Heterocyclic synthesis fullerene reactions

Among the reactions applied in the synthesis of fullerene derivatives cycloaddition reactions such as [2 + 1]-, [2 + 2]-, [3 + 2] and [4 + 2] cycloadditions play a dominant role. In these reactions ring-fused fullerene derivatives are obtained, at least with incorporation of heteroatoms such as oxygen, nitrogen, or silicon. In this section photochemical reactions leading to cycloalkyl ring-fused fullerene adducts will be presented. Photocycloaddition reactions leading to C6o-fused heterocycles will be discussed later. 

As mentioned previously, photocycloaddition reactions are a useful method for obtaining fullerene derivatives fused directly to heterocycles. Such compounds have attracted much interest because they might have interesting properties, e.g., amino acid fullerene derivatives as biologically active compounds or pyrrolidino-fullerenes as key precursors for a great number of fullerenes donor-acceptor bridged dyads and triads. Fullerenes functionalized with silicon compounds are of great interest in materials science. The synthesis of these compounds will be described in this section. 

Under microwave irradiation, carbazole reacted remarkably fast with a number of alkyl halides to give Af-alkyl derivatives of carbazole (82) (Bogdal et al., 1997). The reaction was carried out with high yields by simply mixing carbazole with an alkyl halide, which was adsorbed on potassium carbonate. A facile synthesis of a series of A-alkylpyrrolidino fullerenes (83) by phase transfer catalysis without solvent under microwave irradiation has been described by De la Cruz et al. (1998), while Adamczyk and Rege (1998) have illustrated the dramatic rate acceleration for A-sulfopropylation of heterocyclic compounds using 1,3-propane sultone under microwave irradiation affording the A-sulfopropylated compounds in 68-95% yield. 

Microwave irradiation has been successfully applied in chemistry since 1975 and many examples in organic synthesis have been described [3, 4], Several reviews have been published on the application of this technique to solvent-free reactions [5], cycloaddition reactions [6], synthesis of radioisotopes [7], fullerene chemistry [8] and advanced materials [9], polymers [10], heterocyclic chemistry [11], carbohydrates [12], homogeneous [13] and heterogeneous catalysis [14], medicinal and combinatorial chemistry [15], and green chemistry [16]. All these applications are described elsewhere in this book. 

Chung s research group recently reported a different type of sultine-type precursor [70], The synthesis of heterocyclic sultines 63 has been reported elsewhere [71]. Conventional heating of 2,5-disubstituted thienosultines 63 with [60]fullerene 59 in o-DCB under reflux for 2-24 h produces the biradical intermediates 64 which are subsequently trapped by [60]fullerene 59 to produce the cycloadducts 65 and bis adducts 66 in 37-79% yields in 2 1 to 3 1 ratios (Scheme 11.19). The reaction was highly accelerated under microwave conditions (900 W, <180 °C) for only 4 min. The ratio of mono adducts 65 to bis adducts 66 was also increased from 2-3 1 to 3.5-6 1 when microwaves were applied [18]. 

Hetero-Diels-Alder reactions are useful tools for the synthesis of six-membered heterocycles. As a general rule, fullerene acts as a dienophile at a [6,6] C=C. 

The main use of this reaction is for the synthesis of bowl-shaped polycyclic aromatic hydrocarbons and fullerenes [325], Exploring the boundaries of these reactions resulted in the synthesis of compounds of different geometry, tt-electron structure, aromaticity, and with the presence of heteroatoms [324]. Various cross-conjugated enediynes exert anionic cycloaromatization to form fulvene and fulvalene anions and even heterocycles. In accordance with the concept of aromaticity, the anionic Bergman type cyclization is preferred to the classical Bergman cycloaromatization of linear enediynes. This anionic cyclization differs considerably from the classical Bergman cyclization and related cycliza-tions, as well as from other dianionic cyclizations [326]. 

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