Electrochemical reduction of decafluorobenzil in DMF on a platinum electrode

Article abstract of DOI:10.1134/S1070428007080118

The mechanism of electrochemical reduction of decafluorobenzil on a platinum electrode in DMF was investigated by cyclic voltammetry. The first reduction peak corresponded to a reversible single-electron transfer leading to the formation of a relatively stable anion-radical whose ESR spectrum was registered and characterized. The second peak corresponded to the reduction of the anion-radical into an unstable dianion that quickly reacted with initial decafluorobenzil, and the arising species (or its transformation product) at the given potential underwent further reduction. The effect of fluorine on the potentials and on the mechanism of the electrochemical reduction of decafluorobenzil was considered.

Full text of DOI:10.1134/S1070428007080118

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2007, Vol. 43, No. 8, pp. 1158−1162. © Pleiades Publishing, Ltd., 2007.
Original Russian Text © N.V. Vasil'eva, I.G. Irtegova, 2007, published in Zhurnal Organicheskoi Khimii, 2007, Vol. 43, No. 8, pp. 1165−1169.
Electrochemical Reduction of Decafluorobenzil in DMF
on a Platinum Electrode
N. V. Vasil’eva and I. G. Irtegova
Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences,
Novosibirsk, 630090 Russia
e-mail: vasileva@nioch.nsc.ru
Received September 28, 2006
Abstract—The mechanism of electrochemical reduction of decafluorobenzil on a platinum electrode in DMF
was investigated by cyclic voltammetry. The first reduction peak corresponded to a reversible single-electron
transfer leading to the formation of a relatively stable anion-radical whose ESR spectrum was registered and
characterized. The second peak corresponded to the reduction of the anion-radical into an unstable dianion that
quickly reacted with initial decafluorobenzil, and the arising species (or its transformation product) at the given
potential underwent further reduction. The effect of fluorine on the potentials and on the mechanism of the
electrochemical reduction of decafluorobenzil was considered.
DOI: 10.1134/S1070428007080118
In extension of investigation of electrochemical
reduction of polyfluorinated aromatic compounds in
aprotic media we studied the process by an example of a
representative of α-diketones decafluorobenzil
(C6F5COCOC6F5), that alongside the C–F bonds
contained two carbonyl groups. We formerly investigated
the preparative electrochemical reduction of
decafluorobenzil [1]. This study concerns the
investigation of the mechanism of electrochemical
reduction of decafluorobenzil by cyclic voltammatry
(CVA) and ESR and the revealing the effect produced
by the replacement of the hydrogen atoms in benzil by
fluorine on the mechanism and potentials of the
electrochemical reduction.
The electrochemical reduction of decafluorobenzil
was mentioned in a single publication [13], where
alongside the data on the other p,p’-disubstituted benzils
the reduction potential of decafluorobenzil on a mercury
electrode was reported measured in acetonitrile with
respect to the saturated calomel electrode (Ε1/2 –0.502
V). From the coulometric data for p,p’-di-methoxybenzil
and benzil and the linear dependence of Ε1/2 on ΣσX in
the series of nonfluorinated benzils (X ≠ NO2) a
concusion was made that a single-electron transfer
occurred in reduction of all benzil derivatives. The
detailed mechanism of reduction of benzil and its
derivatives was not discussed in [13].
In this study we investigated the electrochemical
reduction of decafluorobenzil in DM on a platinum
electrode by CVA method.
The electrochemical reduction of unsubstituted benzil
on various electrodes in aprotic [2–8] and water media
in a wide range of pH [6, 8–12] was subjected to
sufficiently detailed study. It was established that in
aprotic environment on a mercury electrode the benzil
underwent successive single-electron reduction into
anion-radical and further into dianion [7, 8]. The
investigation carried out with a glass-carbon electrode
in DMF showed that the benzil anion-radical was stable
whereas the reversibility of the second reduction peak
was observed only at multiple scanning of potential and
in the presence of proton source in the solution [8].
On the cathode branch of the cyclic voltammogram
of decafluorobenzil two reduction peaks were observed
1K
2K
[1K (Εp –0.46 V), 2K (Εp –1.76 V)], on the anode
branch appeared two oxidation peaks (1A and 1a)
1A
(Fig. 1). The first reduction peak 1K is reversible (Εp
–
Εp1K = 0.06 V), it is diffusion-controlled and corresponds
to the transfer of a single electron. The arising anion-
radical, although relatively stable, undergoes further
transformations (at the velocity of the electrode polariza-
tion less than 500 mV/s Ip1A/Ip1K < 1). The one-electron
1158

ELECTROCHEMICAL REDUCTION OF DECAFLUOROBENZIL
1159
Fig. 2. ESR spectrum of decafluorobenzil anion-radical (a)
and its mathematical simulation (b). Constants of superfine
interaction aï–F (2 19F) 0.84, aO–F (4 19F) 0.47, aμ–F (4 19F)
0.38 Gauss, basic linewidth Γo 0.11 Gauss, correlation factor
r 0.999.
Fig. 1. Cyclic voltammogram of decafluorobenzil reduction
in DMF–0.1M solution of Et4NClO4 (v 100 mV/s): reverse
point of potential –2.2 (a), –1.6 V (b) (1 and 2 are numbers of
voltammogram cycles).
at the C–C bond with the formation of pentafluoro-
benzoyl anion and radical [1]. The fragmentation constant
calculated from the ratio of the anode and cathode current
Ip1A/Ip1K was according to [14] kfr 1.1×10–2 s–1 in good
agreement with the value we obtained from the ESR
measurements (kfr 1.2×10–2 s–1).
transfer at the potential of the first peak was confirmed
by registering the ESR spectrum of the anion-radical
obtained by electrochemical generation at the potential
of the first peak (Fig. 2). The character of superfine
19
interactions in the ESR spectrum [ap–F(2 F) 0.84, ao–
We showed in [1] that at the preparative electro-
chemical reduction of decafluorobenzil at the potential
of the first peak the main reaction product was
decafluorobenzophenone, and the most probable
mechanism of its formation was suggested (Scheme 1).
F(4 19F) 0.47, am–F(4 19F) 0.38 Gauss] indicates the
spectral equivalence of the two pentafluorophenyl rings
in the anion-radical of decafluorobenzil and the
occurrence of free rotation of the molecular fragments
around the C–C bonds. The very small values of the
19
However on the cyclic voltammogram of decafluoro-
benzil at the velocity of electrode polarization v 100 mV/s
we did not find any additional peak that could be assigned
to decafluorobenzophenone reduction. In keeping with
the estimated fragmentation constant of the
decafluorobenzil anion-radical it is presumable that at
the used electrode polarization velocity 100 mV/s in the
constants of superfine interactions on the F nuclei of
the pentafluorophenyl rings are due the distribution of
the spin density mainly on the carbonyl groups of the
molecule. The half-life of (τ1/2) of the anion-radical of
decafluorobenzil estimated by ESR method is 57 s. We
formerly demonstrated that the principal mechanism of
the anion-radical decomposition was its fragmentation
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 8 2007

VASIL’EVA, IRTEGOVA
1160
Scheme 1.
_.
_
+e
.
C6F5CO
(1)
(2)
(3)
(4)
C6F5CO
C6F5COCOC6F5
C6F5COCOC6F5
_
C6F5CO
_
_
C6F5
CO
C6F5H
C6F5COC6F5
H+
C6F5
C6F5
_
_.
.
C6F5CO
.
_
_.
C6F5COC6F5 + C6F5COCOC6F5
C6F5COC6F5
(5)
(6)
C6F5COCOC6F5
C6F5COCOC6F5
.
2C6F5CO
time range of CVA the decafluorobenzophenone does
not form in notable amount in the course of registering
the first cycle. The increase in the registering time of
the voltammogram by diminishing the electrode
polarization velocity to 20 mV/s did not result in
appearance of the peak of decafluorobenzophenone. The
reduction peak of decafluorobenzophenone at –1.11 V
we succeeded to observe only by registering several
cyclic voltammogram in succession or by performing
microelectrolysis at the potential of the first peak
(maintaining the potential at Ε –0.6 V for 3–4 min with
subsequent registering of the cyclic voltammogram);
however the current of this peak is very small (~1 μA)
indicating the negligible amount of the product. This
fact can be understood from Scheme 1 on condition of
the short lifetime of the decafluorobenzophenone anion-
radical. According to equation (5) the decafluorobenzo-
phenone formed from the corresponding anion-radical by
a fast electron transfer from these anion-radical to the
initial decafluorobenzil. In the CVAexperiment the deca-
fluorobenzil concentration is 10 times less that in our
experiments of preparative electrolysis, therefore at the
low decafluorobenzil concentration (C 2×10–3 mol l−1)
the unstable decafluorobenzophenone anion-radical
perishes before it meets the molecule of the initial
compound. In the study of decafluorobenzophenone
reduction under identical experimental conditions we
actually showed the instability of its anion-radical. The
first reduction peak of decafluorobenzophenone is
irreversible up till the electrode polarization velocity 100
V/s, at v 100 mV/s the potential of reduction peak equals
Εp1K –1.11 V. Consequently, at the use of low concentra-
tions of decafluorobenzil (under conditions of CVA
registering) no decafluorobenzophenone formed at the
potential of the first reduction peak, and the reaction
occurred further by another mechanism than that of
preparative electrolysis.
Peak 2K on the voltammogram of decafluorobenzil
corresponds to further reduction of decafluorobenzil
anion-radical to dianion since the decafluorobenzil anion-
radical is sufficiently stable in the time scale of CVA
registering (Fig. 1b).
The irreversibility of 2K peak up till electrode
polarization velocity 100 V/s indicated the low stability
of the species formed. This fact and also the relatively
high current of this peak (Ip2K > Ip1K, 59 and 47 μA
Fig. 3. Cyclic voltammogram of benzil reduction in DMF–0.1 Μ solution of Et4NClO4 (v 100 mV/s) on a platinum electrode: reverse
point of potential –2.40 (a), –1.5 V (b) (1 and 2 are numbers of voltammogram cycles; Εp1K –1.13, Εp2K –1.93 V).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 8 2007

ELECTROCHEMICAL REDUCTION OF DECAFLUOROBENZIL
1161
peak (Fig. 1b) and that consequently corresponded to
the oxidation of the product of further chemical and
electrochemical transformations of the decafluorobenzil
dianion. The microelectrolysis at the potential of the
second reduction peak resulted in considerable decrease
in the anode peak 1A in conformity to the diminished
amount of the initial compound near the electrode.
respectively) evidence that the arising dianion is unstable
and suffers fast chemical transformation into an electro-
chemically active species which is reduced further at this
potential. The instability of the decafluorobenzil dianion
may be caused both by its high basicity and by the fast
reaction with proton sources (residual water in the solvent
or solvent itself), or by its nucleophilic characteristics
revealed, for instance, in the reaction with the initial
decafluorobenzil.
Thus the results obtained at several cycles of potential
scanning and on adding various water concentrations as
proton source suggest that peak 2K originates from the
reduction of decafluorobenzil anion–radical to dianion
which quickly reacts with the initial decafluorobenzil.
The species arising in that way or the product of its
transformation undergoes further reduction at the second
peak potential. Consequently, the electrochemical reduc-
tion of decafluorobenzil is a process ΕΕCnΕ (n ≥ 1).
To reveal the effect of hydrogen replacement in benzil
by fluorine on the mechanism and potentials of the
electrochemical reduction we also investigated the
electrochemical behavior of unsubstituted benzil under
identical experimental conditions. The cyclic voltam-
mograms of benzil reduction are presented on Fig. 3.
In order to establish whether the decafluorobenzil
anions perish due to its basic properties we carried out
experiments with water addition to the electrochemical
cell before registering the cyclic voltammograms. The
addition into the cell fivefold and tenfold molar excess
of water with respect to the substarate did not affect the
appearance of the cyclic voltammogram (the reduction
potentials values and the corresponding currents remain
unchanged). Consequently the formation of an electro-
chemically active species at the second reduction peak
potential is not caused by protonation with water traces
in the solvent or by the solvent itself; otherwise the
current of peak 2K should grow.
At scanning of the potential up to –2.2 V on the second
and subsequent voltammogram cycles a very strong
decrease in the current of the first reduction peak
1K was observed (first cycle Ip1K –48 μA, second cycle
Ip1K –10 μA); this was not observeal when the scannig of
potential was performed before the beginning of the
second reduction peak (first cycle Ip1K –48 μA, second
cycle Ip1K –44 μA) (Fig. 1a, b).
This fact suggests a strong reduction of the initial
decafluorobenzil concentration near electrode at the
polarization of the main electrode in the region of the
second peak potential. Consequently, the considerable
decrease in the concentration of the initial decafluoro-
benzil that was observed near the electrode after
formation there of decafluorobenzil dianion originated
from the fast reaction of the forming decafluorobenzil
dianion with the initial decafluorobenzil. The arising
species (or its transformation product) suffered further
reduction at these potential thus increasing the current
of peak 2K compared with the current of peak 1K.
The comparison of reduction potentials of benzil and
decafluorobenzil (Fig. 1a and 3a) shows that the effect
of 10 fluorine atoms in perfluorobenzil on the first reduc-
tion peak potential is fairly essential: on replacement of
hydrogen by fluorine the first peak potential shifts to the
anode region approximately by 0.67 V. This replacement
affects the second peak potential considerably less, the
shift of the potential to the anode region is but 0.17 V.
Note that decafluorobenzil possesses high affinity to
electron: It is reduced at about the same potential as
p-dinitrobenzene [15]. As to the mechanism of the
electrochemical process, in both cases the arising anion-
radicals are sufficiently stable (Fig. 1b and 3b). The path-
ways of these anion-radicals transformations on the
preparative electrolysis scale are different: Whereas the
benzil anion radical gives finally benzoin [3, 16], the
transformations of decafluorobenzil anion-radical result
in decafluorobenzophenone formation [1]. At the
potential of the second peak benzil provides relatively
stable dianion (at v ≥ 200 mV/s the reversibility of peak
2K is clearly seen). The dianion of decafluorobenzil is
unstable, peak 2K of decafluorobenzil is irreversible even
at the high velocity of the electrode polarization
(v 100 V/s). The relatively stable benzil dianion does
not react with the initial compound according to CVA
data (Fig. 3a), whereas this transformation is the main
one for the polyfluorinated derivative. For both com-
At the reverse scanning of the potential from the
region of the second peak potentials on the anode branch
of the cyclic voltammogram of decafluorobenzil beside
the peak of oxidation of decafluorobenzil anion-radical
to the initial compound (peak 1A) a definite anode peak
1a was observed at +0.33 V (Fig. 1a) that was absent at
the scanning of potential only in the region of the first
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 8 2007

VASIL’EVA, IRTEGOVA
1162
working electrode of the cell was placed into the
resonator of ESR spectrometer. The numerical simulation
of the ESR spectra was carried out using WINSIM 32
program with an algorithm of multi-parameter
optimization LMB1.
pounds the anode peak 1a observed at the reverse
scanning of potential is due to the products formed at
the potential of the peak 2K (Fig. 1a, b, 3a, b).
EXPERIMENTAL
Cyclic voltammograms were registered on a modified
electrochemical system SVA-1BM with a triangular pulse
of potential scanning, We used a cell of working volume
5 ml connected to a system by three-electrode scheme
and equipped with a salt bridge with a solution of support-
ing electrolyte in DMF for connecting the working and
reference electrodes. The working electrode was
stationary spherical platinum electrode of area 8 mm2,
the auxiliary electrode was a platinum spiral.As reference
electrode the saturated aqueous calomel electrode was
used. The electrochemical measurements were performed
on solutions of the initial compounds in DMF
(2×10–3 mol l−1), we used as supporting electrolyte
tetraethylammonium perchlorate of concentration
0.1 mol l−1. The oxygen was removed from solvents by
flushing the cell with argon. Anhydrous DMF was
obtained by double distillation in a vacuum over
phosphorus pentoxide collecting the fraction of bp 24ºC
(2 mm Hg).
REFERENCES
1.Vasil’eva, N.V. and Starichenko, V.F., Zh. Org. Khim., 2004,
vol. 40, p. 1180.
2.Kargin, Yu.M. and Kondranina, V.Z., Izv. Akad. Nauk SSSR,
Ser. Khim., 1973, p. 565.
3.Arthur, P., Rulison, D.S., and Berlin, K.D., Anal. Chem.,
1968, vol. 40, p. 1389.
4.Buchta, R.C. and Evans, D.H., Anal. Chem., 1968, vol. 40,
p. 2181.
5.Talant, D.R. and Evans, D.H., Anal. Chem., 1969, vol. 41,
p. 835.
6.Philp, R.H., Flurry, R.L., and Day, R.A., J. Electrochem.
Soc., 1964, vol. 111, p. 328.
7.Philp, R.H., Layloff, T., andAdams, R.N., J. Electrochem.
Soc., 1964, vol. 111, p. 1189.
8.Chandrasekaran, M., Noel, M., and Krishnan, V., J. Chem.
Soc., Perkin Trans. 2, 1992, p. 979.
9.Stapelfeldt, H.E. and Perone, S.P., Anal. Chem., 1968,
vol. 40, p. 815.
In the study were used decafluorobenzil, mp 78–79°C
(according to GC-MS data the content of the main
compound was 98.9%) (mp 79–80ºC [17]), benzil and
decafluorobenzophenone containing no less than 98%
of the main substance (by GC-MS data).
10. Stapelfeldt, H.E. and Perone, S.P., Anal. Chem., 1969,
vol. 41, p. 623.
11. Vincenz-Chodkowska, A., and Grabowski, Z.R.,
Electrochim. Acta, 1964, vol. 9, p. 789.
12. Meunier-Prest, R., Laviron, E., Gaspard, C., and Raveau,
S., Electrochim. Acta, 2001, vol. 46, p. 1847.
13. Clenan, E.L., Speth, D.R., and Barlett, P.D., J. Org. Chem.,
1983, vol. 48, p. 1246.
14. Nicholson, R.S. and Shain, I., Anal. Chem., 1964, vol. 36,
p. 706.
15. Allendoerfer, R.D. and Rieger, P.H., J. Am. Chem. Soc.,
1966, vol. 88, p. 3711.
The ESR spectrum of decafluorobenzil anion-radical
was recorded on a spectrometer Bruker ESP-300,
equipped with double resonator (power of microwave
radiation 256 mW, modulation frequency 100 kHz
modulation amplitude 0.05 Gauss). The reduction of
decafluorobenzil combined with ESR spectrometric
measurementswas done under air-free conditions at
298 K at the potential of the first reduction peak in
a three-electrode cell with a platinum electrode. The
16. Pasternak, H., Helv. Chim. Acta, 1948, vol. 31, p. 753.
17. Chambers, R.D., Clark, M., and Spring, D., J., J. Chem.
Soc., Perkin Trans. 1,1972, p. 2464.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 8 2007