Although the precise mechanism of electrophilic fluorination is currently unclear, highly efficient and stereoselective methods have been developed.
Some common fluorinating agents used for organic synthesis are N-fluoro-o-benzenedisulfonimide (NFOBS), N-fluorobenzenesulfonimide (NFSI), and Selectfluor.
[2] Additionally, radical probe experiments with 5-hexenyl and cyclopropyl enol ethers did not give any rearranged products.
N-fluoroammonium salts of cinchona alkaloids represent the state of the art for reactions of this type.
Although N-fluorosulfonamides are fairly weak fluorinating reagents, N-fluorosulfonimides, such as N-fluorobenzenesulfonimide (NFSI), are very effective and in common use.
The counteranions of these salts, although they are not directly involved in the transfer of fluorine to the substrate, influence reactivity in subtle ways and may be adjusted using a variety of methods.
[13] Similar to other alkenes, cohalogenation can be accomplished either by isolation of the intermediate adduct and reaction with a nucleophile or direct displacement of DABCO in situ.
Metal enolates are compatible with many fluorinating reagents, including NFSI, NFOBS, and sulfonamides.
However, the specialized reagent 2-fluoro-3,3-dimethyl-2,3-dihydrobenzo[d]isothiazole 1,1-dioxide consistently affords better yields of monofluorinated carbonyl compounds in reactions with lithium enolates.
Proton sources or Lewis acids are required to suppress radical formation, and even when these reagents are present, only certain substrates react with high selectivity.
[16] However, difficulties associated with handling and their extreme oxidizing power have led to their replacement with N-F reagents.