A novel reaction in ionic liquids: Selective cyclization of 1-dodecene to cyclododecane under moderate pressure

Article abstract of DOI:10.1016/S0040-4039(03)00158-8

A novel reaction of cyclization of 1-dodecene to cyclododecane with high selectivity, especially under moderate pressure, is found in ethanol buffered chloroaluminate ionic liquids with easy separation of product due to the immiscibility with ionic liquids.

Full text of DOI:10.1016/S0040-4039(03)00158-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 2191–2193
A novel reaction in ionic liquids: selective cyclization of
ꢀ
1-dodecene to cyclododecane under moderate pressure
Kun Qiao and Youquan Deng*
Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, The Chinese Academy of Sciences,
Lanzhou 730000, PR China
Received 12 September 2002; revised 27 December 2002; accepted 10 January 2003
Abstract—A novel reaction of cyclization of 1-dodecene to cyclododecane with high selectivity, especially under moderate
pressure, is found in ethanol buffered chloroaluminate ionic liquids with easy separation of product duo to the immiscibility with
ionic liquids. © 2003 Elsevier Science Ltd. All rights reserved.
Room temperature ionic liquids (RTILs) have recently
triene and then followed by hydrogenation of cyclodo-
received recognition as novel and promising solvents
for synthetic chemistry. They have essentially no vapor
pressure, making them an attractive candidate to
replace many of the volatile organic compounds
dectriene over Ni catalyst to produce cyclododecane.
1
−
Ionic liquids containing chloroaluminate (AlCl ,
4
−
7
Al Cl ) anions could be strong Lewis acids, and there-
2
(
VOCs) that are currently used as solvents in organic
fore, can act as either catalysts or reaction medium for
acid catalyzed processes. It has been demonstrated that
chloroaluminate ionic liquids are suitable solvents for
synthesis. They have also been referred to as ‘designer
solvents’ for their chemical and physical properties
could be adjusted by a careful choice of cation/anion.
Examples of their applications in both reactions and
separations, showing a number of advantages over
conventional organic solvents, have been demonstrated.
9
dimerization and oligomerization of alpha olefins. Tol-
2,3
uene, as cosolvent, is nonetheless necessary for those
reactions to achieve optimum activity and selectivity,
which indicates that cosolvent, in some ways, would
interact with chloroaluminate ionic liquids and play an
important role in the reactions. Only non-protic sol-
vents, such as toluene and heptane, have been used as
cosolvents with chloroaluminate ionic liquids up to
now. The combination of chloroaluminate ionic liquids
with protic solvents has been, however, out of consider-
ation all along duo to the generation of oxychloroalu-
4
,5
The reaction of simple alpha olefin to prepare less
readily available products is important in chemical
industry. By carefully choosing the reaction conditions
and the catalyst, alpha olefin could be converted to
liner internal or highly branched internal olefin via
different reaction route, including isomerization, dimer-
6,7
−
−
ization or oligomerization. Example of direct selective
cyclization of alpha olefin to cycloalkane, however, is
absent from the literatures. For example, no efficient
minate
species
(Al
OCl
3
8
,
Al
3
O Cl
2
6
)
when
chloroaluminate ionic liquids were mixed with the
10
protic solvents. Those oxychloroaluminate species are
Lewis acidic and therefore, there exists the possibility
that the protic solvents buffered chloroaluminate ionic
liquids can still function as either acidic catalysts or
reaction medium. Here we report the first example of
using ethanol buffered chloroaluminate ionic liquids as
reaction medium and catalyst for acid catalyzed isomer-
ization of alpha olefin. It was found that 1-dodecene
proceeded cyclization reaction to cyclododecane in such
a novel reaction medium with high selectivity, especially
under moderate pressure. A recyclable and environmen-
tally friendly process for production and separation of
cyclododecane was also established by addition of etha-
nol to the chloroaluminate ionic liquids (Fig. 1).
catalyst has been reported for direct cyclization of
8
1
-dodecene to cyclododecane. Cyclododecane, an
important intermediate for manufacturing Nylon, was
traditionally prepared by a two-step reaction process,
i.e. 1,4-butdiene was firstly trimerized to cyclododec-
ꢀ
This work was supported financially by The Natural Science Foun-
dation of China (No. 20225309).
*
Corresponding
author.
Fax:
+86-931-8277088;
e-mail:
ydeng@ns.lzb.ac.cn
0
040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00158-8

2
192
K. Qiao, Y. Deng / Tetrahedron Letters 44 (2003) 2191–2193
doubt that oxychloroaluminate, even hydroxychloro-
aluminate species, would generate as the result of mix-
ing chloroaluminate ionic liquid with ethanol.
The acidity of chloroaluminate ionic liquid, as
expected, was also an important factor that affects the
Figure 1. Cyclization of 1-dodecene to cyclododecane in etha-
nol buffered ionic liquids.
reaction. With the molar ratio of AlCl /quaternary
3
ammonium salts decreased from 2:1 to 1:2, i.e. chloro-
aluminate ionic liquids changed from Lewis acid to
Lewis base, the conversion of 1-dodecene and selectiv-
ity to cyclo-dodecane also decreased correspondingly
Results of cyclization of 1-dodecene in the ionic liquid
consisted of AlCl and 1-butyl-3-methyl imidazolium
3
11
chloride ([BMIm]Cl) are listed in Table 1. GC/MS
analysis showed that several products, such as cyclodo-
decane and 9-methyl-2-udencene, etc. were formed, in
which the selectivity for cyclododecane reached 55–
(
entries 2–5), suggesting that acidic environment was
more favorable for cyclo-dodecane formation.
8
3%. For the concentration of 1-dodecene was low in
When the cyclization reaction of 1-dodecene was fur-
ther carried out under moderate inert N2 pressure
ranged from 3.0 to 6.0 MPa, we observed a high
selective cyclization of 1-dodecene (entries 6–9).
Cyclization of 1-dodecene to cyclododecane is a vol-
ume-decreasing reaction (the density of 1-dodecene and
cyclododecane was 0.761 and 0.871 g/ml, respectively).
One can deduce from the prediction of chemical equi-
librium that pressure is favorable for the formation of
cyclododecane. The conversion of 1-dodecene and
selectivity to cyclododecane was, however, almost
unchanged when pressure rose from 3.0 to 6.0 MPa.
The reason was still not clear at this stage.
the reaction mixture, other by-products, including
dimer of 1-dodecene and addition product of 1-
dodecene with ethanol were not detected.
Because chloroaluminate ionic liquids were immiscible
with 1-dodecane, direct addition of 1-dodecene into the
chloroaluminate ionic liquids would result a biphasic
liquid mixture only. In this case, the conversion of
1
-dodecene and selectivity to cyclododecane was rather
low (entry 1). When ethanol, a cosolvent having well
solubility with both of chloroaluminate ionic liquids
and 1-dodecene, was added into the biphasic liquid
mixture, a homogenous liquid mixture formed, which
greatly facilitated the cyclization reaction of 1-
dodecene. The conversions of 1-dodecene and selectiv-
ity to cyclododecane increased remarkably from 4.2 to
Chloroaluminate ionic liquid as reaction medium and
catalyst could be repeatedly used for the cyclization of
1-dodecene. Entry 10 showed that the fifth reaction
12.4% and 73.1 to 82.9%, respectively (entry 2). The
results of reusing of AlCl –BMImCl (1-n-butyl-3-
3
physicochemical state of ethanol buffered chloroalumi-
nate ionic liquid was closely associated with the volume
ratio of ethanol/ionic liquid. When ethanol was mixed
with equal volume of chloroaluminate ionic liquid, for
example, it was a very exothermic reaction and the
colorless ionic liquid turned into being viscous and
opaque after cooling. But a colorless liquid could be
regained if more ethanol was added, i.e. the amount of
ethanol was excessive than that of chloroaluminate
ionic liquid. In our experiments, the volume ratio of
ethanol/ionic liquid was about 10, which could ensure
us to obtain a monophasic reaction medium for cycliza-
tion reaction of 1-dodecene. Therefore, there is no
methylimidazolium chloride). It can be seen that, even
being reused at the fifth time, the conversion of 1-
dodecene and selectivity to cyclododecane still kept at
11.2 and 81.3%, respectively.
Once the reaction was finished, more attention was paid
to recovery of product and recycle of ionic liquid and
ethanol. As demonstrated by Wheeler et al., incorpora-
tion supercritical CO with ionic liquids could form a
2
recyclable process for extraction of non-volatile organic
12
compounds from ionic liquids. We found that addi-
tion of ethanol to chloroaluminate ionic liquids could
also make a similar recycle of ionic liquids and product
Table 1. Results of cyclization of 1-dodecene in chloroaluminate ionic liquid
Entry
AlCl /[BMIm]Cl (mol/mol)
Pressure (MPa)
Time (h)
Conversion (%)
Selectivity (%)
3
1a
2
3
4
5
6
7
8
9
2:1
2:1
1.5:1
1:1
0.5:1
2:1
2:1
2:1
2:1
2:1
N.A.P.c
N.A.P.
N.A.P.
N.A.P
N.A.P.
3
4
5
6
8
8
8
8
8
12
12
12
12
8
4.2
12.4
8.5
4.8
3.2
23.4
24.8
27.2
25.2
11.2
73.1
82.9
75.2
67.9
54.8
93.4
93.1
92.4
93.8
81.3
1
0b
N.A.P.
a
b
c
Ethanol was not added.
Reused five times.
Normal atmospheric pressure.

K. Qiao, Y. Deng / Tetrahedron Letters 44 (2003) 2191–2193
2193
of carbon chains than that of 1-dodecene, could not
form such a suitable conformation and only branched
isomers were obtained.
In conclusion, it is the first time that selective cycliza-
tion of alpha olefin to cycloalkane could proceed in a
novel protic solvent buffered chloroaluminate ionic liq-
uid under moderate pressure with high selectivity. Also,
it was found that ethanol addition into and separation
from the chloroaluminate ionic liquids could make both
cyclization reaction and product separation much more
easier. Such ionic liquid reaction system mediated with
second solvent, which could greatly facilitate both reac-
tion and separation, may give some useful revelation
for efficient organic syntheses and catalytic reactions.
Figure 2. A recyclable ionic liquids/ethanol system for
cyclization of 1-dodecene.
recovery in the case of 1-dodecne cyclization reaction,
and ethanol was more convenient than supercritical
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3
Cyclization of other alkenes with even number carbon
atom, including octene, 1-tetradecene, 1-hexadecene
and 1-octadecene, were also tested in ethanol buffered
ml ethanol and 2 ml 1-dodecene (9.3 mmol) were succes-
sively added into a 90 ml stainless autoclave with mag-
netic stirring. The reaction proceeded at 80–140°C for
6–12 h. Remove ethanol by heating at 80–90°C after
reaction, then the formation of two layers was observed
with the upper layer consisting of products and unreacted
substrate. After removal of products and unreacted sub-
strate by decant, the remaining ionic liquids phase was
purified at 100°C for 30 min under vacuum for the
reusability tests. Composition analysis of the upper layer
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system of GC/MS Chemstation according to the area of
each chromatograph peak.
AlCl –MBImCl chloroaluminate ionic liquids. How-
3
ever, desired products from cyclization reactions were
not observed, and the major products were various
branched isomers of the alkenes. This experimental
results suggested that the selectivity to the desired
cyclized product was closely related to the alkene itself.
Although the detailed mechanism of cyclization is not
clear at this stage, it can be conjectured that the forma-
tion of desired cyclization product was dependant upon
the conformation of alkene molecule. 1-Dodecene just
has the suitable conformation, for example, a head-to-
tail one, that could be easily converted to cyclodode-
cane. Other alkenes with either longer or shorter length
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