Fig. 1 (a) Scanning electron micrograph (SEM) showing the interior of a Pd(OAc)2 containing polyurea microcapsule (magnification 3 2500); (b) TEM of
a sliced Pd(OAc)2 microcapsule after chemical reaction (magnification 3 14000); (c) TEM of a sliced microcapsule showing the distribution of palladium
nanoparticles (dark spots) along a channel formed within the polyurea matrix (magnification 3 46000).
polyphenylene isocyanate (PMPPI, average functionality 2.7, 50 g) in
carried out on polyurea microcapsules (before use in chemical
dichloroethane (70 cm3) was stirred for 1 h at rt. The resulting dark solution
reaction) revealed that the metal was not bound on the surface
was added at a steady rate to an aqueous mixture containing Reax 100 M (10
and contained within the polymeric matrix and core. The
g), Tergitol XD (2.5 g) and Goshenol GL03 (5 g) in de-ionised water (250
Energy Dispersive X-Ray (EDX) pattern along the cross section
cm3) while shearing (using a Heidolph radial flow impeller, 50 mm) at 800
rpm for 2 min. The resulting oil-in-water emulsion was paddle-stirred at rt
for 16 h. The polyurea microcapsules obtained were filtered though a
of a sliced microcapsule showed an even distribution of
palladium. Similarly, the transmission electron micrograph
(TEM) of a sliced microcapsule recovered after a chemical
reaction shows an even distribution of palladium [Fig. 1(b)].
In an attempt to extend the scope of this encapsulation
approach, palladium nanoparticles stabilised by tetraocty-
lammonium bromide were prepared13 and microencapsulated in
polyurea. These nanoparticulate palladium(0) microcapsules
proved just as effective in similar Suzuki-type cross-coupling
reactions as encapsulated Pd(OAc)2. The presence of the
palladium nanoparticles (average size of 5 nm) [Fig. 1(c)] was
detected by TEM; this showed a higher metal concentration
along the channels formed within the polyurea matrix. This
suggested that the palladium nanoparticles were being stabilised
and contained by the urea-linked polymer as the washing
procedure adopted should remove all traces of the tetra-
octylammonium bromide stabiliser.
polyethylene frit (20-micron porosity) and were washed with de-ionised
water, acetone, ethanol, ether and dried.
‡ Representative procedure for Suzuki type cross-coupling reactions using
microencapsulated (MC) Pd(OAc)2 (MC-[Pd]): To a solution of 4-bromoa-
nisole (1 mmol), 4-methoxybenzene boronic acid (1.5 mmol), K2CO3 (3
mmol) in toluene–water–ethanol (4+2+1, 7 cm3) was added MC Pd(OAc)2
(5 mol%, based on palladium content) and the reaction mixture stirred at 80
°C for 12 h. The reaction mixture was diluted with ether (25 cm3) and
filtered though a polyethylene frit (20-micron porosity). The filtrate was
extracted with ether (2 3 20 cm3) and the combined organic layers were
washed with brine (20 cm3) and dried (MgSO4). Evaporation under reduced
pressure and purification by column chromatography gave 4,4’-dimethox-
ybiphenyl (87% yield). 1H NMR (400 MHz, CDCl3): d = 7.47 (d, J = 8.7
Hz, 4H), 6.96 (d, J = 8 Hz, 4H), 3.84 (s, 6H); 13C NMR (100 MHz, CDCl3):
d = 159.1, 157.8, 128.1, 114.5, 55.7; MS(EI): m/z(%): 214(100) [M+];
HRMS(EI): calcd for C14H14O2 [M+] 214.099, found: 214.099.
In summary, palladium species in the form of palladium(II
)
acetate and nanoparticulate palladium have been effectively
entrapped in a polyurea matrix. These microcapsules have
proven to be effective as recyclable catalysts for use in Suzuki
type cross-coupling reactions and do not require expensive
phosphines as co-ligands. The flexibility of this system should
allow the co-encapsulation of specific reaction-enhancing
materials and monomers leading to the generation of bespoke
polymer matrices. The following paper highlights the applica-
tions of these versatile microcap catalysts in chemical synthe-
sis.
We would like to thank I. M. McConvey (AstraZeneca), D.
O’Beirne (AstraZeneca) and D. Pears (Avecia) for discussions;
S. F. Poynter (AstraZeneca) and Dale Mortimer (Avecia) for
XRF and ICP analysis; C. Taylor (Syngenta), A. Bigley
(AstraZeneca), P. Plumb (AstraZeneca) and B. Esmaeili
(Cambridge University) for microscopy. We also thank D.
Tapolczay for early discussions during the development of this
concept. This work was funded by Syngenta.
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Notes and references
† Representative procedure for the preparation of polyurea microcapsules
containing Pd(OAc)2: a mixture of Pd(OAc)2 (5 g) and polymethylene
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