Tetraarylborate anions

The development of cationic catalysts necessitated the development of unreactive counteranions, which typically had been PFg and BF4. Unfortunately, a F can be transferred from either of these counteranions to a highly reactive cationic catalyst [102]. An initial response was to use the tetraphenylborate as a counterior, which eliminated the nucleophilic F" problem, but had the problem of electrophilic addition to the aromatic ring [103]. A solution to these problems is fluorinated tetraarylborates that prevent nucleophilic attack of the F anion as well as electrophilic addition to the aromatic ring [104-107]. Structures 1 and 2 [105], are some examples of these counteranions. 

As counteranions only very robust and extremely weakly coordinating anions [43] such as fluorinated tetraarylborates of the type [B(C6F5 R )4] (R = SiR a, H,... 

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]. 

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