methodology for creating insoluble and highly active cata-
lysts is needed.
the reactions were performed under organic solvent-free
conditions.
For the reasons mentioned above, we decided to focus on
a self-assembled process between non-cross-linked am-
phiphilic copolymer ligands and an inorganic species for the
preparation of highly active, insoluble catalysts.5 For ex-
ample, a tungsten catalyst was formed from phosphotungstic
acid and poly(N-isopropylacrylamide) with an ammonium
salt.5a This catalyst, used in ppm molar equiv, brought about
an efficient epoxidation of allylic alcohols. Since this self-
assembled catalyst exhibited great potential, we consequently
applied the concept to palladium chemistry (Scheme 1). In
A novel palladium-polymer catalyst with structure 3 was
prepared, as shown in Scheme 2.6 Random polymerization
Scheme 2. Preparation of a Recyclable and Assembled
Catalyst 3 for the Suzuki-Miyaura Reaction
Scheme 1. Concept for the Preparation of an Assembled
Catalyst of Palladium and a Non-Cross-linked Amphiphilic
Copolymer
of 4-diphenylstyrylphosphine (4) with 12 mol equiv of
N-isopropylacrylamide (5) in the presence of 4 mol % of
AIBN gave 2 in 89% yield.7 The ratio of the phosphine and
1
the amide units in 2 was 1/10 (determined by H NMR
measurements in CDCl3), and the phosphine unit was hardly
oxidized in this polymerization (as shown by 31P NMR).
Complex 3 was prepared by self-assembly of 2 and
(NH4)2PdCl4 (1) with use of the method for the preparation
of PdCl2(PPh3)2.8 All solvents were degassed by ultrasoni-
cation and argon substitution prior to use. To a well-stirred
solution of 2 (0.36 mmol in phosphine) in THF (72 mL)
was added a solution of 1 (0.12 mmol) in H2O (30 mL), and
the mixture was again degassed. After the mixture was stirred
for 62 h at room temperature, a yellow precipitate A was
formed. Water (30 mL) was added to the suspension, and
THF was removed at 80 °C for 4 h with Dean-Stark
equipment to give a reddish precipitate. This precipitate was
stirred at 100 °C successively in H2O (100 mL) for 12 h, in
THF (100 mL) for 3 h, and in H2O (100 mL) for 12 h to
wash the unreacted palladium species and polymers. After
drying in vacuo (ca. 0.1 mmHg), a dark red solid 3 was
obtained in almost quantitative yield. Complex 3 was
insoluble in water and organic solvents such as acetone,
CH3OH, CH2Cl2, AcOEt, THF, and hexane, whereas 2 was
soluble in organic solvents such as CHCl3, CH2Cl2, and THF.
To obtain further information on the structure of the
catalyst, we analyzed 3 by gel-phase 31P NMR in CDCl3.
The peaks of 2 were observed at -2.9 ppm (ArPh2P) with
trace peaks at 32.1 ppm (ArPh2PdO) and 27.3 ppm. Catalyst
3 swelled so much in CDCl3 that broad peaks were detected
at 32.5 (major) and 26.1 ppm (minor) and assigned as the
signals of ArPh2PdO and PdCl2(PPh2Ar)2, respectively.9 This
this letter, we report the development of a new palladium
catalyst and its application to the Suzuki-Miyaura reaction.
It is noteworthy that the reactions were promoted by only
50-500 ppm mol equiv of the catalyst, which was reused
10 times without any decrease in its activity. Furthermore,
(4) For recent developments and improvements for heterogeneous
catalysis of the Suzuki-Miyaura reaction, see: (a) Marck, G.; Villiger,
A.; Buchecker, R. Tetrahedron Lett. 1994, 35, 3277-3280. (b) Jang, S.-B.
Tetrahedron Lett. 1997, 38, 1793-1796. (c) Fenger, I.; Le Drian, C.;
Tetrahedron Lett. 1998, 39, 4287-4290. (d) Uozumi, Y.; Danjo, H.;
Hayashi, T. J. Org. Chem. 1999, 64, 3384-3388. (e) Zhang, T. Y.; Allen.
M. J. Tetrahedron Lett. 1999, 40, 5813-5816. (f) Li, Y.; Hong, X. M.;
Collard, D. M.; El-Sayed, M. A. Org. Lett. 2000, 2, 2385-2388. (g) Parrish,
C. A.; Buchwald, S. L. J. Org. Chem. 2001, 66, 3820-3827. (h) Mubofu,
E. B.; Clark, J. H.; Macquarrie, D. J. Green Chem. 2001, 3, 23-25. (i)
Akiyama, R.; Kobayashi, S. Angew. Chem., Int. Ed. 2001, 40, 3469-3471.
(j) Cammidge, A. N.; Baines, N. J.; Bellingham, R. K. Chem. Commun.
2001, 2588-2589. (k) Barnard, C. F. J.; Griffin, K. G.; Froelich, J.; Ekman,
K.; Sundell, M.; Peltonen, R. Chem. Ind. 2001, 82, 563-571. (l) Bedford,
R. B.; Cazin, C. S. J.; Hursthouse, M. B.; Light, M. E.; Pike, K. J.; Wimperis,
S. J. Organomet. Chem. 2001, 633. 173-181. (m) Buchmeiser, M. R.;
Schareina, T.; Kempe, R.; Wurst, K. J. Organomet. Chem. 2001, 634, 39-
46. (n) Kosslick, H.; Mo¨nnich, I.; Paetzold, E.; Fuhrmann, H.; Fricke, R.;
Mu¨ller, D.; Oehme, G. Microporous Mesoporous Mater. 2001, 44-45,
537-545. (o) Gordon, R. S.; Holmes, A. B. Chem. Commun. 2002, 640-
641. (p) Sakurai, H.; Tsukuda, T.; Hirao, T. J. Org. Chem. 2002, 67, 2721-
2722.
(5) (a) Yamada, Y. M. A.; Ichinohe, M.; Takahashi, H.; Ikegami, S. Org.
Lett. 2001, 3, 1837-1841. (b) Yamada, Y. M. A.; Ichinohe, M.; Takahashi,
H.; Ikegami, S. Tetrahedron Lett. 2002, 43, 3431-3434.
(6) Bergbreiter and co-workers reported excellent homogeneous pal-
ladium catalysts prepared from poly(N-isopropylacrylamide) copolymers
as soluble and thermoresponsive polymers: (a) Bergbreiter, D. E. Catal.
Today 1998, 42, 389-397. (b) Bergbreiter, D. E.; Liu, Y.-S.; Osburn, P. L.
J. Am. Chem. Soc. 1998, 120, 4250-4251. (c) Bergbreiter, D. E.; Case, B.
L.; Liu, Y.-S.; Caraway, J. W. Macromolecules 1998, 31, 6053-6062. For
examples of polymer-protected palladium nanoparticles for hydrogenation,
see: (d) Chen, C.-W.; Akashi, M. Polym. AdV. Technol. 1999, 10, 127-
133. (e) Chen, C.-W.; Chen, M.-Q.; Serizawa, T.; Akashi, M. Chem.
Commun. 1998, 831-832.
(7) The molecular weight of 2 was wide-ranging (ca. 5000-70000), as
checked by gel-permeation chromatography relative to polystyrene standards.
(8) Mann, F. G.; Purdie, D. J. Chem. Soc. 1935, 1549-1563.
3372
Org. Lett., Vol. 4, No. 20, 2002