Carbene Additions to Fullerenes - Chemical Reviews (ACS Publications)

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Carbene Additions to Fullerenes Michio Yamada,† Takeshi Akasaka,*,‡ and Shigeru Nagase§ †

Department of Chemistry, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan Life Science Center of Tsukuba Advanced Research Alliance, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan § Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103, Japan ‡

3.2.4. Metal-Catalyzed Reactions 3.2.5. Zwitterionic Intermediates 3.3. M@Cs(6)-C82 3.4. M2@Ih(7)-C80 3.4.1. Photolysis of Diazirines 3.4.2. In-Situ Generation of Diazo Compounds 3.5. M2@D3h(5)-C78 3.5.1. Photolysis of Diazirines 3.5.2. Thermolysis of Diazirines 3.6. M2@D2(10611)-C72 3.7. M3N@Ih(7)-C80 3.7.1. In-Situ Generation of Diazo Compounds 3.7.2. Silylenes 3.8. Sc2C2@C3v(8)-C82 3.9. Sc2C2@C2v(5)-C80 3.10. Sc3C2@Ih(7)-C80 3.11. Li@C60 4. Concluding Remarks Author Information Corresponding Author Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Carbene Addition to Empty Fullerenes 2.1. Dihalocarbenes 2.1.1. Dihalocarbene Additions to C60 2.1.2. Dihalocarbene Additions to C70 2.1.3. Dihalocarbene Additions to Fullerene Fragments 2.2. Decomposition of Diazo Compounds 2.2.1. Thermolysis of Diazo Compounds 2.2.2. Photolysis of Diazo Compounds 2.2.3. In-Situ Generation of Diazo Compounds 2.2.4. Isomerization of Fulleroids to Methanofullerenes 2.3. Decomposition of Diazirines 2.3.1. Chemical Probe 2.3.2. Photoaffinity Labeling 2.3.3. Fullerene Sugars 2.3.4. Addition−Elimination Reaction 2.4. Decomposition of Diazonium Salts 2.5. Alkoxycarbenes 2.6. Vinylcarbenes 2.7. Basic Treatment of Malonates in the Presence of Iodine 2.8. Photolysis of Benzocyclobutenediones 2.9. α-Ketocarbenes 2.10. Fischer Carbene Complexes 2.11. Heterocyclic Carbenes 2.12. Zwitterionic Intermediates 2.13. Si and Ge Analogues of Carbenes 2.13.1. Silylene Addition to C60 2.13.2. Silylene Addition to C70 2.13.3. Germylene Addition to C60 3. Carbene Additions to Endohedral Metallofullerenes 3.1. Endohedral Metallofullerenes 3.2. M@C2v(9)-C82 3.2.1. Photolysis of Diazirines 3.2.2. Thermolysis of Diazomethanes 3.2.3. In-Situ Generation of Diazo Compounds © XXXX American Chemical Society

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1. INTRODUCTION Carbon is the most prominent element of state-of-the-art nanoscience. Kroto, Curl, and Smalley were awarded the Nobel Prize in Chemistry in 1996 for discovery of fullerenes.1−4 Another form of carbon, graphene, has also become the center of considerable attention: a Nobel Prize in Physics was awarded in 2010 to Geim and Novoselov for “groundbreaking experiments regarding the two-dimensional material graphene”.5,6 Carbon nanotubes, discovered by Iijima in 1991,7 have also been of special interest. They have been recognized as key materials along with graphenes for use in advanced nanotechnology. From the viewpoint of chemistry, however, fullerenes remain the core of carbon nanoforms because of their homogeneity. Since the discovery of fullerenes, intensive efforts have been undertaken to explore the chemical reactivity and preparation of new functional derivatives. To date, several surveys of the chemistry of fullerenes have been added to the literature. Early studies have been described in many reports.8−26 The aromaticity of fullerenes has been examined as well.27−30 Thilgen and Diederich provided an

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remarkable aspect is that the metal positions and movements are affected by introduction of addends because the electronic structure of the cage is altered by functionalization.56 This part of the review specifically examines the regioselectivity and applicability of carbene additions to various EMFs. Regarding nomenclature for the fullerenes, carbon cages are labeled by their symmetry and the number in accordance with the Fowler−Monolopoulos spiral algorithm to designate isomers, except for C60 and C70. The present review uses a 2fold numbering system, as Popov proposed in his review.53 Usually, a short form of the numbering system is used for IPR isomers, in which only IPR isomers are numbered as given in “An Atlas of Fullerenes”.57 However, isolable non-IPR isomers have been reported to an ever greater degree. They cannot be described using that short and simplified numbering system. In such cases, a full numbering system is adopted for such non-IPR isomers. Additionally, we note that the description of EMFs is not indeed unified in the literature. Conventionally, the symbol @ is used to indicate that atoms to the left are encapsulated within the carbon cage on the right.58 For instance, ‘La@C2v(9)C82’ means that a La atom is encaged in a C2v(9)-C82 cage. On the basis of the International Union of Pure and Applied Chemistry (IUPAC) recommendation,59−61 however, ‘La@C2v(9)-C82’ should be written as ‘i(La)(C82-C2v(9))’. Nevertheless, this is rarely used in the literature. The present review adopts the former conventional style for brevity. Throughout, the diversity and applicability of carbene additions to fullerenes will be exhibited for the reader.

excellent overview of the structural aspects of fullerenes and their derivatives.31 Recent advances of the research on fullerene were reviewed in several reports.32−35 Those numerous studies attest that the chemistry of fullerenes encompasses quite widely diverse chemical reactions including nucleophilic additions, electrophilic additions, electrocyclic reactions, radical additions, metal coordination, and supramolecular complexation. That great diversity of the chemical reactivity promises development of various fullerene-based materials having unique properties for materials and biological sciences. Divalent carbon species, carbenes, are well-known reactive intermediates that enable new carbon−carbon bond formation.36 A typical carbene reaction involves addition to a π bond of olefins, which is also the case for a π bond of fullerenes. It is also important to note that carbenes exhibit diverse reactivity. Singlet carbenes possess both electrophilic and nucleophilic character, whereas triplet carbenes show diradical reactivity. Consequently, widely diverse chemical reactions appear when fullerenes meet carbenes. In this respect, carbene additions to fullerenes are of great importance not only to synthesize useful derivatives but also to clarify the chemical reactivity of fullerenes. Much progress has been made recently in fullerene chemistry, including both experimental and computational contributions. Nevertheless, detailed chemical reactivities of fullerenes toward carbenes and reaction mechanisms have not been reviewed. This lack of detail, representing a gap in the literature, has motivated us to write a review to update the broad, interested readership. This review provides an overview of carbene additions to fullerenes and related reactions, specifically examining reaction mechanisms. Carbene additions to other carbon nanoforms such as carbon nanotubes and graphenes and nitrene additions are beyond the scope of this review. This review includes two major parts. The first deals with carbene additions to empty fullerenes such as C60 and C70. Various carbene species have been applied for synthesis of functional derivatives, providing proof of the superiority of carbenes in fullerene chemistry. In particular, diazirines and diazo compounds deserve special mention as potent carbene precursors. Of importance is that free carbenes and diazo intermediates take part in reactions with fullerenes via different reaction pathways, which lead to different products. To understand the mechanism of product formation, the isomerization process must be considered as well. An outstanding example, phenyl-C61-butyric acid methyl ester ([60]PCBM), prepared by reaction of C60 with a diazo precursor, is a promising candidate for an electron-accepting material in organic solar cells.37 Recent results demonstrate that both electrophilic and nucleophilic carbenes are reactive toward fullerenes, giving various derivatives. In addition, silicon and germanium analogues of carbenes are an option. The second part of this review is devoted to carbene additions to endohedral metallofullerenes (EMFs), the most up-to-date topic.38−44 Recent progress has revealed that chemical reactivities of EMFs differ from those of empty fullerenes.45−50 The difference arises from intramolecular electron transfer from the internal metal atoms to the carbon cages, resulting in a drastic change of their electronic structures.51,52 Consequently, the internal metal atoms play an important role in controlling the chemical reactivities of outer cage carbons. The unique molecular and electronic structures of EMFs also represent a fascinating issue in computational chemistry. Quantum chemical calculations can provide useful information related to the electronic structures, energies, and reactivities of EMFs.53−55 Another

2. CARBENE ADDITION TO EMPTY FULLERENES 2.1. Dihalocarbenes

2.1.1. Dihalocarbene Additions to C60. Dihalocarbene, because it is among the simplest and most representative carbene species, is widely used in synthetic chemistry. Consequently, it is worth beginning this review with the chemistry of dihalocarbenes with fullerenes. In general, the reactive sites on C60 in carbene additions contain carbon atoms with C−C ‘double’ bonds bisecting two hexagon rings, denoted as [6,6] bonds. Neutron powder diffraction,62 electron diffraction,63 and X-ray crystallographic studies64 reveal that the [6,6] bonds (1.39 Å) are shorter than the bonds which bisect pentagon and hexagon rings, denoted as [5,6] bonds (1.45 Å). Consequently, [6,6] bonds of C60 possess olefinic characters. The pyramidalization angle of the individual carbon atom, defined as the p orbital axis vector (POAV), is often used as an index of the local strain.65−67 The POAV angle of C60 (11.64°) induces a large heat of formation ΔHf° = 9.1 kcal mol−1 per carbon atom, which compares with zero for graphite. Pyramidalization contributes to the energies of the molecular orbitals (MOs). Therefore, POAV angles provide a useful guide for understanding the reaction efficiencies. Undoubtedly, fullerenes favor reactions that decrease strain. Pyramidalization reduces overlapping of π orbitals and weakens the π bond, which raises the energy of the bonding π MO and lowers the energy of the antibonding π* MO. In addition, the mixing of s-orbital character with the p orbitals lowers the energy of all the derived π MOs, including both the HOMO and the LUMO. Overall, a major consequence of curvature is a substantial lowering of the energy of the LUMO with a slight change in the HOMO.68 However, frontier orbital theory often encounters difficulty in predicting the kinetically reactive sites of fullerenes because MOs are, generally speaking, strongly delocalized over the spherical surfaces. 69 In fact, the hu B

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HOMOs of C60 are 5-fold degenerate, and the t1u LUMOs are 3fold degenerate. Consequently, the averaged MOs are strongly delocalized. Addition of dichlorocarbene to C60 was first reported by Nogami and his co-workers in 1993.70−72 Pyrolysis of sodium trichloroacetate (1) generated dichlorocarbene, which was subsequently trapped by C60 to afford formation of carbene adduct 2 (Scheme 1). NMR spectra show clearly that 2 possesses

Scheme 3

Scheme 1

resonance at 67.7 ppm was found, which was assigned to the four sp3 fullerene cyclopropane carbons for C121. This value shows agreement with the proposed carbene attack mechanism. Scanning tunneling microscopy (STM) measurements of the samples displayed the dumbbell-like structures clearly, supporting formation of C121 and C122. However, no spectroscopic evidence was found for the existence of a single isomer. Formation of carbene intermediate 5 was also achieved by thermal reaction of diazotetrazole 6 with C60 by Strongin and coworkers.77 Reaction mechanisms of diazo compounds with fullerenes will be explained in section 2.2. At least 14 fullerene products were observed in HPLC analysis, among which fullerene dimer 7 was isolated in, at most, 3% yield (Scheme 4). Mass spectrometric analysis of the other products indicated

a [6,6]-cyclopropanated structure (namely, [6,6] closed methanofullerene). In this respect, the chemical behavior of C60 is similar to those of olefins. The 13C NMR chemical shift of the bridgehead C atoms was found at 80.1 ppm, being indicative of transformation to the 58π electronic structure. Releasing the strain energy of the fullerene curvature could compensate for the loss of the π conjugation. Walton et al. described the synthesis of 1′,1′-dibromo-1,9-methanofullerene (3) and 1′-cyano-1,9-methanofullerene (4) by treating a mixture of C60 and either CHBr3 or CH2BrCN with LDA (Scheme 2).73 Reactions were unsuccessful when NaH was used as base. Scheme 2

Scheme 4

formation of C119 and C121 along with either C61(C6H6) or C61(C7H8), which depended on the choice of benzene or toluene as the reaction solvent. Spectroscopic evidence of the D2hsymmetric structure of 7 was obtained using the 13C-labeling technique. Use of 10−15% 13C-enriched C60 as a starting material enabled preparation of the 13C-labeled NMR sample of 7. The 13C NMR spectrum exhibited 15 out of the 16 expected resonances in the 136−150 ppm region. In addition, the single upfield resonance at 60.1 ppm, integrating to four carbons, was assigned to the skeletal cyclopropane carbons, although the cyclopropylmethylene bridging carbons, at natural abundance, were not observed. Zhu applied an ionic liquid as the reaction medium to the dihalocarbene addition using the ultrasound technique.78 Irradiation of ultrasound to a mixture of C60, haloform (CHCl3, CHBr3, and CHI3), and sodium hydroxide in 1-butyl3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) gave the corresponding methanofullerenes (2, 3, and 8) exclusively in 63−77% yields (Scheme 5). In contrast, low yields of 11−17%

Osterodt and Vögtle used a Seyferth reagent74 for synthesis of 3, in which the [2 + 1] addition mechanism of dibromocarbene to a C60 double bond was proposed because no [5,6] open homofullerene (or fulleroid) isomer was obtained.75 Subsequently, the possibility of synthesizing the carbon clusters C121 and C122 was investigated using various methods via formation of C61 carbene 5 (Scheme 3). Although no dimeric product was isolated, mass spectrometric analysis of the crude reaction products displayed signals of the expected masses 1452 (C121) and 1464 (C122) after the dimerization reaction. Signals were also detected after the Seyferth reaction, implying that C121 and C122 were formed even under those conditions in low quantities. Dragoe et al. also synthesized fullerene dimers C121 and C122 by heating of 3 at 450 °C in the presence of C60.76 In that case, isolation was accomplished using the recycling GPC technique. Sixteen low signal-to-noise lines were observed between 140 and 148 ppm in the 13C NMR spectrum. In addition, a small upfield C

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as occurring in two phases. In the first phase, the carbene approaches suprafacially to the π system to maximize the HOMO(fullerene)−LUMO(carbene) interaction. In the second phase, the nonbonding electron pair of the carbene participates in nucleophilic interaction with the LUMO of the olefin. Nagase, Chen, and co-workers conducted comparative studies of the carbene mechanism and the addition−elimination mechanism in the dichlorocarbene addition to C60 from trichloromethyl anion (Scheme 7).81 Their calculations show that addition of CCl3− to C60 is an exothermic, barrierless process, affording C60(CCl3)− as the exclusive intermediate. The kinetically favorable reaction path for subsequent cyclization involves TS[6,6]‑1 with the cyclization barrier of 14.2 kcal mol−1, which gives [6,6]-C60(CCl2) (2) as a product. However, the other three paths via transition states TS[5,6]‑1 and TS[5,6]‑2, which give [5,6]-C60(CCl2), and TS[6,6]‑2 are kinetically unfavorable (Figure 2). Therefore, calculations indicate that the addition− elimination mechanism is highly regioselective, which is consistent with the results of the reactions of C70 with two different carbene reagents reported by Kiely and co-workers.82,83 They also obtained energy profiles for reaction via the carbene mechanism to compare the addition−elimination mechanism at the same level of theory (Figure 3). They found two reaction paths involving one-step, concerted formation of the [6,6] adduct or the [5,6] adduct, which were similar to the results reported by Bettinger.80 The activation energy barriers in the gas phase with ZPE correction for the two paths are 2.2 and 4.6 kcal mol−1. These two barriers slightly increase, respectively, to 3.2 and 5.5 kcal mol−1 after consideration of the solvent effect of THF. Therefore, the theoretical results of studies suggest that the carbene mechanism slightly prefers the [6,6] adduct to the [5,6] adduct, whereas the addition−elimination mechanism affords the [6,6] adduct as the exclusive product. The energy of CCl3− is lower than the sum of energies of CCl2 and Cl− by 8.0 kcal mol−1, indicating that the concentration of CCl3− is higher than that of CCl2 at equilibrium. The authors also investigated the dependence of solvent dielectric constants on the activation energies of the rate-determining steps E1‡ and E3‡. It is noteworthy that E3‡ greatly decreases as the dielectric constant of solvents increases, whereas E1‡ is little changed. Consequently, highly polar solvents effectively enhance the rate via the addition−elimination mechanism, as found in Zhu’s experiment.78 2.1.2. Dihalocarbene Additions to C70. In C70, the five distinct carbon atoms are named a, b, c, d, and e. Bonds of eight different types named [6,6]-(a−a), [6,6]-(a−b), [5,6]-(b−c), [6,6]-(c−c), [5,6]-(c−d), [5,6]-(d−d), [6,6]-(d−e), and [6,6](e−e) are shown in Figure 4. The alphabetical naming is used as a widely accepted nomenclature conventionally, as employed by Balch et al.84 Among these bonds, [6,6]-(a−b) and [6,6]-(c−c) bonds (1.38 and 1.37 Å, respectively85) are the shortest bonds. They are classifiable as double bonds,86−88 suggesting that they are the most reactive sites. On the other hand, [6,6]-(a−a), [5,6](b−c), [5,6]-(c−d), and [6,6]-(e−e) bonds (1.45−1.46 Å85,87,89−91) are the longest bonds, which are therefore regarded as less reactive.92 Apparently, the equatorial region of C70 has little curvature and lower bond strain. The POAV angles of a, b, c, d, and e are, respectively, 11.96°, 11.96°, 11.46°, 10.06°, and 8.78°.66,93 Experimentally, nucleophilic attack of C70 occurs preferentially at the [6,6]-(a−b) and [6,6]-(c−c) sites.94−98 From a theoretical perspective, recent calculations at the B3LYP/ 6-311G(d,p) level of theory show that the averaged density of the doubly degenerate e1” LUMOs of C70 is delocalized throughout the carbon sphere except in the equatorial regions, in which the a

Scheme 5

were obtained when the same reactions were conducted in THF. Dehalogenation of 2, 3, or 8 in the presence of C60 in [BMIM][BF4] on treatment with magnesium under irradiation of ultrasound gave a fullerene dimer 9 in 84% yield. When the same reactions were operated in THF, the yield was decreased to 21%. The 13C NMR spectrum of 9 consists of 14 resonances in the region of 140.20−147.96 ppm with an additional peak at 67.84 ppm, confirming exclusive formation of the [6,6] closed connection to the C60 cores. Results imply that the carbene mechanism is presumably not dominant in the reaction because the mechanism is known to be hardly affected by solvent polarities. By applying this technique, Zhu et al. synthesized carbene adducts 10 and 11 in 65% and 79% yield, respectively (Scheme 6).79 Scheme 6

Bettinger calculated the energy profiles for addition of dichlorocarbene to C60 by the density functional theory (DFT) method.80 In addition of dichlorocarbene to a [6,6] or [5,6] bond of C60, one transition state for each addition site (TS[6,6] or TS[5,6]), which is 1.8 or 4.1 kcal mol−1 higher in energy than the reactant, was found in both cases (Figure 1). Relative energies of the two transition states are in agreement with the Bell−Evans− Polyani principle. In fact, the [6,6] closed methanofullerene is thermally more stable than the [5,6] open fulleroid is. Addition of the singlet carbene to C60 proceeds similarly to reaction of the carbene with typical alkenes. The reaction is generally regarded D

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Figure 1. Schematic energy profile (in kcal mol−1) for reaction of C60 with singlet dichlorocarbene as computed at the B3LYP/6-311G(d,p)//B3LYP631G(d) level of theory. Zero-point-corrected data are given in parentheses. Transition structures and products computed for addition of singlet dichlorocarbene (chlorine, black spheres) to the [6,6] and [5,6] bonds of C60 at the B3LYP/6-31G(d) level of theory. Bond lengths are given in Angstroms. Reproduced with permission from ref 80. Copyright 2006.

three monoadduct isomers 12, 13, and 14 in a 35:37:29 ratio (Scheme 8).82,83,101 Structures of the isomers were identified using 13C NMR spectroscopy. In addition, structural assignment of 14 was confirmed using X-ray crystallography. It is noteworthy that isomer 14 has a fulleroid structure, whereas isomers 12 and 13 have [6,6] closed methanofullerene structures. Use of sodium trichloroacetate instead of PhHgCCl2Br caused a mixture that included mostly 12, with a small amount of 13 and little to no 14. Trichloromethyl anion is an intermediate in generation of dichlorocarbene from trichloroacetate. Therefore, addition of the anion followed by elimination of chloride anion is regarded as the plausible pathway (addition−elimination mechanism) for formation of methanofullerenes in these reactions. This result also provides evidence for the role of dichlorocarbene in generation of 14 from PhHgCCl2Br. It is likely that the reaction involves initial formation of a [5,6] closed methanofullerene followed by a rapid norcaradiene−cycloheptatriene-like electrocyclic rearrangement to the [5,6] open homofullerene. The enhanced stability of the homofullerene is caused by πhomoaromatic stabilization. 2.1.3. Dihalocarbene Additions to Fullerene Fragments. To shed light on the reactivity of dihalocarbenes to fullerenes, it is worthwhile to exert some effort to understand the reactivity of dihalocarbenes to polycyclic aromatic hydrocarbons

Scheme 7

site possesses the largest coefficient.99 In contrast, the a1” LUMO-1, which lies in very close energy to the LUMO, is distributed mainly to the (c−d) sites. Lee’s DFT calculations at the B3LYP/6-31G* level of theory show that the distribution of the doubly degenerate e1” HOMOs of C70 are also found mainly in the (c−d) sites.100 Kiely et al. demonstrated that reaction of C70 with dichlorocarbene, generated using Seyferth’s phenyl(bromodichloromethyl)mercury reagent, led to formation of

Figure 2. Structures of (a) C60(CCl3)− intermediate and (b−e) transition states at the B3LYP/6-31G(d) level of theory. Bond lengths are given in Angstroms. Arrows of b−e indicate orientations toward which the CCl2 cyclizes or the Cl− leaves in the forward reactions that form C60(CCl2). Reproduced with permission from ref 81. Copyright 2009 American Chemical Society. E

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Figure 3. Energy profiles (in kcal mol−1) reflecting the competition between the carbene and the addition−elimination mechanisms for reaction of CCl3− and C60. Geometry optimization was performed in the gas phase at the B3LYP/6-31+G(d) level. Single-point energies were computed in THF at the same level. Gas-phase energies are given in parentheses. ZPEs (unscaled) are included. Reproduced with permission from ref 81. Copyright 2009 American Chemical Society.

Scheme 9

accumulate as a secondary product (Scheme 10). Diiodocarbene generated from iodoform by the same phase transfer method Scheme 10

Figure 4. Schematic structure of C70. Carbon atoms in C70 are numbered according to IUPAC recommendations.60 Carbon atoms of the five different types in C70 are assigned conventionally as a−e. Double bonds are omitted for clarity.

(PAHs), which can be recognized as fullerene fragments.102,103 In general, chemical reactions of PAHs engender formation of new covalent bonds at carbon atoms located on the outer perimeter of the PAHs but never at interior carbon atoms. For instance, reaction of phenanthrene (15) with dichlorocarbene causes cyclopropanation at the rim double bond to afford 16 (Scheme 9). It is straightforward to infer that phenanthrene is likely to preserve the greatest possible number of benzene rings and introduce the least additional strain into the skeleton. In contrast, Scott and co-workers found that addition of dichlorocarbene to corannulene (17) took place at one of the radial double bonds of corannulene preferentially to give 18, not 19.68,104 As the reaction proceeded, the 2:1 adduct 20 started to

gave a 1:1 adduct similar to 18 in low yield. In addition, dibromocarbene addition occurred at the same bond when Seyferth’s reagent (PhHgCBr3) was used. Such an addition mode (18) disrupts the cyclic conjugation in two benzene rings and introduces more strain than the reasonable mode (19) would have. The authors proposed that cyclopropanation might occur in a stepwise manner. Computa-

Scheme 8

F

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energy in C60; however, such benefits of pyramidalization strain relief contribute to the reactivity and overcome the energetic penalty resulting from the loss of aromaticity in the molecule. In fact, theoretical calculations predict that the [6,6] bond of 24 resembles the [6,6] bond of C60 more closely than it does the π system of planar PAH. DFT calculations indicate that both the HOMO and the LUMO are concentrated on the central C−C bond. Therefore, the bond can be regarded as the kinetically preferred site for attack by both electrophiles and nucleophiles. It is particularly interesting that reactivities of PAHs can be predicted simply using Clar’s aromatic π-sextet theory. According to Clar’s rule,107−109 the Kekulé resonance structure with the largest number of disjoint aromatic π sextets is the most important for characterization of the properties of PAHs.110−112 The Clar structures of 15 and 24 clearly display the reactive sites bearing the olefinic character. For C60, the Clar representation ascribes eight π sextets, leaving six [6,6] double bonds as localized olefins. In that sense, Clar’s rule can provide qualitative information related to the reactivity of C60.113 Moreover, Clar’s rule is applicable for C70. The Clar representation of C70 ascribes nine π sextets, leaving eight [6,6] double bonds as localized olefins, which shows good agreement with the fact that the [6,6](a−b) and [6,6]-(c−c) bonds are the shortest bonds (Figure 6).

tional calculations indicate that the LUMO of the electrophilic carbene interacts most strongly with the largest HOMO coefficient in the π system, that is, the interior carbon atoms. If addition were stepwise, then the resulting zwitterionic intermediate would have structure 21 (Figure 5).

Figure 5. Intermediate 21.

They speculated that most of electrophiles reacted fastest at the interior carbon atoms of corannulene but that initial attack was reversible for stable cations. In that case, these electrophiles can eventually attack a methine position on the rim to generate an arenium ion that can rearomatize by deprotonation. However, in the case of dichlorocarbene addition, the corresponding intermediate 22 is unlikely to release a high-energy carbene. To verify the hypothesis, they investigated the reactivity of corannulene with highly reactive electrophiles. When solid AlCl3 was added to a solution of corannulene in CDCl3 at room temperature, a blue color appeared immediately and the NMR spectra displayed generation of carbocation intermediate 22. Subsequent quenching with methanol and potassium carbonate yielded methyl ester 23 (Scheme 11). Accordingly, these results Scheme 11

Figure 6. Clar representations of 15, 24, C60, and C70. Possible π-sextet migration is not shown.

strongly support the stepwise mechanism. Stepwise addition of dihalocarbenes to cyclopropenes was reported previously as an extreme case by Weber and Brinker.105 Scott et al. reported that C26H12 PAH 24 (diindeno[1,2,3,4defg;1’,2’,3’,4’-mnop]chrysene) also showed fullerene-like reactivity toward dibromocarbene.106 The fullerene segment 24 reacted with dibromocarbene at its central [6,6] bond to give the cyclopropanated derivative 25, although the reaction proceeded very slowly (Scheme 12). In 24, pyramidalization of the central carbon atoms (POAV angle 8.1°) approaches 70% of that of the carbon atoms of C60 (POAV angle 11.64°). In that sense, relief of pyramidalization strain at the four carbon atoms attached to the central [6,6] bond in 24 would be expected by addition. The magnitude of this effect is smaller than that resulting from the relief of the global strain

2.2. Decomposition of Diazo Compounds

2.2.1. Thermolysis of Diazo Compounds. 2.2.1.1. Thermal Reactions of Diazo Compounds with C60. Diazo compounds114 are useful carbene transfer reagents to fullerenes. Therefore, the scope of the reactions of C60 with diazo compounds is quite broad. Wudl and co-workers first demonstrated the reaction of diazo compounds with C60.16,115,116 They found that C60 reacted smoothly with diphenyldiazomethane (26) at room temperature to afford formation of the adduct 27 (Scheme 13). It was proposed that addition of a diphenylcarbene equivalent to a C60 double bond was accomplished by [3 + 2] dipolar addition of diphenyldiazomethane followed by N2 extrusion from the pyrazoline intermediate 28. In this respect, the reaction can be regarded as a ‘formal’ carbene addition. Early reports of the literature suggested that all diazomethane addition products contained π-homoaromatic, [6,6] open structures. However, these assignments were later revised to σ-homoaromatic [6,6] closed methanofullerene structures based on NMR structural analyses and energy minimization calculations.117 Prato, Wudl, and co-workers showed (1) that the initial product of addition was a mixture of methanofullerene and fulleroid and (2) that the

Scheme 12

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Scheme 13

lowest energy isomers of substituted phenlyldiazomethane and substituted diphenyldiazomethane addition were [6,6] closed methanofullerene structures. The first X-ray determination of the cyclopropane structure in methanofullerenes was demonstrated by Vögtle and co-workers.118 They synthesized a series of substituted diphenyldiazomethanes and used them for functionalization of C60 to obtain the corresponding products.119 Among these, single crystals of dimethoxy derivative 29 (Figure 7) were

Scheme 14

membered ring with an eclipsed ethylene bridge. The low-field aromatic doublet at 7.87 ppm, interacting with the multiplet at 7.21 ppm, was caused by the ortho protons H4 and H6 near the spiro carbon. Computational optimization of the structure at the semiempirical MNDO level yielded a preferred boat conformation with rigorous Cs symmetry, which showed complete agreement with the spectroscopic evidence. The deshielded endo ethylene protons H10b and H11b observed at 4.41 ppm are oriented toward the adjacent pentagon of the fullerene sphere. The 1H NMR spectrum of 32 showed that all eight aromatic resonances were differentiated. The two methylene protons resonated as doublets at 4.67 and 6.77 ppm. MNDO calculations display that the acetonide bridge forces the seven-membered ring into a twist-boat conformation, with the endo bridge proton H10 pointing toward the fullerene cage and the exo proton H11 away. The NOE dipolar interactions of exo proton H11 with both protons H1 and H9 and that of endo proton H10 with only H9 were observed, leading to the assignments of doublets at 6.77 and 4.67 ppm to the endo H10 and exo H11 methylene protons, respectively. The endo proton H10, pointing almost exactly toward the center of the adjacent pentagon ring, is 2.1 ppm downfield shifted with respect to the exo proton H11. The segregated ring currents of the fullerene core are quite useful for a straightforward structure determination of diastereomers of fullerene derivatives. The electrochemical properties of substituted diphenylmethanofullerenes are independent of the substituents in the para positions of the phenyl groups. Wudl et al. reported that spiroannelated analogues exhibited remarkable substituent effects on the reduction potentials.132−135 Fluorenomethanofullerenes 33−35 were synthesized through thermolysis of the corresponding diazomethane derivatives in the presence of C60 (Figure 9). The first and second reduction potentials of 34 were shifted to more negative values relative to C60, whereas those of 35 exhibited slightly less negative values than those of C60. Substituent effects are attributed to the through-space orbital interactions between the π systems of the fullerene cages and the spiroannelated addend. Such periconjugation effects were also observed in quinone-type methanofullerenes.

Figure 7. Schematic structure of 29.

obtained as black prisms. Finally, X-ray analysis showed clearly that the product had a [6,6] closed methanofullerene structure. The distance between the two bridged C atoms is 161.4 pm, clearly reflecting transannular bond formation. Oshima and co-workers systematically investigated kinetic substituent and solvent effects of the reactions of metasubstituted and para-substituted diphenyldiazomethanes with C60 and C70.120 The rate constants of addition increased with the increase of the electron-donating ability of the meta substituents and para substituents. In contrast, the rates decreased with the increase of π-electron-donating ability of the solvents because of the ground-state solvation of fullerenes. Because of the synthetic utility, phenyl-substituted diazomethanes have been applied for preparation of various functionalized derivatives such as crown ether-appended derivatives,119,121,122 fullerene-bound dendrimers,123 fullerene− peptide conjugates,124,125 fullerene derivatives for inhibition of HIV-1 enzymes,126,127 C60-substituted stable radicals,128 and soluble fullerene polymers.129 Wudl et al. synthesized structurally constrained diphenylmethanofullerenes 30−32 to obtain experimental evidence indicating the presence of ring currents130 in methanofullerenes (Scheme 14).131 The 1H NMR spectrum of 30 showed a singlet at 7.49 ppm for the olefinic protons and only four signals for the two aromatic rings, thereby confirming exclusive formation of the [6,6]-methano-bridged structure. The 1H NMR spectrum of 31 exhibited two multiplets at 3.33 and 4.41 ppm as an AA′XX′ system for the ethano bridge protons, two multiplets at 7.21 (intensity 4H) and 7.26 ppm (intensity 2H), and a doublet at 7.87 ppm (intensity 2H). The aromatic protons (H1 and H9) orthogonal to the ethylene bridge resonated at 7.26 ppm and displayed relevant NOE dipolar interactions with the high-field multiplet at 3.33 ppm but not with the low-field multiplet at 4.41 ppm. These data ruled out the twist-boat conformation; instead, they confirmed the static boat conformation of the sevenH

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yielded a 4:3 mixture of 41 and 11.149 The methylene protons of 11 resonate at 3.93 ppm as a singlet, whereas the 1H NMR spectrum of 41 contains doublets at 2.87 and 6.35 ppm. On the basis of the ring current arguments, the protons over the pentagon and hexagon structures were assigned to be at 2.87 and 6.35 ppm, respectively.131 1JCH coupling constants between the methylene carbon and its hydrogen atoms determined to be 145.0 and 147.8 Hz are in agreement with the π homoaromaticity of 41. Sternfeld and co-workers investigated the aromaticity of fullerenes by probing the 1H NMR resonances of the hexaanions of the C61H2 isomers.150 The change in the 1H NMR chemical shifts reveals that added electrons are located mainly in the pentagons, converting them from paratropic to diatropic rings. Reduction of methanofullerene 11 caused partial opening of the bond between the hexagons. This phenomenon is explainable by either a nonclassical bond or equilibrium between the open and the closed forms. Compound 11 was also synthesized by reaction of electrochemically generated C60 trianions with dichloromethane.151 It is noteworthy that more reactive dihalomethano compounds such as I2CH(CMe)3 are able to react with C60 dianions152 and that activated dihalomethano compounds such as Br2CHCN are able to react with C60 monoanions.153 Compounds 41 and 11 do not interconvert photochemically or thermally.154 Diederich and co-workers demonstrated stepwise addition of diazomethane to C2v-symmetrical C60 pentakis adducts. They proposed a mechanism for thermal and photochemical N2 extrusion from pyrazoline intermediates to rationalize formation of the products (Scheme 17, in which fullerene cages are shown in part for clarity).155 Starting from pyrazoline 40, be thermolysis provides in an orbital symmetry controlled [π2s + π2s + σ2s + σ2a] rearrangement with high regioselectivity norcaradiene 42 with subsequent valence tautomerization to yield cycloheptatriene-like fulleroid 41. The thermally allowed Woodward−Hoffmann mechanism was also supported by ab initio and density functional calculations of the model systems.156 In contrast, photolysis of 40 proceeds via a diradical mechanism (43 and 44), giving a mixture of 41 and 11. Computational studies show that 41 and 11 are quite close in energy. Raghavachari and Sosa calculated the total energies of 41 and 11 by ab initio and DFT methods.157 At the HF/6-31G* level of theory, 11 was 9.3 kcal mol−1 more stable than 41. The energy difference was smaller: 2.5 kcal mol−1 at the NLSD/DZP level. Diederich et al. calculated the heats of formation of 41 and 11 using the semiempirical PM3 method.142 PM3 calculations showed that the two isomers were almost equal in energy and predicted that 41 was 0.34 kcal mol−1 in favor of 11. Shimotani et al. also conducted DFT calculations and demonstrated that 11 was more stable by 2.8 kcal mol−1 at the B3LYP/6-31G(d) level of theory.158 Unsymmetrical diazo compounds give rise to two different [5,6] open fulleroids. In these cases, the diastereomer with the

Figure 8. MNDO-minimized structures of 31 and 32. Reproduced with permission from ref 131. Copyright 1993 American Chemical Society.

Figure 9. (Left) Schematic structures of 33−35. (Right) Schematic drawing of the HOMO interaction in cyclopentadienofullerene (fullerene shown in part for clarity).

The diazo chemistry has subsequently been extended to other diazo compounds such as diazoalkanes, 136,137 diazoketones,138,139 diazoesters,16,140−142 diazopyruvate,143 diazoamides,144 diazophosphonates,145,146 diazothioates,147 and silyl diazomethanes.148 In general, thermal reactions of diazo compounds to C60 afford a mixture of three regioisomers, identified as the [6,6] closed methanofullerene (36) and the two [5,6] open fulleroids (37 and 38) (Scheme 15). Exclusive formation of the regioisomers is a consequence of the principle of minimization of [5,6] double bonds in the fullerene framework. It is noteworthy that the reaction conditions depend strongly on the diazo compound that is used. For instance, cycloaddition of diazoacetates and diazoamides is conducted in refluxing toluene, whereas addition of diphenyl diazomethanes proceeds at room temperature. In contrast to substituted diazomethanes, the parent diazomethane (39) exhibits unique reactivity toward C60 (Scheme 16). In fact, the pyrazoline intermediate (40) can be isolated and characterized in the reaction of 39 and C60, suggesting that the initial step in addition of diazo compounds to C60 is indeed a 1,3dipolar cycloaddition.51 Thermolysis of 40 in refluxing toluene afforded [5,6] open fulleroid 41, embodying an annulene moiety, in quantitative yield. It is noteworthy that [6,6] closed methanofullerene 11 was not formed. Photolysis of 40 however Scheme 15

I

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Scheme 16

Scheme 17

Figure 10. Equilibrated conformers of a concerted transition state (fullerene shown in part for clarity).

Oshima and co-workers investigated the substituent effects in the reactions of C60 with various unsymmetrical diazoalkanes, Scheme 19 and Table 1.137 In the cases of 49a−c and 49g, formation of the [6,6] closed methanofullerenes (52) was negligible, indicating that the reaction proceeded via a concerted mechanism. In contrast, considerable amounts of [6,6] closed methanofullerenes were obtained when the aromatic diazoalkanes 49d−f, 49i, and 49j were conducted. Therefore, a biradical mechanism was involved in the reactions, as can be inferred from the fact that these diazoalkanes possess substituents that can stabilize the biradical intermediates. The [5,6] open fulleroid with the larger group above the pentagon were formed predominantly. Increasing the bulkiness of the substituents raised the diastereoselectivity. The diastereoselectivity is reasonable even in the biradical mechanism because the repulsive interactions between the substituents of the methylene unit and the azenyl terminus make sense (Figure 11). The cyclopropyl group can act as a more “bulky” group than the p-tolyl group. The reverse diastereoselectivity found in 49f arises from the cooperative π-resonating effects of tolyl and cyclopropyl groups. The sp2 character of the cyclopropane group enforces the coplanar conformation with respect to the spincentered sp2-hybridized plane. In this conformation, the cyclopropane ring will necessarily suffer from the more steric repulsion with the azenyl moiety compared with that of the facing tolyl plane (Figure 12).

larger group over the pentagon predominates.144,159 Monoalkylated diazomethanes 45a−c with C60 proceeded in a remarkably high regioselectivity and diastereoselectivity to yield 46a−c (Scheme 18).136 Formation of the pyrazoline intermediates 47a−c was observable, among which isolation and complete characterization of 47a was successful. Thermal treatment of 47a−c also led to similar diastereoselectivity. Thermal treatment of 46a−c in refluxing toluene for several hours did not engender isomerization to the [6,6] closed methanofullerenes or the diastereomeric fulleroids 48a−c. The PM3 calculated heats of formation of the model compounds C61HMe are 804.3 kcal mol−1 for the [5,6] open fulleroid with the proton over the pentagon, 803.7 kcal mol−1 for the [5,6] open fulleroid with the proton over the hexagon, and 801.7 kcal mol−1 for the [6,6] closed methanofullerene. These experimental and theoretical results suggest that not a thermodynamically controlled process but a kinetically controlled process might be responsible for the diastereoselectivity. Therefore, diastereoselective formation is possibly a consequence of the minimization of repulsive interactions between the substituents on the methylene unit and the N2 moiety during the concerted [π2s + π2s + σ2s + σ2a] process (Figure 10). Scheme 18

J

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Scheme 19

with respect to the central sp2-hybridized plane. Consequently, the bulkiness of the less π-resonating phenyl group is more repulsive toward the azenyl moiety than the π-resonating tolyl or anisyl groups (Figure 13).

Table 1. Product Distributions of the Reaction of C60 with Various Diazoalkanes product ratioa [%] 49

R1

R2

50

51

49a 49b 49c 49d 49e 49f 49g 49h 49i 49j

Et iPr tBu p-tolyl p-tolyl p-tolyl cyclopropyl cyclopropyl p-tolyl p-anisyl

Me Me Me Me iPr cyclopropyl H Me Ph Ph

65 91 91 80 83 26 >99 48 27 15

35 9 5 10 trace 43 trace 44 46 60

52

4 10 17 31 8 27 25

total yieldb [%] 31 23 19 23 42 35 9 36 37 33

Figure 13. Stereoelectronic effects in the diastereoselective ring closure of diazenyl diradical intermediate of 49i and 49j (fullerene shown in part for clarity).

a

Determined using 1H NMR. bOn the basis of C60 used. Reproduced with permission from ref 137. Copyright 2007 American Chemical Society.

Bestmann et al. reported that thermal reaction of C60 with αdiazoketones 53a−e at 100−150 °C in methylnaphthalene or toluene afforded [6,6] closed methanofullerene 54 along with additional products 55 except for reaction with 53b (Scheme 20).138 When 53e was subject to the reaction, the direct reaction product without elimination of AcOH could not be isolated. Instead, trans-alkene 54e was obtained. Formation of dihydrofuranofullerenes 55a and 54d was also achieved by heating the isolated compounds 54a and 54d. Therefore, 55 was probably formed via thermal rearrangement of 54. The preliminary result of the reaction of C60 with alkyl diazoacetates (56) was first described by Wudl et al. in a review.16 A detailed investigation was reported later by Diederich et al.140 In these reactions, evidence of formation of the pyrazoline intermediates was not obtained. They initially stirred equimolar amounts of ethyl diazoacetate and C60 in toluene for several days at room temperature. However, because the reaction proceeded sluggishly and with low conversion, they changed to refluxing toluene, which allowed formation of a mixture of [5,6] open fulleroids (57 and 58) and [6,6] closed methanofullerene 59. Diederich stated that both 1,3-dipolar cycloaddition followed by rapid loss of N2 as well as thermal decomposition of the diazo compound followed by addition of the generated carbene were able to occur concurrently in those conditions. However, Wang et al. performed reaction of C60 with 10 equiv of ethyl diazoacetate in toluene for 10 h at room temperature and obtained pyrazoline 60a in 47% yield with no evidence of formation of 57, 58, and 59 (Scheme 21).160,161 Compound 60 was also prepared through a mechanochemical reaction under

Figure 11. Stereoelectronic effects in diastereoselective ring closure (fullerene shown in part for clarity).

Figure 12. Stereoelectronic effects in diastereoselective ring closure of diazenyl diradical intermediate of 49f (fullerene shown in part for clarity).

Similarly, the reverse diastereoselectivity of diaryl-substituted 49i and 49j can be rationalized by consideration of the πresonating effect. Introduction of electron-donating groups such as methyl and methoxy groups enhances the stability of the diradical intermediates by possessing favorable coplanarization K

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Scheme 20

Scheme 21

Smith III et al.154,162 Treatment of a toluene solution of C70 with parent diazomethane at 0 °C furnished a mixture of pyrazoline adducts 61a, 61b, and 62 in a 12:1:2 ratio (Scheme 23). Reactions appear to be controlled kinetically because addition takes place at the shortest bonds of C70. The observation that the (a−b) bond is more reactive than the (c−c) bond can be attributed to the greater local curvature at the carbons joined by the (a−b) bond. Irradiation of the pyrazoline mixture gave two methanofullerenes 63 and 64 in a 7:1 ratio together with a trace amount ( 500 nm) at room temperature caused quantitative conversion to 130 in 16 h. The photochemical rearrangement was inhibited by O2, indicating participation of a triplet state. On the basis of the results reported above, a light-promoted thermal rearrangement mechanism was V

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Scheme 50

Scheme 51

Scheme 53

Table 2. Rate Constants for Thermal (170°C) First-Order and Photochemical (35°C) Zero-Order Rearrangement of [5,6] Open Fulleroids to [6,6] Closed Methanofullerenes

The reaction rate also increased with temperature, and an activation energy of 22.9 ± 0.3 kcal mol−1 and an entropy of activation of −24.2 ± 0.8 cal mol−1 T−1 for the rearrangement were obtained. The first-order thermal rearrangement pathway has a higher energy barrier. To elucidate the first-order thermal rearrangement pathway, not only the biradical intermediate 133 but also the contribution of zwitterion 134 must be considered (Scheme 52). To Scheme 52

[5,6] open fulleroid

thermal rate constant [s−1]

photochemical rate constant [M s−1]

132 135a 135b 135c 135d 135e 135f 138a 138b

2.27 × 10−4 1.14 × 10−4 8.47 × 10−5 2.29 × 10−5 1.03 × 10−5 1.56 × 10−5 1.37 × 10−5 2.02 × 10−6 1.74 × 10−6

1.38 × 10−6 4.08 × 10−7 a 9.06 × 10−7 a 2.23 × 10−7 2.66 × 10−6 3.49 × 10−8 a

a

Although clean photochemical conversion of [5,6] to [6,6] isomer was observed, the kinetics was not measured. Reproduced with permission from ref 247. Copyright 2001 American Chemical Society.

It is particularly interesting that 138a with three methoxy groups rearranged to 139a more slowly than 135c having one pmethoxy substituent by a factor of more than 100 (Scheme 54). Furthermore, 138b having two o-methoxy groups rearranged to 139b even more slowly. The stereoelectronic effect can be

distinguish between these two possible intermediates, the rates of rearrangement of a series of aryl-substituted [5,6] open fulleroids 132 and 135a−f to the corresponding [6,6] closed methanofullerenes 95 and 136a−f were investigated (Scheme 53 and Table 2). Comparison of the rates of rearrangement of 132 and 135a−d indicated clearly that a decreasing ability to stabilize either a biradical or a zwitterionic intermediate caused decreasing rates of rearrangement. Notably, comparison of the rates of rearrangement of 135c, 135e, and 135f provided evidence in favor of a biradical intermediate. Results show that formation of a biradical intermediate was accelerated by replacement of phydrogen by p-nitrogen. In addition, the rather large variation in rates with changing substituents on the methano bridge appeared to preclude a concerted [1,5] sigmatropic rearrangement via the [5,6] closed methanofullerene similar to 137.

Scheme 54

W

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Scheme 55

M s−1 in the presence of 1 equiv of TCNE. When 1 equiv of chloranil was added to the solution of 132 under the same conditions, the zero-order rate constant of 6.44 × 10−6 M s−1 was obtained. Consequently, rearrangement involves excited-state electron transfer states. The possibility of a radical chain process cannot be ruled out, but an electron-transfer mechanism was proposed. The proposed mechanism involves oxidation of 132 by TCNE, leading to ring opening, followed by back electron transfer (Scheme 57).

rationalized by considering the conformation of the biradical intermediate. To gain the stabilization of the biradical intermediate by the aryl groups, the aromatic π orbitals must become parallel to the bonds connecting the methano bridge. In this conformation, shown as conformer B, the ortho substituents are pointing into the fullerene cage (Scheme 55). Therefore, bulky substituents at the para position make rotation difficult by steric repulsion. The negative entropy for rearrangement is rationalized by the assumption that conformer B is formed before the biradical intermediate. On the basis of the assumption, it is reasonable to consider that fulleroids in which the π system is prealigned in a conformation convert rapidly to the corresponding methanofullerenes in terms of the entropy of activation. In fact, spiro-linked methanofullerenes such as 33 were synthesized, although the corresponding fulleroids were not obtained.135 Hall and Shelvin reported that thermal and photochemical isomerization of aryl-substituted [5,6] open fulleroids to [6,6] closed methanofullerenes were accelerated in the presence of tetracyanoethylene (TCNE).248 As for 132, the TCNEpromoted rearrangement showed first-order kinetics between 60 and 140 °C with an activation energy of 6.7 ± 0.4 kcal mol−1 and an entropy of activation of −58.4 ± 0.5 cal mol−1 T−1. Consequently, TCNE lowers the enthalpy barrier by ca. 26 kcal mol−1 but places a greater entropic demand on the transition state. The possibility of the contribution of a charge-transfer complex between 132 and TCNE can be ruled out because of the absence of any charge-transfer band in the absorption spectrum of the mixture. In addition, the fact that the activation free energy for the TCNE-promoted rearrangement of 132 is 6.7 kcal mol−1, whereas the free energy change for the electron-transfer process requires 28 kcal mol−1 at 298 K, provides evidence against electron transfer in the rearrangement. Consequently, the authors claim that TCNE assists rearrangement by first forming covalently bonded adducts that subsequently generate [6,6] closed methanofullerenes with elimination of TCNE (Scheme 56). Photochemical zero-order rearrangement of 132 to 95 was also assisted by addition of TCNE. The zero-order rate constant of 1.94 × 10−7 M s−1 in the absence of TCNE rose to 2.10 × 10−5

Scheme 57

Oshima and co-workers found that cyclopropyl substituents, much more than aryl substituents, markedly accelerated the rates of thermal rearrangements of [5,6] open fulleroids to [6,6] closed methanofullerenes (Scheme 58).249 It is likely that the πScheme 58

resonating effect of the cyclopropyl group stabilize the biradical intermediate effectively. The sp2 character of the cyclopropane group enforces the coplanar conformation with respect to the spin-centered sp2-hybridized plane. By contrast, such an orbital interaction is inhibited in the case of the aryl-substituted fulleroids because of the congested steric repulsion between the peri-hydrogen and the fullerene cage. Kinetic studies of dicyclopropyl-substituted fulleroid 140 to the corresponding methanofullerene 141 provided an activation energy of 25.4 kcal mol−1 and entropy of activation of −10.3 cal mol−1 T−1 for thermal rearrangement. Apparently, 140 takes advantage of a more favorable entropy of activation than bis(p-anisyl)substituted [5,6] open fulleroid 132. Isomerization of a fulleroid to a methanofullerene is also induced by electrochemical reduction (Scheme 59).250 Wudl et al. synthesized fulleroid 142 by reaction of C60 with the corresponding diazomethane at room temperature. Thermal rearrangement of 142 to methanofullerene 143 was accom-

Scheme 56

X

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Scheme 59

Scheme 61

Scheme 62

plished by heating a chlorobenzene solution to reflux for 48 h. Cyclic voltammetry (CV) measurements of 142 and 143 were conducted in o-DCB, which exhibited four reversible reduction waves. It is noteworthy that 143 displayed a simple reversible reduction behavior at the third reduction step, although the corresponding reduction signal of 142 was composed of two peaks with scan-rate-dependent relative intensities. To prove the electrochemical behavior, electrolysis of 142 at −1.5 V vs Ag/ Ag+, which is just beyond the third wave, was conducted. The absorption spectrum of the sample after electrolysis was superimposable with that of 143, indicating that conversion of 142 to 143 was induced by electrochemical reduction. It is likely that electrochemical isomerization involves electrochemically assisted ring closure followed by a [1,5] shift (Scheme 60). The behavior of the electrochemical isomerization showed good agreement with an Echegoyen’s study, in which his group identified the difference in the electrochemical behavior between [5,6] open fulleroids and [6,6] closed methanofullerenes.251 In contrast, electrochemical oxidation did not engender similar isomerization. Instead, development of polymer film took place on the surface of the electrode.248 Possibly, the radical cation species are reactive, which tends to polymerize on the electrode surface. Acid-catalyzed rearrangement of a [5,6] open fulleroid to [6,6] closed methanofullerenes was reported by Wudl and coworkers.252 Acid treatment of the fulleroid methyl ester 102 using trifluoroacetic acid in o-DCB led to isomerization to the [6,6] closed methanofullerene 100 without hydrolysis (Scheme 61). Under the same acidic condition, the corresponding tertbutyl ester 144 was converted to [6,6] closed methanofullerene carboxylic acid 145 (Scheme 62). However, hydrolysis without rearrangement was performed to yield [5,6] open fulleroid carboxylic acid 146 using 40% (v/v) acetic acid in trifluoroacetic acid with o-DCB as solvent. Rearrangement without hydrolysis was achieved cleanly by photoirradiation to give [6,6] closed methanofullerene tert-butyl ester 147 by di-π-methane rear-

rangement, as described before. Notably, thermolysis of 144 afforded isomerization and additional hydrolysis to give 145 in one step accompanied by loss of isobutylene. The acid-catalyzed rearrangement can be considered a Berson−Willcott rearrangement.246,253,254 In TFA/o-DCB, protonation of the carbonyl group, causing ionization of the tert-butyl group, as well as competitive protonation of a [5,6] open fulleroid double bond would be expected, of which the latter engenders rearrangement of [5,6] open fulleroids to [6,6] closed methanofullerenes (Scheme 63). The protonated carbonyl could mediate delivery of a proton to a fulleroid double bond and initiate isomerization. The ester carbonyl can also assist the process by temporary trapping of the intermediate carbocation. The lower acidity of the medium is insufficient to protonate a fulleroid double bond in TFA/AcOH/o-DCB. 2.3. Decomposition of Diazirines

2.3.1. Chemical Probe. Decomposition of diazirines generates carbenes and diazo intermediates,114 both of which

Scheme 60

Y

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Scheme 63

are reactive toward fullerenes. Of particular importance is that addition of carbenes and diazo intermediates to C60 proceeds via different pathways, giving a mixture of diastereomers with different formation ratios. In general, addition of carbenes to C60 can be regarded as concerted [2 + 1] addition, resulting in formation of [6,6] closed methanofullerenes exclusively. Addition of diazo compounds proceeds via [3 + 2] cycloaddition to form pyrazoline intermediates. Subsequently, N2 extrusion from the pyrazoline intermediates takes place by a thermally allowed Woodward−Hoffmann or a biradical mechanism. The former mechanism leads to [5,6] open fulleroids exclusively, whereas the latter mechanism engenders a mixture of [5,6] open fulleroids and [6,6] closed methanofullerenes. Photochemical or thermal rearrangements of [5,6] open fulleroids to [6,6] closed methanofullerenes must also be considered. Nevertheless, fullerenes act as a useful mechanistic probe for decomposition of diazirines. Such a nonspectroscopic method yields comparable results with those obtained by laser flash photolytic techniques. For instance, photolysis255 or thermolysis256,257 of chlorophenyldiazirine (148) are known to yield chlorophenylcarbene (149) as the sole product. Isomerization to chlorophenyldiazomethane is only a minor process. In fact, photolysis or thermolysis of 148 in the presence of C60 afforded formation of 150 as the sole product (Scheme 64).258−260 In contrast, photolysis of 2-adamantane-2,3′-[3H]-diazirine (151) in the presence of C60 in benzene afforded the corresponding [6,6] closed methanofullerene 152 and [5,6] open fulleroid 153 with a formation ratio of 49:51 (Scheme 65).259 The ratio is comparable with the formation ratio of 2-

Scheme 65

adamantylidene (Ad:) and diazoadamantane from 151 using LFP technique, which is 1:1, reported by Bonneau and Liu.261 It is likely that the in-situ-generated Ad: reacts with C60 to form 152. In contrast, reaction of the in-situ-generated diazo species with C60 forms the corresponding pyrazoline (154) as the intermediate. To gain deeper insight into the reaction mechanism, the decomposition pathway of pyrazoline 154 was studied. 262 Irradiation of a toluene solution of 5′,5′dimethoxyspiro[adamantane]-2,2′-[Δ 3 1,3,4-oxadiazoline] (155) with a high-pressure mercury arc lamp at −78 °C caused formation of 2-diazoadamantane (156) (Scheme 66). To this Scheme 66

Scheme 64

Z

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Scheme 67

solution, a toluene solution of C60 was then added at −78 °C to afford formation of 154. Photoirradiation of the reaction mixture at 15 °C caused formation of 152 and 153 in a 12:88 ratio. The thermal decomposition process of 154 was very slow in comparison with the photochemical decomposition. In addition, interconversion between 152 and 153 under photolytic conditions was not observed. Therefore, the formation ratio of Ad: and diazoadamantane by photolysis of 151 can be estimated as 42:58 from results of chemical probe experiments using C60. In contrast, thermolysis of 151 in the presence of C60 gave 152 and 153 in a 35:65 ratio.260 No interconversion between pure 152 and 153 was observed under thermal conditions. Thermal decomposition of the diazoadamantane to Ad: is negligible under the present experimental conditions because diazoadamantane is trapped rapidly by C60 to give 153. This was supported by the fact that azine species were not formed, although in the absence of C60 diazoadamantane yielded azine by a slow second-order reaction. Therefore, the 152/153 ratio corresponds to the formation ratio of Ad: and diazo compound during thermolysis. When the C−H bond in the α position of the diazirine is weak, a rearrangement in excited states (RIES) is also possible to produce olefins as a decomposition pathway by photolysis. Photolysis of 3-chloro-3-isopropyldiazirine (157) in the presence of C60 at −40 °C gave 1-chloro-2-methyl-1-propene (158), [6,6] closed methanofullerene 159, and [5,6] open fulleroid isomers (160) in relative yields of 82%, 5%, and 13%, respectively (Scheme 67).263 HPLC analysis of the reaction mixture at −40 °C suggested formation of pyrazoline derivative 161 as an intermediate, which was converted to 160 at room temperature by N2 elimination. Pyrazoline derivative 161 was stable at −40 °C under photoirradiation conditions. Bonneau and Liu determined that approximately 13% went to diazo compounds from 157 in the laser flash photolysis.264 Platz reported the quantum yield of the carbene as only 10% for photolysis of 157 and the RIES must be the most important reaction in photolysis of 157 (70−80%).265,266 Therefore, results obtained from chemical probe experiments using C60 showed good agreement with these findings. Akasaka et al. reported that photolysis of 3-chloro-3chloromethyldiazirine (162) in the presence of C60 afforded 1,2-dichloroethane (163) and [6,6] closed methanofullerene 164 with a formation ratio of 64:36 (Scheme 68).263 It is noteworthy that the E/Z ratio of 163 was 28:72. Actually, Bonneau and co-workers reported that photolysis of 162 in the

Scheme 68

absence of olefin reactant yielded ca. 15% of 163-E and ca. 85% of 163-Z.267 They also reported that the relative ratio of the efficiencies of RIES and carbene formation in photolysis of 162 was determined as 52:48 using time-resolved photoacoustic calorimetry (Scheme 69). Additionally, they reported that Scheme 69

thermal rearrangement of free carbene ClCH2CCl gave ca. 6% of 163-E and ca. 94% of 163-Z. In general, photolysis of diazirines gives diazo compounds with variable efficiencies. However, the global yield of formation of 163 via the diazo ground-state pathway is probably negligible for such alkylchlorodiazirines because (1) the efficiency is low and the chlorodiazo derivatives are unstable and (2) thermal decomposition of a diazo compound would give not only the rearranged product (163) but also the carbene which could even be the major product. As a result, Bonneau’s experiments demonstrated that the RIES process gave ca. 25% of 163-E and ca. 75% of 163-Z. The Bonneau’s E/Z ratio (25:75) in the RIES process closely approximates the results from photolysis of 162 in the presence of C60 (i.e., 28:72). Therefore, it was concluded that the photogenerated free carbene was entrapped completely by C60 and the remaining 162* gave 163 exclusively via the RIES process in Akasaka’s reaction. Photolysis of 3-benzyl-3-chlorodiazirine (165) produces benzylchlorocarbene (166), which undergoes 1,2-H migration to form E-β-chlorostyrenes and Z-β-chlorostyrenes (167) AA

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Scheme 70

(Scheme 70).268−270 Upon addition of a carbene trap such as tetramethylethylene (TME), a cyclopropane adduct is formed. Plots of [addition]/[rearr] versus [TME] are curved, suggesting that a second intermediate contributes to formation of chlorostyrenes. Kinetically, both the RIES and a carbene olefin complex (COC) pathway account for the nonlinearity of the plots of [addition]/[rearr] versus [TME]. When photolysis of 165 was conducted in the presence of C60, formation of [6,6] closed methanofullerene 168 (31%) and 167 (57%, E/Z ratio is 4:1) was achieved, although no fulleroid was detected.271 This is consistent with the very low quantum yield for diazo formation reported by Platz and co-workers. Kinetic parameters for reaction of 166 with TME were investigated by Liu and Bonneau, k (−30 °C) = 3 × 109 M−1 s−1.272 To establish the rate constant for reaction of C60 with 166, photolysis of 165 at −30 °C in the presence of an equal amount of C60 and TME was performed. Results suggest that C60 traps 166 4.1 times faster than TME does and that the rate constant for addition of 166 to C60 is on the order of 1010 M−1 s−1. In this respect, C60 trapping is more efficient than 1,2-H migration of 166. Therefore, 167 must come only from the RIES. Consequently, it is reasonable to consider that the formation ratio of carbene/RIES in that reaction was 35:65. Photolysis of 3-tert-butyl-3-chlorodiazirine (169) was studied by Moss et al. They presented that photolysis yielded 2-chloro-3methyl-2-butene (170, 13%) and 2,2-dimethyl-1-chlorocyclopropane (171, 87%) (Scheme 71).273,274 When photolysis was

diazirine. Photolysis of 169 was also applied in the presence of C60 (Scheme 72).275 In that case, 170 (6%), 171 (3%), [6,6] Scheme 72

closed methanofullerene 173 (55%), and azine (174, 6%) were obtained. Because the rate constant for C−H insertion of carbene 175 is slow, 9.3 × 105 s−1,273,274 175 can either be trapped by C60 or react to a lesser extent with 169 to form 174. Therefore, it is reasonable to consider that the C−H insertion pathway from 175 to 171 was suppressed in the presence of C60. As a result, the formation ratio of carbene/RIES in that reaction was estimated as 87:13, which showed good agreement with Moss’s results, although the two methodologies represented completely different approaches. Thermal decomposition of diazirines was investigated using the C60 probe techniques along with computational calculations.260 The most important paths related to the product distribution of diazirines are those for isomerization of diazirines to diazoalkanes (path I), for N2 extrusion from diazirine (path II), and for the rebound of molecular nitrogen to singlet carbene (path III) (Scheme 73). Regarding thermolysis of 3-chloro-3phenyldiazirine, the respective barrier heights for paths I and II were computed as 30.9 and 36.2 kcal mol−1. The barrier for recombination of carbene and nitrogen in path III was computed as 14.3 kcal mol−1. These calculations suggest that the main

Scheme 71

Scheme 73 performed in the presence of 2-methyl-1-butene, cyclopropane adduct 172 was also obtained in addition to 170 and 171. The product distribution of this reaction was studied as a function of the concentration of 2-methyl-1-butene. The ratio of intermolecular/intramolecular product, 172/171, was linear with alkene concentration. In contrast, the yield of 1,2-methyl shift product 170 was independent of the alkene concentration, suggesting that 170 was formed exclusively from the excited AB

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Scheme 74

target molecule through C−C, C−H, O−H, and X−H (X = heteroatom) insertions. Brunner et al. estimated that 3trifluoromethyl-3-phenyldiazirine (180) was rapidly photolyzed on irradiation near 350 nm to yield 65% of the corresponding carbene 181 and 35% of the diazoisomer based on their spectroscopic measurements of the disappearance of the diazirine and the initial accumulation of the diazo compound.281 The diazoisomer can generate carbene upon more severe irradiation (265 nm). In spite of the usefulness of TPDs in PAL, biochemists never checked whether the diazirine was producing 100% carbene or not. In this context, C60 trapping methodology was applied to study photolytic decomposition of 180.282 When 180 was irradiated in the presence of C60, methanofullerene 182 was obtained in 40% yield whereas formation of the corresponding fulleroid isomer was negligible, suggesting that photolysis of 180 yielded only carbene 181 as an intermediate in that condition (Scheme 75).

product of thermolysis of 3-chloro-3-phenyldiazirine is carbene, which is formed directly by extrusion of nitrogen from diazirine. Actually, that is not the case for thermolysis of 3-n-butyl-3phenyldiazirine (176). The barrier for path II is 32.8 kcal mol−1, which is 0.7 kcal mol−1 lower than that of 33.5 kcal mol−1 for path I. It is noteworthy that the dipole moment of the transition state for path II is 2.96 D, whereas that for path I is 1.83 D. This difference in dipole moment implies that the barrier for path II should be lowered relative to path I when the reaction occurs in a polar solvent. The barrier height for path III was computed as 5.6 kcal mol−1. These results suggest that thermolysis of 176 can produce ether carbene or diazoalkane, depending on the reaction condition. In addition, the diazoalkane can be formed from rebinding of molecular nitrogen to carbene. Thermolysis of 176 in o-DCB/C6D6 in the presence of C60 afforded formation of [5,6] open fulleroid 177a in 70% yield. When the reaction was conducted in DMSO as a polar solvent, fulleroid isomer 177b (4%), [6,6] closed methanofullerene (178, 4%), and (E)-1phenyl-1-pentene (179, 13%) were formed along with 177a (76%) (Scheme 74). Apparently, the major decomposition pathway for 176 is formation of diazoalkane. The rate constant for 1,2-migration is not known for n-Bu-C-Ph, but it is expected to be similar to the lifetime for n-propyl-C-Ph in pentane, which was reported as τ = 114 ns at 25 °C.276 With such a long lifetime, the carbene produced by thermolysis of 176 is expected to be trapped quantitatively by C60 to give 178. Therefore, it is concluded that formation of 179 results from direct thermal rearrangement of diazirine without participation of a carbene. In the case of 151, the barrier height for path II is 44.1 kcal mol−1, whereas the barrier for path I is 34.4 kcal mol−1, indicating that diazoadamantane might not be formed directly from isomerization of 151. The barrier for path III is 2.1 kcal mol−1, which is significantly lower than those for other diazirines. These results suggest that a main route for diazoadamantane formation should be made through path III, not through path II. It is expected that in the absence of trapping reagent, diazoadamantane is easily formed from path III. 2.3.2. Photoaffinity Labeling. The concept of photoaffinity labeling (PAL) to study ligand−receptor interactions was introduced by Westheimer in the early 1960s.277 Since then, PAL is widely used in structure−function studies of biological systems.278−280 A probe, which most often is a natural substance analog supplemented with a photoreactive group, is introduced into a system under study and subjected to photoirradiation. Brunner et al. found that 3-trifluoromethyl-3-phenyldiazirines (TPDs)281 meet the criteria for an ideal photoreactive group in PAL. TPDs are small and do not interfere with ligand−receptor interactions. It is noteworthy that TPDs are remarkably stable under a range of different physical and chemical conditions, including heat (80 °C), strong bases, strong acids, oxidizing conditions, and mild reducing agents. Upon irradiation with light around 350 nm, TPDs are rapidly photolyzed to generate a carbene capable of forming a covalent bond with the nearest

Scheme 75

2.3.3. Fullerene Sugars. Diazirines are also useful to append bioactive or functional entities onto fullerenes. The first synthesis of fullerene glycoconjugates was achieved by Vasella, Diederich, and co-workers in 1992.283,284 They used O-protected glycosylidene diazirines (183) as nucleophilic or ambiphilic carbene precursors and obtained the corresponding spiro-linked glycoconjugates (184) in good yields (Scheme 76). NMR studies displayed that the glycoconjugates were not [5,6] open fulleroids but [6,6] closed methanofullerenes. Incorporation of the chiral Scheme 76

AC

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Scheme 77

Scheme 78

2.4. Decomposition of Diazonium Salts

addend was also confirmed by their circular dichroism (CD) measurements. Deprotection is expected to give ambiphilic products with potentially interesting biological and physical properties, but attempts to deprotect 184 have failed under various conditions. To achieve deprotection of fullerene glycoconjugates under reaction conditions that do not affect the C60 moiety, acetal-type protection was adopted for the glycosylidene carbene precursors because it is well known that acetal moieties can be removed easily under mildly acidic conditions. Vasella et al. synthesized 4,6-O-benzylidene-protected manno-diazirine 185 and 2,3:4,6di-O-isopropylidene-protected manno-diazirine 186.284 Subsequently, the diazirines were subjected to fullerene functionalization, resulting in preparation of mannosylidene-fullerenes 187 and 188 (Scheme 77). As expected, deprotection of 187 and 188 were achieved to give partially deprotected 4,6-dihydroxy derivative 187 and unprotected mannosylidenated fullerene 190. 2.3.4. Addition−Elimination Reaction. Hummelen et al. applied carbene addition followed by the β-elimination step to construct new periconjugated structures.285 Thermolysis of benzylchlorodiazirines (165 and 191) in the presence of C60 afforded both [6,6] closed methanofullerenes (168 and 192) and [5,6] open fulleroids (193). After HPLC separation, HCl elimination was conducted for each isomer using KOtBu in oDCB to obtain 194 and 195, although yields of the elimination products were extremely low (Scheme 78).

Reactions of diazonium salts with fullerenes have remained an unexplored subject despite the fact that such reactions with carbon nanotubes have been worthy of remark because the selectivity depends on the electronic structures of carbon nanotubes.286−289 In that case, the reactive species is presumed to be an aryl radical. Wang et al. reported functionalization of fullerenes using 2′,3-dimethylazobenzene-4-diazonium hydrogen sulfate, giving the corresponding [6,6] closed methanofullerene derivative.290 The authors reported that a carbene intermediate can take part in the reaction. 2.5. Alkoxycarbenes

2,2-Dialkoxy-Δ3-1,3,4-oxadiazolines are convenient sources of dialkoxycarbenes by thermolysis in solution. Independently, Diederich et al. and Wudl et al. reported that thermolysis of 2,2dimethoxy-Δ3-1,3,4-oxadiazoline (196) in the presence of C60 gave the methanofullerenone ketal 197 (Scheme 79).141,291 The 13 C NMR chemical shift of the bridgehead C atoms of 197 was found at 84.59 ppm, proving its σ-homoaromatic character. Wudl and co-workers attempted to convert 197 to the corresponding thioketal via hydrolysis, but their attempts failed. Instead, hydrolytic treatment caused formation of the 1,2-dihydrofullerene carboxylate (198). Reaction of C60 with methoxy-[2-(trimethylsilyl)ethoxy]carbene, generated in situ by thermolysis of the corresponding oxadiazoline 199 led to a mixture of 1,2-dihydrofullerene 200 and 1,4-dihydrofullerene 201 in a 5:2 ratio according to HPLC, AD

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of the results, a stepwise mechanism was proposed. Nucleophilic attack of dialkoxycarbene 207, in the Michael sense, affords dipolar intermediate 208, which is possibly in equilibrium with ion pair intermediate 209 (Scheme 82). The β-effect of the

Scheme 79

Scheme 82

trimethylsilyl group plays an important stabilizing role in the β(trimethylsilyl)ethyl cation analogue. The proposed mechanism is in agreement with the nucleophilic nature of dialkoxycarbenes and the electrophilicity of C60. A similar migration of the trimethylsilyl group was also found in the reaction of 202 with Nphenylmaleimide.

although the corresponding dialkoxy-substituted methanofullerene was not obtained (Scheme 80).292 The unusual migration of Scheme 80

2.6. Vinylcarbenes

Cyclopropenone acetal (CPA), a unique source of vinylcarbene species, can be easily generated by its thermolysis.294 Vinylcarbenes are polarized singlet species. For that reason they are able to undergo [2 + 1] or [3 + 2] cycloaddition to electrondeficient olefins. Nakamura et al., after exploring the synthetic utility of CPA, reported that the vinylcarbenes generated by thermolysis of 210 are reactive toward C60, yielding both [2 + 1] and [3 + 2] cycloadducts (211 and 212, respectively) (Scheme 83).295−297 The two intermediates 213 and 214 are in rapid equilibrium mutually. Results obtained from quenching experiments with water and olefins suggest that the terminally substituted intermediate 213 is more reactive than internally substituted intermediate 214 in the reaction of CPAs with C60. Therefore, the kinetic reactivity of the vinylcarbenes plays an important role in regioselectivity. The [3 + 2]/[2 + 1] ratio increases dramatically by raising the reaction temperature. Reaction of C60 with 1.4 equiv of 210a at 80 °C afforded 211a and 212a with the ratio of 8:92. When the temperature was 140 °C, the ratio changed to 93:7. Similar temperature dependence of the [3 + 2]/[2 + 1] ratio was also found for R = Ph. In contrast, reaction of C60 with 210b and 210d proceeded only at 150−170 °C to afford 212b and 212d as the sole products. Thermal isomerization from 211 to 212 was not

the trimethylsilyl group during the reaction was examined using a deuterium-labeling method.293 Reaction of deuterated carbene 202 with C60 produced a difficult to separate mixture of the 1,2addition and 1,4-addition products, among which the 1,2adducts were separated from the mixtures but the 1,4-adducts not (Scheme 81). NMR spectra of the 1,2-adducts showed broad singlets at 1.47 (203) and 3.45 (204) ppm. In contrast, the 1,4adducts exhibited two geminal AB spin systems centered at 1.51 (205) and 3.12 (206) ppm, respectively, because of the diastereotopic relation of the methylene protons. On the basis Scheme 81

AE

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2.7. Basic Treatment of Malonates in the Presence of Iodine

Scheme 83

Nierengarten and Nicoud reported that treatment of C60 with malonic acid monoester 218 in the presence of iodine and DBU in toluene at room temperature for 24 h afforded iodo carboxylate derivative 219 (Scheme 84).303 To prove the reaction mechanism, stepwise preparation of 219 was attempted via the methanofullerene-dicarboxylic acid monoester 220. Compound 220 was synthesized by reaction of C60 with protected malonate ester 221 followed by deprotection. Subsequently, 220 was subjected to the same reaction condition as it was used for preparation of 219 from 218. However, only traces of 219 were detected; compound 222 was the only isolable product. The results indicated that 219 was not formed via formation of 220 followed by decarboxylation.304,305 Therefore, formation of 219 seems to occur not via the addition− elimination mechanism but by the carbene mechanism. The authors presumed that the α-iodocarbanion formed in situ might not be sufficiently nucleophilic to react with C60. Instead, the corresponding diiodomalonate derivative formed. Subsequently, a carbenoid intermediate can be yielded by decarboxylation and iodine displacement. The cyclic analogue of malonate, Meldrum’s acid, is known as an anomalously acidic compound.306 The pKa value of parent Meldrum’s acid is 7.3 in DMSO at 25 °C and more acidic than dimethyl malonate (pKa 15.9 in DMSO at 25 °C).307,308 Jian-Min et al. demonstrated that a series of Meldrum’s acid derivatives (223) react with C60 in the presence of iodine and DBU to give the corresponding [6,6] closed methanofullerenes (224) (Scheme 85).309 The proposed reaction mechanism involves initial formation of α-iodo derivative under the basic condition with subsequent elimination of HI by DBU to form the carbene intermediate, which is entrapped by C60.

observed at 140 °C. The lower reactivity of 210b and 210d compared to those of 210a and 210c is possibly attributable to the high activation energy of the cycloaddition stage. It is considered that the [2 + 1] cycloaddition proceeds via a concerted pathway and that the transition state is rigid. Consequently, the reactivity is affected by the steric congestion. In contrast, Boger et al. reported that the [3 + 2] reaction of CPAs with electron-deficient olefins proceeded via a reversible single electron transfer (SET) process.298 Therefore, it is reasonable to consider that the [3 + 2] cycloaddition involves a SET from the vinylcarbene to C60 to afford generation of a radical ion pair. This consideration is based on the high electron affinity of C60. In that case, the transition state is apparently less rigid than that of the [3 + 2] cycloaddition. However, the temperature dependence of the periselectivity remains unclear. The carbene adduct 215a obtained from hydrolysis of 211a is a useful intermediate for preparation of water-soluble fullerene derivatives, which are suitable for investigation to biological activity of fullerenes. The carboxylic acid derivative 216 was tested for acute toxicity, and the LD50 value of >500 mg kg−1 was obtained (Figure 19).299−301 Oligonucleotide conjugate 217 enables site-selective cleavage of DNA sequence at G sites oxidatively by photoirradiation.302

2.8. Photolysis of Benzocyclobutenediones

Photolysis of benzocyclobutenedione produces two intermediates: bisketene and carbene.310 Tomioka et al. performed photolysis of a solution of benzocyclobutenediones 225 and C60 in toluene.311 In fact, C60 possesses considerably high electron affinity and low ionization potential. Therefore, the bisketene 226 is apparently more reactive than the carbene 227. However, the expected Diels−Alder derivative 228 was not formed. Instead, they obtained a mixture of two isomers of carbene adducts 229 and 230 in a roughly 2:1 ratio (Scheme 86). The reactivity of C60 toward 227 relative to 2,3-dimethylbutene was estimated as being 1/35 per bond basis by competition experiments. The authors reported that C60 reacted with 226 to form 228, but the adduct might be inherently unstable to cause retroaddition to regenerate C60 and 226. Consequently, C60 was reluctantly trapped by 227 to afford the carbene adducts. 2.9. α-Ketocarbenes

Romanova et al. reported that reaction of acenaphthenequinone (231) with hexaethyltriaminophosphine in the presence of C60 engendered formation of [6,6] closed methanofullerene (232)

Figure 19. AF

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Scheme 84

effective trap of the carbene intermediate. In addition to 231, aceanthrenequinone and N-alkylisatins (233) are useful as the αketocarbene reagents for the three-component reactions to achieve cyclopropanated derivatives such as [6,6] closed methanofullerene 234.313

Scheme 85

2.10. Fischer Carbene Complexes

The carbene ligands in Fischer-type metal carbene complexes are known to react with electron-deficient olefins to give cyclopropanes under thermal conditions.314 In principle, the formal [2 + 1] cycloaddition is sensitive to the steric hindrance caused by the number or size of the substituents on the alkene.315 C60 can be regarded as tetra-substituted olefins. However, the pyramidalized characteristics of the double bonds with the high

(Scheme 87).312 The authors proposed that the reaction involved intermediate formation of α-ketocarbene via deoxygenation of the dicarbonyl compound by the P(III) derivative. The dimer of the α-ketocarbene was obtained when the reaction was conducted in the absence of C60. In that sense, C60 can act as an Scheme 86

AG

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Scheme 87

heating of 4,4,5,5-tetramethylimidazolidine-2-thione (239) and (240) with C60 or C70 in dry chlorobenzene gave the corresponding [5,6] open fulleroid 241 (Scheme 90).320,321 The

electrophilicity enable the carbene transfer from Fischer carbene complexes to the C60 surface. Merlic and Bendorf demonstrated cyclopropanation of C60 with [methyl(methoxymethylene)]pentacarbonylchromium (235) took place in vigorously refluxing benzene reflux for 3−4 days, although the phenyl methoxy carbene complex did not react with C60 under various thermal conditions (Scheme 88).316 The reaction mechanism is believed

DL-valine

Scheme 90

Scheme 88

reaction mechanism might involve generation of the imidazolidine-2-ylidene in situ. As shown in Scheme 91, the nucleophilic amino group of valine attacked the carbon atom of the thiocarbonyl group of the thione. The acid−base reaction or acid−base equilibrium followed by elimination of CO2, H2S, and the aldimine engender formation of the N-heterocyclic carbene (NHC, 242). The fulleroid structure of the carbene adduct was confirmed by 13C NMR spectroscopy, in which resonances attributed to two sp3-hybridized carbon atoms on fullerene cages were absent. However, no further details related to the reaction mechanism were forthcoming in that work. Rasmussen and Strongin et al. used thermolysis of 2-diazo-4,5dicyanoimidazole (243) for functionalization of C60.322 The diazo compound, prepared based on Webster’s procedure,323 was added to C60 in benzene. Then the resulting mixture was heated to reflux. NMR spectral studies indicate that the product is neither [5,6] open fulleroid nor [6,6] closed methanofullerene, instead, a 1,4-monoadduct (244) (Scheme 92). The 1H NMR spectrum exhibited a singlet at 4.01 ppm together with a singlet at 13.4 ppm. The former is characteristic of protons attached to C60. The latter corresponds to the acidic heterocycle proton. The broad absorption at 440 nm in the absorption spectrum also supports formation of 1,4-addition. Participation of zwitterionic intermediate 245 or electrophilic NHC 246 was proposed for the reaction. It is noteworthy that C60 can act as a Lewis acid that binds to a Lewis base. Bazan and co-workers described that reaction between a bulky NHC (247) and C60 yielded zwitterionic Lewis acid−base adduct 248 (Scheme 93).324,325 X-ray crystallographic

to be comparable with electron-deficient olefins. Consequently, a reasonable mechanism involves thermally induced loss of a CO followed by coordination of a C60 to the tetracarbonyl intermediate. The resulting η2-C60 chromium complex might rearrange to a metallacyclobutane with subsequent reductive elimination to yield the [6,6] closed methanofullerene 236. Bespalova et al. found that C60 was cyclopropanated via olefin metathesis conditions.317 They applied a homogeneous system WCl6 with 1,1,3,3-tetramethyl-1,3-disilacyclobutane (SCB) in the presence of 1-hexene (237) and C60 to form the corresponding [6,6] closed methanofullerene 238 (Scheme 89). 2.11. Heterocyclic Carbenes

Heterocyclic carbenes have been used successfully as excellent ligands in organometallic chemistry.318,319 Such carbenes are also able to react with fullerenes. Li and co-workers reported that Scheme 89

AH

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Scheme 91

Scheme 92

Scheme 93

the strength of the newly formed C−C single bond is also important: it makes the adduct formation process irreversible. Another Lewis-acid-based adduct was obtained using the NHC with C70 instead of C60. Notably, a single isomer (249) was formed out of five plausible isomers bearing singly bonded NHC fragments. X-ray crystallographic analysis results reveal that the NHC moiety is bound to the a site, which has indeed the largest LUMO density. Akasaka and co-workers encountered that photolysis of antidodecaisopropyltricyclo[4.2.0.02,5]octasilane (250) in the presence of C60 and a trace amount of CS2 caused formation of a spiro-linked C60 derivative containing a cis-fused heterobicycle (251) (Scheme 94).326 The possible reaction mechanism involves generation of cyclotetrasilene followed by formal [3 + 2] cycloaddition with CS2, yielding S-heterocyclic carbene (252) in situ, which indicates that C60 is a good scavenger to verify formation of SHC in the reaction. DFT calculations of the singlet state of 252 reveal that the SHC has dipolar character. Therefore,

analysis of 248 displayed clearly that the NHC moiety is singly bonded to a carbon atom of C60 with bond length of 1.502(16) Å. Natural bond order (NBO) analysis also exhibited the σ singlebond character of the bridging C−C bond between C60 and the NHC moiety. DFT studies indicated that the negative charge of −0.84 e− was spread throughout the C60 cage. Because of the anionic character of the C60 core, the NHC adducts displayed near-IR absorption bands with weak molar absorption coefficients. Steric congestion around the carbene center and delocalization of the positive charge on the N-heterocycle likely plays an important role for single-bond formation instead of cyclopropanation. In addition, the LUMO of the partially formed imidazolium fragment is much higher in energy than the LUMO of less-stabilized carbenium ions. Consequently, the cyclopropanation process can be unfavorable. DFT calculations show that the two potential products, [5,6] open fulleroid and [6,6] closed methanofullerene, are energetically less favorable by 34.6 and 24.1 kcal mol−1, respectively. To explain the stability of 248, AI

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Scheme 94

Figure 20. Resonance structures of the possible reaction intermediate.

Scheme 96

cyclopropanation is believed to be initiated by nucleophilic attack of SHC toward C60. 2.12. Zwitterionic Intermediates

Scheme 97

In 1997, Murata et al. reported that reaction of a 1:1 mixture of triphenylphosphine and dimethyl acetylenedicarboxylate (DMAD) with C60 afforded [6,6] closed methanofullerene 253, which has a stable phosphonium ylide structure (Scheme 95).327 It is reasonable to consider that the carbene (254) or the zwitterion (255), generated by addition of triphenylphosphine to DMAD, was involved in the reaction (Figure 20). 1H and 31P NMR spectra exhibited the presence of two rotational isomers in a 3:2 ratio because of resonance. Wittig reactions of 253 with aldehydes were tested, but no desired product was obtained. The X-ray structure of 253 was solved by Cheng et al. in 1999.328 They also showed that other triarylphosphines and trialkylphosphites were useful reagents to prepare the corresponding fullerene−phosphine ylides. Acidic treatment of 253 and tri(ptolyl)phosphine derivative 256 with hydrobromic acid led to phosphonium salts 257 and 258, respectively, by protonation and a subsequent decarboxylation process (Scheme 96). In contrast, trialkylphosphite derivatives 259 were hydrolyzed in the presence of hydrobromic acid to give phosphonates 260. Further hydrolysis of 260 with hydrobromic acid failed. Instead, treatment of 259 with Me3SiI at 0 °C and then with water led to further hydrolysis and formation of phosphonic acid derivative 261 (Scheme 97).329

Scheme 98

2.13. Si and Ge Analogues of Carbenes

2.13.1. Silylene Addition to C60. Silylation of fullerenes is an effective means of modulating the electronic properties of fullerenes to a great degree.330,331 Recent semiempirical calculations show that a silirane derivative of C60 is expected to increase the open-circuit voltage up to 1 V in BHJ solar cells.332 To date, various types of silylation using silicon compounds92,333−338 have been conducted to introduce silyl substituents into fullerenes. Silylene addition to C60 was first described by Akasaka and co-workers. Photolysis of trisilane derivative 262 with a low-pressure mercury lamp was used for generation of silylene 263, which was subsequently reacted with C60 to yield the silylene adduct 264 (Scheme 98).331,339 The 13C NMR spectrum shows 17 signals for the C60 cage skeleton,

revealing that the adduct has C2v symmetry. In addition, the existence of the 13C signal at 71.12 ppm suggests strongly that 264 is not a [6,6] open fulleroid but a [6,6] closed methanofullerene. AM1 calculations of isomers of a model compound, C60(SiPh2), display that the [6,6] closed methanofullerene is the most stable isomer, whereas the [6,6] open fulleroid is not located on the potential energy surface. However, the [5,6] closed methanofullerene and [5,6] open fulleroid are,

Scheme 95

AJ

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respectively, 19.4 and 10.7 kcal mol−1 more unfavorable than the [6,6] closed methanofullerene. This result contrasts sharply against the fact that the energy difference in the diphenylcarbene adducts between the [6,6] closed methanofullerene and the [5,6] open fulleroid is obtained as only 1.2 kcal mol−1. Su et al. performed DFT calculations to elucidate the chemical reactivity of heavy carbenes Me2X: (M = C, Si, Ge, Sn, and Pb) with C60.340 Those theoretical investigations suggest that a precursor complex for cycloaddition of C60 to a heavy carbene should not exist. Additionally, they concluded that the reaction proceeds in a concerted manner. 2.13.2. Silylene Addition to C70. The generality of the silylene addition was also confirmed by extension of this reaction

Scheme 100

Scheme 99

Scheme 101

to C70 (Scheme 99).330,341 Related to reaction of C70 with 263, 1 H NMR spectra of the reaction mixture before chromatographic separation displayed the existence of two main [6,6] closed methanofullerenes (265 and 266) with a ratio of 2:1. The reaction took place under kinetically controlled processes, and isomerization was not observed under the reaction conditions. The two isomers exhibited almost identical energy at the AM1 level. Instead, the LUMO electron density of C70, obtained at the HF/3-21G level, can explain the regioselectivity in the silylene addition. The authors reported that HOMO(silylene)−LUMO(C70) interaction plays an important role, and nucleophilic attack of the silylene to C70 is involved in the reaction. The LUMO electron densities at positions a, b, and c are 0.10, 0.05, and 0.05, respectively, corresponding to the fact that addition of silylene took place mainly at the [6,6]-(a−b) bond and to a lesser extent at the [6,6]-(c−c) bond. The two silylated derivatives of C70 showed very similar redox potentials in the electrochemical measurements, even though the addition sites are different. 2.13.3. Germylene Addition to C60. Addition of germylene to fullerenes remains an unexplored chemistry. In fact, theoretical calculations suggest that the reactivity of heavy carbene addition to C60 decreases in the order Me2C: > Me2Si: ≫ Me2Sn: > Me2Pb:.340 Kabe and co-workers tested the reaction of C60 with bis(disyl)germylene. However, only an insoluble polymer was obtained. Instead, a formal germylene adduct 267 was accessible together with a germacyclopentane derivative 268 when 1,2digermacyclobutane 269 was exposed to photoirradiation in the presence of C60 (Scheme 100).342 Generation of digermylated C60 intermediates by photolysis of digermacyclobutane followed by elimination of germacyclopropane might be related to the reaction (Scheme 101). The germacyclopentane derivative was produced by addition of the germacyclopropane to C60. It is noteworthy that photochemical reaction of C60 with silacyclopropane yielding the corresponding silacyclopentane derivative

was reported in a similar manner.343 It is particularly interesting that 267 possesses a [5,6] closed methanofullerene structure. Computational calculations suggest that the [5,6] open fulleroid is less stable than the [5,6] closed methanofullerene at both the PM3 and the HF/STO-3G levels of theory. Replacement of methylene by germylene can destabilize a transannular homoconjugation of [5,6] open annulenes.

3. CARBENE ADDITIONS TO ENDOHEDRAL METALLOFULLERENES 3.1. Endohedral Metallofullerenes

Fullerene cages can entrap metal atoms or clusters to form endohedral metallofullerenes (EMFs). In general, electronic structures of EMFs are entirely different from empty fullerenes because of electron transfer from encaged metal atoms to fullerene cages. For example, the electronic structure of a representative EMF M@C2v(9)-C82 is ‘formally’ described as M3+(C2v(9)-C82)3−. The ionic model is very useful in some cases to explain spectroscopic and structural properties of EMFs. However, the metal−cage interaction is indeed not purely ionic, and covalent contribution should be also considered. Calculated bond lengths of La@C2v(9)-C82 display that the classification of single- and double-bond character is less clear than that of empty fullerenes (Figure 21). The bonding characteristics of the carbon cage in La@C2v(9)-C82 anion were investigated experimentally using the 2D incredible natural abundance double quantum transfer experiment (INADEQUATE) NMR studies.344 Actually, 1JCC is well known to be greater at shorter bond lengths and increased π-bond character.345−347 Apparently, the observed AK

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hemisphere, showing the importance of topology for the electronic structure of fullerene cages. In this respect, endohedral metal atom doping or exohedral functionalization is necessary to stabilize the radical character for successful extraction and isolation. 3.2. M@C2v(9)-C82

3.2.1. Photolysis of Diazirines. An excellent example of the regioselective carbene addition to M@C2v(9)-C82 (M = La) was first reported in 2004 (Figure 23).350 Photolysis of adamantane diazirine 151 as a carbene source with M@C2v(9)-C82 (M = La,350 Ce,351 Gd,352 and Y353) afforded predominant monoadducts 270a−273a together with minor isomers 270b−273b (Scheme 102). Structures of both the major and the minor

Figure 21. Histogram of [5,6] and [6,6] ring-fusion bond lengths calculated for C60, C70, C2v(9)-C82, and La@C2v(9)-C82 at the B3LYP/6311+G(d) level. Reproduced with permission from ref 349. Copyright 2005.

Scheme 102

1

JCC values of the [5,6] bonds were distributed in the same range as that of the [6,6] bonds. Consequently, specific reactive sites are not predictable in terms of bond lengths because both [6,6] bonds and [5,6] bonds possess generally similar bond lengths in EMFs. In fact, C2v(9)-C82 is not available yet as the empty form. DFT calculations indicate that C2v(9)-C82 prefer the triplet configuration to the singlet configuration.348 Apparently, C2v(9)-C82 is a non-Kekulé molecule that has radical substructures, as shown in Figure 22. Indeed, phenalenyl-like motifs are visible in the

products were determined unambiguously using X-ray crystallography. Addition occurred at the [6,6]-{C(1)−C(2)} site in 270a−273a and [6,6]-{C(2)−C(12)} site in 270b−273b. In both cases, the C−C bonds on the sites of addition are cleaved by adding the electrophilic carbene to form open-cage structures. The structure is in sharp contrast with that of the carbene adduct of C60, which possess a [6,6] closed form. Possibly, bond cleavage by carbene addition in M@C2v(9)-C82 is associated with the ambiguous bonding character of the C−C bonds and the delocalization of the π electrons. One can imagine that release of the cage strain is the driving force engendering the bond cleavage. Apparently, the POAV angles (Table 3) of the carbon atoms at the site of addition decrease in a large degree by the carbene

Figure 22. Schlegel diagram of one possible Clar structure of C2v(9)-C82.

Figure 23. Two orthogonal views of the schematic structure of La@C2v(9)-C82. Carbon atoms in C2v(9)-C82 are numbered according to IUPAC recommendations.358 AL

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Table 3. Charge Densities and POAV Angles of M@C2v(9)-C82 charge density

POAV

atom

La

Ce

Gd

Y

Sc

La

Ce

Gd

Y

Sc

1 2 8 9 10 11 12 25 26 27 28 29 30 47 48 49 50 51 52 68 69 70 71 81

−0.136 −0.170 −0.045 −0.071 −0.092 −0.037 −0.099 −0.061 −0.021 −0.020 −0.006 −0.047 −0.022 −0.036 0.000 −0.012 −0.006 0.004 −0.027 −0.026 0.002 0.006 −0.006 −0.025

−0.152 −0.206 −0.012 −0.102 −0.072 −0.011 −0.136 −0.040 −0.009 −0.002 0.001 −0.034 0.004 −0.029 0.004 −0.010 −0.004 0.006 −0.021 −0.023 0.004 0.008 −0.004 −0.023

−0.123 −0.175 0.007 −0.060 −0.058 0.002 −0.086 −0.039 −0.008 −0.003 0.002 −0.033 0.002 −0.028 0.005 −0.008 −0.004 0.007 −0.020 −0.023 0.005 0.008 −0.004 −0.023

−0.135 −0.172 −0.008 −0.076 −0.071 −0.018 −0.096 −0.047 −0.012 −0.010 −0.001 −0.036 −0.007 −0.030 0.002 −0.010 −0.004 0.005 −0.022 −0.023 0.004 0.008 −0.004 −0.023

−0.188 −0.215 −0.021 −0.089 −0.066 −0.014 −0.110 −0.044 −0.008 −0.006 0.002 −0.030 −0.002 −0.027 0.005 −0.008 −0.003 0.006 −0.018 −0.021 0.005 0.008 −0.004 −0.022

11.20 11.28 9.16 9.62 9.06 10.49 8.82 8.89 10.57 10.66 10.89 7.66 9.94 8.24 11.00 10.62 10.91 11.01 7.36 8.36 10.50 10.74 10.73 8.31

11.74 11.54 8.61 9.39 8.88 10.32 8.41 8.78 10.62 10.68 10.98 7.75 9.83 8.25 11.13 10.68 10.90 11.01 7.44 8.39 10.49 10.72 10.72 8.31

11.74 11.47 8.78 9.19 9.00 10.45 8.24 8.88 10.64 10.69 10.92 7.68 10.03 8.23 11.07 10.64 10.89 11.00 7.41 8.37 10.51 10.73 10.72 8.32

11.68 11.45 8.91 9.16 9.05 10.52 8.23 8.88 10.65 10.69 10.92 7.66 10.04 8.22 11.05 10.64 10.89 11.00 7.38 8.39 10.48 10.73 10.72 8.32

11.66 11.20 8.98 9.04 9.16 10.48 8.37 9.21 10.52 10.66 10.76 7.89 10.03 8.48 10.93 10.70 10.77 10.86 7.56 8.58 10.44 10.58 10.69 8.56

addendum far from the reactive sites does not interfere with the carbene addition. The reactivity of Ad: toward Sc@C2v(9)-C82 provided deeper insights. In that case, four monoadduct isomers 274a, 274b, 274c, and 274d were obtained, respectively, in yields of 40%, 25%, 25%, and 10% (Scheme 103).359 X-ray crystallographic

addition to obtain planarity. A similar [6,6] open structure was found in a Bingel−Hirsch-type derivative of Y3N@Ih(7)-C80.354 However, the formation mechanism of such [6,6] open structures remains unclear. The authors attempted to compute the [6,6] closed structure of the Bingel−Hirsch derivative, but the optimization procedure invariably yielded the [6,6] open structure. In contrast, [6,6] closed methanofullerene structures were found in Bingel−Hirsch derivatives of Sc3N@D3h(5)C78.355 Nevertheless, the absorption features of the monoadducts 270−273 resemble those of pristine M@C2v(9)-C82, showing that carbene addition scarcely alters the π-electron systems. It is difficult to explain the regioselectivity based on MOs because both α-HOMO and α-HOMO-1 are mainly delocalized on the cage carbons. Instead, it is reasonable to consider that charge densities and POAV angles play an important role in the regioselectivity because the carbon atoms at the sites of addition have the greatest charge densities and POAV angles. Theoretical calculations revealed that 270b is 2.5 kcal mol−1 more stable than 270a. In this context, 270a can be regarded as a kinetically favored product. Although no conversion was observed by heating an o-DCB solution of 270a to reflux for 12 h, thermal rearrangement to 270b was finally achieved by heating a powder sample at 523 K for 12 h.356 Vis−NIR absorption spectra of the major isomers and minor isomers are almost identical to those of pristine M@C2v(9)-C82. This similarity suggests that these adducts retain the essential electronic structures despite the C−C bond cleavage by Ad: addition. The adducts also exhibited similar redox potentials to those of M@C2v(9)-C82, verifying that the Ad group has no important influence on the electron-accepting properties. The same regioselectivity of Ad: was found in the reaction of 151 with a La@C2v(9)-C82-1,2,3,4,5-pentamethylcyclopendadiene (Cp*) adduct, in which the Cp* moiety is connected to the [6,6]-{C(69)−C(70)} site.357 Results demonstrate that the

Scheme 103

analysis of 274a clarified that the addition site was the same as those in 270a−273a, i.e., the [6,6]-{C(1)−C(2)} site. To explain the lower selectivity of Sc@C2v(9)-C82 against M@ C2v(9)-C82 (M = La, Ce, Gd, and Y), charge densities of the cage carbons were calculated. In Sc@C2v(9)-C82, not only C(2) but also C(1) have much higher charge density values than the others. The difference in the charge densities on the cage carbons can be associated with a certain degree of back-donation of electrons from the carbon cage to the Sc ion. Consequently, the charge densities play a key role in the regioselectivity of the carbene addition. In M@C2v(9)-C82, the encaged metal atom is located at an off-center position adjacent to a hexagonal ring along the C2 axis of the cage.360 Localization of the metal atom results in an inhomogeneous distribution of the charge densities over the fullerene surface. In other words, the encaged metal atom has a directional effect controlling the reactivity of the cage AM

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Scheme 104

Among the isolated products, the structure of 275c was determined using X-ray crystallography, showing that addition took place at the [6,6]-{C(2)−C(12)} site with an open form, which was identical to those for the minor isomers of M@C2v(9)C 82 (Ad) (270b−273b). Regarding 275c, the two NIR absorptions at around 986 and 1525 nm are fully consistent with those seen for 270b. In contrast, 275a,b and 277a,b exhibited characteristic NIR absorptions at 1010 and 1456 nm, which was identical to those of 270a. On the basis of the similarity in the absorption spectra, it is deduced that 275a,b and 277a,b have the same addition pattern and the same addition sites with 270a. In this respect, additions conceivably took place via a carbene mechanism. 3.2.4. Metal-Catalyzed Reactions. Copper(I)-catalyzed cycloaddition of Tb@C2v(9)-C82 using diazoesters 278 was reported by Gu et al. (Scheme 105).367 In the reported reaction

carbons via metal−cage charge transfer. On the basis of the charge density distribution, the authors assigned that the addition sites of the other three isomers could be [5,6]{C(1)−C(6)}, [5,6]-{C(1)−C(9)}, and [6,6]-{C(2)−C(12)} sites. Absorption spectra of 274a−d resemble that of Sc@C2v(9)C82, showing that the electronic structure of Sc@C2v(9)-C82 is not altered significantly by Ad addition. Shiga et al. applied the paired interacting orbital (PIO) analysis for Ad: addition to La@C2v(9)-C82.361 There are 165 occupied MOs, SOMO, and 171 unoccupied MOs in the extended Hückel MOs of La@C2v(9)-C82. Similarly, there are 27 occupied MOs and 27 unoccupied MOs in the same MOs of Ad:. The 337 × 54 orbital interactions are represented compactly in 54 PIOs, among which only PIO-1 mainly expresses the interaction between La@ C2v(9)-C82 and Ad:. Consequently, consideration of the overlap population of the PIO-1 and the localization ratio of the PIO-1 of each atom provides a favorable addition site, which is coincident with the experimentally obtained results. Chlorophenyldiazirine 148 was applied for the functionalization of La@C2v(9)-C82.362 EPR spectrum of the purified product suggested the existence of two inseparable isomers in a ratio of 57:43. By analogy with Ad: addition to La@C2v(9)-C82, the site of addition was proposed to be the same position (C(1)−C(2) site) with 270a−274a. The difference in total energy of 0.12 kcal mol−1 for the two C1-symmetric diastereomers was obtained using DFT calculations. It is noteworthy that incorporation of the Ad: moiety to La@ C2v(9)-C82 drastically improves its crystallinity. Using the characteristics, FET properties363 and carrier mobilities364 of single crystals of 270a have been investigated. 3.2.2. Thermolysis of Diazomethanes. Thermal reaction of diphenyldiazomethane with La@C2v(9)-C82 was investigated by Kato et al. in 1995.365 However, the reaction took place in a nonspecific manner. EPR spectra showed that the product contained several isomers of monoadducts. 3.2.3. In-Situ Generation of Diazo Compounds. Akasaka et al. synthesized PCBM analogues of La@C2v(9)-C82 (Scheme 104).366 Through subsequent multistage HPLC separation, three isomeric monoadducts (275a, 275b, and 275c) were isolated in 15%, 15%, and 6% yields based on consumed La@C2v(9)-C82. Similarly, free-base tetraphenylporphyrin (H2TPP)-appended derivatives were prepared using the same strategy with tosylhydrazone 276, resulting in successful isolation of three isomeric monoadducts (275a, 275b, and 275c) in conversion yields of 10%, 10%, and 5% based on consumed La@C2v(9)-C82.

Scheme 105

conditions, formation of multiadducts (279) up to hexakis adducts was observed based on results of MALDI-TOF mass spectrometry. XPS studies show that carbene addition has little effect on the valence of an endohedral metal atom, as suggested by Kato et al.365 3.2.5. Zwitterionic Intermediates. A Dy@C2v(9)-C82 derivative bearing a phosphonium ylide moiety was synthesized by regioselective reaction of Dy@C2v(9)-C82with DMAD and triphenylphosphine (Scheme 106).368 The reaction proceeded smoothly at room temperature to afford a single product 280. Apparently, carbene 254 or zwitterion 255 took part in the reaction as the intermediate. The preferred formation of the [6,6] open structure was supported by DFT calculations. The exothermicity for [2 + 1] addition of the zwitterionic intermediate to Dy@C2v(9)-C82 was obtained as 16 kcal mol−1. The authors also tried to optimize the [6,6]-{C(2)−C(12)}closed structure, but geometry optimization led eventually to the [6,6]-{C(2)−C(12)}-open structure. AN

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Scheme 106

Table 4. Charge Densities and POAV Angles of La@Cs(6)-C82

3.3. M@Cs(6)-C82

Chemical reactivity of M@Cs(6)-C82 has not been explored well because of the low yield, low cage symmetry, and low stability in air. La@Cs(6)-C82 has 44 nonequivalent carbon atoms (Figure 24). Therefore, its reactions can produce a greater number of regioisomers than those of M@C2v(9)-C82. In the Ad: addition of La@Cs(6)-C82, two monoadducts were isolated and characterized.369 The calculated charge densities and POAV angles of carbon atoms (Table 4) suggest that C(81) and C(82) are most reactive toward the electrophilic carbene. However, structural elucidation of the products was not achieved. The authors inferred that the isolated products were possibly [6,6]-{C(81)− C(82)} and [6,6]-{C(74)−C(82)} isomers, which were 6 and 9 kcal mol−1 more stable than the [5,6]-{C(77)−C(81)} isomer. Absorption spectra of the two monoadducts are very similar to that of La@Cs(6)-C82, showing that the electronic structure of La@Cs(6)-C82 is retained after addition of Ad, irrespective of the addition site.

atom

charge density

POAV

atom

charge density

POAV

2 3 4 12 13 14 15 16 17 18 19 20 31 32 33 34 35 36 37 38 39 40

−0.017 −0.026 0.005 −0.006 0.006 −0.007 −0.008 0.001 −0.003 −0.030 −0.004 −0.008 −0.010 −0.007 0.005 −0.030 −0.036 0.003 −0.011 −0.018 −0.009 −0.042

7.48 7.77 9.74 11.56 10.83 10.69 10.54 10.97 10.49 8.56 10.55 10.91 11.58 11.43 10.86 7.60 7.53 10.91 10.93 10.89 11.01 8.47

41 42 53 54 55 56 57 58 59 60 61 62 63 72 73 74 75 76 77 78 81 82

−0.004 −0.013 −0.020 −0.003 −0.051 −0.031 −0.035 −0.066 −0.018 −0.039 −0.033 −0.073 −0.023 −0.092 −0.048 −0.145 −0.082 −0.136 −0.113 −0.068 −0.155 −0.165

10.60 10.68 10.38 10.86 7.95 9.20 9.41 8.06 10.77 10.09 10.51 9.11 10.50 9.08 10.16 9.98 10.10 9.47 10.73 9.11 11.89 12.01

and 282 (M = Ce).373 X-ray crystallographic analysis results indicated that the carbene attached at the (a−b) site in which the C−C bond was cleaved by addition, as found in carbene adducts of M@C2v(9)-C82. Remarkably, a drastic change of the movement of the metal atoms was observed in 281 and 282. Crystallographic data reveal that two metal atoms are collinear with the spiro carbon of the adduct, unlike the three-dimensional delocalization in pristine M2@Ih(7)-C80. The La−La distance in 281 is highly elongated to 4.03 Å, whereas the calculated La−La distance in La2@Ih(7)-C80 is 3.83 Å. The authors claimed that the remarkable La−La elongation results from expansion of the inner space of the cage by the bond cleavage, which reduces the electrostatic repulsion between the positively charged La atoms. Results reveal that chemical functionalization enables regulation of the position of the encaged metal atoms, leading to alternation of the distribution of the charge densities over the cage. Consequently, the regioselectivity of the second Ad: addition can

3.4. M2@Ih(7)-C80

3.4.1. Photolysis of Diazirines. An icosahedral C80 cage possesses two nonequivalent carbon atoms (designated as a and b) which provide C−C bonds of two kinds (denoted as [5,6]-(a− a) and [6,6]-(a−b) bonds) (Figure 25). In principle, encapsulation of two metal atoms can reduce the molecular symmetry. However, 13C NMR spectra of M2@Ih(7)-C80 (M = La370,371 and Ce372) display only two carbon signals with an intensity ratio of 3:1, indicating that delocalization of the two metal atoms makes the symmetry icosahedral (Ih) for the whole molecule. Photochemical Ad: addition of M2@Ih(7)-C80 proceeded in a regioselective manner to afford single products 281 (M = La)

Figure 24. Two orthogonal views of the schematic structure of La@Cs(6)-C82. Carbon atoms in Cs(6)-C82 are numbered according to IUPAC recommendations.358 AO

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Figure 25. Two orthogonal views of the schematic structure of La2@Ih(7)-C80 with D2h configuration. Carbon atoms in Ih(7)-C80 are numbered according to IUPAC recommendations.358 Two nonequivalent carbon atoms are labeled as a and b in order of the IUPAC numbering system.

Scheme 107

appear cooperatively. To prove the hypothesis, Akasaka et al. demonstrated the two-step double addition of carbenes to La2@ Ih (7)-C 80 (Scheme 107). 374 At the first step reaction, chlorophenylcarbene, generated by photoirradiation of diazirine, reacted with La2@Ih(7)-C80 to afford a single product 283. X-ray analysis reveals that the molecular structure of 283 has [6,6]-(a− b)-open structures as 281 does. Subsequently, the second carbene toward 283 was performed using Ad:. After multistep HPLC separation, three isomers of the bis-adducts 284a−c were obtained. NMR characterization of 284a−c failed because of the poor solubility. However, X-ray crystallographic analysis of one of the adducts displays clearly that the two carbenes are attached at the two poles, and the two spiro carbons of the carbene addend are almost in line with the metal atoms. The addition sites, C(73) and C(74), satisfy the criteria in charge densities and POAV angles (see Figure 26 and Table 5). Furthermore, the addition sites of the other two isomers are predictable based on the criteria. Because of the expansion of the inner space, the La−La distance is further elongated to be 4.16 Å. Absorption spectra of La2@Ih(7)-C80 and its derivatives are featureless. They resemble each other. Therefore, it is difficult to discuss the difference in the electronic structures based solely on the absorption characteristics. Synthesis of EMF glycoconjugates was first reported in 2011.375 The glycosilydene carbene generated in situ from diazirine 183a was highly reactive toward La2@Ih(7)-C80 at room temperature to afford two inseparable diastereomers of the monoadducts (285) in 62% yield based on consumed La2@

Figure 26. Schematic structure of 283.

Ih(7)-C80. NMR spectroscopic results of studies suggest that both diastereomers possess [6,6]-(a−b)-open structures. AP

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the ability to reverse the direction of intramolecular charge transfer in the excited states of M2@Ih(7)-C80−ZnTPP in polar media, forming (M2@Ih(7)-C80)•+−(ZnTPP)•−.

Table 5. Charge Densities and POAV Angles of Selected Carbon Atoms in 283 atom

charge density

POAV

atom

charge density

POAV

54 55 56 57 58 59 60 61 62 63 64 65 68

−0.047 −0.036 −0.068 −0.115 −0.071 −0.036 −0.049 −0.100 −0.160 −0.036 −0.028 −0.070 −0.069

9.52 9.73 10.31 8.94 10.26 9.68 9.47 10.57 9.38 9.84 10.55 8.74 8.75

69 70 71 72 73 74 75 76 77 78 79 80

−0.028 −0.034 −0.157 −0.095 −0.180 −0.167 −0.180 −0.181 −0.128 −0.085 −0.084 −0.126

10.54 9.91 9.31 10.57 9.80 12.79 9.95 11.80 10.93 9.04 9.05 10.84

3.5. M2@D3h(5)-C78

3.5.1. Photolysis of Diazirines. In M2@D3h(5)-C78, two metal atoms are located at two positions directed toward the hexagonal rings at the C3 axis of the ellipsoidal D3h(5)-C78 cage.378,379 There are only eight nonequivalent carbon atoms (a−h) on the cage (Figure 28). However, the regioselectivity of La2@D3h(5)-C78 toward Ad: is lower than that of M@C2v(9)-C82. Photochemical reaction of with 151 afforded four monoadduct isomers (290a−d).380 The photochemical reaction proceeded very rapidly, forming multiadducts easily. NMR structural analyses show that the three major isomers 290a−c have Cs symmetries, whereas the structure of the other isomer (290d) remains unclear. Candidates for addition in 290a−c to satisfy the NMR patterns are [6,6]-(a−a), [5,6]-(a−a), [6,6]-(b−b), [6,6](e−e), and [5,6]-(g−g) sites. Among the isolated monoadducts, one of the major isomers (290a) was subjected to X-ray crystallographic analysis, revealing that 290a possesses a [5,6](g−g) open structure. The g carbon has large POAV angles, but it has a very small charge density (−0.0263) (see Table 6). In this

3.4.2. In-Situ Generation of Diazo Compounds. PCBM analogues of M2@Ih(7)-C80 (M = La,376 and Ce377) were synthesized using diazo precursor 101 (Figure 27). Both X-ray

Table 6. Charge Densities and POAV Angles of La2@D3h(5)C78

Figure 27.

structures were solved, confirming that addition took place at the [6,6]-(a−b) sites followed by C−C bond cleavage. The reactions proceeded regioselectively to afford La2@Ih(7)-C80-PCBM (286) in 55% and Ce2@Ih(7)-C80-PCBM (287) in 62% yield based on consumed M2@Ih(7)-C80. Similarly, M2@Ih(7)-C80− zinc porphyrin (ZnTPP) conjugates (M = La, 288; M = Ce, 289) were prepared successfully in moderate yields.376,377 It is particularly interesting that photophysical reports have described

atom

charge density

POAV

atom

charge density

POAV

3 4 5 13 14 15 16 17 18 19 20

−0.1483 −0.1471 −0.1479 −0.0723 −0.1071 −0.1116 −0.1092 −0.1099 −0.0700 −0.1092 −0.1108

12.6 12.7 12.6 9.4 9.7 8.5 8.6 9.6 9.5 9.7 8.5

31 32 33 34 35 36 37 38 39 40 41

−0.0832 −0.0269 −0.0279 −0.0249 −0.0242 −0.0293 −0.0263 −0.0844 −0.0264 −0.0290 −0.0243

8.2 10.8 10.5 11.0 11.0 10.5 10.8 8.2 10.8 10.5 11.0

context, the addition site is not consistent with the criteria in charge densities and POAV angles. Addition of Ad: to La2@ D3h(5)-C78 does not modify the absorption onset of pristine La2@D3h(5)-C78 but changes its main absorption positions and

Figure 28. Two orthogonal views of the schematic structure of La2@D3h(5)-C78. Carbon atoms in D3h(5)-C78 are numbered according to IUPAC recommendations.358 Eight nonequivalent carbon atoms are labeled by a−h in order of the IUPAC numbering system. AQ

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Figure 29. Two orthogonal views of the schematic structure of La2@D2(10611)-C72. Carbon atoms in D2(10611)-C72 are numbered according to IUPAC recommendations.358 No contiguous helical numbering pathway exists in D2(10611)-C72.

fact that the e sites possess highest POAV angles (14.9°) and sufficient charge densities (−0.124). DFT calculations show that the HOMO is distributed mainly on the carbon cage, but it does not contain the [5,5]-{C(24)−C(46)} bonds. Consequently, the lower distribution of the HOMO at the [5,5] sites might be responsible for the lower reactivity compared to that at any site close the [5,5]-{C(24)−C(46)} sites. In contrast, the LUMO is completely localized on C(24)−C(46) and C(35)−C(36) sites and the metal atoms. Accordingly, the C(24)−C(46) and C(35)−C(36) sites can be reactive toward nucleophiles. Vis− NIR absorption spectra of 291a−f resemble that of La2@ D2(10611)-C72, indicating that the π-electron system of La2@ D2(10611)-C72 is not changed markedly by Ad addition. Accordingly, the similarities suggest that the six isomers are expected to have open-cage structures. By extending the photoirradiation time, formation of the bisadducts was induced. HPLC analysis suggested that the fraction contained more than 15 isomers, among which only seven isomers (292a−g) were isolated and characterized.387 In particular, the structure of the most abundant isomer 292a was determined using X-ray crystallography. X-ray analysis shows that Ad: is attached to the [5,6]-{C(45)−C(46)} site located at one side and the [5,6]-{C(35)−C(36)} site located at the other side. It is particularly interesting that no tris addition or more multiple additions occurred even if the reaction mixture was irradiated further for 20 min in the presence of an excess amount of 151. This finding led to the speculation that only the two fused-pentagon sites were reactive toward the carbene and that no further reaction occurred after the two active sites were occupied. However, this speculation is not able to explain the reactivity of La2@D3h(5)-C78.380

relative absorbance. In particular, vis−NIR absorption spectra of 290a and 290b differ from that of La2@D3h(5)-C78, although 290a has an open structure. 3.5.2. Thermolysis of Diazirines. Thermolysis of 151 in the presence of La2@D3h(5)-C78 yielded more than seven monoadduct isomers (290a−g), of which four isomers were the same as those produced by the photochemical reaction. 3.6. M2@D2(10611)-C72

M2@D2(10611)-C72381,382 is known not to conform to the isolated pentagon rule (IPR),383,384 which states that each pentagonal ring on a fullerene cage is expected to be surrounded by hexagonal rings in stable fullerenes. Such non-IPR fullerenes are generally unstable for empty fullerenes. However, results of recent studies have revealed that non-IPR structures can be stabilized by exohedral derivatization or encapsulation of metal atoms.385 In fact, the two metal atoms in M2@D2(10611)-C72 (M = La and Ce) are located near the two fused pentagons in both sides (Figure 29). The cage encloses 18 nonequivalent carbon atoms. Photochemical Ad: addition yielded sixmonoadduct isomers (291a, 291b, 291c, 291d, 291e, and 291f) with relative abundances of 8%, 9%, 40%, 36%, 4%, and 3%.386 The similarities in the absorption spectra of 291a−f suggest that 291a−f have open cage structures. Among them, Xray crystallographic analysis reveals clearly that 291b, 291c, and 291d have [5,6]-{C(23)−C(44)}-open, [5,6]-{C(44)−C(45)}open, and [5,6]-{C(23)−C(24)}-open structures, respectively. The reaction sites are consistent with the criteria in charge densities and POAV angles (Table 7). Consequently, it can be deduced from the results that one of the remaining three minor isomers could possess a [6,6]-{C(44)−C(45)}-open structure. The [5,5]-{C(24)−C(46)} adduct was not obtained, despite the

3.7. M3N@Ih(7)-C80

3.7.1. In-Situ Generation of Diazo Compounds. Nitride cluster fullerenes (NCFs) such as M3N@C2n are EMFs of another type.388−392 The encaged clusters critically influence the chemical reactivity of the whole molecule.393 For instance, our group found that La2@Ih(7)-C80 reacts with 1,1,2,2-tetramesityl1,2-disilirane under thermal condition to give the corresponding adduct, although Sc3N@Ih(7)-C80 does not (Figure 30).394 The difference in the reactivity derives from the difference in the electronic structures. In this context, electrochemical measurements are important to characterize the electronic structures.51 In 2009, Drees et al. demonstrated that high-yield charge separation can occur with OPV systems that have a reduced donor−acceptor LUMO offset using Lu3N@Ih(7)-C80 derivatives with P3HT.395,396 The Lu3N@Ih(7)-C80-PCBX analogues

Table 7. Charge Densities and POAV Angles of La2@ D2(10611)-C72 atom

charge density

POAV

atom

charge density

POAV

4 5 6 7 18 19 20 21 22

−0.0625 −0.0278 −0.0412 −0.0931 −0.0286 −0.0097 −0.1054 −0.0576 −0.1299

8.7 10.1 10.3 11.2 10.5 10.1 8.5 10.7 10.8

23 24 40 42 43 44 45 63 64

−0.1386 −0.1239 −0.0242 −0.0548 −0.1305 −0.1598 −0.1347 −0.0583 −0.1765

13.1 14.9 10.9 10.3 8.7 11.5 13.1 10.2 9.3 AR

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Figure 30. Schematic structure of M3N@Ih(7)-C80. Carbon atoms in Ih(7)-C80 are numbered according to IUPAC recommendations.358 Two nonequivalent carbon atoms are labeled using a and b in order of the IUPAC numbering system.

(293−296) were prepared according to the standard protocol used for synthesis of [60]PCBM. However, modification of the reaction condition was required because the reactivity of Lu3N@ Ih(7)-C80 differs from that of empty fullerenes. For instance, the reagents had to be increased to 25 times the amounts used for empty fullerenes; the reaction was run at 120 °C instead of 70 °C. The highest yield was obtained after 25 min rather than after 22 h. The lower reactivity derives from the increased HOMO− LUMO gap. In contrast, the high overall OPV device power conversion efficiency (PCE) of the P3HT/294 device is attributed to a better positioned LUMO level of 286, resulting in increasing open-circuit voltage (VOC) to 260 mV above reference devices made with [60]PCBM acceptor. Factors influencing the short-circuit current (JSC) in blends containing Lu3N@Ih(7)-C80-PCBEH (297) were investigated by Dyakonov et al.397 Dorn and co-workers synthesized M3N@Ih(7)-C80PCBM (M = Sc (298) and Y (299)) and fully characterized them.398 They used a reaction temperature of 70 °C for 24 h with subsequent chromatographic procedures and obtained 298 and 299 in 30% and 34% yields, respectively, based on consumed M3N@Ih(7)-C80. Molecular structures of 298 and 299 were determined using X-ray crystallography, showing the possession of [6,6]-(a−b)-open structures (see Figure 31). Akasaka, Guldi, and co-workers prepared Sc3N@Ih(7)-C80− porphyrin conjugates (300 and 301)376 and investigated their photophysical properties. Both conjugates possess [6,6]-(a−b)open structures similar to M3N@Ih(7)-C80-PCBX. The conjugates are stable. Moreover, they can be stored in the dark under Ar without showing any decomposition for 6 months, despite the fact that related reports described recently have demonstrated the intrinsic instability of Lu3N@Ih(7)-C80 (M = Sc and Y) based electron donor−acceptor conjugates.399,400 Synthesis of 300 took nearly 20 h, which contrasted sharply against the fact that synthesis of the corresponding La2@Ih(7)-C80−H2TPP conjugate requires a reaction time of 2 h under similar conditions. To explore the electron-donating power of EMFs, a covalently linked Lu3N@Ih(7)-C80-perylenebisimide (PDI) conjugate (302) was synthesized.401 Spectroscopic and kinetic studies, including femtosecond transient absorption measurements, corroborated that a photoinduced electron transfer event evolved from the ground state of Lu3N@Ih(7)-C80 to the excited state of the PDI moiety.

Figure 31.

3.7.2. Silylenes. In general, silylenes can be generated under harsh conditions that are not applicable for reactions with EMFs. Initial discovery of thermal silylene generation using hexamethylsilirane was reported by Seyferth and co-workers.402 Other cyclic silanes, including cyclotrisilanes403−406 and cyclohexene silacyclopropanes,407−409 have been employed for rather mild silylene generation. Akasaka et al. reported silylene addition to Lu3N@Ih(7)-C80 using thermolysis of 9,9-bis(2,6-diethylphenyl)-9-silabicyclo[6.1.0]nonane (303) (Scheme 108).410 Addition Scheme 108

proceeded regioselectively to afford a single monoadduct (304a) in the dark. 1H and 13C NMR spectra together with vis−NIR absorption spectra show that 304a possesses a [6,6]-(a−b)closed structure. However, 304a readily undergoes photochemical conversion to afford 304b under ambient light, during which time degradation to yield pristine Lu3N@Ih(7)-C80 also occurs. The [5,6]-(a−a)-open structure of 304b was determined unambiguously using X-ray crystallography. Although 303 efficiently generates 263 by photoirradiation using a mediumpressure mercury lamp, photolysis of 303 in the presence of Lu3N@Ih(7)-C80 caused consumption of the starting materials, without formation of monoadducts. The insufficient photoreactivity is attributable to the instability of the silylene adducts under photoirradiation. The vis−NIR absorption spectrum of 304a shows a distinctive absorption maximum around 700 nm, AS

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Figure 32. Two orthogonal views of the schematic structure of Sc2C2@C3v(8)-C82. Carbon atoms in C3v(8)-C82 are numbered according to IUPAC recommendations.358 There is no contiguous helical numbering pathway in C3v(8)-C82.

respectively, as proved using 13C NMR spectroscopy and X-ray crystallographic analyses of the 1,3-dipolar cycloadducts.

which is in agreement with those of the reported [6,6]-methanobridged Lu3N@Ih(7)-C80 derivatives. On the other hand, 304b exhibits a broad and gently sloping absorption around 700 nm with two obscure absorption maxima, reflecting that the vis−NIR absorption spectra are associated with the addition patterns. In contrast, preparation of the silylene adduct of Sc3N@Ih(7)C80 was not successful under similar conditions, although formation was confirmed only by MALDI-TOF mass spectrometry. Apparently, the encaged metal atoms influence the reactivity of M3N@Ih(7)-C80 toward silylenes, which suggests the possibility that the difference in the distributions of HOMOs of M3N@Ih(7)-C80 is responsible for the reactivities, in which 263 can act as a weak electrophile.411

3.9. Sc2C2@C2v(5)-C80

As for “Sc2C82” species, two isomers (designated as Sc2C82(I) and Sc2C82(II)) were isolated in 1999.418 “Sc2C82” isomers were thought to be dimetallofullerenes. However, 13C NMR spectroscopic investigations using the corresponding 13Cenriched samples showed clearly that Sc2C82(I) is actually Sc2C2@C2v(5)-C80 (Figure 33).419 Similarly, structural reassignment of Sc2C82(II) to Sc2@C3v(8)-C82 was performed in 2012 by Akasaka and co-workers using 13C NMR spectroscopy.420 Among them, the chemical reactivity of Sc2C2@C2v(5)-C80 toward Ad: was investigated.421 Photochemical reaction of Sc2C2@C2v(5)-C80 with 151 afforded five monoadduct isomers (306a, 306b, 306c, 306d, and 306e) with respective abundances of 20%, 40%, 25%, 5%, and 10% (Scheme 109). The addition sites of the monoadducts except for 306d were determined using X-ray crystallographic analyses, showing that 306a, 306b, 306c, and 306e, respectively, possess [6,6]-{C(21)−C(22)}-open, [5,6]-{C(5)−C(6)}-open, [6,6]-{C(4)−C(5)}-open, and [5,6]{C(11)−C(12)}-open structures. Calculated charge densities and POAV angles predict that the most reactive sites are possibly C(32), C(13), C(12), and C(33). However, the additional sites of the products do not meet the criteria except for the minor isomer 306e. In fact, the 13C NMR spectrum of Sc2C2@C2v(5)C80 at 298 K shows that the molecule possesses Cs symmetry, which means that the rotation of the scandium carbide cluster is slower than the NMR time scale and that the cluster lies on the mirror plane at 298 K. When the temperature increases to 413 K, the rotation of the cluster becomes faster than the NMR time scale, leading to the symmetry of the whole molecule to be C2v symmetric. Dynamic motion was also observable in the variabletemperature 45Sc NMR spectra. On the basis of the coalescence temperature (TC) of 373 K, the free energy of activation (ΔG‡) was estimated as 15.1 kcal mol−1. In this context, we must draw attention to the fact that the rotation of the cluster is not considered in the calculated charge densities and POAV values (Table 9). Accordingly, the cluster rotation in the photoreaction process might be responsible for formation of the monoadducts, although the reaction took place at 298 K. Another explanation for the addition sites concerns the distributions of the LUMO of Sc2C2@C2v(5)-C80. In fact, the LUMO spreads mainly over the hemisphere around C(1)−C(30) sites. In contrast, the HOMO is distributed to the other side (Figure 34). Vis−NIR absorption

3.8. Sc2C2@C3v(8)-C82

Among the three isomers of isolated “Sc2C84” EMFs (designated as Sc2C84(I), Sc2C84(II), and Sc2C84(III)), the major isomer “Sc2C84(III)” was originally regarded as dimetallofullerene Sc2@ D2d(23)-C84. Structural assignment was based on 13C NMR spectroscopic analysis412 and the maximum-entropy method (MEM)/Rietveld analysis of synchrotron X-ray powder diffraction data.413 However, in 2006, Akasaka et al. obtained an improved 13C NMR spectrum and revised the structure of “Sc2C84(III)” to be Sc2C2@C3v(8)-C82 (Figure 32).414 To verify the molecular structure, Ad: adducts of “Sc 2C84” were synthesized, which were subject to X-ray analysis.415 The photochemical reaction afforded a mixture of several monoadduct and bis-adduct isomers. Among them, one major isomer (305) of the monoadducts was isolated and characterized successfully. X-ray analysis of 305 identified beyond question that Ad: was reacted not with Sc2@D2d(23)-C84 but Sc2C2@ C3v(8)-C82, forming the corresponding [5,6]-{C(1)−C(2)}open adduct. However, the selectivity of Ad: toward the D2d(23)C84 cage was not discussed in the literature.413 In fact, charge densities at the addition sites (C(1) and C(2)) are low despite the fact that Ad: is an electrophile. It is noteworthy that the Sc2C2 cluster can rotate rapidly on the NMR time scale in pristine Sc2C2@C3v(8)-C82 and even in 305. In such a ‘dynamic’ system it is difficult to predict the most reactive site based on the charge density and POAV criteria (Table 8). In this respect, we must note that the regioselectivity of EMFs depends also on the encapsulated metal type and metal position, as well as the dynamic motion of the internal species. The other isolated isomers, “Sc2C84(I)” and “Sc2C84(II)” were shown to be Sc2C2@Cs(6)-C82416 and Sc2C2@C2v(9)-C82,417 AT

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Photochemical reaction of Sc3C2@Ih(7)-C80 with 151 proceeded smoothly, and 307 was obtained regioselectively. X-ray analyses of 307 and nBu4N+[307]− salt displayed that carbene addition took place at the (a−b) site, leading to the [6,6]-(a−b)-open structure.

Table 8. Charge Densities and POAV Angles of Sc2C2@ C3v(8)-C82 atom

charge density

POAV

atom

charge density

POAV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

−0.0055 −0.005 −0.0003 −0.0006 −0.0013 −0.001 0.0113 −0.014 −0.033 −0.0043 −0.025 −0.0017 −0.0249 −0.0016 0.0117 −0.0024 0.011 −0.0248 −0.0243 0.0105 −0.0026 −0.0033 0.0078 −0.0147 −0.0012 −0.0347 −0.1371 −0.1707 −0.1082 −0.1627 −0.0936 −0.0238 0.0069 −0.0087 0.0032 −0.0063 −0.0122 −0.0519 −0.1332 −0.1096 −0.0421

11.75 11.68 11.38 11.62 11.69 11.31 11.00 7.32 7.43 10.78 11.14 10.74 7.56 7.37 11.07 10.72 10.82 7.28 7.37 10.93 10.74 10.76 10.95 7.68 10.22 9.99 7.11 10.30 9.25 9.81 7.51 10.10 10.11 7.44 11.27 10.75 11.24 7.85 8.08 7.93 7.82

42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82

−0.0076 −0.0153 0.0035 −0.0324 −0.0014 −0.0091 −0.112 −0.2316 −0.1924 −0.1866 −0.2352 −0.0735 −0.0074 −0.0027 −0.0413 −0.0035 −0.0304 −0.1883 −0.1862 −0.1895 −0.1915 −0.1403 −0.0625 −0.0197 −0.0067 −0.0584 −0.0298 −0.1516 −0.1288 −0.0316 −0.0572 −0.0089 −0.0354 −0.0986 −0.1936 −0.2231 −0.1872 −0.0285 −0.0806 −0.0091 −0.0455

11.34 10.55 10.40 8.69 10.34 10.35 9.15 10.17 13.37 12.92 9.36 9.65 10.46 10.35 8.82 10.41 10.04 8.99 11.32 13.93 10.16 8.99 10.73 10.65 10.81 8.81 10.21 9.78 10.04 10.25 8.85 10.50 10.23 10.56 9.24 11.96 8.41 10.19 8.76 10.76 9.95

3.11. Li@C60

Encapsulation of the lithium ion inside fullerene cages was first demonstrated by Campbell and co-workers using low-energy ion bombardment of fullerene films.424−426 However, the structure and properties of Li@C60 in pure form had not been characterized sufficiently because of its insolubility and difficulty in purification. Possibly, the strong charge transfer interaction between Li@C60 and C60 causes formation of insoluble clusters. Finally, Sawa and Tobita et al. accomplished complete isolation and X-ray diffraction studies in 2010.427,428 They used a plasma method to carry out lithium ion collision to fullerene films. Successive oxidation of the fullerene deposit with tris(4bromophenyl)ammonium hexabromoantimonate in a mixture of o-DCB and acetonitrile at 100 °C provided monomeric [Li+@ C60], which was isolated subsequently as an SbCl6− salt. It is noteworthy that electron transfer from the Li atom to the C60 cage does not occur in Li@C60. Therefore, [Li+@C60] requires a counteranion in the exterior of the C60 cage to be a neutral species. Electrochemical investigations showed that not the internal Li+ but the C60 cage is responsible for electron uptake and release in the electrochemical events. In this context, the fullerene cage in Li@C60 retains the π-electronic structure of C60 itself, although an electrostatic interaction exists between the C60 cage and Li+. An analogue PCBM derivative of Li@C60 salt was synthesized by Matsuo et al.429 The initial attempt to prepare the PCBM analogue by the established procedure using tosylhydrazone 101 with NaOMe in a pyridine o-DCB solution engendered a complex mixture because of the higher reactivity of Li+@C60 than that of C60 as well as the ionic characteristics of the counteranion. Instead, the desired product was obtained using reaction of [Li+@C60]PF6− with the corresponding diazoalkane (308), which was readily prepared by treatment of 101 with KOtBu in o-DCB followed by heating to eliminate potassium tosylate (Scheme 110). The 1H NMR spectrum showed that the product was a [5,6]-isomer (309). In fact, the difference in the 1H resonances of 309 and 101 is quite small (up to 0.08 ppm), indicating that the encapsulated Li ion induces no significant magnetic effect on the fullerene surface. Thermal isomerization of 309 to [6,6] closed methanofullerene 310 was accomplished at lower temperature (90 °C) than in the case of empty [60]PCBM, in which isomerization was achieved at 180 °C. The favorable rearrangement can be an analogy with the acidcatalyzed isomerization mechanism.252 The possible reaction mechanism involves addition of the internal lithium ion to the fullerene double bonds (Scheme 111). The X-ray structure of 310 suggests that such lithium ion−carbon interaction is possible in the intermediate state. UV−vis absorption spectra of these Li+encaged compounds are similar to those of the empty species except for slight broadening. The broader peaks are most likely attributable to partial aggregation of Li+-encaged species in oDCB. In contrast, electrochemical studies reveal that 309 and 310 possess the first reduction potentials at −0.37 and −0.43 V vs Fc/Fc+, respectively, which are close to that of [Li+@C60]PF6− (−0.37 V). These values are much higher than those of empty species (−1.08 V for C60, −1.16 V for 102, and −1.18 V for 100). Overall, encapsulation of Li+ does not affect the light absorption

spectra of 306a−e are roughly similar, but certain differences are visible among the distinct absorptions between 600 and 800 nm, as well as the optical insets. The difference in the absorption characteristics might be associated with different addition sites. It is particularly interesting that 306b converted to 306c under thermal condition (100 °C). Thermal isomerization also proceeded in the solid state. DFT calculations suggest that 306b is 14.8 kcal mol−1 higher in energy than 306c. On the basis of the fact that both 306b and 306c contain the common bonding site C(5), it can be inferred that isomerization proceeds via a pirouette-like pathway. 3.10. Sc3C2@Ih(7)-C80

“Sc3C82” was long believed to be Sc3@C3v(7)-C82 based on the MEM/Rietveld analysis reported by Takata et al.422 However, the structure was later corrected to be Sc3C2@Ih(7)-C80 (see Figure 35) based on 13C NMR measurement of its anion and Xray crystallographic analysis of the Ad: adduct (307).423 AU

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Figure 33. Two orthogonal views of the schematic structure of Sc2C2@C2v(5)-C80. Carbon atoms in C2v(5)-C80 are numbered according to IUPAC recommendations.358

Scheme 109

Figure 34. HOMO and LUMO of Sc2C2@C2v(5)-C80. Reprinted with permission from ref 421. Copyright 2012 American Chemical Society.

Table 9. Charge Densities and POAV Angles of Sc2C2@ C2v(5)-C80421 atom

charge density

POAV

atom

charge density

POAV

1 2 3 4 5 6 12 13 14 15 16 17 18 19 20 21 32 33 34 35 36

−0.019 −0.051 −0.059 −0.009 0.014 0.000 −0.174 −0.187 −0.129 −0.059 0.001 0.005 −0.031 0.010 0.000 −0.004 −0.191 −0.180 −0.061 −0.053 −0.006

10.62 10.69 7.40 7.68 10.49 10.34 11.04 12.41 9.45 8.18 9.56 10.12 7.99 10.77 11.17 11.85 14.19 9.37 8.97 8.53 10.76

37 38 39 40 41 42 54 55 56 57 58 59 60 61 62 72 73 74 75 76

−0.028 −0.043 −0.119 −0.033 −0.001 −0.004 −0.102 −0.011 −0.009 −0.021 −0.103 −0.096 −0.094 −0.113 −0.104 −0.010 −0.011 −0.108 −0.107 −0.138

10.97 10.31 8.90 10.29 10.90 11.50 8.64 10.67 11.17 10.74 8.60 9.21 9.07 9.63 8.79 10.75 10.20 8.50 10.39 13.83

Figure 35. Schematic structure of Sc3C2@Ih(7)-C80. Carbon atoms in Ih(7)-C80 are numbered according to IUPAC recommendations.358 Two nonequivalent carbon atoms are labeled using a and b in order of the IUPAC numbering system.

carbene precursors are useful for reactions. The availability of widely diverse carbene precursors enriches the fullerene chemistry. Both electrophilic and nucleophilic carbenes are applicable reagents, giving the corresponding fullerene derivatives. The reaction mechanisms depend on the precursors and reaction conditions applied. In this context, computational calculations contribute to a rational understanding of the reaction mechanism. Future efforts shall be devoted specifically to rational design of carbene adducts for applications such as organic photovoltaics. In particular, new compounds showing superior PCEs to PCBM are strongly desired. In addition, Bazan

property of the C60 cage but exerts a heavy influence on its electron affinity.

4. CONCLUDING REMARKS We provided a comprehensive view of carbene additions to fullerenes and related reactions. Carbenes are unquestionably powerful intermediates to functionalize fullerenes. Various AV

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Scheme 110

Scheme 111

Notes

et al. shed light on the new aspect of fullerenes, which is related to the concept of frustrated Lewis pairs.430,431 Further studies of fullerene-based Lewis acid−base adducts can be expected to pave the way to development of new carbon-based catalytic systems.325 Recent studies have also demonstrated clearly that carbene additions of several types are applicable to functionalize EMFs in a regioselective manner. In most cases, carbene adducts of EMFs prefer open structures to closed structures, which contrasts sharply with the corresponding carbene adducts of C60. The increasing quantity and quality of X-ray crystallographic data of carbene derivatives of EMFs enables us to determine the addition sites and forms. Regioselectivity was discussed based on the criteria in the charge density and POAV angles. Careful investigations have been undertaken to unveil the reaction mechanism and regioselectivity and ascertain the mysterious reactivity of EMFs. Because of their excellent ability of electron release and uptake, EMF-based compounds are anticipated for use as promising materials for active components in new photovoltaic and photosynthetic systems. In summary, extensive investigations have been made of carbene additions to fullerenes. Experiments were performed in which new compounds have been identified and by which mechanistic pathways have been elucidated. As a result, studies of carbene additions to fullerenes have become better understood. We believe that studies of carbene additions to fullerenes will continue to gain increasing significance to achieve comprehensive understanding of the reactivity of fullerenes and develop functional materials for applications.

The authors declare no competing financial interest. Biographies

Michio Yamada was born in Osaka, Japan, in 1981. He received his Ph.D. degree from the University of Tsukuba in 2008 under his advisor Prof. Takeshi Akasaka. He also received a JSPS Research Fellowship for Young Scientists (2007−2009). During his Ph.D. studies, he spent 3 months as an exchange student with the group of Prof. François Diederich at ETH Zürich, Switzerland. In 2008, he moved again to the Diederich group and spent 2 years as a postdoctoral fellow working with axial chirality in push−pull-substituted chromophore−fullerene conjugates. Since 2010, he has been an Assistant Professor in the Department of Chemistry at Tokyo Gakugei University, Japan. His research interests lie in the chemistry of carbon-rich architectures such as fullerenes, carbon nanotubes, and acetylenic scaffolds. He was awarded the Osawa Award of the Fullerenes and Nanotubes Research Society in 2008.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. AW

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Takeshi Akasaka received his Ph.D. degree from the University of Tsukuba, Japan, in 1979. After working as a Postdoctoral Fellow at Brookhaven National Laboratory, he returned to the University of Tsukuba in 1981. In 1996, he moved to Niigata University as a professor. Since 2001, he has been Professor of the Tsukuba Advanced Research Alliance (TARA Center), University of Tsukuba. His research interests include the chemistry of fullerenes, endohedral metallofullerenes, and carbon nanotubes.

Shigeru Nagase received his Ph.D. degree from Osaka University in 1975. After working as a postdoctoral fellow (1976−1979) at the University of Rochester and Ohio State University, he returned to the Institute for Molecular Science in 1979. In 1980, he was appointed Associate Professor of Yokohama National University where he was promoted to Professor in 1991. He moved to Tokyo Metropolitan University in 1995 and the Institute for Molecular Science in 2001 as Professor. From 2012, he started work at Kyoto University as a Senior Research Fellow. He has great interest in developing new molecules and reactions through close interplay between theoretical predictions and experimental tests.

ACKNOWLEDGMENTS This work was supported in part by Grants-in-Aid for Scientific Research on Innovative Areas (Nos. 20108001 and 20108007, “pi-Space”), Grants-in-Aid for Scientific Research (A) (No. 20245006) and (B) (No. 24350019), The Next Generation Super Computing Project (Nanoscience Project), Nanotechnology Support Project, Grants-in-Aid for Scientific Research on Priority Area (Nos. 20036008, 20038007), and Specially Promoted Research (No. 22000009) from MEXT of Japan. We thank Prof. Michael T. H. Liu (Professor Emeritus, University of Prince Edward Island) for valuable comments related to the manuscript. AX

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