Fullerenes chemistry heterofullerenes

Heterofullerene chemistry is still a very young discipline within synthetic organic chemistry and even within fullerene chemistry. So far, it is restricted to mono-azafullerenes. However, the potential of structural diversity within heterofullerenes is enormous (Figure 12.2). Preparative challenges for the future are, for example. 

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 ... 

Beside the fact that azide addition to C50 is a convenient method for functionalizing fullerenes, the iminofullerenes also serve as precursors in the synthesis of nitrogen heterofullerenes [171]. Their chemistry is described in more detail in Chapter 12. 

The ratio of the constitutional isomers 14-16 as determined by HPLC was 16 3 1. The fact that mixed dimers are also easily accessible reflects another aspect of the great diversity within heterofullerene chemistry. As with the aza[60]fullerenes, alkoxy substituted monomers 17 and 18 are formed together with the dimers 14-16 (Scheme 12.4) [3]. These two monomers are formed in a ratio of 7 1. Also, for these higher heterospheres NMR spectroscopy reveals a closed structure. For example, the signal of the sp C-atom of 17 appears at 5 = 96.42. 

Since heterofullerene chemistry began, as well as 8, 17 and 18 various further derivatives formed during the formation of azafullerenes or from (C59N)2 (2) itself have been synthesized. Heterofullerene transformation of the ketolactam 6 in the presence of a 15-fold excess of hydroquinone leads to the parent hydroaza[60]-fullerene 25 [14]. The hydroquinone is assumed to reduce the C59N radical intermediate (Scheme 12.6). 

The synthetically most valuable intermediate in heterofullerene chemistry so far has been the aza[60]fulleronium ion C59N (28). It can be generated in situ by the thermally induced homolytic cleavage of 2 and subsequent oxidation, for example, with O2 or chloranil [20-24]. The reaction intermediate 28 can subsequently be trapped with various nucleophiles such as electron-rich aromatics, enolizable carbonyl compounds, alkenes and alcohols to form functionalized heterofullerenes 29 (Scheme 12.8). Treatment of 2 with electron-rich aromatics as nucleophilic reagent NuH in the presence of air and excess of p-TsOH leads to arylated aza[60]fullerene derivatives 30 in yields up to 90% (Scheme 12.9). A large variety of arylated derivatives 30 have been synthesized, including those containing cor-annulene, coronene and pyrene addends [20, 22-25]. 

The accessibility of the fullerenes [1] in macroscopic quantities [2] opened up the unprecedented opportunity to develop a rich three-dimensional chemistry of spherical and polyfunctional all carbon molecules. [3-8] A large multitude of fullerene derivatives like exohedral covalent addition products, salts, cluster opened and defined degradation products, heterofullerenes and endohedral derivatives can be imagined and numerous examples, especially of covalent adducts have been synthesized and characterized. [3-8] Within a few years the fullerenes became essential building blocks in organic chemistry. Most of the chemistry of fullerenes has so far been carried out with Ceo (1) with little work on C70 and few experiments with C76 and Cg4. This is simply due to the fact that Ceo... 

The state of the art in heterofuUerene chemistry and physics is reviewed with emphasis on azafullerenes. The macroscopic synthetic methods that have been developed for aza[60] fullerene compounds since 1995 have led to a whole new and rich area in the science of fuUe-renes cage modification chemistry. The synthetic routes towards aza[60]fuUerene and its derivatives are reviewed in Sect. 2. The synthetic routes for aza[70]fuUerene and its derivatives are summarized in Sect. 3. Section 4 comprises the theoretical and experimental work on the physicochemical properties of azafuUerene compoimds. Finally, in Sect. 5, the literature regarding heterofullerenes other than monoazafuUerenes is reviewed. 

The prototype of all fullerenes is Buckminsterfullerene ,. Since it is the most abundant fiillerene obtained from macroscopic preparation procedures such as the classical Kratschmer-Huffman method, its chemical and physical properties were developed in very quick sucession soon after it became available in 1990. The icosahedral football shaped Buckminsterfrdlerene 60 is now the most intensively studied molecule of all. Many principles of the chemistry of C , are known. These allowone to tailor design new fuUerene derivates with specific properties useful for biological applications or as new materials. The main types of fullerene derivative are exohedral addition products, endohedral fullerenes, heterofullerenes and cluster opened systems. Whereas many examples of the first two groups are already known, sophisticated methods for the synthesis of the latter two groups have started to emerge only recently. 

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