Generating Carbocations in Superacids Systems

In contrast to the rather well-defined trivalent ( classical ) carbenium ions, nonclassical ions 26 have been more loosely defined. In recent years, a lively controversy centered on the classical-nonclassical ion problem.27-37 The extensive use of dotted lines in writing carbonium ion structures has been (rightly) criticized by Brown, 31 who carried, however, the criticism to question the existence of any o-delocalized (nonclassical) ion. For these ions, if they exist, he stated ... a new bonding concept not yet established in carbon structures is required.  

unequivocal experimental evidence has by now been obtained for nonclassical ions such as the norbomyl cation.38 10 The bonding concept required to define nonclassical ions is simply to consider them as penta(or higher)-coordinated carbonium ions involving at least one two-electron three-center (or multicenter) bond, of which CH5+ (themethoniumion-carbonium ion) is the parent, asCH3+ (the methenium ion, methyl cation, carbenium ion) is the parent for trivalent carbenium ions. An example of a hexacoordinate carbonium ion is the pyramidal dication of Hogeveen.41 

Methods for the generation of very reactive carbocations have been developed most notably by Ahlberg45 and Saunders et al.46 The former group describes an ion-generation apparatus that consists of a Schlenk tube-like vessel attached to an NMR tube in which the carbocation is prepared at low temperatures. This methodology has been further improved.47,48 

Kelly and Brown51 describe a method for the preparation of concentrated solutions of carbocations ( 1 M) in SbFs-S02ClF. This method, which employs a syringe technique, allows quantitative conversion of precursors soluble in SO2CIF at —78°C into the corresponding carbocations. 

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