Chemistry Reference
In-Depth Information
N
Pd(OAc)
2
NaBH
4
CHCl
3
n
-C
6
H
13
N
N
N
N
N
n
-C
6
H
13
N
PS
N
+
PS
N
+
N
PS = polystyrene
37
38
Scheme 14.37
(PANI).
119
The Pd-PANI catalyst system catalyzes the Suzuki coupling of aryl
chlorides in water at 100 1C with 0.05 mol% palladium. Even aryl fluorides
can be coupled under these conditions. The catalyst is recyclable for 10 re-
action cycles with palladium leaching at the ppm level. The PANI material
seems to be highly effective in preventing agglomeration of the active pal-
ladium particles. Palladium nanoparticle catalyst 38 supported on poly-
styrene modified with an imidazolium-substituted triazine (37) was effective
in the Suzuki coupling of aryl bromides and showed no decrease in activity
over five reaction cycles (Scheme 14.37).
120
Polymers with anionic substituents can also effectively host palladium
nanoparticles. Palladium nanoparticles supported on poly(4-styrenesulfonic
acid-co-maleic anhydride) (PSSA-co-MA) were applied to the Suzuki coupling
of aryl bromides in water.
121
The Pd-PSSA-co-MA catalyst system was used for
six reaction cycles with the reaction yield decreasing from 96% to 86%.
Palladium nanoparticles supported on poly(N-isopropylacrylamide-co-
potassium methacrylate) hydrogel are effective catalysts for the Suzuki
coupling of aryl iodides and bromides.
122
The hydrogel has a large water
uptake capacity. When palladium(II) ions are added, they act as cross-linking
agents and shrink the gel. Reduction of the palladium to give nanoparticles
returns the water-swelling ability of the gel. The insoluble polymer could be
recovered and reused for seven cycles with minimal decrease in activity.
A core-shell nanosphere comprising a polystyrene core and a polyacrylate
shell serves as a pH-responsive support for palladium nanoparticles.
123
Between pH 3 and 11, the shell is highly dispersed in water, allowing for
ecient interaction between the catalyst and substrates. At low pH, the shell
contracts and the particles can be separated by filtration. The catalyst was
applied to the Suzuki coupling of aryl bromides and iodides and could be
used for four reaction cycles, all of which gave quantitative yields.
Natural polymers are attractive and sustainable supports for catalysts as
alternatives to synthetic polymers. In addition, biopolymers are water-
compatible materials. Cellulose is a cheap and widely available plant-derived
material. Palladium nanoparticles supported on bacterial cellulose prepared
by fermentation of Acetobacter xylinum were highly active for Suzuki coup-
lings of aryl halides, including chlorides, in water.
124
With a catalyst loading
of 0.05 mol%, the catalyst gave high yields for four reaction cycles. The
cellulose-palladium catalyst could also be used in the Heck coupling of aryl
iodides in DMF. Chitosan [poly(b-
D
-glucosamine)]-supported nanoparticles
were applied to the Suzuki coupling of aryl iodides and bromides in water at
150 1C under microwave heating.
125
The catalyst could be used for five re-
action cycles with no decrease in product yield. Wool-supported palladium
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