Reactivity of o-Carborane

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 

It is known that a carborane framework involves three-center two-electron bonding and is thus an elelctron-deficient cluster. Shatenshtein76 reported that the pKa value of the C—H proton of carborane is 23. Therefore, a proton attached to the carbon of o-carborane could be deprotonated easily by a weak base, and the resulting 

SCHEME 1. Mechanism for the palladium catalyzed carborany lation of aldehydes. 

The connection of the three-dimensional carboranes and classical aromatic compounds, the two-dimensional benzenes, has been a fertile area for exploration. 

2- Dehydrobenzene or benzyne 158 can be trapped by all manner of species. 1,2-Dehydro-o-carborane 159 has been shown to undergo many of the same reactions as its two-dimensional relative, 1,2-dehydrobenzene. Although dehydroaromatic molecules can be formed in a variety of ways, synthetic pathways to 1,2-dehydro-o-carborane are quite limited. An effective procedure reported so far78 first forms the dianion by deprotonation of o-carborane with 2 equiv. of butyllithium. Precipitated dilithium carborane is then treated with 1 equiv. of bromine at 0°C to form the soluble bromo anion 160. Thermolysis of 160 with anthracene, furan, and thiophene as substrates leads to the adducts 161-164.79 80 1,2-Dehydro-o-carborane reacts with norbomadiene to give both homo 2+4 and 2+2 addition, leading to three products 165-167, in a 7 1 ratio79. An acyclic diene, 2,3-dimethyl- 

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