Near-Infrared-Absorbing Chiral Open [60]Fullerenes

Though [60]fullerene is an achiral molecular nanocarbon with I_h symmetry, it could attain an inherent chirality depending upon a functionalization pattern. The conventional chiral induction of C₆₀ relies mainly upon a multiple addition affording a mixture of achiral and chiral isomers while their chiral function would be largely offset by the existence of pseudo-mirror plane(s). These are major obstacles to proceed further study on fullerene chirality and yet leave its understanding elusive. Herein, we showcase a carbene-mediated synthesis of C₁-symmetric chiral open [60]fullerenes showing an intense far-red to near-infrared absorption. The large dissymmetry factor of |g_abs|=0.12 was achieved at λ=820 nm for circular dichroism in benzonitrile. This is, in general, unachievable by other small chiral organic molecules, demonstrating the potential usage of open [60]fullerenes as novel types of chiral chromophores.     

Individual (f,tA)‐ and (f,tC)‐Fullerene‐Based Nickel(II) Glycinates: Protected Chiral Amino Acids Directly Linked to a Chiral π‐Electron System

Stereoselective electrosynthesis of the first individual (^f,tA)‐ and (^f,tC)‐1,4‐fullerene derivatives with a non‐inherently chiral functionalization pattern is described, as well as the first example of an optically pure protected primary amino acid directly linked to the fullerene through only the chiral α‐amino‐acid carbon atom. An application of an auxiliary chiral nickel‐Schiff base moiety as derivatizing agent allowed separation of (^f,tA)‐ and (^f,tC)‐1,4‐fullerene derivatives using an achiral stationary phase, a separation which has never been done before.     

Selective Encapsulation and Chiral Induction of C60 and C70 Fullerenes by Axially Chiral Porous Aromatic Cages

Chiral induction has been an important topic in chemistry, not only for its relevance in understanding the mysterious phenomenon of spontaneous symmetry breaking in nature but also due to its critical implications in medicine and the chiral industry. The induced chirality of fullerenes by host–guest interactions has been rarely reported, mainly attributed to their chiral resistance from high symmetry and challenges in their accessibility. Herein, we report two new pairs of chiral porous aromatic cages (PAC), R- PAC-2 , S- PAC-2 (with Br substituents) and R- PAC-3 , S- PAC-3 (with CH₃ substituents) enantiomers. PAC-2 , rather than PAC-3 , achieves fullerene encapsulation and selective binding of C₇₀ over C₆₀ in fullerene carbon soot. More significantly, the occurrence of chiral induction between R- PAC-2 , S- PAC-2 and fullerenes is confirmed by single-crystal X-ray diffraction and the intense CD signal within the absorption region of fullerenes. DFT calculations reveal the contribution of electrostatic effects originating from face-to-face arene-fullerene interactions dominate C₇₀ selectivity and elucidate the substituent effect on fullerene encapsulation. The disturbance from the differential interactions between fullerene and surrounding chiral cages on the intrinsic highly symmetric electronic structure of fullerene could be the primary reason accounting for the induced chirality of fullerene.     

Investigations in the field of higher fullerenes

Halogenation of higher fullerene mixtures or their perfluoroalkylation with R^FI followed by HPLC separation of R^F derivatives and subsequent synchrotron X-ray crystallographic study made it possible to confirm cage connectivities of higher fullerenes and, in addition, to receive information concerning their reactivity in radical addition reactions. The data obtained are compared with theoretical predictions for higher fullerenes. Addition patterns of higher fullerene derivatives are discussed. Skeletal rearrangement of the D ₂-C₇₆ cage during chlorination has been observed for the first time.     

Low-Cost Synthesis of Fullerene Derivatives

Refined mixed fullerenes were used as a reagent in known organic reactions instead of the pure fullerene C₆₀ with aim to find an alternative, low-cost method for the synthesis of fullerene derivatives potentially exhibiting photoconductive properties. The isolation of C₆₀ or C₇₀ in clean form without admixtures requires the use of large quantities of toluene or other nonpolar solvents, polluting the environment and multiplying the production cost. 1,3-Dipolar cycloaddition of azomethine ylide to fullerite was chosen because this reaction is one of the most widely used for fullerene functionalization, producing material possibly presenting photoinducing behavior. The data showed that the use of the cheaper mixed fullerenes instead of pure C₆₀ leads to the isolation of the same expected products with similar yields. The photoelectric properties of mixed fullerenes and their organic derivatives were also examined. A slightly semiconductive behavior was confirmed as well as a noticeable photoresponse.

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Synthesis,Separation, and Characterization of Optically Pure C76 Mono-Adducts

Nucleophilic Bingel cyclopropanation of D₂-C₇₆ with bis[(S)-1-phenylbutyl] 2-bromomalonate in toluene in the presence of base yielded three constitutionally isomeric pairs of diastereoisomeric mono-adducts together with one other constitutional isomer. All seven mono-adducts were isolated in optically pure form by prep. HPLC on a (S,S)-Whelk-O1 chiral stationary phase. They represent the first optically pure adducts of an inherently chiral fullerene. Characterization by UV/VIS, CD, ¹³C- and ¹H-NMR spectroscopy allowed identification of pairs of stereoisomers and symmetry assignments: the two pairs of diastereoisomers which were isolated as the major product possess C₁ symmetry, whereas the third pair of diastereoisomers, which is a minor product, is C₂-symmetrical. The circular dichroism spectra of the optically active C₇₆-adducts showed very pronounced Cotton effects resulting from strong chiroptical contributions of the chiral fullerene chromophore with the maximum observed Δε values being twice as high than those previously measured for optically active adducts of achiral fullerenes with a chiral addition pattern. Whereas the regioselectivity of mono-additions to C₇₀ correlates with the degree of local bond curvature and the regioselectivity of multiple Bingel cyclopropanations of C₆₀ with electronic parameters such as coefficients of the lowest unoccupied molecular orbital (LUMO), no such simple predictive correlations exist for the nucleophilic addition to C₇₆. Despite full spectral characterization, an unambiguous structural assignment of the isolated compounds was not possible, except for the two C₂-symmetrical isomers. Based on considerations of local bond curvature and the previous experiences with the chemistry of C₇₀, the structures of the C₂-symmetrical stereoisomers were assigned as (S,S,^fC)- 3 and (S,S,^fA)- 3 , resulting from addition to the polar α-type C(1)? C(6) bond.     

Geometrical and Electronic Rules in Fullerene‐Based Compounds

The discovery of buckminsterfullerene C₆₀ opened up a new scientific area and stimulated the development of nanoscience and nanotechnology directly. Fullerene science has since emerged to include fullerenes, endohedral fullerenes (mainly metallofullerenes), exofullerenes, and carbon nanotubes as well. Herein, we look back at the development of fullerene science from the perspective of epistemology by highlighting the proposed main rules or criteria for understanding and predicting the structures and stability of fullerene‐based compounds. We also point out that a rule or criterion may contribute significantly to the corresponding discipline and suggest that two unsolved issues in fullerene science are the addition patterns of fullerene derivatives and the structures and stability of nonclassical fullerenes.     

Stereoanalysis of fullerenes with a chiral molecular framework

Stereoanalysis of three fullerene molecules with a chiral molecular framework C₃₂, C₇₆, and C₇₈ and achiral fullerene C₆₀ molecule was carried out. Comparative quantitative analysis of the degree of chirality showed topology to be the major factor governing the chirality of fullerenes. A procedure for determining the relative contribution of topological chirality to the total chirality of the molecule is proposed. Structural fragments responsible for chirality are found. The title fullerenes are assigned to the corresponding subclasses of homochirality. A classification system of isomeric fullerenes is proposed.     

A new method for the functionalization of [60] fullerene: an unusual 1,3-dipolar cycloaddition pathway leading to a C60 housane derivative

[reaction: see text] A variant of the Huisgen 1,3-dipolar cycloaddition reaction provides a new and convenient functionalization of fullerenes. This method complements the widely used Prato and Bingel-Hirsch reactions. The derived, highly functionalized cyclopentenone and cyclopentenamine fullerene compounds upon hydrolysis are suitable for further functionalization and may serve well in the synthesis of new C60 derivatives possessing uncommon and interesting properties.     

Furan‐Derived Chiral Bicycloaziridino Lactone Synthon: Collective Syntheses of Oseltamivir Phosphate (Tamiflu), (S)‐Pipecolic acid and its 3‐Hydroxy Derivatives

A unified synthetic strategy for oseltamivir phosphate (tamiflu), (S)‐pipecolic acid, and its 3‐hydroxy derivatives from furan derived common chiral bicycloaziridino lactone synthon is described here. Key features are the short (4‐steps), enantiopure, and decagram‐scale synthesis of common chiral synthon from furan and its first‐ever application in the total synthesis of biologically active compounds by taking the advantages of high functionalization ability of chiral synthon.     

Exohedral Addition Chemistry of the Fullerenide Anions C602− and C60⋅−

A systematic screening study of the exohedral reactivity of the reduced fullerenes (fullerenides) C₆₀²⁻ and C₆₀^⋅− is reported. These doubly and singly negatively charged carbon cages were prepared by two-fold reduction of C₆₀ with potassium, leading to K₂C₆₀, or by in situ monoreduction with the radical anion of benzonitrile PhCN^⋅−, respectively. Several series of electrophiles, including geminal and distant dihalides, benzyl bromides, and diazonium compounds, were employed as addition partners. In general, the investigated bromides proved to be the most suitable reaction partners. A series of fullerene adducts and cycloadducts involving either 1,2- or 1,4-addition patterns, depending on the precise architecture and the steric demand of the addends, were synthesized and fully characterized. Some of the reaction products are unprecedented and inaccessible forms of neutral C₆₀. The fullerenide chemistry presented here closely resembles related reactions of graphenides and carbon nanotubides, which are the most powerful methods for the functionalization of these macromolecular forms of synthetic carbon allotropes (SCAs). Activation of C₆₀ by negative charging represents a little explored concept of fullerene chemistry, providing both new insights of fullerene reactivity itself and new types of exohedral derivatives.     

Retro-Bingel reaction in the electrochemical reduction of bis(dialkoxyphosphoryl)methanofullerenes

Four steps of reduction were detected for bis(diethoxyphosphoryl)- and bis(diisopropoxyphosphoryl)methano[60]fullerenes (1, 2) and bis(diethoxyphosphoryl)methano[70]fullerene (3) by cyclic voltammetry in the o-dichlorobenzene—DMF (3 : 1, v/v)/Bu₄NBF₄ (0.1 mol L^–1) system on a glass-carbon electrode. At the first step the reversible transfer of one electron affords stable radical anions 1 and 2 (g = 1.9999, H = 1.9 G). When two electrons per molecule are transferred, the methano fragment is rapidly eliminated (retro-Bingel reaction). This process involves the step-by-step cleavage of two C—C bonds of exo-carbon with the fullerene shell in combination with the stepwise transfer of other two electrons and a proton to form finally the carbanion of the methano fragment and fullerene dianion. For all studied compounds, the elimination rate is much higher than that for bis(alkoxycarbonyl)- and dialkoxyphosphoryl(alkoxycarbonyl)methano[60]fullerenes, which makes it possible to propose bisphosphorylmethane groups as protective in synthesis of new fullerene derivatives.     

Electrochemistry of Mono- through Hexakis-adducts of C60

The first systematic electrochemical study by cyclic voltammetry (CV) and rotating-disk electrode (RDE) of the changes in redox properties of covalent fullerene derivatives ( 2 – 11 ) as a function of increasing number of addends is reported. Dialkynylmethanofullerenes 2 – 4 undergo multiple, fullerene-centered reduction steps at slightly more negative potentials than C₆₀ ( 1 ; see Table and Fig. 1). The two C-spheres in the dumbbell-shaped dimeric fullerene derivative 4 show independent, identical redox characteristics. This highlights the insulating character of the sp³-C-atoms in methanofullerenes which prevent through-bond communication of substituent effects from the methano bridge to the fullerene sphere. In the series of mono- through hexakis-adducts 5 – 11 , formed by tether-directed remote functionalization, reductions become increasingly difficult and more irreversible with increasing number of addends (see Table and Fig. 2). Whereas, in 0.1M Bu₄NPF₆/CH₂Cl₂, the first reduction of mono-adduct 5 occurs reversibly at ?1.06 V vs. the ferrocene/ferricinium couple (Fc/Fc⁺), hexakis-adduct 11 is reduced irreversibly only at ? 1.87 V. Hence, with incremental functionalization of the fullerene, the LUMO of the remaining conjugated framework is raised in energy. Reduction potentials are also dependent on the relative spatial disposition of the addends on the surface of the fullerene sphere. Observed UV/VIS spectral changes and changes in the chemical reactivity along the series 5 – 11 are in accord with the results of electrochemical measurements. Further, with increasing number of addends, the oxidation of derivatives 5 – 11 becomes more reversible. Whereas oxidations are increasingly facilitated upon going from mono-adduct 5 (+1.22 V) to tris-adduct 7 (+0.90 V), they occur at nearly the same potential (+0.95 to +0.99 V) in the higher adducts 8 – 11 . This indicates that the oxidations occur in these compounds at a common sub-structural element, for which a ‘cubic’ cyclophane is proposed (see Fig. 3). This sub-structure is fully developed in hexakis-adduct 11 .     

Optically Active Macrocyclic cis‐3 Bis‐Adducts of C60: Regio‐ and Stereoselective Synthesis,Exciton Chirality Coupling,and Determination of the Absolute Configuration,and First Observation of Exciton Coupling between Fullerene Chromophores in a Chiral Environment

A series of optically active cis‐3 bis‐adducts, such as (R,R,^fC)‐ 16 (Scheme 6), was obtained regio‐ and diastereoselectively by Bingel macrocyclization of C₆₀ with bis‐malonates, which contain optically active tethers derived from 1,2‐diols. The absolute configuration of the inherently chiral addition pattern in cis‐3 bis‐adducts had previously been determined by comparison of calculated and experimental circular dichroism (CD) spectra. Full confirmation of these earlier assignments was now obtained by an independent method based on semiempirical AM1 (`Austin Model 1') and OM2 (`Orthogonalization Method 2') calculations combined with ¹H‐NMR spectroscopy. It was found computationally that bis‐malonates [CHR(OCOCH₂COOEt)]₂, which contain (R,R)‐ or (S,S)‐butane‐2,3‐diol derivatives as optically active tethers, preferentially form out‐out cis‐3 bis‐adducts of C₆₀ as a single diastereoisomer in which the alkyl groups R adopt a gauche conformation, while the two glycolic H‐atoms are in an antiperiplanar (ap) and the ester linkages to the fullerene in a gauche relationship (Figs. 2 and 5). In contrast, in the less favorable diastereoisomer, which should not form, the alkyl groups R adopt an ap and the H‐atoms a gauche conformation, while the ester bridges to the fullerene remain, for geometric reasons, locked in a gauche conformation. According to the OM2 calculations, the geometry of the fully staggered tether in the free bis‐malonates closely resembles the conformation of the tether fragment in the bis‐adducts formed. These computational predictions were confirmed experimentally by the measurement of the coupling constant between the vicinal glycolic H‐atoms in the ¹H‐NMR spectrum. For (R,R,^fC)‐ 16 , ³J(H,H) was determined as 7.9 Hz, in agreement with the ap conformation, and, in combination with the calculations, this allowed assignment of the ^fC‐configuration to the inherently chiral addition pattern. This conformational analysis was further supported by the regio‐ and diastereoselective synthesis of cis‐3 bis‐adducts from bis‐malonates, including tethers derived from cyclic glycol units with a fixed gauche conformation of the alkyl residues R at the glycolic C‐atoms. Thus, a bis‐malonate of (R,R)‐cyclohexane‐1,2‐diol provided exclusively cis‐3 bis‐adduct (R,R,^fC)‐ 20 in 32% yield (Scheme 7). Incorporation of a tether derived from methyl 4,6‐O,O‐benzylidene‐α‐D ‐glucopyranoside into the bis‐malonate and Bingel macrocyclization diastereoselectively produced the cis‐3 stereoisomer (α,D ,^fA)‐ 22 (Scheme 8) as the only macrocyclic bis‐adduct. If the geometry of the alkyl groups R at the glycolic C‐atoms of the tether component deviates from a gauche relationship, as in the case of tethers derived from exo cis‐ and trans‐norbornane‐2,3‐diol or from trans‐cyclopentane‐1,2‐diol, hardly any macrocyclic product is formed (Schemes 5 and 9). The absolute configurations of the various optically active cis‐3 bis‐adducts were also assigned by comparison of their CD spectra, which are dominated by the chiroptical contributions of the inherently chiral fullerene chromophore (Figs. 1, 3, and 4). A strong chiral exciton coupling was observed for optically active macrocyclic cis‐3 bis‐adducts of C₆₀ with two appended 4‐(dimethylamino)benzoate ((S,S,^fC)‐ 26 ; Fig. 6) or meso‐tetraphenylporphyrin ((R,R,^fC)‐ 28 ; Fig. 7) chromophores. Chiral exciton coupling between two fullerene chromophores was observed for the first time in the CD spectrum of the threitol‐bridged bis‐fullerene (R,R)‐ 35 (Fig. 9).     

Fullerene Derivatives Embedded in Hybrid Sol-Gel Glasses: Nonlinear Optical Properties and Optical Limiting Performances

Hybrid organic-inorganic materials are investigated as suitable materials for inclusion of fullerene derivatives and for fabrication of laser protection devices. A specific synthesis has been developed in order to optimize non-linear optical performances of fullerene derivatives. 3-glicydoxypropyltrymethoxysilane has been used as an inorganic and organic network former to obtain the host material. The sol-gel synthesis consists of the hydrolysis and condensation in acidic conditions of the inorganic network. Epoxy polymerization has been achieved by using zirconium or BF₃ alkoxides precursors. Bulk and multilayer materials doped with a fullerene derivative have been fabricated. They show good optical requirements: high fullerenes concentration, high microstructural homogeneity, high laser damage threshold and high optical limiting efficiency. Optical limiting (OL) mechanisms have been investigated. The most effective in the sol-gel materials is the reverse saturable absorption (RSA) one. However, different mechanisms, like non-linear (NL) scattering and NL refraction contribute to a different extent. Open- and closed-aperture OL and z-scan measurements on sol-gel samples show the contribution of NL scattering and NL refraction at 690 nm. Laser damage threshold has been characterized as a function of the structure of the samples and of the optical configurations (f/66 and f/5).     

Asymmetric Tandem 1,5‐Hydride Shift/Ring Closure for the Synthesis of Chiral Spirooxindole Tetrahydroquinolines

The direct functionalization of sp³ C?H bonds through a tandem 1,5‐hydride shift/ring closure is described. Various optically active spirooxindole tetrahydroquinoline derivatives bearing contiguous quaternary or tertiary stereogenic carbon centers were readily synthesized. A chiral scandium complex of N,N′‐dioxide promoted the reactions in good yields (up to 97 %) with excellent diastereoselectivities (>20:1) and enantioselectivities (up to 94 % ee). Kinetic isotope effect (KIE) experiments and internal redox reactions of chiral substrates were conducted, and the results provided intriguing information that helped clarify the mechanism of the reaction.     

Tribenzotriquinacene Receptors for C60 Fullerene Rotors: Towards C3 Symmetrical Chiral Stators for Unidirectionally Operating Nanoratchets

The synthesis of a stereochemically pure concave tribenzotriquinacene receptor ( 7 ) for C₆₀ fullerene, possessing C₃ point group symmetry, by threefold condensation of C₂‐symmetric 1,2‐diketone synthons ( 5 ) and a hexaaminotribenzotriquinacene core ( 6 ) is described. The chiral diketone was synthesized in a five‐step reaction sequence starting from C_2h‐symmetric 2,6‐di‐tert‐butylanthracene. The highly diastereo‐discriminating Diels–Alder reaction of 2,6‐di‐tert‐butylanthracene with fumaric acid di(?)menthyl ester, catalyzed by aluminium chloride, is the relevant stereochemistry introducing step. The structure of the fullerene receptor was verified by ¹H and ¹³C NMR spectroscopy, mass spectrometry and single crystal X‐ray diffraction. VCD and ECD spectra were recorded, which were corroborated by ab initio DFT calculations, establishing the chiral nature of 7 with about 99.7 % ee, based on the ee (99.9 %) of the chiral synthon ( 1 ). The absolute configuration of 7 could thus be established as all‐S [(2S,7S,16S,21S,30S,35S)‐( 7 )]. Spectroscopic titration experiments reveal that the host forms 1:1 complexes with either pure fullerene (C₆₀) or fullerene derivatives, such as rotor 1′‐(4‐nitrophenyl)‐3′‐(4‐N,N‐dimethylaminophenyl)‐pyrazolino[4′,5′:1,2][60]fullerene ( R ). The complex stability constants of the complexes dissolved in CHCl₃/CS₂ (1:1 vol. %) are K([ C₆₀ ? 7 ])=319(±156) M ^?1 and K([ R ? 7 ])=110(±50) M ^?1. With molecular dynamics simulations using a first‐principles parameterized force field the asymmetry of the rotational potential for [ R ? 7 ] was shown, demonstrating the potential suitability of receptor 7 to act as a stator in a unidirectionally operating nanoratchet.