Preparation of alkyl adipates

Alkyl adipates are prepared by reacting an alcohol and carbon monoxide with an alkyl pentenoate in the presence of (i) a catalytically effective amount of a cobalt catalyst, (ii) a tertiary nitrogen base, and (iii) hydrogen, with the hydrogen comprising at least 0.1% by volume of the carbon monoxide, and said reaction being carried out in (iv) an aromatic hydrocarbon or substituted aromatic hydrocarbon reaction medium.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the preparation of alkyl adipates from 
alkyl pentenoates, and, more especially, to the selective preparation of 
alkyl adipates by reacting carbon monoxide and an alcohol with an alkyl 
pentenoate. 
2. Description of the Prior Art 
It is well known to this art [compare Bulletin of the Chemical Society of 
Japan, 46, pp. 526-527 (1973)] that a mixture containing dialkyl esters, 
and in particular an alkyl adipate, is obtained by reacting carbon 
monoxide and an alcohol with an alkyl pent-3-enoate, under high pressure 
and at elevated temperature, in the presence of cobalt carbonyl and an 
aromatic heterocyclic nitrogen base. However, the industrial-scale 
development of a technique of this type, the value of which is not 
contested in principle, is greatly jeopardized not only by the low 
efficacy of the catalyst system, but also by the substantial proportion of 
alkyl pentanoate formed, even though the reaction is carried out in the 
absence of hydrogen. 
Furthermore, it too is well known to this art that the presence of small 
amounts of hydrogen in the reaction medium tends to increase the efficacy 
of cobalt-based catalysts in processes for the synthesis of esters by 
reacting an alcohol and carbon monoxide with an olefinic compound. 
It has nevertheless also been found that, in the process in question, this 
favorable effect associated with the presence of small amounts of hydrogen 
is accompanied by an adverse influence on the selectivity of the process 
in respect of alkyl adipates, which are the specifically desired products. 
In fact, it has been observed that the presence of hydrogen not only tends 
to increase the proportion of hydrogenation products in the reaction 
mixture, but is also capable of reducing the proportion of adipate in the 
diesters formed. 
This adverse effect greatly detracts from the economics of the subject 
process, insofar as the utilization of the branched diesters and the alkyl 
pentanoates is uncertain or even nonexistent. In other words, the 
formation of these products, which are destroyed in practice, corresponds 
to an intolerable loss of starting material. Furthermore, hydrogen can be 
formed in situ from the traces of water which may be present in 
technical-grade reactants, according to the well known reaction: 
EQU H.sub.2 O+CO.fwdarw.CO.sub.2 +H.sub.2 
It would be desirable, for obvious economic reasons, to be able to employ 
technical-grade carbon monoxide containing hydrogen, without this 
detracting from the selectivity of the process in respect of alkyl 
adipates, which are the desired diesters. It would also be desirable, for 
the same reasons, to be able to use reactants containing traces of water, 
without this resulting in a loss of starting material. 
SUMMARY OF THE INVENTION 
Accordingly, it has now surprisingly been found, and which is a major 
object of the present invention, that alkyl adipates are selectively 
prepared by reacting an alcohol and carbon monoxide with an alkyl 
pentenoate in the presence of a metal catalyst selected from the group 
comprising cobalt and its compounds, and in the further presence of a 
tertiary nitrogen base and hydrogen, the hydrogen representing at least 
0.1% by volume of the carbon monoxide, provided that the reaction is 
carried out in an aromatic hydrocarbon which, if appropriate, can contain 
from 1 to 3 substituents independently selected from among alkyl, aryl and 
aralkyl radicals containing at most 20 carbon atoms.

DETAILED DESCRIPTION OF THE INVENTION 
More particularly according to the present invention, an alcohol and carbon 
monoxide are therefore reacted with an alkyl pentenoate. Alkyl pentenoates 
can be obtained by reacting an alcohol and carbon monoxide with butadiene, 
in a manner which is in itself known. Alkyl pent-3-enoates are obtained as 
the principal products. Within the scope of the present process, it is 
also possible to use alkyl pent-2-enoates, which are obtained, for 
example, by isomerization of the corresponding pent-3-enoates, these 
.alpha.,.beta.-unsaturated esters proving more reactive. 
An alcohol is also used to carry out the present process. This other 
starting material can be represented by the formula R'OH, in which R' is 
an alkyl radical containing at most 12 carbon atoms and optionally 
substituted by one or two hydroxyl groups, or a cycloalkyl radical having 
from 5 to 7 carbon atoms, or an aralkyl radical having from 7 to 12 carbon 
atoms, or a phenyl radical. 
Exemplary of alcohols which can be used according to the present invention, 
representative are methanol, ethanol, isopropanol, n-propanol, 
tert.-butanol, n-hexanol, cyclohexanol, 2-ethylhexan-1-ol, dodecan-1-ol, 
ethylene glycol, hexane-1,6-diol, benzyl alcohol, phenylethyl alcohol and 
phenol. 
It is preferred to use an alkanol having at most 4 carbon atoms; methanol 
and ethanol are suitable for carrying out the present process. It is 
advantageous to use the alcohol corresponding to the alkyl radical of the 
pentenoate selected as the starting material. 
The alcohol and the alkyl pentenoate can be used in stoichiometric amounts. 
However, it is preferable to use an excess of alcohol in a proportion of 1 
to 10, or even more preferable to use from 2 to 5 mols of alcohol per mol 
of alkyl pentenoate. 
The reaction is carried out in the presence of a metal catalyst selected 
from among cobalt and compounds thereof. Any source of cobalt which is 
capable of reacting with carbon monoxide in the reaction medium to give 
cobalt carbonyl complexes in situ can be used within the scope of the 
invention. 
Examples of typical sources of cobalt are finely divided cobalt metal, 
inorganic salts, such as cobalt nitrate or carbonate, and organic salts, 
in particular carboxylates. Cobalt carbonyls or hydrocarbonyls can also be 
used; dicobalt octacarbonyl is suitable, for example, for carrying out the 
present process. 
The molar ratio of the alkyl pentenoate to the cobalt generally ranges from 
10 to 1,000. This ratio is advantageously set at a value ranging from 20 
to 300. 
The process according to the present invention also requires the presence 
of a tertiary nitrogen base having a pK.sub.a ranging from 3 to 10. 
Preferably used are heterocyclic nitrogen compounds comprised of 5 to 6 
ring members, which can contain one or two substituents selected from 
among alkyl or alkoxy groups having at most 4 carbon atoms, the hydroxyl 
group and halogen atoms, which optionally contain 2 or 3 double bonds and 
which can furthermore be fused to a benzene nucleus, if appropriate, 
provided that the ring members adjacent to the nitrogen hetero-atom are 
neither substituted nor common to two rings. 
6-Membered heterocyclic nitrogen compounds having a pK.sub.a of between 4 
and 7, in particular pyridine, 4-picoline, isoquinoline and 3,5-lutidine, 
are more particularly preferred for carrying out the present process. 
The amount of tertiary nitrogen base used is generally such that the molar 
ratio N/Co ranges from 1 to 50. To carry out the invention with especially 
good results, it is preferred that this ratio be set at a value ranging 
from 2 to 25. 
One of the essential characteristics of the present process is the use, as 
a solvent, of an aromatic hydrocarbon which, if appropriate, can contain 
from 1 to 3 substituents independently selected from among alkyl, aryl and 
aralkyl radicals containing at most 20 carbon atoms. 
More preferably, the solvents used can be represented by the formula: 
##STR1## 
in which a is a benzene or naphthalene nucleus and R.sub.1, R.sub.2 and 
R.sub.3, which are identical or different, represent hydrogen or an alkyl, 
aryl or aralkyl radical containing at most 20 carbon atoms. Preferably, at 
least one of the radicals R.sub.1 to R.sub.3 is a hydrogen atom and the 
other two radicals are selected from among hydrogen, alkyl radicals having 
at most 10 carbon atoms and the phenyl radical. 
Preferably, a is a benzene nucleus, two of the radicals R.sub.1 to R.sub.3 
represent a hydrogen atom and the third radical is an alkyl radical 
containing at most 4 carbon atoms or a phenyl radical. 
Of course, it too is possible to use mixtures of several of these aromatic 
compounds, and in particular mixtures which are commonly available 
commercially. 
Examples of solvents suitable for carrying out the present process are: 
benzene, naphthalene, toluene, ethylbenzene, cumene, tert.-butylbenzene, 
n-nonylbenzene, n-octadecylbenzene, methylnaphthalenes, 
isobutylnaphthalenes, diphenylmethane, biphenyl, xylenes, 
dimethylnaphthalenes, p-ethyltoluene, p-heptyltoluene, diethylbenzenes, 
methylisopropylbenzenes (cymenes), methylbiphenyls, 4,4'-dimethylbiphenyl, 
terphenyls, mesitylene and its isomers, ethylxylenes, 
trimethylnaphthalenes and the like. 
Benzene and monoalkylbenzenes in which the alkyl radical contains at most 4 
carbon atoms are the preferred, on the one hand because of the 
satisfactory results which they produce, and on the other hand because of 
their greater availability. 
The amount of solvent, which influences the selectivity of the reaction, 
will generally be more than 20% (by weight) of the initial reaction 
mixture, and, to carry out the present process with good results, it will 
range from 30 to 60% (by weight) of the said mixture. 
The process according to the present invention is also carried out in the 
presence of hydrogen, the hydrogen representing at least 0.1% (by volume) 
of the carbon monoxide. 
To carry out the invention with good results, the hydrogen will represent 
at most 3% (by volume) of the carbon monoxide, and it will preferably 
represent from 0.5 to 2% (by volume) of the carbon monoxide. 
Of course, although the hydrogen can be introduced into the reaction medium 
conveniently in the form of a mixture with the carbon monoxide, it can 
also be supplied separately. 
The reaction is carried out in the liquid phase at a temperature above 
120.degree. C., there being no advantage in exceeding 200.degree. C., 
under a carbon monoxide pressure which is at least 50 bars and can be as 
much as 1,000 bars. The reaction is preferably carried out at a 
temperature on the order of 130.degree. to 180.degree. C. and under a 
carbon monoxide pressure on the order of 100 to 300 bars. 
In addition to hydrogen, the carbon monoxide used can contain impurities 
such as carbon dioxide, methane and nitrogen. 
Upon completion of the reaction, or when the desired degree of conversion 
has been attained, the alkyl adipate is recovered by any suitable means, 
for example, by distillation or liquid-liquid extraction. 
In order to further illustrate the present invention and the advantages 
thereof, the following specific examples are given, it being understood 
that same are intended only as illustrative and in nowise illustrative. 
In said examples to follow, the following conventions are used: 
The compounds resulting from the position isomerism of the olefinic double 
bond are not included in the products formed. 
The products formed are essentially the diesters and the alkyl pentanoate, 
the latter resulting from the hydrogenation of the starting material 
ester. 
A denotes the activity expressed in mols of products formed per hour and 
per gram atom of cobalt. 
X(%) denotes the number of mols of diesters per 100 mols of products 
formed. 
Y (%) denotes the number of mols of alkyl adipate per 100 mols of products 
formed. 
Z (%) denotes the numbers of mols of alkyl pentanoate per 100 mols of 
products formed. 
EXAMPLES 1 to 18 
Control Experiments (a) to (j) 
A series of experiments was carried out according to the following 
procedure: 
Methyl pent-3-enoate (P3), methanol, dicobalt octacarbonyl (DCOC), 
isoquinoline and, if appropriate, a solvent were introduced into a 125 ml 
stainless steel autoclave purged under argon. 
The autoclave was then purged with a stream of carbon monoxide, if 
appropriate containing hydrogen. The autoclave was then heated to the 
temperature T under a pressure P. After a reaction period (designated by t 
and expressed in hours) at this temperture, the autoclave was cooled and 
degassed. The reaction mixture was analyzed by gas chromatography. The 
particular conditions and also the results obtained are respectively 
reported in Tables (A) and (B) below: 
In Table A, the ratios MeOH/P3, P3/Co and N/Co denote, respectively, the 
molar ratio of the methanol to the pent-3-enoate (Example 12 was carried 
out starting from methyl pent-2enoate), the ratio of the number of mols of 
pent-3-enoate to the number of gram atoms of cobalt, and the ratio of the 
number of mols of isoquinoline to the number of gram atoms of cobalt. 
Control experiments (a) to (d) clearly show that, in the absence of 
solvent, the presence of hydrogen results in an increase in the efficiency 
of the cobalt-based catalyst and in a substantial drop in selectivity with 
respect to dimethyl adipate. 
Control experiment (e) shows that, in the absence of hydrogen, the presence 
of benzene in the reaction medium made it possible to obtain dimethyl 
adipate with a noteworthy selectivity. However, the efficacy of the 
cobalt-based catalyst was very low under these conditions. Examples 1 to 4 
show that the simultaneous presence of benzene and hydrogen made it 
possible to obtain dimethyl adipate selectively and efficiently. 
TABLE (A) 
__________________________________________________________________________ 
H.sub.2 
P3 MeOH 
DCOC (% by 
P T 
Ref. 
mmol. 
mmol. 
mmol. 
MeOH/P3 
P3/Co 
N/Co 
volume) 
bars 
.degree.C. 
__________________________________________________________________________ 
a 50.1 
109 0.96 
2.17 24.6 
4 0 130 
160 
b 49.7 
99 0.88 
1.99 28.5 
4.5 0.7 " " 
c 99.8 
198 2.00 
1.98 25.0 
12 0.9 " " 
d " 200 2.01 
2.00 24.8 
" 2.6 " " 
e 50.8 
43 1.01 
0.85 25.1 
3.9 0 " " 
1 50.2 
40 1.00 
0.80 " 4.0 0.8 " " 
2 50.1 
41 1.03 
0.82 24.4 
4.0 2 " " 
3 50.6 
102 1.00 
2.01 25.3 
11.9 
0.8 " " 
4 50.3 
" 0.95 
2.03 26.4 
12.6 
2 " " 
f 50.3 
104 1.00 
2.06 25.2 
3.9 0.8 " " 
5 49.9 
100 0.99 
2.00 25.1 
3.9 " " " 
6 49.2 
101 0.92 
2.05 27.3 
4.3 " " " 
7 50.2 
100 0.98 
1.99 25.7 
4.2 " " " 
8 48.8 
100 0.97 
2.05 25.7 
4.1 " " " 
g 100 198 2.00 
1.98 24.9 
2.1 " " " 
9 50.1 
102 1.01 
2.03 24.7 
2.0 " " " 
h 101 196 1.97 
1.94 25.6 
8.1 " " " 
10 50.4 
97 0.94 
1.92 26.7 
8.4 " " " 
i 49.6 
104 0.98 
2.09 25.0 
20.6 
" " " 
11 50.1 
101 1.02 
2.01 24.6 
19.5 
" " " 
12 50.2 
100 1.02 
1.99 25 3.9 0.7 " " 
13 50.2 
102 1.90 
2.03 13.2 
4.2 0.8 " " 
14 99.9 
201 1.03 
2.01 48.5 
7.9 " 250 
180 
15 100 102 1.00 
1.02 50.1 
7.9 " " " 
j 49.7 
98,4 
1.01 
1.98 24.5 
7.9 " 130 
" 
16 50.0 
99,0 
1.02 
" 24.4 
7.9 " " " 
17 50.2 
102 1.01 
2.03 24.8 
3.9 1 " 160 
18 49.7 
99 1.02 
1.99 24.4 
3.8 " " " 
__________________________________________________________________________ 
TABLE (B) 
______________________________________ 
SOLVENT 
(% by X Y 
Ref. nature weight) t A (%) (%) Z (%) 
______________________________________ 
a -- 0 1 3.7 95.1 79.4 4.9 
b -- 0 " 10.4 89.0 74.6 10.4 
c -- 0 " 1.9 94.7 75.5 4.9 
d -- 0 " 3.3 92.5 74.7 6.8 
e benzene 51 2 1.2 97.3 83.5 2.7 
1 " " " 4.6 97.0 83.7 2.1 
2 " " " 5.3 92.4 79.6 6.7 
3 " 41 " 5.2 94.7 80.0 5.0 
4 " 42 " 5.0 93.8 79.3 5.6 
f -- 0 " 7.9 92.3 76.4 7.0 
5 benzene 20 " 6.9 92.5 77.6 6.4 
6 " 46 " 4.5 95.2 82.4 4.2 
7 toluene 46 " 2.9 95.3 81.9 4.1 
8 t-butylbenzene 
47 " 3.4 95.1 81.7 4.1 
g -- 0 " 5.6 94.8 78.2 4.5 
9 benzene 48 " 2.1 94.1 80.0 4.4 
h -- 0 " 2.4 94.9 76.6 4.6 
10 benzene 44 " 5.3 95.1 80.5 4.0 
i -- 0 " 1.1 90.1 73.7 8.8 
11 benzene 38 " 3.9 94.5 77,2 5.2 
12 " 46 " 7.6 95.3 81.6 4.2 
13 " 60 " 2.6 95.2 81.7 4.2 
14 " 31 " 13.9 95.1 80.8 4.4 
15 " 52 " 5.4 95.5 78.3 3.9 
j -- 0 " 5.6 88.0 74.8 11.6 
16 benzene 44 " 7.4 91.6 79.3 8.1 
17 naphthalene 50 " 4.6 96.3 82.4 3.7 
18 biphenyl 50 " 5.4 95.0 81.3 4.3 
______________________________________ 
While the invention has been described in terms of various preferred 
embodiments, the skilled artisan will appreciate that various 
modifications, substitutions, omissions, and changes may be made without 
departing from the spirit thereof. Accordingly, it is intended that the 
scope of the present invention be limited solely by the scope of the 
following claims.