Polyfluoroalkylation of 2,6-di-tert-butylphenol by 1,2-dibromotetrafluoroethane activated by sulfur dioxide

Article abstract of DOI:10.1016/j.jfluchem.2006.07.002

It was shown that it was possible to perform fluoroalkylation of both sterically-hindered phenolates and phenols with the use of 2,6-di-tert-butylphenol as an example. The role of sulfur dioxide as an activator of the interaction process was established.

Full text of DOI:10.1016/j.jfluchem.2006.07.002

Journal of Fluorine Chemistry 127 (2006) 1242–1244
www.elsevier.com/locate/fluor
Polyfluoroalkylation of 2,6-di-tert-butylphenol by
1,2-dibromotetrafluoroethane activated by sulfur dioxide
*
Vyacheslav G. Koshechko, Lydiya A. Kiprianova , Ludmyla I. Fileleeva,
Ludmyla I. Kalinina, Vasylii A. Khizhnyi
L.V. Pisarzhevsky Institute of Physical Chemistry of the National Academy of Sciences of Ukraine, Pr.Nauky 31, Kiev 01039, Ukraine
Received 28 April 2006; received in revised form 29 June 2006; accepted 2 July 2006
Available online 5 July 2006
Abstract
It was shown that it was possible to perform fluoroalkylation of both sterically-hindered phenolates and phenols with the use of 2,6-di-tert-
butylphenol as an example. The role of sulfur dioxide as an activator of the interaction process was established.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Polyfluoroalkylation; 2,6-di-tert-butylphenol; Sulfur dioxide
1. Introduction
oxidation [15]. Polyfluoroalkyl substituted phenols can also be
useful as physiologically active substances.
Recently the problems of halocarbon activation and the
search for new ways of their use have attracted considerable
attention. The use of chlorofluorocarbons (CFCs) CF2Cl2,
CF2ClCFCl2, CCl3CF3, bromofluorocarbons BrCF2CF2Br, etc.
as fluoroalkyl synthons for the synthesis of different fluorine-
containing compounds is of special interest. Such simple
fluorine-containing substituents containing other halogen
atoms besides fluorine can be transformed to more complex
groups [1–10].
2. Results and discussion
Among sterically-hindered phenols the para-derivatives of
2,6-di-tert-butylphenol are in considerable use [11,16], so we
have chosen this phenol as the object of the research.
We have found that these phenols, including 2,6-di-tert-
butylphenol, are inert to CFCs as well as to 1,2-dibromotetra-
fluoroethane, and direct alkylation of phenols is not observed in
Me2SO. We supposed that in the case of phenolates the
situation is different. Phenolate anions of sterically-hindered
phenols can act not only as active nucleophiles but also as
monoelectronic reducing agents [11]. If such a process takes
place in the interaction of 2,6-di-tert-butylphenolate with
BrCF2CF2Br we may suppose that monoelectronic reduction of
1,2-dibromotetrafluoroethane can initiate the radical fluoroalk-
ylation reaction (Scheme 1).
The aim of our study was to elucidate the incorporation of
polyfluoroalkyl groups of BrCF2CF2Br during the interaction
with sterically-hindered phenols. Such phenols are important,
as they are widely used as antioxidants in motor oils, stabilizers
of thermo- and photodestruction of rubbers [11]. Despite
significant progress in the field, data concerning the synthesis
and properties of sterically-hindered phenols with fluorine-
containing substituents are scarce [12–14]. The possibility of
fluoroalkylation has not been discussed in the literature. The
incorporation of polyfluoroalkyl groups can influence the
stability (and inhibition ability) of free radical formation during
Taking into account that the unpaired electron of the phenoxyl
radical possesses the greatest spin density in the para-position of
ꢀ
phenyl cycle [15] the recombination of BrCF2CF2 with
phenoxyl is expected to occur in the para-position (Scheme 2).
To verify this assumption we have studied the interaction of
sodium 2,6-di-tert-butylphenolate with BrCF2CF2Br.
It was found that the interaction of 2,6-di-tert-butylpheno-
late-BrCF2CF2Br changes the color of solution from the green
* Corresponding author. Fax: +380 44 5256216.
E-mail address: lkipr@inphyschem-nas.kiev.ua (L.A. Kiprianova).
0022-1139/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2006.07.002

V.G. Koshechko et al. / Journal of Fluorine Chemistry 127 (2006) 1242–1244
1243
reactions among the products of the reaction (even in the
presence of water).
Contrary to common phenolates the sterically-hindered
phenols in Me2SO are not able to react with BrCF2CF2Br; that
may result from the fact that the phenol ionization potential is
significantly higher than that of the common phenolate. The
addition of organic bases (such as pyridine and picoline) to
promote dissociation of phenol was unsufficient to carry out the
fluoroalkylation of 2,6-di-tert-butylphenol by BrCF2CF2Br.
It looked reasonable to activate the interaction of 2,6-di-tert-
butylphenol with 1,2-dibromotetrafluoroethane by electron
transfer mediator. We as well as other authors [2,3 and
references therein] have demonstrated that sulfur dioxide can
serve as an electron transfer mediator in some electrochemical
and chemical fluoroalkylation processes. The appearance of
catalytic currents in the cyclic voltammogram of sulfur dioxide
on the addition of the investigated fluorocarbon to the solution
is evidence for the possibility of the electron transfer from
phenol to BrCF2CF2Br (Fig. 1).
Scheme 1.
It was found that the interaction of 2,6-di-tert-butylphenol
with BrCF2CF2Br in Me2SO in the presence of sulfur dioxide
upon addition of bases (pyridine or b-picoline) occurs rather
easily, yielding 59% of the phenol, fluoroalkylated by the group
-CF2CF2Br in the para-position of phenyl nucleus (Table 1).
We have shown that this process (as for the case of 2,6-di-tert-
butylphenolate) is inhibited by the presence of radical trap—
dinitrobenzene, proving its radical nature. Also there are no
phenols containing the –CF2CF2H group among the reaction
products.
Scheme 2.
to almost black. In this case two extended triplets appear in the
19F NMR spectrum at 69.4 and 106.9 ppm (relative to CCl3F),
corresponding to the –CF2CF2Br and –CF2CF2Br groups
respectively. The yield of polyfluoralkylated product was 32%
(Table 1).
In the reaction with addition of SO2 besides polyfluoro-
alkylated phenol, formation of BrCF2CF2H (10–15%) is also
The fact that this interaction occurs by the radical
pathway has been supported by our studies elucidating the
influence of the inhibitors of free-radical processes. The
addition of the trap of free radicals such as dinitrobenzene to
the reaction mixture leads to the stoppage of the process
(Table 1).
As was demonstrated earlier [17,18], non-hindered potassium
phenolates can react with BrCF2CF2Br forming PhOCF2CF2Br
and PhOCF2CF2H ethers by a ionic (bromophile) mechanism.
Additionalevidence of radical, ratherthan an ionicmechanism in
our case is the absence of monohydrated phenol derivatives with
aHCF2CF2–substituentinthepara-positiontypicalforhalophile
Table 1
Product yields of polyfluoralkylation of 2,6-di-tert-butylphenol and phenolate
by BrCF2CF2Br [phenol] = 5 Â 10À4 mol; Me2SO; t0 = 35 8C
Reagent
[BrCF2CF2Br], mol [SO2], mol Product yield, %a
Sodium phenolate
2 Â 10À3
–
–
32
–
Sodium phenolateb 2 Â 10À3
Phenolc
Phenolc
Phenolc
Phenolb,c
1 Â 10À3
1 Â 10À3
1 Â 10À3
1 Â 10À3
–
–
5 Â 10À4
1 Â 10À3
1 Â 10À3
32
59
–
a
Measured by NMR.
Fig. 1. Cyclic voltammogram of sulfur dioxide in the absence (1) and in the
presence (2) of BrCF2CF2Br, (Pt, Me2SO, [SO2] = 0.5 Â 10À4 mol;
[BrCF2CF2Br] = 1.0 Â 10À4 mol. [Bu4NBF4] = 1 Â 10-2 mol, 25 8C).
b
c
Dinitrobenzene added.
b-Picoline added.

1244
V.G. Koshechko et al. / Journal of Fluorine Chemistry 127 (2006) 1242–1244
observed. Its formation may result from the hydrꢀogen
abstraction from the solvent by the radical of BrCF2CF2 .
As is known [11,16] the oxidation of sterically-hindered
phenols leads to the formation of phenoxyl radicals with life
times depending on electron structure of phenols. We have
established that the oxidation of the product of the interaction of
2,6-di-tert-butylphenol with BrCF2CF2Br by lead dioxide in
toluene at room temperature formed a relatively stable radical.
The ESR spectrum of this radical contains a triplet with 1:2:1
distribution of equidistant components that reflects an
interaction of unpaired electron with two nuclei of fluorine
of para-substituent of phenol nucleus (aF = 1.25 mT).
Hence our study with the use of 2,6-di-tert-butylphenol as
the example has shown the possibility of fluoroalkylation of
both sterically-hindered phenolates and phenols by
BrCF2CF2Br. The process with phenols may be activated by
sulfur dioxide.
19F NMR (CCl3F) d: 69.4 (tr, 2F, J = 5.6 Hz, CF2Br); 106.9 (tr,
2F,J = 5.6 Hz,C–CF2).Anal.Calcd.forC16H21F4OBr:C49.9;H
5.5. Found: C 49.9; H 5.5.
3.3. Interaction of 2,6-di-tert-butylphenol with
BrCF2CF2Br in the presence of SO2
1,2-Dibromotetrafluoroethane (0.258 g, 1 Â 10À3 mol) and
sulfur dioxide (0.4 ml of SO2 solution in Me2SO,
1 Â 10À4 mol) were added to a solution of 2,6-di-tert-
butylphenol (0.103 g, 5 Â 10À4 mol) in a mixture of Me2SO
(1 ml) with b-picoline (0.5 ml) under argon. The sealed
ampoule was kept at 35 8C for 3–4 h. Further treatment of
reaction products and their determining were carried out
similarly to the above mentioned.
The yield of polyfluoralkylation product was 59% (deter-
mined by 19F NMR). This product had the same characteristics
and was found to be the compound, obtained by interaction of
sodium 2,6-di-tert-butylphenolate with BrCF2CF2Br–2,6-di-
tert-butyl-4-(2-bromotetrafluoroethyl)-phenol.
3. Experimental
3.1. General experimental procedure
Acknowledgments
1
Melting point temperature was uncorrected. H NMR and
19F NMR spectra were recorded using a Bruker CXP-90
(90 MHz) instrument. 1= and 19F chemical shifts were recorded
in d (ppm) values relatively to hexamethyl disiloxane (1H) and
CCl3F (19F) as internal standard. ESR spectrums were recorded
using a Varian E-9 instrument using Mn2+ as external standard.
Cyclic voltammetry experiments were carried out in the
classical three electrode analytical cell; working electrode—
disk cathode of platinum; auxiliary electrode—platinum wire;
reference electrode—Ag/AgNO3. The solvent of analytical
grade (10 ml) contained supporting salt (Bu4NBF4). The cell
was purged for 15 min with nitrogen. The research was carried
out using EP 20A potentiostate- and PC-based computerised
electrochemical facilities. The elemental analysis was carried
on elemental analyzer ‘‘Carlo Erba’’, model 1106.
We express our gratitude to Fundamental Researches Fund
of Ukraine for the partial financial support of the work, and also
to Prof. Vitaly D. Pokhodenko for the discussion of data
obtained.
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1
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