Camouflaged Polyhedral Species [pdf]

          The three isomers of the icosahedral C₂B₁₀H₁₂ (1,2- or ortho-; 1,7- or meta-; and 1,12- or para-) carboranes and the isoelectronic polyhedral borane dianion B₁₂H₁₂^2- are uniquely suited to play the role of building-blocks in the construction of molecular scaffolding and stereochemically rigid platforms. These versatile components often provide sites for further functionalization of the macrostructure or to add other interesting chemical properties. This versatility has lead to the formation of several different types of structural motifs, including carborods, mercuracarborands, carboracycles, and camouflaged carboranes and polyhedral borane anions. This section of the web site will focus on the latter of these structural types, the camouflaged carboranes and polyhedral borane anions. The term camouflaged carborane or polyhedral borane describes a molecule in which all, or very nearly all, the B-H vertices have been substituted by a typical organic substituent. The resulting polyhedral borane derivative, which has a borane core stabilized by multicenter electron delocalization, is shrouded by a periphery of organic substituents. Hence, camouflaged carboranes demonstrate chemical properties indicative of their hybrid nature between that of the thermally and chemically stable carboranes and boranes and the substitutionally versatile aliphatic hydrocarbons.
          Electrophilic substitution of hydrogen by halogenation or alkylation at B-H vertices of the carboranes is a well-known reaction. However, it was our initial report of the high-yield synthesis of per-B-methyl para-carborane by the methylation of para-carborane with methyl triflate and a catalytic amount of triflic acid that initiated the development of a new class of unprecedented molecules, the camouflaged carboranes. This remarkable chemoselectivity leaves the C-H vertices free for further functionalization using other means. The complete methylation of all the vertices of para-carborane can be accomplished if 1,12-dimethyl-1,12-dicarba-closo-dodecaborane (12) is methylated using methyl triflate and triflic acid, producing dodecamethyl-1,12-dicarba-closo-dodecaborane(12) in excellent yield, 91%. Figure 1a presents a view of a space-filling model of this novel icosahedral hydrocarbon-like molecule. Radical photochlorination of decamethyl-para-carborane, by chlorine in carbon tetrachloride under UV irradiation forms decakis(dichloromethyl)-1,12-dicarba-closo-dodecaborane(12) in high yield, 88%. This more extensively camouflaged carborane is depicted in Figure 1b as a ball and stick structure showing the rigorous stereochemical orientation of the -CHCl₂ substituents. The extensive steric crowding observed in the space-filling diagram of decakis(dichloromethyl)-para-carborane accounts for the selective chlorination of only two of the three hydrogen atoms present on each methyl group. These camouflaged carboranes are currently under investigation as modules for the formation of the molecular assemblies (carborods, carboracycles, mercuracarborands, and pharmacophore groups) described in other sections of this web page.

          Another application which is of particular interest is the possible adoption of these camouflaged icosahedral carboranes as the bases of an inventory of large, hydrophobic and biodegradation-resistant pharmacophore groups and their potential functionalization for use in drug discovery. A comparison of the calculated van der Waals diameters of dodecamethyl-para-carborane (990 pm), decakis(dichloromethyl)-para-carborane (1440 pm) and a well-known hydrophobe, C₆₀ (1070 pm) shows the similar volumes of these three molecules. In order to permit either of these camouflaged carboranes to be used as pharmacophore groups, reactions which provide further functionalization of the carboranes needed to be developed. The photochlorination results demonstrate that reactions characteristic of aliphatic hydrocarbons can be utilized to functionalize the organic shroud surrounding the delocalized polyhedral cage bonding of the carborane scaffolding. This hydrocarbon-like reactivity was further exploited through adaptation of the Barton photochemical oximation reaction.

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