Metathesis of hex-1-ene in ionic liquids catalyzed by WCl6

Article abstract of DOI:10.1007/s11172-005-0096-8

Metathesis of hex-1-ene in ionic liquids catalyzed by WCl₆ was studied. The metathesis is preceded by isomerization of hex-1-ene to hex-2-ene, from which the main reaction product, viz., oct-4-ene, is derived. The WCl ₆-1-butyl-3-methylimidazolium tetrafluoroborate (BMIM·BF ₄) system efficiently catalyzes metathesis of linear olefin, the ionic liquid serving as the reaction medium by forming a stable homogeneous catalytic system with WCl₆. The yields of the metathesis products increase with increasing reaction temperature. The addition of tin-containing promoters leads to a substantial increase in the reaction rate. In the WCl ₆-BMIM·BF₄-SnBu₄ system, the selectivity of the formation of oct-4-ene is significantly enhanced.

Full text of DOI:10.1007/s11172-005-0096-8

Russian Chemical Bulletin, International Edition, Vol. 53, No. 10, pp. 2187—2191, October, 2004
2187
Metathesis of hexꢀ1ꢀene in ionic liquids catalyzed by WCl6
ꢀ
A. V. Vasnev, A. A. Greish, and L. M. Kustov
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 137 2935. Eꢀmail: greish@ioc.ac.ru
Metathesis of hexꢀ1ꢀene in ionic liquids catalyzed by WCl6 was studied. The metathesis is
preceded by isomerization of hexꢀ1ꢀene to hexꢀ2ꢀene, from which the main reaction prodꢀ
uct, viz., octꢀ4ꢀene, is derived. The WCl6—1ꢀbutylꢀ3ꢀmethylimidazolium tetrafluoroborate
(BMIM•BF4) system efficiently catalyzes metathesis of linear olefin, the ionic liquid serving as
the reaction medium by forming a stable homogeneous catalytic system with WCl6. The yields
of the metathesis products increase with increasing reaction temperature. The addition of
tinꢀcontaining promoters leads to a substantial increase in the reaction rate. In the
WCl6—BMIM•BF4—SnBu4 system, the selectivity of the formation of octꢀ4ꢀene is signifiꢀ
cantly enhanced.
Key words: metathesis, ionic liquids, tungsten hexachloride, 1ꢀbutylꢀ3ꢀmethylimidazolium
cation, hexꢀ1ꢀene, octꢀ4ꢀene, isomerization.
Ionic liquids based on dialkylꢀsubstituted imidazoles
with the AlCl4– and BF4– anions find use as solvents and
catalysts for paraffin isomerization, Friedel—Crafts alkyꢀ
lation and acylation, nitration of aromatic hydrocarbons,
Diels—Alder reactions, etc.1 It was of interest to examine
the possibility of performing such an important process as
olefin metathesis in ionic liquids.2,3 It should be noted
that there are a few publications on metathesis in ionic
liquids. In particular, metathesis of functionalized acyclic
dienes with isolated double bonds in the presence of Ru
alkylidene complexes (Grubbs catalysts)4—7 and metatheꢀ
sis of pentꢀ2ꢀene in the presence of tungstenꢀcontaining
catalysts8 was investigated.
The aim of the present study was to examine metatheꢀ
sis of hexꢀ1ꢀene in the presence of WCl6 as a catalyst.
Ionic liquids based on the 1ꢀbutylꢀ3ꢀmethylimidazoꢀ
lium (BMIM) cation, viz., neutral (BMIM•BF4 and
BMIM•PF6) and acidic (BMIM•Cl—AlCl3, 1.0 : 1.05,
and 1 : 2) systems, were used as the
Experimental
Hexꢀ1ꢀene, Nꢀmethylimidazole, AlCl3, WCl6, NH4BF4,
Bu4Sn, PPh3, Bu2SnO, SnCl4, Ph3SnCl, SbCl5, and SbCl3 were
purchased from Aldrich, and BMIM•PF6 was purchased from
Merck.
Preparation of ionic liquids
1ꢀButylꢀ3ꢀmethylimidazolium chloride (BMIM•Cl). A mixꢀ
ture of Nꢀmethylimidazole (0.3 mol) and butyl chloride (0.3 mol)
was refluxed on a water bath under argon for 20 h. The reaction
product was isolated as an individual phase. The resulting salt
was washed with acetone (3×100 mL), and BMIM•Cl was isoꢀ
lated as a white crystalline substance in 86% yield (0.26 mol).
1ꢀButylꢀ3ꢀmethylimidazolium tetrafluoroborate (BMIM•
•BF4). Ammonium tetrafluoroborate (0.25 mol) was added to a
solution of BMIM•Cl (0.25 mol) in anhydrous acetone (120 mL)
under argon, and the reaction mixture was stirred at ~20 °C for
72 h. The precipitate (NH4Cl) was filtered off. The solution of
BMIM•BF4 in acetone was concentrated first on a rotary evapoꢀ
rator and then using an oil pump. The ionic liquid was obtained
in 84% yield (0.21 mol).
reaction medium.
Particular attention was given to
the acidic properties of the ionic liqꢀ
uids. The rates of side processes, such
as the double bond migration, oligoꢀ
1ꢀButylꢀ3ꢀmethylimidazolium
tetrachloroaluminate
(BMIM•Cl—AlCl3). Aluminum trichloride (0.114 mol) was
added to BMIM•Cl (0.057 mol) during 3 h (for the 1 : 2
BMIM•Cl—AlCl3 system) or AlCl3 (0.06 mol) was added durꢀ
ing 2 h (for the 1 : 1.05 BMIM•Cl—AlCl3 system), the temperaꢀ
ture of the reaction mixture being maintained at no higher than
25 °C. The ionic liquid was obtained in a yield of 0.57 mol. In
both cases, the yield of the reaction product was 100%.
merization, and polymerization of
the starting olefin, would be expected
to increase in acidic ionic liquids. Hence, it was of interꢀ
est to examine the influence of the amount of aluminum
chloride in ionic liquids on the ratio of the yields of the
products of both the metathesis and side processes.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2094—2097, October, 2004.
1066ꢀ5285/04/5310ꢀ2187 © 2004 Springer Science+Business Media, Inc.

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Vasnev et al.
Metathesis of hexꢀ1ꢀene
than decꢀ5ꢀene as the major product, in spite of the fact
that the formation of the latter would be expected proꢀ
vided that the starting hexꢀ1ꢀene enters into the reaction
(Scheme 1).
General procedure. Experiments were carried out in a 15ꢀcm3
cylindrical reactor, which was preliminarily purged with nitroꢀ
gen for 30 min. The components of the reaction mixture, viz., a
ionic liquid, a catalyst, hexꢀ1ꢀene, and a promoter (in particular
experiments), were placed in the reactor with continuous purgꢀ
ing. The reactor was covered with a lid, which provided comꢀ
plete hermiticity of the reaction volume. Then the reaction mixꢀ
ture was vigorously stirred using a magnetic stirrer for a specified
time (standard conditions: T = 20 °C, t = 1 h, the stirring rate
was 500—600 rpm). After completion of the experiment, the
reactor was cooled to 0 °C. Then a hydrocarbon layer was
sampled under a stream of argon and analyzed by GLC.
The WCl6—BMIM•BF4 system. The catalyst (0.2 mmol,
0.08 g) and hexꢀ1ꢀene (2.5 mmol, 0.3 mL) were added to the
ionic liquid (2.0 mmol, 0.45 g).
Scheme 1
This situation can be attributable to the fact that
isomerization of hexꢀ1ꢀene to hexꢀ2ꢀene proceeds much
more rapidly than its metathesis. As a result, metathesis
proceeds primarily with the involvement of hexꢀ2ꢀene
and, probably, hexꢀ3ꢀene. However, in the latter case,
degenerate metathesis can take place. According to the
data published in the literature,9 the concentrations of
hexꢀ1ꢀene, hexꢀ2ꢀene, and hexꢀ3ꢀene in an equilibrium
mixture of isomers are 1, 66, and 33% at 20 °C and 2, 66,
and 32%, respectively, at 50 °C (see Scheme 1). Actually,
hexꢀ2ꢀene and hexꢀ3ꢀene were detected in small amounts
in the final reaction mixture.
As can be seen from Table 1, metathesis proceeds
in two catalytic systems, viz., WCl6—BMIM•BF4 and
WCl6—BMIM•PF6. The reactions with the use of these
systems produced mixtures containing also small amounts
of heptenes and nonenes (~0.2 and 0.1% yields, respecꢀ
tively), which are the соꢀmetathesis products of different
isomers of hexene. In the case of acidic ionic liquids
containing the AlCl4– anions, the main reaction pathway
involves, apparently, oligomerization or even polymerꢀ
ization of hexene giving rise to resinous products. This
Addition of a promoter to the WCl6—BMIM•BF4 system.
The catalyst (0.2 mmol, 0.08 g), the promoter (0.07 mmol), and
hexꢀ1ꢀene (2.5 mmol, 0.3 mL) were added to the ionic liquid
(2.0 mmol, 0.45 g).
The WCl6—BMIM•PF6 system. The catalyst (0.2 mmol,
0.08 g) and hexꢀ1ꢀene (2.5 mmol, 0.3 mL) were added to the
ionic liquid (2.0 mmol, 0.56 g).
The WCl6—BMIM•Cl—AlCl3 system. The catalyst
(0.2 mmol, 0.055 g) was added to the ionic liquid (2.0 mmol,
0.63 g) and the reaction mixture was stirred at 20 °C for 30 min.
Then hexꢀ1ꢀene (2.5 mmol, 0.3 mL) was added and the mixture
was magnetically stirred for a specified time.
Analysis of the reaction products. The metathesis products
were analyzed by GLC using packed (3 m × 3 mm, SEꢀ30 on
Chromosorb) and capillary (30 m, SEꢀ30; 40 m, PEGꢀ20M)
columns. The yields of the metathesis products were determined
by the internal standard method.
Results and Discussion
The experiments demonstrated that metathesis of hexꢀ
1ꢀene occurs in systems involving WCl6 (Table 1). In
most experiments, the reactions afforded octꢀ4ꢀene rather
Table 1. Yields of the metathesis products of hexꢀ1ꢀene in various catalytic systemsa
Catalytic
system
Conversion
of hexꢀ1ꢀene (%)
Yieldb (mol.%)
Other
products
octꢀ4ꢀene decꢀ5ꢀene
WCl6—BMIM•BF4
WCl6—BMIM•PF6
WCl6—BMIM•Cl—AlCl3
(1 : 1.05)
12.8c
17.5
100.0
8.4
10.8
—
4.1
6.5
—
—
—
Resin
WCl6—BMIM•Cl—AlCl3
(1 : 2)
100.0
—
—
Resin
a Here and in Tables 2—4, the stoichiometry of the formation of ethylene and butꢀ2ꢀene
was maintained in all reactions.
b The accuracy of the determination of the yields of the metathesis products was 3—5 rel.%.
c In addition to the metathesis products, the reaction mixture contained small amounts of
compounds formed in the side dimerization and oligomerization processes.

WCl6ꢀcatalyzed metathesis in ionic liquids
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 10, October, 2004
2189
situation has been observed earlier.10 In the reaction mixꢀ
tures obtained in the experiments with the use of chloroꢀ
aluminate systems, not only the products of metathesis,
dimerization, and trimerization of the substrate but also
hexꢀ1ꢀene were absent.
1ꢀene layer and forming no stable catalytic systems
with WCl6. In the case of the WCl6—BMIM•BF4 system,
the hexꢀ1ꢀene layer remains colorless after the addition of
the second portion of the olefin, the yield of octꢀ4ꢀene
remains virtually unchanged, whereas the yield of decꢀ5ꢀ
ene decreases by a factor of ~2.5. Consequently, the seꢀ
lectivity of the formation of octꢀ4ꢀene increases, whereas
the total yield of metathesis products decreases only
slightly. Presumably, in this case, the already present acꢀ
tive form of the catalyst interacts with the ionic liquid to
form a stable homogeneous catalytic phase, which can
provide olefin metathesis involving preliminary isomerꢀ
ization of hexꢀ1ꢀene. A decrease in the conversion of
hexꢀ1ꢀene can be explained as follows. After the replaceꢀ
ment of the olefin, the reaction proceeds at the interface,
whereas, during the first hour, the reaction proceeds parꢀ
tially under conditions of homogeneous catalysis and parꢀ
tially under conditions of phase transfer catalysis.
Therefore, of all the systems under consideration, the
WCl6—BMIM•РF6 system exhibits the highest activity in
olefin metathesis. In this system, the upper liquid layer
containing hexꢀ1ꢀene turned darkꢀviolet, which is charꢀ
acteristic of solutions of WCl6 in olefins. It should be
noted that the color of the ionic liquid layer remained
unchanged, and a suspension of WCl6 was visually obꢀ
served in this layer. In the case of the WCl6—BMIM•BF4
system, the upper hexꢀ1ꢀene layer initially also turned
violet, but then the color disappeared. The ionic liquid
layer formed a green homogeneous solution. Presumꢀ
ably, the catalyst in the former case is distributed beꢀ
tween the hexꢀ1ꢀene and ionic liquid layers and disꢀ
solves only in hexꢀ1ꢀene. Apparently, the reaction in the
WCl6—BMIM•PF4 system is the case of homogeneous
catalysis of metathesis in the presence of WCl6. By conꢀ
trast, the reaction in the WCl6—BMIM•BF4 system proꢀ
ceeds at the interface of two liquids (homogeneous cataꢀ
lytic phase, viz., catalyst—ionic liquid, and the olefin
phase).
In another series of experiments, the olefin was reꢀ
placed with a new portion of hexꢀ1ꢀene within one hour
after the beginning of the reaction. The results of these
experiments are given in Table 2.
In the WCl6—BMIM•PF6 system, the total yield of
the metathesis products after the replacement of the oleꢀ
fin decreases by a factor of two, the selectivity of the
process being retained (see Table 2). Interestingly, afꢀ
ter the replacement of the olefin, the hexꢀ1ꢀene layer
also turned darkꢀviolet. Apparently, the ionic liquid
BMIM•PF6 plays an insignificant role as the medium for
metathesis and serves as a diluent of the catalyst, thus
preventing the latter from going completely into the hexꢀ
An increase in the temperature can influence not only
the reaction rate but also the miscibility of two liquid
phases. Hence, we carried out additional experiments
at 50 °C. The results of these experiments are given in
Table 3.
It appeared that the rise of the temperature from 20
to 50 °C has only a slight effect on the yield of the metꢀ
athesis products in the WCl6—BMIM•PF6 system. In the
case of the WCl6—BMIM•BF4 system, an analogous inꢀ
crease in the temperature leads to an increase in the yields
of octꢀ4ꢀene and decꢀ5ꢀene by a factor of 2 and 1.5,
respectively. This fact is attributable to an increase in
miscibility of hexꢀ1ꢀene and the ionic liquid with increasꢀ
ing temperature, and the viscosity of the ionic liquid thereꢀ
with decreases. Both these factors exert a positive effect
on the kinetics of metathesis. These results confirm the
assumption that the WCl6—BMIM•PF6 system provides
homogeneous catalysis of metathesis involving tungsten
hexachloride, whereas the WCl6—BMIM•BF4 system
provides phase transfer catalysis involving this homogeꢀ
neous catalytic phase.
For both systems, we estimated the activation enerꢀ
gies of the process (Ea). For the WCl6—BMIM•PF6
Table 2. Characteristics of the reuse of catalytic systems in metꢀ
athesis of hexꢀ1ꢀene
b
Catalytic
system
Timea
/h
α
Yield (mol.%)
Sc
(%)
Table 3. Temperature dependence of the yields of the metathesis
products
(%)
octꢀ4ꢀene decꢀ5ꢀene
Catalytic
system
T/°С α (%)
Yield (mol.%)
S (%)
WCl6—BMIM•PF6
WCl6—BMIM•BF4
1
2
1
2
17.5 10.8
6.5
3.1
4.2
1.7
61.7
60.9
65.6
79.8
8.7
12.8
9.9
5.3
8.4
7.9
octꢀ4ꢀene decꢀ5ꢀene
WCl6—BMIM•PF6
WCl6—BMIM•BF4
20
35
50
20
35
50
17.5
19.5
20.6
12.8
19.8
22.5
10.8
12.5
13.1
8.4
6.5
6.8
7.1
4.2
4.7
5.8
61.7
64.1
63.6
65.6
71.7
72.4
a Within one hour after the beginning of the reaction, the olefin
was replaced with a new portion of hexꢀ1ꢀene.
b Here and in Tables 3 and 4, the conversion of hexꢀ1ꢀene.
c Here and in Tables 3 and 4, the selectivity with respect to
octꢀ4ꢀene.
14.2
16.4

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Russ.Chem.Bull., Int.Ed., Vol. 53, No. 10, October, 2004
Vasnev et al.
Y (%)
Table 4. Influence of promoters on metathesis of hexꢀ1ꢀene in
the WCl6—BMIM•BF4 system
30
20
10
a
1
2
Promoꢀ
ter
α (%)
Yield (mol.%)
S (%)
octꢀ4ꢀene decꢀ5ꢀene
3
—
12.8
25.0
23.8
20.7
19.7
18.8
12.7
4.3
8.4
24.3
18.8
15.7
15.2
14.5
8.5
4.1
0.6
4.8
4.9
4.3
4.2
4.0
2.0
65.6
97.2
78.9
75.8
77.2
77.1
66.9
48.8
Bu4Sn
SnCl4
SbCl5
Ph3SnCl
Bu2SnO
SbCl3
PPh3
0
1
2
3
4
t/h
Y (%)
b
1
2
30
20
10
2.1
In modern catalytic systems for olefin metathesis, W,
Re, or Moꢀbased catalysts are used together with tinꢀ,
leadꢀ, antimonyꢀ, or aluminumꢀcontaining chlorides or
organometallic compounds as promoters. To examine
the effect of such promoters on the activity of the
WCl6—BMIM•BF4 system, we carried out a series of
experiments with Bu4Sn, SnCl4, SbCl5, SbCl3, Bu2SnO,
Ph3SnCl, and Ph3P as additives.
3
0
1
2
3
4
t/h
Fig. 1. Dependence of the yields of the metathesis products of
hexꢀ1ꢀene (Y ) on the reaction time (t) at 20 (a) and 50 °C (b):
the total yield (1), the yield of octꢀ4ꢀene (2), and the yield of
decꢀ5ꢀene (3).
The results of these experiments (Table 4) demonꢀ
strated that Bu4Sn is the most efficient promoter. It should
be noted that the composition of the metathesis products
obtained with the use of tetrabutyltin differs substantially
from those obtained in the reactions with the use of all
other compounds. In the presence of Bu4Sn, the selectivꢀ
ity with respect to the metathesis products of hexꢀ2ꢀene
increases essentially (to ~98%). In the presence of other
promoters, the selectivity with respect to octꢀ4ꢀene is
substantially lower, and the yield of decꢀ5ꢀene is virtually
the same (4—5%). Based on these data, it can be conꢀ
cluded that the addition of Bu4Sn leads to rapid activaꢀ
tion of the catalyst to form a complex soluble in the ionic
liquid, and octꢀ4ꢀene appears in the ionic liquid meꢀ
dium, in which hexꢀ1ꢀene undergoes isomerization to
hexꢀ2ꢀene.
The addition of triphenylphosphine sharply reduces
the activity of the catalyst. The yields of octꢀ4ꢀene and
decꢀ5ꢀene decrease by a factor of 4 and 2, respectively,
which is, apparently, attributable to the fact that triphenylꢀ
phosphine is involved in the coordination environment of
tungsten and the resulting complex is inactive in metatheꢀ
sis reactions.
and WCl6—BMIM•BF4 systems, Ea are 4.4 0.2 and
15.1 0.2 kJ mol–1, respectively. These results suggest that
the pathway of the reaction involving the latter system,
most likely, differs from that observed for homogeneous
metathesis. Subsequent experiments were carried out for
the WCl6—BMIM•BF4 system, which is most efficient in
hexene metathesis.
It was of interest to elucidate how the yields of the
products correlate with the thermodynamic equilibrium
concentrations of the metathesis products. For this purꢀ
pose, we performed experiments at 20 and 50 °C, in which
the reaction time was varied.
As can be seen from Fig. 1, the yields of octꢀ4ꢀene and
decꢀ5ꢀene were 23 and 5% at 20 °C and 28 and 6%
at 50 °C, respectively, i.e., the reaction performed at 20 °C
for one hour produced the metathesis products, viz.,
octꢀ4ꢀene and decꢀ5ꢀene, in 37 and 82% yields, respecꢀ
tively, of the maximum possible yields. An increase in the
temperature to 50 °C led to an increase in both the reacꢀ
tion rate and the yields of the metathesis products. During
5 h, the total yield increased from 28 to 34% (28% for
octꢀ4ꢀene and 6% for decꢀ5ꢀene, i.e., after one hour the
yields are 59 and 92% of the maximum values, respecꢀ
tively).
The yield of decꢀ5ꢀene in the WCl6—BMIM•BF4 sysꢀ
tem both at 20 and at 50 °C remains virtually unchanged
with increasing reaction time, the maximum yield being
obtained after one hour. This fact suggests that the major
amount of hexꢀ1ꢀene undergoes isomerization within one
hour after the beginning of the reaction and then excluꢀ
sively hexꢀ2ꢀene is involved in metathesis.
To summarize, we found that in WCl6ꢀcontaining ionic
liquids, hexꢀ1ꢀene undergoes isomerization to hexꢀ2ꢀene,
which is then subjected to metathesis. Of the catalysts
under study, the WCl6—BMIM•BF4 system is most effiꢀ
cient in metathesis of hexꢀ1ꢀene. In the latter case, the
ionic liquid together with WCl6 forms a stable homogeꢀ
neous system, which acts under conditions of phase transꢀ
fer catalysis. The addition of tetrabutyltin leads to an
increase in the yield of metathesis products. In the

WCl6ꢀcatalyzed metathesis in ionic liquids
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 10, October, 2004
2191
6. N. Audic, H. Clavier, M. Maudeit, and J.ꢀC. Guillemin,
J. Am. Chem. Soc., 2003, 125, 9248.
7. Q. Yao and Y. Zhang, Angew. Chem., Int. Ed., 2003,
42, 3395.W
8. Eur. Pat. 1035093; Chem. Abstr., 2000, 133, 237853.
9. Yu. M. Zhorov, Izomerizatsiya olefinov [Isomerizaꢀ
tion of Olefins], Khimiya, Moscow, 1977, 204 pp. (in
Russian).W
WCl6—BMIM•BF4 system, the selectivity with respect
to octꢀ4ꢀene increases to 98%.
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Received December 8, 2003;
in revised form April 15, 2004