Selective anodic fluorination

Anodic fluorination of a-phenylthioacetate 1 in ionic liquid Et3N 3HF proceeds smoothly without anode passivation under ultrasonication to provide the a-mono-fluorinated product 2 in high yield and with high current efficiency. Notably, anodic difluorination of 1 can be also achieved in the same ionic liquid under ultrasonication as shown in Scheme 8.4 [15]. Thus the current efficincy for the anodic difluorination of 1 was greatly improved as compared with that without ultrasonication. As described above, Et3N 3HF ionic liquid is highly useful for selective anodic fluorination. 

As mentioned above, liquid fluoride salts like EtaN nHF (n = 3-5) and Et4NF-4HF have proved to be highly useful as the electrolytic media and fluoride ion source for selective anodic fluorination. However, this solvent-free method has an atom economy problem because of the use of an excessive amount of liquid salts in place of a solvent. 

Selective anodic fluorination of aldehyde and cyclic ketones can be successfully carried out in Et3N-5HF to give the corresponding acyl fluorides and fluoroacyl fluorides, respectively, in good yields, as shown in Eqs. (10) and (11) [41]. In these reactions, fluoride salts such as Et3N-3HF lead to lower yields, reflecting the fact that this salt is discharged prior to oxidation of the carbonyl compounds. 

Though partially fluorinated heterocycles are focus of much biological interest, limited examples of selective anodic fluorination of heterocycles have been reported [Eqs. (23-27)] [62-66]. These processes are limited to nitrogen- and oxygen-containing heterocycles, and the yields are generally quite low. In addition, no successful anodic fluorination of sulfur-containing heterocycles has been reported up to 1991. 

Fuchigami and coworkers have developed conditions for highly selective anodic fluorination reactions of various heterocyclic compounds including sulfur-containing heterocycles, as discussed later [3,6]. 

Selective anodic fluorination of dithioacetal (Et4NBr + EtsN.SHF in CH2CI2)... 

Early studies in electrofluorination were conducted by fluorination of chiral benzylic esters at a platinum anode in EtaN x 3HF/CH3CN moderate diastereomeric excesses up to 60% were recorded (Scheme 44.29). The diastereo-selective anodic fluorination of A -protected thiazolidines derived from L-cysteine or sulfides gave the corresponding fluoro derivatives in a Pummerer-type reaction mechanism in moderate yields and high to very high diastereoselectiv-ities, up to 99% (Scheme 44.30). ... 

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