Regioselectivity fullerenes

Regioselective and stereoselective template synthesis in chemistry of fullerenes annelated with heterocycles or linked with heterocyclic fragments 99ACR537. 

In 2000, it was proposed that the regioselectivity of the [3 + 2] cycloaddition of fullerenes could be modified under microwave irradiation. Under conventional heating, N-methylazomethine yhde and fullerene-(C7o) gave three different isomeric cycloadducts because of the low symmetry of C70 vs. Ceo. Using microwave irradiation and o-dichlorobenzene as a solvent, only two isomers were obtained, the major cycloadduct 114 being kinetically favored (Scheme 39) [75]. The same authors had previously reported the 1,3-dipolar cyclo addition of pyrazole nitrile oxides, generated in situ, to Geo under either conventional heating or microwave irradiation. The electrochemical characteristics of the cycloadduct obtained with this method made this product a candidate for photophysical apphcations [76]. 

Thioselenation of alkenes,209,210 1,3-dienes,207 and vinylcyclopropanes197 occurs by using the binary system of (PhS)2/(PhSe)2 to afford the adducts regioselectively in good yield (Table 6). Addition of a disulfide to fullerene[60] is achieved using this binary system.211... 

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

F. Diederich, R. Kessinger, Templated Regioselective and Stereoselective Synthesis in Fullerene Chemistry , Acc Chem Res. 1999, 32, 537-545. 

J.-P. Bourgeois, P. Seiler, M. Fibbioli, E. Pretsch, F. Diederich, L. Echegoyen, Cyclophane-Type Fullerene-dibenzo [18] crown-6 Conjugates with trans-1, trans-2, and trans-3 Addition Patterns Regioselective Templated Synthesis, X-ray Crystal Structure, Ionophoric Properties, and Cation-Complexation-Dependent Redox Behavior , Helv. Chim Acta 1999, 82,1572-1595. 

Beuerle F, Chronakis N, Hirsch A (2005) Regioselective synthesis and zone selective deprotection of [60]fullerene tris-adducts with an e,e,e addition pattern. Chem. Commun. 3676-3678. 

Zhong Y-W, Matsuo Y, Nakamura E (2006) Convergent synthesis of a polyfunctionalized fullerene by regioselective five-fold addition of a functionalized organocopper reagent to C60. Org. Lett. 8 1463-1466. 

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. 

Silylmethylation of fullerenes can be achieved by Grignard reaction of Cjq with Me2Si(0 Pr)MgCl (Scheme 3.4). Two different types of silylmethylated 1,2-fullerene adducts are formed [8]. The use of different solvents leads either to the 1,2-product 12 or to the unprecedented 1,4-addition product 13. These compounds react easily with various alcohols or phenols, which makes silylmethylated fullerenes a versatile starting material for regioselectively defined fullerene compounds [10]. Some examples are shown in Scheme 3.4. 

The chiral fullerene C75 was also asymmetrically osmylated using the chiral ligands and (Scheme 8.8) [63]. In this way an optically active allotrope of a pure element was prepared. C75 contains 15 different types of [6,6] bonds. The pronounced regioselectivity of C7Q towards osmylation [58] suggests that specific bonds in C75 may be favored for an attack by OSO4. An analysis of the ab initio calculated curvature of 75 shows that two of the five pyracylene-type bonds are particularly distorted, which could enhance their reactivity [64]. Indeed HPLC analysis of C75[Os04L ] shows that two regioisomers are predominantly formed upon osmylation of C75 [63]. 

Cycloadditions to [6,6]-double bonds of Cjq are among the most important reactions in fullerene chemistry. For a second attack to a [6,6]-bond of a C q monoadduct nine different sites are available (Figure 10.1). For bisadducts with different but symmetrical addends nine regioisomeric bisadducts are, in principle, possible. If only one type of symmetrical addends is allowed, eight different regioisomers can be considered, since attack to both e - and e"-positions leads to the same product. Two successive cycloadditions mostly represent the fundamental case and form the basis for the regioselectivity of multiple additions. In a comprehensive study of bisadduct formations with two identical as well as with two different addends, nucleophilic cyclopropanations, Bamford-Stevens reactions with dimethoxybenzo-phenone-tosylhydrazone and nitrene additions have been analyzed in detail (Scheme 10.1) [3, 9, 10]. 

The introduction of two [5,6]-aza bridges shows a remarkable regioselectivity even if segregated alkylazides are used [17]. The diazabishomofullerenes 23 (Scheme 10.3) are by far the major products and only traces of one other bisadduct with unidentified structure are found if, for example, a twofold excess of azide is allowed to react with CgQ at elevated temperatures [17]. To obtain clues on the mechanism of this most regioselective bisadduct formation process in fullerene chemistry a concentrated solution of an azahomofullerene precursor 24 was treated with an alkyl azide at room temperature. 

A very versatile tool for the synthesis of monomeric aza[60]fullerene derivatives 32 is the Mannich fimctionalization (Scheme 12.11), where 2 is typically treated in ODCB with an excess of the carbonyl compoimd and 40 equiv. TsOH at 150 °C in a constant stream of O2 [21, 22, 26]. After 15 min the conversion is usually completed and the Mannich bases are formed in good yield and complete regioselectivity. 

F. Regioselective Self-sensitized Oxygenation of Fullerene Adducts. 857... 

TABLE 15. Regioselectivity in the self-sensitized oxygenation of fullerene adducts"... 

Examination of the possible transition states TSi and TS2, leading to the minor and the major allylic hydroperoxide, respectively, provides an insight into this regioselectivity (Scheme 27). In transition state TS2 the non-bonding interactions, involving the large fullerene moiety, are smaller than those at the transition state TSi leading to the minor product. As the size of the substituents on both sides of the double bond become similar. 

Ortiz AL, Rivera DM, Athans AJ et al (2009) Regioselective addition of N-(4-thiocyanato-phenyl)pyrrolidine addends to fullerenes. Eur J Org Chem 3396-3403 S3396/l-S3396/25... 

---