Silyl carborane anions

Another milestone discovery in the held of silyl cations chemistry was achieved by Reed and co-workers in the same year, 1993, when Lambert published his Et3Si study. Reed synthesized his t-PrjSi (CBuHsBrg), (2+ (CBnH5Br5), by the hydride transfer reaction of /-PrjSiH and Ph3C (CBnH6Br6) in toluene,taking advantage of the very low nucleophilicity of the carborane anion" (Scheme 2.9). 

In a series of papers Reed and colleagues present the solid state structures of a series of simple trialkyl-substituted silyl cations with halogenated carborane anions... 

In 2008, protonation of 2,4,6-tri-ter-butylphosphinine 61 (as well as the methylation and silylation of 1,3,5 triphosphabenzene) was achieved by using the toluenium salt of a carborane anion [40], The X-ray crystal structure of the corresponding phosphininium salt 62 has been recorded and it was shown that, upon protonation, the phosphorus atom gains an hypervalent character (Scheme 14). 

Much research has been carried out in many contexts to create anions with low nucleophilicity. In the specific context of low valent silyl cations, the anion families that have proved most successful have been perfluoroaromatics introduced by Lambert and Zhang and carboranes introduced by Reed and coworkers, reported in adjacent publications in 1993. Prior to use of these anions, unsuccessful investigations were reported with anions such as perchlorate, tetrafluoroborate, tetraphenylborate, and hexachloroantimonate. All successful preparations of stannylium cations have used perfluoroaromatic anions, particularly tetrakis(pentafluorophenyl)borate (TPFPB). 

Use of extremely weakly coordinating anions (including several based on carboranes, whose structures we will not discuss for reasons of space), hereafter simply referred to as [WCA] , allows the synthesis of even relatively unhindered silyl cations, such as the triethylsilyl cation ... 

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