Control of self-aldol condensation by pressure manipulation under compressed CO2

Article abstract of DOI:10.1039/b307647b

Under supercritical CO₂ conditions, simple adjustment of the pressure was found to successfully control the ratio of aldol to enal product in the self-aldol condensation of aldehyde, in which the enal product was obtained in a maximum selectivity of 94% at the critical pressure of 12MPa, whereas 85% selectivity to the aldol product was achieved at the subcritical region.

Full text of DOI:10.1039/b307647b

Control of self-aldol condensation by pressure manipulation under
compressed CO2†
Keitaro Matsui, Hajime Kawanami,* Yutaka Ikushimaab and Hiromichi Hayashia
a
ab
a
Supercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology,
4-2-1 Nigatake, Miyagino-ku, Sendai, Miyagi 983-8551, Japan. E-mail: h-kawanami@aist.go.jp;
Fax: +81-22-237-5215; Tel: +81-22-237-5211
CREST, Japan Science and Technology Corporation (JST), Honcho, Kawaguchi 332-0012, Japan
b
Received (in Cambridge, UK) 4th July 2003, Accepted 12th August 2003
First published as an Advance Article on the web 22nd August 2003
Under supercritical CO
2
conditions, simple adjustment of
2
scCO . Such a high selectivity can be successfully achieved by
simple pressure manipulation without the improvement of
catalysis by the addition of other metal cations.
The self-aldol condensation of propionaldehyde (Scheme 1)
was conducted in a batchwise operation.13 The results are
summarized in Table 1. In the presence of MgO,‡ the total
conversions remarkably drop from 95% (run 2) to 20% (run 9)
the pressure was found to successfully control the ratio of
aldol to enal product in the self-aldol condensation of
aldehyde, in which the enal product was obtained in a
maximum selectivity of 94% at the critical pressure of
1
2MPa, whereas 85% selectivity to the aldol product was
achieved at the subcritical region.
with increasing CO
was inactivated by forming carbonic acid on the MgO surface,
and the reactant was diluted by liquid and scCO with
2
pressure, because the basic site of MgO
From the view point of greener processes, the design of organic
synthetic methods using supercritical carbon dioxide (scCO
has become of much interest in recent years, because scCO
2
)
2
2
is
increasing pressure which is disadvantageous for the bimo-
lecular reaction. However, the highest selectivity of 93% to
considered as an environmentally benign and cheap synthetic
–7
medium.1 It has several advantages for chemical syntheses
3-hydroxy-2-methylpentanal 2a was obtained at a CO
of 5 MPa (run 5). The addition of water to the MgO system at
a fixed CO pressure of 5MPa retained a good selectivity to 2a
2
pressure
such as relatively moderate physical properties (T
c
= 304.2 K,
P
c
= 7.38 MPa) and easily tunable physicochemical properties
2
such as density and solubility for substrate by adjusting the
(run 6), but increasing the pressure up to 12 MPa led to
significant changes in selectivity for both MgO and MgO b
water system (runs 9, 10), in particular, in the MgO + water
system the selectivity of 3a showed a dramatic increase from 15
to 94%, as opposed to the drop in selectivity to 2a from 85 to 8%
(runs 6, 10).
This enhancement in the selectivity to 3a with pressure may
be due to the dehydration of 2a to 3a, being catalyzed by
carbonic acid as a Brønsted acid catalyst which was derived
pressure and temperature.7
Though quite a large number of homogeneous catalytic
reactions have been reported so far, it has suffered from a
serious disadvantages of the separation of reactant/product and
of side reactions like decomposition, which could occur during
the distillation after the reaction. Hence, solid heterogeneous
catalyst characteristics, such as thermostability and easy
catalyst/product separation, are advantageous compared with
homogeneous catalysts for reuse.
2
from a small amount of water and CO . Because addition of dil.
The aldol reaction is one of the most important reactions to
form C–C bonds. Many attempts have been made to obtain
HCl as a Brønsted acid instead of water, the selectivity of 3a
was remarkably improved from 15% to 57% at 5MPa (runs 6,
7), and high selectivity and improved conversion of 1a from
37% to 99% at 12 MPa (runs 10, 11). Furthermore, in only the
presence of water, selectivity of 3a at 12 MPa (40% in run 12)
is also higher than that in the absence of water (22% in run 8).
This would be due to the generation of carbonic acid derived
8
aldol or enal products in high selectivity from unmodified
aldehydes using heterogeneous catalysts such as alkaline
oxides, alkaline earth oxides,9 zeolite, and hydrotalcites.
,10
11
12
However, it is difficult to obtain the aldol or enal product in
satisfactory selectivity, because of the high reactivity of
aldehyde group. In this work, we thus demonstrate that the self-
aldol condensations of unmodified aldehydes can produce aldol
derivatives and unsaturated aldehydes in high selectivity in
†
Electronic supplementary information (ESI) available: aldol reactions of
aldehyde 1b in the presence of MgO catalyst. See http://www.rsc.org/
suppdata/cc/b3/b307647b/
Scheme 1 Self-aldol reaction of propionaldehyde.
Table 1 Aldol reactions of propionaldehyde in the presence of MgO catalyst.
Yield of
Selectivity
Yield of
Selectivity
Run
Substrate
Pressure/MPa
Conditions
Conv. (%)
2a (%)
to 2a (%)
3a (%)
to 3a (%)
1
2
3
4
5
6
7
8
9
propionaldehyde
0
0
0
5
5
none
MgO
MgO + water
none
MgO
MgO + water
MgO + dilHCl
none
27
95
95
18
59
39
92
18
20
37
99
16
17
61
51
11
55
33
4
10
9
2
60
64
54
61
93
85
5
55
45
5
2
34
44
0
4
6
52
4
10
35
87
7
5
36
46
0
7
5
5
15
57
22
50
94
88
40
12
12
12
12
12
MgO
1
1
1
0
1
2
MgO + water
MgO + dilHCl
water
9
6
9
36
2
502
CHEM. COMMUN., 2003, 2502–2503
This journal is © The Royal Society of Chemistry 2003

from water and carbon dioxide, which catalyze the dehydration
of 2a to 3a. Thus the product distribution as the 2a/3a ratio can
selectivity to aldol 2a. In supercritical conditions, above 8 MPa,
14,15
water is more soluble than gaseous CO
2
,
and the solvated
be varied by simply controlling the pressure of CO
2
.
water reacts with CO likely to generate carbonic acid which
2
Fig. 1 shows the CO pressure dependence of the total
2
could promote the dehydration of 2a as a Brønsted acid. Thus,
under supercritical conditions, the selectivity to 3a would be
higher than that under subcritical conditions in the lower
pressure region. We also investigated another aldol reaction of
acetaldehyde under the same conditions in the presence of
MgO, and a similar pressure dependence on the product
selectivity was observed.§
In conclusion, we found that the selectivity of the self-aldol
condensation of propionaldehyde can be tuned simply by
2
adjusting the pressure of CO in the presence of MgO catalyst
with a small amount of water. At the supercritical region the
unsaturated aldehyde 3a was obtained in 94% high selectivity
around the critical pressure of 12 MPa, while the aldol
derivative 2a was attained in a satisfactory selectivity of 85% at
the subcritical region. Furthermore, acetaldehyde can be also
converted into aldol or unsaturated aldehyde in good selectivity
conversion of 1a in MgO in the presence of a small amount of
water. One can see an interesting pressure dependence, in which
the conversion decreases, reaching a minimum at 8 MPa, and
then increases to the maximum at the near-critical pressure
around 14 MPa. Taking into account the error rate in Fig. 1, this
pressure dependence showing extrema in the conversion is
considered to be convincing. Such a pressure dependence was
2
previously reported when CO was used as a solvent or a
reactant.2 The pressure dependence of the selectivity was
further investigated. Interestingly the selectivities toward 2a
and 3a which vary in opposite manners with pressure can be
seen in Fig. 2. In the lower pressure range below about 5 MPa,
an increase in pressure accelerated only the formation of 2a and
led to the maximum selectivity to 2a of 85% around 5 MPa,
when the dehydration for the formation of 3a from 2a was
depressed. However, further increase in pressure above 5 MPa
resulted in higher selectivity to 3a, attaining the maximum value
of 94% at the critical pressure of 12 MPa. This pressure
dependence would be due to the changing property on the
,3
2
by pressure manipulation of CO .
Notes and references
surface of the MgO catalyst caused by adsorption of CO
water at each pressure. At lower pressures below 8 MPa, CO
adsorbed on the surface of MgO catalyst, and carbonic acid is
formed by the reaction of CO with added water which is also
2
and
‡
MgO (JRC-MGO-4 100A) used in this study was purchased from The
2
is
Catalysis Society of Japan, and its physical properties are as follows:
2
21
particle size: ca. 14 nm, BET surface area; 120 m g . MgO was heated at
00 °C under air for 1 h before the reaction.
2
1
adsorbed on the surface of MgO, and the Brønsted acid derived
from generated carbonic acid is considered to be neutralized
with the basic sites of MgO, resulting in the increase in
§ 3-Hydroxybutyraldehyde and but-2-enal were also obtained from the
aldol reaction of acetaldehyde. At 5 MPa, 3-hydroxybutyraldehyde was
obtained in a maximum selectivity of 73%, whereas but-2-enal was obtained
in 96% selectivity at 12 MPa. For details see ESI.†
1
2
J. Hyde, W. Leitner and M. Poliakoff, in High Pressure Chemistry, ed.
R. Eldik and F.-G. Klärner, Wiley-VCH, Weinheim, 2002, p. 369.
S. Fujita, B. M. Banage, Y. Ikushima and M. Arai, Green Chem., 2001,
3
, 87.
3
4
5
6
H. Kawanami and Y. Ikushima, Chem. Commun., 2000, 2089.
H. Kawanami and Y. Ikushima, Tetrahedron Lett., 2002, 43, 3841.
H. Kawanami and Y. Ikushima, J. Jpn. Petrol. Inst., 2002, 45, 321.
H. Kawanami, A. Sasaki, K. Matsui and Y. Ikushima, Chem. Commun.,
2
003, 896.
7
Y. Ikushima and M. Arai, in Chemical Synthesis Using Supercritical
Fluids, ed. P. G. Jessop and W. Leitner, Wiley-VCH, Weinheim,
1
999.
8
9
W. Carruthers, Some Modern Methods of Organic Synthesis – 3rd edn.,
Cambridge Univ. Press., New York, 1986.
K. Tanabe, G. Zhang and H. Hattori, Appl. Catal., 1989, 48, 63.
1
1
0 H. Tsuji, F. Yagi, H. Hattori and H. Kita, J. Catal., 1994, 148, 759.
1 E. Dumitriu, V. Hulea, I. Fechete, A. Auroux, F.-F. Lacaze and C.
Guimon, Microporous Mesoporous Mater., 2001, 43, 341.
Fig. 1 Pressure dependence of the total conversion of 1a in the presence of
MgO and water.
1
2 (a) Y. Anzai, M. Goto, A. Kodama and T. Hirose, Proc. 14th Symp. On
Chem. Eng., Kyushu-Taejon/Chungnam, Taejon Univ., Korea, Dec. 1st
2
001, p. 13; (b) M. Goto, Y. Anzai, A. Komada and M. Yoshida, Chem.
Eng. Trans., 2002, 2, 85.
1
3 The typical experiment procedure is as follows: acetaldehyde (10 mmol)
or propionaldehyde (10 mmol), and MgO (50 mg) were charged into a
2
5 cm3 stainless steel reactor at room temperature. In the case of
addition of water, water (0.1 mL) was mixed with aldehydes and MgO.
The reactor was heated at 80 °C and CO was subsequently charged into
2
the reactor using a high-pressure liquid pump and compressed to the
desired pressure. Pressure control was achieved by a back-pressure
regulator. The reactions were started by stirring the mixture, continued
for 6 h. After reaction, the reactor was cooled to 0 °C with ice and the
pressure was released slowly. The MgO was removed by filtration, and
the products and yields were determined by GC-MS (HP 6890 GC
System and 5973 Mass Selective Detector) with tridecane as an internal
reference.
Fig. 2 Pressure dependence of the selectivity of 2a and 3a in the presence
of MgO and water.
14 R. Weibe and V. L. Gaddy, J. Am. Chem. Soc., 1941, 63, 475.
15 R. Wiebe, Chem. Rev., 1941, 29, 475.
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