Activation of free-radical polyfluoroalkylation of organic substrates with freon BrCF2CF2Br using a system of organic base-electron transfer mediator

Article abstract of DOI:10.1016/j.jfluchem.2009.11.003

A polyfluoroalkylation of pyrrole and phenols with freon BrCF₂CF₂Br using sulfur dioxide-substituted pyridine activating system proceeds at the aromatic nucleus by a free-radical mechanism. The essential influence of the basicity of the medium is demonstrated and discussed.

Full text of DOI:10.1016/j.jfluchem.2009.11.003

Journal of Fluorine Chemistry 131 (2010) 208–211
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Activation of free-radical polyfluoroalkylation of organic substrates with freon
BrCF2CF2Br using a system of organic base-electron transfer mediator
*
Vyacheslav G. Koshechko, Lydiya A. Kiprianova , Ludmyla I. Kalinina
L.V. Pisarzhevsky Institute of Physical Chemistry of the National Academy of Sciences of Ukraine, Pr. Nauky 31, Kiev 01039, Ukraine
A R T I C L E I N F O
A B S T R A C T
Article history:
A polyfluoroalkylation of pyrrole and phenols with freon BrCF2CF2Br using sulfur dioxide-substituted
pyridine activating system proceeds at the aromatic nucleus by a free-radical mechanism. The essential
influence of the basicity of the medium is demonstrated and discussed.
ß 2009 Elsevier B.V. All rights reserved.
Received 20 July 2009
Received in revised form 6 November 2009
Accepted 9 November 2009
Available online 14 November 2009
Keywords:
Polyfluoroalkylation
Pyrrole
Hydroquinone
Sulfur dioxide
1. Introduction
phenols with dibromotetrafluoroethane [16], involving an organic
base and an electron transfer mediator. Interestingly, the process
Polyfluoroalkylation of aromatic and heterocyclic compounds
by freons attracted considerably less attention than their
fluoroalkylation with iodoperfluoroalkanes, which is largely
caused by the lower reactivity of freons as compared to that of
perfluoroalkyliodides. However, polyhalogenfluoroalkanes seem
interesting as, firstly, frequently cheaper sources of fluoroalkyl
groups than iodoperfluoroalakanes and, secondly, as they enable
the fluoroalkyl substituent to be further modified by halogen
replacement with other groups [1–7].
Interaction of thiophenols, phenols and some heterocyclic
compounds with freons is dependent on the state of the substrate
(either the starting substrate itself or its salt), on the nature of the
freon, and on the reaction conditions. Salts of the above-mentioned
compounds mainly undergo halophilic ionic reactions [3,8–13].
For certain freons, in particular CF2ClBr and CF2Cl2, such reactions
may proceed by a radical pathway, which suggests a common free
radicals-involving step for the halophilic and radical processes
[9,10].
feasibility and the products yield have been essentially dependent
on the strength of the bases – substituted pyridines – added to the
solution [14].
This paper has explored a possibility of employing the above-
mentioned ‘‘electron transfer mediator-organic base’’ activating
system for polyfluoroalkylation with freons of other classes of
organic compounds such as phenols and nitrogen heterocycles, in
particular pyrrole. We have payed attention especially to the effect
of the strength of the added bases on the fluoroalkylated product
yields as one major contributor to the implementation of such
processes.
One more interesting question is whether the activating
system makes it possible to introduce the fluoroalkyl group from
the freon into the alkylated substrates’ aromatic (heterocyclic)
nucleus, rather than to the heteroatom. This would expand the
synthetic capability of such processes. For phenols and pyrrol in
the presence of an electron transfer mediator and organic bases
to promote the radical route of the process, one would expect C-
polyfluoroalkylation rather than ionic halophilic O- or N-
alkylation [3,8–13]. Such possibility may be related to the
phenolates being harder bases than the thiophenolates and
consequently less efficient heteroatom-involving free-radical
traps. In the case of nitrogen heterocycles, in particular
imidazoles, the maximum electronic density [18,19] is mostly
concentrated on C- rather than N-atoms, which may result in C-
alkylated products.
The possibility of similar free-radical processes has been shown
also for the mild polyfluoroalkylation of thiophenols with freons
BrCF2CF2Br and CICF2CFCl2 [14,15] and of sterically hindered
* Corresponding author. Tel.: +380 44 525 54 14; fax: +380 44 525 62 16.
E-mail addresses: tkachenko_yuri@ukr.net, lkipr@inphyschem-nas.kiev.ua
(L.A. Kiprianova).
0022-1139/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2009.11.003

V.G. Koshechko et al. / Journal of Fluorine Chemistry 131 (2010) 208–211
209
Table 2
C- but not O- or N-alkylated products had been observed earlier
in the electrochemical perfluoroalkylation of phenols [17] at fairly
low yields and of nitrogen heterocycles [18] with iodoperfluor-
oalkanes, as well as in the perfluoroalkylation of these compounds,
promoted with sodium dithionite [19] and in the presence of
metals [20], zinc and sulfur dioxide [21], for which the radical
mechanism had been assumed.
Peak potential dependence of electrochemical reduction of sulfur dioxide on the pKa
of the pyridines added to the solution [SO2] = 7 Â 10À5 mol; [pyridi-
ne] = 2.5 Â 10À3 mol; DMSO; Pt; Bu4NClO4 0.1N; Ag/Ag+; 25 8C.
2-Acetylpyridine
Pyridine
2,5-Lutidine
g
-Collidine
pKa
3.18
5.23
6.25
7.60
Ep, V
À1.34
À1.16
À0.98
À0.88
2. Results and discussion
radical absorption band appears solely in the simultaneous
presence of pyrrole and
b-picoline, whereas without b-picoline
Under mild conditions, phenols and pyrrole unlike their salts
[3,8–13] do not react with BrCF2CF2Br. In order to activate electron
transfer from these substrates to the freons to form active
fluoroalkyl radicals, the electron-donating ability of starting
phenols and pyrrole should be enhanced without being converted
to salts to avoid halophilic transformations. We have supposed
that the electron-donating ability of the substrates could be
enhanced by adding strong organic bases to the solution in order to
drive the substrate to ‘‘the semi-ionised state’’. To verify this
assumption we have added substituted pyridines to solutions of
the phenols and pyrrole in aprotic solvents (DMFA, DMSO), as
pyridines are capable of forming complexes connected by
hydrogen bonds with the above-mentioned substrates [22]. As
expected, the electron-donating ability of such complexes
significantly increases in comparison with the starting substrates
as ascertained by cyclic voltammetry. This indicates a decrease in
the oxidation potential of these compounds when adding pyridine;
and the higher the pKa of the added pyridine, the more dramatic
the drop will be. For example, Table 1 demonstrates the peak
potential (Ep) dependence of electrochemical oxidation of p-
methylphenol in an acetonitrile-substituted pyridine mixture on
the pKa of the pyridine. In the absence of pyridine for p-
methylphenol, Ep = 0.91 V.
ꢀ
there is no sulfur dioxide reduction with pyrrole to form S2O4À . By
adding the freon to the solution under investigation, the
Àꢀ
absorption band of the S2O4 anion radical rapidly disappears,
which may be relevant to its interaction with the freon. Evidence
for subsequent electron transfer from SO2 anion radical to freon
Àꢀ
can be provided by the results of our previous electrochemical
investigations into cathodic SO2 reduction in the presence of the
freons [14–16], which demonstrates significant catalytic currents
in SO2 cyclic voltammograms when adding freon BrCF2CF2Br to the
solution.
Nitrogen-containing heterocyclic bases critically influence not
only the electron-donating properties of alkylated substrates but
also the ability of the SO2 mediator to accept an electron from the
substrate–pyridines complexes. The last fact could also be an
important reason for the influence of the basicity of the medium on
the efficiency of fluoroalkylation processes using the ‘‘electron
transfer mediator-base’’ activating system. As follows from Table
2, increasing basicity of the pyridines, added to the solution,
markedly increases the electron-accepting capacity of the sulfur
dioxide too, which, for its part, should facilitate the freon activation
process.
We have elucidated the ability of our activating system to
activate fluoroalkylation of nitrogen-containing nucleophiles,
pyrrole in particular. We have ascertained that the reaction of
pyrrole with BrCF2CF2Br in aprotic solvents in the presence of
sulfur dioxide and pyridine leads to pyrrole fluoroalkylation to
form 2-(2-bromotetrafluoroethyl)pyrrole by a procedure similar
to alkylation of thiophenols with this freon [14]. The yields of the
fluoroalkylated pyrroles depend significantly on the sulfur
dioxide concentration (Table 3) and on the pKa of the added
base (Table 4).
There is a linear dependence between the Ep of p-methylphenol
oxidation and the pKa of the pyridines (Ep = À0.057 pKa + 0.944,
r = 0.997). A similar dependence has been observed by us for
thiophenols, in particular for p-chlorothiophenol Ep = À0.083
pKa + 1.272, r = 0.971) [14]. As the process of electrochemical
oxidation of pyrrole in the presence of pyridines is complicated due
to the pyrrole polymerisation on the electrode to form
a
passivating polymer film, therefore it has been rather difficult to
determine the precise values of Ep for the pyrrole.
Adding pyridines such as 2-chloropyridine or 2-acetylpyridine
to the solution would not ensure the pyrrole’s sufficient electron-
donating capacity for freon activation and the fluoroalkylation
process fails. With further increasing pKa of pyridines, the
fluoroalkylation process proceeds more efficiently and the yield
of 2-(2-bromotetrafluoroethyl)pyrrole becomes practically quan-
Enhancing the donor strength of phenols and pyrrole via
addition of the pyridines to the aprotic solvent is a necessary but
insufficient condition for the substrate-to-freon electron transfer
and the progress of fluoroalkylation. To further activate the process
we have added sulfur dioxide as an effective electron transfer
mediator, which we have used previously for the fluoroalkylation
of some organic substrates with freons [14–16]. The ability of SO2
to accept an electron from phenol or pyrrole complexes with
nitrogen bases has been confirmed by the results of our spectro-
photometric investigations: addition of sulfur dioxide to a solution
titative when adding 2,5-lutidine. In the case of
g-collidine the
desired product yield drops, which obviously accounts for the
debromination of the freon with formation of tetrafluoroethylene,
of pyrrole with
b-picoline in a deaerated DMSO causes a blue
Table 3
colouring of the solution, and concurrently in the electron
spectrum we observe an absorption band (580 nm), attributed
Product yields of pyrrole–BrCF2CF2Br reaction in the presence of SO2 [pyrro-
le] = 1 Â 10À3 mol; DMSO; 35 8C.
Àꢀ
Àꢀ
to an S2O4 anion radical, i.e. dimerisation product of the SO2
Àꢀ
[SO2]
(Â10À3 mol)
[
b
-picoline]
[BrCF2CF2Br]
Product
and starting sulfur dioxide [23]. Importantly, the S2O4 anion
(Â10À2 mol)
(Â10À3 mol)
yields (%)
–
1.0
1.0
1.0
1.0
1.0
1.0
1.0
–
4.0
4.0
4.0
4.0
4.0
4.0
1.0
4.0
–
Table 1
0.5
1.0
2.0
4.0
2.0
1.0
1.0
18
51
85
55
Peak potential (Ep) dependence of electrochemical oxidation of p-methylphenol on
the pKa of the pyridines added to the system [p-CH3C6H4OH] = 0.001 M;
[pyridine] =2.5 Â 10À3 M; Pt; NaClO4 0.1N; CH3CN, Ag/Ag+ 25 8C.
a
–
2-Chloropyridine Pyridine
b
-Picoline 2,4-Lutidine
g-Collidine
78
–
pKa
0.72
5.23
0.66
5.97
0.60
6.82
0.55
7.60
0.51
Ep (V) 0.90
a
p-Dinitrobenzene is added.

210
V.G. Koshechko et al. / Journal of Fluorine Chemistry 131 (2010) 208–211
Table 4
Dependence of the yield of the polyfluoroalkylated pyrrole on the basicity of the
pyridines added to the system [pyrrole] = 0.5 Â 10À3 mol; [SO2] = 0.44 Â 10À3 mol;
[BrCF2CF2Br] = 0.8 Â 10À3 mol; [substituted pyridine] = 1.0 Â 10À2 mol; DMSO, 35 8C.
Base
pKa
Product yield (%)
CF25CF2
2-Chloropyridine
2-Acetylpyridine
Pyridine
0.72
3.18
5.23
5.97
6.25
7.60
5.97
–
–
–
–
–
–
–
8
–
78
89
95
62
–
b
-Picoline
2,5-Lutidine
-Collidine
g
b
-Picolinea
Both the bromide ion and the pyridine may act as the proton
acceptor.
a
p-Dinitrobenzene is added.
As seen from the above-mentioned mechanism, the base is very
important not only in the pyrrole oxidation step (Scheme 1a),
enhancing the ability of the pyrrole to donate the electron to the
mediator and then to the freon, but also in the proton abstraction
step (1b). With increasing pKa of the base, its proton-accepting
ability is increasing, too.
Table 5
Dependence of the yield of the polyfluoroalkylated p-methylthiophenol on the
basicity of the pyridines added to the system. [p-methylthiophenol] = 5 Â 10À4 mol;
[BrCF2CF2Br] = 1 Â 10À3 mol; [SO2] = 7 Â 10À4 mol; DMFA; 25 8C.
Base
pKa
Yield of p-CH3C6H4SCF2CF2Br (%)
However, it cannot be ruled out that the fluoroalkylation
process may proceed through a radical-nucleophilic mechanism
2-Chloropyridine
2-Acetylpyridine
Pyridine
0.72
3.18
5.23
6.25
7.60
5.23
–
–
SRN1 [24–27] that, as follows from Refs. [1,3,18], may lead to C-
78
98
99
28
fluoroalkylated nitrogen-containing heterocycles.
2,5-Lutidine
We studied the possibility of mild fluoroalkylation of phenols
with freon BrCF2CF2Br using the above-mentioned activating
system. As pointed out above, the fluoroalkyl radical attack was
expected to proceed at the o- or p-positions of the aromatic ring
forming fluoroalkylated phenols rather than by phenolate fluor-
oalkylation yielding ethers [10–12,28]. Using hydroquinone as one
of the most active electron donors among the phenols, we explored
a phenols–BrCF2CF2Br reaction in the presence of SO2 as the
g
-Collidine
Pyridinea
a
p-Dinitrobenzene is added.
and presumably for the precipitation of sulfur dioxide–
salt.
g-collidine
Similarly, the significant influence of the pyridine basicity on
the fluoroalkylation process using the ‘‘sulfur dioxide-substituted
pyridine’’ activating system has been observed by us for
fluoroalkylation of thiophenols with BrCF2CF2Br (Table 5).
As follows from Tables 4 and 5, for the pyrrole and thiophenol
alike the maximum yields of fluoroalkylation product are obtained
in the range of pKa = 5–6 for pyridines used to raise the electron-
donating capacity of the substrates.
In the case of fluoroalkylation of both pyrrole (Table 4) and
thiophenols (Table 5) addition of p-dinitrobenzene as a free-
radicals trap to the reaction mixture sharply inhibits the formation
of the desired products, which may support the hypothesis that
these processes proceed by a free-radical route.
electron transfer mediator and of
b-picoline used to enhance the
electron-donating capacity of the fluoroalkylated substrates. The
process was run at 35 8C for 3–4 h in sealed ampoules by adding
sulfur dioxide to a solution containing hydroquinone, freon and b-
picoline in oxygen-free organic solvents, i.e. dimethylformamide
or dimethylsulphoxide. It has been ascertained that using the
‘‘sulfur dioxide-substituted pyridine’’ activating system, under the
conditions given above, hydroquinone can be polyfluoroalkylated
in the aromatic ring with BrCF2CF2Br (Scheme 2):
Last but not least the radical nature of the processes involving
the ‘‘electron transfer mediator-base’’ activating system is
supported by the observation that the fluoroalkylation of
thiophenol with ClCF2CFCl2 gave thioethers p-XC6H4SCFClCF2Cl
(X = CH3, H, Cl) [15], rather than the alkylation products of
The yield of the polyfluoroalkylated hydroquinone in the
thiophenol potassium salt, p-XC6H4SCF2CFCl2, formed by
a
presence of equimolar amounts of sulfur dioxide and excess b-
halophilic mechanism [10].
picoline in dimethylformamide at room temperature stands at 66%
and gets practically quantitative (92%) at twofold SO2 excess.
Addition of p-dinitrobenzene as a radical trap markedly inhibited
the process, which again supports its radical nature.
The probable mechanism of pyrrole fluoroalkylation with
BrCF2CF2Br in the presence of pyridines and sulfur dioxide can
be represented by Scheme 1a and 1b.
The ‘‘sulfur dioxide-substituted pyridine’’ activation system
helps to introduce BrCF2CF2-group of freon not only to the
aromatic ring of hydroquinone, but also into phenol to form para-
and ortho-fluoroalkylated products with a combined yield of 51% at
35 8C. These results will be subject of our next publication.
3. Conclusion
Thus, a system of an organic base (substituted pyridines) and an
electron transfer mediator (sulfur dioxide) enables mild and
efficient polyfluoroalkylation of the aromatic ring of phenols and
pyrrole with 1,2-dibromotetrafluoroethane. Complexation of
substituted pyridines with phenols and pyrrole enhances the
electron-donating properties of the substrates, activates freon
The way in which the process unfolds further is not so clear. The
nascent active radicals CF2CF2Br may then react with the pyrrole
cation radical (its complex with substituted pyridines), which is
accompanied by proton abstraction to form 2-(2-bromotetrafluoro-
ethyl)-pyrrole:
ꢀ

V.G. Koshechko et al. / Journal of Fluorine Chemistry 131 (2010) 208–211
211
using a mediator to form active radicals and makes the process
proceeding by a radical route.
products and their determining were carried out similarly to the
above mentioned.
4. Experimental
4.1.4. Reaction of hydroquinone with BrCF2CF2Br in the presence
of sulfur dioxide and
b-picoline
4.1. General
1,2-Dibromotetrafluoroethane (0.2 mL (2.0 Â 10À3 mol) and
sulfur dioxide (0.17 mL of SO2 solution in DMFA, 5 Â 10À4
mol SO2) were added to a solution of hydroquinone (0.055 g,
Melting points were uncorrected. 1H NMR and 19F NMR spectra
were recorded in
d
(ppm) using a Bruker-CXP-90 (90 MHz)
5 Â 10À4 mol) in a mixture of DMFA (2.5 mL) with
b-picoline
spectrometer (DMSO-d6, vs. TMS (1H) and CCl3F (19F) as internal
standard), the 13C NMR spectra using a Bruker AVANCE 400
spectrometer (CDCl3 vs. TMS). The elemental analysis was carried
out on elemental analyzer ‘‘Carlo Erba’’, model 1106. Cyclic
voltammetry experiments were carried out in the undivided three
electrode analytical cell; working electrode: disk cathode of
platinum; auxiliary electrode: platinum wire; reference electrode:
Ag/AgCl. The solvent of analytical grade (10 mL) contained support-
ing salt (0.1 M Bu4NBF4). The cell was purged for 15 min with
nitrogen. The research was carried out using EP 20A potentiostate-
and PC-based computerized electrochemical facilities. The yields of
(0.2 mL, 2.0 Â 10À3 mol) under an argon atmosphere. The sealed
ampoule was kept at 35 8C for 3–4 h. The reaction mixture was
diluted with 30 ml of 17% solution of HCl, three times extracted
with hexane or chloroform, organic layer washed with water,
dried and solvent distilled. The reaction product p-HO-
C6H3(CF2CF2Br)-OH was isolated by silica gel column chromato-
graphy with ether–dichloroethane (1:1) as eluent; yield 85%
respectively to the hydroquinone; colorless crystals, m.p. 76 8C.
1H NMR : 6.5–6.8 (m, 3H, C6H3); 19F NMR
d d: 68.5 (t, 2F, CF2,
JFF = 4.8 Hz), 109.7 (t, 2F, CF2Br, JFF = 4.8 Hz); Anal. Calcd. for
C6H5BrF4O2: C, 33.2; H, 1.7. Found: C, 33.2; H, 1.7.
polyfluoroalkylated products (Tables 3–5) were calculated from 19
NMR spectra respectively to the pyrrole and hydroquinone.
F
References
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b-picoline
1,2-Dibromotetrafluoroethane (0.48 mL, 4.0 Â 10À3 mol) and
sulfur dioxide (SO2 solution in DMFA, 2 Â 10À3 mol) were added to
a solution of pyrrole (0.067 g, 1 Â 10À3 mol) in a mixture of DMFA
(2.5 mL) with
b
-picoline (0.4 mL, 4.0 Â 10À3 mol) under an argon
atmosphere. The sealed ampoule was kept at 35 8C for 4–5 h. The
reaction mixture was diluted with 30 ml of 17% solution of HCl,
three times extracted with hexane or chloroform, organic layer
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bromotetrafluoroethyl)pyrrole was distilled under reduced pres-
sure. b.p. 66–68 8C (15 Torr); yield 82% (based on pyrrole); 1H NMR
d
: 6.2 (s, 1H (4)); 6.5 (s, 1H (3)); 7.0 (s, 1H (5)); 11.9 (s, 1H, N–H); 19
NMR : 65.7 (t, 2F, CF2, JFF = 5.6 Hz); 102,8 (t, 2F, CF2Br, J = 5.6 Hz).
F
d
Anal. Calcd. for C6H4BrF4N: C, 29.3; H, 1.6; N, 5.7. Found: C, 29.5; H,
1.7; N, 5.8.
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The test was carried out similarly to the above-mentioned
protocol (Section 4.1.1) however, p-dinitrobenzene (0.0336 g,
2 Â 10À4 mol) was added to the reaction mixture. Further
treatment of reaction products and their determining were carried
out similarly to the above mentioned. Products containing fluorine
were not revealed.
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4.1.3. Reaction of pyrrole with BrCF2CF2Br in the presence of sulfur
dioxide and different substituted pyridines
The tests were carried out similarly to the above-mentioned
protocol (Section 4.1.1), however, different substituted pyridines
(2 Â 10À4 mol) mentioned in Table 4 were added instead of
b-
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picoline to the reaction mixture. Further treatment of reaction