Difluorinated carbocation

It is interesting to see completely different regiochemistries in nucleophilic and electrophilic reactions to 1,1-difluoroethene as a model. Scheme 1.55 shows a typical electronic effect of the fluorine atom as a substituent. Electrophiles mostly attack the (3-carbon of 1,1-difluoroethene generating a-difluorinated carbocations as intermediates in contrast to regioselective nucleophilic additions on the a-carbon of 1,1-difluoroethene generating (3-fluorocarbanions. 

The thallium trinitrate-mediated ring contraction of frani-decal-2-ones has opened up a new route to the hydrindane system, and fluorinative ring contraction of cyclic alkenes to afford difluorocycloalkanes has been induced by iodotoluene difluoride and EtsN-HF. A possible mechanism is shown in Scheme 78. The double bond of the cyclohexene ring is attacked by iodotoluene difluoride activated by HF from the axial direction, followed by the addition of a fluoride ion from the trans direction. Reductive elimination of iodotoluene from the resulting adduct, ring contraction and the addition of the fluoride ion to the carbocation stabilized by fluorine then take place to give the ring-contracted difluorinated product. 

The selective monofluorination of difluorinated alkanes with cobalt(III) fluoride at 25 C to give trifluoroalkanes 10 has been reported. Fluorinations with cobalt(lll) fluoride are considered to proceed via carbocations and the selectivity of the fluorination is explained by the relative stabilities of the intermediate carbocations. Although treatment of alkanes with chlorine trifluoride results in fluorination. it is of little practical use due to over-fluorinalion. chlorination and in some cases isomeri7ation of the starting alkane. ... 

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