Electrophilic fluorination reagents molecular fluorine

Scheme 2.86 Electrophilic fluorination reagents with an improved ratio of electrophilic fluorine to molecular weight. The respective ratios are ...

Most reactions with electrophilic fluorinating agents involve transformation of a carbon-hydrogen bond to a carbon-fluorine bond at a carbon atom that is either sp hybridised or that bears a negative charge. Selective electrophilic fluorination at a tertiary sp carbon atom can be achieved with molecular fluorine but is very difficult with N-F type reagents. 

The introduction of the halogens onto aromatic rings by electrophilic substitution is an important synthetic procedure. Chlorine and bromine are reactive toward aromatic hydrocarbons, but Lewis acid catalysts are normally needed to achieve desirable rates. Elemental fluorine reacts very exothermically and careful control of conditions is required. Molecular iodine can effect substitution only on very reactive aromatics, but a number of more reactive iodination reagents have been developed. 

As has been exemplified in this chapter, fluorinations with fluorine-18 can be classified into two categories (1) the nucleophilic reactions, which usually involve no-carrier-added [ F]fluoride of high-specific radioactivity as its Kf FIF-K complex and include substitutions in the aliphatic and the /lomoaromatic series and (2) the electrophilic reactions, which mainly use moderately low-specific radioactivity molecular [ F]fluorine, or other reagents prepared from it, such as acetyl [ F] hypofluorite, and include addition across double bonds, reactions with carbanions and especially fluorodehydrogenation and fluorodemetallation reactions. 

Whether fluorine can activate or deactivate an aromatic ring relative to hydrogen depends on the nature of the attacking electrophile. When the reagent is less reactive (late transition state) such as in molecular chlorinations and brominations [42, 43] (Table 4.6), resonance stabilisation of the Wheland-like transition state becomes far more important and so fluorine activates the system. On the other hand, nitration of fluoro-benzene is slower than the corresponding reaction with benzene. 

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