Carboranes aryl-substituted

Depending on the temperature and the reaction time, the solvent-free iodination of 1 with I2 in a sealed tube allowed the regioselective synthesis of 9-iodo-o-carborane (17, 97% yield), 9,12-diiodo-o-carborane (18, 62% yield), and 8,9,10,12-tetraiodo-o-carborane (26, 93% yield. Scheme 6.4) (Barbera et al., 2008 Vaca et al., 2006). Shorter reaction times were reqnired when mono/di-C-alkyl- and mono/di-C-aryl-substituted o-carboranes were iodinated nsing the same method (Barbera et al., 2008). Tetraiodination of 1,2-diphenyl-o-carborane (27) and 1,2-dimethyl-o-carborane (28) required only 3.5 h and 2.5 h, respectively, to give 29 and 30. These results were in agreement with previous studies showing that the presence of methyl substituents at the carbon atoms increased the electron density at the boron atoms of 1 (Boer et al., 1966 Pbtenza and Lipscomb, 1966). 

This method is applicable to a wide variety of alkynes (e.g., R,R = H, alkyl, aryl, alkoxy, alkenyl, alkynyl), but reagents containing OH or COOH moieties degrade the boron cage and cannot be employed. However, carboranes containing these functional groups can be obtained via electrophilic substitution on the parent compound as outlined below. 

The phenomenon of atropisomerism (formation of rotational isomers) in porphyrins with meso aryl substituents has been first described in 1969 by Gottwald and Ullman [78] with tetra-me o-o-hydroxyphenylporphyrins. This concept has been exploited with me o-tetra amino-phenylporphyrins to design porphyrin scaffolds with different symmetry [79-81]. The atropisomerism phenomenon in porphyrin is caused by the steric hindrance induced by the presence of a substituent in ortho position of meso aryl substituents. In contrast, wieta-substitution of the meso aryl substituents very rarely shows restricted aryl rotation in tetra- and di-arylporphyrins. Few examples of atropisomerism in meta position are nevertheless found in literature (such as porphyrin carborane [82], fuUerene porphyrins [83, 84] and multimetallic porphyrin monomers [85, 86]. In these examples, the large size of the meso aryl substituents (carborane, fiiUerene...) despite substituted in meta position, hinders rotation around the Cme -Caryi bond leading to rotational isomers. 

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