Improvement of catalyst activity in the Ru-catalyzed dimerization of acrylonitrile by using diphenyl ether as a solvent

Article abstract of DOI:10.1246/cl.2007.1384

For the catalyst system of ruthenium and carboxylic acid, which is useful for the efficient tail-to-tail dimerization of acrylonitrile, the TON increases as the ruthenium catalyst concentration is decreased. Furthermore, the addition of aromatic solvents of equal volume to that of acrylonitrile improves the catalyst activity. Especially, the use of diphenyl ether leads to a 1.7 time improvement of the TON. Copyright

Full text of DOI:10.1246/cl.2007.1384

1
384
Chemistry Letters Vol.36, No.11 (2007)
Improvement of Catalyst Activity in the Ru-catalyzed Dimerization
of Acrylonitrile by Using Diphenyl Ether as a Solvent
Ã1;2
1
1
1
Kohichi Kashiwagi,
1
Ryoji Sugise, Toshihiro Shimakawa, and Tunao Matuura
Ube Industries, Ltd., 1978-5 Kogushi, Ube 755-8633
2
Department of Chemistry, Graduate School of Science, Hiroshima University,
1-3-1 Kagamiyama, Higashi-hiroshima 739-8526
(Received September 3, 2007; CL-070948; E-mail: 25988u@ube-ind.co.jp)
For the catalyst system of ruthenium and carboxylic acid,
Table 1. Effect of the molar ratio of AN/Ru on the dimeriza-
tion of AN using the combination of Ru-based catalyst and o-
benzoylbenzoic acid
which is useful for the efficient tail-to-tail dimerization of acryl-
onitrile, the TON increases as the ruthenium catalyst concentra-
tion is decreased. Furthermore, the addition of aromatic solvents
of equal volume to that of acrylonitrile improves the catalyst
activity. Especially, the use of diphenyl ether leads to a 1.7 time
improvement of the TON.
a
Combined yield7
of dimers 1–3/%
21.4
Conversion
of AN/%
AN/Ru
TON8
792
3
7
700b
28.8
c
c;9
c
(
0.430/0.479/0.014/0.041/0.021/0.015)
400b
15.5
0.433/0.482/0.012/0.038/0.019/0.015)
13.1 10.3
13.0
960
1030
1077
1240
997
(
The dimerization of acrylonitirile (AN) has provided an at-
tractive alternative route to conventional industrial methods for
the preparation of hexamethylenediamine, which is one of the
d
1
0000
(
0.441/0.485/0.012/0.030/0.017/0.015)
9.0 7.2
0.460/0.486/0.011/0.026/0.014/0.013)
7.2 6.2
(0.460/0.485/0.010/0.023/0.010/0.011)
4.9 4.0
0.462/0.483/0.010/0.023/0.011/0.011)
d
1
,2
15000
monomers used in the production of Nylon-6,6. Ruthenium-
based catalysts have been important candidates for this reaction
due to their high reactivity and selectivity for linear dimers 1–
c
c
c
(
2
2
0000d
5000d
3,4
3 In the Ru-catalytic system, the inevitable use of hydrogen
as an extra agent to keep the reaction catalytic has resulted
.
(
in the formation of a large amount of propionitrile (PN) (5) as
3
an undesirable by-product.
Recently, we have found that a new catalytic system
aReaction conditions: (General procedure) AN 282.7 mmol,
anisole 18.5 mmol, DMSO 6.4 mmol, CH CH CO Na
3
2
2
[
gives linear dimers of AN without the formation of PN
RuCl2 (DMSO)4 /CH3 CH2 COONa/DMSO/carboxylic acid]
0.306 mmol, RuCl2(DMSO)4 0.0283 mmol, AN/Ru =
10000 (molar ratio), o-benzoylbenzoic acid 5.70 mmol,
ꢀ
150 C, 9 h. See Supporting Information for details. See also
Ref. 10. Reaction time 6 h. Molar ratio of dimers 1–3: cis-
5
(
Scheme 1). The reaction could be carried out neat, which
would be an advantage in a large-scale operation.
b
c
d
1/trans-1/cis,cis-3/cis,trans-3/trans,trans-3/2.
time 9 h.
Reaction
In order to improve catalyst efficiency, we studied the effect
of ruthenium catalyst concentration on the TON values for the
dimerization of AN. Table 1 shows the effect of the molar ratio
ꢀ
of Ru/AN on the dimerization of AN at 150 C in the presence of
o-benzoylbenzoic acid, which was the most suitable carboxylic
acid for the catalytic system [RuCl2(DMSO)4/CH3CH2COO-
AN/Ru was 20000. This result shows that lower catalyst concen-
tration induces higher catalyst efficiency. However, contrary to
our expectation, the TON for dimers 1–3 decreased when the
molar ratio of AN/Ru was higher than 20000. Lower catalyst
concentration induced slight increase of the production ratio of
2,4-hexadienedinitrile (DCBD 3). DCBD 3 may affect the cata-
lyst activity. The TON for dimers 1–3 went down to under 1000
when the molar ratio of AN/Ru was 25000. In this case the
dimerization reaction had poor reproducibility.
5
,6
Na/DMSO/carboxylic acid]. Products were analyzed by gas
chromatography using anisole as an internal standard.
Decreasing the molar ratio of AN/Ru gave poorer yields of
the dimers 1–3. Conversely, the TON for dimers 1–3 increased
with the reduction of ruthenium catalyst concentration. The
TON for dimers 1–3 rose up to 1240 when the molar ratio of
Thus, we examined the addition of a volume of solvent
equal to that of AN to the reaction mixture in order to decrease
catalyst concentration, while maintaining the molar ratio of
AN/Ru (10000). Table 2 shows the additive effect of the sol-
vents on the dimerization of AN. The dimerization of AN in sat-
urated hydrocarbon solvents such as hexane and cyclohexane
gave slightly reduced TONs compared to that without added sol-
vent. Nevertheless, the addition of hydrocarbon solvents gave
little adverse influence on the dimerization of AN. Unsaturated
hydrocarbons such as cyclohexene, and toluene provided some-
what better results. Diphenyl ether turned out to be the most
favorable solvent for the catalytic dimerization of AN in this
experiment. The combined yield of dimers 1–3 became 18.1%
Ru-based catalyst
CN
CN
CN
NC
NC
RCO H
2
2
-Hexenedinitrile Hexanedinitrile
1
2
CN
R
O
CN
CN
NC
O
2
,4-Hexadienedinitrile 2-Cyanoethylcarboxylate Propionitrile
3
4
5
Scheme 1.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.11 (2007)
1385
Table 2. Additive effect of solvents on the dimerization of AN
using the combination of Ru-based catalyst and o-benzoylbenzo-
ic acid
References and Notes
1
G. W. Parshall, S. D. Ittel, Homogeneous Catalysis, 2nd ed.,
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Patent 4504674, 1985.
a
Combined yield7
of dimers 1–3/%
6.7
Conversion
of AN/%
Solvent
TON8
675
t-Butanol
8.7
2
3
J. M. Thomas, R. Raja, G. Sankar, R. G. Bell, Acc. Chem.
Res. 2001, 34, 191; W. Niu, K. M. Draths, J. W. Frost, Bio-
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b
(
0.441/0.506/0.010/0.025/0.014/0.004)
10.8 8.9
0.472/0.475/0.009/0.024/0.014/0.007)
11.5 9.7
0.473/0.475/0.009/0.024/0.012/0.007)
11.5 9.9
0.473/0.476/0.009/0.023/0.013/0.006)
Cyclohexene 12.9 10.8
0.474/0.475/0.009/0.022/0.013/0.007)
13.3 11.3
0.473/0.477/0.008/0.023/0.013/0.006)
Diphenylether 20.3 18.1
0.475/0.477/0.008/0.022/0.012/0.006)
Reaction conditions: reaction mexture (AN/Ru = 10000,
THF
890
972
b
b
b
b
b
b
(
Hexane
(
3
435.
A. Misono, Y. Uchida, M. Hidai, H. Kanai, Chem. Commun.
London) 1967, 357; J. D. McCure, R. Owyang, L. H.
(
Cyclohexane
986
Slaugh, J. Organomet. Chem. 1968, 12, 8; E. Billig, C. B.
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mun. (London) 1968, 704; A. Misono, Y. Uchida, M. Hidai,
H. Shinohara, Y. Watanabe, Bull. Chem. Soc. Jpn. 1968, 41,
(
1083
1125
1797
(
Toluene
(
3
1
96; W. Strohmeier, A. Kaiser, J. Organomet. Chem. 1976,
14, 273; D. T. Tsou, J. D. Burrington, E. A. Maher, R. K.
(
Grasselli, J. Mol. Catal. 1985, 30, 219; D. T. Tsou, J. D.
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a
ꢀ
Table 1)/solvent = 1 (v/v), 150 C, 9 h. See Supprting Informa-
tion for details. See also Ref. 10. Molar ratio of dimers 1–3:
cis-1/trans-1/cis,cis-3/cis,trans-3/trans,trans-3/2.
b
4
Rhone-Poulenc, Neth. Appl. NL 6603115, 1966; Chem.
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and the TON for dimers 1–3 rose up to 1797. It was 1.7 times
higher than the TON in the solvent-free system. In addition, it
was higher than when the TON was AN/Ru = 20000 under sol-
vent-free condition. It seems that diphenyl ether has a favorable
influence on the rate-determining step, in which the carboxylic
acid allegedly cleaves the carbon–ruthenium bond of an inter-
mediate ruthenium complex by means of protonolysis.6,11 Ali-
phatic ether THF gave reduced TON. It contrasts with aromatic
ether diphenyl ether. Alcoholic solvents, which have been often
used in the catalytic dimerization of AN under hydrogen atom-
splere, were not suitable for the catalytic dimerization of AN us-
ing the combination of the ruthenium-based catalyst and a car-
boxylic acid, as exemplified by t-butanol. No dimer was formed
at all when N,N-dimethylformamide was used as the solvent.
Judging from these results, it seems that the transition state of
the protonolysis step is not very polar. This postulate is consis-
tent with our previous results, which showed that the protonoly-
5
K. Kashiwagi, R. Sugise, T. Shimakawa, T. Matsuura, M.
Shirai, F. Kakiuchi, S. Murai, Organometalics 1997, 16,
2233.
K. Kashiwagi, R. Sugise, T. Shimakawa, T. Matsuura, M.
Shirai, Chem. Lett. 2006, 35, 186.
6
,11
sis step has a very early transition state.
Addition of solvents
6
7
8
induced slight increase of the production ratio of DCBD 3. It
seems that the addition of solvents gives the formation of 2-
hexenedinitrile (1), which is produced via protonolysis step, a
slight advantage over the formation of DCBD 3.
In conclusion, in the Ru-catalyzed dimerization reaction of
AN, it was found that lower catalyst concentration gives higher
TON for dimers 1–3 and the addition of a volume of diphenyl
ether equal to that of AN to the reaction mixture increases the
TON by 1.7 times (TON = 1797).
Combined yield of dimers 1–3 = 100 Â [2 Â (combined
mols of dimers 1–3)]/[mols of charged AN].
TON = [2 Â (combined mols of dimers 1–3)]/[mols of
Ru]. The values are averages of three runs of which deviation
was within 50.
The molar ratio of produced dimers 1–3 did not change
largely even at a short reaction time of 0.5 h. See Supporting
Information for details. See Ref. 10.
9
Further investigations on the additive effect of solvent are in
progress.
10 Supporting Information is also available electronically on
the CSJ-Journal Web site, http://www.csj.jp/journals/chem-
lett/index.html.
We thank Dr. S. Kojima of Hiroshima University for reading
the manuscript and making helpful suggestions.
11 K. Kashiwagi, R. Sugise, T. Shimakawa, T. Matsuura, M.
Shirai, J. Mol. Catal. A: Chem. 2007, 264, 9.
Published on the web (Advance View) October 20, 2007; doi:10.1246/cl.2007.1384