Fluonnated carbocations

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. 

P-Fluonne or fluorine further removed from the cation center always inductively destabilizes carbocations [115, 116] No simple P-fluoroalkyl cations have been observed in either the gas phase or solution, and unlike the cases of the other halogens, there is no evidence for formation of alkyl fluoronium ions (5) in solution [117, 118], although (CH3)2F+ is long-lived in the gas phase [7791 The only P-fluonnated cations observed m solution are those that benefit from additional conjugativc stabilization, such as a-trifluoromethylbenzyl cations [772] and per-fluonnated allyl [729], cyclopropenium [772], and tropyliiim [727] ions... 

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