Some Aryl-Carboranes

We pointed out in Section 3.4 that the skeletal bonding in closo carboranes arose primarily from a- and tt-type interactions between tangentially oriented p AOs on adjacent skeletal atoms. This makes carborane cages sensitive to the n systems of substituents such as aryl groups, which have been found to be able to communicate electronically with, indeed through, carborane cages. -  

By contrast, the compound commonly referred to as benzocarborane, a derivative of orf/io-carborane in which the two carbon atoms are linked through four methine units (CH)4, shows little evidence in its structure of delocalization of the exo-ring jr-electrons into the exo C-C bonds, which remain resolutely single bonds (Fig. 3.16). It had been thought that this compound, formally sharing two carbon atoms between a benzenoid ring and the carborane icosahedron, might have shown some multiplicity in the exo C-C 

However, the external (CH)4 link between the carborane hypercarbon atoms shows the alternation in C-C bond distances (1.48,1.34,1.45, 

following the external ring from exo bond to exo bond) appropriate for a diene. Indeed, benzocarborane can undergo Diels-Alder addition reactions if its cage is converted from closo into nido form to allow room for such reactions in the parent compound, the approach of dienophiles will be inhibited by adjacent BH units in the icosahedral cage (Fig. 3.16). 

That exo multiple bonding to an orz/zo-carborane cage hypercarbon atom is possible (but with orientational constraints that prevent its occurrence in benzocarborane, and with fascinating imphcations for the use of ortho-carborane as a 7t-acidic probe by which to measure the n-donor capacity of substituents) has been demonstrated by a series of studies on derivatives of phenyl-orz/io-carborane PhCBioHioCX bearing assorted zi-donor C-attached 

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C2BioHi2 derivatives with, e.g., R = ester, amine, amide, acyl halide, halogen, ether, alkyl, aryl, alkenyl or alkynyF , although a dialkyne will lead to some bis-carborane ... 

Wilbur and coworkers concentrating on the use of /t3b-carborane derivatives for astatination of biotin foimd that /rfo-carborane can be directly labeled in 70-80% yield and the biotin derivatives containing astatinated ratJb-carborane moieties are more stable to in vivo deastatination than aryl astatine derivatives however, the astatinated wzVfo-carborane derivatives undergo some deastatination [249,250,254]. 

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