Electrophilic fluorination

Electrophilic fluorination is the combination of a carbon-centered nucleophile with an electrophilic source of fluorine to afford organofluorine compounds. Although elemental fluorine and reagents incorporating an oxygen-fluorine bond can be used for this purpose, they have largely been replaced by reagents containing a nitrogen-fluorine bond.

Electrophilic fluorination offers an alternative to nucleophilic fluorination methods employing alkali or ammonium fluorides and methods employing sulfur fluorides for the preparation of organofluorine compounds. Development of electrophilic fluorination reagents has always focused on removing electron density from the atom attached to fluorine; however, compounds containing nitrogen-fluorine bonds have proven to be the most economical, stable, and safe electrophilic fluorinating agents. Electrophilic N-F reagents are either neutral or cationic and may possess either sp²- or sp³-hybridized nitrogen. Although the precise mechanism of electrophilic fluorination is currently unclear, highly efficient and stereoselective methods have been developed.

The most common fluorinating agents used for organic synthesis are N-fluoro-o-benzenedisulfonimide (NFOBS), N-fluorobenzenesulfonimide (NFSI), and Selectfluor.

The mechanism of electrophilic fluorination remains controversial. At issue is whether the reaction proceeds via an S_N2 or single-electron transfer (SET) process. In support of the S_N2 mechanism, aryl Grignard reagents and aryllithiums give similar yields of fluorobenzene in combination with N-fluoro-o-benzenedisulfonimide (NFOBS), even though the tendencies of these reagents to participate in SET processes differ substantially. Additionally, radical probe experiments with 5-hexenyl and cyclopropyl enol ethers did not give any rearranged products.

On the other hand, the lifetime of radicals in the SET process is predicted to be four orders of magnitude shorter than the detection limit of even the most sensitive of radical probes. It has been postulated that after electron transfer, immediate recombination of the fluorine radical with the alkyl radical takes place.

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