Carborane ring

Scheme 3 Synthesis of the electron-precise carborane ring.

Acetylation of the benzene nuclei of a number of l-phenyldicarba-cto o-dodecabor-anes [e.g. (21)] by acetyl chloride in the presence of ttifluoromethanesulfonic acid gave mainly para-acylated products in spite of the strongly electron-withdrawing effect of the carborane skeleton. Substitution of a methyl group at the 2-position of the carborane ring caused acetylation mainly at the meta position. 

Potential areas of apphcation for carboranes include adhesives, gaskets, and O-rings (27). Research has been reported on improving the oxidation resistance and strength of carbon materials by coating with a carborane siloxane polymer (36). 

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). 

When 1,2-bis(chlorodimethylsilyl)carborane 2 was treated with ammonia, an amminolysis reaction occurred to give the cyclic tetramethyldisilaazane 3. The ease with which this reaction occurs to form five-membered ring bears notice. Hydrolysis of 2 gives cyclic siloxane compound 4. Utilization of the same general... 

A novel cyclooctaphane (94) (Fig. 57) was assembled by the condensation reaction between the C,C -dicopper(I) derivative of m-carborane and l,2-bis(4-iodophenyl)-o-carborane.124 The macrocycle was found to adopt a butterfly (dihedral angle 143°) confirmation with the o-carborane units at the wingtips and the phenylene ring planes roughly perpendicular to the wing planes of the confirmation. 

Many other carboracycles and carborands, another group of organic ring systems, have been reported and are discussed in the section on carborane supramolec-ular chemistry later in this chapter. 

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. 

Bipyridylnickel(II) o-carborane-l,2-dicarboxylate eliminated two carbon dioxide molecules in refluxing decane and formed a complex with a QNi three-membered ring [Eq. (99), M = Ni] (107). Both carbon dioxide... 

These figures are based on published and submitted papers. Among others, carboranes, donor-acceptor complexes of boron, ring molecules with silicon, phosphoranes, sul-fones have received much attention. The lack of gas-phase structural studied of other classes of compounds, e. g., sulfuranes, is also noteworthy. The above elements were selected as most typical. Much interest is concentrated however on other elements as well, which may have been somewhat neglected in the past. The increase of the amount of structural data is also facilitating demand for further extension of the circle of compounds studied. 

The halogen atoms remaining can then be replaced by organic residues such as trifluoroethoxy units. High polymers can also be prepared by ring-opening polymerization of the chlorocyclo-phosphazene, XXVIII. Compounds of this type can be converted to nldo-carboranes in the presence of base, but these do not form metallo-derivatlves, presumably for sterlc reasons (29). 

However, separation of the carborane cage from The phos-phazene ring or chain by a methylene spacer group allows metals to be Inserted into the open face of the carborane. These syntheses were accomplished by the reaction routes shown in Schemes 3 and A. High polymeric analogues of these transformations have also been accomplished following polymerization of XXX. The rhodium-bound cyclophosphazenes and polyphosphazenes are catalysts for the hydrogenation of 1-hexene. In this, they show a similar behavior to metallocarboranes linked to polystyrene... 

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