1 (a) I. V. Kozhevnikov, Catalysis by Polyoxometalates, Wiley,
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3 G. Maayan, R. H. Fish and R. Neumann, Org. Lett., 2003, 5,
3547–3550.
4 A. Haimov and R. Neumann, Chem. Commun., 2002, 876–877.
5 W. Leitner, Acc. Chem. Res., 2002, 35, 746–756.
6 For general discussion of oxidation in scCO2, see: N. Theyssen, Z. Hou
and W. Leitner, Chem.–Eur. J., 2006, 12, 3401–3409 and references
therein.
7 Selected examples for the use of scCO2 with typical heterogeneous
alcohol oxidation catalysts: (a) J.-D. Grunwaldt, M. Caravati, M. Ramin
and A. Baiker, Catal. Lett., 2003, 90, 221–229; (b) A. M. Steele, J. Zhu
and S. C. Tsang, Catal. Lett., 2001, 73, 9–13; (c) R. Gla¨ser, R. Jos and
J. Williardt, Top. Catal., 2003, 22, 31–39.
catalyzed by the polyoxometalate H5PV2Mo10O40. Even though
the catalytic activity is as yet lower than that of alcohol oxidation
catalysts based on Pd nanoparticles,8 the cheap and readily
available polyoxometalates are a promising alternative with a
distinct and complementary substrate specifity. Surprisingly, the
best performance of the catalyst was observed when it was used in
unsupported solid form. The simple product isolation and the high
catalyst stability make this system quite attractive for the use as a
‘‘catalyst cartridge’’14 and/or for continuous-flow operation.
The research was supported by the German Federal Ministry
of Education and Research (BMBF) within the framework of
the German–Israeli Project Cooperation (DIP-G7.1) and the
Helen and Martin Kimmel Center for Molecular Design. R. N.
is the Rebecca and Israel Sieff Professor of Organic Chemistry.
W. L. thanks the Fonds der Chemische Industrie for financial
support.
Notes and references
8 For alcohol oxidation with PEG-stabilized Pd-nanoparticles in scCO2,
see: Z. Hou, N. Theyssen, A. Brinkmann and W. Leitner, Angew.
Chem., Int. Ed., 2005, 44, 1346–1349.
9 P. Licence, J. Ke, M. Sokolova, S. K. Ross and M. Poliakoff, Green
Chem., 2003, 5, 99–104.
10 (a) D. J. Cole-Hamilton, Science, 2003, 299, 1702; (b) W. Leitner, Pure
Appl. Chem., 2004, 76, 635–644.
11 For an in-depth discussion of catalyst immobilization using scCO2, see:
Multiphase Homogeneous Catalysis, ed. B. Cornils, W. A. Herrmann,
D. Vogt, I. Horvath, H. Olivier-Bourbigon, W. Leitner and S. Mecking,
Wiley-VCH, 2005, ch. 7.
12 For example: (a) S. Kainz, D. Koch, W. Baumann and W. Leitner,
Angew. Chem., Int. Ed. Engl., 1997, 36, 1628–1630; (b) S. Kainz and
W. Leitner, Catal. Lett., 1998, 55, 223; (c) G. B. Coombes, F. Dehghani,
F. P. Lucien, A. K. Dillow and N. R. Foster, in Reaction Engineering for
Pollution Prevention, ed. R. P. Hesketh, Elsevier, Amsterdam, 2000; (d)
Y. Kayaki, Y. Noguchi and T. Ikariya, Chem. Commun., 2000, 2245.
13 A. M. Khenkin, L. Weiner, Y. Wang and R. Neumann, J. Am. Chem.
Soc., 2001, 123, 8531–8542.
{ H5PV2Mo10O4 (0.23 g) was dissolved in acetonitrile and stirred with
2.34 g alumina suspended in chloroform for 2 h until the solution became
colorless. The catalyst was then filtered off and dried overnight under high
vacuum.
§ H5PV2Mo10O4 (0.23 g) was dissolved in acetonitrile and PEG-1000
(1.15 g) was dissolved in chloroform. The solutions were mixed; 2.34 g
alumina was added and the suspension was stirred for 2 h until the solution
became colorless. The catalyst was then filtered off and dried overnight
under high vacuum. PEG-1000 was used because PEG with lower
molecular weight (,900) has a significant solubility in scCO2 and would be
extracted with the medium during workup (see ref. 8 and references
therein). Although PEG with high molecular weights are waxy solids at
room temperature, they show a significant melting point depression under
CO2 pressure making them available as liquid catalyst phases under mild
conditions.
" A 10 mL metal autoclave was charged with 5 mmol catalyst, 0.5 mmol
benzyl alcohol, 2 bar oxygen and ca. 6 g liquid CO2 and sealed under
pressure. The pressure was increased to ca. 170 bar upon heating to 100 uC
as the liquid becomes scCO2. After 16 h, the system was cooled down and
the pressure was released by letting CO2 slowly expand through a double
cold trap. The reaction was analyzed by GLC and GLC-MS.
14 M. Solinas, J. Jiang, O. Stelzer and W. Leitner, Angew. Chem., Int. Ed.,
2005, 44, 2291–2295.
2232 | Chem. Commun., 2006, 2230–2232
This journal is ß The Royal Society of Chemistry 2006