Tetrahedron Letters
Electrochemical hydrodefluorination of fluoroaromatic compounds
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Wen-Bin Wu, Mei-Li Li, Jing-Mei Huang
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China
a r t i c l e i n f o
a b s t r a c t
The BHÀ4 - promoted electrochemical hydrodefluorination of fluoroaromatic compounds was reported.
Using platinum as electrodes in an undivided cell, the electrolysis was carried out at constant current
at room temperature under air without the need of pretreatment of the solvent. This reaction could pro-
ceed smoothly on both nonactivated monofluoroarenes and perfluoroarenes with high yields and good
selectivities.
Article history:
Received 3 December 2014
Revised 31 January 2015
Accepted 6 February 2015
Available online 13 February 2015
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Electrochemistry
Fluoroarenes
Hydrodefluorination
Borohydride
The fluorinated organic compounds (FOCs) have attracted great
attention during the last few decades due to their extensive applica-
tions in modern pharmaceuticals, agrochemicals, materials science,
and polymer chemistry.1 C–F bonds are of high thermodynamic sta-
bility and kinetic inertness as the consequence of the small size
showed that the reaction was catalyzed by the ‘amalgam’ formed
from the mercury and the dimethylpyrrolidinium ion (DMP+).9 In
Langer and Yurchak’s study, fluorobenzene underwent electro-
chemical hydrogenation on platinum black electrodes to give equal
amounts of benzene and cyclohexane by using the H2 as a reduc-
tant.10 In 2009, Trnková studied the electrochemical processes of
hydrodefluorination of fluorobenzene on a platinum polycrystalline
electrode in sulfuric acid by differential electrochemical mass spec-
trometry.11 However, general, highly efficient, and selective electro-
chemical methods for hydrodefluorination of fluoroaromatics are
still desirable. With our continuous interest in electrochemical syn-
thesis,12 herein we reported a highly efficient electrochemical
method for hydrodefluorination of fluoroaromatics by using Pt
electrodes.
We chose the hydrodefluorination of fluorobenzene as a model
reaction. The reaction was initially investigated in a one-compart-
ment cell with platinum foil as the anode and a platinum minigrid
electrode (represented as Pt (g)) as the cathode in THF–0.2 M
TBABF4 (tetrabutylammonium tetrafluoroborate) solution at room
temperature. After electrolysis for 3 h under a constant current
(20 mA), only trace amount of benzene and about 90% of unreacted
substrate were obtained (Table 1, entry 1). To our surprise, when
NaBH4 (2 equiv) was added, the yield of benzene increased to
39% (Table 1, entry 2). Then several other solvents were screened.
The results showed that 1,4-dioxane, acetonitrile, and ethanol
could give only trace amounts of desired product. Among all the
tested solvents, diglyme turned out to be the best choice, giving
the desired product in an excellent yield of 98% (Table 1, entries
2–9). It is important to note that NMP (N-methyl-2-pyrrolidone)
could work as a convenient alternative solvent (Table 1, entry 8).
(rW = 1.47 Å) and high electronegativity (v = 4) of the fluorine
atom.2,5 As a result, the organofluorine molecules are highly persis-
tent in the environment and resistant to aerobic degradation.3,6a,n
The simplest and most efficient activation of the C–F bond in this
regard is hydrodefluorination (HDF), by converting a C–F bond into
the C–H bond. In the early studies of hydrodefluorination, efforts
were focused on the relatively reactive perfluorobenzene deriva-
tives, while nonactivated monofluoroarenes were less studied.4
Over the past two decades, the main group5 and transition metals6
have been employed and substantial progress has been made in the
reaction of hydrodefluorination of fluoroarenes. Despite the tre-
mendous progress made, it is still necessary to explore new
approaches which are more simple, mild, and economical for
hydrodefluorination.
Electrochemical methods have attracted great interest because
of their significant environmental and economical advantages.7,8
However, electrochemical hydrodefluorination of simple fluoroaro-
matic compounds is less explored. Kariv-Miller developed an elec-
trochemical
method
for
hydrodefluorination
of
1,3-
difluorobenzene and fluorobenzene by using platinum foil anode
and mercury pool cathode. The hydrodefluorination of fluoroben-
zene afforded benzene in a moderate yield of 76%. Their studies
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