Electrochemical fluorination of N,N-dimethylperfluoroacylamides

Article abstract of DOI:10.1016/S0022-1139(01)00545-0

The electrochemical fluorination (ECF) of N,N-dimethylperfluoroacylamides gives the corresponding perfluoro-N,N-dimethylacylamides in low yield. With increase of the number of carbon atoms in the perfluoroacyl radical the yield of the required perfluoro-N

Full text of DOI:10.1016/S0022-1139(01)00545-0

Journal of Fluorine Chemistry 113 (2002) 201–205
Electrochemical fluorination of N,N-dimethylperfluoroacylamides
Nikolai V. Ignat’eva,*, Michael Schmidta, Udo Heidera, Andriy Kucherinab,
Peter Sartorib, Fatiah M. Helmyb,1
aMerck KGaA, Frankfurter Str. 250, D-64271 Darmstadt, Germany
b
¨
Institut fur Synthesechemie, Gerhard-Mercator-University of Duisburg, Lotharstrasse 1, D-47048 Duisburg, Germany
Received 2 July 2001; accepted 28 September 2001
Abstract
The electrochemical fluorination (ECF) of N,N-dimethylperfluoroacylamides gives the corresponding perfluoro-N,N-dimethylacylamides
in low yield. With increase of the number of carbon atoms in the perfluoroacyl radical the yield of the required perfluoro-N,N-
dimethylacylamides is slightly increased. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Electrochemical fluorination; Simons process; N,N-dimethylperfluoroacylamides; Perfluoro-N,N-dimethylacylamides
1. Introduction
2. Results and discussion
N,N-bis(trifluoromethyl)perfluoroalkanesulfonamides are
convenient starting compounds for the introduction of
(CF3)2N– groups into organic molecules [1,2]. These per-
fluorinated sulfonamides can be prepared in a one step
procedure by means of the Simons process [3]. The starting
materials for this process, N,N-dimethylperfluoroalkanesul-
fonamides, can easily be synthesized from readily available
chemicals.
In the literature, only one example of electrochemical
fluorination (Simons process) of N,N-dimethylperfluoroacy-
lamides, namely of N,N-dimethyltrifluoroacetamide, is
given [4]. The reported yield of perfluoro-N,N-dimethyla-
cetamide, CF3CON(CF3)2 is very low; 48 g of this substance
are obtained from 569 g of starting material, corresponding
to a yield of 4.8%. Decomposition products, among them
bis(trifluoromethyl)carbamyl fluoride, (CF3)2NCOF, are
preferably formed.
We have repeated this experiment and also obtained a very
low yield of the required perfluoro-N,N-dimethylacetamide.
We believe that the low yield of perfluoro-N,N-dimethyla-
cetamide is related to the high solubility of this product in
HF which leads to its decomposition during the Simons
process. We decided to investigate the ECF of homologues
of N,N-dimethyltrifluoroacetamide with longer perfluori-
nated chains to decrease the solubility of perfluoro-N,N-
dimethylacylamides in HF.
CF3SO2F þ 2HNðCH3Þ2
! CF3SO2NðCH3Þ þ ðCH3Þ2NHþ2 FÀ
The disadvantage of this method is the high price of tri-
fluoromethanesulfonyl fluoride. From this point of view the
application of perfluoroacyl fluorides in place of trifluor-
omethanesulfonyl fluoride as starting material is more rea-
sonable.
In the present study we have investigated the electroche-
mical fluorination (ECF) of N,N-dimethylperfluoroacyla-
mides, which can easily be obtained from the correspond-
ing perfluoroacyl fluorides or chlorides.
Thus, electrochemical fluorination of N,N-dimethylpenta-
fluoropropioamide, C2F5CON(CH3)2 and N,N-dimethylhep-
tafluorobutyramide, C3F7CON(CH3)2 was the subject of the
present study.
* Corresponding author.
E-mail address: ignatiev@unidui.uni-duisburg.de (N.V. Ignat’ev).
1 Visiting professor from Department of Chemistry, College of
Education for Girls, Al-Bagdadeye-Madaris Street, Jeddah, Kingdom of
Saudi Arabia.
0022-1139/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 0 1 ) 0 0 5 4 5 - 0

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N.V. Ignat’ev et al. / Journal of Fluorine Chemistry 113 (2002) 201–205
Table 1
Composition of the mixturea after electrochemical fluorination of C2F5C(O)N(CH3)2
Compound
Boiling point (8C)
49–50
19F, d (ppm)b
Content in the mixture (wt.%)
32.6
À55.06 t (6F3,4
)
À81.76 m (3F1)
À118.75 m (2F2)
5JF3;F2 ¼ 8:0 Hz
À26.5 [5]
þ23.73 tq (1F3)
À83.39 d (3F1)
À121.90 d (2F2)
4JF3;F1 ¼ 4:5 Hz
3JF2;F3 ¼ 7:7 Hz
65.2
13–15 [4,6]
þ5.38 sp (1F1
)
2.2
À56.47 d (6F2,3
)
4JF1;F3 ¼ 17:0 Hz
a Trapped at À78 8C.
b Spectra were measured at À40 8C.
2.1. Electrochemical fluorination of C2F5CON(CH3)2
solution the same irrespective of addition of starting mate-
rial.
Ninety grams of N,N-Dimethylpentafluoropropioamide,
C2F5CON(CH3)2, were used as starting material for the ECF
process and 23.0 g of a mixture of perfluorinated products
were trapped at À78 8C (see Section 3). The composition of
the mixture, estimated by means of 19F NMR spectroscopy,
is given in Table 1. The content of perfluoro-N,N-dimethyl-
propioamide, C2F5CON(CF3)2 is still very low (non-isolated
yield of 5.3%). Probably the yield can be raised due to the
saturation of HF in the cell with fluorinated products if a
larger amount of starting material is used for the ECF.
The presence of partially fluorinated compounds such as:
C3F7CON(CF3)(CHF2) and C3F7CON(CHF2)(CHF2) in the
reaction mixture, confirms again that ECF is a step-wise
process [9].
3. Experimental details
3.1. Apparatus
A stainless-steel Simons-type cylindrical cell (total
volume 360 cm3) with an array of nickel anodes (effective
anodic area: S ¼ 4:58 dm2) and cathodes with the same
effective area was used for the ECF of N,N-dimethylper-
fluoroacylamides. The cell was equipped with a stainless-
steel condenser and two FEP-traps cooled to À78 8C.
2.2. Electrochemical fluorination of C3F7CON(CH3)2
This compound was fluorinated by means of ECF under
two different protocols (see Section 3). The main difference
in these two experiments is the temperature and the time-
table for filling of the cell with starting material. In experi-
ment (II) the cell was re-filled with N,N-dimethylheptafluor-
obutyramide every 2 h to keep the Ni-electrodes active with
the fluorination of starting material but not with fluorination
of the desired perfluoro-N,N-dimethylbutyramide. This pro-
cedure helps to increase the contents of C3F7CON(CF3)2 in
the resultant mixture from 10.2% in experiment (I) to 15.7%
in experiment (II) (Table 2), corresponding to non-isolated
yield of 7.6%. However, the main product in the reaction
mixture is still perfluorobutyryl fluoride.
This is explainable by the comparatively high solubility of
C3F7CON(CF3)2 in HF, which undergoes subsequent fluor-
ination even in the presence of the starting material
C3F7CON(CH3)2. That is the reason why, after every re-
filling of the cell with starting material, the cell voltage,
which strongly decreases at the beginning of the experiment,
remains practically unchanged at the end of the experiment
(Fig. 1). At this point the HF in the cell is saturated with
soluble materials, which keeps the conductivity of the
3.2. Analytical procedures
19F and 1H NMR spectra were measured for a neat liquids
on the Brucker WP 80 SY spectrometer. Spectra were
recorded using FEP sample tube inside a 5 mm thin walled
NMR tube with an acetone-D6 film as an external lock and
CCl3F or TMS as internal references.
Gas chromatography was undertaken with a Perkin-Elmer
gas chromotograph using a column (4% OV 101 on Chro-
mosorb GAW DMCS) of length 2.5 m and 2 mm i.d. at
40 8C. The temperature of the injector was 100 8C and the
temperature of the FID detector was maintained at 250 8C.
The carrier gas was He.
3.3. Chemicals
N,N-dimethylpentafluoropropionamide, C2F5CON(CH3)2,
was synthesized by the interaction of pentafluoropropionyl

N.V. Ignat’ev et al. / Journal of Fluorine Chemistry 113 (2002) 201–205
203
Table 2
Composition of the mixture after electrochemical fluorination of C3F7C(O)N(CH3)2
Compound
Boiling point (8C)
19F, d (ppm)
1H, d (ppm)
Contents in the mixture (wt.%),
experiment
I
II
72–73
À54.90 t (6F4,5
)
10.2
15.7
À80.18 t (3F1)
À115.75 m (2F3)
À124.57 m (2F2)
4JF1;F3 ¼ 9:5 Hz
5JF5;F3 ¼ 7:8 Hz
90–91
À54.82 m (3F5)
7.57 t (1H)
6.1
4.4
À80.12 t (3F1)
JH,F ¼ 55.3 Hz
À99.56 dqt (2F4)
À114.30 m (2F3)
À124.79 m (2F2)
4JF1;F3 ¼ 9:5 Hz
4JF4;F5 ¼ 10:9 Hz
5JF4;F3 ¼ 4:0 Hz
–
À80.07 t (3F1)
7.47 t (2H)
7.2
2.7
À99.78 dt (4F4,5
)
JH,F ¼ 57.02 Hz
À113.99 m (2F3)
À125.07 m (2F2)
4JF1;F3 ¼ 9:5 Hz
5JF4;F3 ¼ 5:0 Hz
13–15
(Table 1)
0.7
5.3
a
7–9 [7]
þ24.70 t (1F4)
À80.90 t (3F1)
À119.20 m (2F3)
À127.00 m (2F2)
4JF1;F3 ¼ 8:0 Hz
62.8
48.1
(starting material)
See Section 3
–
13.2
a
CF3CF2CF3
À38 [8]
À82.30 s (6F)
À131.00 s (2F)
1.3
4.0
6.6
Others
11.7
a
19F NMR spectra were recorded at À40 8C.
fluoride (obtained in 27% yield by the ECF of propionyl
chloride) with dimethylamine. 102.0 g (0.614 mol) of pen-
tafluoropropionyl fluoride were bubbled through the stirred
solution of 75.0 g (1.66 mol) of dimethylamine (‘‘Aldrich’’,
>99%)in500 cm3ofdryethercooledinice-water.Thereaction
mixture was kept overnight at room temperature, the white
deposit was filtered off and washed with three portions
(50 cm3 each) of ether. The ether was distilled off and the
remainderwasdistilledatreducedpressure.Eighty-fivegrams
of N,N-dimethylpentafluoropropionamide (bp 98–100 8C
at 18.4 kPa) [10] was obtained. Yield: 72.4%. 19F NMR
spectrum (solvent: acetone-D6; standard: internal CCl3F), d
(ppm): À82.04 s (CF3), À115.00 m (CF2). 1H NMR spectrum
(solvent: acetone-D6; standard: internal TMS), d (ppm): 3.04 t
(CH3), 3.23 t (CH3), 4JH;F ¼ 1:0 Hz, 4JH;F ¼ 2:4 Hz.
N,N-dimethylheptafluorobutyramide was prepared from
heptafluorobutyryl chloride by the reaction with dimethyla-
mine similarly. The 132.5 g (0.57 mol) of heptafluorobutyryl
chloride were slowly (ꢀ1.5 g per min) added to the stirred
solution of 71.0 g (1.58 mol) of dimethylamine (‘‘Aldrich’’,
>99%) in 500 cm3 of dry ether cooled in ice-water. After
addition, the reaction mixture was refluxed for 1 h and left
overnight at room temperature. The white deposit was filtered
off and washed with three portions (50 cm3 each) of ether.
After distilling off the ether, the rest was distilled at reduced
pressure to give 121.7 g of N,N-dimethylheptafluorobutyra-
mide (bp 107 8C at 14.7 kPa) [11]. Yield: 88.6%. 19F NMR
spectrum (solvent: acetone-D6; standard: internal CCl3F), d
(ppm): À79.51 t (CF3), À111.65 m (CF2), À125.04 s (CF2),
4JF;F ¼ 9:5 Hz. 1H NMR spectrum (solvent: acetone-D6;

204
N.V. Ignat’ev et al. / Journal of Fluorine Chemistry 113 (2002) 201–205
Fig. 1. Cell voltage during the ECF of C3F7C(O)N(CH3)2. Experiment II.
standard: internal TMS), d (ppm): 3.06 t (CH3), 3.23 t (CH3),
3.5. Electrochemical fluorination
of N,N-dimethylheptafluorobutyramide
4
4JH;F ¼ 0:8 Hz, JH;F ¼ 2:4 Hz.
3.4. Electrochemical fluorination
of N,N-dimethylpentafluoropropionamide
3.5.1. Experiment (I)
The temperature of the cell body in this experiment was
maintained at À15 8C and the temperature of the condenser
was kept at À30 8C.
The temperature of the cell body in this experiment was
maintained at 0 8C and the temperature of the condenser was
kept at À30 8C.
The 140.4 g of N,N-dimethylheptafluorobutyramide was
added as shown in the Table 4 to 338 g of liquid HF
previously electrolyzed in the cell for 17 h. The gaseous
products from the cell were passed through a condenser and
two FEP traps at À78 8C. The electrolysis, which proceeded
at a cell voltage of 4.6–5.5 Vand a current density of 0.66 A/
dm2, was completed after consumption of 226.5 Ah (121.0%
of the theoretical amount calculated on a 12 electron pro-
cess). The 70.5 g of a transparent liquid was obtained from
the trap after separation from the HF layer. This mixture was
analyzed by 19F NMR spectroscopy.
Ninety grams of N,N-dimethylpentafluoropropionamide
was added as shown in Table 3 to 337 g of liquid hydrogen
fluoride previously electrolyzed in the cell for 25 h. The
gaseous products from the cell were passed through a
condenser and two FEP traps at À78 8C. The electrolysis,
which proceeded at a cell voltage of 4.3–5.1 Vand a current
density of 0.66 A/dm2, was completed after consumption of
183.7 Ah (121.3% of the theoretical amount calculated on a
12 electron process). Twenty-three grams of a transparent
liquid was obtained from the trap after separation from the
HF layer.
Dilution of the HF-solution from the cell with ice-water
gave in addition 32.2 g of transparent liquid, which was
dried over magnesium sulphate and analyzed by means of
gas chromatography and 19F NMR spectroscopy methods.
According to 19F NMR spectra this mixture contains three
main substances (Table 1). By the fractional distillation
perfluoro-N,N-dimethylpropioamide was isolated (3.6 g)
as a pure compound; bp 49–50 8C.
Table 4
Table 3
Weight of C3F7CON(CH3)2 (g)
Electricity consumed (Ah)
22.4
27.0
22.0
27.0
22.0
20.0
0
30.2
Weight of C2F5CON(CH3)2 (g)
Electricity consumed (Ah)
25.0
25.0
20.0
20.0
0
73.6
54.0
86.8
128.4
102.0
141.6
175.6

N.V. Ignat’ev et al. / Journal of Fluorine Chemistry 113 (2002) 201–205
205
The data in Table 2 show the contents of fluorinated
compounds in the mixture, isolated from the trap and from
the cell after ECF of N,N-dimethylheptafluorobutyramide.
By fractional distillation of the material from the trap
(after warming up to room temperature 4.9 g remain)
together with the product obtained from the cell, the follow-
ing compounds were isolated as pure substances (for NMR
data see Table 2):
calculated on a 12 electron process). An amount of 57.1 g of
a transparent liquid was obtained from the trap after separa-
tion from the HF layer. Dilution of the HF-solution from the
cell with ice-water gave in addition 35.4 g of a transparent
liquid, which was dried over magnesium sulphate.
Analyses of both those samples and separation of the
individual compounds were carried out as in Experiment (I).
Data are given in the Table 2.
1. Perfluoro-N,N-dimethylbutyramide, C3F7CON(CF3)2;
bp 72–73 8C (isolated yield: 3.0%).
2. N-(trifluoromethyl)-N-(difluoromethyl)heptafluorobu-
References
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