Anions A Review

RUSSIAN RESEARCH CENTRE KURCHATOV INSTITUTE, MOSCOW 123182, RUSSIA 

By the time the alkylation studies were started, the reactivity of the dicarbollide anions toward electrophilic agents had been studied mainly on insertion reactions of boron, or heteroatom, or transition metal into the place of the missing vertex of the icosahedron, restoring its structure. A broad area of metallocarboranes was developed as a result of these studies, which was the subject of many articles and reviews. 

It was found that 7,8- and 7,9-dicarbollide dianions, formed on abstraction of a bridging endo proton from the corresponding dicarba-nzrfo-undecaborate monoanion with sodium metal solution or sodium amide in liquid ammonia or butyl lithium in THF, very easily interact with alkyl halides to yield B-alkyl-7,8- and -7,9-dicarba-nz tfoundecabo-rates(l-).4,5 Formally the reaction can be represented by a simple scheme  

The alkylation reaction is very selective. It yields quantitatively pure B-monoalkyldi-carba-nn/o-undecaborate, independent from the excess of alkylating agent. The most active alkyl halides are alkyl iodides and then - bromides. For the alkylation to be complete, 5-10 percent excess of alkyl halide is usually used. And even greater excess makes no difference, since a monoanion formed does not react with RX, and dialkylation does not occur. As a parallel process there could be the proton abstraction from the formed alkylated monoanion with the dicarbollide that has not yet entered the alkylation reaction  

In order to avoid this possible side reaction and ensure its selectivity there was used either fast addition of alkyl halide or reversed order of reagent addition. The latter was used particularly with less active alkyl halides. 

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Skeletal Rearrangements following Electrophilic Alkylation of 7,8- and 7,9-Dicarbollide Anions (A Review)... 

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