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that with Pd/Al2O3.
Then, the catalysts were applied to carbon–carbon
cross-coupling reactions (Eqs. 1–4). In the Suzuki–Miy-
aura reaction between phenylboronic acid and bromo-
benzene, the activity of 1 was about two times higher
than that of 2. It is also higher than those of palladium
nanospheres3b and palladium clusters.3k Thus, the other
coupling reactions were carried out with 1. While 1 was
effective for the activation of aryl iodides and aryl bro-
mides, it was not active enough toward aryl chloride.
Except the Stille reaction, the yields of the coupling reac-
tions were over 80%. The reusability of 1 was tested in
the Suzuki–Miyaura reaction between phenyl boronic
acid and methyl 4-bromobenzoate; the catalyst could
be reused three times without losing activity.
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3543.
In summary, we developed a simple method for prepar-
ing recyclable palladium catalysts from readily available
reagents, and demonstrated their catalytic activities in
the hydrogenation of various alkynes and alkenes and
in the carbon–carbon cross-coupling reactions. Other
metal-catalyzed reactions including asymmetric ones
are under study.
1 (100 mg, 0.75 mol %)
K3PO4 (2 mmol)
R
Ph
(eq 1)
Ph B(OH)2
(1.5 mmol)
Toluene, 110 oC, 5 h
X = I, R = COOMe (100%)
X = Br, R = H (86 % after 12 h)
X = Br, R = COOMe (100%)
X = Br, R = CHO (98 %)
X = Br, R = CF3 (98 %)
X = Cl, R = COOMe (<5%)
1 (100 mg, 0.75 mol %)
X
R
CuI (4.0 mol %)
Ph
(eq 2)
R
Ph
Et3N/DMF
110 oC, 20 h
+
8. During the preparation of 1 and 2 triphenylphosphine was
oxidized into triphenylphosphine oxide, which was recov-
ered from the filtrate in more than 90% yields. Meanwhile,
most of the tetra(ethylene glycol) was incorporated in the
catalysts, and less than 10% of the employed amount was
(1.5 mmol)
X = I, R = COOMe (87%)
1 (100 mg, 0.75 mol %)
NaOAc (2 mmol)
Ph
Ph
(eq 3)
(eq 4)
DMF, 120 oC, 17 h
(1.5 mmol)
1
R
detected by H NMR in the filtrate.
X = I, R = COOMe (80%)
9. The inductively coupled plasma (ICP) analysis revealed
that the palladium contents of 1 and 2 are 0.59 and
0.79wt%, respectively, and that palladium is entrapped in
the catalysts in more than 95% yields.
1 (100 mg, 0.75 mol %)
CsF (2.2 mmol)
Bu3Sn
R
Dioxane, 120 oC, 40 h
(1.5 mmol)
X = I, R = COOMe (68%)
10. Following the procedures similar to that for 1, various
oligo(ethylene glycol)s were employed to make the corre-
sponding gels to compare the catalytic activity: no
ethylene glycol (30% of the activity of 1 in the hydrogen-
ation of trans-stilbene), ethylene glycol (3%), di(ethylene
glycol) (57%), tri(ethylene glycol) (12%), tetra(ethylene
glycol) (100%), penta(ethylene glycol) (12%), hexa(ethyl-
ene glycol) (36%), poly(ethylene glycol) (Mn 400: 18%; Mn
550: 29%).
Acknowledgements
This work was supported by KOSEF through the SRC
program and by MOE through the BK21 project.
11. When the amount of Pd(PPh3)4 was reduced to 50% with
maintaining the amounts of other components, the activity
of the resulting catalyst decreased to about 50% of 1.
Meanwhile, increasing the amount of Pd(PPh3)4 did not
increase the activity.
References and notes
1. Negishi, E.-I. In Handbook of Organopalladium Chemistry
for Organic Synthesis; Negishi, E., Ed.; John Wiley &
Sons: New York, 2002.
12. The palladium on charcoal would cause ignition when it is
exposed to air after the use under hydrogen.
13. In the filtrate palladium was not detected by ICP analysis.