Lewis-acid assisted cross metathesis of acrylonitrile with functionalized olefins catalyzed by phosphine-free ruthenium carbene complex

Article abstract of DOI:10.1039/b510776h

The exchange of the PPh₃ ligand in the complex [1,3-bis(2,6-dimethylphenyl)4,5-dihydroimidazol-2-ylidene](PPh ₃)-(Cl)₂Ru=CHPh (7) for a pyridine ligand at ambient temperature leads to the formation of the stable phosphine-free carbene ruthenium complex [1,3-bis(2,6-dimethylphenyl)4,5-dihydroimidazol-2-ylidene] (C₅H₅N)₂(Cl)₂ Ru=CHPh (8). The resulted ruthenium complex exhibits highly catalytic activity for the cross metathesis of acrylonitrile with various functionalized olefins under mild conditions, and its activity can be further improved by the addition of a Lewis acid such as Ti(O′Pr)₄. In the mixture products, the Z-isomer predominates. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b510776h

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A R T I C L E
Lewis-acid assisted cross metathesis of acrylonitrile with
functionalized olefins catalyzed by phosphine-free ruthenium
carbene complex
Chen-Xi Bai, Xiao-Bing Lu, Ren He,* Wen-Zhen Zhang and Xiu-Juan Feng
State Key Laboratory of Fine Chemicals, Dalian University of Technology, No. 158 Zhongshan
Road, Dalian, 116012, PR China. E-mail: dlengima@yahoo.com.cn; Fax: +86-411-8363-3080;
Tel: +86-411-8899-3861
Received 29th July 2005, Accepted 21st September 2005
First published as an Advance Article on the web 17th October 2005
The exchange of the PPh3 ligand in the complex [1,3-bis(2,6-dimethylphenyl)4,5-dihydroimidazol-2-ylidene](PPh3)-
=
(Cl)2Ru CHPh (7) for a pyridine ligand at ambient temperature leads to the formation of the stable phosphine-free
=
carbene ruthenium complex [1,3-bis(2,6-dimethylphenyl)4,5-dihydroimidazol-2-ylidene](C5H5N)2(Cl)2 Ru CHPh
(8). The resulted ruthenium complex exhibits highly catalytic activity for the cross metathesis of acrylonitrile with
various functionalized olefins under mild conditions, and its activity can be further improved by the addition of a
Lewis acid such as Ti(OiPr)4. In the mixture products, the Z-isomer predominates.
Introduction
Results and discussion
Recently, olefin metathesis has attracted much attention as
a powerful tool for C–C bond formation.1 The commercial
availability of well-defined transition metal catalysts (Fig. 1),
such as the molybdenum alkoxyimidoalkylidene 1,2 ruthenium
benzylidene catalysts 2 and 3,3 and ether-tethered ruthenium
alkylidene derivative catalyst 4,4 has made olefin metathesis
practical for application to synthetic organic chemistry.
Synthesis of complex 8
In contrast to complex 3 and 4, the synthesis of the ruthenium
complex 8 was relatively simple and easily for scale operation,
because it was not heavily reliant on column chromatography
purification steps and the use of expensive PCy3. The ruthenium
complex 7 was prepared by treatment of complex 6 and 5
with potassium tert-butoxide in toluene. Complex 7 can be
purified by several washes with hexane and isolated as brown
microcrystalline solids in good yields of 77% (Fig. 1). A
bispyridine ruthenium complexes 8 can be prepared by adding
an excess pyridine to 7 (Fig. 2). These reactions are complete
within minutes, in the absence of any solvent. The product 8 is
isolated in 91% yield by precipitation with hexane and without
further purification. The resulted complex 8 exhibits good air-
and moisture stability.
Fig. 1 Olefin metathesis catalysts, Mes = 2,4,6-trimethylphenyl.
On the other hand, cross-metathesis (CM), a method for
the intermolecular formation of carbon–carbon double bonds,
has been underutilized in comparison with other metathesis
reactions. This is primarily due to the lack of reaction selec-
tivity and olefin stereoselectivity.5 The discovery of the highly
active and stable ruthenium-based “second generation” Grubbs’
Fig. 2 Synthesis of bispyridine complex 8.
=
catalyst 3, (H2IMes)(PCy3)(Cl)2Ru CHPh (where H2Imes =
1,3-dimesityl-4,5-dihydroimidazol-2-ylidene), has dramatically
advanced the utility of CM.6 However, the original synthetic
route of complex 3 was not practical for large scale operation,
because it heavily relied on column chromatography purification
steps and the use of expensive PCy3.7
Although several examples of selective Mo- and Ru-
catalyzed acrylonitrile CM have appeared in the literature,8
these complexes are very air- and moisture-sensitive and
show a restricted tolerance of several heteroatom function-
alities. The presence of acids, reactive carbonyl groups and
alcohols significantly leads to the catalyst inactive.9 Most
of phosphine-ligated ruthenium catalysts have given poor
results for this transformation.10 More recently, (H2IMes)(3-
Cross-metathesis reaction
We initially investigated the use of the complexes 8, 2, 3, 4
and 7 for the self-CM reaction of acrylonitrile (Table 1). With
the exception of the complex 8, other complexes showed no
or low activity for this reaction. With catalyst 8, we observed
a maximum 39% conversion of acrylonitrile to 1,4-dicyano-
2-butene product with 2 mol% catalyst loading. Though the
yield is not high, it has already been the best result at present.
Furthermore, the reaction was performed in different solvents
(replacing dichloromethane with toluene) at higher temperature
with a higher catalyst loading of (10 mol% vs. 2 mol%), but
the yield did not improve. We suspect that the moderately
strong coordination of cyano group with the ruthenium center
resulted in deactivating the catalyst during self-CM reaction of
acrylonitrile. In order to testify this idea, we also explored the
use of the carbene ruthenium complex 8 for the metathesis of
1-hexene in various polar solvents with coordination ability. The
results are summarized in Table 2. Obviously, the stronger the
=
bromopyridine)2(Cl)2Ru CHPh was found to be more effective
than 3.11 This result stimulated us to develop a economical
and convenient method for preparing the similar phosphine-
free carbene complex on a large scale. In this paper, we report an
inexpensive and highly efficient ruthenium complex 8 to perform
acrylonitrile CM with various functionalized olefins.
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 1 3 9 – 4 1 4 2
4 1 3 9
©

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Table 1 Ru-catalyzed CM of acrylonitrile
expected product 9 was obtained in 60% yield (Table 3, entry 5).
Decreasing the amount of Ti(Oi-Pr)4 only had little effect on this
reaction. However, prolonged the reaction time and elevated the
reaction temperature were beneficial for improving the yield.
Furthermore, we were delighted to find that the complex 8
could be used in CM of acrylonitrile with different functional-
ized olefins, such as a,b-unsaturated esters, acids and aldehydes.
The results are summarized in Table 4. The cross metathesis can
be performed in good yields and the Z-isomer predominates
in the mixture products. Among these substances, but-3-en-1-
ol is more reactive for CM with acrylonitrile in the presence
of Ti(Oi-Pr)4, and a yield of 92% was achieved (Table 4, entry
4). On the contrary, conjugate allyl alcohol (Table 4, entry 2)
or allylic substituted alcohol (Table 4, entry 6) were proved
to be less efficient. This is due primarily to the electronic and
steric characteristic of the allyl alcohol. Further investigation
showed that the olefins with various functional groups, such
as hydroxide, carbonyl, ester and carboxyl resulted in different
results. Generally, the CM of acrylonitrile with functional
olefins including aldehyde or alcohol group is faster than that
including ester or carboxylic acid group. We suspect that the
ruthenium complex 8 can be decomposed in the presence of
proton hydrogen of carboxylic acid.
Catalyst
Solvent Temperature/◦C Time/h Yield of 9a (%)
2 (2 mol%)
CH2Cl2
45
45
45
45
45
45
45
12
24
12
24
24
24
24
12
0
0
2
12
21
15
39
38
2 (10 mol%) CH2Cl2
3 (2 mol%)
3 (10 mol%) CH2Cl2
4 (2 mol%) CH2Cl2
7 (10 mol%) CH2Cl2
8 (2 mol%) CH2Cl2
CH2Cl2
8 (10 mol%) Toluene 110
a Isolated yield.
Table 2 Influence of solvent on 1-hexene metathesis reactiona
Entry
Solvent
TOF
Conversion (%) of 1-hexeneb
1
2
3
4
5
6
7
Neat
1-Butanol
THF
142.8
73.5
113.8
57
31.3
47.3
1.3
85.7
44.1
68.3
34.2
18.8
28.4
7.8
Acetone
Acetic acid
Ethyl acetate
Acetonitrile
Conclusions
In conclusion, we have demonstrated that the phosphine-free
carbene ruthenium complex 8 can efficiently catalyze the cross
metathesis of acrylonitrile with various functionalized olefins.
The cross metathesis can be performed in good yields and the Z-
isomer predominates in the mixture products. The coordination
solvents have great negative effect on the reaction. The existence
of a Lewis acid such as Ti(Oi-Pr)4 can effectively improve the
reaction rate as well as the yield. The CM of acrylonitrile with
functional olefins including aldehyde or alcohol group is faster
than that including ester or carboxylic acid group.
a Reaction conditions: 8 (0.05 mmol), 1-hexene (50 mmol), solvent (50
mmol), 60 ◦C, 2 h. b Yields are based on GC.
coordination ability of the solvent is, the lower the yield is. For
example, substitution of acetic acid for THF as solvent, results
in the conversion of 1-hexene from 68.3% decreasing into 18.8%.
In situ NMR spectra further showed existence of a cyano-
substituted alkylidene during metathesis of acrylonitrile using
the carbene ruthenium complexes 8. The cyanocarbene moiety
1
Experimental
is distinguished by a H NMR resonance at d 18.31 for the
carbene proton, a 13C NMR resonance at d 237 for the carbene
resonance, a 13C NMR resonance at d 115 for the cyano group.
The species has a slower initiation rate than 2, which suggests
that a cyanocarbene intermediate, if trapped by phosphine, only
reenters the catalytic cycle with difficulty.8c
All manipulations were carried out under an inert atmosphere of
nitrogen on a vacuum line using standard Schlenk techniques.
All chlorinated solvents were dried from CaH2 and all non-
halogenated solvents were distilled from sodium or potassium
benzophenone ketyl. NMR spectra were recorded on a VAR-
IAN INOVA 400 MHz NMR spectrometer using CDCl3 as the
solvent. Mass spectral determinations were made on a Q-TOF
mass spectrometry (Micromass, England). Analytical thin-layer
chromatography (TLC) was performed using silica gel 60 F254
precoated plates (0.25 mm thickness).
In order to prevent the coordination of the cyano group
towards the ruthenium carbene intermediate, some Lewis acids
as co-catalyst were introduced to the reaction system. We
intended to form a complex of Lewis acids with the cyanogroup
and thus the metathesis reaction of acrylonitrile should be
improved. The metathesis of acrylonitrile in the presence of
Lewis acids was performed and the results are shown in Table 3.
Lewis acids have a dramatic influence on the yield of the cross-
metathesis reaction. Strong Lewis acids such as AlCl3 and
AlEt2Cl would decompose the catalyst in a short time, whereas
the addition of Al(Oi-Pr)3 did not affect the CM of acrylonitrile
(Table 3, entry 3). The systematic studies indicated that Ti(Oi-
Pr)4 was the best promoter for this reaction. When the reaction
was carried out in the presence of 1 equiv. of Ti(Oi-Pr)4, the
[1,3-Bis(2,6-dimethylphenyl)-4,5-dihydroimidazol-2-
=
ylidene](PPh3)(Cl)2Ru CHPh (7)
A solution of KOtBu (117 mg, 1.04 mmol) in dry THF (20 ml)
was slowly added to a solution of 1,3-bis-(2,6-dimethylphenyl)-
4,5-dihydroimidazolinium chloride12 (437 mg, 1.02 mmol) in
THF (20 ml) at ambient temperature under N2. The suspension
was stirred at room temperature for 2 h to give a yellow solution.
Table 3 Metatheses of acrylonitrile in the presence of Lewis acidsa
Entry
Lewis acid
Temperature/◦C
Time/h
Yield of 9b(%)
1
2
3
5
6
7
AlEt2Cl (1 equiv.)
AlCl3 (1 equiv.)
Al(Oi-Pr)3 (1 equiv.)
Ti(Oi-Pr)4 (1 equiv.)
Ti(Oi-Pr)4 (50 mol%)
Ti(Oi-Pr)4 (20 mol%)
45
45
45
25
45
45
1
1
12
2
2
12
0
0
41
60
61
68
a Reaction conditions: 8 (0.04 mmol), acrylonitrile (2 mmol), CH2Cl2 (20 ml), 60 ◦C, 2 h. b Isolated yield.
4 1 4 0
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 1 3 9 – 4 1 4 2

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Table 4 The CM of acrylonitrile with functionalized olefinsa
Entry
1
Cross partner
Lewis acid
None
Product
Yield (%)b
56
Z : E ratioc
3 : 1
2
3
Ti(Oi-Pr)4
None
80
61
3 : 1
4 : 1
4
5
Ti(Oi-Pr)4
None
92
52
4 : 1
2 : 1
6
Ti(Oi-Pr)4
73
2 : 1
7
8
None
61
84
35
70
4 : 1
4 : 1
3 : 1
3 : 1
Ti(Oi-Pr)4
None
9
10
Ti(Oi-Pr)4
11
12
13
None
44
75
36
4 : 1
4 : 1
1 : 1
Ti(Oi-Pr)4
None
14
Ti(Oi-Pr)4
61
1 : 1
a Reaction conditions: 8 (0.04 mmol), 1 equiv. cross partner, acrylonitrile (2.10 mmol), Ti(Oi-Pr)4 (20 mol%), CH2Cl2 (20 ml), 45 ◦C, 12 h. b Isolated
yield. c The (Z/E)-ratio was determined by 1H NMR.
The solution was then added (by way of a stainless steel cannula
filtered, and the resulting solution was cooled to −50 ◦C. After
1 h, the solution was filtered to obtain the product as brown
microcrystals, which were washed with cold hexane and dried
under vacuum to give complex 7 as a light brown powder. Yield:
211 mg, 77%. 1H NMR (400 MHz, CDCl3): d = 19.25 (s, 1 H,
Ru–CH), 7.69–6.56 (multiple peaks, 24H, PPh3, para CH, meta
13
=
fitted with a filter) to (PPh3)2Cl2Ru CHPh 6 (0.200 g, 0.243
mmol) suspended in toluene (40 ml). The reaction mixture
was stirred for 30 min at 70 ◦C, resulting in a clear dark
brown solution. The solvent was completely removed under
vacuum. The residue was dissolved into 10 mL of hexane and
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 1 3 9 – 4 1 4 2
4 1 4 1

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CH, and 2,6-dimethylphenyl aromatic CH), 7.67 (d, 2H, ortho
CH, J = 7.2 Hz), 4.12 (t, 2H, CH2CH2, J = 7.2 Hz), 3.95 (t,
2H, CH2CH2, J = 7.2 Hz), 2.64 (s, 12H, ortho CH3). 13C NMR
7 Hz, 1H cis), 6.40 (d, J = 17 Hz, 1H trans), 6.37 (d, J = 11 Hz,
1H cis). 14: 1H-NMR (400 MHz, CDCl3): d = 11.0 (d, J = 7 Hz,
1H), 6.68 (dt, J = 17 Hz, J = 7 Hz, 1H trans), 6.45 (dt, J =
11 Hz, J = 7 Hz, 1H cis), 5.39 (d, J = 17 Hz, 1H trans), 5.31 (d,
=
(100 MHz CDCl3): d = 292.3 (d, Ru CHPh), 219.7, 152.6,
1
139.7, 138.3, 137.4, 134.2, 132.2, 130.5, 129.4, 129.2, 128.7,
128.6, 128.4, 127.8, 127.6, 125.5, 51.9, 50.2, 21.6, 18.9. 31P NMR
(161.9 MHz, CDCl3): d = 37.29 (s). Q-TOFMS: calculated:
767.1896 [M–Cl]+; found: 767.1916 [M–Cl]+.
J = 7 Hz, 1H cis). 15: H-NMR (400 MHz, CDCl3): d = 6.75
(dt, J = 17 Hz, J = 7 Hz, 1H trans), 6.64 (dt, J = 11 Hz, J =
7 Hz, 1H cis), 6.51 (dt, J = 17 Hz, J = 2 Hz, 1H trans), 6.43 (dt,
J = 11 Hz, 2H cis), 3.84 (s, 3H).
[1,3-Bis(2,6-dimethylphenyl)-4,5-dihydroimidazol-2-
ylidene](C5H5N)2(Cl)2Ru CHPh (8)
Acknowledgements
=
This research was supported by a grand from China National
Petroleum Corporation (CNPC).
Pyridine (2.0 ml, 25 mmol) was added to complex 7 (2.0 g,
2.5 mmol) in a 20 mL vial with a screw cap. The solution was
stirred in air at room temperature for 10 min, during which time
a color change from brown–red to bright green was observed.
The reaction mixture was cannula transferred into 50 mL of cold
(−5 ◦C) hexane, and a green solid precipitated. The precipitate
was filtered, washed with 4 × 20 ml of hexene, and dried under
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General procedure for CM of acrylonitrile and functionalized
olefins
To a mixture of cross partner (1.05 mmol) and acrylonitrile
(112 mg, 2.10 mmol) dissolved in dichloromethane (20 mL)
was added Ti(Oi-Pr)4 (60 mg, 0.21 mmol) under nitrogen
atmosphere by syringe. After stirring for 1 h at room tem-
perature, ruthenium catalyst 8 (70 mg, 0.1 mmol) dissolved in
dichloromethane was added by syringe. After 12 h of reflux,
the reaction was complete as indicated by TLC. Saturated
sodium bicarbonate was added to quench the reaction, the
organic layer was separated and the aqueous layer was extracted
with dichloromethane. The combined organic layers were dried
over anhydrous magnesium sulfate for several hours, and then
filtrated. The solution was concentrated via rotavapor. Flash
column chromatography (hexane–EtOAc) of the crude oil gave
the corresponding products. All compounds gave satisfactory
spectroscopic and analytical data. Selected data for compounds
1
are included. 9: H-NMR (400 MHz, CDCl3): d = 6.18 (s, 1H
cis), 6.27 (s,1H trans). 10: 1H-NMR (400 MHz, CDCl3): d = 6.81
(dt, J = 17 Hz, J = 3 Hz, 1H trans), 6.48 (dt, J = 16 Hz, J =
7 Hz, 1H cis), 5.71 (m, 1H), 4.34 (dd, J = 7 Hz, J = 2 Hz, 2H
trans), 4.23 (dd, J = 7 Hz, J = 2 Hz, 2H cis), 2.11 (s, 1H). 13:
1H-NMR (400 MHz, CDCl3): d = 9.70 (d, J = 7 Hz, 1H), 6.90
(dt, J = 17 Hz, J = 7 Hz, 1H trans), 6.71 (dt, J = 11 Hz, J =
13 P. Schwab, R. H. Grubbs and J. W. Ziller, J. Am. Chem. Soc., 1996,
118, 100–110.
4 1 4 2
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 4 1 3 9 – 4 1 4 2