Preparative electrochemical reduction of decafluorobenzil

Article abstract of DOI:10.1023/B:RUJO.0000045893.52310.94

Preparative electrochemical reduction of decafluorobenzil in DMF on a platinum electrode at the potential of the first peak afforded decafluorobenzophenone as the principal product. The reaction mixture lacked products of hydrodefluorination of the decafluorobenzil or of its reduction at the C=O group.

Full text of DOI:10.1023/B:RUJO.0000045893.52310.94

Russian Journal of Organic Chemistry, Vol. 40, No. 8, 2004, pp. 1132 -1135. Translated from Zhurnal Organicheskoi Khimii, Vol. 40, No. 8, 2004,
pp. 1180-1183.
Original Russian Text Copyright Ó 2004 by Vasil’eva, Starichenko.
Preparative Electrochemical Reduction of Decafluorobenzil
†
N.V. Vasil’eva and V.F. Starichenko
Vorozhtsov Novosibirsk Institute of Organic Chemistr, Siberian Division, Russian Academy of Sciences,
Novosibirsk, 630090 Russia
e-mail: vasileva@nioch.nsc.ru
Received December 26, 2003
Abstract—Preparative electrochemical reduction of decafluorobenzil in DMF on a platinum electrode at the
potential of the first peak afforded decafluorobenzophenone as the principal product. The reaction mixture lacked
products of hydrodefluorination of the decafluorobenzil or of its reduction at the C=O group.
Electronic structure and reactivity of anion-radicals
from haloaromatic compounds were recently extensively
investigated [1–4]. The anion-radicals of this type attract
interest to a large extent because their fragmentation
furnishing aryl radicals and haloanions constitutes an
elementary stage of nucleophilic substitution of the SRN1
type and of reductive dehalogenation.
In the preparative electrochemical reduction of benzil
both in aprotic and water media a reduction of a single
carbonyl group was observed, and benzoin was obtained
as the reduction product [7, 17]. In going to perfluorinated
benzil it was presumable that alongside C=O group
reduction might occur also the reduction of C–F bonds
for these bonds were known to be reduced in the case of
polyfluorinated aromatic compounds containing electron-
withdrawing substituents giving as a result products of
hydrodefluorination [5, 18].
One way to investigate the reactivity of electro-
chemically-generated anion-radicals is analysis of com-
pounds arising at preparative electrolysis. We formerly
studied preparative electrochemical reduction of fluorin-
ated benzonitriles [5] in aprotic media. In extension of
the research in the field of electrochemical reduction of
polyfluorinated aromatic compounds in DMF we turned
our attention to the study of decafluorobenzil
(C6F5COCOC6F5) that alongside C–F bonds contained
additionally two potential reaction sites like C=O groups.
As to decafluorobenzil, the only publication concern-
ing this compound [19] contained the reduction potential
of the decafluorobenzil in acetonitrile on the mercury
electrode measured with respect to the saturated calomel
electrode (E1/2 –0.502 V). Laev and Shteingarts basing
on coulometric data for p,p'-dimethoxybenzil and benzil
and on the linear dependence of E1/2 on SsX in the series
of nonfluorinated benzils (X ¹ NO2) concluded that the
reduction of all benzil derivatives occurred with
transition of one electron. The preparative electrolysis
was not performed in [19].
The electrochemical reduction of unsubstituted benzil
on various electrodes in aprotic [6–12] and water media
at different pH[10, 12–16] is sufficiently well document-
ed. In aprotic media it undergoes successive one-elec-
tron reduction into anion-radical and further into di-
anion [11,12]. In the course of the benzil reduction in
acidic and neutral water solutions a single reduction wave
is observed on the polarogram, and in alkaline solutions
two waves appear [10]. Philp et al. indicated that the
second wave appeared at the potential where under
similar condition benzoin was reduced [10]. The fact of
existence of the second wave at the use of alkaline
solutions was ascribed to acceleration in this medium of
rearrangement into benzoin suffered by the endiol formed
at the first wave potential [10].
Taking into consideration the presence in the
decafluorobenzil of several probable reaction sites we
investigated the process of its electrochemical reduction
by preparative electrolysis in DMF on platinum electrode
aiming to elucidate which among the possible pathways
is realized. To determine the conditions of the preparative
electrolysis we studied by cyclic voltammetry the first
reduction peak of decafluorobenzil in DMF on platinum
electrode. At the rate of electrode polarization 100 mVs-1
on the cyclic voltammogram of this compound a
reduction peak Ep1 –0.46 V was observed that was
†
Deceased.
1070-4280/04/4008-1132Ó2004 MAIK “Nauka/Interperiodica”

1133
PREPARATIVE ELECTROCHEMICAL REDUCTION OF DECAFLUOROBENZIL
a
diffusion-controlled, electrochemically reversible (Ep –
also detected in the reaction mixture. This result permits
a conclusion that the processes shown on Scheme 1 and
also reduction at C=O group do not operate or at least
are not dominant. It is presumable that the electrolysis
products arise after fragmentation of the deca-
fluorobenzil anion-radical, but the rupture occurs not
at the C–F, but at C–C bond. As a result form
pentafluorobenzoyl anion and a radical [equation (8)]
(Scheme 2).
ê
Ep 0.06, Ep/2 – Ep 0.06 V), and corresponded to one-
electron transfer*. On the anode branch of the cyclic
voltammogram a peak was clearly seen corresponding
to the oxidation of the arising anion-radical of
decafluorobenzil into the initial compound.
The ratio of the current in anode and cathode peaks
Ip /Ip for the first reduction peak of decafluorobenzil on
a
k
the cyclic voltammogram at relatively low polarization
rate of electrode is less than unity (v 100 mVs-1, Ip /Ip
Scheme 2.
a
ê
0.77) testifying to insufficient stability of the arising
anion-radical that evidently suffers chemical
transformations. The unstable anion-radicals forming at
one-electron reduction of fluoroaromatic compounds
[equation (1)] are known to undergo commonly
dimerization [equation (2)], fragmentation at the C–F
bond [equation (3)], or in the presence of proton sources
get protonated giving radical species that further are
reduced into anion which through fluoride anion
elimination affords the hydrodefluorinated product
[equation (4)] (Scheme 1) [20].
H
& ) &2&2& )
& ) &2&2& )
& ) &2ꢀꢀꢁꢀ& ) &2
& ) ꢀꢀꢁꢀ&2
ꢁꢉꢃ
ꢁꢂꢃ
& ) &2
& ) ꢀꢀꢁꢀ+
& ) +
ꢁꢄꢅꢃ
ꢁꢄꢄꢃ
& ) ꢀꢀꢆꢀ& ) &2
& ) &2& )
& ) &2& ) ꢁꢀ& ) &2&2& )
ꢁꢀ& ) &2&2& )
& ) &2& ) ꢀꢁ
ꢁꢄꢇꢃ
ꢁꢄꢈꢃ
ꢇꢀ& ) &2
& ) &2&2& )
The pentafluorobenzoyl anion decomposes further
into pentafluorophenyl anion and CO. The arising
pentafluorophenyl anion may react with the pentafluoro-
benzoyl radical to yield decafluorobenzophenone anion-
radical [equation (11)], or with a proton source (residual
water in the solvent, solvent or supporting electrolyte
[22]) to give pentafluorobenzene [equation (10)].
Inasmuch as the decafluorobenzophenone has a more
negative reduction potential (E1/2 –1.21 V with respect
to the saturated calomel electrode in THF [23]) than
decafluorobenzil, the forming decafluorobenzophenone
anion-radical interacting with the initial decafluorobenzil
transmits the excessive electron to the latter [equation
(12)]. As a result the decafluorobenzil anion-radical
appears again in the reaction mixture, this time without
participation of the electrode. The pentafluorobenzoyl
radical besides the reaction with pentafluorophenyl anion
can undergo dimerization to the initial decafluorobenzil,
and the pentafluorophenyl anion can react with the initial
decafluorobenzil affording addition products [24].
Scheme 1.
$U)ꢀꢁꢀH
ꢂꢀ$U)
$U)
$U)
ꢃꢆꢅ
ꢃꢂꢅ
B
B
$Uꢀꢀꢀ$Uꢀꢀꢀꢂꢀ)
>$U)ꢀꢀꢀ$U)@ꢀꢀ
$Uꢀꢀꢁꢀ)
ꢃꢄꢅ
ꢃꢇꢅ
ꢃꢈꢅ
ꢃꢉꢅ
ꢃꢊꢅ
H
$U)
>$U)+@
>$U)+@
$U+ꢀꢀꢀ)
ꢀꢀ+
ꢀꢀH
$U
$U
$U
$U
$U+ꢀꢀꢀ6RO
$U+
ꢀꢀ+6RO
ꢀꢀ+
Inasmuch as at recording several cycles on the cyclic
voltammogram of decafluorobenzil in the more positive
potential region no additional peaks appeared that could
be attributed to the reduction of dimeric reaction products
[21] the expected reduction sites in preparative
electrochemical reduction should be either C=O group
or C–F bond.
However after the preparative electrolysis of
decafluorobenzil at the potential of the first peak (E –
0.55 V) the reaction mixture lacked products of
decafluorobenzil hydrodefluorination or its reduction at
the C=O group: The main reaction product was
decafluorobenzophenone. Some pentafluorobenzene was
According to GC-MS data the reaction mixture apart
from decafluorobenzophenone and pentafluorobenzene
contained in small amounts compounds whose molecular
mass (558 and 538) corresponded to addition of one C6F5
fragment to decafluorobenzil (cf. [24]) or C6F5CO
fragment to decafluorobenzophenone.
* Detailed study of perfluorobenzil electrochemical reduction in DMF
by cyclic voltammetry and of the structure of anion-radical formed
is planned to be performed further.
The fact of predominant reaction of the pentafluoro-
phenyl anion with pentafluorobenzoyl radical and not with
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 40 No. 5 2004

1134
VASIL’EVA, STARICHENKO
the initial decafluorobenzil which was present in a larger
amount [24] indicated that the process apparently
occurred prevailingly in a “cage”.
and the reference electrode was the saturated calomel
electrode.
In the electrochemical measurements were used
–3
The number of electrons transmitted per substrate
molecule in the course of reduction estimated from the
quantity of electricity consumed amounted 0.2 in a good
agreement with the suggested mechanism. The reaction
mixture also contained in conformity to Scheme 2 some
reaction products (in overall amount ~5 wt%) that
according to the molecular mass and the mass spectral
data were tentatively regarded as compounds generated
by reduction of the decafluorobenzophenone at the C–F
bonds (mono- and dihydrodefluorinated derivatives) and
also at the C=O bond. These compounds formed
apparently concurrent with reaction (12) from the anion-
radical of the decafluorobenzophenone.
solutions of initial compounds in DMF of 2 ´10
M
concentration, as supporting indifferent electrolyte
tetraethylammonium perchlorate was applied of 0.1 M
concentration. The oxygen was removed from the cell
by flushing with argon. Anhydrous DMF was obtained
by two-fold vacuum distillation over phosphorus pent-
oxide collecting the fraction boiling at 24°C (2 mm Hg).
The preparative electrolysis was carried out with the
use of potentiostat P-5848 in potentiostatic mode in a
three-electrode cell.As cathode a platinum plate was used
of 1 cm2 area, a platinum spiral served as anode. As
reference electrode was used the saturated calomel
electrode. On completing the electrolysis the reaction
mixture was analyzed by GC-MS method on a Hewlett-
Packard G 1801 A instrument including a gas
chromatograph HP 5890 of series II and mass-selective
detector HP 5971, capillary column HP-5MS, 30 m ´
0.25 mm ´ 0.25 mm, carrier gas helium, flow rate 1 ml
per min, oven temperature programmed as follows:
2 min at 50°C, from 50 to 280°C at a rate 10 deg/min,
10 min at 280°C, vaporizer temperature 280°C; ion
source temperature 175°C. Mass scanning was perform-
ed from 30 to 650 m/z.
The concurrent elimination of CO from the penta-
fluorobenzoyl radical cannot be absolutely excluded, but
if this process is operative, it apparently is not important.
It should be noted that the preparative electrochemical
reduction of decafluorobenzil proceeds sufficiently rapid-
ly and completely. In 1 h the control voltammograms
did not contain even a trace of the reduction peak of the
initial compound and an extended reduction peak appear-
ed in the more negative potential region (Ep –1.10...
–1.20 V). This peak corresponds presumably to deca-
fluorobenzophenone reduction: This assumption is
supported by the composition of the reaction mixture,
and also by published value of the reduction potential of
the decafluorobenzophenone (E1/2 –1.21 V with respect
to the saturated calomel electrode in THF [23]).Another
among the main reaction products, pentafluorobenzene,
is reduced at notably more negative potential [Ep(C6F5H)
–2.31 V] [25]. It is worth noting that on the cyclic
voltammograms of decafluorobenzil no reduction peak
appeared at potentials –1.1...–1.2 V showing that
apparently during the measurement of voltammogram
the decafluorobenzophenone had not yet enough time to
form.
The preparative electrolysis and cyclic voltammetry
was performed using decafluorobenzil, mp. 78–79°C
(according to GC-MS the content of the main substance
was 99%) (publ.: mp 79–80°C [26]), as reference
compounds were applied pentafluorobenzene and
decafluorobenzophenone containing no less than 98%
of the main compound (GC-MS data).
Preparative electrochemical reduction of deca-
fluorobenzil. Into an electrochemical cell was charged
0.1 M solution of tetraethylammonium perchlorate in
DMF (5 ml into cathode space and 4 ml into anode space),
into the cathode space an appropriate amount of deca-
fluorobenzil (39 mg, C 0.02 mol l-1) was added, and the
cell was charged at E –0.55 V. The electrolysis was
carried out at this voltage with stirring at room
temperature. Due to volatile products formation the cell
was flushed with argon after charging the decafluoro-
benzil, and electrolysis was further continued under argon
atmosphere. The reaction progress was monitored by
cyclic voltammograms of the reaction mixtures measured
intermittently in the course of the process. After 1 h of
the process on the test voltammograms the reduction peak
of decafluorobenzil already disappeared, but the
EXPERIMENTAL
Cyclic voltammograms were registered on a modified
electrochemical system CBA-1AM with a triangular
pulse of potential scanning. The cell used was of net
volume 5 ml and was connected to the system by three-
electrode connection diagram. As working electrode
served a stationary needle platinum electrode of area 8
mm2, as auxiliary electrode a platinum spiral was used,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 40 No. 8 2004

1135
PREPARATIVE ELECTROCHEMICAL REDUCTION OF DECAFLUOROBENZIL
p. 2181.
electrolysis was still continued for 30 min to ensure
complete reduction of the initial compound. On
completing the electrolysis the reaction mixture from the
cathode space was poured into 20 ml of water (pH of the
solution was ~ 7), the reaction products were extracted
into ethyl ether (5´7 ml), the extract was washed from
DMF traces with distilled water (3´10 ml), and dried on
magnesium sulfate. Then the ether was evaporated to
residual solution volume ~4 ml, and the products were
analyzed by GC-MS method in comparison with
authentic samples. Residue after complete removal of
solvent weighed 20.2 mg and contained 85.6% of
decafluorobenzophenone (GC-MS data).
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