Note: Descriptions are shown in the official language in which they were submitted.
Process_for the pre~aration of
methyl 2-formylbutyrate
The invention relat~s to a process for the preparation of
methyl 2-formylbutyrate by reaction of methyl crotonate
with hydrogen and carbon monoxide.
2-Formylbutyric acid esters are useful intermediate
products in chemical syntheses. They can be converted by
reduction into hydroxy compounds, which are used, for
example, for the preparation of polyesters. Reaction with
ammonia and hydrogen gives esters of aminomethylbutyric
acids, and oxidation of the formylbutyric acid esters
leads to half-esters of dicarboxylic acids.
The hydroformylation of crotonic acid esters always leads
to product mixtures which essentially contain 2-, 3- and
4-formylbutyric acid esters and, by hydrogenation of the
crotonic acid ester, also butyric acid esters. The
reaction has been investigated repeatedly.
Adkins et al. in J.Am.Chem.Soc. 71 (1949~, page 3051 et
seq. thus r~port on the reaction of ethyl cro~onate with
watergas (C0 : H2 = 1: 1) at 120 to 125C under a total
pressure of 200 to 300 atmospheres in the presence of
cobalt as a catalyst and ben2ene as the reaction medi.um.
The reaction gives ethyl 3-formylbutyrate in a yield of
71%.
According to Piacenti et al., Ullmanns Encyklopadie der
technischen Chemie (Ullmann's Encyclopedia of Industrial
Chemistry) 62, Volume 13, page 65, ethyl 2-, 3- and 4-
formylbutyrate are obtained in a ratio of 15 : 15 : 70 in
70% yield from ethyl crotonate, evidently in the presence
of a CO catalyst.
Falbe et al. have publ.ished the results of the hydro-
formyl~tion of ethyl crotonate in the presence of 1~ by
weight of Rh2O3 in a two-stage reacti.on in Brennstoff
~ ~2~
Chemie 48 (1967), page 46 et seq. Thus, a reaction
mixture which essentially consists of ~-methyl-~-butyrol-
actone, ethyl butyrate, ~ -valerolactone and ethyl c~_
hydroxymethylbutyrate thus results at 135C under a
pressure of 200 atmospheres in the first stage and at
200~C under a pressure of 300 atmospheres in the second
stage.
Lai and Ucciani (Adv.Chem.Ser. 1974, (132), page 1 et
seq.) hydroformylated methyl crotonate under various
conditions and ~ound that the ~electivity of the reaction
decreases if rhodium is used as the catalyst in compari-
son with the cobalt-catalyzed reaction. The selectivity
becomes e~en lower if rhodium is used together with
triphenylphosphine. The main product of the reaction
catalyzed by cobalt is methyl 4-formylbutyrate. Rhodium
alone gives the 3-formyl compound as the main product,
and rhodium with triphenylphosphine gives predominantly
the 2-formyl compound, but the selectivity of the reac-
tion is completely unsatisfactory when rhodium cat~lysts
are used.
According to Tanaka et al., Bull.Chem.Soc.Jap. 50 ~1977~,
2351 et seq., the product distribution on hydrofonnyla-
tion of methyl crotonate in the presence of rhodium
depends very greatly on the nature of the ligands used.
If triphenylphosphine is used as the ligand, 3-formyl-
butyric acid ester is formed as the main product in a
moderate yield, and the 2-formyl compound is formed only
in a minor amount. Using (H5C6)2P(CHz),,P(C~H5)z as the
ligand, ethyl 2-formylbutyrate is preferably obtained.
Finally, Okano et al., Bull.Chem.Soc.Jap. 54 (1981), 3799
et seq. found that mainly butyric acid and in addition
small amounts o methyl 2- and 3-formylbutyrate are
formed in the reaction of methyl crotonate with water in
the presence of rhodium-phosphine comple~ compaunds as
catalysts on the basis of the watergas equilibrium which
is established. This reaction route also does not offer
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the possibility of an economically satisfactory prepara-
tion of methyl 2-formylbutyrate on an industrial scale.
There was therefore the object of developing a process
which allows hydroformylation of methyl crotonate to give
methyl 2-formylbutyrate in a high yield with a high
selectivity using readily accessible catalysts.
This objec~ is achi.eved by a process for the preparation
of methyl 2-formylbutyrate by hydroformylation of methyl
crotonate in the presence of catalysts containing rhodium
and organic phosphines. It comprises carrying out the
reaction at temperatures of 80 to 120C, under pressures
of 20 to 30 MPa, with a rhodium concentration of 10 to
S00 ppm, based on the methyl crotonate employed, and in
an organic solvent as the reaction medium.
Methyl crotonate is employed as the starting compound for
the preparation of methyl 2-formylbutyrate. It can be
used in the pure form, i.e. in the distilled form.
However, it has been found that crude products which also
contain other constituents in addition to methyl croton-
ate can also be employed without a disadvantage to theyield and selectivity. Thus, for ex~nple, mixtures which
are obtained on esterification of crotonic acid with
methyl alcohol and contain 40 to 60% by weight of methyl
crotonate, 30 to 40% by weight of methanol and 5 to 15%
by weight of water are successfully used directly as the
starting suhstances.
. .
Carbon monoxide and hydrogen are in general employed in
~ha form of synthesis gas, which is obtained by partial
oxidation of carbonaceous material in the presence of
water. It contains carbon monoxide and water in a ratio
of about 1:1. However, mixtures in which one of the
components is present in excess can also successfully be
used as reaction partners. In practice, CO/H2 mixtures
which contain 0.8 to 1.2 mol of CO per mol of H2 have
proved to be suitable. It goes without saying that
2~2~91~
-- 4
i~purities which may lead to poisoning of the catalyst
must b~ removed from the gas. Thus, for e~ample, sulfur
can be tolera~ed only up to a maximum concentration of
about 2 ppm.
The starting substances are reacted in the presence of a
catalyst system comprising rhodium and an organic phos-
phine. The concentrati.on of the rhodium, based on the
methyl crotonate originally employed, is 10 to 500 ppm,
preferably 50 to 200 ppm and in particular 80 to 120 ppm.
The rhodium is employed as the metal, advantageously in
finely divided form, or as a compound. In practice, Rh 2-
ethylhexanoate has proved to be a particularly suitable
starting substance for the catalyst. The second constitu-
ent of the catalyst system is an organic phosphine. These
are understood as being alkyl- and arylphosphines. Tri-
n-butylphosphine and triphenylphosphine have proved to be
particularly suitahle. 1 to 50, in particular 2 to 20 mol
of phosphine are used per g-atom of rhodium. The catalyst
system can be added to the reaction mixture in finished
form. In this case it is obtained in its own process step
independent of the actual reaction. However, the catalyst
can also be prepared in situ, i.e. in the reaction
mixture under hydroformylation conditions, with an
equally good result.
It is an essential feature of the invention that certain
pressure and temperature ranges are observed during the
hydroformylation, i.e. 80 to 120C and 20 to 30 MPa. It
is advantageous to use temperatures of 90 to 110C, and
in particular 90 to 100C, and pressures of 200 to 300
bar. Temperatures higher than those mentioned above lead
to increased hydrogenation, and lower temperatures lead
to a reduction in the conversion.
Finally, the novel process comprises the use of organic
solvents as the reaction medi~m. Aliphati.c hydrocarbons
have proved to be particularly suitable, and cyclohexane
is preferably employed. The proportion of the reaction
-- 5
medium in the reaction mixture can vary within wide
ranges and can be from 20 to 80~ by weight. About 50~ by
weight are preferred.
The process according to the invention can be carried out
discontinuously or continuously. In the case of a discon-
tinuous procedure, the ester, the reaction mediurn and the
catalyst, which is formed beforehand or employed in the
forJn of its components, are introduced into the reactor
and the desired pressure is established by forcin~ in
carbon monoxide and hydrogen, while heating at the same
time. It is advisable to stir the reaction mixture if
adequate mixing is not already achieved by the introduc-
tion of carbon monoxide and hydxogen. Synthesis gas is
fed to the reactor at the rate at which it is consumed by
the reaction. The reaction has ended as soon as no
further uptake of gas occurs.
The reaction is preferably carried out continuouslyO In
this case, synthesis gas, methyl crotonate and reaction
medium are introduced continuously into the reactox
containing the catalyst system. The reaction product and
some of the reac~ion medium are removed continuously. A
stationary state is maint~ined in the reactor by approp-
riate adjustment of the feed rate of the reaction part-
ners and the removal rate of the products. Interruption
of the reaction only becomes necessary when the catalyst
stability has decreased significantly. This point in time
can be delayed by replacing some of the spent catalyst
by fresh catalyst from time to time.
The methyl 2-formylbutyrate prepared by the process
claimed is purified in a kno~l manner by distillation.
The yields are more than 85%. In addition to methyl 2-
formylbutyrate, the crude product of the hydroformylation
contains about 2 to 5~ of methyl 3-formylbutyrate and 5
to 10% of methylbutyrate.
The novel process is described in more detail in the
-- 6 --
examples which follow.
~:xamPle 1
500 g of methyl crotonate, 500 g of cyclohexane, 0.6 g of
triphenylphosphine and 100 ppm of Rh (as Rh 2-ethyl-
hexanoate) are initially introduced into a 2 l autoclavewith a piston stirrer. The mixture is allowed to react at
90C under a CO/H2 pressllre of 27 MPa for about 8 hours,
the con~ents of the autoclave are then cooled and the
reaction mixture is analyzed by gas chromatography.
It has the following composition (in % by weight, without
taking into account cyclohexane):
methyl butyrate 4.23
methyl crotonate 0.59
methyl 2-formylbutyrate91.67
methyl 3-formylbutyrate2.05
other substances 1.46
xample 2
250 g of methyl crotonate, 250 g of cyclohexane, 0.83 g
of triphenylphosphine and 100 ppm of Rh (as Rh 2-ethyl-
hexanoate) are initially introduced into a 1 l autoclavewith a piston stirrer. The mixture is allowed to react at
909C under a CO/H2 pressure of 27 MPa for about 7 hours,
the contents of the autoclave are then cooled and the
reaction mixture is analyæed by gas chromatography.
It has the following composition (in ~ by weight, without
taking into account the cyclohexane):
methyl butyrate 5.23
methyl crotonate 0.11
methyl 2-formylbutyrate88.82
methyl 3-formylbutyrate4.21
other substances 1.63
2~2~
Example 3
250 g of methyl crotonate, 250 g of cyclohexane, 0.83 g
of triphenylphosphine and 100 ppm of ~h (as Rh 2-ethyl-
hexanoate) are initially introduced into a 1 1 autoclave
with a piston stirrer. The mixture is allowed to react at
100C under a CO/H2 pressure of 28 MPa for about 7 hours,
the contents of the autoclave are then cooled and the
reaction mixture is analyzed by gas chromatography.
It has the following composition (in % by weight, without
taking into account the cyclohexane):
methyl butyrate 7.11
methyl crotonate 0.08
methyl 2-formylbutyrate87.55
methyl 3-formylbutyrate4.01
other substances 1.25
