Preparative Electrochemical Reduction of Fluorinated Benzonitriles

Article abstract of DOI:10.1023/A:1025588119221

Electrochemical reduction of fluorinated benzonitriles in aprotic media is accompanied by concurrent processes of dimerization and fragmentation of the corresponding anion-radicals. With 2-fluoro- and 3,4,5-trifluorobenzonitriles the defluorinated products can be obtained at direct electrochemical reduction; for the other compounds studied have been found mediators providing a possibility to obtain in high yield defluorinated products at preparative electrochemical reduction. A fundamental possibility was demonstrated of providing products by nucleophilic fluorine substitution by SBN mechanism at electrochemical reduction of fluorinated benzonitriles in aprotic solvents in the presence of mediators.

Full text of DOI:10.1023/A:1025588119221

Russian Journal of Organic Chemistry, Vol. 39, No. 2, 2003, pp. 212
pp. 235 241.
-218. Translated from Zhurnal Organicheskoi Khimii, Vol. 39, No. 2, 2003,
-
Original Russian Text Copyright © 2003 by Efremovà, Podryvanova, Vasil’evà, Starichenko, Shteingarts.
Preparative Electrochemical Reduction of Fluorinated
Benzonitriles
N. V. Efremova, L. V. Podryvanova, N. V. Vasil’eva, V. F. Starichenko, and V. D. Shteingarts
Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences,
Novosibirsk, 630090 Russia
Received October 23, 2002
Abstract¾ Electrochemical reduction of fluorinated benzonitriles in aprotic media is accompanied by concurrent
processes of dimerization and fragmentation of the corresponding anion-radicals. With 2-fluoro- and 3,4,5-
trifluorobenzonitriles the defluorinated products can be obtained at direct electrochemical reduction; for the other
compounds studied have been found mediators providing a possibility to obtain in high yield defluorinated prod-
ucts at preparative electrochemical reduction. A fundamental possibility was demonstrated of providing products
by nucleophilic fluorine substitution by SBN mechanism at electrochemical reduction of fluorinated benzonitriles
in aprotic solvents in the presence of mediators.
Recently the electronic structure and reactivity of
The anion-radicals with hydrogen atom in the para-
anion-radicals from haloaromatic compounds are ex- position to cyano group undergo fragmentation at the
tensively studied (see, e.g. [1- 4]). The interest to an- ortho-position, and concurrent dimerization at the para-
ion-radicals of this type is due to a great extent to the
fact that their fragmentation to aryl radicals and halide
anions constitutes the elementary stage of nucleophilic
substitution of the SRN1 type and of reductive haloge-
nation.
position is here reversible (Scheme 2). Basing on these
data we investigated in this work the possibility of
preparative electrochemical reduction of fluorinated
benzonitriles.
It follows from the above stated that the electro-
chemical reduction of fluorinated benzonitriles can
provide defluorination product only if the parallel
dimerization process of the corresponding anion-
radicals is reversible , as with 2-fluorobenzonitrile, and
if the latter process is irreversible as with compounds
having a para-loicated fluorine, it should be consid-
erably slower than fragmentation. Actually, the prepa-
rative electrolysis of 2-fluorobenzonitrile furnishes as
main product the benzonitrile in 80% yield.*
We investigated previously by means of cyclic
voltammetry the electrochemical reduction of a large
series of fluorinated benzonitriles in DMF [5- 7]. It
was established that all fluorinated benzonitriles stud-
ied underwent in DMF one-electron electrochemical
reduction providing anion-radicals of different stabil-
ity. For instance, anion-radicals of 3-fluoro- and 3,5-
difluorobenzonitriles are relatively stable in DMF: On
the cyclic voltammetric curves of these nitriles in
the system DMF-tetraethylammonium perchlorate
From 3,4,5-trifluorobenzonitrile we also obtained
the product of anion-radicals fragmentation, 3,5-di-
fluorobenzonitrile (59%).
(TEAP) reversible one-electron reduction peaks are
present, and at reduction of the nitriles in a cell placed
in the probe of an ESR-spectrometer well resolved
ESR spectra of the corresponding anion-radicals are
observed [5].
The fraction of dimerization products obtained in
the mentioned processes depends on the rates ratio of
concurrent transformations of anion-radicals that in its
turn is affected by the concentration of the initial com-
___________
Anion-radicals of benzonitriles with a fluorine atom
located in the para-position to the cyano group suffer as
concurrent processes elimination of the para-located
fluorine and dimerization at this position [6] (Scheme 1).
* Here and hereinafter the yield of products is given with respect
to the initial compound.
1
070-4280/00/3902-0212$25.00 Ó 2003 MAIK “Nauka/Interperiodica”

PREPARATIVE ELECTROCHEMICAL REDUCTION OF FLUORINATED BENZONITRILES
213
Scheme 1.
(
(
1)
2)
Scheme 2.
lower than that of the substrate. Therefore the concen-
tration of the arising anion-radicals would be signifi-
cantly lower than that required for notable competition
of dimerization with fragmentation.
In selection of mediator the following criteria
should be used [10, 11]:
(
(
3)
4)
(
1) the reduction potential of the mediator should be less
negative than that of the compound under study;
2) stability of the anion-radical form of mediator Q;
3) sufficiently high rate of electron transfer from
(
(
the Q form to substrate.
Criterion (1) means that electron transfer from the
anion-radical of mediator to the substrate is thermody-
namically unfeasible, but irreversibility of reaction (7)
ensures the occurrence of the process.
pound. Therefore with certain fluorinated benzonitriles
it is possible to choose concentration where the frag-
mentation of anion-radicals is the prevailing process.
For instance, at concentration of 4-fluorobenzonitrile
of 10 M the main product of electrochemical reduc-
tion is benzonitrile [8]. However not always the de -
crease in concentration completely prevents dimeriza-
tion of anion-radicals.
-
4
We proved the possibility to apply mediator for
prevailing occurrence of fragmentation by an example
of 4-fluorobenzonitrile. The direct electrochemical
-
3
reduction of the latter at c > 10 M is accompanied by
dimerization [8]. The efficiency of mediator was esti-
mated by cyclic voltammetry. On the figure is shown a
cyclic voltammogram of a potential mediator, anthra-
cene. The curve contains a reversible one-electron
It was presumable that the ratio of fragmentation
and dimerization products may be changed also at the
use of mediators. The principal stages of the mecha-
nism of indirect reductive electrolysis may be repre-
sented by reactions sequence (5)- (9) [9].
peak of anthracene reduction into anion-radical (I
). The addition of 4-fluorobenzonitrile to the system
results in increase of the cathode wave and in decrease
of the I /I ratio as a consequence of electron transfer
a
/I
c
=
1
(
(
(
(
(
5)
6)
7)
8)
9)
a
c
from the anion-radical of anthracene to the 4-fluoro-
benzene molecule with anthracene recovery along re-
action (6). This result evidences that anthracene can be
applied as mediator. Actually, the main product of
4-fluorobenzene electrolysis in the presence of anthra-
cene was benzonitrile (see Table 1) that corresponded
to the fragmentation of the anion-radical as the prevail-
ing process.
Here P and Q are respectively neutral and anion-
radical forms of mediator, and SH is solvent.
In this case the electron transfer to substrate occurs
not at the electrode surface but in solution of the me-
diator which is present in a concentration considerably
The electrochemical reduction of 2,4-difluoro-
benzonitrile and 2,6-difluorobenzonitrile furnished
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 2 2003

2
14
EFREMOVA et al.
It was presumable that application of mediator
would decrease the probability of dimerization. Actu-
ally, the electrochemical reduction in the presence of
4-cyanopyridine as mediator resulted in reaction of
2,4-difluorobenzonitrile in a product mixture contain-
ing 70% of benzonitrile and no products with a double
molecular weight. Besides the properly selected me-
diator and the use of redox-catalysis permits preven-
tion of reduction of the primary defluorination product
since the equilibrium constant of the homogeneous
electron transfer (K) depends essentially on the differ-
ence between the standard reduction potentials of the
mediator and substrate, Ê = exp(–FÄE
even small (0.1- 0.2 V) changes in the ÄE
o
/RT) [12], and
va lu e sig-
î
nificantly affect the rate of electron transfer. In keep-
ing with above at electrochemical reduction of 2,6-di-
fluorobenzene with 9,10-anthraquinone as mediator at
reduction potential corresponding to reduction of the
anthraquinone anion-radical to dianion a single
fluorine atom was successfully eliminated, and
-
1
Cyclic voltammogram of 10 M anthracene solution
in CH3CN, supporting electrolyte 0.1 Ì TEAP, v
0 mV sec (1) with no 4-fluorobenzonitrile, (2) in the pre-
-
1
5
sence of 4-fluorobenzonitrile.
2-fluorobenzonitrile was obtained (Scheme 3).
Scheme 3.
As showed the experiment with pentafluorobenzo-
nitrile, the accumulation of fluorine atoms in the ben-
zene ring led to complication of the process by polym-
erization and tarring on the electrodes. As a result it is
impossible to carry out the reaction in a preparative
mode. It was attempted to overcome these difficulties
by application of a mediator. To the requirements
given above for mediators correspond nitrobenzene,
4-nitrotoluene, 9,10-anthraquinone, 9- nitroanthracene.
Although at electrochemical reduction of pentafluoro-
benzonitrile in the presence of first two of the men-
tioned mediators the product of fluorine elimination
from the para-position with respect to cyano group
was obtained, but its yield was very small (see Table 1).
This fact apparently shows that the yield of reaction
Scheme 4.
products depends not only on the difference in E of
0
the mediator and the substrate but also on the structure
of the anion-radicals used. On the contrary the electro-
chemical reduction of the pentafluorobenzene in the
presence of 9-nitroanthracene resulted in successful
selective elimination of fluorine atom only from the
para-position at complete conversion of the initial
compound (Scheme 4).
benzonitrile. In both cases no product of a single fluo-
rine elimination , 2-fluorobenzonitrile, was found. Ap-
parently the latter product underwent reduction under
experimental conditions for the reduction potentials of
2,4-difluoro- and 2-fluoronitriles are virtually equal (in
DMF E - 2.04 V), and those of 2,6-difluoro and
9
2
-
-fluorobenzonitriles are sufficiently close (in DMF
1.90 V and - 2.04 V respectively). In the first case in
Thus it was established that in preparative mode of
electrolysis 2-fluoro and 3,4,5-trifluorobenzonitrile
provide the desired product at direct electrochemical
the products mixture were found also unidentified sub-
stances that apparently originate from further electro- reduction. For the other studied benzonitriles were se-
chemical reduction of dimerization products (Scheme 3).
lected mediators that provide a possibility to direct the
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 2 2003

PREPARATIVE ELECTROCHEMICAL REDUCTION OF FLUORINATED BENZONITRILES
215
Table 1. Experimental conditions and products of preparative electrochemical reduction of fluorinated benzonitriles
C6FnH5–nCN
Amount of
compound, mg
stituents (concentration, Ered, V lysis
Composition of reaction mixture (ac-
cording to GLC data)
Sub-
Electro-
Mediator
[amount, mg, (concentrationèÿ,
M)]
initial
compound,
%
defluorina- dimeriza- Solvent
tion tion
product, % product, %
Ì )
time, h
–
2
2
4
-F
-F
60 (5 ´ 10 ) –2.10
3
80
DMF
MeCN
DMF
–
2
43.5 (7.2 ´ 10 ) –2.20
3.25
3.5
3.25
65.1
–
1
–3
6
1.2 (1 ´ 10 ) –1.85
Anthracene [2 (1 ´ 10 )]
52
15
43
21.5
2.6
–
2
2
2
,4-F2
,6-F2
74 (5.3 ´ 10 ) –2.20
Polymeric MeCN
(benzonitrile) products
–
2
7
0 (5 ´ 10 ) –1.80
3
4-Cyanopyridine [5.2 (5 ´
15
23
70
(benzonitrile)
77
MeCN
DMF
–
3
1
0 )]
–
2
71 (7 ´ 10 ) –1.86
4.25
(
benzonitrile)
–
2
2
0.7 (2 ´ 10 ) –1.45
8.25
3
3.5
3.5
3
2.25
2.25
4.5
3
9,10-Anthraquinone [4 (2 ´
95.7
59.3
1.1
1.5
0
10.5
16
0
DMF
–
2
–3
3
F5
,4,5-F3 100 (9.1 ´ 10 ) –1.85
10 )]
37
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
–
2
–3
67.5 (5 ´ 10 ) –1.27
4–Nitrotoluene [1.4 (10 )]
Nitrobenzene [1.2 (10 )]
9,10-Anthraquinone [2 (10 )]
17.7
18.5
50
84.1
67.5
34.3
–
2
–3
6
6
6
7.5 (5 ´ 10 ) –1.23
–
2
–3
7.5 (5 ´ 10 ) –0.95
–
2
–3
7.5 (5 ´ 10 ) –0.94
4,4'-Dinitrodiphenyl [2.5 (10 )]
–
–
1.05
1.10
–
2
–3
6
1.5 (5 ´ 10 ) –1.10
9-Nitroanthracene [2 (10 )]
69
Table 2. Experimental conditions and products of preparative electrochemical reduction of fluorinated benzonitriles
C6FnH5–nCN in the presence of 0.1 M tetrabutylammonium thiophenolate
Amount of
Mediator
Composition of reaction mixture (according to
Substitu- compound, mg
Electrolysis [amount, mg,
GLC data)
product of
ents
(concentration, Ered, V
Ì )
time, h
(concentration, dimerization defluorination
Solvent
M)]
product, % product, %
nucleophilic
substitution, %
20
–
2
2
2
-F
,4-F2
60 (5 ´ 10 )
–2.10
–2.10
3
3
80
DMF
MeCN
–
2
70 (5 ´ 10 )
+
+
benzonitrile)
15
(
–
2
7
0 (5 ´ 10 )
–1.80
3
4-Cyanopyridine
70
MeCN
–3
[5.2 (5· 10 )]
(benzonitrile)
2,4-
(PhS)2 C6H3CN
Scheme 5.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 2 2003

2
16
EFREMOVA et al.
transformation of anion-radicals on the fragmentation
route and thus to obtain in sufficiently high yield de-
fluorination products.
addition complication consists in irreversible dimeriza-
tion as show compounds with a double molecular weight
among the products. It was presumable that the encoun-
tered complications might be avoided by using a media-
tor since in this case the reduction of nitrile would occur
in solution and not on the electrode. Then the probability
of reduction of the cyanophenyl radical produced by frag-
mentation of the primary anion-radical would be a lot
smaller, and its main reaction would be that with thio-
phenolate anion. The reaction of 2,4-difluorobenzonitrile
with the thiophenolate anion initiated electrochemically
was carried out with 4-cyanopyridine as mediator in ace-
tonitrile solution at the reduction potential of the cyano-
pyridine. As the main product was obtained in 70% yield
the compound originating from the double substitution,
2,4-diphenylthiobenzonitrile. This result can be rational-
ized as follows: The intermediately arising anion-radical
of 2-fluoro-4-phenylthiobenzonitrile is unstable and un-
dergoes fragmentation with elimination of fluorine in the
ortho-position with respect to cyano group transforming
into 4-phenylthiocyanophenyl radical that further reacts
with the thiophenolate anion giving the product of double
substitution (Scheme 5).
As was already mentioned, the fragmentation of
anion radicals of the haloaromatic compounds is the key
stage of nucleophilic substitution reactions of SRN
1
type [13].
Many examples are known of the use of com-
pounds ArHlg (Hlg = I, Cl, Br) in this type reactions,
but with substituted fluoroarenes exist only few exam-
ples [14]. With fluorinated benzonitriles the reactions
of nucleophilic substitutions proceeding along SRN
1
mechanism may be complicated due to irreversible
dimerization of anion-radicals, but we hoped that this
can be avoided by using mediators.
Dimerization of anion-radicas of 2-fluorobenzo-
nitrile is reversible and therefore should not bring about
the mentioned complications. Therefore we tried to
carry out the nucleophilic fluorine substitution in
Thus the results obtained demonstrate the promising
prospects of applying mediators in reactions of reductive
2
-fluorobenzonitrile at direct electrochemi cal reduc- defluorination and aromatic nucleophilic substitution along
S
RN 1 mechanism for fluorinated aromat ic compounds.
tion. The cyclic voltammogram showed that in the
electrochemical reduction of this compound in the
presence of tetrabutylammonium thiophenolate oc-
curred fluorine substitution with a thiophenyl group as
evidenced by appearance of oxidation wave of
EXPERIMENTAL
Cyclic voltammograms were registered on a modi-
fied polarograph LP-7e equipped with three-electrode
2
-phenylthiobenzonitrile [7]. The preparative elec- cell. As the main electrode served a platinum disk of
2
trolysis of 2-fluorobenzonitrile in the presence of
0.07 cm surface area, the supplementary electrode
Bu
4
NSPh as supporting electrolyte furnished benzoni-
was of a platinum spiral, and as reference electrode
was used a saturated calomel electrode. Electrochemi-
cal measurements were performed with solutions of
the initial compounds in DMF or acetonitrile of con-
trile and 2-phenylthiobenzonitrile in 80 and 20% yield
respectively. The low yield of the product of nucleo-
philic substitution is caused by competition in DMF
between the trapping of the cyanophenyl radical with
the nucleophile and hydrogen rupture from the solvent
molecule by this radical [15].
-
3
centrations (1- 5) ´ 10 M, supporting electrolyte
.1 M tetraethylammonium perchlorate. To remove
0
oxygen the cell and solutions were flushed with argon.
Anhydrous DMF was prepared by double vacuum dis-
tillation over phosphorus pentoxide collecting the frac-
tion of bp 34°C (1 mm Hg). The acetonitrile was puri-
fied by distillation over potassium permanganate fol-
lowed by double distillation over phosphorus pentox-
ide. The solvents were stored over molecular sieves.
The direct preparative electrochemical reduction of
2,4-difluorobenzonitrile did not give any product of
nucleophilic substitution at all. This is apparently due
to the high rate of fragmentation of the anion-radical
2
-1
from 2,4-difluorobenzonitrile (k = 10 s [6]). As a re-
sult the anion-radical arising on th e electrode has not
enough time to pass into the volume of the solution, and
the cyanophenyl radical formed close to the surface of
the electrode further undergoes reduction into anion. An
The preparative electrolysis was carried out with
the use of a potentiostat P-5848 in a potentiostatic
mode in a three-electrode cell. As catode served a
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 2 2003

PREPARATIVE ELECTROCHEMICAL REDUCTION OF FLUORINATED BENZONITRILES
217
2
platinum plate of 1 cm surface area, as anode a plati-
num spiral.
was placed into the cell together with the substrate,
and the electrocynthesis was carried out at the reduc-
tion potential of the mediator. More detailed experi-
mental conditions of reduction of benzonitriles under
study are given in Table 1. For benzonitriles reduced
with mediators were carried out control runs without
mediators under the same experimental conditions. In
all such control runs the final mixture contained only
the initial nitrile.
As a reference electrode was used a saturated aque-
ous calomel electrode. After treatment the reaction
mixture containing the products of electrochemical
synthesis were analyzed by GLC on a chromatograph
LKhM-72 under the following conditions: stationary
phase 15% of SKTFB-803 on Chromosorb W, oven
temperature programmed in the range from 80
to 250°C at a rate 19 deg min , detector tempera-
ture 250°C, vaporizer temperature 250°C, column
000 ´ 4 mm. IR spectra were recorded on spectropho-
tometer UR-20. Precise values of molecular ions mass
were measured by high resolution mass spectroscopy
on Finnigan MAT 8200 in strument. H NMR spectra
were registered on spectrometer Bruker WP-200SY, as
internal reference were used residual protons in deu-
terochloroform solvent (d 7.24 ppm).
-
1
Ele ctrochemical reduction of fluorinated benzo-
nitriles in the presence of thiophenolate anion. The
electrochemical reduction of fluorinated benzonitriles
in the presence of thiophenolate -anion was carried out
as described above, but the 0.1 M TEAP solution was
2
1
replaced by 0.1 M Bu NSPh solution. The electrolysis
4
conditions and the composition of reaction mixtures
are given in more detail in Table 2. The electrolysis
products were separated by TLC on silica ge l- gypsum
plate (silica gel of L100-160 m grade), eluent chloro-
form.
Fluorinated benzonitriles and tetrabutylammonium
thiophenolate were prepared by published methods
[
16- 20, 6]. The characteristics of all compounds ob-
2
-Fluorobenzonitrile. The 2-phenylthiobenzo-
tained are consistent with the published data. As me-
diators were used anthracene, mp 216- 217°C, 9,10-
anthraquinone, mp 285–286°C, 4-cyanopyridine, mp 79–
8
8
nitrile obtained was purified by recrystallization from
diluted methanol., bp 57- 59°C (cf. [22]). IR and
NMR spectra of compound obtained were fully in
agreement with the assumed structure.
1°C, 4-nitrotoluene, mp 51–53°C, nitrobenzene, bp
5.4± 0.2°C/10 mm Hg.; 9-nitroanthracene, mp. 146–148°
C; bp 239°C (8 mm Hg) (cf. [21]).
2,4-Difluorobenzonitrile . According to GC-MS
data the reaction mixture contained alongside benzoni-
trile also compounds with molecular weight about twice
as large as that of the initial nitrile, and a number of
unidentified reaction products of minor content; the lat-
ter compounds were similar to those obtained in the ab-
sence of the nucleophile. No products of nucleophilic
substitution were detected.
Ele ctrochemical reduction of fluorinated benzo-
nitriles. Into an electrrochemical cell was poured 0.1
M solution of TEAP in an aprotic solvent (7 ml into
the cathode chamber and 4 ml into the anode
chamber), into the cathode chamber was charged
the required amount of fluorinated benzonitrile
(
c 0.05- 0.1 M). The current was supplied to the cell at
the voltage required for the electrosynthesis. The proc-
ess was carried out at a constant potential, at room
temperature, with stirring by a magnetic stirrer, and at
constant argon stream through the cell. The reaction
was monitored by intermittent recording of voltammo-
grams of the reaction mixture. On completion of the
reaction the reaction mixture from the cathode cham-
ber was poured into 20 ml of water and depending on
pH of the resulting mixture it was neutralized either
with hydrochloric acid or sodium hydrogen carbonate.
The reaction products were extracted into ethyl ether
2
,4-Difluorobenzonitrile in the presence of me-
diator. By means of TLC as described above were
separated from the reaction mixture benzonitrile and
the main fraction of mp 80- 130°C that on recrystalli-
zation from hexane afforded colorless powder of mp
1
08- 110°C.
1
9
According to the data of F NMR spectroscopy the
compound contained no fluorine. As found from mass
spectrum:
Ì
319.0485;
C
19
H
13NS
2
.
Calculated
319.049. Mass spectrum, m/z (Iîòí., %) 319 (100); 210
(
5 ´ 7 ml) and dried with calcined magnesium sulfate.
(22.9); 209 (42.6), 183 (6.5); 139 (6.7), 109 (12.5); 77
The mixture obtained on evaportion of ether was
sybjected to GLC analysis. At the use of mediator it
(17.5); 65 (11.5); 51 (19.2). IR spectrum (in CHCl
cm : 3070–3030, 1100–1050 (Cað–H); 2230 (Cº N);
3
), n,
–
1
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 2 2003

2
18
EFREMOVA et al.
1
1
580, 1500-1450 (C=Cað). H NMR spectrum (CDCl
3
),
12. Mairanovskii, V.G., Elektrosintez monomerov. Pr o-
gress elektrokhimii organicheskikh soedinenii (Elec-
trosynthesis of Monomers. Progerss of Electrochem-
istry of Organic Compounds) Moscow: Nauka, 1980,
p. 247.
d, ppm: 6.59 d (1H, CHað, JH,H 1.8 Hz), 6.88 d.d (1H,
CHað, JH,H 1.8, 8.2 Hz), 7.40 d (1H, CHað, JH,H 8.2 Hz),
7
.1–7.5 m (10H, 10CHað).
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