Recently we described a robust and recyclable polyurea-
encapsulated palladium catalyst [Pd EnCat] that was shown
images revealed that most of the particles are very small
with a diameter of 2 nm or less. The particle indicated by
the right arrow shows 1D fringes with a d spacing of about
0.20 nm. Since the common structure of Pd metal is cubic
close packed with the unit cell parameter a ) 0.388 nm,
this d spacing corresponds to the (200) crystal planes. The
particle on the left side indicated by another arrow shows a
2D lattice image. Despite the small particle size (less than 2
nm), defects can still be observed. The relatively highly
ordered area in the left half of the particle can be regarded
as an image viewed down the [011] zone axis of the Pd unit
cell.
0
to be a highly efficient transfer hydrogenation catalyst for
chemoselective reduction of a wide range of aryl ketones to
7
0
benzyl alcohols. [Pd EnCat] was prepared by reduction of
the polyurea-coordinated Pd(OAc) with HCOOH. Reduction
with H in place of HCOOH, however, did not produce an
2
2
active catalyst. Here we disclose the characterization and
comparison of the microencapsulated palladium particles
prepared under the two different reducing conditions (H or
2
HCOOH). Further application of this catalytic system in the
reductive ring-opening of benzylic epoxides via transfer
hydrogenation was investigated. Hydrogenolysis of terminal
epoxides with conventional hydrogenation conditions was
also examined.
This difference in atomic structure could contribute to the
0
superior properties of the newly developed catalyst [Pd EnCat]
in reduction of aryl ketones. To expand the applications of
this catalyst, reductive ring-opening of epoxides by hydro-
genolysis was investigated. With use of trans-stilbene oxide
as a substrate, the hydrogenolysis reaction with 4 equiv of
The most frequently used methodology for the synthesis
of metallic nanoparticles involves the reduction of the
precursors of metal salts or complexes stabilized in homo-
geneous form. It has been reported that the atomic structure
of Pd particles in terms of morphology is dominated by the
3
Et N and HCOOH in EtOAc at 23 °C reached completion
after 5 h to give the corresponding alcohol in 99% isolated
yield. It is worth noting that further hydrogenolysis of the
alcoholic C-O bond was not observed at a detectable level
even with prolonged reaction time, illustrating a clear
advantage over Pd/C in terms of chemoselectivity.
8
reduction speed. Our rationale for using HCOOH was based
on the expectation that the polyurea-coordinated Pd(OAc)
PdEnCat] would undergo anionic ligand exchange with
2
[
formate resulting in efficient local reduction to deposit fine
nanoparticles in the support material with less agglomeration.
To test this hypothesis, two samples prepared from [PdEnCat]
The efficiency and stability of this catalyst was examined
with trans-stilbene oxide as a substrate. It was pleasing to
find that the microcapsules could be recovered by simple
filtration and reused without loss of activity. Also of
importance is that the level of palladium in the reaction
mixture after filtration was lower than the detection threshold
of 5 ppm by inductively coupled plasma analysis (ICP). The
2
by reduction with H and HCOOH respectively were
examined with use of a JEOL JEM-2010 electron microscope
operating at 200 keV. The resolution of the microscope is
approximately 0.19 nm, allowing direct imaging of the
arrangement of heavy atoms in crystalline particles even
when the particle size is very small. In the sample prepared
hydrogenolysis reaction reaches completion giving excellent
by reduction with H
2
, the majority of the particles are larger
isolated yields through 10 successive recycle runs (Table 1).9
than 5 nm in size. These particles are single domain and
well ordered (Figure 1a).
Table 1. Recycling Experimentsa
a
0
Reagents and conditions: [Pd EnCat] (5 mol %), HCOOH (4 equiv),
Et3N (4 equiv), EtOAc, 23 °C, 5 h.
A variety of other benzylic epoxides were then subjected
to the same hydrogenolysis conditions and good yields of
the homobenzylic alcohol were consistently obtained (Table
Figure 1. (a) Pd(0) particles prepared by reduction of [PdEnCat]
0
with H
2
. (b) Pd(0) particles [Pd EnCat] prepared by reduction of
[PdEnCat] with HCOOH.
2
). It is known that the regioselectivity of epoxide opening
by metal hydride depends sensitively on the epoxide
structure. For example, the reduction of cis-â-methylstyrene
oxide with LiAlH proceeds with opposite regioselectivity
4
The structure of the sample prepared by reduction with
HCOOH was drastically different (Figure 1b). HRTEM
(
7) Yu, J. Q.; Wu, H. C.; Ramarao, C.; Spencer, J. B.; Ley, S. V. Chem.
Commun. 2003, 678.
8) Lu, W.; Wang, B.; Wang, K.; Wang, X.; Hou, J. Langmuir 2003,
9, 5887.
(9) Since no obvious drop of reactivity was observed, the upper limit of
the recyclablity remains to be tested. TEM examinations of the samples
after 1 cycle and 10 cycles observed no significant change in the particle
size or distribution.
(
1
4666
Org. Lett., Vol. 5, No. 24, 2003