Angewandte
Chemie
[3] For recent examples, see: a) K. Yamaguchi, N. Mizuno, Angew.
Experimental Section
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Safety warning: Experiments using large amounts of compressed
gases, especially molecular oxygen, are potentially hazardous and
must only be carried out by using appropriate equipment and safety
precautions.
The palladium cluster 3 was prepared under argon according to a
known procedure.[10a] [Pd(acac)2] (4), Brij 35, PEG-1000, and 1a–h
were obtained from commercial suppliers. The oxidation products
were identified by comparison of 1H NMR spectra and GC data with
those of authentic samples. A preformed mixture of CO2 and O2
(molar ratio 92:8) was prepared in a temperature-controlled 5 L
autoclave at 358C and 20.0MPa and used throughout most of the
oxidation experiments.
[4] For the use of supercritical CO2 with heterogeneous Pd catalysts
for alcohol oxidation, see: a) G. Jenzer, D. Sueur, T. Mallat, A.
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[8] The immobilization of Wilkinsonꢃs catalyst in PEG for biphasic
hydrogenation in scCO2 has been described recently: D. J.
Heldebrant, P. G. Jessop, J. Am. Chem. Soc. 2003, 125, 5600 –
5601.
Batchwise catalytic oxidation of 1a–h: A solution of 3 (9.98 ꢀ
10À5 mol) in acetic acid (1 mL) was introduced into a 10 mL window-
equipped stainless-steel high-pressure reactor. The solvent was
evaporated; PEG-1000 (2.40 g) and then the appropriate substrate
1a–h (2.0 ꢀ 10À3 mol) were added. The reactor was pressurized with
the CO2/O2 mixture (5.50 g) and heated to the desired temperature
under vigorous stirring for the given reaction time (see Figure 1 and
Table 1). After the reaction, CO2 at 808C and 14.5 MPa was flushed
through the liquid PEG phase for 7 h by use of a capillary with a
compressor at an outlet flow-rate of approximately 5 LhÀ1. The
products were collected from the gas stream in two serial traps kept at
À358C and analyzed by 1H NMR spectroscopy and GC. Before
starting the next run, the reactor was vented to ambient pressure and
then new substrate was charged, followed by the CO2/O2 mixture.
Continuous-flow catalytic oxidation of 1d: The catalytically
active material formed from 3 or 4 and PEG-1500 (3.5 g) was charged
into a 36 mL stainless-steel high-pressure reactor which was kept at
808C. A prewarmed (508C) continuous stream of CO2/O2 was passed
through the liquid PEG phase at a flow rate of approximately 5 LhÀ1
by use of a capillary (exit flow at ambient conditions, total pressure of
15.5 MPa and 13.2 MPa in the reactor, respectively; Figure 2). The
substrate was fed into the CO2 stream before it entered the reactor
with an HPLC pump at a rate of 0.5 mLhÀ1. The mixture of 1d and 2d
was isolated in two sequential cold traps (À358C) from the exit flow
upon depressurization of the supercritical solvent. The cold traps
were replaced periodically after about 2 h and their combined content
was analyzed by 1H NMR spectroscopy and GC.
[9] Ru and Pd nanoparticles embedded in high density polyethylene
as catalysts for hydrogenations with scCO2: H. Ohde, M. Ohde,
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Received: July 30, 2004
Revised: October 15, 2004
Published online: January 26, 2005
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Keywords: aerobic oxidation · multiphase catalysis ·
nanoparticles · palladium · supercritical fluids
.
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Angew. Chem. Int. Ed. 2005, 44, 1346 –1349
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