Selective anodic fluorination monofluorination

Generally, anodic benzylic substitution reactions take place readily. However, anodic benzylic fluorination does not always occur. The major competitive reaction is acet-amidation when MeCN is used as a solvent [24-26]. In contrast to these cases, triphenylmethane is selectively monofluorinated in 80% yield [27]. When Et4NF-/7HF n = 3,4,5) is used in the absence of MeCN, anodic fluorination of toluene and mono-fluoromethylbenzene provides mainly mono- and difluoromethylbenzenes, respectively [22]. On the other hand, difluoromethylbenzene affords only products fluorinated in the benzene ring [Eq. (4)] [28]. 

Fuchigami T, Konno A, Nakagawa K, Shimojo M (1994) Electrolytic partial fluorination of organic compounds. 12. Selective anodic monofluorination of fluoroalkyl and alkyl sulfides. J Org Chem... 

Anodic monofluorination of the side chain of various heterocyclic compounds has been systematically studied [82-84]. The active methylenethio group attached to heterocycles is selectively fluorinated to give the corresponding a-fluorinated products in moderate to good yields [Eqs. (36) and (37)] [82-87]. In these reactions, the choice of solvent and supporting fluoride salt is of great importance in order to achieve efficient fluorination reaction [84-87]. 

Anodic monofluorination of 4,4-dimethyl-2-ethoxycarbonyl-3-thiolanones 49 proceeds smoothly without passivation of the anode in EtjN-SHF/MeCN to provide the corresponding 6-fluorinated products 50 in good yields (Scheme 38). When R in 49 is hydrogen, fluorination takes place at the 2-position selectively. 

Highly regioselective anodic monofluorination of 2-aryl-4-thia-zolidinones 53 can also be promoted by using pulse electrolysis in EtjN-SHF/MeCN as shown in Scheme 41. In this case, benzylic fluorination does not take place and fluorination occurs at the 5-position selectively. The regioselectivity of this process appears also to be controlled kinetically as shown in Scheme 40. The transformation of the sulfones 55 derived from fluorinated products 54 into monofluoro P-lactams 56 is readily performed in excellent yields by thermolysis (Scheme 41). 

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