Practical and scalable preparation of 2-methyleneglutaronitrile via an efficient and highly selective head-to-tail dimerization of acrylonitrile catalysed by low-loading of tricyclohexylphosphine

Article abstract of DOI:10.1039/c4ra02810d

By carefully evaluating the effect of solvent, ligand, temperature, and time, the commercially available tricyclohexylphosphine was found to be a particularly effective catalyst for head-to-tail dimerization of acrylonitrile, providing an efficient, scalable, and highly selective method for preparation of the useful 2-methyleneglutaronitrile with a low catalyst loading. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02810d

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Practical and scalable preparation of
2-methyleneglutaronitrile via an efficient and
highly selective head-to-tail dimerization of
acrylonitrile catalysed by low-loading of
tricyclohexylphosphine†
Cite this: RSC Adv., 2014, 4, 19122
Received 29th January 2014
Accepted 11th April 2014
DOI: 10.1039/c4ra02810d
Lei Yu, abcd Jun Wang,b Xu Zhang,ab Hongen Cao,b Genlin Wang,abc Kehong Ding,abc
*
abc
*
www.rsc.org/advances
Qing Xu
and Mark Lautensd
By carefully evaluating the effect of solvent, ligand, temperature, and useful synthetic methods8 and in a cooperation project with
time, the commercially available tricyclohexylphosphine was found to chemical industry, we became interested in the synthesis of the
be a particularly effective catalyst for head-to-tail dimerization of useful 2-MGN1 and 1,4-dicyanobutenes (DCBs),9 which may be
acrylonitrile, providing an efficient, scalable, and highly selective obtained by selective head-to-tail2–6 and tail-to-tail10–12 dimer-
method for preparation of the useful 2-methyleneglutaronitrile with a ization of AN, respectively. Herein we report a PCy3-catalysed
low catalyst loading.
highly selective head-to-tail dimerization of AN, which provided
a practical method for preparation of the useful 2-MGN by using
a low loading of the phosphine catalyst under mild conditions.
Transition metal-catalysed methods are usually of high effi-
ciencies and can provide higher selectivities and yields of the
target products especially in addition type reactions.13 Since
previous studies have shown that Ru-catalysed methods can
give 2-MGN3 or DCBs10 in high selectivities, we initially intend to
modify the methods to achieve highly selective procedures for
preparation of 2-MGN or DCBs by ne-tuning of the more
available non-noble metal catalysts and ligands. Thus, a neat
mixture of AN and catalytic amounts of Ni(cod)2 and PCy3 was
2-Methyleneglutaronitrile (2-MGN) is an important synthetic
intermediate abundantly used in pharmaceutical and agricul-
tural chemistry and polymers and materials science, such as the
synthesis of bromothalonil, lactams, halogenated pyridines,
antimicrobial, herbicides and pesticides, polymeric nano-
particles, nitrile rubbers or plastics, etc.1 Therefore, the prepa-
ration of 2-MGN has attracted the interest of synthetic
chemists,2–7 among which head-to-tail dimerization of the
readily available and inexpensive acrylonitrile (AN) is an ideal
strategy to obtain 2-MGN.2–6 However, the known methods have
drawbacks such as high reaction temperatures, high pressure,
the use of air sensitive, odorous, and toxic phosphine catalysts,
or the use of sensitive transition metal catalysts and haloge-
nated solvents, high catalyst loadings, or low selectivity and low
yields of 2-MGN. These issues largely impeded the larger scale
production of 2-MGN and consequently limited its applications.
Therefore, highly efficient and selective, high yielding, and
practical methods for 2-MGN synthesis is still of great signi-
cance. With the intension to develop green and potentially
ꢀ
heated at 70 C under nitrogen. The reaction tarred in 10 h to
give 19% yield of the head-to-tail dimerization product 2-MGN
as conrmed by GC-MS and 1H NMR analysis, without detecting
the isomers of DCBs (eqn (1), run 1). Aware that this is in effect
the Rauhut–Currier reaction, known since 1963 (Scheme 1),2,14 it
occurred to us that PCy3 alone may also be able to catalyse the
reaction. A control reaction using PCy3 as the catalyst was
carried out under the same conditions to conrm our hypoth-
esis. The reaction afforded 21% yield of 2-MGN (run 2), a similar
aZhejiang Key Laboratory of Carbon Materials, College of Chemistry and Materials
Engineering, Wenzhou University, Wenzhou, Zhejiang, 325035, China. E-mail:
qing-xu@wzu.edu.cn; Fax: +86-577-86689302; Tel: +86-138-57745327
bSchool of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou,
Jiangsu 225002, China. E-mail: yulei@yzu.edu.cn; Fax: +86-514-87975244; Tel: +86-
136-65295901
cJiangsu Yangnong Chemical Group Co. Ltd, Yangzhou, Jiangsu 225009, China
dDavenport Chemistry Laboratories, Department of Chemistry, University of Toronto,
80 St. George Street, Toronto, ON, M5S 3H6, Canada
† Electronic supplementary information (ESI) available: Experimental details,
condition screening tables, GC spectra of the reactions, characterization data,
and 1H and 13C NMR spectra 2-MGN. See DOI: 10.1039/c4ra02810d
Scheme
1 The Rauhut–Currier dimerization of activated alkenes
(EWG ¼ electron-withdrawing group).
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yield to that obtained using the nickel/phosphine catalyst (run 6). It was then found that, although other primary
(run 1). This result implied that metal catalysts might not be and secondary alcohols afforded only low yields of 2-MGN (runs
needed at all in the reaction.
7–9), t-BuOH is the best solvent, ensuring not only a complete
conversion of AN but also a good 85% yield of 2-MGN (run 10).
Ready tarring of the neat reactions also indicated that
solvent volume or the reaction concentration may affect the
reaction. Thus, concentration effects of the reaction by using
different volumes of t-BuOH were next investigated (Fig. 1). The
results showed that addition of 1–2 equiv. of t-BuOH to AN
could greatly enhance the yield of 2-MGN from 21% when run
neat to 85% using 2 equiv. of t-BuOH. More solvent gradually
led to lower yields of 2-MGN possibly due to lower concentra-
tions of AN and thus slower reactions.
In the search for the best organophosphorous catalyst, a
series of alkyl and aryl phosphines, phosphinates, and phos-
phites were then investigated (Fig. 2). PMe3 exhibited only a low
activity. PBu3, a phosphine catalyst frequently used in Rauhut–
Currier dimerization of activated alkenes,2,14 also gave a low
yield of 2-MGN. In contrast, cyclohexyl-substituted PCy3 and
PCy2Ph gave much better results under the same conditions.
PCy3 was still found to be the most effective phosphine catalyst,
ensuring complete conversion of AN to afford the highest 85%
yield of 2-MGN. In contrast, electron-rich and -decient
(1)
A literature survey showed that, although various phos-
phines including PCy3 has been tested as catalysts in Rauhut–
Currier type head-to-tail dimerization of AN, PCy3 was not an
effective catalyst under the reported conditions and the known
methods still have certain drawbacks (vide supra).2,14 Mean-
while, we also noticed that PCy3 has been reported as an
effective ligand in Ru- and Ni-catalysed reactions,3,4 producing
73% yield of 2-MGN with approximately 10–20% polymeric
products by using 3–5 mol% of the expensive and sensitive Ru
catalyst C5Me5(PCy3)RuH3,3 or requiring harsher conditions by
using sensitive Ni(0) complexes as catalysts, chlorobenzene as
the solvent, and alkyl halides as necessary additives.4 Therefore,
the above nding that PCy3 alone can act as the AN dimerization
catalyst in the absence of metals (eqn (1)) and literature back-
ground2,14 led us to investigate the reaction in more detail,
hoping to develop a practical procedure for 2-MGN synthesis.
To avoid potential oligomerization or polymerization of AN,
the solvent was screened prior to the catalyst. As shown in
Table 1, in comparison with the neat reaction (run 1), addition
of polar organic solvents such as THF, acetone, acetonitrile, and
DMF enhanced the conversion of AN and the yield of 2-MGN in
varying degrees (runs 2–5). Aware that polar and protonic
solvents are commonly used in Rauhut–Currier type reac-
tions,2,14 we then investigated the effect of water and alcohols,
but water only gave a similar result with the neat reaction
Fig. 1 Solvent volume and reaction concentration screening (see
Table 1 for similar conditions).15
Table 1 Solvent screeninga
Run
Solvent
AN conv.b (%)
2-MGN%b
1
2
3
4
5
—
THF
Acetone
CH3CN
DMF
68
81
82
92
90
21
44
30
27
24
19
0
6
7
H2O
EtOH
74
47
8
9
10
i-PrOH
c-C6H11OH
t-BuOH
86
87
>99
43
31
85
a
AN (1 mL, 15.2 mmol) and PCy3 (0.08 mmol 0.5 mol%) in a solvent (2
b
mL) was heated at 70 ꢀC for 10 h under N2. Determined by GC with
Fig. 2 Organophosphorous catalyst screening (see Table 1 for similar
conditions).15
biphenyl used as the internal standard.
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triarylphosphines PAr3, phosphinate Ph2P(OEt), or phosphites
P(OEt)3 and HP(O)(OEt)2 were not effective.
We next investigated the effect of temperature. As shown in
Fig. 3, the reaction proceeded smoothly even at room temper-
ature (ca. 25 ꢀC), but it gave only a moderate yield of 2-MGN. The
product yield could be enhanced at higher temperatures (40–
70 ꢀC), but it began to decrease at temperatures higher than
70 ꢀC, most possibly due to easier formation of byproducts. As a
ꢀ
result, 60–70 C is the most suitable temperature zone for the
reaction, affording 83–85% yields of 2-MGN.
We next examined the progress of the reaction as a function
of time and temperature (Fig. 4). It seems the progress of the
reaction at each temperature differed, but the product yields of
these reactions all began to decrease in over times. At 50 ꢀC, the
reaction proceeded slowly and gave only a moderate yield of
2-MGN in 10–15 h; while at 80 ꢀC, the reaction proceeded
quickly but gave no higher than 70% yield of 2-MGN in 5 h. In
Fig. 5 Screening on catalyst loading under the optimized conditions
(60 ꢀC, 15 h; see Table 1 for similar conditions).15
0.1–0.2 mol% of PCy3 gave much lower yields of 2-MGN than the
standard reaction of 0.5 mol% PCy3. Then, 2-MGN’s yields were
enhanced greatly by using 0.3–0.5 mol% of PCy3, with 0.5 mol%
PCy3 giving the highest 90% yield; whereas, even more loadings
of PCy3 (0.6–1.0 mol%) only led to decrease of the product yields
gradually, most possibly due to faster reactions and formation
of byproducts. Therefore, 0.5 mol% of PCy3 is still the optimal
catalyst loading.
During the studies, we also noticed that DCBs were not
detected in the present reactions, with 2-MGN being detected as
the only product. To understand more about the reaction, we
also measured the progress of the standard reaction by GC at
5 and 15 h, respectively. As shown in Fig. 6, 75% and 90% yields
of 2-MGN were detected as the only product in the GC trace
while a trace amount of AN still remained aer 5 h (Fig. 6a) and
complete conversion of AN aer 15 h (Fig. 6b). In both chro-
matograms, no DCBs were detected. These results revealed that
the present method is a highly head-to-tail selective dimeriza-
tion reaction, leading to the only formation of 2-MGN and
possibly the easy purication of 2-MGN.
ꢀ
contrast, 60–70 C was found to be the most suitable tempera-
tures zone, at which the product yields increased more quickly
and reached the highest point in 10–15 h, giving 2-MGN in 90%
and 85% yields, respectively. Therefore, 60 ꢀC and 15 h are the
most appropriate temperature and time for the reaction.
Moreover, although the present loading of PCy3 (0.5 mol%) is
low, we continued to reduce the loading in order to obtain the
least but most effective catalyst condition under the optimized
conditions (60 ꢀC, 15 h). As shown in Fig. 5, the reactions using
Fig.
3 Temperature effect screening (see Table 1 for similar
conditions).15
Fig. 4 Time effect of reactions at 50–80 ꢀC (see Table 1 for similar Fig. 6 GC chromatograph of the standard reaction at 60 ꢀC:15 (a) 75%
conditions).15,16
yield in 5 h; (b) 90% yield in 15 h.
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With the optimized reaction parameters in hand, we nally
examined scalability of the method for large scale preparation
of 2-MGN. As shown in eqn (2), a 760 mmol scale reaction of AN
under the optimized conditions (0.5 mol% PCy3, 100 mL t-
BuOH, 60 ꢀC, 15 h) readily afforded 31.0 gram of 2-MGN (79–86
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1
ꢀC/0.6 mmHg, 77% isolated yield, >98% purity by H NMR) by
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(2)
In summary, by carefully evaluating the reaction parameters
and by using tricyclohexylphosphine (PCy3) as the effective
catalyst, we developed an efficient, practical, scalable, highly
selective, and potentially useful head-to-tail dimerization of
acrylonitrile (AN) for the preparation of useful 2-methyl-
eneglutaronitrile (2-MGN). In comparison with the known
processes,2–6 this new method has obvious advantages,
including a low loading of the commercially available, stable,
and readily-disposable solid phosphine catalyst, low dosage of
the cheap and halogen-free t-BuOH solvent, and thus high
concentration and conversion of AN, an efficient reaction under
mild conditions, and high selectivity and good yield of 2-MGN.
Further extension of the method to other activated alkenes and
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Acknowledgements
We thank NNSFC (21202141, 20902070), ZJNSF for Distin-
guished Young Scholars (R14B020009), Opening Foundation of
the Key Laboratory of Environmental Materials and Engineering
of Jiangsu Province (K11024, K090030), and Opening Founda-
tion of the Key Laboratory of Green Pesticide and Agricultural
Bioengineering, Ministry of Education, Guizhou University
(2010GDGP0106) for nancial support.
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