Microencapsulation of osmium tetroxide in polyurea

Article abstract of DOI:10.1021/ol020225+

(Matrix presented) Osmium tetroxide has been microencapsulated in a polyurea matrix using an in situ interfacial polymerization approach. These microcapsules have been effectively used as recoverable and reusable catalysts in the dihydroxylation of olefins.

Full text of DOI:10.1021/ol020225+

ORGANIC
LETTERS
2003
Vol. 5, No. 2
185-187
Microencapsulation of Osmium
Tetroxide in Polyurea
,†
Steven V. Ley,* Chandrashekar Ramarao,† Ai-Lan Lee,† Niels Østergaard,†
Stephen C. Smith,‡ and Ian M. Shirley‡
UniVersity Chemical Laboratory, Lensfield Road, Cambridge, CB2 1EW, UK, and
Syngenta, Jealott’s Hill International Research Center, Bracknell,
Berkshire, RG42 6EY, UK
sVl1000@cam.ac.uk
Received November 4, 2002
ABSTRACT
Osmium tetroxide has been microencapsulated in a polyurea matrix using an in situ interfacial polymerization approach. These microcapsules
have been effectively used as recoverable and reusable catalysts in the dihydroxylation of olefins
Transition-metal-based catalytic processes are of vital im-
portance to pharmaceutical, agrochemical, and fine chemical
industries. A vast proportion of such catalytic metal species
are often expensive and toxic, thereby making operational
handling potentially hazardous. The current trend toward
“clean and rapid” synthesis has driven the generation of a
number of new strategies for reagent immobilization to
enable easy recovery, reuse, and disposal at an acceptable
economic cost.1 Microencapsulation, the process of entrap-
ping material in a polymeric coating, has recently been
demonstrated to be a useful alternative strategy for reagent
immobilization.2,3 Microcapsules that efficiently entrap active
material can be manufactured in sizes ranging from a few
microns to 4000 µm using a plethora of techniques.4 Polyurea
microcapsules5 are prepared by an in situ interfacial poly-
merization approach, which involves the dispersion of a
solution of isocyanates and the reagent to be encapsulated
in a suitable organic solvent into an aqueous mixture
containing surfactants. The resultant oil-in-water micro-
emulsion is allowed to cure, during which time polymeri-
zation occurs at the oil-water interface. The resultant
polyurea microcapsules are completely insoluble in aqueous
and organic solvents and have been proven to be extremely
robust without any degradation under normal reaction
conditions. It has been demonstrated that metal species such
as palladium(II) acetate can be microencapsulated in polyurea
(Pd EnCat) and used as recoverable and reusable catalysts
without significant leaching or loss of activity.2,6 It is thought
that the urea functionality, which forms the backbone of the
polymer, ligates and retains the metal species within the
polymeric matrix. The ease with which these polyurea
microcapsules could be recovered and reused prompted
investigations to encapsulate other catalytic species such as
osmium tetroxide.7
† University Chemical Laboratory.
‡ Syngenta.
(1) For example, see: (a) Ley, S. V.; Baxendale, I. R.; Bream, R. N.;
Jackson, P. S.; Leach, A. G.; Longbottom, D. A.; Nesi, M.; Scott, J. S.;
Storer, R. I.; Taylor, S. J. J. Chem. Soc., Perkin Trans. 1 2000, 3815. (b)
Kirschning, A.; Monenschein, H.; Wittenberg, R. Angew. Chem., Int. Ed.
2001, 40, 650. (c) Clapham, B.; Reger, T. S.; Janda, K. D. Tetrahedron
2001, 57, 4637.
Osmium tetroxide is the most efficient catalyst for the
synthesis of syn-diols from olefins.8 The high toxicity7 and
(2) Ramarao, C.; Ley, S. V.; Smith, S. C.; Shirley, I. M.; DeAlmeida,
N. Chem. Commun. 2002, 1132.
(3) Akiyama, R.; Kobayashi, S. Angew. Chem., Int. Ed. 2002, 41, 2602.
(4) Mars, G. J.; Scher, H. B. In Controlled DeliVery of Crop Protecting
Agents; Wilkens, R. M., Ed.; Taylor and Francis: London, 1990; p 65. (b)
Jain, R. A. Biomaterials 2000, 21, 2475. (c) Shimofure, S.; Koizumi, S.;
Ichikawa, K.; Ichikawa, H.; Dobashi, T. J. Microencapsulation 2001, 18,
13. (d) Uludag, H.; De Vos, P.; Tresco, P. A. AdV. Drug DeliVery ReV.
2000, 42, 29. (e) Tsuji, K. J. Microencapsulation 2000, 18, 137.
(5) Scher, H. B. US Patent 4,285,720, 1980.
(6) (a) Ley, S. V.; Ramarao, C.; Gordon, R. S.; Holmes, A. B.; Morrison,
A. J.; McConvey, I. F.; Shirley, I. M.; Smith, S. C.; Smith, M. D. Chem.
Commun. 2002, 1134. (b) Brenmeyer, N.; Ley, S. V.; Ramarao, C.; Shirley,
I. M.; Smith, S. C. Synlett 2002, 1843.
(7) Hammond, C. R. In CRC Handbook of Chemistry and Physics, 81st
ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL; pp 4-21.
10.1021/ol020225+ CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/24/2002

Figure 1. Scanning electron micrograph (SEM) of the polyurea microcapsule sections showing the OsO4 distribution (bright spots) within
the polyurea matrix.
expense of this volatile reagent is a deterrent for use in large-
scale operations. To overcome these issues, several groups
have adopted different strategies to immobilize this reagent
on soluble and insoluble supports, achieving varying levels
of success.9
distribution of the metal within the flocculent polyurea matrix
(Figure 1, bright spots).12 These microcapsules proved to be
effective in the dihydroxylation of a range of olefins (Table
1). The reactions were carried out at room temperature using
The preparation of polyurea microcapsules containing
osmium tetroxide (Os EnCat) is straightforward. A mixture
of osmium tetroxide (0.396 g) and polymethylene poly-
phenylene diisocyanate (SUPRASEC 5025, average func-
tionality of 2.7, 7 g) in Solvesso 20010 (10 g) was dispersed
(at 800 rpm using a Heidolph 33 mm rotary flow impeller)
into an aqueous solution containing sodium lignosulfonate
(Reax 100 M, 1.8 g),11 poly(vinyl alcohol) (Goshenol GL03,
0.6 g),11 and the polyoxypropylene polyoxyethylene ether
of butyl alcohol (Tergitol XD, 0.3 g)11 in deionized water
(45 mL). This operation resulted in an oil-in-water micro-
emulsion with a particle size range of 20-250 µm that was
gently paddle-stirred for 36 h to yield the insoluble polyurea
microcapsules. The microcapsules were filtered and washed
with deionized water and a range of organic solvents and
dried.12
Table 1. Dihydroxylations of Olefins Using Os EnCata
The use of scanning electron microscopy (SEM) helped
ascertain the metal distribution within the polymeric matrix.
The electron micrograph (backscattered mode) shows the
(8) (a) Schroder, M. Chem. ReV. 1980, 80, 187. (b) Kolb, H. C.;
VanNieuwenhze, M. S.; Sharpless, K. B. Chem. ReV. 1994, 94, 2483.
(9) (a) Cainelli, G.; Contento, M.; Manescalchi, F.; Plessi, L. Synthesis
1989, 45. (b) Nagayama, S.; Endo, M.; Kobayashi, S. J. Org. Chem. 1998,
63, 6094. (c) Salvadori, P.; Pini, D.; Petri, A. Synlett 1999, 1181. (d)
Kobayashi, S.; Endo, M.; Nagayama, S. J. Am. Chem. Soc. 1999, 121,
11229. (e) Kobayashi, S.; Ishida, T.; Akiyama, R. Org. Lett. 2001, 3, 2649.
(f) Severeyns, A.; De Vos, D. E.; Fiermans, L.; Verpoort, F.; Grobet, P. J.;
Jacobs, P. A. Angew. Chem., Int. Ed. 2001, 40, 586. (g) Choudary, B. M.;
Chowdari, N. S.; Jyothi, K.; Kantam, M. L. J. Am. Chem. Soc. 2001, 123,
9220. (h) Yao, Q. Org. Lett. 2002, 4, 2197.
(10) Solvesso 200 is a high-boiling (230-257 °C) mixture of aromatics
(predominantly naphthalenes) and a product of the Exxon Mobil Corpora-
tion.
(11) Polymethylene polyphenylene diisocyanate (SUPRASEC 5025) was
purchased from Huntsman ICI Polyurethanes; Reax 100M, Goshenol GL03,
and Tergitol XD were bought from Westvaco, British Traders and Shippers,
Ltd., and Union Carbide, respectively.
a Reagents and conditions: Os EnCat 5 mol %, NMO, 10:1 acetone/
H2O, rt, 12-24 h. a Based on isolated yields.
5 mol % (loading ) ca. 0.2 mmol/g, based on the metal
content and the assumption that there is no loss of OsO4
during the encapsulation process) of the polyurea catalyst
in a 10:1 acetone-water solvent system with N-methyl
morpholine N-oxide as the co-oxidant.13 The microcapsules
were recovered by a simple filtration and reused five times,
showing no significant loss in activity. (Table 2).
(12) see Supporting Information.
186
Org. Lett., Vol. 5, No. 2, 2003

of the formed glycols using sodium periodate (NaIO4).15 This
transformation can also be carried out using Os EnCat.
Treatment of a range of olefins with Os EnCat (2 mol %)
and NaIO4 (3 equiv) in 2:1 THF-H2O gave the required
carbonyl compounds in good yields (Table 3).12 Os EnCat
was easily recovered by a simple filtration and reused several
times.
Table 2. Recycling Experiments with Os EnCata
Table 3. Oxidative Cleavage of Olefins Using Os EnCata
entry
R1
R2
R3
yield%
1
2
3
4
5
6
7
8
n-C6H13
Ph
Ph
H
Ph
Ph
n-C6H13
Ph
H
H
H
Ph
H
H
H
Ph
H
H
Me
Ph
Ph
CO2Me
n-C6H13
H
66d (100)c
92b
79
99
98b
a Reagents and conditions: Os EnCat 5 mol %, NMO, 10:1 acetone/
95b
H2O, rt, 12-24 h. b Based on isolated yields.
55d (100)c
99b
No sign of cross-contamination was evident when the
microcapsules were used in recycling experiments with
different substrates. This suggests that the reagents and
products are not imbibed and retained in the polymer matrix
after the workup procedure (washings). When used after
storage for 10 months without any special precautions (such
as an inert atmosphere), the microcapsules were just as
effective in these oxidations. Qualitative leach test experi-
ments were carried out by stirring the microcapsules in
solution for 24 h. The microcapsules were filtered, and the
solution was used in catalytic osmylation experiments. No
reaction occurred, suggesting that significant leaching of an
active osmium species had not taken place.14
a Reagents and conditions: Os EnCat 2 mol %, NaIO4 (3 equiv), 2:1
THF/H2O, rt, 1-8 h. b Isolated and identified as the phenyl hydrazone.c GC
yield. d Isolated yield of the alcohol formed upon treatment of crude product
with PhMgBr.
In summary, another polyurea microcapsule-based catalytic
system has been developed that has proved to be effective
and easy to handle. The manufacture of these microcapsules
is cost-effective and can be easily translated to meet large-
scale requirements for industrial applications. Current work
involves the extension of this system to asymmetric dihy-
droxylation reactions. The incorporation of other chelating
and ligating functional groups within the polyurea framework
will help ascertain if any rate enhancement in these reactions
can be achieved.
Osmium tetroxide is used as a catalyst for the generation
of carbonyl compounds from olefins by oxidative cleavage
Acknowledgment. The authors thank J. Skepper (Multi
Imaging Center, Cambridge University) for microscopy. This
work was funded by Syngenta Limited.
(13) General Procedure for Dihydroxylations using Os EnCat: Os
Encat (5 mol %) was added to a solution of olefin (1 mmol) and N-methyl
morpholine N-oxide (NMO) (1.5 mmol) in 10:1 acetone-H2O (10 mL),
and the reaction mixture was stirred at room temperature for 12-24 h
(monitored by thin-layer chromatography (TLC)). The reaction mixture was
filtered, and the recovered microcapsules were washed with H2O and
acetone. The filtrate was treated with saturated aqueous sodium metabisulfite
(20 mL), extracted with ethyl acetate, and dried (MgSO4). Evaporation under
reduced pressure and purification by column chromatography gave the
products.
(14) (a) The authors refrain from referring to this catalyst system as a
heterogeneous system because the possibility of these microcapsules
functioning as a reservoir and an efficient scavenger of homogeneous
catalytic species exists. (b) Davies, I. W.; Matty, L.; Hughes, D. L.; Reider,
P. J. J. Am. Chem. Soc. 2001, 123, 10139.
Supporting Information Available: Experimental pro-
cedures and NMR data of the products. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL020225+
(15) (a) Pappo, R.; Allen, D. S.; Lemieux, R. U.; Johnson, W. S. J. Org.
Chem. 1956, 21, 478. (b) Cainelli, G.; Contento, M.; Manescalchi, F.; Plessi,
L. Synthesis 1989, 47.
Org. Lett., Vol. 5, No. 2, 2003
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