Carborate anion

Direct bridgehead alkynylation of 1-iodoadamantane has been achieved by treatment with a variety of silver acetylides at reflux in IV-methylmorpholine.114 The methodology has been extended to the direct bridgehead substitution of methyl-bicyclo[2.2.2]octane and a carborate anion (Scheme 1.50).115... 

WEAKLY COORDINATING ANIONS - CARBORATES, FLUOROALKOXYALUMINATES AND FLUOROALKOXYBORATES... 

Many of the anions used for ionic liquids belong to the group of weakly coordinating anions as they show a minimal interaction with the cation [30]. The most weakly coordinating anions such as carborates, dodecaborates or (fluoroalkoxy)aluminates are usually used to stabilise reactive cations [31-33]. Taking this into consideration, it appeared logical to test these types... 

It was already pointed out in the introductory remarks that non-nucleophilic reaction conditions are one prerequisite for the synthesis of silylium ions in the condensed phase. The use of weakly coordinating anions is particularly decisive. Halogenated closo-carborate monoanions and - more recently - closo-borate dianions have found the most widespread use besides fluorinated tetraarylborates. While with bulky substituents these anions form silylium ions salts with well-separated cations and borate anions [10, 20, 22, 42], silylium ions with smaller alkyl substituents tend to coordinate directly to the anion forming zwitterionic compounds [7, 31, 32, 47]. For example, upon reaction of trimethylsUane with trityl-cZoso-carborate 86 in excess trimethylsUane the trimethylsUylium carborate 87 is formed (Scheme 27) [44]. 

A structural study for 87 and several related halogenated trialkylsUylium carborates and borates reveals in each case a close contact between one halogen atom of the anion and the silicon atom, which leads to a distorted tetrahedral coordination environment for the silicon atom [7, 9, 31, 32, 47]. The molecular stmcture of sUylium carborate 87 serves here as a typical example for this type of compounds. It is characterized by a long Si-F bond (190.2 pm, see Fig. 15a), which... 

---