Carborane

Die Kohlenstoffatome nehmen immer Postionen niedrigster Konnektivitat ein. 

Enthalt eine Verbindung mehr als ein Kohlenstoffatom, so ist das thermodynamisch stabilste Pro-dukt jenes, in dem die Kohlenstoffatome so weit wie moglich voneinander entfemt sind. 

Umlagerungen in Carboranen Carborane unterliegen folgenden Umlageningen, 

Wahrend dieser Reaktion geht ein Ikosaeder iiber einen kuboktaedrischen Ubergangszustand in einen anderen Ikosaeder iiber ( Diamond-Square-Diamond -Mechanismus). 

Mit Hilfe des gezeigten Mechanismus kann man Umlagerungen von 1,2- zu 1,7-Carboranen verste-hen, aber er erklart nicht die Umformung von 1,7- zu 1,12-Carboranen. Dafiir wurden weitaus kompliziertere Mechanismen entwickelt, auf die an dieser Stelle nicht naher eingegangen wird. 

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Allylation of the 10-carborane 236 (pKa = 18-22) with diallyl carbonate is possible under neutral conditions to give 237[146], Allylation and rearrangement of the trialkylalkynylborane 238 affords the trisubstituted alkene 239 stereoselectively [ 147],... 

Placement of Extra Hydrogens. The placement of extra hydrogens plays a cmcial role ia determining the stmctures adopted by boranes and carboranes. However, the exact position of extra hydrogens sometimes depends on the physical state of the molecule, eg, the... 

G in the presence of a catalytic amount of a Lewis base such as dimethylether, (GH2)20. In addition to the gas-phase pyrolysis of diborane, can be prepared by a solution-phase process developed at Union Garbide Gorp. Decaborane is a key intermediate in the preparation of many carboranes and metaHa derivatives. As of this writing, this important compound is not manufactured on a large scale in the western world and is in short supply. Prices for decaborane in 1991 were up to 10,000/kg. 

Garboranes. The term carborane is widely used in American Hterature as a contraction of the lUPAC approved nomenclature carbaborane. The first carboranes, isomers of C2B2H, and were prepared in the mid-1950s at Olin Mathiesen. These carboranes were obtained as a mixture... 

The discovery (116) of the base-promoted degradation of the isomeric closo-Q,. . cages provided one of the most important carborane anion systems, the isomeric nido-Q,. ][ , anions,... 

Aside from their extensive use in metaHacarborane chemistry, the dicarboUide anions are important intermediates in the synthesis of other carborane compounds. For example, aqueous ferric chloride oxidation of the 1 anion results in the 10-vertex cage nido- b ()-(Z, 2 (H8) and the... 

Nonicosahedral carboranes can be prepared from the icosahedral species by similar degradation procedures or by reactions between boranes such as B H q and B H with acetylenes. The degradative reactions for intermediate C2B H 2 species (n = 6-9) have been described in detail (119). The small closo-Qr Yi 2 species (n = 3-5 are obtained by the direct thermal reaction (500—600°C) of B H using acetylene in a continuous-flow system. The combined yields approach 70% and the product distribution is around 5 5 1 of 2,4-C2B3H2 [20693-69-0] to l,6-C2B Hg [20693-67-8] to 1,5-C2B3H3 [20693-66-7] (120). A similar reaction (eq. 60) employing base catalysts, such as 2 6-dimethylpyridine at ambient temperature gives nido-2 >-(Z, ... 

As with the simple boranes, the closo carboranes are generally more thermally stable than the corresponding nido and arachno species. Thermal decomposition of nido and arachno carboranes often leads to one or more closo carborane. For example, pyrolysis of 2,3-C2B4Hg is another route to 2,3-C2B3H2 [30347-95-6], l,2-C2B4Hg [20693-68-9] and l,6-C2B4Hg [20693-67-8], and 1,5-C2B3H3 [20693-66-7] (123). 

A readily accessible carborane is nido-1 -(]5 i])- l- 1( 12 [I d-5], a 2witterionic species formally derived from [CB2qH23] by replacement of a by NH3. It has been shown (124) that this monocarbaborane can be obtained in excellent yield by treatment of B2QH24 with CN followed by passage through an acidic ion-exchange column (eq. 61). 

The related mono-A/-alkylated carboranes, 7-(NH2R)-7-CB2qH22, can be prepared by treatment of decaborane(14) with alkyl isocyanides (125). 

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