Fullerenes derivatization

Yang J, Alemany LB, Driver J, Hartgerink JD, Barron AR (2007a) Fullerene-derivatized amino acids Synthesis, characterization, antioxidant properties, and solid-phase peptide synthesis. Chem. Eur. J. 13 2530-2545. 

The main type of chemical fullerene derivatizations are addition reactions. Regardless of the relatively many possible reaction sites, addition reactions show a remarkable regioselectivity, especially when the number of addends is small. This is another fulfilled requirement, which makes these molecular spheres exciting objects for synthetic chemists. 

The tether-directed remote-functionalization methodology has proved to be a very powerful synthetic tool, due to its high regio- and stereoselectivity. Since the first description of such a reaction in 1980 [6] a variety of regioselective protocols have been developed for fullerene derivatization, and these were recently reviewed [7], so this section constitutes a selective overview mainly focused on the pertinent features relevant to this chapter. 

The functionalization of fullerenes is predominantly through addition reactions [70]. With many double bonds in a fullerene molecule (30 for Cgo), multiple additions to the same fiillerene cage are quite common in fullerene derivatization. For Cgo derivatives, there is a suggestion that the monoadducts... 

In the case of 12, electronic factors exceed the normal characteristic of fullerene derivatization that is the tendency to avoid endocyclic double bonds in five-membered rings. Prior to this work. Banks claimed the existence of a [5,6]... 

Fullerene derivatization is also possible by structural modification of the core. The simplest case consists in the insertion of a carbon atom or a heteroatom into a C-C bond and is encountered in homo[60]- [28-32, 75] and -[70] fullerenes [27,75-77]. Only a few deeper modifications, creating small holes in the fullerene shell are known of Cgo [31,75] and C70 [75,78]. Looking at structurally modified fullerenes in a broader sense and from the viewpoint not of cage degradation but assembly, it is clear that much more work - mostly aimed at buckybowls [79] or the total synthesis of buckminsterfuUerene [80] - has been dedicated to this topic. 

Nucleophilic addition of organozinc reagents is crucial step in Reformatsky reaction which was employed for fullerene derivatization. Ethylbromoacetate reagent (5 equiv.) in conjunction with zinc dust (20 equiv.) reacts with fullerene to obtain product 48 in high yield. In this process, known cyclopropane molecule 49 was formed as a side product, as well as two products unknown in the literature, 50 and 51 (Scheme 7.14) [26]. Formation of these products is rationalized by mechanism which starts with formation of organozinc reagent, which is followed by nucleophilic addition to fullerene. 

Fullerene-derivatized silica Better tolerance to high salt concentration... 

Azadienes 89, generated in situ by thermolysis of the corresponding o-aminobenzylalcohols, have been used for the derivatization of [60]-fullerene through C-N bond formation leading to tetrahydropyrido [60]-fullerenes [93]. Theoretical calculations predicted these cycloadditions to be HOMO azadiene-controlled (Equation 2.25). 

In conclusion, such three-dimensional carbon-based nanostructures as fullerenes and nanotubes constitute scaffolds whose efficient functionalization and derivatization... 

The demonstration that the 1,3-dipolar cycloaddition process with azomethine ylides works with nanotubes implies that similar reactions developed for use with fullerenes also may be successful with carbon nanotubes. In particular, the cyclopropanation reactions discussed previously for the modification of Cg0, likely will work for derivatization of SWNTs and MWNTs (Zakharian et al., 2005). 

Fullerene The reaction of 1 with an equimolar amount of fullerene- 60 in toluene at room temperature gives the first fullerene complex of titanium Cp2Ti(q2-C60) 121 [60]. An X-ray diffraction study of this complex has shown that it has the structure of a titanacyclopro-pane derivative, which should have a high potential for further derivatization of the fullerene. 

Schinazi RF, Sijbesma RP, Srdanov G, Hill CL, Wudl F (1993) Synthesis and virucidal activity of a water soluble, configurationally stable, derivatized C60 fullerene. Antimicrob. Agents Chemother. 37 1707-1710. 

The two most commonly used derivatization methods for exohedral functionalization are the nucleophilic cyclopropanation with malonates (Bingel, 1993) and the formation of fulleropyrrolidines (Maggini et al., 1993). Both of these protocols have been used extensively to produce water-soluble fullerenes for biomedical applications. Other stable water-soluble fullerene adducts have also been reported (Hirsch and Brettreich, 2005). Sections 3.2.2-3.2.5 will give a short overview on the state-of-the-art of water-soluble fullerene derivatives and outline some general trends for designing such molecular structures. 

Fig. 10.3 Examples of water-soluble derivatized fullerenes with 1, 3, and 18 carboxyl groups attached (Reprinted from Bosi et al., 2003. With permission from Elsevier)...

Reaction ofcarboxy- or hydroxy-substituted o-quinodimethanes such as 52, 54 or deprotected 58 lead to Cjq adducts with a functionalizable group. They can be derivatized for example by forming an ester [46,49,50] from the acid or by addition of the hydroxy funchonality to double bonds [49], Addihon products of the para-dihydroxy derivahve 65 can be easily oxidized to the corresponding benzoquinone-linked fullerene with DDQ [48]. The cycloadduct 73 of Cgg with 4-(4-fluoro-3-... 

Reaction of 175 with Cgg yields a hydroxy-functionalized fullerene that can be further derivatized. This hydroxy-fullerene was coupled with a porphyrine unit via a polyethyleneglycol-Hnker. This linker can be arranged similarly to a crown-ether to complex metal cations. Complexation is used to tune the distance between the porphyrin imit and the Cgg-moiety and thus tune the donor-acceptor properties of this porphyrin-fuUerene hybrid [177]. 

Derivatizing Cjq with a strong acceptor, such as the tetracyanoquinodimethane 231 yields organofullerenes that can accept up to eight electrons in solution [263, 278]. The acceptor ability of the quinodimethane moiety in the fullerene adduct is... 

Figure 14.1 Possible derivatizations of Qq- ( ) fullerene salts, (b) exohedral adducts, (c) open cage fullerenes, (d) quos/Tullerenes, (e) heterofullerenes, (f) endohedral fullerenes.

Fullerenes can be derivatized by various means. For example, reaction with fluorine gas proceeds stepwise to the formation of colorless CeoFeo, which, according to the 19F nuclear magnetic resonance (NMR) spectrum, contains just one type of F site and so evidently retains a high degree of symmetry.9 In view of the low adhesion typical of fluorocarbons, this spherical molecule is expected to have extraordinary lubricant properties. Curiously, bromination of Ceo is reversible on heating otherwise, the reactions of fullerenes resemble those of alkenes or arenes (aromatic hydrocarbons). 

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