Table 1 Study on Suzuki coupling reactions using different aryl halidesa
R1
X
Conversion with catalyst A
Conversion with catalyst B
CHO
OMe
C(O)CH3
OH
Br
Br
Br
I
99%
71%
99.7%
92.5% (24 h)
98.4%
72.2%
87.6%
92.4% (24 h)
a
Dilute conditions: 0.275 mmol phenyl boronic acid, 0.25 mmol aryl halide, 0.95 mmol Na2CO3 in 10 mL 1 : 1 EtOH–water mixture, catalyst A
[PSt–PDMAA–Pd(II)] and catalyst B [PSt–PHPA–Pd(II)] at room temperature, for 8 h unless specified, with catalyst loading of ca. 0.06 mol% with
respect to aryl halide.
(Scheme 1).18,19 In a control experiment where Pd(OAc)2 was
used as catalyst, the formation of Pd black was observed,
indicating that without appropriate ligands or supports, Pd
black formation and precipitation are inevitable. In contrast,
using the polymer composite supported Pd catalyst
PSt–PHPA–Pd(II), even after 10 reaction cycles, no Pd black
was formed. This suggests that the polymer composites act not
only as supports for Pd catalysts but also as stabilizers. The
catalyst demonstrated no loss in catalytic activity even after
recycling 9 times. From XPS studies of the Pd composites after
the 1st run and after the tenth run nearly identical surface
loading of Pd was observed. The Heck coupling of a variety
of alkenes was investigated with PSt–PHPA–Pd(II) and
representative results are shown in Table 2. The catalyst is
highly effective for all the reactions involving iodobenzene as
the arylating agent. For the olefins examined, good to excellent
yields were obtained.
Table 2 Representative Heck coupling reactionsa
R1
R2
Time/h
Conversion (%)
H
H
H
2-Me
3-MeO
CO2 n-Bu
CO2H
Ph
CO2Et
CO2 n-Bu
2
2
6
6
2
499
99
85
92
99
a
Standard conditions: 1 mmol iodobenzene (or derivative), 1.5 mmol
acrylate, 1.25 mmol NEt3, catalyst PSt–PHPA–Pd(II) with catalyst
loading of ca. 0.6 mol% with respect to aryl iodide, in 1.5 mL NMP.
catalysts to be encapsulated or the reactions to be examined.
This new class of organic crosslinked functional polymers
shows promise in terms of atom economic reactions and
recyclability and thus will open new avenues for research in
the areas of catalysis and organic synthesis.
Notes and references
For Heck reactions, it is reported that even parts per trillion
levels of Pd are sometimes sufficient to catalyze the coupling.20
A filtration test was used to detect any trace amounts of
leached Pd from the supports. After the completion of the
1st Heck reaction, the Pd composites were separated by
filtration. To the filtrate were added the reactants and the
reaction mixture was heated at 90 1C for 5 h. Analysis of the
1H NMR spectrum of the reaction mixture showed the intact
presence of unreacted starting materials.
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In summary, macroporous composite resins comprising
poly(styrene-co-divinylbenzene) microspheres embedded with
poly(2-hydroxypropyl acrylate-co-glycerol dimethacrylate)
and poly(N,N-dimethylacrylamide-co-glycerol dimethacrylate)
are stable and robust supports for the immobilization of metal
catalysts. The design of the polymer composite is unique and
versatile. The hard skeleton can vary in rigidity and porosity
through the choice of monomers, diluents, porogens, cross-
linkers and the control of the degree of crosslinking. The
functional ‘soft’ gel can be specially chosen to suit the metal
14 R. Akiyama and S. Kobayashi, Angew. Chem., Int. Ed., 2001, 40,
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20 A. S. Gruber, D. Pozebon, A. L. Monteiro and J. Dupont,
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Scheme 1 General scheme for Heck coupling reactions.
5532 | Chem. Commun., 2009, 5530–5532
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This journal is The Royal Society of Chemistry 2009