Fluoroalkylation of thiophenols with Freons using conjugated electron transfer mediator systems composed of methylviologen-SO2 and I2-SO2

Article abstract of DOI:10.1016/S0022-1139(99)00069-X

Methylviologen-sulfur dioxide and iodine (iodide)-sulfur dioxide have been shown to mediate electron transfer from thiophenols to Freons and perform the fluoroalkylation of thiophenols under mild conditions.

Full text of DOI:10.1016/S0022-1139(99)00069-X

Journal of Fluorine Chemistry 96 (1999) 163±166
Fluoroalkylation of thiophenols with Freons using conjugated electron
transfer mediator systems composed of methylviologen-SO and I -SO
2
2
2
*
V.G. Koshechko , L.A. Kiprianova, L.I. Fileleeva, K.G. Tsanov
L.V. Pisarzhevsky Institute of Physical Chemistry of the National Academy of Sciences of Ukraine, Pr. Nauky 31, Kiev 252039, Ukraine
Received 30 November 1998; accepted 11 March 1999
Abstract
Methylviologen-sulfur dioxide and iodine (iodide)-sulfur dioxide have been shown to mediate electron transfer from thiophenols to
Freons and perform the ¯uoroalkylation of thiophenols under mild conditions. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Fluoroalkylation; Thiophenols; Freons; Methylviologen; Sulfur dioxide; Catalysis; Radical-chain process
1
. Introduction
It is rather well known that the popular use of Freons as
aerosols, coolants, etc. has caused a signi®cant reduction of
the stratospheric ozone layer. However, an alternative use of
these harmful chemicals involves their utilization in the
preparation of intermediates in organic synthesis, particu-
larly in the synthesis of ¯uoroalkyl compounds [1±3]. It has
been shown that methylviologen can serve as an effective
catalyst in the preparation of per¯uoroalkylsul®des from the
reaction of per¯uoroalkyliodides with thiophenols [4]. A
direct transfer of electron from thiophenol to Freons in the
presence of viologen has been deemed not possible. How-
F13B1 (CF Br) and F113 (CFCl CF Cl), and thiophenol
2
3
2
and p-thiocresol were used in this study as Freons and
nucleophiles, respectively. This reaction was conducted in
a 1 : 1 mixture of dimethylformamide (DMF) and pyridine
to enhance the nucleophilicity of thiophenols.
2. Results and discussion
ever, it has been demonstrated that SO can serve as an
2
‡ꢀ
effective mediator in electron transfer from a cathode to
CF Br and thereby lead to the electrochemical tri¯uoro-
As shown by spectrophotometric results, both MV
iodides and chlorides react with thiophenols via the forma-
tion of the corresponding radical cation (Diagram 1, step 1).
The immediate appearance of the intense absorption bands
3
methylation of thiophenols [5,6]. In view of the poor af®nity
of SO for electrons, it is dif®cult for SO to accept an
electron from thiophenol and transfer it to CF Br in the
2
2
ꢀ
‡
at ꢀmax ˆ 398 and 609 nm, characteristic of MV when
3
2
‡
absence of potential, high pressure and temperature. In so far
as each of the above-mentioned mediators perform only half
of the function, namely either to accept an electron from the
nucleophile (thiophenol) or to transfer it to Freon, it was
considered interesting to explore the mutually complemen-
tary use of these mediators to activate Freons via a cascade
MV was added to solutions of thiophenols, serves as the
evidence for this phenomenon. The rate of single-electron
reduction of methylviologen diiodide with thiophenols to
ꢀ
ꢀ
‡
‡
MV , spectrophotometrically recorded for MV by the
ꢀmax ˆ 609 nm adsorption band intensity, is described by a
kinetic equation of the second-order with ®rst-order for each
of the reagents:
electron transfer from the nucleophile to Freons (R X). The
F
highly reactive R radicals thus generated can then be used
F
v ˆ k ‰PhSHŠ‰MV2‡2I� Š
2
in the per¯uoroalkylation of thiophenols under mild homo-
geneous conditions as shown below:
3
2
0
for C H SH and p-CH C H SH, with k ˆ 4.6 and
6
5
�
3
6
4
1
� 1
7
.2 l mol
s , correspondingly. The kinetic data show that
2‡
the interaction of thiophenols with MV proceeds with a
rather high rate. When adding F13B1 and F113 Freons into
*
Corresponding author. Tel.: +380-44-265-66-65; fax: +380-44-265-62-
1
0
6; e-mail: ipcukr@sovamsu.sovusa.com
022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 0 6 9 - X

1
64
V.G. Koshechko et al. / Journal of Fluorine Chemistry 96 (1999) 163±166
SO is re¯ected in the results obtained spectrophotometri-
2
ꢀ
‡
cally. During the addition of SO to the MV cation radical
2
ꢀ
‡
solution, a rapid disappearance of the characteristic MV
absorption band and the appearance of a new absorption
band as ꢀmax ˆ 434 nm can be seen. It is possible to
ꢀ
‡
regenerate the MV cation radical electrochemically or
2‡
chemically, for example, by adding zinc to reduce MV to
‡
ꢀ
MV
.
According to the results of the examination of the elec-
trochemical processes involved (Fig. 1(c)), the addition of
2
‡
CF Br to the system containing MV
and SO2 will
decrease the catalytic current at the reduction potential of
3
ꢀ
‡
MV , which is probably caused by the interaction of the
SO anion radical being formed with Freon (Diagram 1, step
2
III). A possibility of realizing this electron transfer channel
has already been described by us [6] and others [5].
This system of two mutually complementary mediators,
2‡
MV -SO , is able to activate Freons under mild conditions
2
via a cascade of electron transfer from the nucleophile to
Freon (Diagram 1) and can be used on a preparative scale.
When bubbling F13B1 Freon through the solution of thio-
phenol or p-thiocresol in the mixture of DMF and pyridine
2‡
�
containing catalytic amounts of MV 2I (3±4% of thio-
phenol) and SO , the yields of C H SCF and p-
2
6
5
3
CH C H SCF were 74% and 41%, respectively. The simul-
3
6
4
3
taneous formation of other compounds such as tri¯uoro-
methanesul®nic acid (CF SO H) was also noticed. As
shown in the case of thiophenol (Table 1), the variation
3
2
in PhSH and SO concentration ratios allow an activation
2
process directed towards the synthesis of both C H SCF
3
6
5
and CF SO H in rather high yields. The following mechan-
3
2
‡
�
2
Fig. 1. Cyclic voltammetry of MV 2I -SO
� 4
2
in DMF ‡ Py ‡ 0.1 M
2‡ �
� 3
2‡
�
ism describes the involved processes:
Bu
(
4
NBF
4
at 258C. (a) MV 2I (5 Â 10 mol) alone; (b) MV 2I
�
4
ꢀ
ꢀ
5 Â 10 mol) after addition of SO
2
(2.5 Â 10 mol); and (c) after
�
2‡
‡
1
2
3
. PhS ‡ MV ! PhS ‡ MV
�
1
further bubbling of the excess of CF
3
Br. Scan rate: 0.2 v s
.
ꢀ
ꢀ
‡
2‡
�
. MV ‡ SO @ MV ‡ SO
2
2
ꢀ
� ꢀ
ꢀ
3
�
�
. SO ‡ CF Br ! SO ‡ CF Br ! CF ‡ Br
2
3
2
3
ꢀ
3
� ꢀ
�
‡ꢀ
4. CF ‡ PhS ! ‰CF3SPhŠ
the solution of the MV cation radical, the concentration of
‡
� ꢀ
‡ꢀ
2
‡
ꢀ
5. ‰CF3SPhŠ ‡ MV ! CF
3
SPh ‡ MV
MV does not change.
� ꢀ
ꢀ
�
2
6
7
. ‰CF3SPhŠ ‡ SO ! CF SPh ‡ SO
2
3
This observation suggests that in the absence of other
mediators, the radical cation cannot transfer the electron
from thiophenol to Freon. To see whether the electron
ꢀ
ꢀ
�
�
. CF ‡ SO ! CF SO
3
2
3
2
Steps 1±3 show a cascade of electron transfer from the
nucleophile to Freons and step 4±6 show the interaction
between trifluoromethyl radicals formed during Freon acti-
vation with thiophenolate by radical-chain mechanism
transfer can occur from methylviologen to SO , cyclic
2
voltammetry has been used to monitor the transfer process
‡
ꢀ
for MV iodides and chlorides both in the presence and
absence of SO . As shown in Fig. 1(a), MV dication in the
2‡
2
absence of SO is reversibly reduced in DMF to the corre-
2
sponding cation at the potential of � 0.4 V. On adding SO ,
Table 1
2
The dependence of trifluoromethylphenylsulfide and trifluoromethylsul-
which is dif®cult to reduce under similar conditions
E ˆ � 0.9 V) from the MV cyclic voltagram, there is
2
‡
finic acid yields on sulfur dioxide concentration in the F13B1 activation
2‡
(
an increase in the cathode current peak and a decrease in
p
process using a system of electron transfer mediators, MV -SO
2
SH] ˆ10� 3 M, [MV 2Cl ] ˆ 5  10 M)
2‡
�
� 5
(
[C
6
H
5
ꢀ
‡
anodic current relative to the MV cation radical
(Fig. 1(b)). The observed changes suggest that at the elec-
trochemical stage in the homogenous reaction, the MV
[
SO
2
] (M)
C
6
H
5
SCF
3
yields (%)
CF SO
3
2
H yields (%)
ꢀ
‡
� 4
5 Â 10
12
74
65
56
5
21
78
76
�
3
1
1
 10
�
cation radical can reduce the SO molecule and as such can
2
3
.4 Â 10
serve as a mediator of electron transfer from cathode to SO .
2
� 3
ꢀ
2.0 Â 10
‡
The ability of MV cation radical to transfer an electron to

V.G. Koshechko et al. / Journal of Fluorine Chemistry 96 (1999) 163±166
165
conditions, it was observed that molecular iodine possesses
a redox potential close to that exhibited by the methylviolo-
gen dication. As shown in Fig. 2, I can be reduced rever-
2
sibly at a potential of � 0.39 V. In the presence of SO ,
2
considerable growth of the reduction wave of I and practi-
2
cally a complete disappearance of the reverse oxidation
wave (Fig. 2(b)) are seen. As demonstrated spectrophoto-
metrically, the observed catalytic effect can be caused by the
electron from reduced I getting added to SO . It was found
2
2
that the addition of an excess of SO to KI leads to a yellow
2
colored solution and increases the absorption of molecular
iodine in electronic spectra ꢁꢀmax ˆ 366 nm†.
Molecular iodine, on the other hand, can also react with
�
thiophenol, generating I . This phenomenon is demon-
strated by a disappearance of the colour of the solution
and absorption band of I when double the amount of
2
thiophenol is added to the iodine solution. The above results
show the conditions of mutually complementary mediation
for electron transfer from iodide ion or reduced by thio-
phenolate-I2 to SO2 to activate Freons. Indeed as was
observed in the preparative scale reaction, upon bubbling
of CF Br into the solution containing equimolar quantities
3
of thiophenol, SO and 12% I , C H SCF and CF SO H
2
2
6
5
3
3
2
were formed in 31% and 9% yields, respectively. Instead of
I2, if iodide ion is used as a co-catalyst in the reaction of
CF Br with thiophenol in the presence of KI and SO
3
2
(
C H SCF and CF SO H drops to 20% and 7%, respec-
[PhSH] ˆ [SO ] ˆ [KI] ˆ 0.002 mol), the formation of
2
6 5 3 3 2
tively. Lower yields of C H SCF and CF SO H are
2
6
5
3
3
�
obtained using I -SO and I -SO catalytic systems as
2
2
2
2
‡
compared to MV -SO . The molecular I in the I -SO
2
2
2
2
�
and I -SO catalytic systems can serve to trap free radicals
2
in the radical chain process of tri¯uoromethylation, the
example being CF radicals derived from F13B1 Freon
3
activation. This inference is con®rmed by the formation
of CF I in these systems along with C H SCF and
CF SO H. Thus, in the case of the reaction of
3
6
5
3
3
2
�
C H SH ‡ CF Br with I -SO and I -SO systems, CF I
6
5
3
2
2
2
3
is formed in 16% and 10% yields, respectively. The above
mentioned transformation can be described schematically as
follows:
ꢀ
�
�
1
. 2C H S ‡ I ! 2C H S ‡ 2I
6
5
2
1
2
6
5
Fig. 2. Cyclic voltammetry of I
2
2
-SO
2
in DMF ‡ Py ‡0.1 M Bu
4 4
NBF at
ꢀ
�
�
2
4
� 3
2. I ‡ SO @ I ‡ SO
58C. (a) I
SO . Scan rate: 0.2 v s
2
(1 Â 10� mol) alone and (b) after addition of 2 Â 10 mol
2
2
ꢀ
� ꢀ
ꢀ
3
�
1
�
�
2
.
3. SO ‡ CF Br ! SO ‡ CF Br ! CF ‡ Br
2
3
2
3
�
3
ꢀ
ꢀ
�
4
5
. CF ‡ C H S ! C H SCF
3
6
5
6
5
.
SRN1. The interaction between trifluoromethyl radicals
either with thiophenolate or with SO depends on the ratio
2
of the concentration of thiophenol and SO , and hence leads
2
ꢀ
ꢀ
�
�
6
7
8
. CF ‡ SO ! CF SO
3
2
3
2
to a preferable formation of either C H SCF or CF SO H.
6
ꢀ
ꢀ
5
3
3
2
. CF ‡ I ! CF I ‡ I
2‡
3
2
3
That MV -SO is able to activate the interaction of thio-
2
ꢀ
. 2I ! I
Here, Steps 1 and 2 serve as the initiation of the chain
2
phenols with F13B1 and F113 Freons under mild conditions
was definitely ascertained. In the latter case,
C H SCF CFCl was obtained in 34% yield.
process, while Steps 3±5 and Steps 6±8 represent chain
propagation and chain termination, respectively. Instead of
I2, if KI is used, then the initiation reaction begins with Step
6
5
2
2
In the search for the catalytic systems for successive
electron transfer from the nucleophile to Freon under mild

1
66
V.G. Koshechko et al. / Journal of Fluorine Chemistry 96 (1999) 163±166
2
system of mutually complementary mediators MV -SO2
. The results of the present investigation show that the
2
3.1. Perfluoroalkylation of thiophenols in the presence of
methylviologen and sulfur dioxide
‡
�
and I /I -SO are capable of realizing a subsequent electron
2
2
2
‡
�
� 3
transfer from the nucleophile to F13B1 and F113 Freons.
This process permits the ¯uoroalkylation of thiophenols
under mild conditions, and especially if methylviologen
diiodide is used as a co-catalyst, two channels of electron
To the solution of MV 2I (3 Â 10 mol) in 3 ml DMF
and 1.5 ml of pyridine under argon, was added
�
3
1 Â 10 mol of thiophenol or p-thiocresol (blue colour),
�
3
1 Â 10 mol of sulfur dioxide (yellow colour), and an
excess of F13B1 bubbled for 4±6 h. From 19F-NMR, the
yields of C H SCF (ꢁ ˆ 120 ppm) and CF SO H
transfer to SO can be opened: one from thiophenol through
2
ꢀ
‡
MV intermediate cation radical and the second with
6
5
3
3
2
2
‡
MV iodide ion. Apparently, the above-described system
of complementary mediators activates Freons through a
path of electron transfer not only from thiophenols but
also from other nucleophiles, provided the nucleophile
oxidation potential is close to or more negative as compared
to the reduction potential of the mediator conjugated with
SO2.
(ꢁ ˆ 76 ppm) have been calculated. The reaction mixture
was diluted with water, extracted with ether or chloroform,
and the organic layer washed with alkali water, then acid-
i®ed, dried and fractionated. The physical parameters (such
1
as boiling or melting point), refractive indices, spectra ( H-
and 19F-NMR) of the tri¯uoromethylalkylsul®des agreed
with the literature data [3].
The ¯uoroalkylation of thiophenols with F113 Freon and
the isolation of the products were similarly conducted but
�
2
� 2
3
. Experimental
with 1 Â 10 ±2 Â 10 mol of F113 instead of CF Br. The
3
same technique was used for the tri¯uoromethylation of
thiophenols with F13B1 Freon in the presence of iodine and
SO . The concentration of the starting substrates were as
Electrochemical investigation has been carried out using
the potentiostat PI-50-1 and programmer PR-8 in a thermo-
stated three-electrode cell with spot platinum electrode
(0.7 mm), the reference electrode Ag/AgCl, and supporting
electrolyte, Bu NBF (0.1 M), recrystallized from an ethy-
lacetate±pentane mixture. The NMR- F spectra were
recorded on a 235-MHZ Bruker spectrometer CXP-90,
and the 19F chemical shifts are given in ppm by reference
to C F . The yields of the products have been determined by
NMR-spectroscopy. The electron spectra have been
recorded with the aid of spectrophotometer Specord M-
2
�
3
follows: [PhSH] (or [p-CH C H SH]) ˆ 2  10 mol;
3
6 4
[I ], 10±12% of the quantity of thiophenol; [SO ] ˆ
2
2
�
3
� 2
� 2
2  10 mol; and [CF Br] ˆ 1  10 ±1.5  10 mol.
4
4
3
19
Acknowledgements
6
6
The authors of the present paper thank the Fund for
Fundamental Research, Ministry of Science and Engineer-
ing of Ukraine for partial ®nancial support of the present
work.
1
0 in 10 mm quartz cuvettes in dimethylformamide. Thio-
phenol was puri®ed by distillation and p-thiocresol was
puri®ed by acid precipitation from its alkaline solution and
subsequent recrystallization. DMF and pyridine were pur-
i®ed as described elsewhere [7].
References
Cyclic voltammograms were recorded over the range
0.1±� 0.6 V under pure argon in a cell containing 3 ml
[1] N. Ishikava, Y. Kobayashi, Fluorine. Chemistry and Application,
Mir, Moscow, 1982.
�
2‡ �
[2] L.M. Yagupolskii, Aromatic and Heterocyclic Compounds with
Fluorine Substituents, Naukova Dumka, Kiev, 1988.
DMF ‡ 2 ml of pyridine for methylviologen (MV 2X ,
2
‡
�
� 4
X ˆ Cl, I; [MV 2I ] ˆ 5  10 mol) whilst adding SO2
[
[
3] C. Wakselman, M. Tordeux, J. Org. Chem. 50 (1985) 4047.
4] V.G. Koshechko, L.A. Kiprianova, L.I. Fileleeva, Tetrahedron Lett.
33 (1992) 6677.
�
3
to this solution (2.5 Â 10 mol) and also whilst bubbling to
2
‡
MV -SO solution of an excess of F13B1 Freon
2
�
2
� 2
[
[
5] C.P. Andrieux, L. Gelis, J.-M. Saveant, J. Am. Chem. Soc. 112
(
1 Â 10 ±2 Â 10 mol). An analogous technique was
used to record the cyclic voltammograms of I2
(
1990) 786.
6] V.G. Koshechko, L.A. Kiprianova, L.I. Fileleeva, Z.Z. Roshkova,
J. Fluorine Chem. 70 (1995) 277.
�
4
� 3
(
1  10 mol), I ‡ SO (2  10 mol) and I ‡ SO ‡
2
2
2
2
�
2
2
CF Br (excess, 1 Â 10 ±2 Â 10 mol) in the same mixture
3
[7] A.J. Gordon, R.A. Ford, The Chemist's Companion, Mir, Moscow,
1976.
of solvents.