Current Organic Chemistry - Volume 16, Issue 9, 2012

Chemical functionalization of fullerene is vital to its applications in nanomedicine, biomedical imaging, organic photovoltaics and advanced materials. A number of efficient reactions have been proposed in the preparation of fullerene monoadducts and multiple adducts. Many articles and book chapters have reviewed the majority of fullerene structures and reactions. Two important areas in fullerene chemistry are emerging rapidly, attract a great degree of attention across the fullerene and general chemistry community but have not been reviewed in detailed accounts. One area is the endohedral metallofullerene. One or more metal atoms or metal clusters can be entrapped inside a fullerene cage, called endohedral metallofullerene, which has very broad and promising applications being a new generation of magnetic resonance imaging contrast agent and an efficient electron acceptor in organic solar cell. Of particular interest is the trimetallic nitride endohedral metallofullerene (TNT metallofullerene) pioneered by Dr. Harry Dorn and being commercialized by Luna nanoWorks. In this special issue of Current Organic Chemistry, one excellent article is presented to review the recent progress in the synthesis, separation, molecular structure and characterization of metallofullerenes including conventional endohedral fullerenes and endohedral clusterfullerenes, and a second article is to review the chemical functionalization methods for metallofullerenes and the influence of the choice of functionalization on the electronic properties of TNT metallofullerenes. The second area covered in this special issue is the use of radical reactions to functionalize fullerenes. Radical reaction is one of the first investigated reactions in fullerene chemistry. There are three classes of radical reactions: direct radical addition to fullerene, single electron transfer and oxidation of fullerene anions. However in many cases radical reactions lead to the formation of a mixture of fullerene isomers, which are not acceptable for either biomedical or OPV applications. In this special issue of COC two articles are presented to review the recent progress of fullerene radical reactions. One article focuses on metal-mediated radical reactions that can be controlled to synthesize isomerically pure fullerene adducts, and the second article reviews the recent advancement in halogenation that is a highly chemoselective and regioselective multiple radical addition reaction. The guest editor and all authors hope to use this platform to increase the awareness of these important areas in fullerene chemistry.

Halogenated fullerenes have been subjects of intensive investigations since the discovery of fullerenes. This paper reviews the synthesis and molecular structures of fullerene halides, covering earlier work in this field and progress in synthetic methods developed in the last few years. The use of inorganic chlorides and oxychlorides as well as (TiCl4 + Br2) mixtures for the chlorination of fullerenes allowed the preparation of new compounds that have been characterized by single-crystal X-ray diffraction, IR spectroscopy, and confirmed by theoretical calculations. The maximum degrees of halogenation and compound stability increase from the bromides to the fluorides. The larger sizes of Cl and Br atoms and the lower C–Hal bond energies account for significant differences in the addition patterns of the C60Haln and C70Halm molecules in relation to those of the fluorides. Investigation of chlorides of lower (C50 – C68) and higher (up to C90) fullerenes contributed significantly to the new chemistry of fullerenes including those with non-IPR and non-classical cages.

Endohedral fullerenes represent a novel type of nanostructures, which are characterized by a robust fullerene cage with atoms, ions, or clusters trapped in its hollow. Since the first separation of the endohedral metallofullerene La@C82, a large variety of endohedral structures have been isolated and their endohedral nature has been proved by experimental studies. The world of endohedral fullerenes was significantly enlarged within the past decade by the clusterfullerenes including nitride clusterfullerenes and the new carbon cages including non-IPR (IPR=isolated pentagon rule) structures. With the classification of endohedral fullerenes presented first, we review the synthesis methods of endohedral fullerenes focusing on the new synthetic routes of nitride clusterfullerenes, the extraction and separation of endohedral fullerenes. Finally the intriguing molecular structures of endohedral fullerenes are addressed as well.

Since the serendipitous discovery of C60 and subsequent finding of other fullerenes, scientists have focused on the discovery of the physical and chemical properties of these nanomaterials and their incorporation in biomaterials and nanodevices [1,2]. The development of a particular family of fullerenes which incarcerates a metal, metals, or a metallic cluster in the interior of their carbon cages, the endohedral metallofullerenes, is quickly becoming one of the most interesting research areas within the field of carbon-based compounds because of the scope of electronic properties not accessible with empty-cage fullerenes [3-7]. Even though mass spectrometric evidence of their existence was already available in the first paper reporting the discovery of Buckminsterfullerene in 1985 [8, 9], there was controversy about the location of the metals, since fullerenes with exohedral metals were also observed. It was not until 1991 that Smalley and coworkers unequivocally showed that a lanthanum atom was encapsulated in a C82 fullerene shell as La@C82 [10], and since then, many analogous species have been isolated. However, the exploration of the physical, chemical and electronic properties of endohedral metallofullerenes has become a reality only in the last few years when technological advances have been made in the production of macroscopic quantities of these nanomaterials [11, 12]. In this section, an overall view of the effect of the encapsulated metal species on the exohedral reactivity of the carbon cages will be addressed and the differences between empty cage fullerenes and endohedral metallofullerenes will be emphasized. It is our intent to suggest that endohedral metallofullerenes are the fullerenes of the future.

Various transition metal salts have been employed to the radical reactions of [60]fullerene (C60). The present review article covers the reactions of C60 with active methylene compounds, methyl ketones, β-enamino carbonyl compounds, carboxylic acid derivatives, and diallylamines in the presence of Mn(OAc)3·2H2O affording the singly bonded fullerene dimers, 1,4-adducts and 1,16-adduct of C60, C60-fused dihydrofurans, methanofullerenes, C60-fused pyrrolines, C60-fused lactones, fullerenyl ester derivatives, and C60-fused pyrrolidines as well as the transformation of ArC60-H into ArC60-OAc by Mn(OAc)3·2H2O. Radical reactions of C60 promoted by other transition metal salts including copper(II), lead(IV), iron(III), ruthenium(II/III), tungsten(VI), cerium(IV) salts are also described.

The Suzuki cross-coupling reaction, which is commonly referred to simply as the “Suzuki reaction” and uses palladium as a catalyst, has become an efficient tool for constructing C–C bonds over the past two decades. This paper reviews recent advances in the use of this reaction with heterogeneous palladium catalysts. The heterogenization of homogeneous catalysts by incorporation into an inorganic support (silica, mainly) or an organic polymer is currently of one the hottest research topics. Although heterogeneous catalysts based on uncomplexed Pd(II) or Pd(0) deposited on an inorganic supports such as carbon, silica or a zeolite remain in wide use, new catalysts consisting of Pd(0) nanoparticles entrapped in or deposited onto an organic or inorganic support are being increasingly used in the Suzuki reaction.

Conformational studies of symmetric diesters, dimethyl maleate, dimethyl 5,6-isopropylidenedioxybicyclo[2.2.1]hepta-2-ene- 2,3-dicarboxylate, and dimethyl succinate have been performed. It was found that the conformation wherein one of the carbonyl oxygens interacts with the carbon of the other carbonyl is the most stable in all the cases. In particular, in dimethyl maleate and dimethyl 5,6- isopropylidenedioxybicyclo[2.2.1]hepta-2-ene-2,3-dicarboxylate, it was found that the structures wherein one of the carbonyl groups is oriented near-perpendicular with respect to the C=C bond and the other carbonyl group is oriented near-parallel with respect to the C=C bond have the lowest energies, showing good agreement with the X-ray crystal structures. Bonding interaction between the carbonyl oxygen and the other carbonyl carbon was also found in all these structures. These results suggest the existence of the n->π* interaction between the two carbonyl groups and are expected to provide insight into the non-covalent interaction between the two carbonyl groups in these symmetric diesters.

A convergent chiral pool approach for the total synthesis of xestodecalactones B and C was demonstrated in which intramolecular acylation reaction constituted the key step. Synthetic and spectroscopic studies reported herein, suggested that the previously assigned structures of xestodecalactone B and C be interchanged.