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Si‐Tethered Bis‐ and Tris‐Malonates for the Regioselective Preparation of Fullerene Multi‐Adducts
Abstract
Bis‐ and tris‐malonates constructed around a silicon atom have been prepared by reaction of malonate derivatives bearing an alcohol function with di‐ tert ‐butylsilyl bis(trifluoromethanesulfonate) and tert ‐butyl(trichloro)silane, respectively. These compounds have been used for the regioselective bis‐ and tris‐functionalization of C 60 under Bingel conditions. By changing the nature of the linker between the central Si atom and the reactive malonate groups, the malonate precursors have been optimized to produce specific bis‐ and tris‐adducts with excellent regioselectivity. A complete understanding of the electronic and stereochemical factors governing the regioselectivity has been obtained by combining computational studies with a complete analysis of the by‐products formed during the reactions of the Si‐tethered tris‐malonates with C 60 . Finally, desilylation reactions of the resulting fullerene bis‐ and tris‐adducts have been carried out to generate the corresponding acyclic fullerene bis‐ and tris‐adducts bearing alcohol functions.
The addition of two unsymmetric malonate esters to the Buckminster fullerene C60 can lead to 22 spectroscopically distinguishable isomeric products and therefore represents a formidable synthesis challenge. In this work, we achieve 87 % selectivity for the formation of a single (in,out‐trans‐3) isomer by combining three approaches: (i) we use a starting material, in which the two malonates are covalently connected (tether approach); (ii) we form the strong supramolecular complex of C60 with the shape‐persistent [10]CPP macrocycle (template approach) and (iii) we embed this complex further within a self‐assembled nanocapsule (shadow mask approach). Variation of the spacer chain shed light on the limitations of the approach and the ring dynamics in the unusual [2]catenanes were studied in silico with atomistic resolution. This work significantly widens the scope of mechanically interlocked architectures comprising cycloparaphenylenes (CPP).
The addition of two unsymmetric malonate esters to the Buckminster fullerene C60 can lead to 22 spectroscopically distinguishable isomeric products and therefore represents a formidable synthesis challenge. In this work, we achieve 87% selectivity for the formation of a single (in,out‐trans‐3) isomer by combining three approaches: (i) we use a starting material, in which the two malonates are covalently connected (tether approach); (ii) we form the strong supramolecular complex of C60 with the shape‐persistent [10]CPP macrocycle (template approach) and (iii) we embed this complex further within a self‐assembled nanocapsule (shadow mask approach). Variation of the spacer chain shed light on the limitations of the approach and the ring dynamics in the unusual [2]catenanes were studied in silico with atomistic resolution. This work significantly widens the scope of mechanically interlocked architectures comprising cycloparaphenylenes (CPP).
A series of macrocyclic bis‐malonates incorporating either di‐tert‐butylsilylene or tetra‐iso‐propyldisiloxane subunits have been prepared and used for the regioselective bis‐functionalization of [60]fullerene by double Bingel cyclopropanations. By systematically changing the length and the rigidity of the spacer units linking the malonate moieties to the silylated protecting groups, fullerene bis‐adducts with different addition patterns have been obtained. Finally, the bridging di‐tert‐butylsilylene groups have been cleaved to afford the corresponding acyclic fullerene bis‐adducts bearing four alcohol functions.
Macrocyclic tris‐malonates incorporating cleavable di‐tert‐butylsilylene protecting groups have been prepared by a stepwise approach and used for the regioselective tris‐functionalization of C60. Fullerene tris‐adducts with an e,e,e addition pattern have been thus obtained. The bridging di‐tert‐butylsilylene protecting groups have been cleaved to liberate the terminal alcohol functions of the fullerene tris‐adduct. This key building block has been further functionalized to generate a clickable fullerene [3:3]‐hexa‐adduct scaffold allowing the successive introduction of two azide reagents.
Molecular Russian dolls (matryoshkas) have proven useful for testing the limits of preparative supramolecular chemistry but applications of these architectures to problems in other fields are elusive. Here we report a three-shell, matryoshka-like complex—in which C60 sits inside a cycloparaphenylene nanohoop, which in turn is encapsulated inside a self-assembled nanocapsule—that can be used to address a long-standing challenge in fullerene chemistry, namely the selective formation of a particular fullerene bis-adduct. Spectroscopic evidence indicates that the ternary complex is sufficiently stable in solution for the two outer shells to affect the addition chemistry of the fullerene guest. When the complex is subjected to Bingel cyclopropanation conditions, the exclusive formation of a single trans-3 fullerene bis-adduct was observed in a reaction that typically yields more than a dozen products. The selectivity facilitated by this matryoshka-like approach appears to be a general phenomenon and could be useful for applications where regioisomerically pure C60 bis-adducts have been shown to have superior properties compared with isomer mixtures.
The chemo- and regioselective functionalization of fullerenes is a long-standing problem of organic synthesis. Over the past five years, this fundamental challenge has gained technological relevance, because studies on single bis-adduct isomers in new-generation solar cells have demonstrated that the widespread use of isomer mixtures leads to suboptimal power conversion efficiencies. Herein, we review recent work on supramolecular approaches for achieving chemo- and regioselective syntheses of multiply functionalized derivatives of C60.
Multi‐functionalization and isomer‐purity of fullerenes are crucial tasks for the development of their chemistry in various fields. In both current main approaches—tether‐directed covalent functionalization and supramolecular masks—the control of regioselectivity requires multi‐step synthetic procedures to prepare the desired tether or mask. Herein, we describe light‐responsive tethers, containing an azobenzene photoswitch and two malonate groups, in the double cyclopropanation of [60]fullerene. The formation of the bis‐adducts and their spectroscopic and photochemical properties, as well as the effect of azobenzene photoswitching on the regiochemistry of the bis‐addition, have been studied. The behavior of the tethers depends on the geometry of the connection between the photoactive core and the malonate moieties. One tether lead to a strikingly different adduct distribution for the E and Z isomers, indicating that the covalent bis‐functionalization of C60 can be controlled by light.
A trigonal-bipyramidal covalent organic cage compound serves as an efficient host to form stable 1:1-complexes with C 60 and C 70 . Fullerene encapsulation has been comprehensively studied by NMR and UV/Vis spectroscopy, mass...
Fullerenes and their derivatives are of tremendous technological relevance. Synthetic access and application are still hampered by tedious purification protocols, peculiar solubility and limited control over regioselective derivatization. We present a modular self-assembly system based on a new low-molecular-weight binding mo-tif, appended by two palladium(II)-coordinating units of different steric demands, to either form a [Pd2L14]4+ cage or unprecedented [Pd2L23(MeCN)2]4+ bowl (with L1 = pyridyl, L2 = quinolinyl donors). The former was used as a selective induced-fit receptor for C60. The latter, owing to its more open structure, also allows binding of C70 and fullerene derivatives. By exposing only a fraction of the bound guests’ surface, the bowl acts as fuller-ene protecting group to control functionalization, as demonstrated by exclusive mono-addition of anthracene. In a hierarchical manner, sterically low-demanding dicarboxylates were found to bridge pairs of bowls into pill-shaped dimers, able to host two fullerenes. All hosts allow transferring bound fullerenes into a variety of organic solvents, extending the scope of possible derivatization and processing methodologies.
Since fullerenes are available in macroscopic quantities from fullerene soot, large efforts have been geared toward designing efficient strategies to obtain highly pure fullerenes, which can be subsequently applied in multiple research fields. Here we present a supramolecular nanocage synthesized by metal-directed self-assembly, which encapsulates fullerenes of different sizes. Direct experimental evidence is provided for the 1:1 encapsulation of C60, C70, C76, C78 and C84, and solid state structures for the host-guest adducts with C60 and C70 have been obtained using X-ray synchrotron radiation. Furthermore, we design a washing-based strategy to exclusively extract pure C60 from a solid sample of cage charged with a mixture of fullerenes. These results showcase an attractive methodology to selectively extract C60 from fullerene mixtures, providing a platform to design tuned cages for selective extraction of higher fullerenes. The solid-phase fullerene encapsulation and liberation represent a twist in host-guest chemistry for molecular nanocage structures.
A three-fold Bingel reaction between C60 and t-butyl(trialkoxy)silane derivatives bearing three
malonate substituents gave access to fullerene e,e,e tris adducts. Subsequent desilylation with
HF.pyridine or BF3.Et2O afforded the corresponding fullerene tris-adducts bearing three alcohol
functions.
The reaction of readily available optically active Si-tethered tris(malonates) with C60 gave easily separable diastereoisomers differing by the absolute configuration of the inherently chiral addition pattern on the fullerene core. The absolute configuration of the e,e,e addition pattern has been unambiguously determined using X-ray crystal structure analysis.
The synthesis and flash column chromatographic separation of enantiomerically pure tris-adducts of C60 with an e,e,e-addition pattern is achieved via cyclopropanation with chiral D3-symmetrical cyclo-tris(malonate) tethers.
Imagine you were given an isotropic ball-shaped object of nanometer dimensions and you’d have to neatly decorate it in specific positions. This would certainly be a difficult task, even more so if the ball were rotating rapidly. In this issue of Chem, Ribas and co-workers use a nanocage for regioselective functionalization of fullerenes.
Isomer-pure poly-functionalized fullerenes are required to boost the development of fullerene chemistry in all fields. On a general basis, multi-adduct mixtures with uncontrolled regioselectivity are obtained, and the use of chromatographic purification is prohibitively costly and time consuming, especially in the production of solar cells. Single-isomer poly-functionalized fullerenes are only accessible via stoichiometric, multistep paths entailing protecting-unprotecting sequences. Herein, a nanocapsule is used as a supramolecular tetragonal prismatic mask to exert full control on the reactivity and the equatorial regioselectivity of Bingel-Hirsch cyclopropanation reactions of a confined C 60 guest. Thus, equatorial bis-, tris-, and tetrakis-C 60 homo-adducts are exclusively obtained in a stepwise manner. Furthermore, isomer-pure equatorial hetero-tetrakis-adducts or hetero- Th-hexakis-adducts are synthesized at will in one-pot synthesis for the first time. This work provides a synthetically valuable path to produce a plethora of new pure-isomer poly-functionalized C 60-based compounds as candidates for testing in solar cell devices and biomedical applications.
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In this handbook, the leading experts in the field presents important and fundamental aspects of the organic and organometallic chemistry of fullerenes. Naturally they also cover the applications in material and medicinal science for these fascinating molecules. Completely self-contained, the book is logically arranged such that information is easy to retrieve, and the style lends itself to effortless reading and to learning more about the chemical properties of this family of molecules. A definitive "must" for everyone working in this ever-expanding sphere.
The self-assembly of 4-fold-symmetric Porphyrins with FeII or ZnII gave a new cubic MII8L6 cage framework with electron-deficient walls. This cage bound C60-indene or C60-anthracene bisadducts selectively, whereas unfunctionalized fullerenes and monoadducts were not encapsulated. The FeII8L6 cage also enabled the reaction of C60 and anthracene to yield the bisadducts selectively under conditions where no reaction was observed in the absence of the cage. These findings have relevance in the context of polymer solar cells, where C60 bisadducts have found use as electron acceptors, because these adducts currently require laborious and timeconsuming syntheses and purification.
A new class of C3-symmetrical fullerene building blocks with a well-defined spatial arrangement of addends has been developed. They comprise compounds with a e,e,e-trisaddition pattern and a mixed octahedral hexakisaddition pattern. Fullerenophosphates with a e,e,e-addition pattern were used for the spatial preorganization of the addends. After deprotection seven different trisbromides (12–16, 15a, 16a) with varying chain lengths and orientation of the functional groups were obtained. Acetylenes could be coupled to these compounds by click chemistry through an intermediate trisazide. For the synthesis of mixed hexakisadducts 19–21 a template system based on a central benzyl moiety was attached to the free hemisphere of the trisadduct fullerenophosphates. It was shown that the phosphate and the benzyl template can be removed independently. Fully deprotected building block 24 bearing both terminal bromide and hydroxy moieties was prepared. In addition, rather complex hybrid architecture 29 bearing three porphyrin units in the polar addend zone was successfully synthesized.
We have synthesized and isolated two C2-symmetric enantiomerically pure trisadducts of C60 with an inherently chiral trans-3,trans-3,trans-3 addition pattern with the aid of an enantiopure cyclo-tris-malonate tether. An unexpected stereoisomerism was observed which was attributed to the spatial arrangement of spacers in the tether.
A macrocyclic bis-malonate incorporating two rigid di(phenylethynyl)silane moieties has been prepared and used to functionalize C60 in a double Bingel cyclopropanation. Two regioisomeric bis-adducts have been thus obtained, namely the trans-1 and trans-3 isomers. The formation of these specific isomers is favored by both the size of the starting macrocycle and the minimum of strain in the final products.
The very regioselective twofold cyclopropanation of C60 with a porphyrin bismalonate leads to a C2 symmetrical [60]fullerene–porphyrin dyad with pronounced electronic interactions between the two π–π stacked chromophores.
Hexaphenylbenzene derivatives bearing two aldehyde groups were prepared by reaction of 4,4′-(ethyne-1,2-diyl)dibenzaldehyde with bisarylalkynes in the presence of a catalytic amount of Co2(CO)8. These synthetic intermediates were used to produce the corresponding bismalonates. Finally, macrocyclization of the bismalonates with C60 afforded equatorial fullerene bisadducts. The relative position of the two cyclopropane rings on the C60 core was determined on the basis of the molecular symmetry (C1) deduced from the 1H and 13C NMR spectra and further confirmed by the characteristic features seen in their UV/Vis spectra.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
The macrocyclization between buckminsterfullerene, C60, and bis-malonate derivatives in double Bingel reaction provides a versatile and simple method for the preparation of covalent bis-adducts of C60 with high regio- and diastereoselectivity. A combination of spectral analysis, stereochemical considerations, and X-ray crystallography (Fig. 2) revealed that out of the possible in-in, in-out, and out-out stereoisomers, the reaction of bis-malonates linked by o-, m-, or p-xylylene tethers afforded only the out-out ones (Scheme 1). In contrast, the use of larger tethers derived from 1,10-phenanthroline also provided a first example, (±)-19 (Scheme 2), of an in-out product. Starting from optically pure bis-malonate derivatives, the new bis-functionalization method permitted the diastereoselective preparation of optically active fullerene derivatives (Schemes 4 and 5) and, ultimately, the enantioselective preparation (enantiomeric excess ee > 97%) of optically active cis-3 bis-adducts whose chirality results exclusively from the addition pattern (Fig. 6). The macrocyclic fixation of a bis-malonate with an optically active, 9,9′-spirobi[9H-fluorene]-derived tether to C60 under generation of 24 and ent-24 with an achiral addition pattern (Scheme 4) was found to induce dramatic changes in the chiroptical properties of the tether chromophore such as strong enhancement and reversal of sign of the Cotton effects in the circular dichroism (CD) spectra (Figs. 4 and 5). By the same method, the functionafized bis-adducts 50 and 51 (Schemes 10 and 11) were prepared as initiator cores for the synthesis of the fullerene dendrimers 62, 63, and 66 (Schemes 12 and 13) by convergent growth. Finally, the new methodology was extended, to the regio- and diastereoselective construction of higher cyclopropanated adducts. Starting from mono-adduct 71, a clipping reaction provided exclusively the all-cis-2 tris-adduct (±)-72 (Scheme 14), whereas the similar reaction of bis-adduct 76 afforded the all-cis-2 tetrakis-adduct 77 (Scheme 15). Electrochemical investigations by steady-state voltammetry (Table 2) in CH2Cl2 (+0.1M Bu4NPF6) showed that all macroeyciic bis(methano)fullerenes underwent multiple reduction steps, and that regioisomerism was not much influencing the redox potentials, All cis-2 bis-adducts gave an instable dianion which decomposed during the electrochemical reduction. In CH2Cl2, the redox potential of the fullerene core in dendrimers 62, 63, and 66 is not affected by differences in size and density of the surrounding poly(ether-amide) dendrons. The all-cis-2 tris- and tetrakis(meihano)fullercnes (±)-72 and 77, respectively, are reduced at more negative potential than previously reported all-e tris- and tetrakis-adducts with methane bridges that are also located along an equatorial belt. This indicates a larger perturbation of the original fullerene π-chromophore and a larger raise in LUMO energy in the former derivatives.
An aromatic crown ether tethered to C60 is a template for the formation of the [2]catanene shown on the right, in which cyclobis(paraquat-p-phenylene) cyclophane becomes mechanically interlocked with the fullerene-appended macrocyclic polyether. Since the hydroquinone ring connecting the bis(p-phenylene)-[34]crown-10 to C60 behaves as a π-electron-rich exo donor and undergoes additional interactions with the electron-accepting C60 moiety, this first fullerene catenane boasts an unprecedented intramolecular acceptor-donor-acceptor-donor-acceptor stack.
Reaction of C60 with bis(β-keto esters) in the presence of iodine and diazabicyclo[5.4.0]undec-7-ene (DBU) in toluene provides covalent bisadducts of C60 with high regioselectivity.
An overall yield of 35% based on C60 was obtained in the synthesis of hexaadduct 1. First, bis- and trisadducts of C60 were prepared regiospecifically by tether-directed remote functionalization. Further functionalization of the trisadduct via a single tetraadduct gave the pseudooctahedral hexaadduct 1 regiospecifically. All intermediates were isolated in pure form without the use of tedious, scale-limiting HPLC methods.
The direct treatment of C60 with malonates in the
presence of CBr4 and diazabicyclo[5.4.0]undec-7-ene (DBU)
leads to an efficient conversion to methanofullerenes, which is
demonstrated with the syntheses of monoadducts and highly symmetric
hexaadducts in good yields.
A versatile and simple method provides access to covalent bisadducts of C60 with high regio- and diastereo-selectivity. Starting from optically pure bis(malonate) derivatives and C60, a double Bingel reaction afforded optically active cis-3 bisadducts (ee > 97%), whose chirality results exclusively from the addition pattern.
The Bingel addition of a trimalonate derived from a chiral, cyclotriveratrylene (CTV)-based tripodal tether to C60 was reinvestigated. The present use of enantiomerically pure (P)- and (M)-configured CTV units in the tether allowed the isolation of a total of four enantiomerically pure C3-symmetric trisadducts of [60]fullerene (two from each CTV enantiomer). With the support of NMR and UV/Vis spectroscopy, electronic (ECD) and vibrational (VCD) circular dichroism spectroscopic analysis (comparison to ECD data of known compounds
and to ECD and VCD data calculated by TD-DFT or ZINDO methods as part of this work) allowed a definitive
determination of the following structural features for all four CTV-C60 conjugates: (i) The constitution of the fullerene addition pattern, (ii) its absolute configuration, and (iii) the absolute configuration of the CTV moiety. It is concluded that the triple Bingel addition proceeded with complete regioselectivity and negligible diastereoselectivity, affording only the trans-3,trans-3,trans-3 fullerene addition pattern in both enantiomeric forms, whereas the previously reported e,e,e motif was not formed.
The ins and outs of fullerene chemistry: Fullerenophosphates with a C(3) -symmetrical e,e,e-trisaddition pattern were synthesized with high regioselectivity. In- (see structure, red O, yellow P, blue C) and out-invertomers, based on the orientation of the PO group, were isolated and their structures were confirmed by X-ray-crystallography.
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Regioselective Bingel macrocyclization of C60 with a bis-malonate containing a novel dibenzo[18] crown-6 tether provides a versatile access to trans-1 fullerene bis-adducts such as (±)-1. Complexation of a potassium ion by (±)-1 has a pronounced effect on the redox properties of the carbon sphere as a result of the close proximity of the fullerene surface to the crown ether bound cation, which is enforced by the double bridging.
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--D-glucopyranoside into the bis-malonate and Bingel macrocyclization diastereoselectively produced the cis-3 stereoisomer (,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).
Representatives of two classes of hexakis-adducts of C60 were prepared by templated synthesis strategies. Compound 8 with a dipyridylmethano addend in a pseudo-octahedral addition pattern was obtained by DMA-templated addition (DMA=9,10-dimethylanthracene; Scheme 1) and served as the starting material for the first supramolecular fullerene dimer 2. Hexakis-adduct 12 also possesses a pseudo-octahedral addition pattern and was obtained by a sequence of tether-directed remote functionalization, tether removal, and regioselective bis-functionalization (Scheme 2). With its two diethynylmethano addends in trans-1 position, it is a precursor for fascinating new oligomers and polymers that feature C60 moieties as part of the polymeric backbone (Fig. 1). With the residual fullerene π-electron chromophore reduced to a `cubic cyclophane'-type sub-structure (Fig. 4), and for steric reasons, 8 and 12 no longer display electrophilic reactivity. As a representative of the second class of hexakis-adducts, (±)-1, which features six addends in a distinct helical array along an equatorial belt, was prepared by a route that involved two sequential tether-directed remote functionalization steps (Schemes 3 and 5). In compound (±)-1, π-electron conjugation between the two unsubstituted poles of the carbon sphere is maintained via two (E)-stilbene-like bridges (Fig. 4). As a result, (±)-1 features very different chemical reactivity and physical properties when compared to hexakis-adducts with a pseudo-octahedral addition pattern. Its reduction under cyclic voltammetric conditions is greatly facilitated (by 570 mV), and it readily undergoes additional, electronically favored Bingel additions at the two sterically well-accessible central polar 6-6 bonds under formation of heptakis- and octakis-adducts, (±)-30 and (±)-31, respectively (Scheme 6). The different extent of the residual π-electron delocalization in the fullerene sphere is also reflected in the optical properties of the two types of hexakis-adducts. Whereas 8 and 12 are bright-yellow (end-absorption around 450 nm), compound (±)-1 is shiny-red, with an end-absorption around 600 nm. This study once more demonstrates the power of templated functionalization strategies in fullerene chemistry, providing addition patterns that are not accessible by stepwise synthetic approaches.
Three series of regioisomeric bisadducts of C60, namely, C62(anisyl)4 and the mixed systems C62(anisyl)2-(COOEt)2, and C61(COOEt)2(NCOOEt), were synthesized starting from the 1,2-monoadducts C61(COOEt)2 (1), C61-(anisyl)2 (2), and C60(NCOOEt) (4) by using the Bingel and Bamford-Stevens reactions, and nitrene additions. In the case of C61(COOEt)2(NCOOEt) the complete series of nine possible regioisomers were isolated for the first time. For steric reasons the cis-1 isomers of C62(anisyl)4 and C62(anisyl)2(COOEt)2 were not formed. The transannular [6,6] bonds in the cis-1 isomer 42 of C61(COOEt)2(NCOOEt) are closed. The properties and regioselectivities of formation of these bisadducts and their monoadduct precursors were compared with those of the series C62-(COOEt)4 and C60(NCOOEt)2, which we synthesized previously. In the additions to 1, 2, and 4 the preferred positions of attack are e and trans-3 for sterically demanding addends (e.g., combinations of C(anisyl)2 and C(COOEt)2) and cis-1, e, and trans-3 for sterically less demanding addends (e.g., combinations of N(COOEt) and C(COOEt)2). A detailed analysis of the MO structures, the experimental and calculated geometries of monoadduct precursors, and the stabilities of reaction products leads to the conclusion that the addend-independent cage distortion itself is responsible for the observed regioselectivities of bisadduct formations.
The synthesis of the cyclophane-type molecular dyads 1 and 1 . Zn was accomplished by Bingel macrocyclization of porphyrin-tethered bis-malonates 5 or 5 . Zn, respectively, with C60 (Scheme). In these macrocycles, the doubly bridged porphyrin adopts a close, tangential orientation relative to the surface of the C-sphere. The porphyrin derivatives 6 and 6 . Zn with two appended, singly-linked C60 moieties were also formed as side products in the Bingel macrocyclizations. The trans-1 addition pattern of the fullerene moiety in 1 and 1 . Zn was unambiguously established by 1H- and 13C-NMR spectroscopy. Due to the close spatial relationship between the fullerene and porphyrin components in 1 and 6 and the corresponding ZnII complexes, the porphyrin fluorescence is efficiently quenched as compared to the luminescence emitted by 5 and 5 . Zn, respectively (Fig. 2). Cyclic-voltammetry studies show that the mutual electronic effects exerted by the fullerene on the porphyrin and vice versa in 1 and 1 . Zn are relatively small despite the close proximity between the porphyrin donor and the fullerene acceptor (Fig. 3).
A computational (AM1) analysis is presented which shows that the observed regioselectivities of multiple cyclopropanations of C60 are due to orbital-allowed processes. Next to the coefficients of frontier orbitals the characteristic cage distortions of the oligomethanofullerenes are discussed. For further experimental corroboration the synthesis and characterization of an unsymmetrical trisadduct with a I, eI, III*-addition pattern are described. Moreover, since a systematic and descriptive assignment of addition patterns in derivatives of C60 and higher fullerenes becomes more and more important, a simple and general algorithm is introduced, which enables unambiguous bond labeling of both [6,6]- and [5,6]-bonds as well as the assignment of the absolute configuration of any fullerene derivative.
A comprehensive series of multiple adducts of C60 was prepared by tether-directed remote functionalization. When the tether-reactive-group conjugates 2 and 10 were attached to methano[60]fullerenecarboxylic acid ( = cyclopropafullerene-C60-Ih-carboxylic acid) and C60, respectively, the e-bis-adducts 4 and 9 (Schemes 1 and 2) were obtained with complete regioselectivity as predicted by semi-empirical PM3 calculations (Fig. 2). Attachment of the anchor-tether-reactive-group conjugate 13 to C60 by Bingel reaction, followed by double intramolecular Diels-Alder cycloaddition afforded the tris-adduct 12 (Scheme 3). Starting from 12, a series of selective e-additions led to the tetrakis-adducts 16 and 19 (Scheme 4), pentakis-adducts 20–23 (Scheme 5), and, ultimately, to hexakis-adducts 24 and 25 (Scheme 6), and 29 and 30 (Scheme 7) with a pseudo-octahedral addition pattern on the fullerene core. Oxidative cyclization of diethynylmethanofullerene 30 under Eglinton-Glaser conditions afforded the trimeric and tetrameric acetylenic macrocycles 26, with three, and 27, with four appended C60 moieties, respectively (Scheme 8). These multinanometer-sized compounds are the first soluble derivatives of C195 and C260, two members of a new class of fullerene-acetylene hybrid C-allotropes with the general formula Cn(60 + 5). The matrix-assisted laser-desorption time-of-flight mass spectra of 26 and 27 showed a remarkable fragmentation; the sequential loss of fullerene spheres led to the formation of ions corresponding to mono-fullerene adducts of the cyclocarbons cyclo-C15 and cyclo-C20 (Fig. 4). Large solvent effects were observed in the Bingel addition of 2-bromomalonates to higher adducts of C60, with the use of polar solvents enhancing the reaction rate without loss of regioselectivity. Experimental evidence for the enhanced reactivity of eface over eedge bonds was obtained, which had previously been predicted in computational studies. The correlated series of mono- to hexakis-adducts of C60 allowed identification of the changes in reactivity and physical properties that occur, when the conjugated π-electron chromophore of the fullerene is reduced as a result of increasing functionalization; this analysis is the subject of the directly following paper.
By the tether-directed remote functionalization method, a series of bis- to hexakis-adducts of C60, i.e., 1–7 (Fig. 1), had previously been prepared with high regioselectivity. An efficient method for the removal of the tether-reactive-group conjugate was now developed and its utility demonstrated in the regioselective synthesis of bis- to tetrakis(methano)fullerenes ( = di- to tetracyclopropafullerenes-C60-Ih) 9–11 starting from 4, 5, and 7, respectively (Schemes 2, 4, and 5). This versatile protocol consists of a 1O2 ene reaction with the two cyclohexene rings in the starting materials, reduction of the formed mixture of isomeric allylic hydroperoxides to the corresponding alcohols, acid-promoted elimination of H2O to cyclohexa-1,3-dienes, Diels-Alder addition of dimethyl acetylenedicarboxylate, retro-Diels-Alder addition, and, ultimately, transesterification. In the series 9–11, all methano moieties are attached along an equatorial belt of the fullerene. Starting from C2v-symmetrical tetrakis-adduct 15, transesterification with dodecan-1-ol or octan-1-ol yielded the octaesters 16 and 17, respectively, as noncrystalline fullerene derivatives (Scheme 3). The X-ray crystal structure of a CHCl3 solvate of 11 (Fig. 3) showed that the residual conjugated π-chromophore of the C-sphere is reduced to two tetrabenzopyracylene substructures connected by four biphenyl-type bonds (Fig. 5). In the eight six-membered rings surrounding the two pyracylene (= cyclopent[fg]acenaphthylene) moieties, 6–6 and 6–5 bond-length alteration (0.05 Å) was reduced by ca. 0.01 Å as compared to the free C60 skeleton (0.06 Å) (Fig. 4). The crystal packing (Fig. 6) revealed short contacts between Cl-atoms of the solvent molecule and sp2- and sp3-C-atoms of the C-sphere, as well as short contacts between Cl-atoms and O-atoms of the EtOOC groups attached to the methano moieties of 11. The physical properties and chemical reactivity of compounds 1-11 were comprehensively investigated as a function of degree and pattern of addition and the nature of the addends. Methods applied to this study were UV/VIS (Figs. 7–11), IR, and NMR spectroscopy (Table 2), cyclic (CV) and steady-state (SSV) voltammetry (Table 1), calculations of the energies of the lowest uunoccupied mmolecular orbitals (LUMOs) and electron affinities (Figs. 12 and 13), and evaluation of chemical reactivity in competition experiments. It was found that the properties of the fullerene derivatives were not only affected by the degree and pattern of addition but also, in a remarkable way, by the nature of the addends (methano vs. but-2-ene-1, 4-diyl) anellated to the C-sphere. Attachment of multiple thano moieties along an equatorial belt as in the series 8–11 induces only a small perturbation of the original fullerene π-chromophore. In general, with increasing attenuation of the conjugated fullerene π-chromophore, the optical (HOMO-LUMO) gap in the UV/VIS spectrum is shifted to higher energy, the number of reversible one-electron reductions decreases, and the first reduction potential becomes increasingly negative, the computed LUMO energy increases and the electron affinity decreases, and the reactivity of the fullerene towards nucleophiles and carbenes and as dienophile in cycloadditions decreases.
Macrocyclic C(60)-(pi-conjugated oligomer) dyads have been prepared from the corresponding bis-malonates by a macrocyclization reaction on the C(60) sphere. In both multicomponent systems, the fullerene moiety and the pi-conjugated oligomer subunit are at the van der Waals contact due to the cyclic structure. Interestingly, the characteristic pi-pi* electronic transition band of the conjugated system is significantly red-shifted in both dyads with respect to the corresponding model compounds lacking the fullerene unit (Delta lambda(max) = 24 to 34 nm). Whereas the absorption properties are dramatically affected by the intramolecular electronic interactions between the conjugated bridging system and the accepting C(60) spheroid, cyclic voltammetry revealed only small changes in their redox potentials. However, these intramolecular interactions have a significant influence on the electronic coupling of the two terminal aniline redox units of the conjugated system in the dyads. Actually, when compared to the corresponding model compound 14(center dot+), delocalization of the positive charge in the mixed-valence species derived from the dyad 1 is more difficult due to the pi-pi interactions of the conjugated system with the electron-withdrawing fullerene group. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
The Bingel functionalisation of C(60) with a structurally novel tether equipped with three reactive malonate groups afforded a C(2v)-symmetrical e(edge),e(face),trans-1 trisadduct in a complete regioselective manner and in an excellent yield of 65%. The [60]fullerene trisadduct showed pronounced ability to crystallise and gave X-ray quality single crystals for analysis.
The Bingel reaction between C(60) and an enantiopure bismalonate tether equipped with two acetonide moieties led to the synthesis and successful column chromatographic isolation of the enantiomerically pure (f,s)C and (f,s)A bisadducts with the inherently chiral trans-3 addition pattern. Acidic deprotection of the acetonide groups gave access to novel chiral fullerene compounds which combine the inherent chirality of the fullerene core with the functional glycol groups located on the tether.
e,e,e-Trisadducts 13 and 15 have been prepared by a highly regioselective threefold cyclopropanation of tripodal malonates 10 and 12 with C60. The yield and regioselectivity depend on the length and structure of the tethers that connect the malonate units to the focal benzene core of 13-15. As a consequence of the template-directed synthesis, all e,e,e-trisadducts were formed as in/out isomers exclusively and contain two spherically well-defined addend zones with equatorial and polar orientation, respectively. By variation of the outer malonate termini of the tethers, selective functionalization of the equatorial addend zone could be achieved, thus leading to fine-tuning of intermolecular interactions, such as solubility or aggregation phenomena. After removal of the focal benzene moiety in 14 and 15, selective functionalization of the polar addend zone could be achieved. Strong intramolecular hydrogen-bonding networks of the polar substituents in the polar addend zone could be observed by 1H NMR spectroscopic analysis. By orthogonal functionalization of both addend zones, fullerene derivatives 44-48 could be synthesized as one single in/out isomer, thus greatly enhancing the potential of e,e,e-trisadducts as building blocks in supramolecular architectures.
New macrocyclic malonates 2-5 have been prepared by reaction of malonyl dichloride with alkanediols. Reactions of these cyclo-[n]-alkylmalonates with C60 are highly regioselective. The macrocycles containing identical alkyl spacers selectively form bis- and trisadducts of C60 with rotational symmetry. The addition pattern of the regioselectively formed oligoadducts is determined by the size of the alkyl spacer within the macrocyclic malonate. A variety of bis-, tris-, tetra-, and hexaadducts have been synthesized to show the scope of this approach. "Exotic" addition patterns such as trans-4,trans-4,trans-4, which has been synthesized and completely characterized for the first time, are also accessible by this method. The regioselectivity is ruled by the even distribution of the strain within the macrocyclic malonates containing spacer alkane chains of identical lengths: addition patterns with rotational symmetry provide exactly identical distances of the malonate oxygen atoms and are thus exclusively formed by this method. In contrast, when macrocycles with two different alkyl spacer lengths are used, such as 9 and 10, the reaction exclusively yields C(s)-symmetric bisadducts.
We have applied a modified macrocyclic tether approach to control multiple additions to C60. The technique of 3He NMR was used to confirm the selective formation of specific C60 multiple adducts by the macrocyclic tether approach. An oligoglycol was used as a flexible linker to produce macrocyclic polyether-linked malonates 5, 6, 8, and 9 under solid-liquid PTC (phase-transfer-catalysis) conditions. The formation of a single C60 tris-adduct, 3, from macrocyclic malonate 1 and 3He@C60 was proven by 3He NMR. Similarly, multiple additions to C60 of macrocyclic polyether malonate 5 gave C60 bis-adduct 10 selectively, while the reaction of C60 with macrocyclic malonate 8 gave bis-adducts 11 and 12. A similar process with macrocyclic malonate 6 gave tris-adduct 13 with high selectivity as well. Saponification of these C60 multiple adducts gives the corresponding polyacids that are potentially useful in biological applications. Macrocyclic polyether fullerenes are a new class of ionophores, which could be interesting for molecular recognition and for the development of biosensors.
D(3h)-symmetrical tripodal tris(malonate) tethers have been used for the synthesis of [60]fullerene tris-adducts with an e,e,e addition pattern bearing topologically distinct polar and equatorial addend zones that can selectively be deprotected.
The synthesis of isomerically multiple adducts of C(60) with a defined three-dimensional structure is still one of the most challenging tasks of exohedral fullerene chemistry. The inherent regioselectivity of successive additions of addends such as malonates to the fullerene's [6,6]-double bonds is only moderate. In most cases difficult-to-isolate mixtures of regioisomers are obtained. The regioselectivity can be significantly improved if multifunctional addends able to undergo two or more additions are allowed to react with C(60). Preorganization and minimization of strain energy within the addend skeleton reduce the number of sterically allowed addition patterns. Improved concepts for highly regio- and stereoselective bis- and triscyclopropanations of C(60) are described. Two examples of the bisadditions with complete regioselectivity leading to trans-2- and cis-2 are presented. Here, the two malonate binding sites are linked by rigid tetraphenylporphyrin and calix-[4]-arene spacers. Selective trisadditions were achieved with the easy-to-synthesize and easy-to-modify tripodal addends 5-7, where the malonates are held together by a focal aryl moiety. Another very elegant approach for bis- and trisadditions involves cyclo-[n]-alkylmalonates. Selection between addition patterns with and without rotational axes is possible by choosing the right combinations of the flexible alkyl chains connecting the malonates. If alkyl chains of identical lengths are used bis- and trisadducts such as 19-21 and 25 with rotational symmetry are formed with high regioselectivity. These addition patterns are avoided if cyclo-[n]-malonates containing alkyl chains of different lengths are employed. In this case adducts such as 26 and 27 with C(s)-symmetry are formed. The use of the chiral cyclo-[3]-malonate 28 allows for the regio- and stereoselective synthesis of the enantiomerically pure e,e,e-trisadducts 29 and 30 containing an inherently chiral addition pattern with C(3)-symmetry.
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