Electrochemical fluorination of trialkylphosphines

Article abstract of DOI:10.1016/S0022-1139(99)00253-5

The electrochemical fluorination (ECF) of trialkylphosphines results in the formation of the corresponding tris(perfluoroalkyl)difluorophosphoranes in good yield. In the case of ECF of tri(iso-butyl)phosphine, isomerisation of iso- into n-alkyl compound occurs.

Full text of DOI:10.1016/S0022-1139(99)00253-5

Journal of Fluorine Chemistry 103 (2000) 57±61
Electrochemical ¯uorination of trialkylphosphines
*
Nikolai Ignat'ev, Peter Sartori
Institut f uÈ r Synthesechemie, Gerhard-Mercator-University of Duisburg, Lotharstrasse 1, D-47048 Duisburg, Germany
Received 7 June 1999; accepted 4 October 1999
Abstract
The electrochemical ¯uorination (ECF) of trialkylphosphines results in the formation of the corresponding tris(per¯uoroalkyl)di-
uorophosphoranes in good yield. In the case of ECF of tri(iso-butyl)phosphine, isomerisation of iso- into n-alkyl compound occurs.
¯
#
2000 Elsevier Science S.A. All rights reserved.
Keywords: Electrochemical ¯uorination; Simons process; Trialkylphosphines; Tris(per¯uoroalkyl)di¯uorophosphoranes
1
. Introduction
of tris(per¯uoroalkyl)di¯uorophosphoranes in high yield
3].
[
Since its discovery at the beginning of the 1940s electro-
chemical ¯uorination in anhydrous HF using nickel electro-
des (Simons process) has been applied for the ¯uorination of
different kinds of organic compounds [1], but only one
example of electrochemical ¯uorination (ECF) of organo-
phosphorus compounds, namely of trialkylphosphine oxi-
des, is described in the literature [2]. By this process, the
yield of the corresponding tris(per¯uoroalkyl)di¯uoropho-
sphoranes was 24±46%.
R ˆ C H ꢀI†; C H ꢀII†; n-C H ꢀIII†
2
5
3
7
4
9
RF ˆ C2F5ꢀIa†; C3F7ꢀIIa†; n-C4F9ꢀIIIa†
The maximum yield (74%) of per¯uorinated material was
obtained in the case of ECF of triethylphosphine (I). The
crude product, which was collected from the bottom of the
electrochemical cell, was practically pure tris(penta¯uor-
oethyl)di¯uorophosphorane, which in contrast to partially
¯uorinated phosphoranes is practically insoluble in HF. As
can be expected, the yield of tris(per¯uoroalkyl)di¯uoropho-
sphoranes diminishes with increasing number of CH2
groups in the aliphatic chain of starting trialkylphosphine.
The presence of small amounts of compounds bearing i-
C F and t-C F groups in the crude product which was
RF ˆ CnF2n‡1; n ˆ 2 � 6
The main disadvantages of this method are the low yield
of tris(per¯uoroalkyl)di¯uorophosphoranes and the forma-
tion in parallel of the toxic gas, OF , as a by-product, which
2
can also cause severe explosions.
In order to avoid these dif®culties we decided to apply
trialkylphosphines as starting materials for the ECF process.
4
9
4 9
obtained after ECF of tri(n-butyl)phosphine (III), is due to
impurities with iso- and tert-butyl structures in the starting
material.
Tris(per¯uoroalkyl)di¯uorophosphoranes (Ia±IIIa) which
were obtained by the ECF of corresponding trialkylpho-
sphines (I±III) are practically free from partially ¯uorinated
2
. Results and discussion
2
.1. Electrochemical fluorination of tri(n-alkyl)phosphines
compounds. As main by-products PF and per¯uoroalkanes
5
The electrochemical ¯uorination of trialkylphosphines
in HF solution proceeds smoothly and leads to the formation
are formed during the ECF of phosphines (I±III).
The solutions of trialkylphosphines (I±IV) in HF for the
ECF were prepared by slow addition of the corresponding
phosphine to anhydrous HF in a PFA ¯ask cooled with dry
ice. Good stirring of the solution is important for the smooth
*
Corresponding author. Fax: ‡49-203-3792231.
E-mail address: nicolai@lims.uni-duisburg.de (P. Sartori).
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 2 5 3 - 5

5
8
N. Ignat'ev, P. Sartori / Journal of Fluorine Chemistry 103 (2000) 57±61
reaction of trialkylphosphines with HF.
In our previous publication we have reported that com-
pounds bearing CH and CH F groups bonded to a nitrogen
‡ �
R PH F
3
3
2
R3P
I� IV†
‡
HF
!
atom are not stable and split off HF with the formation of
cyclic products [7]. This is not an astonishing result, because
it is well known that some mono¯uoro compounds loose HF
surprisingly readily [8]. Probably the same process takes
place in the case of compounds of type (VI) leading to the
formation of a cyclo-propyl system of type (VII). We hope
to obtain experimental evidences for this hypothesis by ECF
of compounds containing the cyclo-propyl structure in the
starting substance. The results of this work will be published
separately.
ꢀ
Trialkylphosphines are protonated in HF and give stable,
non-¯ammable solutions, which have a good conductivity
and are very convenient for the ECF. Usually, they are used
in 35±55% concentration, which is convenient for the
storage of these solutions and adding them into the cell.
31
The P NMR parameters of some trialkylphosphonium
uorides are listed in the following table.
¯
3
1
1
Trialkylphosphonium
fluoride
P Chemical shift,
d (ppm) (in HF
JP,H
solution at � 308C)
(
(
(
CH ) PHF
3
� 4.0 dm (at � 408C)
501 Hz
478 Hz
474 Hz
3
C H ) PHF
2
21.0 dm
11.5 dm
5 3
Certainly, the formation of the cyclopropane ring can
proceed not only from the stage of mono-¯uoroalkyl com-
pounds of type (VI) but also under the participation of
compounds bearing two or even more ¯uorine atoms in
positions available for the ring-closing reactions.
n-C H ) PHF
9 3
4
2
.2. Electrochemical fluorination of tri(i-butyl)phosphine
Electrochemical ¯uorination of tri(i-butyl)phosphine
IV) results in the formation of a complex mixture of
Taking into account that ECF proceeds with the partici-
pation of a very active ¯uorinating agent, NiF [7,9], it is
(
3
isomeric tris(per¯uorobutyl)di¯uorophosphoranes together
with a small amount (less than 20 mol%) of partially
¯
clear, that cyclo-propane systems have low chances to
survive during the Simons process. The cleavage of C±C
bond and opening of the cyclopropane ring can occur in two
possible ways: 1 and 2. The way 1 leads to the formation of
the alkyl radicals with a linear structure and the way 2 gives
the alkyl radicals with iso-structure (Scheme 1). Only
these two types of the compounds have been found in the
reaction mixture after the ECF of tri(i-butyl)phosphine (see
Section 3).
uorinated compounds.
Scheme 1 re¯ects the ring-opening reaction which
takes place in the case of a partially ¯uorinated compound
bearing three ¯uorine atoms in the alkyl radical. But it is
clear that this reaction can happen at any stage of ¯uorina-
tion process, in other words in parallel with replacement
of hydrogen atoms by ¯uorine atoms during the Simons
process.
The proposed mechanism of the isomerisation (iso- !n-)
of the alkyl groups during the Simons process is in a good
agreement with the product distribution pattern during the
ECF of tri(i-butyl)phosphine. In contrast, the carbocationic
mechanism of the isomerisation (iso- ! n-), proposed by
Gambaretto et al. [5], or migration of alkyl groups during
The isomerisation (iso- ! n-) of the alkyl group is a
known phenomenon [4±6], and the formation of compound
(
IIIa) during the ECF of tri(i-butyl)phosphine is not surpris-
ing. The amount of (IIIa) in the crude product was estimated
to be ca. 15 mol%. The presence of crossed isomers of type
IVb) indicates that the isomerisation of the starting material
proceeds in parallel with the Simons process and partially
uorinated compounds having the structure of type (IVb)
(
¯
are the intermediates on the way from phosphine (IV) to
tris(n-per¯uorobutyl)di¯uorophosphorane (IIIa).
Recently, we have shown [7] that ECF is a step by step
process and during every step only one hydrogen atom is
replaced by a ¯uorine atom. According to this mechanism,
the formation of compounds of type (V) and (VI) is the ®rst
step in the ECF of tri(i-butyl)phosphine.
Scheme 1.

N. Ignat'ev, P. Sartori / Journal of Fluorine Chemistry 103 (2000) 57±61
59
the ECF process, proposed by Plashkin et al. [4], cannot
explain the absence of compounds with the structure (VIII)
given below) in the crude product, obtained by ECF of
tri(i-butyl)phosphine.
3.3. Chemicals
(
Trimethyl- and tri(n-propyl)phosphine (Aldrich), tri(n-
butyl)phosphine (Fluka), and tri(i-butyl)phosphine (Strem
Chemicals) and triethylphosphine (Chemikalien Klaus F.
Meyer GmbH) were used as delivered.
3
(
.4. Electrochemical fluorination of trialkylphosphines
I±IV)
2
.3. Electrochemical fluorination of trimethylphosphine
Tris(penta¯uoroethyl)di¯uorophosphorane (Ia): A stain-
3
less-steel cylindrical cell (total volume 360 cm ) with an
2
In comparison to ECF of triethylphosphine the ECF of
trimethylphosphine (IX) gives a low yield of tris(tri¯uor-
omethyl)di¯uorophosphorane (X) (see Section 3).
array of nickel anodes (effective anodic area: S ˆ 4.58 dm )
and cathodes with the same effective area was used for this
experiment. The temperature of the cell body was main-
tained at 08C and the temperature of the condenser was kept
at � 308C.
The solution of 158.2 g, with the concentration of
triethylphosphine in HF of 36%, was added to 310 g of
liquid HF previously electrolysed in the cell for 48 h as
follows:
Per¯uorinated substance (X) is quite soluble in HF,
enabling further ¯uorination to the ®nal product PF , caus-
ing a low yield of the desired product (X). The formation of
5
Weight of the solution
added (g)
Electricity consumed
(A h)
19
PF during the ECF process was con®rmed by the F and
5
3
1
�
P NMR spectra which show the presence of PF6 anion
1
27.8
30.4
31.9
31.1
37.0
0
81.8
168.0
256.4
371.3
3
(
P NMR in HF solution at � 408C, d: � 147.8 sep;
J
ˆ 701 Hz) after addition of LiF to the HF-phase trapped
P,F
at � 788C.
The presence of the partially ¯uorinated compounds
CH (CF ) PF (XI) and CH CF PF (XII) in the HF solu-
3
3 2
2
3
3
3
tion in the traps again con®rms that ECF is a stepwise
process and ¯uorination of trimethylphosphine proceeds in
parallel with the decomposition of the starting material and
intermediates.
The gaseous products from the cell were passed through a
condenser and two FEP traps at � 788C. The electrolysis,
which proceeded at a cell voltage of 4.0±5.1 Vand a current
2
density of 0.44±0.55 A/dm , was completed after consump-
tion of 517 A h (133.8% of the theoretical amount of
electricity calculated on 32 electron process). At this point
the current density dropped. A liquid product, which col-
lected on the bottom of the cell during the electrolysis, was
periodically removed from the cell and some amount of the
HF (in total 131 g) was added into the cell to keep the
volume of the liquid in the cell constant. After completion of
the electrolysis the liquid phase was removed from the cell
into a separating funnel and the per¯uorinated product was
collected from the bottom. Altogether 146.1 g of a trans-
parent liquid, which according to the F and P NMR
spectra is practically pure tris(penta¯uoroethyl)di¯uoropho-
sphorane, was obtained. 5 g of this substance was obtained
additionally from the trap after separation from the HF layer.
The total yield of tris(penta¯uoroethyl)di¯uorophosphorane
is 73.5%.
3
. Experimental details
3
.1. Apparatus
Stainless-steel Simons-type electrochemical cells of dif-
ferent volumes were used for the ECF of trialkylphosphines.
Each cell was equipped with a stainless-steel condenser and
two FEP traps kept at � 788C.
19
31
3
.2. Analytical procedures
1
9
31
F and P NMR spectra were measured for neat
liquids without solvent using
a
special technique
(CD COCD ®lm) on the Brucker DRX 500 spectrometer
3 3
1
(470.6 MHz for F and 202.5 MHz for P). CCl F and
9
31
3
1
9
8
the NMR measurements were made from FEP and placed
5% H PO were used as external references. The tubes for
F NMR, d: � 49.76 dm (2F, PF ); � 82.27 t (9F, 3CF );
3
4
2
3
1
2
� 113.81 dt (6F, 3CF ); J ˆ 1003 Hz; J ˆ 122 Hz;
F,F F,F
2 P,F P,F
31
3
4
inside a glass sleeve containing acetone-d with dissolved
6
J
These spectra coincide with the spectra of the tris(penta-
ˆ 12.5 Hz; J ˆ 9.5 Hz. P NMR, d: � 47.95 tsep.
reference substance.

6
0
N. Ignat'ev, P. Sartori / Journal of Fluorine Chemistry 103 (2000) 57±61
¯
[
uoroethyl)di¯uorophosphorane described in the literature
2,10].
Tris(hepta¯uoro-n-propyl)di¯uorophosphorane (IIa): A
Weight of the tri-n-butylphosphine
added (g)
Electricity
consumed (A h)
3
4
3
3
1.8
8.0
8.0
0
stainless-steel cylindrical cell (total volume 310 cm )
with an array of nickel anodes (effective anodic area:
291.3
623.8
930.6
1430.0
1939.0
2414.9
2
S ˆ 3.75 dm ) and cathodes with the same effective area
was used for this experiment. The temperature of the cell
body was maintained at 08C and the temperature of the
condenser was kept at � 258C.
35.1
41.8
35.8
37.5
The solution containing 133.0 g of tri-n-propylphosphine
in HF, with a concentration of 37.6%, was added to 230 g of
liquid HF previously electrolysed in the cell for 50 h as
follows:
The electrolysis, which proceeded at a cell voltage of 4.5±
2
5.2 V and a current density of 0.32 A/dm , was completed
after consumption of 2918.4 A h (146.5% of the theoretical
amount of electricity calculated on 56 electron process).
During the electrolysis a liquid product, which was col-
lected on the bottom of the cell, was periodically removed
from the cell and some amount of HF (in total 1164 g) was
added into the cell to keep the volume of the liquid in the cell
constant. After completion of the electrolysis, the liquid
content was removed from the cell into a separating funnel
and the per¯uorinated product was collected from the
bottom. Altogether 470 g of a transparent liquid, which
Weight of the solution
added (g)
Electricity consumed
(A h)
3
3
3
3
3.0
1.0
2.0
7.0
0
91.8
189.8
282.3
The gaseous products from the cell were passed through a
condenser and two FEP traps at � 788C. The electrolysis,
19
31
according to the F and P NMR spectra is practically
pure tris(nona¯uoro-n-butyl)di¯uorophosphorane, was
obtained from the cell. Yield: 48.8%.
which proceeded at a cell voltage of 4.0±5.1 Vand a current
2
density of 0.37±0.53 A/dm , was completed after consump-
1
9
tion of 476.3 A h (129.4% of the theoretical amount of
electricity calculated on 44 electron process). A liquid
product, which was collected on the bottom of the cell
during the electrolysis, was periodically removed from the
cell and some amount of HF (in total 135 g) was added into
the cell to keep the volume of the liquid in the cell constant.
After completion of the electrolysis the liquid phase was
removed from the cell into a separating funnel and the
per¯uorinated product was separated from the HF-layer. All
together 95.6 g of a transparent liquid, which according to
F NMR, d: � 46.97 dm (2F, PF ); � 83.36 m (9F, 3CF );
2
3
� 109.43 dm (6F, 3CF ); � 121.88 m (6F, 3CF ); � 127.61 m
2 2
1
2
31
(6F, 3CF ); J ˆ 1049 Hz; J ˆ 124.7 Hz. P NMR, d:
2
P,F
P,F
� 43.50 tsep. These spectra coincide with the spectra of the
tris(nona¯uoro-n-butyl)di¯uorophosphorane described in
the literature [2,10].
Tris(nona¯uoro-i-butyl)di¯uorophosphorane (IVa): This
compound was obtained by the process using the electro-
chemical cell described in the previous experiment.
The solution containing 499 g of tri(i-butyl)phosphine in
HF with the concentration of 42.9% was added to 1075 g of
liquid HF previously electrolysed in the cell for 138 h as
follows:
1
9
31
the F and P NMR spectra is practically pure tris(hepta-
uoropropyl)di¯uorophosphorane, was obtained from the
cell. Yield: 53.2%.
¯
1
9
F NMR, d: � 47.42 dm (2F, PF ); � 82.49 m (9F, 3CF );
2
3
1
�
110.40 dm (6F, 3CF ); � 125.77 s (6F, 3CF ); J
ˆ
2
2
P,F
Weight of the solution
added (g)
Electricity consumed
(A h)
2
3
31
1
041 Hz;
J
ˆ 124 Hz
J
F,F
ˆ 14 Hz.
P NMR, d:
43.89 tsep. These spectra coincide with the spectra of
P,F
�
the tris(hepta¯uoropropyl)di¯uorophosphorane described in
the literature [2,10].
104.0
96.0
102.0
99.0
98.0
0
315.2
699.9
983.6
1373.4
Tris(nona¯uoro-n-butyl)di¯uorophosphorane (IIIa): A
stainless-steel cylindrical cell (total volume 1.5 l) with an
2
array of nickel anodes (effective anodic area: S ˆ 15.6 dm )
and cathodes with the same effective area was used for this
experiment. The temperature of the cell body was main-
tained at 08C and the temperature of the condenser was kept
at � 208C.
The electrolysis, which proceeded at a cell voltage of 4.5±
2
5.5 V and a current density of 0.20±0.35 A/dm was
completed after consumption of 2377.2 A h (149.5% from
the theoretical amount of electricity calculated on 56 elec-
tron process). During the electrolysis a liquid product
which was collected on the bottom of the cell was periodi-
cally removed from the cell and some amount of the HF
Tri-n-butylphosphine weighing 268 g, previously dis-
solved in the HF (concentrations of two different solutions
34.8 and 43.6%) was added to 1125 g of liquid HF pre-
viously electrolysed in the cell for 100 h as follows:

N. Ignat'ev, P. Sartori / Journal of Fluorine Chemistry 103 (2000) 57±61
61
(
in total 690 g) was added into the cell to keep the volume
tdec. These spectra coincide with those described in the
literature [11].
of the liquid in the cell constant. After completion of
the electrolysis the liquid phase was removed from the
cell into a separating funnel and the product was separated
from the HF layer. Altogether 440 g of a transparent liquid,
which according to the F and P NMR spectra was the
mixture of tris(nona¯uoro-iso-butyl)di¯uorophosphorane
Some amount of tris(tri¯uoromethyl)di¯uorophosphor-
ane (X) (approximately 10 g) was found to be dissolved
3
in the HF (70 cm ) collected above the bottom layer in the
19
31
FEP trap. Besides the compound (X) the HF layer also
contains partially ¯uorinated compounds: CH (CF ) PF
2
3
3 2
(
(
ꢁ35 mol%), tris(nona¯uoro-n-butyl)di¯uorophosphorane
(XI) and CH CF PF (XII).
3 3 3
31
ꢁ15 mol%),
[bis(nona¯uoro-n-butyl)](nona¯uoro-iso-
CH (CF ) PF (XI): P NMR in HF solution at � 408C,
3
3 2
2
2
butyl)di¯uorophosphorane (ꢁ5 mol%), [bis(nona¯uoro-
iso-butyl)](nona¯uoro-n-butyl)di¯uorophosphorane (ꢁ25
mol%), and partially ¯uorinated compounds with 1±3
hydrogen atoms in the molecule (ꢁ20 mol%), was obtained
from the cell (yield: 57.2% calculated on per¯uorinated
product). By fractional distillation in a Te¯on-apparatus
tris(nona¯uoro-iso-butyl)di¯uorophosphorane can be iso-
lated as the main product.
d: � 36.0 tsepq; JP,H ˆ 18.3 Hz; (Ref. [12]: d: � 35.9;
2
19
ˆ 19.3 Hz). F NMR in HF solution at � 408C, d:
J
P,H
1
� 48.5 dsepq (2F, PF); � 68.0 dt (6F, 2CF ); J ˆ 832 Hz;
3
P,F
2
3
3
J
[12]:
ˆ 156 Hz;
J
ˆ 17.1 Hz;
J
F,H
ˆ 12.1 Hz; (Ref.
F,F
P,F
F,F
1
2
3
J
ˆ 840 Hz;
J
ˆ 154 Hz;
J
ˆ 16.5 Hz;
P,F
P,F
3
JF,H ˆ 12.3 Hz).
CH CF PF (XII): P NMR in HF solution at � 408C, d:
19
3
1
3
3
3
2
� 19.6 qqq; JP,H ˆ 18.3 Hz; (Ref. [13]: d: � 11.2;
1
9
2
F NMR, d: � 45.98 dm (2F, PF ); � 74.07 m (18F,
J
ˆ 17.5 Hz). F NMR in HF solution at � 408C, d:
2
P,H
1
6
CF ); � 99.20 dm (6F, 3CF ); � 180.49 m (3F, 3CF);
� 80.3 br,d (2F, PF); � 70.7 dq (3F, CF ); J ˆ 946 Hz;
3
2
3
P,F
1
2
31
2
3
1
J
tsep.
ˆ 1087 Hz; J ˆ 124.9 Hz. P NMR, d: � 41.35
P,F
J
ˆ 161.7 Hz;
J
ˆ 13.1 Hz; (Ref. [12]:
J
P,F
ˆ
P,F
P,F
F,F
2
3
955 Hz; J ˆ 156 Hz; J ˆ 12.0 Hz). The ratio of the
P,F
F,F
compounds (X), (XI) and (XII) in the HF solution trapped at
788C is 75, 10 and 15 mol%, respectively.
�
3
.5. Electrochemical fluorination of trimethylphosphine
ECF of trimethylphosphine was done in the same way as
described above. A stainless-steel cylindrical cell (total
Acknowledgements
3
volume 310 cm ) with an array of nickel anodes (effective
The authors thank Merck KGaA (Germany) for the
®nancial support of this work.
2
anodic area: S ˆ 3.75 dm ) and cathodes with the same
effective area was used for this experiment. The temperature
of the cell body was maintained at 08C and the temperature
of the condenser was kept at � 258C.
References
The solution containing 121.0 g of trimethylphosphine in
HF, with the concentration of 41.3%, was added in ®ve
equal portions (every 36 h) to 220 g of liquid HF previously
electrolysed in the cell for 45 h. The gaseous products from
the cell were passed through condenser and two FEP traps at
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[
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�
788C. The electrolysis, which proceeded at a cell voltage
[
3] P. Sartori, N. Ignat'ev, DE 19 846 636, 1998.
2
of 4.2±4.8 V and a current density of 0.53 A/dm , was
completed after consumption of 437.5 A h (124.2% of the
theoretical amount of electricity calculated on 20 electron
process).
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(
[
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1
9
31
FEP traps was investigated by F and P NMR spectro-
scopies. The ®rst in the line of FEP trap contained two
layers: bottom layer and HF solution above. The bottom
layer (7 g of clear liquid) which was separated from the HF
solution consisted of practically pure tris(tri¯uoromethyl)-
di¯uorophosphorane (X) (yield: 3.9%).
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[11] M. G oÈ rg, G.-V. R oÈ schenthaler, A.A. Kolomeitsev, J. Fluorine Chem.
9 (1996) 103.
[12] K.I. The, R.G. Cavell, Inorg. Chem. 16 (1977) 1463.
13] N.T. Yap, R.G. Cavell, Inorg. Chem. 18 (1979) 1301.
1
9
7
F NMR (neat liquid, � 408C), d: � 60.8 ddec (2F, PF);
31
1
2
�
65.0 dt (9F, 3CF );
3
J
ˆ 989 Hz;
J
P,F
ˆ 167 Hz;
P,F
[
3
JF,F ˆ 17.2 Hz. P NMR (neat liquid, � 408C), d: � 62.2