Linear relationship between activity of a new Ru-catalyst and acidity of substituted benzoic acids in the dimerization of acrylonitrile

Article abstract of DOI:10.1246/cl.2006.186

A new type of catalyst system using ruthenium and carboxylic acid is useful for the tail-to-tail dimerization of acrylonitrile, proceeding without the formation of undesired by-product propionitrile. Carboxylic acids having pK _a 3.5-5 are suitable as co-catalysts for the dimerization of acrylonitrile. The relationship between the logarithm of the relative rate in the dimer formation and the pK_a of m- and p-substituted benzoic acids (Bronsted plot) was linear (R² = 0.946) with a slope of -0.199. The role of the carboxylic acids can be considered to be effective protonation in the protonolysis of the carbon-ruthenium bond of an intermediate Ru complex. Copyright

Full text of DOI:10.1246/cl.2006.186

1
86
Chemistry Letters Vol.35, No.2 (2006)
Linear Relationship between Activity of a New Ru-catalyst and Acidity
of Substituted Benzoic Acids in the Dimerization of Acrylonitrile
ꢀ
1
1;2
1
1
1
1
Kohichi Kashiwagi,
Ryoji Sugise, Toshihiro Shimakawa, Tunao Matuura, and Masashi Shirai
UBE INDUSTRIES, LTD., 1978-5 Kogushi, Ube 755-8633
2
Department of Chemistry, Graduate School of Science, Hiroshima University,
-3-1 Kagamiyama, Higashi-hiroshima 739-8526
1
(Received November 7, 2005; CL-051381; E-mail: 25988u@ube-ind.co.jp)
ꢂ
A new type of catalyst system using ruthenium and carbox-
Table 1. TON for dimerization of acrylonitrile at 150 C for 6 h
a
ylic acid is useful for the tail-to-tail dimerization of acrylonitrile,
proceeding without the formation of undesired by-product
propionitrile. Carboxylic acids having pKa 3.5–5 are suitable
as co-catalysts for the dimerization of acrylonitrile. The relation-
ship between the logarithm of the relative rate in the dimer
in the presence of substituted benzoic acids
b
Carboxylic acid
pKa
TON
636
c
PhCO2H
p-Substituted benzoic acid
p-ClC6H4CO2H
4.20
c
c
c
3.83
4.24
4.47
616
355
346
formation and the pKa of m- and p-substituted benzoic acids
2
p-MeC H CO H
6
4
2
(
ꢁ
Brꢀnsted plot) was linear (R ¼ 0:946) with a slope of
p-MeOC6H4CO2H
m-Substituted benzoic acid
m-NCC6H4CO2H
m-ClC6H4CO2H
m-MeOC6H4CO2H
m-MeC6H4CO2H
0:199. The role of the carboxylic acids can be considered to
be effective protonation in the protonolysis of the carbon–ruthe-
nium bond of an intermediate Ru complex.
c
c
c
c
3.60
3.99
4.09
4.34
778
705
443
379
The regioselective tail-to-tail dimerization of acrylonitirile
AN) has provided an attractive alternative route toward the for-
(
mation of hexamethylenediamine, which is one of the monomers
o-Substituted benzoic acid
o-PhCOC6H4CO2H
o-MeC6H4CO2H
d
c
3.54
3.91
4.09
960
442
195
1
,2
used in the production of Nylon-6,6. Ruthenium-based cata-
lysts are important candidates for this reaction due to the high
reactivity [the catalyst turnover number (TON) has been about
c
o-MeOC6H4CO2H
aReaction conditions: AN 283 mmol, anisole (internal stand-
ard for gas chromatography) 18.5 mmol, DMSO 6.4 mmol,
3
,4
1
000] and the high selectivity for the linear dimers 1–3.
Komiya and his co-workers found that zero-valent ruthenium
complexes are effective catalyst precursors for the tail-to-tail di-
merization of AN. In their Ru-catalytic system, the inevitable
use of hydrogen as an extra agent to keep the reaction catalytic
resulted in the formation of a large amount of propionitrile 5 as
RuCl2(DMSO)4 0.0381 mmol, sodium propionate 0.306
ꢂ
b
mmol, acid 5.70 mmol, 150 C, and 6 h. TON ¼ ½2 ꢃ
c
(combined mol of dimers 1{3)ꢄ=[mol of Ru]. pKa values
d
taken from Ref. 6. pKa values taken from Ref. 7.
3
i
The main product was 1,4-dicyanobutene.
an undesirable by-product.
Recently, we found that the catalytic system of [RuCl2-
DMSO)4/CH3CH2COONa/DMSO/carboxylic acid] gives lin-
The TON in the catalyzed reactions by m- and p- substituted
benzoic acids increased with smaller pKa (stronger acid). The ef-
fective acidity of carboxylic acids for optimal TON is around
(
ear dimers of AN (TON ¼ 1206) without the formation of 5
Scheme 1).5
pK 3.5 and the use of o-benzoylbenzoic acid as the co-catalyst
a
(
gave rise to the highest TON of all the carboxylic acids exam-
ined in the present experiments. Phenol as well as p-toluene sul-
Ru-based catalyst
NC
,4-dicyanobutene 1,4-dicyanobutane
CN
CN
fonic acid and trifluoroacetic acid were ineffective as co-cata-
8
NC
CN
RCO H
lysts for the dimerization. It can be assumed that the active
2
1
Ru-catalyst for the dimerization is produced in the presence of
carboxylic acids of pKa ¼ 3:5{5, while in the presence of strong
acids such as sulfonic acid and trifluoroacetic acid, such active
catalysts either are not formed or are destroyed. A plausible
mechanism containing both the hydrogenation and the proton-
1
2
CN
R
O
CN
NC
CN
O
1
,4-dicyanobutadiene 2-cyanoethylcarboxylate propionitrile
5
ation steps is shown in Scheme 2.
ꢂ
3
4
5
The reaction at 120 C proceeded for 24 h at the same
rate without deactivation of catalyst. The relationship between
the logarithm of VX=VH [VX: dimer production per second
(mmol/s) for m- and p-substituted benzoic acids, VH: dimer
production per second (mmol/s) for benzoic acid] and the pKa
of carboxylic acids is shown in Figure 1. The plots for m- and
p-substituted benzoic acids showed a good linear correlation
with a slope of ꢁ0:199. Since the slope is related to the free
energy of activation in the rate-determining step, it could be
Scheme 1.
In order to investigate the protonolysis step, we have studied
the relationship between catalytic activity and the acidity of car-
boxylic acids in this catalytic system. Reported herein are the re-
sults. Table 1 shows TON values for the Ru-catalyzed dimeriza-
tion of AN at 150 C for 6 h in the presence of various substituted
benzoic acids. Products were analyzed by gas chromatography.
ꢂ
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.2 (2006)
187
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[
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CN
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Ru
H
[
RuH2 ]
[ H-Ru-O CR ]
2
AN
Path a
Path b
protonation
3
β-elimination
1
H
Ru
1
without H2
RCO H
2
NC
H2
hydrogenation
with H2
CN
Scheme 2.
4
5
6
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0
.2
.15
.1
o-PhCO
7
0
0
y = −0.199x + 0.820
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2
R = 0.946
0
m-Cl
m-MeO
.05
0
H
p-Cl
120, 191146. h) T. Yorisue, S. Kanejima, Jpn. Kokai Tokkyo
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p-Me
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−
0.05
p-MeO
−0.1
K. Kashiwagi, R. Sugise, T. Shimakawa, T. Matsuura, M.
Shirai, F. Kakiuchi, S. Murai, Organometallics 1997, 16,
2233. Previously, we reported some results of lesser produc-
tion of 2-cyanoethyl carboxylate than that of 1,4-dicyanobu-
tene. The production of the equimolar amount of 1,4-dicyano-
butene and 2-cyanoethyl carboxylate was needed for the
mechanism shown in Scheme 2. Therefore, we proposed the
different mechanism previously. But we found subsequently
the regeneration of carboxylic acid and AN from 2-cyanoethyl
carboxylate under the dimerization conditions. This results
make possible to explain the production of 2-cyanoethyl car-
boxylate lesser than that of 1,4-dicyanobutene, thus we pro-
pose the mechanism (Path b) corresponding to that under H2
pressure (Path a) this time. An alternative possibility, which
is shown in Ref. 5, is not shown in Scheme 2 for simplicity.
Further investigation on this reaction is in progress.
3
.4
3.6
3.8
4.0
4.2
4.4
4.6
pKa
Figure 1. Relationship between logðVX=VHÞ and pKa of substi-
ꢂ
tuted benzoic acids at 120 C. : m- and p-substituted benzoic
acid, : o-benzoylbenzoic acid.
supposed that proton-transfer in the protonolysis process is
rate-determining. The small value for the slope (ꢁ0:199) in
the Brꢀnsted relationship suggests that the protonolysis step
has a very early transition state. The deviation of the plot for
o-benzoylbenzoic acid from the fitted line is probably related
to favorable steric factors in the protonation step.
Further investigations on the mechanism, reaction condi-
tions and the catalysts in the acrylonitrile dimerization are in
progress.
E. A. Braude, F. C. Nachod, Determination of Organic
Structures by Physical Methods, Academic Press, New York,
We thank Dr. S. Murai, Professor Emeritus of Osaka Uni-
versity, and Dr. K. Ohkata, Professor of Hiroshima University,
for their valuable discussions and suggestions. We also thank
Dr. Satoshi Kojima for reading the manuscript and making
several helpful suggestions.
1
955, p. 567.
7
8
G. B. Bray, J. E. Doppy, S. R. C. Hughers, J. Chem. Soc. 1957,
265.
The dimer of AN was not formed at all when p-toluene sulfon-
ic acid and trifluoroacetic acid were used as the acid. Phenol
gave the same results as in the case of the absence of hydrogen
and carboxylic acid. The Ru-catalyzed dimerization of AN
in the absence of hydrogen and carboxylic acid proceeded
to afford a small amount of 1,4-dicyanobutadiene 3 as the
major dimer.
References and Notes
1
a) G. W. Parshall S. D. Ittel, Applied Homogeneous Catalysis,
nd ed., Wiley-Interscience, New York, 1992, p. 242. b) K.
Weissermel, H.-J. Arpe, Industrial Organic Chemistry,
2
Published on the web (Advance View) February 5, 2006; DOI 10.1246/cl.2006.186