Ortho-carborane

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. ... 

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

The research field of ortho-carborane chemistry continues to expand rapidly mainly due to the commercial availability of ort/fo-carborane itself and the recent interest of these derivatives as agents for BNCT and other applications. Synthesis of these carboranes is still dominated by reactions of decaborane with various alkynes as shown in Table 19 (Figure 2). 3-Substituted derivatives are made by capping the /[Pg.71]

Table 19 Synthetic routes to c/oso-1, 2-C2BioH- 2 (ortho-carborane) derivatives from decaborane and alkynes...

Table 20 Synthetic routes to closo-1,2-C2B10H12 (ortho-carborane) derivatives from 7,8-C2B9H12 anions...

Table 21 C-H to C-X and B-H to B-X conversions in closo-1,2-C2B10H-i2 (ortho-carborane) derivatives... 

Commonly P - Cl bonds enter rapid reactions with - or N -nucleophiles. However, this could not be found for ortho-carborane (12) derivatives where the carbon atoms carry P - Cl moieties, see Fig. 5. Even in the presence of methanol and triethylamine as a base, low yields and rates were observed (162). The carborane group can be excluded as cause of the observation because the alcoholysis was found for meto-carboranes or mono-substituted ortho-carboranes. Therefore, a mechanistic study was carried out to reveal the agent of the unexpected experimental observation. 

Fig. 5. Illustration of the inhibited methanolysis of P - Cl bounds observed for ortho-carborane (12) derivatives.

The acetylene may be unsubstituted (R = H) or substituted, in which case the reaction proceeds even more readily. The resulting compound is known as 1.2-dicarbtw/aY -dodecaborane(l2), or the ortho carborane, and is isoelectronic and isostructur.il with [BI2HI2]2-. It is stable to both heat and air, but it isomerizes at high temperatures to the 1,7 ( meta or neo isomer) and the 1,12 ( para isomer) (Fig. 16.51). The mechanism of isomerization, thought to be intramolecular, has been discussed for many years.133... 

The Step 3 product (4.00 mmol) was dissolved in 100 ml of THF and then cooled to —78°C and treated with 2.7ml of n-BuLi (4.32mmol 1.6M in hexane). It was stirred for 30 minutes, followed by 4 additional hours of stirring at ambient temperature, and then concentrated. The residue was washed twice with 15 ml of n-hexane, dissolved in 150 ml of THF containing 1% Merrifield s resin (2.0 g 3.94 mmol Cl), and stirred at ambient temperature for two days. The mixture was refluxed for 4 hours and then quenched with 3.0 ml of methanol and re-concentrated. The residue was washed twice with 10 ml of deionized water and twice with 20 ml of n-hexane. The solid was dried, and 2.40 g polystyrene-supported ortho-carborane were isolated as a pale yellow solid. 

TABLE 1. Effectiveness of polystyrene-supported zirconinm-ortho-carborane as a polymerization catalyst for selected monomers. 

In the presence of air at 20°C, the reaction of the iminophosphorane 5 with ortho carborane 1 resulted in deboronation to yield the H2NP(NMe2)3+ salt 6 of the nido anion 7,8-C2B9Hi2 and boric acid.7... 

Through this study we suggest a useful synthetic procedure for good candidate compounds of a new cage-cluster. The ort/io-carboranes are very stable chemicals in this research. Carboranyl acetic ester can be easily introduced to the ortho- carborane skeleton and then many types of cyclisation of carboranyl cluster can be made to occur. The reactivity and reaction mechanism for cyclisation of carborane cluster have been discussed. The mechanisms for boron cluster expansion reaction and cyclisation at carboranyl edge are summarized as follows. 

The C-allylation to ortho-carborane is accelerated by palladium catalysts. 

C-allylation is dependent on the ligand of palladium catalyst which assists the reaction of C-allylation to ortho-carborane. 

The solvent effect is high in the case of C-allylation to ortho-carborane cluster. In non-polar solvents the C-allylation is accelerated, while in polar solvents the C-allylation is decelerated. 

In the presence of Pd (OAc >2, the cyclisation at carboranyl edge occurs effectively through the reaction of ortho-carborane cluster with diazomethyl ethyl ketone as shown in Scheme 4. 

However, totally different reaction products are formed when ortho-carborane cluster reacts with phenyl diazomethane in the presence of Pd(OAc>2 and without any other irradiation as shown in Scheme 5. 

The complex Cp 2Yb forms complexes with ortho-, meta-, and 1,2-dimethyl-nrt o-carborane. The sterically and electronically unsaturated ytterbium center in this complex is expected to coordinate any molecule with a permanent dipole moment. The dipole moment of ort o-carborane is 4.31 D. When ortho-carborane and Cp 2Yb are mixed in hexane, an immediate precipitation of a green solid is observed. The solid can be recrystallized from hot toluene to give dark green needles in good yield. This complex shows thermochromism at lower temperatures the complex is green and turns reversibly orange at 130 °C. Carboranes with smaller dipole moments like meta-catbotanc and 2-dimethyl-ort o-carborane also form complexes with Cp 2Yb, which are more soluble and can be crystallized from hexane at —25 °C.517... 

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... 

The breakthrough came with the discovery that the nido anion C 2B<)1 In p (an icosahedral fragment prepared by removal of a BI P unit from ortho-carborane l,2-C2BioHi2, by means of its reaction with sodium methoxide in methanol) could coordinate strongly to transition metal cations in a manner similar to the cyclopentadienide anion, (11 (Scheme 3.7)11.40,198-202... 

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