Relatively stable fluorinated carbocations

The now-classic technique pioneered by Nobel Laureate George Olah and co-workers [52, 53] for preparing relatively stable long-lived carbocations, and their direct observation in solution by NMR, has been applied to the study of a number of classes of fluorinated carbocationic species [52-55], including alkyl, aryl, allyl and tropylium cations (Table 4.9). 

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NMR spectroscopy is ideal for detecting charged fluorinated intermediates and has been applied to the study of increasingly stable carbocation and carbanion species. Olah [164, 165] has generated stable fluorocarbocations m SbFj/SOjClF at low temperatures The relatively long-lived perfluoro-rerr-butyl anion has been prepared as both the cesium and tris(dimethylamino)sulfonium (TAS) salts by several groups [166, 167, 168], Chemical shifts of fluonnated carbocations and carbanions are listed m Table 23. 

The difluorobenzyl cation (Figure 4.20) is stable under conditions in which the benzyl cation undergoes rapid polymerisation [57] and and NMR studies [58] have shown that, as the electron demand of the aromatic ring increases (i.e. R is electron-withdrawing), the TT(p-p) donation of fluorine to the carbocation centre also increases, leaving the electron density at relatively constant for a variety of aromatic substituents. 

A computational study was undertaken to evaluate the stabilizing effects of a-sulfur and a-fluorine groups at carbocation centers. This was done by calculating the relative energies for protonation, methylation, and bromination of HFC = CH(SMe) - comparing the regioisomers of electrophilic attack. In general, the a-SMe carbocations were found to be considerably more stable. 

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