Fig. 2 Water–surfactant emulsion in supercritical conditions: a) P , 100 atm, T = 70 uC; b) 120 atm , P , 150 atm, T = 90 uC; c) P . 165 atm, T =
90 uC.
was increased, the conversion reached a maximum of 93% at an
H2O/2 ratio of 14 (entry 5, Table 1). In all these cases the selectivity
in acids remained at 80% which is a better result than the one
obtained with the organic and aqueous systems.4 At higher
pressure, when the system is soluble in scCO2, the conversion
decreased (44% conversion in entry 7, Table 1) as happened
without the addition of surfactant.
1 were placed in a 100 ml autoclave equipped with sapphire
windows, and the system was pressurized up to 30 atm of CO.
Then, the pressure and temperature were gradually increased. At
180 atm and 70 uC a single yellow phase was formed.
Once the conditions in which the system became soluble had
been determined, we performed the catalytic experiments using the
in situ formed catalyst precursor [PdCl2(PhCN)2]/1 (P/Pd = 4) in
the presence of oxalic acid and added water.{ The results are
summarized in Table 1.
Further experiments are in progress to optimize the parameters
and obtain more information about the species formed under
catalytic conditions.
Conversion was very low (6%) at 90 uC and 200 atm of total
pressure, although the system was soluble in these conditions
(entry 1, Table 1). The chemoselectivity was also low and only 74%
of the products were acids, the remaining products being the result
of isomerization (2- and 3-octenes). Increases in conversion (up to
55%), selectivity (up to 90%) and regioselectivity in the linear
isomer were observed when the total pressure was decreased to
150 atm (entry 2, Table 1). In these conditions visual inspection
through the sapphire windows shows a suspension. When these
results are compared with those obtained with [PdCl2(PhCN)2]/
PPh3 under the same conditions (entry 3, Table 1), it can be
observed that there is a clear improvement in the selectivity if
scCO2 is used, although the conversion is lower. The selectivity
obtained with our Pd/1 system is better than the selectivity
reported for aqueous biphasic catalysis with Pd/TPPTS. For
instance, for 1-hexene 40% of isomerization was reported in the
biphasic aqueous system.4
In summary, we present the first example of the hydrocarboxy-
lation of 1-octene in scCO2 using the Pd/1 catalyst precursor. The
use of scCO2 means that the selectivity in acids is higher than for
the organic and aqueous systems. The addition of a perfluorinated
surfactant enhances the activity and the regioselectivity to the
linear acid although the selectivity decreases.
We gratefully acknowledge the Generalitat de Catalunya
(DURSI and Fons Social Europeu) for a Fellowship (FI) to C.
T. and the Ministerio de Educacion y Ciencia (CTQ2004-03831/
PPQ) for financial support. We also thank Brugarolas, S.A. for
supplying the Krytox1 FS-L 57. Krytox1 is a trade mark of
DuPont.
Notes and references
{ Catalysis: [PdCl2(PhCN)2] (0.025 mmol) was mixed with 1.56 mmol of
H2C2O4?2H2O, 0.1 mmol of 1 and 2 when used (mmol depending on the
experiment according to Table 1). The mixture was loaded into a 25 ml
stainless steel reactor vessel and the system was purged. Degassed water
(0.23 ml), 1-octene (1.56 mmol) and undecane (0.13 ml) as GC internal
standard were mixed and charged in vacuum. Then the CO gas was
charged, the reactor pressurized to 30 atm, and the liquid carbon dioxide
introduced. The contents were heated to 90 uC. The compressed carbon
dioxide was introduced to attain the desired reaction pressure and
magnetically stirred (750 rpm). After the reaction, the vessel was cooled
with ice water to 0 uC and slowly depressurized to atmospheric pressure
through a cold trap. The reaction mixture was extracted with diethyl ether
and analyzed by gas chromatography.
The fact that the conversion obtained was low was attributed to
a mass transfer problem of the water to the scCO2 phase. Thus, we
decided to add an ammonium salt of perfluoropolyether
carboxylic acid 221 (Krytox1). We first examined the solubility
of the [PdCl2(PhCN)2]/1/2 system under scCO2 (Fig. 2). To do so,
0.05 mmol of [PdCl2(PhCN)2], 0.20 mmol of 1, 3.125 mmol of
oxalic acid, 18.75 mmol of water and 1.78 mmol of 2 were placed
in a 100 ml autoclave equipped with sapphire windows, and then
the same method as for [PdCl2(PhCN)2]/1 was followed. At 70 uC
and below 100 atm, the scCO2 phase was colorless and transparent
(Fig. 2a). When the temperature and pressure were increased to
90 uC and 120 atm, respectively, we observed the formation of a
suspension (Fig. 2b) that was white up to 165 atm and then turned
yellow up to 180 atm (Fig. 2c)
1 R. A. Sheldon, J. Mol. Catal., 1996, 107, 75.
2 B. Hinz, C. P. Dorn, Jr., T. Y. Shen and K. Brune, in Ullmanns
Encyclopedia of Industrial Chemistry, ed. W. Gerharzt, VCH, Weinheim,
2002.
3 I. del R´ıo, C. Claver and P. W. N. M. van Leeuwen, Eur. J. Inorg.
Chem., 2001, 2719; G. Kiss, Chem. Rev., 2001, 101, 3435.
4 S. Tilloy, E. Monflier, F. Bertoux, Y. Castanet and A. Mortreux, New J.
Chem., 1997, 21, 529.
The addition of surfactant 2 (H2O/2 = 28) to the Pd/1 system
increased both the conversion (entry 4, Table 1) and the
regioselectivity to the linear acid to 80%. When the amount of 2
5 G. Papadogianakis, G. Verspui, L. Maat and R. A. Sheldon, Catal.
Lett., 1997, 43.
2790 | Chem. Commun., 2006, 2789–2791
This journal is ß The Royal Society of Chemistry 2006