Notes and references
{ In a typical procedure, 10 mM H5PV2Mo10O40 in water was treated with
an excess of zinc powder under argon. An excess of Zn0 was used to ensure
complete reduction of the polyoxometalate. After the color of the solution
turned blue indicating formation of [PVIV2Mo10O40]72 the solution was
filtered to remove the remaining Zn0 and the filtrate was diluted to 1 mM.
The solution of the reduced polyoxometalate was treated under sonication
with the appropriate amount of metal salt (see the experimental section for
full details). After 10 min the color turned yellow, indicating the re-
oxidation of the polyoxometalate and the formation of metallic
nanoparticles. The solutions remained clear to the eye.
§ The identity of the specific counteractions on the polyoxometalate
cannot be determined absolutely in solution. However, since eventually the
catalyst is supported on a matrix and the elemental analysis by EDS
indicated that one equivalent of zinc was indeed needed to reduce the
polyoxometalate, we use the formulation as indicated in Scheme 1
although it is clear that Zn2+ cations may exchange with H+ cations in the
solution.
Fig. 4 Aerobic epoxidation of 1-octene by M–POM/a-Al2O3 (black
bars–conversion (mol% 1-octene reacted), grey bars–mass balance (mol%
1-octene oxide/mol % all products-1-octene and non-detected)).
be made: (i) Most importantly, at the beginning of the reaction,
epoxides were formed very effectively with little co-formation of
additional products, for example a y25% yield for 1-octene oxide
with .90 mass balance, a y60% yield for cyclododecene oxide at
y100% mass balance, and 40–50% yield for cyclohexene oxide at
a 40–50% mass balance. (ii) There is a tendency that as the
conversion increased (especially noticeable for less reactive
terminal alkenes), the selectivity of the epoxidation reaction
decreased to approximately y50%. There was no significant
change in selectivity upon carrying out the reaction at a higher
temperature (190 uC) or lower temperature (150 uC). (iii) There
was a noticeable induction period for reactions catalyzed by Agn–
POM/a-Al2O3. (iv) POM/a-Al2O3 and Ptn–POM/a-Al2O3 (results
not shown) were inactive for alkene epoxidation. (v) The reduced,
i.e. less than 100%, mass balance was due to formation of
polymeric by-products.** (vi) Control experiments 1-octene/
solvent and 1-octene/a-Al2O3/solvent showed no reaction after
1 h and 35–40% reaction with y35% epoxide selectivity after 4 h.
This is in line with typical thermal autooxidation profiles in the
absence of metal catalysts.13 (vii) a-Al2O3 impregnated with
AgNO3, RuCl3, Agn and Run showed the formation of allylic
oxidation products and reaction profiles similar to those involving
1-octene/a-Al2O3/solvent. (viii) IR spectra after the reaction did
not show the typical absorption peaks of the polyoxometalate and
therefore indicated the decomposition (no polyoxometalate was
found in solution) of the polyoxometalate species on the alumina
surface. Thus, also a recycled catalyst did not show the same
activity/selectivity as a pristine sample.
" Reaction conditions: 0.5 mmol cyclohexene, 15 mg (5% w/w) Mn–POM/
a-Al2O3, 0.05 mmol anisole (internal standard), 0.5 mL trifluoromethyl-
benzene (solvent), 2 atm O2, 160 uC, 1 h. The non-detected products were
quantified by % missing compound relative to anisole. Anisole was shown
to be nonreactive both in situ and separately. Its use as external standard
gave statistically identical results.
I Reaction conditions: 0.5 mmol 1-methylcyclohexene, 15 mg (5% w/w)
Mn–POM/a-Al2O3, 0.05 mmol anisole (internal standard), 0.5 mL
trifluoromethylbenzene (solvent), 2 atm O2, 160 uC, 1 h. The reaction
with Run–POM/a-Al2O3 was carried out at 130 uC. The non-detected
products were quantified by % missing compound relative to anisole.
** Alkenes did not react under anaerobic conditions and oxides
(cyclohexene oxide, 1-octene oxide) were stable under reaction conditions.
Also a reaction workup involving filtration of the catalysts, evaporation of
the volatile reaction components (substrate, detected products and solvent)
followed by dissolution of the remainder in deuterated DMSO yielded an
1H NMR spectrum with peaks in the area of 3–4 ppm indicative of
oxygenated compounds as non-volatile products. These remaining
compound(s) could not be eluted under any GC/MS conditions; likely
they are polymers. The mass of these remaining compound(s) is in
accordance with the computed mass balance.
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Importantly, one can therefore conclude that it would appear
that both the Run–POM/a-Al2O3 and Agn–POM/a-Al2O3 cata-
lysts were capable of catalyzing the direct epoxidation of alkenes
with oxygen in the liquid phase. The decrease in epoxide selectivity
over time was apparently due to competing autooxidation, which
is inhibited for only a limited period of time. This may be related to
the loss of the polyoxometalate structure during the reaction.
Notably, however, addition of more H5PV2Mo10O40 to the
catalysts did not ameliorate the situation. At this time, it would
still be very presumptuous to give a mechanistic explanation for
the initial high selectivity obtained in these direct epoxidation
reactions. Given the high initial selectivity for epoxidation of
alkenes, the next stage of the research will be to carry out the
reactions in the gas phase.
11 R. Neumann and M. Dahan, J. Chem. Soc., Chem. Commun., 1995,
171–173.
12 R. A. Sheldon and J. K. Kochi, Metal-Catalyzed Oxidations of Organic
Compounds, Academic Press, New York, N.Y., 1981.
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The research was supported by the Israel Science Foundation
(Grant No. 315/03), the Minerva Foundation, and the Kimmel
Center for Molecular Design. R.N. is the Rebecca and Israel Sieff
Professor of Organic Chemistry.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 4595–4597 | 4597