Note: Descriptions are shown in the official language in which they were submitted.
2~9~693
W0 92/05299 PCT/EP91/0173
Description
Process for the preparation of halogenated acrylic acids
Pcrylic acid derivatives have a very broad field of
application as organic intermediate~. They permit acces~
to a multiplicity of useful compounds, but in particular
they are suitable for the preparation of pla~tics.
Halosenated and deuterated acrylic acid derivatives and
also m~thacrylic acid derivatives have been of particular
interest for some tLme since ~ubst~nces of this type are
suitabls for the preparation of special plastics having
special properties.
Thus, for example, ~-halogenoac~ylic acid esters are us~d
for the preparation of radiation-sen~itive protective
layers in resist technology. ~-Fluoroacrylic acid esters
are suitable, for example, for the preparation of
~ynthetic glasse~ for aerospace technology and are also
suitable s~arting materials for polymer beam waveguides,
deuterated derivatives being of particular interest
becau~e of theix better optical characteristics.
3-Chloro-2-1uoroacrylic acid and its derivatives are
used, for example, in order to Lmprove the pre-
vulcanization process in the prepara~ion of elastomer
mixtures (US-~ 4 254 013) or in the synthesis of
~5 fluorlne-sl~bstituted pyr~throid esters having
in~ecticidal properties (Australian Patent Application
~04813).
Various processes are available for the ~ynthesis of
3-chloro-2-fluoroacrylic acid and its derivatives.
3-Chloro-2-fluoroacrylic acid i~ formed in yields of
73-76% on heating 2-bromo-3-chloro-2-fluoro-propionic
acid esters or 2,3-dichloro-2-fluoropropionic acid ~sters
with 40% strength pota~sium hydroxide solution at
2 ~ 3
60-70C. The pyrolysis of 2,3-dichloro-2-fluoropropionyl
chloride i~ effected in quantitative yield by passing
throuyh a quartz tube filled with active charcoal and
heated to 440C. The problem with this method is the
costly synthesis of the selectively chlorinated or
chlorinated and brominated 2-fluoro-propionic acid
derivatives which are required 26 Btarting materials.
In the case of the reaction of 1,2-dichloro-3,3-
difluorocyclopropene with methanolic sodium methyla~e,
methyl 3-chloro-2-fluoroacrylate is formed only in a
yield of 12% (J. Fluorine Chem. 10 (1977), 4, 2613.
3-Chloro-2-fluoroacrylic acid i~ al~o formed only a3 a
minor by-product in the case of the electrochemical
dehalogenation of the readily acces~ible, completely
halogenated 2-fluoropropionic acid~ to halogenated
fluoroa~rylic acids. (EP-A 0 308 838). Thus, in invention
Example 7, in the case of the electrolysis of 2,3,3,3-
tetrachloro-2-fluoropropionic acid on graphite electrodes
in ~ catholyte which al~o contain~ in solution, in
addition to NaO~, Pb(OOCCH3)2 as the salt of a metal
having a hydrogen overvoltage of at least 0.25 V (based
on a current density of 300 mA~cm2), the yield of
3,3-dichloro-2-fluoroacrylic acid i8 97.2%; 3-chloro-2-
fluoroacrylic acid is formed in this reaction as a by-
product in a yield of only 2.1~.
The electrochemical replacement of halogen atoms ~yhydrogen or deu~erium atoms in 3,3-dichloro-2~fluoro-
acrylic acid and its derivatives in principle offers
access to 3-chloro-2-fluoroacrylic acid and to 3-chloro-
3-deutero-2-fluoroacrylic acid and their derivatives.
However, in thi6 process the reaction can be stopped only
incompletely at the 3-chloro-2-fluoroacrylic acid stage
and in the main 2-fluoroacrylic acid or its derivatives
are formed. Thus, it can be seen from the invention
~5 ~ - examples in EP-A-0 2g0 120 that 3-chloro-2-fluoroacrylic
acid is passed through as an intermediate stage in the
2~2~3
dehalogenation of 3,3-dichloro-2-fluoroacrylic acid to
2-fluoroacrylic acid. 3-Chloro-2-fluoroacrylic acid is,
however, at best formed in a yield of 62.8%, but is
present only in a purity of at most 7S~ alongside
the two acids 3,3-dichloro~2-fluoroacrylic acid and
2-fluoroacrylic acid, which are difficult to separate off
(Example 3).
The preparation of 3-chloro-2-fluoroacryllc acid by ~his
process is unselective and uneconomic since a costly
purification of the electrolysis product i necessary.
The aim of findinq a proces~ for the preparation of
2-fluoroacrylic acid derivati~es, in particular
2-fluoroacrylic acid derivatives halogen-substituted or
deuterium-s~bstituted in the 3-position, which makes
possible a selective and economic synthesis of thes~
compounds therefore resulted from the prior art.
It has now been found, surprisingly, that the ob~ect can
be achieved if the electrolysis is carried out in water
or deuterium oxide, if appropriate with the addition of
an organic solvent, on carbon elec~rodes in the presence
of iron salts.
This finding is particularly surprising since it is known
from the prior art that the presence of iron salts during
the electrolysis is to be avoided since, even in very low
concentrations (0.1 ppm), these salts cause complete
poisoning of the cathode within a short time ~F. Beck,
Elektroorganische Chemie (Electroorganic Chemistry),
Weinheim, 1974, 95). A5 a result of such poisoning the
ca~hode loses its hydrogen overvoltage, i.e. the ability
to reduce organic compounds in the presence of protons
i.e. under acid conditions.
Only the thermodynamically more favorable reduction of
protons to hydrogen then still proceeds at the cathode.
Since this reaction drastically impairs the current yield
20~12~3
-- 4 --
of the proce~s concerned, the proces~ becomes uneconomic.
For thi reason, the presence of iron ions iB carefully
excluded from alectrochemical reductions in accordance
with the prior art.
The in~ention thus relate~ to a proce~ for the
preparation of compound~ of the formula (I)
~2
C = C (I3
R3~ ~ R4
wherein
R1 is a fluorine atom or a methyl or deuteromethyl
group, in particular a fluorine atom,
R2 and ~3 are identical or different and axe a fluorine,
chlorine, hydrogen or deuterium atom, preferably a
chlorine, hydrogen or deuterium atom, and
R4 is a
o
Il 1~
- C - R'
group, in which
R5 is -OH, -0~, Cl-C4-alkoxy or -OMe, where Me = alkali
metal, alkaline earth metal or NH4 ion,
by electrolytic reduction of compounds of the formula
(I
R2 Rl
R3 - C ~ 4
R6 R6 (II~
in which Rl, R2, R3 and R4 have the abovementioned meaning
-- 5 --
and R6 is a chlorine atom, or compounds of the formula
(III)
R6 ,~ Rl
C = C \ (III~
in which Rl, R4 and R6 have the abovementioned meaning, in
a divided or undivided cell in an electrolysis.liquid
consisting of - in each case based on the total amount of
the el~ctrolyte in the undivided cell or of the catholyte
in the clivided cell - 0 to 100~ by weight o~ deuterium
oxide or water and 100 to 0% by weight of one or more
organic solv~nts, at a temperature in the range from
-10C up to the boiling point of the electrolyte liquid,
at a pH value of below 6 and current densities of lO to
500 mA/cm2, in the presence of iron salt~, which are
presen~ in the electrolyte in a concentration of 1 to
lS 5000 ppm, at a carbon cathode.
The process according to the invention is carried out in
divided or undivided cells. The customary diaphragms
whlch are stable in the electrolyte and ara made of
polymers, preferably perfluori.nated pol~mers~ or other
orS~anic or inorganic material~, such aq, for example,
gla~s or ceramic, but pre~erably ion exchange membrane~
arP used to divide the cells into anode ~pace and cathode
space. Preferred ion exchange membranes are cation
exchange membrane~ made of polymer~, preferably
perfluorinated polymers containing carbo~yl and/or
sulonic acid groups. The u~e of stable anion exchange
membranes i~ also possibleO
The electroly~is can be carried out in all customary
electrolysis cells, such as, for example, in beaker or
plate and frame cells or cells containing fi~ed bed or
fluidized bed electrodes. Both monopolar and bipolar
switchin~ of the electrode~ can be employed.
2 ~ 3
Preferred anolyte liquids are aqueous mineral acids or
solutions of their salts, such as, for ex~nple, dilute
sulfuric acid, concentrat~d hydrochloric acid, sodium
sulfate or sodium chloride solutions and solutions of
hydrogen chlorid2 in alcohol.
The catholyte liguid consists of water or deuterium oxide
or can contain these.
In the case of electrolysis in the presence of deuterium
oxide, all active protons and all water present,
including the water of crystallization, in ~he anolyte
and catholyte must be replaced ~y deuterium atomq or
deuterium oxide.
The catholyte liquid can also consist of organic solvents
which can contain water or deu~erium oxide. Examples of
organic solvents are short-chain aliphatic alcohols, such
as m~thanol, ethanol, propanols or butanols, diols, such
as ethylsne glycol or propanediol, but also polyethylene
glycols and their eth~rs, ethers such as tetrahydrofuran
or dioxane, amides such as dimethylformamide or N-methyl-
2-pyrrolidinone, nitriles such as acetonitrile, ketones
such as acetone, or other solvents.
The proportion of organic solvents in ~he catholyte
liquid is 0~ by weight in the case of electrolysis in
water or in deuterium oxide or 100% by weight if the
electrolysis is carried out under anhydrous conditions.
If the electrolysis is carried out in mixtures of wa~er
or deuterium oxide and organic sol~Qnts, the proportion
of organic solvents is 5 to 95% by weight and pref2rably
10 to 90~ by weight of the catholyte liquid.
The iron salts contained according to the invention in
the electrolyte solutions can be present in the oxidation
s'cates 2-~ and 3+, including alongside one another. The
nature of the anion is not problematical, but iron salts
~2~
containing the anions halide, so42-, HSO4-, NO3-, BF4- and
CH3COO- are preferably employed. The concentra~ions of the
iron salts in the electrolysis solutions are 1 to
5000 ppm, preferably 10 to 3000 ppm and in particular 100
to 1000 ppm.
The electrolysis is carried out at pH valuss of below 6,
preferably below 5, and in particular at pH value~ below
3.
In order to adjust the pH to the value favorable for the
electrolysis and to increase the conductivity,
dissociating compounds, for example inorganic or organic
acids, preferably acids such as hydrochloric acid, boric
acid, phosphoric acid, sulfuric acid or tetrafluoroboric
acid and/or formic acid, acetic acid or citric acid
and/or their salts, can be added to the catholyte in the
divided cell or to the ~lectrolyte in the undivided cell.
Tetraalkyl ammonium salts, such as, for example,
tetramethyl-, tetraethyl~-, tetrapropyl- and
tetrabutylammonium salt containing halide, sulfate,
~0 hydrogen sulfate, tetrafluoroborate, nitrate or acetate
anions, are also particularly ~iuitable for increasing the
conducti~ity. The salts should be so selected that no
sparingly soluble compounds are formed.
The electrolysis is carried out at current den~ities of
10 to 500- A/cm2, preferably at current densities o 20 to
400 mA/cm2 and in particular at 50 to 300 mA/cm2.
Furthermore, salts of metals having a hydrogen
overvoltage of at least O.25 V (based on a current
density of 300 mA/cm2) and optionally dehalogenating
properties can be added to the electrolyte in the
undivided cell or to the catholyte in th~ divided cell.
Suitable salts are, in particular, the soluble salts of
Cu, Ag, Au, Zn, Cd, Hg, Sn, Pb, Tl, Ti, Zr, ~i, V, Ta, Cr
or Ni, preferably the soluble Pb, Zn, Cd, Cu and Sn
2 6 9 ~
salts, in particular the soluble lead salts. The
preferred anions in the salts are CI-, SO~--, NO3- and
CH3COO-.
The salts can be added directly ~o ~he elec~rolysis
solution or can also be produced in the solution, for
example by adding oxides, carbonates and the like - and
in a few cases al50 the metals themselves ~i soluble).
The salt concentration in the catholyte in the divided
cell is appropriately ad~usted to about 0.1 to 5000 ppm,
preferably 10 to 1000 ppm.
Suitable carbon cathodes are, in principle, all possible
carbon electrode materials, such as, for example:
electroda graphite, impregnated graphite materials and
also vitreous carbon.
The anode material used can be the same material as used
for the cathode, or all materials at which the anode
reactions known per se take place. Examples are lead,
lead oxide on lead or other supports, platinum, or
titanium dioxide doped with noble metal oxides, for
example platinum oxide or ruthenium dioxide, on titanium,
or other materials for the evolution of chlorine from
aqueous alkali metal chloride ~olutions or a~ueous or
alcoholic hydrogen chloride solutions
The electrolysis temperature is in the range from -10C
up to the boiling point of the electrolyte liquid,
preferably from 10 to 90C and in particular from 15 to
80C
It is possible to carry out the electrolysis either
continuously or discontinuously. A method in divided
electrolytic cells in which the cathode reaction is
carried out discontinuously and the anode reaction is
carried out conti~uously is particularly appropriate. If
the anolyte con~ains hydrogen chloride, Cl- is
2~g26~3
- 9 -
continually consumed by the anodic evolution of chlorine
and this can be compensated by continuous replenishment
of gaseous HCl or of aqueous hydrochloric acid.
The electrolysis product is woxked up in a known manner,
for example by distillation or extraction of the
products.
The compounds added to the catholyte can thus be fed back
into the process.
The process according to the inventLon can also be
carried out at high current densities and low p~ values,
without it being possible to observe a fiignifican~
evolution of hydrogen at the cathode. The high
selectivity of the reaction is retained even under these
condi~ions.
An increase in the current yield and in the selectivity
of the reaction, and thus in the economy of the process,
when carrying out the electrolysis in the pre~ence of
iron salts was in no way to be expected according to the
prior art.
Examples
The electrolyses were carried out in divided plate and
frame cells (Sigri, Meitingen, Germany).
Electrode surface area: 1000 cm2
Electrode spacing: 4 mm
Cation exchange membrane: ~Nafion 423,
tSingle-layer memh~ane of a copolymer of a
perfluorosulfonylethoxyvinyl ether and
tetrafluoro2thylene)
Turbulence intensifier; Polyethylene gratings
Anolyte: 20% HCl in water
Flow rate: 800 l/h
2~2~3
-- 10 --
Electrolytic cell 2:
As electrolytic cell 1 except for the following
differences.
Electrode surace area; 200 cm2
Anode: Titanium electrode coated with TiO2 and activated
with RuO2.
Anolyte: 2~ strength D2SO4 in 300 g of D20 or 5~ strength
H2SO4 in HzO
Flow rate: 300 l/h
The catholyte composition, the electrolysis conditions
and the electrolysis resul~s can be taken from the
following table.
Example~ 1 2 3
Electrolytic cell 1 1 2
Catholyte:
CCl3CFClCOOH [g] 1937 914 -CCl3CFClCOONa [g]
_ _ 42
CCl3CFClCOOCH3 [g~
H2O [q] 5930 3538
D2O [g] _ _ 500
CH3OH - - -
FeCl3 [g~ (ppm) 1.6 ~218) 2 (449) 0.2 (363) Pb(NO3)2
[g] (ppm) - 1 ~224) 0.3 (535)
PH(start) 0.72 -0.1
PH(end) 0.08 -0.28
Voltage [V] 4.4-2.7 4.4 7.5
Current density [mA/cm2~ 150-40 150 200
Charge consumption [Ah] 720 428 38.8
Temperature [C] 37-39 40 17-30
Hydroqen [1] 23.7 21
Deuteriwm ~1] - - 14.6
2 ~ 3
11
Examples 1 2 3
El~ctrolysis result:
[ g (yield)]
CHCl=CFCOOH 635.5 (89.4~) 388 (80.5)
CDCl=CFCOOD - - 18.8(90.4%)
CDCl=CFCOOCH3 - - _
CCl2=CFCOOH 64.2 21.1
CCl2=CFCOOCH3
CH2=CFCOOH14.3 (2.6%) 12.9 (3.7%~ -
CD2=CFCOOD - - 1.2 (7-2%)
CD2=CFCOOCH3
Current yield 75.5% 71.2% 61.4%
