Functionalization of carboranes

On the other hand, carboryne, l,2-dehydro-t -carborane, is a three-dimensional relative of benzyne (Fig. 7.1) [6]. It can react with alkenes, dienes, and alkynes in [2-1-2], [4-1-2] cycloaddition and ene-reaction patterns [7], similar to those of benzyne [8], Although these reactions show the potential for the preparation of functionalized carboranes in a single operation, they are complex and do not proceed in a controlled manner. In view of the spectacular role of transition metals in synthetic chemistry, we envisage that the aforementioned reactions may work efficiently and in a controlled way with the help of transition metals. In this connection, we initiated a research program to develop transition-metal-mediated/catalyzed synthetic methodologies for the functionalization of carboranes. This chapter summarizes the recent progress in this research area. 

The hydrogen atoms of the two CH groups are acidic, in contrast to those of the BH groups, which are hydridic. Therefore, selective functionalization of the carbon atoms is possible by electrophilic substitution or deprotonation with strong bases followed by subsequent reaction with electrophiles. However, the steric bulk of the clusters requires appropriate reactivity of the electrophile to obtain the desired product in acceptable yields. Phosphorus-containing compounds such as halophosphines are suitable for this type of functionalization of carboranes. In 1963, the first reaction with phosphines was described and until today it has ranained the method of choice for phosphorus functionalization of carboranes. 

This review will restrict itself to boron-carbon multiple bonding in carbon-rich systems, as encountered in organic chemistry, and leave the clusters of carboranes rich in boron to the proper purview of the inorganic chemist. Insofar as such three-dimensional clusters are considered at all in these review, interest will focus on the carbon-rich carboranes and the effect of ring size and substituents, both on boron and carbon, in determining the point of equilibrium between the cyclic organoborane and the isomeric carborane cluster. A typical significant example would be the potential interconversion of the l,4-dibora-2,5-cyclohexadiene system (7) and the 2,3,4,5-tetracarbahexaborane(6) system (8) as a function of substituents R (Eq. 2). 

Scheme 21 Preparation of carborane complexes comprising the/ac-[99mTc(CO)3]+ moiety and different functionalities for linking to biomolecules...

The rapid development of carborane chemistry is mainly due to their practical applications. For instance, the potential utility of carborane polymers as gaskets, O-rings, and electrical connector inserts has been reported. Their functionality for solvent extraction of radionuchdes as well as the potential medicinal value of the isoelectronic and isostructural boron analogues of biologically important molecules has been the subject of many review articles. For example, a number of boron compounds have been found to possess anti-inflammatory and antiarthritic activity in animal model studies. Boron compounds have also been implicated in studies designed to probe the importance of the so-called anionic subsite of acetylcholine esterase and Ach receptors. But, by far the most interesting practical apphcations of carboranes are in areas of boron neutron capture therapy (BNCT) and supramolecular assembly. 

The copolymer composition is a function of the comonomers reactivities. For example, the carborane concentration in a poly (diene-co-acryloyloxymethyl carborane) is higher than that of the carborane monomer in the initial solution 20>. Chloroprene, which has a higher reactivity than the other monomers, is an exception. 

Incorporation of carborane moieties into silicone chains also leads to improved thermal properties of such semiorganic polymers. The stationary phases based on carborane polymers have been available commercially since the early 1970 s, when the first analytical results were also reported [86] for some biological compounds. Although such phases were believed to be stable up to about 400°C, only moderately successful separations have been reported. While the carborane cages can be differently incorporated into siloxane polymers to influence their consistency, functional groups (phenyl, cyanoethyl, etc.) may also be varied to influence selectivity. 

Recent research activity in carborane chemistry has been directed toward expanding the use of carborane clusters in materials science including, among others, molecular recognition systems, display devices, modular construction systems, NLO materials and special polymers. Nonetheless, their use in developing functional materials such as olefin polymerization catalysts and luminescent materials is still limited. 

While the synthesis of a variety of single macrocycles containing 1,2- and 1,7-CaBioHio carboranylene linkers has been described, more recent work has focused upon functionalization of the carborane cages from which these hydrophobic species are comprised and the discovery of methods by which such molecules could be made water-soluble. Water solubility would place a hydrophobic cavity bounded by carborane cages in aqueous media and provide a means to support the solubilization of hydrophobic hydrocarbon or carborane-rich structures. Derivatization of B-vertices of carboracycles derived from 1,2- or l.T-CaBioHio carboranylene groups is easily accomplished by well established methods. ... 

PAR Parrott, M.C., Valliant, J.F., and Adronov, A., Thermally induced phase separation of carborane functionalized aliphatic polyester dendrimers in aqueous media,... 

An alternative approach explored functionalization of SWNT with substituted car-borane cages to develop a new delivery system for an efficient boron neutron capture therapy [54]. Indeed, these studies showed that some specific tissues contained carbor-ane following intravenous administration of the CNT conjugate and, more interestingly, that carborane was concentrated mainly at the tumor site. 

Kurkiewicz et al. have demonstrated that MQ NMR measurements of isotropic quadrupolar shifts are a simple way to probe nanosecond time-scale motions in solids. They have measured the B (spin 7= 3/2) shifts of the resolved boron sites in ort/jo-carborane as a function of temperature and have interpreted the results in terms of the known rapid tumbling dynamics. 

Synthesis and Characterization of Carborane Functionalized Dendronized Polymers as Potential Boron Neutron Capture Therapy Agents... 

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