Electrolytic Partial Fluorination of Organic Compounds XL. Solvent Effects on Anodic Fluorination of Heterocyclic Sulfides

Abstract

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.

Unique solvent effects on selective electrochemical fluorination of organic compounds

“…In a solution such as DME, Et 4 NFÁ4HF may dissociate to Et 4 N + , F(HF)n À (n = 1-3), and HF [30][31][32][33]. Moreover, it was also found that DME was oxidized simultaneously during the electrolysis of substrates, which can suppress overoxidation of both the substrates and products, particularly at a later stage of the electrolysis [27].…”

“…On the other hand, we recently found that ethereal solvents like 1,2-dimethoxyethane (DME) were much more suitable than MeCN for the anodic fluorination of various heterocyclic sulfides as shown in Schemes 8 and 9 [27][28][29]. The use of MeCN results in extremely low yields or no formation of fluorinated products, while the use of DME markedly increases the yields.…”

Electrochemical partial fluorination of organic compounds: 81. Regioselective anodic fluorination of O-methyl S-aralkyl thiocarbonates

“…The yield of 2a also decreased when the supporting electrolyte Et 3 N-5HF was used comparing with the case of Et 4 NF-4HF (Table 2, Run 10). It is known that dimethoxyethane (DME) is an excellent electrolytic solvent for anodic fluorination in some cases [20]. However, DME was not effective for the anodic fluorination of 1a regardless of supporting salt since DME is more easily oxidized than 1a.…”

“…This can be explained as follows. Since DME is more easily oxidized than CH 3 CN, DME was oxidized simultaneously during the electrolysis of the substrate 1b, which can suppress further oxidation of the product 2b, particularly at a later stage of the electrolysis [20]. However, a larger excess amount of electricity was required to obtain higher yield of 2b in DME due to the competitive anodic oxidation of DME (Runs 6 and 8).…”

Electrolytic partial fluorination of organic compounds

“…We have systematically studied anodic monofluorination at the side chain of various heterocyclic compounds [7][8][9][10]. For example, selective anodic fluorination at the side chain of various heterocyclic compounds such as six-membered (2-pyridyl-and 4-pyrimidyl-), and five-membered (2-thiadiazolyl, 2-oxadiazolyl and 2-triazolyl) heterocycles was comparatively studied (Schemes 1 and 2) [11][12][13][14][15][16]. Thus, the active methylenethio group attached to various heterocyclic rings is selectively fluorinated to give the corresponding a-fluorinated products in moderate to good yields.…”