Biphasic hydroformylation of 1-hexene with carbon dioxide catalyzed by ruthenium complex in ionic liquids

Article abstract of DOI:10.1246/cl.2004.14

Hydroformylation of 1-hexene using carbon dioxide as carbonyl carbon source attained high yield and good chemoselectivity in heptanols when a ruthenium complex was used in biphasic ionic liquid-toluene system.

Full text of DOI:10.1246/cl.2004.14

14
Chemistry Letters Vol.33, No.1 (2004)
Biphasic Hydroformylation of 1-Hexene with Carbon Dioxide Catalyzed
by Ruthenium Complex in Ionic Liquids
Ken-ichi Tominaga and Yoshiyuki Sasaki
National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba 305-8569
(Received October 7, 2003; CL-030949)
Hydroformylation of 1-hexene using carbon dioxide as car-
Table 1. Hydroformylation of 1-hexene with CO2 catalyzed by
a ruthenium complex in ionic liquidsa
bonyl carbon source attained high yield and good chemoselectiv-
ity in heptanols when a ruthenium complex was used in biphasic
ionic liquid–toluene system.
Entry Ionic
liquid
Co-
Conv.
/% Alcohol Aldehyde Alkane
Yield/%b
solv.
1
[bmim]Cl toluene 97
NMP 98
84
64
82
60
20
66
79
60
80
63
3
0
5
0
0
0
0
0
0
0
0
0
0
11
24
12
14
13
8
2c
3d
4e
5
À
Carbon monoxide (CO) is one of the most useful raw chemi-
cal materials for the production of hydrocarbons, aldehydes, al-
cohols, and organic acids on an industrial scale, but with a major
disadvantage of high toxicity. In contrast, carbon dioxide (CO2)
is an ideal raw material, because it is safe, abundant, and inex-
pensive. In our attempts to use CO2 as a substitute for CO,1–6
we recently reported a novel hydroformylation using CO2 as car-
bonyl carbon source in the presence of ruthenium complex and
halide salts catalyst system.6 This catalysis was effectively per-
formed with several alkenes to give corresponding alcohols in
good yields. However, some problems remained unsolved with
this catalytic system; alkene hydrogenation proceeded prior to
hydroformylation with terminal alkenes; effective solvents such
as N-methyl-2-pyrolidone (NMP) were with so high boiling
points that it might cause some difficulties of separating the
products from the solvent by distillation, when it would be put
into industrial practice.
In view of those situations, we have taken up ionic liquids,
on which much attention has been focused as a new reaction me-
dia in recent years. One of the attractive characters of ionic liq-
uids is immiscibility with a number of organic solvents, enabling
their use as a nonaqueous alternative solvent in two-phase sys-
tems.7,8 The catalyses using two-phase system have been known
to have advantages that products can be separate from a catalyst
phase by simple extraction and that it is possible to modulate the
reaction selectivity. Here we report an improved hydroformyla-
tion of 1-hexene with CO2 using ruthenium complex in a bipha-
sic catalytic system consisting of ionic liquid and organic solvent
(Eq 1).
[bmim]Cl toluene 96
[bmim]Cl toluene 78
[bmim]Cl C6H12
[bmim]Cl Et2O
[bmim]Cl THF
37
78
88
6
7
9
8
[emim]Cl toluene 76
[omim]Cl toluene 98
[bmim]BF4 toluene 96
[bmim]PF6 toluene 95
[bmpy]Cl toluene 98
12
16
26
86
10
9
10c
11c
12
65
aThe reaction was performed as described in Ref. 9. bBased on
1-hexene. c[PPN]Cl (0.4 mmol) was added. dThe second run. eThe
fourth run.
rized in Table 1.9 In the toluene/[bmim]Cl system (bmim = 1-n-
butyl-3-methylimidazolium), the reaction proceeded smoothly
to give a mixture of C7 alcohols in 84% yield (Entry 1), while
the yield of these alcohols remained 64% and the corresponding
aldehydes were also formed with NMP (Entry 2). Unfortunately,
the regioselectivity was not improved by using ionic liquid; both
reaction systems gave mixtures of almost equimolar amounts of
n-heptanol and 2-methylhexanol. Once the reaction reached
completion, the mixture was spontaneously separated into or-
ganic and ionic liquid layers. Although the produced alcohols
were present in both layers, they could be easily extracted using
diethyl ether. The remaining mixture of ruthenium complexes
and [bmim]Cl could be reused with almost the same activity
for the second run (Entry 3), while its catalytic activity was con-
siderably decreased by the fourth run, probably due to the accu-
mulation of water that suppressed the CO formation (Entry 4).
It is necessary that organic co-solvent should be miscible
with ionic liquid under the reaction conditions so that the sub-
strate alkenes could contact with catalysts present in ionic liquid
phase. Many factors are related to the miscibility of organic
compounds with ionic liquids.10 In general, the miscibility in-
creases in the order of polarity of organic compounds but it also
depends on their aromaticity, because aromatic compounds have
CHÁÁÁꢀ or ꢀ–ꢀ interaction with ionic liquids resulting in their
relatively high miscibility. The conversion of 1-hexene was thus
affected by the miscibility of the co-solvents with [bmim]Cl and
increased in the order of cyclohexane < Et2O < THF < toluene
(Entries 1, 5–7).
C4H9
OH
+ CO2 + 3H2
(1)
C4H9
+ H2O
OH
cat.
in ionic liquids
C4H9
Hydroformylation with CO2 proceeds in two steps: CO2 is
converted initially into CO, which in turn becomes the reagent
of hydroformylation of the substrate.6 For the first conversion
of CO2 to CO, the key process is deprotonation of a ruthenium
hydride complex with halide anions to form an active species
that coordinates to CO2.3 Among the halide anions examined
as an additive, chloride salts are the most effective ones because
of their high proton affinity.3,6 On the basis of those findings, we
chose 1,3-dialkylimidazolium chloride salts as ionic liquids. Al-
though most of them are solid at room temperature, they melt at
around 100 ꢀC or below. Some representative results are summa-
Another important factor affecting the alkene reactivity is
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.1 (2004)
15
the lipophilicity of imidazolium cations that increases with in-
creased alkyl chain length. Thus, the conversion of alkene was
relatively low in the reaction with [emim]Cl (emim = 1-ethyl-
3-methylimidazolium) due to its less miscibility (Entry 8). On
the other hand, a more miscible salt, [omim]Cl (omim = 1-n-oc-
tyl-3-methylimidazolium), afforded a single phase, which
seemed responsible for the decreased chemoselectivity (Entry
9).
Ionic liquids with anions other than chloride were much less
effective; even in the presence of a small amount of a chloride
salt, the hydroformylation in [bmim]BF4 gave lower yield and
chemoselectivity in alcohols (Entry 10). In [bmim]PF6, a degra-
dation of the catalyst was observed as an unknown yellow com-
plex precipitated, resulting in only a slight formation of hydro-
formylation products (Entry 11). A pyridinium chloride (bmpy
= 1-n-butyl-4-methylpyridinium) gave a considerable amount
of hydroformylation products, but unfortunately degraded as
the alcohols were produced (Entry 12).
In conclusion, 1,3-dialkylimidazolium chlorides can make a
useful media for hydroformylation with CO2. They provide bi-
phasic systems and high concentration in chloride anion, which
may be the cause of the improved chemoselectivity in favor of
hydroformylation. Moreover, the extraction of products and
the recycling of catalysts become much easier compared with
the conventional method. Further investigation is in progress
to improve the catalyst recycling.
Saito, J. Chem. Soc., Chem. Commun., 1993, 629.
K. Tominaga, Y. Sasaki, M. Saito, K. Hagihara, and T.
Watanabe, J. Mol. Catal., 89, 51 (1994).
K. Tominaga, Y. Sasaki, K. Hagihara, T. Watanabe, and M.
Saito, Chem. Lett., 1994, 1391.
K. Tominaga, Y. Sasaki, T. Watanabe, and M. Saito, Bull.
Chem. Soc. Jpn., 68, 2837 (1995).
K. Tominaga, Y. Sasaki, T. Watanabe, and M. Saito, Stud.
Surf. Sci. Catal., 114, 495 (1998).
K. Tominaga and Y. Sasaki, Catal. Commun., 1, 1 (2000).
T. Welton, Chem. Rev., 99, 2071 (1999).
J. Dupont, R. F. de Souza, and P. A. Z. Suarez, Chem. Rev.,
102, 2002 (2002).
In a typical experiment, a solution of Ru3(CO)12 (0.1 mmol),
[bmim]Cl (1.0 g), and 1-hexene (5.0 mmol) in toluene
(5.0 mL) was placed in a 50-mL stainless-steel autoclave.
CO2 (4.0 MPa) and H2 (4.0 MPa) were introduced at room
temperature and then the reactor was heated to 140 ꢀC and
held at that temperature for 30 h with stirring. After the reac-
tion was completed, the resulting reaction mixture was sep-
arated into a toluene phase and an ionic-liquid phase. GC
analysis indicated that the former contained 2.7 mmol of
C7 alcohols, 0.6 mmol of hexane and 0.1 mmol of 1-hexene
and the latter contained 1.5 mmol of C7 alcohols. The total
yield of C7 alcohols was 84%. After evaporating the vola-
tiles and then extracting the ionic liquid several times with
diethyl ether, the crude mixture of C7 alcohols was recov-
ered. This mixture was then purified by Kugelrohr distilla-
tion (isolated yield = 80%).
2
3
4
5
6
7
8
9
This study was supported by Industrial Technology
Research Grant Program in ’02 from NEDO of Japan.
10 J. D. Holbrey, Ann E. Visser, and R. D. Rogers, in ‘‘Ionic
Liquids in Synthesis,’’ ed. by P. Wasserscheid and T.
Welton, Wiley-VCH, Weinheim (2003), Chap. 3, p 68.
References and Notes
1
K. Tominaga, Y. Sasaki, M. Kawai, T. Watanabe, and M.
Published on the web (Advance View) December 1, 2003; DOI 10.1246/cl.2004.14