Kunitaka Momota

Electrochemical fluorination of aromatic compounds in liquid R4NF·mHF—part I. Basic properties of R4NF·mHF and the fluorination o

Abstract A series of new electrolytes, R 4 NF· m HF (R: CH 3 , C 2 H 5 and n -C 3 H 7 , m > 3.5), has been utilized for electrochemical fluorination of organic compounds. These electrolytes are non-viscous liquids at room temperature, and have high electrolytic conductivity and high electrochemical stability. The electrolysis gave high current densities and high current efficiencies for the fluorination without any film formation at the anode. Benzene, fluorobenzene and 1,4-difluorobenzene were electrochemically fluorinated on a platinum anode at 2.5 V (vs. Ag/Ag + (0.01 mol dm −3 )) in (C 2 H 5 ) 4 NF· m HF. Electrochemically stable 3,3,6- trifluoro-1,4-cyclohexadiene(I) and 3,3,6,6-tetrafluoro-1,4-cyclohexadiene(II) were obtained as the final products for the fluorination of benzene and fluorobenzene. The electrolysis of 1,4-difluorobenzene gave II in ca . 90% yield. The current efficiency for the fluorination was sufficiently high; benzene, fluoro- benzene and 1,4-difluorobenzene were fluorinated in 66–70%, 71–76% and 88–90% current efficiences, respectively.

Electrochemical fluorination of aromatic compounds in liquid R4NF · mHF—Part IV. fluorination of chlorobenzene

Electrochemical fluorination of chlorobenzene (1) in neat Et4NF · mHF (Et = C2H5: m = 4.0, 4.45 and 4.7) was carried out on a platinum anode at 2.3 or 2.5V vs. AgAg+ (0.01 M; M = mol dm−3). As the primary products, 1-chloro-2-fluorobenzene (2) and 1-chloro-4-fluorobenzene (3) were obtained, whose molar ratio (23) was about 1:3. The product 2 was further fluorinated to 1-chloro-3,6,6-trifluoro-1, 4-cyclohexadiene (2a) with a high yield. On the other hand, the electrochemical fluorination of 3 yielded 3-chloro-3,6,6-trifluoro-1,4-cyclohexadiene (3a), 2-chloro-5,5,6-trifluoro-1,3-cyclohexadiene (3b) and 3,3,6, 6-tetrafluoro-1,4-cyclohexadiene (6a). A larger amount of 6a was obtained at the final stages of the electrolyses of 1 and 3. The yield of 6a varied with the electrolysis conditions.

Electrochemical fluorination of benzene in acetonitrile solutions

Abstract Benzene was electrochemically fluorinated at platinum anode at 2.5 V [vs. Ag/Ag+ (0.01 mol/dm3)] in acetonitrile solutions containing R4NF · nHf (R = CH3, C2H5, n-C3H7 and nC4H9, n = 2, 3 and 4) and (C2H5)3N · 3HF, as supporting electrolytes and fluorine sources. The present electrolyte solution systems were mild and suitable for the fluorination of benzene. The yields of fluorobenzenes were in the range of 20–45%. The current density during the macro-electrolysis, ie the reaction rate, was significantly varied with the composition of the electrolyte solution.

Electrochemical fluorination of aromatic compounds in liquid R4NF·MHF—Part II. Fluorination of di- and tri-fluorobenzenes

Abstract Di- and tri-fluorobenzene were electrochemically fluorinated at a platinum anode at 2.5 V [vs. Ag/Ag+ (0.01 mol dm−3)] in Et4NF·mHF (Et = C2H5, m = 4.0, 4.35). The fluorinated cyclohexadienes were obtained as the major final products in high yield, and neither deposition of a polymeric film on the anode surface nor a coloration of the electrolyte solution was observed. Some 1,2,4-trifluorobenzene ( 4 ) or 1,2,3,5-tetrafluorobenzene ( 6 ) was produced in the course of the fluorination of 1,3-difluorobenzene ( 2 ) or 1,3,5-trifluorobenzene ( 5 ), respectively. These were produced chemically by the dehydrofluorination of 1,3,3,6-tetrafluoro-1,4-cyclohexadiene ( 2a ) or 1,3,3,5,6-pentafluoro-1,4-cyclohexadiene ( 5a ), which was produced by the anodic fluorination, and large portions of the resulting 4 and 6 were further fluorinated electrochemically to the corresponding fluorinated cyclohexadienes. The reaction paths to the major products were explained by the combination of electrochemical reactions, 1,4- or 1,2-addition of two fluoride anions to substrate compounds by an ECEC mechanism (which yielded the fluorinated cyclohexadienes) and a chemical reaction (the dehydrofluorination of some fluorinated cyclohexadienes) which yielded fluorobenzenes.

Electrolytic partial fluorination of organic compounds. Part 27. Regioselective anodic monofluorination of 2-substituted 1,3-dithiolanon-4-ones using Et4NF·4HF and Et3N·3HF

Abstract Highly regioselective anodic monofluorination of 2-substituted 1,3-dithiolan-4-ones was successfully carried out by using novel supporting electrolyte, Et 4 NF·4HF while a conventional supporting electrolyte, Et 3 N·3HF gave poor yields and/or low current efficiencies owing to severe passivation of the anode during the electrolysis.

Competitive fluorination on methyl-group and benzene-ring during the anodic fluorination of fluorotoluenes in Et4NF · mHF

Abstract Electrochemical fluorinations of 2-fluorotoluene (2a), 3-fluorotoluene (3a) and 4-fluorotoluene (4a) in neat Et4NF · mHF (Et = C2H5, m = 3.5 or 4.0) were carried out on a platinum anode. The fluorination of 2a, 3a and 4a occurred competitively both on the side-chain (formation of monofluoromethylfluorobenzenes) and on the benzene-ring (formation of methyltrifluoro-1,4-cyclohexadienes). These results are explained by the facility of proton eliminaticn from the methyl groups of the radical cations which have been formed by the anodic one-electron transfer reactions of 2a, 3a and 4a. A cyclic voltammogram of 4a showed three anodic current peaks which correspond to the oxidation of 4a, 1-fluoromethyl-4-fiuorobenzene (4b) and 1-difluoromethyl-4-fluorolbenzene (4c). On the other hand, voltammograms of 2a and 3a showed two anodic current peaks and no difluoromethylfluorobenzenes was obtained during the fluorination of 2a and 3a.

Electrochemical fluorination of aromatic compounds in liquid R4NF · mHF part V—A study on side-reactions during the fluorination of halobenzenes

Abstract The formation mechanism of 1,4-difluorobenzene ( 5 ) has been studied during the electrolyses of chlorobenzene ( 1 ), 1-chloro-4-fluorobenzene ( 2 ), bromobenzene ( 3 ) and 1-bromo-4-fluorobenzene ( 4 ) in Et 4 NF · m HF. The mechanism consists of a cathodic dehalogeno-defluorination of 3-chloro-3,6,6-trifluoro-1,4- cyclohexadiene ( 2a ) (or 3-bromo-3,6,6- trifluoro-1,4-cyclohexadiene ( 4a )) which was produced by anodic fluorination of 1 and 2 (or 3 and 4 ). The reaction should compete with the cathodic evolution of hydrogen. The ratio of the dehalogeno-defluorination and the hydrogen evolution varied with the cathode potential, the content of HF ( m ) in the electrolyte Et 4 NF · m HF and the concentration of 2a or 4a in the electrolyte solution. The chloride and bromide anions produced through the cathodic dehalogeno-defluorination are anodically oxidized to chlorine and bromine radicals, respectively. The radicals would further react with the substrate compounds or the fluorinated products, so that many kinds of halogenated compounds were also produced as well as the fluorinated compounds.