Carboranes formation

Derivatives of decaborane are listed in Table V. Carborane formation from decaborane and acetylenes is summarized in Section III. 

The reaction of (fluoroalkenyl)carboranes with potassium permanganate in acetone leads to formation of a-diketones [119. These compounds react by photochemical reaction in which the radical formation at boron is followed by addition to the double bond [120] (equation 92). 

The formation of boron-group IB bonds succeeds in two ways by transfer of a boryl group from metal-boron compounds to other metals, and by reaction of anionic boranes or carboranes with transition-metal halides. 

Carboranes also offer the capacity for Co111—C bond formation. Perhaps the best-known example of a mixed cyclopentadienyl-metallocarborane sandwich complex is the cobalt(III) compound (Cp)Co(l,2-C2B9Hn), which serves as a precursor for other species.544... 

The first examples of DNA metallointercalators containing a 1,2- or 1,7-carborane thiolato ligand have been reported.349 For example, treatment of [Pt(OTf)(terpy)]+ (terpy = 2,2 6, 2 -terpyr-idine) with l-HSCH2-l,2-C2B10Hii leads to the formation of [Pt(l-SCH2-l,2-C2BioHn)(terpy)]+ (143). The DNA-binding properties of several complexes have also been described. 

The addition of o-carborane to electrophiles is one of the most important reactions to synthesize carboranes containing organic functional groups. Lithiocarbo-ranes which are readily prepared from butylithium with carboranes are widely utilized for C—C bond formation involving various functional groups. Recently, Yamamoto and co-workers have developed addition reactions of lithio-,73... 

During heat treatment between 250 and 500°C, the formation of three-dimensional networks by diene and addition reactions of Ph—C C, C=C, and m-carborane groups in this system has been characterized by nB MQ-MAS NMR and 13C and 29 Si CP-MAS NMR methods. 

This effect has also been observed in polyphosphazenes containing alkyl- or phenyl-carborane as pendent groups.12 A typical synthetic route to poly(phenyl-carboranyl-di-trifluoroethoxy-phosphazene) having pendent phenyl-carborane groups is shown in scheme 4. A substantial improvement in the thermal stability of the polymer was observed. This is attributed to a retardation of the ring-chain de-polymerization mechanism due to steric hindrance effects of the carborane units, inhibiting helical coil formation. 

An alternative route to poly(m-carborane-siloxane) rubbers is via the condensation reaction between w-carborane di-hydrocarbyl-disilanol and a bis-ureidosilane.20 This mild reaction allows the incorporation of desired groups into the polymer via both the dihydrocarbyl-disilanol and the bis-ureidosilane (see scheme 8). The first step involves the formation of the carborane silanol from the butyl lithium carborane derivative. The bis-ureidosilane is prepared from the phenyl isocyanate (see step 2), and the final step involves reacting the dihydrocarbyl-disilanol with bis-ureidosilane. 

The preparation of poly(m-carborane-siloxane) polymers has also been successfully achieved directly from the carborane monomer.22 The reaction used is shown in scheme 9. Here, the direct salt elimination reaction between dilithiocarborane and a dichlorosiloxane (e.g., 1,5-dichlorohexamethyltrisiloxane) results in the formation of linear polymers with a molecular-weight (M ) typically of 6800 dalton. However, the reported literature detailing this approach is very limited indeed, and the reaction has not found significant use. This is most probably because only relatively low molecular-weight polymers can be produced, ultimately restricting the flexibility to produce materials of controlled mechanical properties. 

The final class of polymers containing carboranyl units to be mentioned here is the polyphosphazenes. These polymers comprise a backbone of alternating phosphorous and nitrogen atoms with a high degree of torsional mobility that accounts for their low glass-transition temperatures (-60°C to -80°C). The introduction of phenyl-carboranyl units into a polyphosphazene polymer results in a substantial improvement in their overall thermal stability. This is believed to be due to the steric hindrance offered by the phenyl-carborane functionality that inhibits coil formation, thereby retarding the preferred thermodynamic pathway of cyclic compound formation (see scheme 12). 

When one considers the potential high-energy release on rupture of a carborane unit, together with the thermodynamic stability of combustion products, it is hardly surprising that there is a body of literature that reports on the use of carbo-ranes within propellant compositions. Their use in energetic applications is to be expected when the enthalpy of formation (AH/) data for the products of combustion for boron are compared to those of carbon. Thermodynamic data for the enthalpy of formation of o-carborane and of typical boron and carbon combustion products is shown in Table 4. Measurements of the standard enthalpy of combustion32 for crystalline samples of ortho-carborane show that complete combustion is a highly exothermic reaction, AH = — 8994 KJmol. ... 

Table 4 Enthalpy of Formation for o-Carborane and Combustion Products...

Scheme 13 Formation of nitrato-carborane via dissolution in nitric acid.

Figure 9 Alternative C-C bond formation in ortho-carboranes.

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