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• W3000000650 abstract "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. 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. In 1985, fullerene C60 was described by Kroto, Curl, and Smalley, who were honored by a Nobel Prize in 1996.1Kroto W.H. Heath R.J. O’Brien C.S. Curl F.R. Smalley E.R. C60: buckminsterfullerene.Nature. 1985; 318: 162-163Crossref Scopus (13971) Google Scholar The attractive football-shaped molecule inspired countless scientists to deeply investigate its properties, homologs, reaction products, and applications. Nowadays, fullerenes are applied in organic electronics and photovoltaics as electron-accepting and -transporting materials. Derivatives are further examined for nano-medicinal purposes, e.g., as antioxidants and photodynamic agents. Despite enormous progress regarding their synthetic modification, it remains difficult to control the regioselective chemical modification of fullerenes, particularly when several substituents are sought to be installed in defined positions. There is, however, a clear demand for multi-functionalized fullerene derivatives given that regioisomeric compounds have been clearly shown to differ in their electronic properties2Umeyama T. Imahori H. Isomer effects of fullerene derivatives on organic photovoltaics and perovskite solar cells.Acc. Chem. Res. 2019; 52: 2046-2055Crossref PubMed Scopus (88) Google Scholar and (liquid)-crystal-packing patterns,3Sawamura M. Kawai K. Matsuo Y. Kanie K. Kato T. Nakamura E. Stacking of conical molecules with a fullerene apex into polar columns in crystals and liquid crystals.Nature. 2002; 419: 702-705Crossref PubMed Scopus (370) Google Scholar both of which are determining factors in materials research. A classic way to access specific regioisomers is based on the covalent introduction of protecting or directing groups on the fullerene surface, followed by the desired modification with simultaneous control over the steric and/or electronic factors. However, this approach can suffer from tedious preparation and deprotection steps, lengthening the overall synthetic route.4Isaacs L. Haldimann R.F. Diederich F. Tether-directed remote functionalization of buckminsterfullerene: regiospecific hexaadduct formation.Angew. Chem. Int. Ed. 1994; 33: 2339-2342Crossref Scopus (182) Google Scholar,5Schwenninger R. Müller T. Kräutler B. Concise route to symmetric multiadducts of [60]fullerene: preparation of an equatorial tetraadduct by orthogonal transposition.J. Am. Chem. Soc. 1997; 119: 9317-9318Crossref Scopus (83) Google Scholar Furthermore, the level of regiocontrol achieved by these methods is still limited to certain substitution patterns, e.g., depending on the degree to which the frontier orbital coefficients can be adjusted. Recently, an alternative strategy for gaining control over the functionalization of fullerenes has emerged from basic research activities in host-guest chemistry. Although fullerenes were encapsulated inside covalent macrocyclic hosts, such as calixarenes, decades ago, progress in the area of metal-mediated self-assembly of three-dimensional cavitands with tunable shape and size has spurred exciting new developments. Nowadays, the coordination-driven assembly of a wide range of organic building blocks into rings and cages with an accessible cavity follows elaborate design principles accounting for denticity and shape, as well as functional-group arrangement in the ligands. A vast number of hosts have been prepared in this way for various guest types, but the number of systems that selectively bind fullerenes in their interior remains scarce.6García-Simón C. Costas M. Ribas X. Metallosupramolecular receptors for fullerene binding and release.Chem. Soc. Rev. 2016; 45: 40-62Crossref PubMed Google Scholar In general, hosts for C60 and its homologs and derivatives have relatively large cavities lined by extended aromatic π-systems, such as porphyrins, anthracenes, or triptycene-derived panels. Fullerene binding is then mainly promoted by extensive π-π interactions and can be supported by C–H∙∙∙π contacts and solvophobic effects. Motivation to encapsulate fullerenes inside nanoscopic hosts—including selective recognition for sensing and purification purposes, solubilization in more polar solvents than are usually feasible, and modulation of the fullerenes’ electronic and optical properties—is multifarious. Beyond such mere host-guest recognition phenomena, the utilization of self-assembled hosts to modulate the chemical reactivity of their incorporated guests has moved into focus. So far, only a small number of reports have tackled the modification of C60 confined inside a cavity. For example, Nitschke and co-workers described the selective addition of two anthracenes to a C60 molecule with both substrates co-encapsulated inside a cubic cage.7Brenner W. Ronson T.K. Nitschke J.R. Separation and selective formation of fullerene adducts within an M(II)8L6 cage.J. Am. Chem. Soc. 2017; 139: 75-78Crossref PubMed Scopus (103) Google Scholar Clever and co-workers constructed a bowl-shaped assembly that was able to mask most of the surface area of a bound C60, thus limiting the addition of a single anthracene to the fullerene’s unprotected face.8Chen B. Holstein J.J. Horiuchi S. Hiller W.G. Clever G.H. Pd(II) coordination sphere engineering: pyridine cages, quinoline bowls, and heteroleptic pills binding one or two fullerenes.J. Am. Chem. Soc. 2019; 141: 8907-8913Crossref PubMed Scopus (83) Google Scholar Also note that Guldi and von Delius directed trans-functionalization of a C60 derivative embedded in an organic [10]cycloparaphenylene ring to realize a unique fullerene-based rotaxane.9Xu Y. Kaur R. Wang B. Minameyer M.B. Gsänger S. Meyer B. et al.Concave–Convex π–π Template Approach Enables the Synthesis of [10]Cycloparaphenylene–Fullerene [2]Rotaxanes.J. Am. Chem. Soc. 2018; 140: 13413-13420Crossref PubMed Scopus (66) Google Scholar What these and related studies have shown is that the design of hosts capable of controlling the covalent modification of fullerenes can be tricky, given that two potentially contradictory prerequisites must be fulfilled: (1) a large binding constant for the fullerene guest is helpful to make the host-guest complex survive the administered reaction conditions, and (2) a suitably sized, free patch of fullerene surface has to be left where reactants can approach the encapsulated fullerene. A tightly binding host might not leave enough space for the desired chemical reaction, whereas a lean receptor poses the risk of only loosely binding the fullerene and hence sacrificing strict selectivity. In 2014, Ribas and co-workers reported a purification method for fullerene C60 by using a peculiar coordination cage design based on four macrocyclic pillars, each chelating two Pd(II) cations, bridged by two zinc porphyrin panels, to give a cuboid box-shaped host.10García-Simón C. Garcia-Borràs M. Gómez L. Parella T. Osuna S. Juanhuix J. Imaz I. Maspoch D. Costas M. Ribas X. Sponge-like molecular cage for purification of fullerenes.Nat. Commun. 2014; 5: 5557Crossref PubMed Scopus (125) Google Scholar In a subsequent study, this host was further shown to allow the recognition and purification of endohedral fullerenes.11Fuertes-Espinosa C. Gómez-Torres A. Morales-Martínez R. Rodríguez-Fortea A. García-Simón C. Gándara F. Imaz I. Juanhuix J. Maspoch D. Poblet J.M. et al.Purification of uranium-based endohedral metallofullerenes (EMFs) by selective supramolecular encapsulation and release.Angew. Chem. Int. Ed. 2018; 57: 11294-11299Crossref PubMed Scopus (48) Google Scholar In their new work published in this issue of Chem, Ribas and co-workers vastly extend the utility of their proven host system to exert an unprecedented degree of control over the multi-functionalization of an encapsulated fullerene guest.12Fuertes-Espinosa C. García-Simón C. Pujals M. Garcia-Borràs M. Gómez L. Parella T. Juanhuix J. Imaz I. Maspoch D. Costas M. Ribas X. Supramolecular fullerene sponges as catalytic masks for regioselective functionalization of C60.Chem. 2020; 6: 169-186Abstract Full Text Full Text PDF Scopus (35) Google Scholar The structure of the host-guest complex shown in Figure 1A shows a striking detail with regard to the accessibility of the bound guest: whereas the major contribution to the tight association between host and guest surely derives from the sandwich-type arrangement involving the upper and lower porphyrin panels,13Tashiro K. Aida T. Zheng J.-Y. Kinbara K. Saigo K. Sakamoto S. Yamaguchi K. A cyclic dimer of metalloporphyrin forms a highly stable inclusion complex with C60.J. Am. Chem. Soc. 1999; 121: 9477-9478Crossref Scopus (310) Google Scholar the lateral pillars leave four openings to allow access to the bound fullerene by reagents in a square-planar fashion. In search of suitable reagents able to attack the C60 through these four channels, the authors were successful with the Bingel-Hirsch methodology, which is based on a bromo-malonate as a reactive intermediate that nucleophilically attacks the bound fullerene and the subsequent intramolecular SN2 ring closure under ejection of bromide to yield a cyclopropanated product. When a fullerene encapsulated by the coordination cage is treated, the latter serves as a mask to direct muti-functionalization with high selectivity (Figure 1A). As a result, four malonate substituents are installed along the equator of the ball-shaped guest, delivering one regioisomer in quantitative yields. It should also be mentioned that the reaction can be performed in polar media (such as acetonitrile), which are usually not suited for the solubilization of C60. Therefore, the cage serves a second role as a solubilizing agent that greatly enhances the scope of reactions that can be applied.8Chen B. Holstein J.J. Horiuchi S. Hiller W.G. Clever G.H. Pd(II) coordination sphere engineering: pyridine cages, quinoline bowls, and heteroleptic pills binding one or two fullerenes.J. Am. Chem. Soc. 2019; 141: 8907-8913Crossref PubMed Scopus (83) Google Scholar The authors were able to expand their methodology toward a targeted hetero-functionalization of the fullerene substrate. Therefore, the initial addition of a first malonate reactant was restricted to only 1 or 2 equiv, the latter of which gave only the cis-adduct for electronic reasons; subsequently, saturation of the equatorial substitution sites with a second, different malonate yielded hetero-functionalized products with determined stoichiometry and regiochemistry. Furthermore, the tetrakis-adducts obtained by the above-mentioned route could be reacted with excess amounts of a different nucleophile, inducing its decomplexation from the cage and the installation of two further malonates, thereby allowing the synthesis of a hexakis-adduct with distinguishable substituents in equatorial and axial positions (Figure 1B). Finally, Ribas and co-workers were able to perform the regioselective functionalization reaction under cyclic turnover conditions12Fuertes-Espinosa C. García-Simón C. Pujals M. Garcia-Borràs M. Gómez L. Parella T. Juanhuix J. Imaz I. Maspoch D. Costas M. Ribas X. Supramolecular fullerene sponges as catalytic masks for regioselective functionalization of C60.Chem. 2020; 6: 169-186Abstract Full Text Full Text PDF Scopus (35) Google Scholar (Figure 1C): they added excess amounts of sulfate, which acted as a counter anion to the cationic host-guest complex and caused its transfer into the water layer of a biphasic reaction mixture. Conversely, the addition of tetraarylborates triggered re-transfer into the organic layer. Meanwhile, the Bingel-Hirsch reaction was confined to the aqueous phase, and once the reaction product was handed back to the organic layer, the tetrakis-adduct was replaced by pristine C60 given that the latter has a higher affinity to the host. Within a few years, the host-guest chemistry of fullerene-binding coordination cages has matured significantly, including the elaboration of new molecular design strategies, binding principles, and a range of applications. Systems have proven to be useful for recognizing different fullerene derivatives, changing their solubility and electronic properties, and enabling their separation from raw mixtures, such as industrial soot. Only now, even more sophisticated applications aimed at the highly selective, even stepwise multi-functionalization of fullerenes to produce products with complex substitution patterns have come into reach. The research presented by Ribas and co-workers in this issue of Chem is certainly a major step in this direction. S.H. thanks the German Academic Exchange Service (DAAD) for a PhD fellowship. This work was supported by the research training group “Confinement-Controlled Chemistry,” funded by the Deutsche Forschungsgemeinschaft (DFG) under grant GRK2376/331085229. Supramolecular Fullerene Sponges as Catalytic Masks for Regioselective Functionalization of C60Fuertes-Espinosa et al.ChemNovember 11, 2019In BriefAn unprecedented and straightforward supramolecular mask strategy to prepare exclusively equatorial bis-, tris-, and tetrakis-cyclopropanated-C60 Bingel-Hirsch derivatives is reported. By taking advantage of the high affinity for fullerene of tetragonal prismatic supramolecular cages, a highly stable C60⊂1a·(BArF)8 host-guest complex is submitted to Bingel-Hirsch cyclopropanation reaction conditions. Regioselectivity is strictly dictated by the four cross-shaped apertures of the nanocapsule in a controlled fashion. Moreover, stepwise-cyclopropanated adducts up to tetrakis additions are obtained in excellent yields and purities. Full-Text PDF Open Archive" @default.
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• W3000000650 title "Metallo-supramolecular Shell Enables Regioselective Multi-functionalization of Fullerenes" @default.
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