Hydroformylation of hex-1-ene in supercritical carbon dioxide catalysed by rhodium trialkylphosphine complexes

Article abstract of DOI:10.1039/a803855d

Triethylphosphine complexes of rhodium catalyse the hydroformylation of hex-1-ene in supercritical carbon dioxide at rates similar to those obtained in toluene, but with a slightly improved n : i ratio; using trioctylphosphine, much lower rates are observed.

Full text of DOI:10.1039/a803855d

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Hydroformylation of hex-1-ene in supercritical carbon dioxide catalysed by
rhodium trialkylphosphine complexes
Ingrid Bach and David J. Cole-Hamilton*†
School of Chemistry, University of St. Andrews, St. Andrews, Fife, Scotland, UK KY16 9ST
Triethylphosphine complexes of rhodium catalyse the hy-
droformylation of hex-1-ene in supercritical carbon dioxide
at rates similar to those obtained in toluene, but with a
slightly improved n : i ratio; using trioctylphosphine, much
lower rates are observed.
(up to 250 bar). They should only be attempted in autoclaves
that have been specially designed to withstand such pressures.]
Comparison of the pressures within the autoclave during the
heating period (10 min), with those obtained during heating an
identical solution in the absence of catalyst shows that the onset
of reaction occurs at 80 °C, and visual inspection through a
sapphire window shows that the mixture becomes homoge-
neous (monophasic pale yellow solution) at ca. 70 °C. Table 1
lists the results of a series of different experiments and shows
that the rate is very similar to that obtained with the same
catalyst under identical conditions using toluene as the solvent,
Supercritical carbon dioxide is potentially an excellent solvent
for carrying out homogeneous catalytic reactions because it is
easily separated from the catalyst and the products and is
environmentally benign.1 In addition, the high solubility of
,2
2
gases (totally miscible with scCO ) means that problems
associated with transport of gases across interfaces are removed
and the gas-like nature of the medium means that diffusion is
much faster than in solution reactions.
2
but the n : i ratio is slightly higher in scCO . The reaction is
retarded by addition of excess of PEt
increased pCO or pH2
In the solution phase system, we have previously shown that
3
, but enhanced by
.
9
Extensive studies have been made of hydroformylation of
3,4
2
alkenes using [Co (CO)
8
]
but there have been problems with
7
C alcohols can be products of the hydroformylation of hex-
studying the more active and selective rhodium based species
because the triarylphosphine complexes that are usually
1-ene, either formed in a sequential reaction (thf as solvent) or
as primary products (alcoholic solvents). In the hydroformyla-
tion of hex-1-ene carried out in scCO , C -alcohols are products
2 7
employed in liquid phase reactions have very low solubility in
5
scCO
2
. A brief report has appeared in which this problem has
(ca. 8%) after 2 h reaction and become significant (31%)
products after longer reaction times. Adding ethanol to the
mixture does not increase the amount of alcohol produced, but
been overcome by derivatising the aryl groups with fluorinated
chains.6
9
We now report that simple trialkylphosphines such as PEt
3
,
somewhat reduces the overall rate. We have shown that, in
which are readily available, do not involve the use of costly
fluorinated derivatives and have low molecular masses so the
relatively small amounts can be employed, can provide highly
liquid phase reactions, protonation of an acyl intermediate by
ethanol is the key step that leads to C
presumably the inability of scCO to solvate the ionic species
formed by the protonation, that prevents the direct formation of
alcohols when the reaction is carried out in scCO in the
presence of ethanol. Interestingly, addition of the fluorinated
alcohol, C CH OH, does lead to an increase in the
7
alcohol production. It is
2
active catalysts for the hydroformylation of hex-1-ene in scCO
2
.
Trimethylphosphine complexes of ruthenium have been used
C
7
2
5
for the hydrogenation of CO
from CO , hydrogen and dimethylamine in scCO
Using a catalyst prepared in situ from [Rh (OAc)
complete conversion to C
2
and for the formation of DMF
7,8
2
2
.
6
F13CH
2
2
2
4
] and PEt
3
,
amount of alcohol produced and to a higher reaction rate.
We have also studied other trialkylphosphine ligands.
7
aldehydes, with a trace of C
7
alcohols [total straight to branched (n : i) ratio = 2.4] is
obtained within 2 h at 100 °C.‡ [SAFETY WARNING: All
reactions involving scCO are carried out at very high pressures
2
Replacing one ethyl group in PEt
(–CH CH 13) increases the reaction rate, presumably
because of electron density changes on the rhodium rather than
3
with a fluorinated chain
2
2 6
C F
Table 1 Conditions and yields of products from the hydroformylation of hex-1-ene catalysed by rhodium complexes‡
[
dm
Rh]/mmol
Aldehydesa Heptanol
TOF/
2
3
Rh h21
21
Solvent
Toluene
Phosphine
pCO/bar
p 2
H
/bar
t/h
(%)
(%)
n : i
PEt
PEt
PEt
PEt
PEt
PEt
PEt
PEt
PEt
PEt
PEt
3
3
3
3
3
3
3
3
3
3
2
6.58
6.54
6.48
6.43
6.57
6.48
6.52
6.58
6.48
6.42
6.45
6.57
6.56
6.69
6.61
20
10
10
10
5
20
20
20
20
20
20
5
20
20
20
10
20
20
20
30
20
20
20
10
20
10
10
1
1
1
1
1
1
2
22
1
1
74
56
23
38
35
82
89
54
60
82
81
8.2
12
17
20
4.8
1
—
—
—
2.1
2.4
2.2
2.5
2.6
2.4
2.5
2.5
2.4
2.5
2.4
3.3
3.9
2.8
2.8
53
38
16
27
24
57
scCO
scCO
scCO
scCO
scCO
scCO
scCO
scCO
scCO
scCO
scCO
scCO
scCO
scCO
2
2
2
2
2
2
2
2
2
2
2
2
2
2
b
2.3
8.1
c
28
/EtOHd
/R
3
11
—
—
—
1.5
2.3
44
62
58
2.7
4.2
6.3
7.5
d,e
f
OH
e,f
R
f
1
2
2
2
P(C
P(C
P(C
P(C
8
8
8
8
H
17
H
17
H
17
H
17
)
3
3
3
3
)
)
)
5
10
15
2
a
Traces of isomerised alkenes are also observed. b 0.2 cm3. c 2-Methylhexanol (3%) and hexane are also observed. d 2 cm3. e
R = C F13CH CH .
f 6 2 2
f
90 °C.
Chem. Commun., 1998
1463

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changes in solubility (both reactions are fully homogeneous).
Using tri-n-octylphosphine, the rate is very much reduced, two
phases are apparent in the system and higher n : i ratios (up to
throughout. After the reaction, the autoclave was allowed to cool in dry ice
3
to 250 °C. The CO
2
was vented and the liquid product, typically 1.8 cm ,
was collected and analysed by GLC (quantitative analysis) and GC–MS
(
identification of products). 31P NMR studies of the final liquid indicated
3
.9) are obtained with low pCO, although the conversion is low.
9
2 3 2
that the major species in solution is [RhH(CO) (PEt ) ].
We have shown that higher n : i ratios can be obtained at low
For reactions in toluene, the reaction was carried out in the same
9
conversion in the solution phase system.
3
autoclave using half the amounts of all the reagents, 17 cm of toluene and
We conclude that hydroformylation of hex-1-ene can be
successfully carried out in scCO using rhodium based catalysts
2
the same gas pressure. This ensures that all the concentrations are the same
as in the scCO2 experiments and that the pressure drop for the same
percentage conversion of substrate is also the same.
containing cheap, readily available trialkylphosphine ligands,
and that different chemoselectivities can be observed from those
observed under some conditions in organic solvents.
We thank the EPSRC and the EC TMR programme for a
Fellowship (I. B) and Professor M. Poliakoff for helpful
discussion.
1
2
3
R. Noyori, Science, 1995, 269, 1065.
P. G. Jessop, T. Ikariya and R. Noyori, Chem. Rev., 1995, 95, 259.
J. W. Rathke, R. J. Klinger and T. R. Krause, Organometallics, 1991, 10,
1
350.
4
5
R. J. Klinger and J. W. Rathke, J. Am. Chem. Soc., 1994, 116, 4772.
P. G. Jessop, Y. Hsiao, T. Ikariya and R. Noyori, J. Am. Chem. Soc., 1996,
Notes and References
1
18, 344.
†
‡
E-mail: djc@st-and.ac.uk
Reactions were carried out in a 36 cm Hastelloy autoclave fitted with a
6 S. Kainz, D. Koch, W. Baumann and W. Leitner, Angew. Chem., Int. Ed.
Engl., 1997, 36, 1628.
3
mechanical stirrer and a sapphire base for visual observation of the contents.
All manipulations were carried out under nitrogen. [Rh (OAc) ] (0.052 g,
(0.1 cm , 0.67 mmol) to give a red
7 P. G. Jessop, T. Ikariya and R. Noyori, J. Am. Chem. Soc., 1994, 116,
8851.
8 P. G. Jessop, Y. Hsiao, T. Ikariya and R. Noyori, J. Am. Chem. Soc., 1996,
118, 344.
9 J. K. MacDougall, M. C. Simpson, M. J. Green and D. J. Cole-Hamilton,
J. Chem. Soc., Dalton Trans., 1996, 1161.
2
4
3
0
.1176 mmol) was dissolved in PEt
3
3
solution. Degassed hex-1-ene (2 cm , 16 mmol) was added and the mixture
transferred into the autoclave, pressurised with CO and H and stirred at
room temperature for 1 h. Liquid CO was then transferred into the
autoclave using a cooled head hplc pump to give a total pressure 65 bar
above that of the CO–H . The autoclave was then heated for the desired
reaction time, with the pressure and temperature being monitored
2
2
2
Received in Cambridge, UK, 21st May 1998; 8/03855D
1464
Chem. Commun., 1998