Covalent fullerenes

DIederIch F and Thilgen C 1996 Covalent fullerene chemistry Sc/e/ ce271 317-23... 

Aspects of Covalent Fullerene Chemistry Regioselective Multiple Functionalization, Optically Active Carbon Allotropes, and Electroluminescent Devices (LEDs)... 

F. Diederich, C. Thilgen, Covalent Fullerene Chemistry , Science 1996, 271, 317-323. 

The Fullerenes form particularly strong complexes with porphyrins as exemplified by the X-ray crystal structure of the covalent Fullerene-porphyrin conjugate 15.8 (Figure 15.29).48 This property allows fullerenes and porphyrins to form extended supramolecular arrays (even when not covalently linked) and has been used to engineer host-guest complexes in which a Fullerene is sandwiched in between a pair of porphyrins, and ordered arrays involving interleaved porphyrins and Fullerenes. Applications include the use of porphyrin solid phases in the chromatographic separation of Fullerenes and potential applications in porous frameworks and photovoltaic devices.49... 

Here, a first review is provided which summarizes the more recent development framework modified fullerenes like cluster opened structures and heterofullerenes. The key steps for such framework modifications are always defined activations of the fullerene cluster due to specific covalent addition reactions. Therefore, the principles of covalent fullerene chemistry [3-8] will be considered first ... 

Chirality, in the field of fullerenes, is not limited to the chiral carbon cages themselves. In 1992, the first enantiomerically pure covalent fullerene adducts, Cso-sugar conjugates, were prepared [39]. With the explosive development of covalent fullerene chemistry during the last few years [27-32], an increasing number of chiral C50 and C70 derivatives has been published [27, 33], and the need for an appropriate configurational description became pressing [34]. 

A fluid micelle which dissolves apolar porphyrins but can nevertheless be isolated as a material and characterized by transmission electron (TEM) and atomic force microscopy (AFM) has kanamycin as a head group. This is a trisaccharidelike cyclohexane derivative containing several OH- and four amino groups. One of them was amidated with stearic acid, and upon sonication spherical micelles with a diameter of about 6 nm were formed. TEM shows a large assembly of spheres which look similar to covalent fullerene spheres, but are ten times larger. ... 

Diederich, R, and Thilgen, C. Covalent Fullerene Chemistry." Science, 271,317-323 (1996). 

An alternative approach envisages the stimulating idea to produce an all-carbon fullerene polymer in which adjacent fullerenes are linked by covalent bonds and align in well characterized one-, two- and tliree-dimensional arrays. Polymerization of [60]fullerene, with the selective fonnation of covalent bonds, occurs upon treatment under pressure and relatively high temperatures, or upon photopolymerization in the absence of a triplet quencher,... 

Maggini M, Done A, Scorrano G and Prato M 1995 Synthesis of a [60]fullerene derivative covalently linked to a ruthenium (II) tris(bipyridine) complex J. Chem. Soc., Chem. Commun. 845-6... 

Nie B., Rotello V. Reversible Covalent Attachment of Fullerenes to Polymers and Materials Proc. - Electrochem. Soc. 1996 96-10 1212-1217... 

In 1995, Boyd and co-workers <95TL7971 > covalently linked a porphyrin to fullerene Cgo through a 1,3-dipolar cycloaddition reaction involving the porphyrinic azomethine ylide 28 (Scheme 8). The ylide was generated in situ from befa-formyl-meso-tetraphenylporphyrin 27 and A -methylglycine, and provided the porphyrin-C6o diad 29 in good yield. 

The covalent methods previously discussed for fullerene modification using cycloaddition reactions also can be applied to carbon nanotubes. This strategy results in chemically linking molecules to the graphene rings on the outer surface of the cylinder, resulting in stable... 

Treatment of isoxazoline-fiised [60]fullerene 48 with NaOMe in the presence of MeOH gave the p-hydroxy nitrile derivative 49 in good yield <00SL361>. The synthesis of the enantiomerically pure cyclopropane amino acid 51 covalently attached to a fulleroisoxazoline has been achieved . 

The synthesis of C60-based dyads in which the Ccm core is covalently attached to a strong electron acceptor moiety, has been carried out by 1,3-dipolar cycloaddition of in situ generated nitrile oxides with C(,o- As expected, the obtained adducts show reduction waves of the fullerene core that are anodically shifted in comparison with the parent Cr>o. This indicates that they are remarkably stronger acceptors than Ceo-The electron acceptor organic addend also undergoes an anodic shift due to the electronic interaction with the C(,o moiety (545). 

Following the discovery of a bulk fullerene preparation process in 1990, the covalent chemistry of these carbon allotropes has developed at a phenomenal pace. Frontier orbital (LUMO) and tether-directed functionalization concepts have been successfully applied to the regio- and stereoselective preparation of multiple covalent adducts of C60. These have found increasing applications in the construction of functional supramol-ecules. More recently, the sequence of Bingel reaction - retro-Bingel reaction has provided an elegant access to isomerically pure higher fullerenes and, in particular, to pure carbon enantiomers. 

For origins of fullerene chirality and definitions of configurational description factors fC and fA (f = fullerene, C = clockwise, A = anticlockwise), see C. Thilgen, A. Herrmann, F. Diederich, The Covalent Chemistry of Higher Fullerenes C70 and Beyond , Angew. Chem Int. Ed. Engl. 1997,36,2269-2280. 

Dendrimers have also been found useful in the construction of thin films containing isolated fullerenes [85, 86]. Such materials could eventually find practical application in sensors and/or optoelectronic devices [87]. Generally, monolayers prepared from C60 are ill-defined due to the aggregation tendency of fullerenes. The covalent attachement of C60 to bulky dendritic frameworks... 

Diels-Alder reactions have been used to synthesize and functionalize polymers, as reported by several groups. Rotello and coworkers112, for example, covalently attached [60]fullerene to furan and cyclopentadiene substituted resins. The reaction with the furan substituted resin proved reversible. The resin was recovered by heating the fullerene functionalized resin. 

A paclitaxel fullerene derivative has been obtained by covalent linkage of the dmg to the C60 by means of an ester, the hydrolysis of which presents a favorable kinetic profile, with consequent release of paclitaxel (Zakharian et al., 2005). The in vitro tests show a good anticancer activity, holding out hope for enhancing the dmg efficacy... 

In principle, there are four basic strategies to compensate for the repulsive effects between the hydrophobic fullerene surface and water (a) encapsulation in the internal hydrophobic moiety of water-soluble hosts like cyclodextrins (Andersson et al., 1992 Murthy and Geckeler, 2001), calixarenes (Kunsagi-Mate et al., 2004) or cyclotriveratrylenes (Rio and Nierengarten, 2002) (b) supramolecular or covalent incorporation of fullerenes or derivatives into water-soluble polymers (Giacalone and Martin, 2006) or biomolecules like proteins (Pellarini et al., 2001 Yang et al., 2007) (c) suspension with the aid of appropriate surfactants and (d) direct exohe-dral functionalization in order to introduce hydrophilic moieties. 

C60 is also a highly versatile synthetic scaffold that can easily be functionalized by the methods of synthetic organic chemistry. The formation of C60 derivatives (i.e., covalently modified C60) nearly always involves the addition of a functional group (addend) across one or more of its 30 double bonds. When only one addend is attached, the fullerene derivative is called a monoadduct, with two, a bisadduct, etc. The ability to sensitize molecular oxygen in the presence of visible light is retained in the simple derivatives of C60 (i.e., mono-, bis-, and trisadducts). 

One of the examples of inclusion complexes formed by intermolecular forces is the C60/PVP complex for the first time described by Yamakoshi et al. (1994). The non-covalent intermolecular nature of forces that stabilizes this complex confirms the fact that fullerene can be quantitatively extracted from its water solution by toluene. 

As with fullerenes, carbon nanotubes are also hydrophobic and must be made soluble for suspension in aqueous media. Nanotubes are commonly functionalized to make them water soluble although they can also be non-covalently wrapped with polymers, polysaccharides, surfactants, and DNA to aid in solubilization (Casey et al., 2005 Kam et al., 2005 Sinani et al., 2005 Torti et al., 2007). Functionalization usually begins by formation of carboxylic acid groups on the exterior of the nanotubes by oxidative treatments such as sonication in acids, followed by secondary chemical reactions to attach functional molecules to the carboxyl groups. For example, polyethylene glycol has been attached to SWNT to aid in solubility (Zhao et al., 2005). DNA has also been added onto SWNT for efficient delivery into cells (Kam et al., 2005). 

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