Electrochemical fluorination of toluene and benzotrifluoride in the presence of triallylamine

Article abstract of DOI:10.1134/S1070427207120169

Electrochemical fluorination of benzotrifluoride in the presence of triallylamine as depolarizer was studied. The possibility of preparing perfluoromethylcyclohexane and its isomers with high current efficiency and substance yield in combination with perfluorinated tertiary amines was examined.

Full text of DOI:10.1134/S1070427207120169

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2007, Vol. 80, No. 12, pp. 2090 2092.
Pleiades Publishing, Ltd., 2007.
Original Russian Text
V.A. Matalin, G.I. Kaurova, V.V. Berenblit, V.I. Gribel’, 2007, published in Zhurnal Prikladnoi Khimii, 2007, Vol. 80,
No. 12, pp. 2012 2014.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Electrochemical Fluorination of Toluene and Benzotrifluoride
in the Presence of Triallylamine
V. A. Matalin, G. I. Kaurova, V. V. Berenblit, and V. I. Gribel’
Prikladnaya Khimiya Russian Scientific Center, Federal State Unitary Enterprise, St. Petersburg, Russia
Lebedev Research Institute of Synthetic Rubber, Federal State Institution, St. Petersburg, Russia
Received Decemver 25, 2006; in final form, October 2007
Abstract Electrochemical fluorination of benzotrifluoride in the presence of triallylamine as depolarizer
was studied. The possibility of preparing perfluoromethylcyclohexane and its isomers with high current ef-
ficiency and substance yield in combination with perfluorinated tertiary amines was examined.
DOI: 10.1134/S1070427207120169
Electrochemical fluorination (ECF) in anhydrous
HF on nickel anodes is one of frequently used meth-
ods for conversion of hydrocarbons and partially
fluorinated organic compounds into their perfluori-
nated analogs, with the preservation of the functional
group or structure of the starting compound contain-
ing a heteroatom. This method was described for
the first time in the late 1940s by Simons and then
was developed by many researchers [1] who used it
for fluorination of organic compounds containing
C O, C N, C S, or C P bonds.
Nyberg [2 4] has reported on a synthesis of per-
fluoromethylcyclohexane (PFMCH) in poor yield by
ECF of toluene. With benzotrifluoride as starting
compound, the yield is 47%, but the electrolysis can-
not be performed in the continuous mode because
of the electrolyte tarring [5].
As shown in [6], these effects can be restricted
by adding a depolarizer fixing the steady-state elec-
trolysis mode. This is particularly important when
the ECF substrate is insoluble in HF and nonconduct-
ing, because this additive provides both the conduc-
tivity of the electrolyte and the solubility of the sub-
strate in it.
Perfluorocycloalkanes are not usually mentioned
among numerous functional perfluorinated compounds
that have found practical use after they became ac-
cessible by ECF, because attempts to prepare these
compounds by electrolysis of their hydrocarbon ana-
logs, which are insoluble in the electrolyte, have been
not very successful.
In this study, we made an attempt to use ECF
for preparing perfluorinated cyclic paraffins using
an electrolytic additive whose electrolysis occurs
without tarring and yields products that are of prac-
tical interest.
However, the development of a method for pre-
paring these compounds remains an important prac-
tical problem, because they exhibit unusual combi-
nations of physicochemical properties: low boiling
points, critical temperatures, and critical pressures at
high molecular weights, densities in the liquid state,
and coefficients of expansion and compressibility.
The last two provide a very low speed of sound in
these media. These compounds have considerably
lower, compared to other liquids, surface tension and
refractive index. They have excellent electric-insula-
tion properties and unique chemical and biochemical
stability, making them virtually nontoxic.
In this context, we performed a comparative study
of the dependence of the PFMCH yield in ECF of
toluene and benzotrifluoride in the presence of a de-
polarizing additive, triallylamine (CH2=CHCH2)3N
(TAA), whose ECF occurs in the steady-state mode
under conditions of continuous electrolysis and gives
in high yield a mixture of perfluorotripropylamine
with nitrogen-containing heterocycles, in particular,
perfluoro-N-(n-propyl)-cis-3,4-dimethyl- and perflu-
oro-N-(n-propyl)-trans-3,4-dimethylpyrrolidines and
perfluoro-N-(n-propyl)-3-methylpiperidine [7].
2090

ELECTROCHEMICAL FLUORINATION OF TOLUENE AND BENZOTRIFLUORIDE
2091
Table 1. Signals in the 19F NMR spectra of isomers present in the target fraction
Signal no.
CFCl3, ppm
Multiplet
JF F, Hz
Number of F
Structure
Perfluoro-1,1-dimethylcyclopentane
C9F2 C16F2
C9F2 C16F2
x
1
9
16
59.6
114.6
123.3
q.q.(m)
septet
m
1F 9F 12; 1F F, 7
6
(C1F3)2C
9F 1F, 12
4
4
Perfluoro-cis-1,2-dimethylcyclopentane and perfluoro-trans-1,2-dimethylcyclopentane
3
4
70.6
70.8
t
m
d
d.m
d
d
s
s
6
6
4
4
2
2
2
2
12F F15
12F 3F, 277
15F 12F, 277
18F 19F, 274
19F 18F, 274
F18
F19
12
15
18
19
24
24
119.4
123.3
124.2
128.2
183.0
183.0
C3;4F3C24;29F C
C
cis-trans
C3;4F3C24;29F C
12F
F15
Perfluoro-cis-1,3-dimethylcyclopentane and perfluoro-trans-1,3-dimethylcyclopentane
16F F17
x
5
6
16
17
25
26
71.4
71.6
122.4
124.0
183.3
184.0
t
5F F, 15
6
6
4
4
2
2
x
C5;6F3C25;26F C
sextet
d.m
d
s
s
6F F, 7, 6
x
16F F, 271
cis-trans
C5;6F3C25;26F C
16F
x
16F F, 271
F17
Perfluoroethylcyclopentane
CF2 CF2
x
C7F3C19F2CF27
7
90.0
t
7F F, 11
3
CF2 CF2
EXPERIMENTAL
a mixture of toluene with triallylamine, the excess
amount of the consumed electricity (Q) (over the cal-
culated value) was considerably larger than that in
ECF of benzotrifluoride.
As starting products for preparing perfluoromethyl-
cyclohexane by ECF we used compounds formed in
electrolysis of mixtures of benzotrifluoride or toluene
with triallylamine as electrolytic additive.
The ECF products, whose density was higher than
that of the electrolyte, were collected in the lower
part of the electrolyzer and drained at equal intervals.
The crude product was discharged beneath a water
layer and neutralized with sodium carbonate. After
that, the crude product was purified by treatment with
aqueous-alcoholic alkali to remove incompletely fluo-
rinated products and dried over silica gel. The dried
crude product was distilled on a column to obtain
three fractions: head fraction (bp 53 54 C), target
fraction (bp 76 78 C), and heavy fraction (bp 129
131 C).
The products in the fractions were identified by 19F
NMR on a Bruker Spectrospin AM-500 spectrometer
(operating frequency 470.572 MHz) in C6F6 as sol-
vent and internal reference. The products were also
analyzed by GLC on a Tsvet-100 device equipped
with a thermal conductivity detector, using a column
The electrochemical fluorination was performed in
a 660-ml carbon steel Simons electrolyzer equipped
with a water-cooled coil and a stack of nickel elec-
trodes with the surface area of the anodes of 500 cm2.
The electrolyzer was also equipped with a reflux con-
denser for condensing HF entrained by electrolysis
gases and returning it into the electrolyte. The cur-
rent during the experiments was constant, 15 A, and
2
the current density was 0.034 A cm .
As an electrolyte we used a solution of 5 wt % ben-
zotrifluoride or toluene and 5 wt % triethylamine in
liquid anhydrous HF. The concentration of the or-
ganic substrate in the electrolyte was maintained con-
stant by feeding a mixture of the starting organic com-
pounds once or twice an hour in amounts correspond-
ing to their consumption coefficients. In ECF of
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 12 2007

2092
MATALIN et al.
Table 2. Electrochemical fluorination of mixtures of toluene and benzotrifluoride with triallylamine
1
Loaded, g (mol)
Obtained in fractions, g (mol)
Yield, mol % [g (A h) ]
Q,
A h
head
target
heavy C9F21N + cyclo-C6F12
+
C9F21N +
cyclo-C9F19N
CH3C6H5 N(CH2CHCH2)3
cyclo-C6F12 cyclo-C7F14
cyclo-C9F19N
cyclo-C6F12
62.0(0.68)
CF3C6H5 N(CH2CHCH2)3
75.0(0.55)
788
2.5(0.013)
C6F12
5.0(0.021)
C7F14
32.2(0.081)
C9F21N + C9F21N cyclo-C6F12
cyclo-C6F12
5.1(0.017)
8.1(0.073)
C9F21N +
cyclo-C9F19N
24(0.105)
+
127.4(0.87)
121.0(0.89)
440 41.4(0.14) 236(0.67)
82.2(0.21)
93(0.35)
with 20% , , -tricyanoacetophenone on Silokhrom-
80 as stationary phase. The composition determined
by GLC is well consistent with the signal intensities
in the 19F NMR spectra.
the freezing point of the mixture down to 65 C, i.e.,
by 30 C relative to PFMCH.
Because the electrolysis process is stable in time,
our study opens up prospects for producing in a com-
mon process a series of organofluorine products for
medicine and engineering in high yields by ECF of
benzotrifluoride in a mixture with triallylamine.
In the head fraction with the empirical formula
cyclo-C6F12, we identified two compounds: perfluo-
rocyclohexane and the product of ring contraction,
perfluoromethylcyclopentane.
CONCLUSIONS
The target fraction with the empirical formula
cyclo-C7F14, boiling at 76 78 C, contained up to
80% perfluoromethylcyclohexane CF3-cyclo-C6F11,
whose 19F NMR spectrum agrees with that described
in [7], and its isomers, products of ring contrac-
tion: perfluoro-1,1-dimethylcyclopentane, perfluoro-
ethylcyclopentane, and stereoisomeric perfluoro-1,2-
dimethyl- and perfluoro-1,3-dimethylcyclopentanes.
The characteristic signals in the 19F NMR spectra of
the isomers present in the target fraction are listed in
Table 1.
(1) The possibility of preparing perfluorometh-
ylcyclohexane by electrochemical fluorination in
the presence of triallylamine as electrolytic additive
was demonstrated. The process occurs with a good
current efficiency and is stable in time and practically
feasible.
(2) The electrolysis additionally yields perfluori-
nated tertiary amines, which are widely used in indus-
try, medicine, and defense technology.
The heavy fraction was a mixture containing per-
fluorotri(n-propyl)amine (20.3 wt %) and cyclic prod-
ucts of the empirical formula C9F19N, in particular,
perfluoro-N-(n-propyl)-cis-3,4-dimethylpyrrolidine
(41.2 wt %), perfluoro-N-(n-propyl)-trans-3,4-dimeth-
ylpyrrolidine (18.7 wt %), and perfluoro-N-(n-propyl)-
3-methylpiperidine (19.3 wt %). Their spectra were
described in [7].
REFERENCES
1. Abe, T. and Nagase, S., Preparation, Properties and
Industrial Application of Organofluorine Compounds,
ch. 1: Electrochemical Fluorination (Simons Process)
as a Route to Perfluorinated Organic Compounds
of Industrial Interest, Banks, R.E., Ed., Horwood:
Chichester, 1982, pp. 19 43.
2. Nyberg, K., Acta Chem. Scand., 1970, vol. 24,
The conditions and results of ECF of mixtures of
toluene and benzotrifluoride with triallylamine are
listed in Table 2.
pp. 1609 1618.
3. Nyberg, K., Acta Chem. Scand., 1971, vol. 25,
pp. 534 537, 2499 2503, 2983 2985.
4. Nyberg, K., Acta Chem. Scand., 1973, vol. 27,
The observed substance yields and current efficien-
cies do not contradict the published data [2 5], and
the use of n-triallylamine as an electrolytic additive
allows the yield of useful fluorination products to be
raised to 78%.
pp. 530 509.
5. Yonekura, M., Nagase, S., Baba, H., et al., Bull. Chem.
Soc. Jpn., 1976, vol. 49, pp. 1113 1122.
6. Berenblit, V.V., Grachev, V.I., Dolgopol’skii, I.M.,
and Davydov, G.A., Zh. Prikl. Khim., 1974, vol. 47,
no. 3, p. 590.
The target fraction from ECF contains up to 20%
products of contraction of the cyclohexane ring. These
isomers, forming a eutectic with PFMCH, depress
7. Jolley, K.W., Satcliffe, L.H., and Walker, S.M., Trans.
Faraday Soc., 1968, vol. 64, pp. 269 277.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 12 2007