radicals to fullerene are two orders of magnitude higher than those for addition of radicals to a wide class of monosubstituted unsaturated compounds; (ii) the multiple addition of free radicals to C60 results in stable fullerenyl radicals with an allylic or cyclopentadienyl structural pattern; (iii) the unpaired electron in the RC60• radical adducts is essentially confined to specific carbon atoms incorporated into the two hexagons adjacent to the radical center and is not extensively delocalizated over the C60 sphere; (iv) fullerenyl AV


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radicals demonstrate a tendency to dimerize in a “head-to-head” type; (v) there is hindered rotation about the R−C60• bond in alkyl fullerenyl radicals, whereas the rotation barrier is lower for the Si−C60• radicals due to the slightly longer bond of Si−C60 compared to C−C60. From a practical point of view, this review seeks to provide the readers with a detailed overview of the existing applications of the radical reactivity of fullerenes in various fields ranging from synthetic organic chemistry and material sciences to biomedicine. Thus, as a first practical aspect, this review provides a systematic summary of free-radical methods for the functionalization of fullerenes. Such reactions include the addition of radicals of different chemical nature, including C-, Si-, O-, S-, P-, N-, and metal-centered radicals, as well as hydrogen and halogen atoms to fullerenes C60 and C70, endohedral metallofullerenes (EMFs), and heterofullerenes. Mechanistic aspects related to these transformations are also illustrated. Another important practical aspect that has been covered in this review concerns the application of fullerenes and their derivatives as novel antioxidant agents in both biological systems and polymeric materials. Thus, it has been shown that fullerenes act as stabilizers of polymers by scavenging efficiently chain radical processes and thus inhibiting their degradation. Besides, a plethora of both in vitro and in vivo studies have shown that the radical scavenging properties of fullerenes can be exploited for the development of novel therapeutic agents in biological systems. Major efforts in this field have focused on the potential treatment of various free-radical-induced biological disorders including mostly neurodegenerative diseases and other cytotoxic processes caused by oxidative stress; furthermore, these properties have already been exploited in a practical use within skin rejuvenation cosmetics. With this work, we provide the first comprehensive review of the radical reactivity of fullerenes that will be valuable to scientists who are interested in the synthetic, material, biological, and pharmacological applications of fullerene compounds. Some of the most recent developments outlined in this review, as well as ongoing research interest into the radical reactions of fullerenes suggest that there are more, still unexplored radical reactions and applications of fullerenes that are likely to emerge in the near future.

Biographies

Manolis D. Tzirakis was born in 1980 and grew up in Heraklion, Crete, Greece. In 2000, he began his undergraduate studies at the University of Crete, during which he was first introduced to organic chemistry in the laboratory of Prof. Michael Orfanopoulos. After graduating in 2004 with a B.A. degree in Chemistry, he began his graduate studies under the guidance of Prof. Michael Orfanopoulos and received his M.Sc. and Ph.D. degrees in 2006 and 2009, respectively. His doctoral research focused on the development of new synthetic methodologies and mechanistic studies in fullerene functionalization chemistry, as well as on the applications of fullerenes and polyoxometalates in heterogeneous photocatalysis. After a short postdoctoral period at the University of Crete, he moved to ETH Zurich, Switzerland, joining the laboratory of Prof. François Diederich, where he has been working on the design and synthesis of enantiopure alleno-acetylenic molecules for the construction of helical oligomers and macrocycles, with special emphasis on the amplification of their chiroptical properties in supramolecular assemblies.

Michael Orfanopoulos was born in Patras, Greece. He received his B.Sc. degree from the University of Patras in 1971, M.Sc. degree from the University of Toledo, and Ph.D. degree from Case Western Reserve University in 1979, under the direction of Prof. L. M. Stephenson. He was a postdoctoral fellow at Stanford University with Prof. H. S. Mosher and at UCLA with Prof. C. S. Foote. In 1981, he was appointed as a Research Scientist at the National Centre of Scientific Research ‘‘Demokritos’’ in Athens, Greece, and in 1985, he joined the University of Crete, where he is currently Professor of Organic Chemistry. His research interests focus on the chemistry and photochemistry of fullerene-based carbon nanostructures, mechanistic studies on the thermal and photochemical reactions of various enophiles, as well as photocatalysis involving polyoxometalates and reactive oxygen species.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (M.D.T.); orfanop@ chemistry.uoc.gr (M.O.). Present Address †

Department of Chemistry and Applied Biosciences, ETH Zurich, HCI G 312, Wolfgang-Pauli-Strasse 10, 8093 Zurich, Switzerland. Notes

The authors declare no competing financial interest. AW


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ACKNOWLEDGMENTS This work was supported by the Greek State Scholarship Foundation (I.K.Y.) through a postdoctoral research fellowship awarded to M.D.T. We are grateful to Dr. Tamsyn Montagnon (University of Crete) for critical and scholastic reading of the manuscript. We also thank Dr. Mariza N. Alberti (University of Crete; currently at ETH Zurich) for valuable comments and discussions during manuscript preparation. LIST OF ACRONYMS AND ABBREVIATIONS AIBN 2,2′-azo(bisisobutyronitrile) ALS amyotrophic lateral sclerosis BHT 3,5-di-tert-butyl-4-hydroxytoluene BNAH 1-benzyl-1,4-dihydronicotinamide CL chemiluminescence DCA 1,4-dicyanoanthracene DMAIB dimethyl azo(bisisobutyrate) DMAP 4-dimethylaminopyridine DMF dimethylformamide dppe bis(diphenylphosphino)ethane DPPH 2,2-diphenyl-1-picrylhydrazyl EDTA ethylenediaminetetraacetic acid EMFs endohedral metallofullerenes ESR electron spin resonance HAT hydrogen-atom transfer HSVM high-speed vibration milling i-PP isotactic polypropylene IPR isolated pentagon rule ISC intersystem crossing KIE kinetic isotope effect MMA methyl methacrylate NADH nicotinamide adenine dinucleotide, reduced NCS N-chlorosuccinimide NIR near-infrared NMA+PF6− N-methylacridinium hexafluorophosphate o-DCB 1,2-dichlorobenzene PET photoinduced electron transfer PHHJs pentagon−hexagon−hexagon junctions PPHJs pentagon−pentagon−hexagon junctions PS polystyrene p-TsOH p-toluenesulfonic acid PVP polyvinylpyrrolidone RNS reactive nitrogen species ROS reactive oxygen species SCE saturated calomel electrode SET single electron transfer SOD superoxide dismutase TBADT tetrabutylammonium decatungstate [(nBu4N)4W10O32] TBHP tert-butyl hydroperoxide TCB 1,2,4-trichlorobenzene TEMPO 2,2,6,6-tetramethylpiperidine-1-oxyl TFA trifluoroacetic acid TGA thermogravimetric analysis THJs triple-hexagon junctions TPP+BF4− 2,4,6-triphenylpyrylium tetrafluoroborate UV ultraviolet REFERENCES (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (2) Krätschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. AX


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BH


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