Solvent effects on electrolytic partial fluorination of ␣-(2-benzoxazolylthio)-, ␣-(2-benzothiazolylthio)-, and ␣-(2-pyrimidylthio)acetates were investigated systematically using acetontrile (MeCN) and 1,2-dimethoxyethane (DME) containing Et 4 NFиnHF (n ϭ 3, 4) as a supporting electrolyte and a fluoride ion source. MeCN was not suitable for the fluorination of these heterocyclic sulfides since formation of nonconductive polymers on the anode took place to result in low yields or no fluorination of fluorinated products. In contrast, the use of DME did not cause such polymer film formation and provided much better or excellent fluorinated product yields. The superiority of DME can be explained in terms of its ability to solvate the cationic part of the fluoride salt in which a reactive fluoride ion is left for easy attack on the anodically generated cationic intermediate. It is noted that addition of DME into MeCN electrolytic solution markedly increased the product yield and current efficiency since the polymer film formation on the anode and overoxidation of the fluorinated products were effectively suppressed.
Electrochemical fluorination of 2-benzothiazolyl and 5-chloro-2-benzothiazolyl sulfides was successfully carried out using Et 4 NF‚3HF as a supporting electrolyte and fluoride ion source in dimethoxyethane to provide the corresponding R-monofluorinated sulfides in good yields. Fluorination took place selectively at the position R to the sulfur atom of the sulfides, and the heterocyclic moieties were not fluorinated at all.
Highly regioselective monofluorination of
1-aryl-3-(phenylthio)oxindoles and 2-substituted-3-oxo-4-(phenylthio)-1,2,3,4-tetrahydroisoquinolines can be successfully
carried out in the presence of
Me4NF·4HF or Et4NF·3HF as a supporting
electrolyte to provide the corresponding 3-fluorinated
oxindole and 4-fluorinated isoquinoline derivatives in good yields.
Carbon anodes as well as a
platinum anode were found to be effective for the fluorination when
Me4NF·4HF was used as the
supporting electrolyte.
Highly regioselective electrochemical fluorination of 2-pyrimidyl sulfides having an electronwithdrawing group (EWG) at the position R to the sulfur atom was successfully carried out using Et 4 NF‚nHF (n ) 3, 4) or Et 3 N‚3HF as a supporting electrolyte and a fluoride ion source in 1,2dimethoxyethane (DME) in an undivided cell. 2-Methylthiopyrimidine devoid of an EWG was also selectively fluorinated in DME to provide 2-(fluoromethylthio)pyrimidine in a moderate yield as 63%, while corresponding 2-methylthiopyridine was less selectively fluorinated in lower yield along with R,R-difluorinated product. In contrast, the corresponding 2-quinazolinonyl sulfides underwent similarly R-fluorination along with unexpected ipso-fluorination through anodic desulfurization.
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