Boron carborane rearrangement

The C2B10H12 molecule is the most stable carborane and takes its common name from the name of the entire class. Only three positional isomers are possible, 1,2- 1,7-, and 1,12-, for which the notations of ortho, meta, and para (o-, m-, P-) are usually used. The least stable isomer (o-carborane) rearranges to m-carborane at a temperature of 450 °C (an activation barrier of 260 kJ mol has been reported for this process), while /n-carborane rearranges to p-carborane at the higher temperature of 600 °C. Various mechanism have been suggested (Table 5). In addition, at about 350 °C there is a process that scrambles boron vertices in o-carborane but not in m-carborane. 

Interestingly, depending on the catalyst, the diboration of vinylboranes with B2cat2 leads via seqnential diboration and dehydroboration steps either to 1,1,1-triborylalkanes (20) or to 1,1,2-triborylalkanes (21). 1,1,1-Triborylalkanes, 1,1,2,2-, and 1,1,1,2-tetraborylethane species have been proposed as intermediates in the reaction of borylated alkynes with excess Et2BH ( hydride bath ) that eventnally leads to the formation of small carboranes. Indeed, isolated triboryhnethane species with alkyl or hahde snbstituents on boron have recently been shown to rearrange to small carboranes upon thermal treatment. ... 

Rearrangements are well known in the carborane family. The initial carborane product from the reaction of an acetylene and boron hydride is often a carbon cage with adjacent carbon atoms. The carbon-adjacent form is often a kinetic product and will rearrange to a more stable form if heated sufficiently. The initially formed closo carboranes and the more stable thermodynamic products are listed in Table 4. 

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