particles of metal complex, forming capsules. A solvent is
then added to harden the capsule walls, any excess of the
metal complex is removed by washing, and then the capsules
are used.
To test the MC-VO(acac)2 as a catalyst for epoxidation
reactions, we chose first to investigate its use in the TBHP-
mediated epoxidation of geraniol. This reaction is well
represented in the literature3 and has been the focus of other
studies using supported analogues of MC-VO(acac)2, proving
to be a useful yardstick for determining the effectiveness of
our immobilized catalyst. The solvents most used for the
metal-catalyzed homogeneous epoxidation of allylic alcohols
are either benzene10,11 or, with better results, chloroalkanes22
such as dichloromethane or chloroform. We considered these
chlorocarbons to be unsuitable for our study due to expected
solvation of the MC-VO(acac)2 and therefore screened a
range of other solvents. We found that acetonitrile, toluene,
and diethyl ether led to low yields of product and pronounced
metal leaching but, somewhat surprisingly, in hexane a 93%
yield of product was obtained when using MC-VO(acac)2.
This is particularly interesting when bearing in mind the
observation both by ourselves and others20 that aliphatic
hydrocarbons are not effective as solvents for oxidation
reactions catalyzed by 1. What is equally noteworthy is that
these results are obtained at room temperature as compared
to the elevated temperatures often used in the case of
epoxidations using 1 or other supported oxyvanadium
complexes in low-polarity solvents such as benzene. There-
fore, the activity of 1 is totally changed by encapsulation.
Being able to perform the reaction in hexane has significant
advantages over the use of benzene or chlorocarbons in terms
of ease of use, environmental impact, and toxicity. The MC-
VO(acac)2 is easily removed at the end of the reaction by
simple filtration. To show the reusability of MC-VO(acac)2,
we used the same batch four times in the epoxidation of
geraniol, with a simple washing between runs. As shown in
Table 1, the activity was only slightly decreased after the
In this Letter we report the synthesis of microencapsulated
VO(acac)2 [MC-VO(acac)2] and its use in epoxidation
reactions. VO(acac)2 1 is one the best and most extensively
used catalysts for epoxidation of unsaturated alcohols.3
Mainly studied by Sharpless et al., 1/tert-butyl hydroperoxide
(TBHP) was shown to be a highly regio- and stereoselective
system for the epoxidation of allylic10-12 and homoallylic
alcohols.13
There have been a number of reports of immobilization
of 1 and VO3- onto derivitized polystyrene resins. Examples
of supports used include acetylacetone,14-16 phosphono-
methyl,17,18 and hydroxyethylamino17,18 derivitized poly-
styrenes. The vanadium complexes have also been attached
to ion-exchange resins.19,20 In all these cases, the catalytic
activity of the heterogeneous complexes has been found to
be superior to that of the homogeneous oxovanadium
complexes; however, there are problems associated with the
supported complexes. Often it is not possible to recycle them,
and leaching of the metal off the support during the course
of the reaction is also highlighted. We felt that by immobiliz-
ing 1 using microencapsulation it may be possible to
maximize the advantages of heterogenization of increased
activity and ease of use and minimize the disadvantages of
leaching and nonrecyclability.
MC-VO(acac)2 was prepared using the Kobayashi protocol
using 1 g of polystyrene and 200 mg of VO(acac)2.21
Measurement of the mass increase together with elemental
analysis of the resultant MC-VO(acac)2 indicate that 160 mg
of VO(acac)2 is encapsulated. Once prepared, the MC-VO-
(acac)2 can be stored in air at room temperature for several
months without loss of activity.
Table 1. Reuse of MC-VO(acac)2 in the Epoxidation of
Geraniol and Determination of Metal Leaching
(10) Sharpless, K. B.; Michaelson, R. C. J. Am. Chem. Soc. 1973, 95,
6136.
(11) Sharpless, K. B.; Verhoeven, T. R. Aldrichimica Acta 1979, 12,
63.
(12) Itoh, T.; Jitsukawa, K.; Kaneda, K.; Teranishi, S. J. Am. Chem. Soc.
1979, 101, 159.
(13) Mihelich, E. D.; Daniels, K.; Eickhoff, D. J. Am. Chem. Soc. 1981,
103, 7690.
(14) Bhaduri, S.; Khwaja, H.; Khanwalkar, V. J. Chem. Soc., Dalton
Trans. 1982, 445.
run
yield of epoxide (%)
metal leaching
(15) Bhaduri, S.; Ghosh, A.; Khwaja, H. J. Chem. Soc., Dalton Trans.
1981, 447.
(16) Kumar, A.; Das, S. K. J. Catal. 1997, 166, 108.
(17) Yokoyama, T.; Nishizawa, M.; Kimura, T.; Suzuki, T. M. Chem.
Lett. 1983, 1703.
(18) Yokoyama, T.; Nishizawa, M.; Kimura, T.; Suzuki, T. M. Bull.
Chem. Soc. Jpn. 1985, 58, 3271.
(19) Linden, G. L.; Farona, M. F. Inorg. Chem. 1977, 16, 3170.
(20) Pesiri, D. R.; Moritia, D. K.; Walker, T.; Tumas, W. Organometallics
1999, 18, 4916.
1
2
3
4
93
84
83
81
5
0.097%
550 ppm
275 ppm
118 ppm
5a
a Using 1 (0.1 mol %) in the place of MC-VO(acac).2.
(21) Polystyrene (1.000 g, purchased from Aldrich Ltd., average Mw
)
280 000) was dissolved at 70 °C in cyclohexane (50 mL), and to this solution
was added VO(acac)2 (0.2 g). This mixture was stirred for 1 h at this
temperature and then slowly cooled to 0 °C with Vigorous stirring. The
polystyrene solidified around the metal catalyst dispersed in the solution.
Hexane (30 mL) was added to harden the capsule walls. The mixture was
stirred at room temperature for 1 h, and the capsules were washed with
hexane several times, to remove unencapsulated VO(acac)2, and then dried
under vacuum for 18 h at rt.
fourth run. The leaching of the metal in each run was
determined by ICP analysis and found to decrease from
0.097% in the first run down to 118 ppm in the fourth. To
(22) Sheldon, R. A.; Van Boorn, J. A.; Schram, W. A.; De Jong, A. J.
J. Catal. 1973, 31, 438.
1520
Org. Lett., Vol. 4, No. 9, 2002