Polystyrene-supported recyclable palladacycle catalyst for Heck, Suzuki and Sonogashira reactions

Article abstract of DOI:10.1016/j.tetlet.2003.08.069

A new type of soluble polystyrene-supported palladium complex as an excellent and recyclable palladacycle catalyst was discovered for carbon-carbon bond formation in Heck, Suzuki and Sonogashira reactions. Precipitation and filtration process for recycling the catalyst was also achieved.

Full text of DOI:10.1016/j.tetlet.2003.08.069

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 7565–7568
Polystyrene-supported recyclable palladacycle catalyst for Heck,
Suzuki and Sonogashira reactions
Chih-An Lin and Fen-Tair Luo*
Institute of Chemistry, Academia Sinica, Nankang, Taipei 11529, Taiwan, ROC
Received 30 June 2003; revised 13 August 2003; accepted 20 August 2003
Abstract—A new type of soluble polystyrene-supported palladium complex as an excellent and recyclable palladacycle catalyst was
discovered for carbonꢀcarbon bond formation in Heck, Suzuki and Sonogashira reactions. Precipitation and filtration process for
recycling the catalyst was also achieved.
© 2003 Elsevier Ltd. All rights reserved.
Palladacycles have been known for over 20 years; yet
they have been used as catalysts recently.1 Nitrogen-,
phosphorus-, and sulfur-containing palladacycles are
emerging as a new family of palladium catalyst precur-
sors, and they have recently become the most simple
and efficient catalyst in applying to Heck–Mizoroki,
Suzuki–Miyaura and Sonogashira reactions.1g Herr-
mann and his co-workers have demonstrated that pal-
ladacycle 1 is thermally stable and efficient catalyst for
Heck–Mizoroki, Suzuki–Miyaura, and Sonogashira
reactions.2 It also had been shown to have high reactiv-
ity toward arylation of olefins with aryl chlorides.2e
Although the palladacycle catalyst, as Herrmann
reported, have high reactivity to promote a lot of
reactions, only a few reports on the recovery of pallada-
cycle catalysts were reported in the literature.3 Berg-
bretier and his co-workers have synthesized the
improve its recyclabilities. But, it was deactivated after
the second run.3e On the other hand, phosphorous-con-
taining PS was generally used as the ligand to trap and
reuse Pd catalysts after the reaction.4 Herein, we report
the synthesis of a new PS-supported palladacycle cata-
lyst 4 which can be used as an efficient recyclable
palladium catalyst for running Heck–Mizoroki,
Suzuki–Miyaura and Sonogashira reactions.
The synthesis of the PS-supported palladacycle catalyst
is shown in Scheme 1. Thus, trans-di(m-acetato)-bis[2-
(diphenylphosphino)-4-vinylbenzyl]dipalladium(II)
6
was synthesized from 3-(diphenylphosphino)-4-methyl-
styrene 5 by heating with Pd(II) acetate in toluene at
50°C.2a Compound 5 was prepared in moderate yield
by bromination (Br2/AlCl3)5a of 4-methylacetophenone
followed by the reduction (LAH/ether), dehydration
(KHSO4/D)5b and coupling with chlorodiphenylphos-
phine (Mg/THF; ClPPh2).5c Considering the proper
space around the palladium in the catalyst, the pallada-
cycle precursor 6 was copolymerized with 6 equiv. of
styrene at 70°C. The phosphorous-containing com-
pounds were analyzed by solution phase 31P NMR in
palladacycle
2 with (SCS)-type tridentate ligand
bounded by thermomorphic polymer. They have dis-
covered that the recyclability of 2 has been enhanced
significantly; yet its reactivity is not effective with aryl
bromides.3a–d Nowotny has suggested a new cyclopalla-
dated imine catalyst 3 bounded by polystyrene (PS) to
Keywords: polystyrene-supported palladacycle catalyst; recyclable catalyst; Suzuki coupling reaction; Heck reaction; Sonogashira reaction.
* Corresponding author. Fax: 886-2-27888199; e-mail: luoft@chem.sinica.edu.tw
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.08.069

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C.-A. Lin, F.-T. Luo / Tetrahedron Letters 44 (2003) 7565–7568
Scheme 1. Synthesis of PS-supported pallacycle catalyst 4.
CDCl3. The chemical shift of phosphorus in 5 was
appeared at −12 ppm as a singlet, while it was shifted to
17 ppm as a singlet in 6. On the other hand, the methyl
protons (3H) in 5 appeared at 2.3 ppm as a singlet, while
the methylene protons (4H) in 6 appeared at 3.25 ppm
as a broad singlet. The 31P signal of catalyst 4 was
observed at 53 ppm before and after Heck reaction, while
the 31P signal of catalyst 6 observed at 17 ppm before
Heck reaction but appeared at 17 ppm along with a new
peak at 53 ppm after Heck reaction (the peaks ratio at
either electron-rich or electron-withdrawing groups (Eq.
(1)), which produced only one major product as deter-
mined from GC and crude H NMR spectral analysis.
1
The PS-supported catalyst 4 prepared with different ratio
of styrene (mole ratio of styrene/6=3 or 9) were also
synthesized and were run as the catalyst in the above
Heck reactions. No obvious difference was observed
among these PS-supported catalyst prepared from differ-
ent ratio of styrene/6. In addition, we also tested the
applicability of catalyst 4 in Suzuki cross-coupling and
Sonogashira reactions. The Suzuki cross-coupling of
4-bromoacetophenone and phenylboronic acid (1.5
equiv.) in the presence of 0.2 mol% of catalyst 4 and
K2CO3 (1.5 mol) was proceeded well in o-xylene at 130°C
(Eq. (2)). We could isolate only the desired product in
high yields (>99%) after 5 h. The Sonogashira reaction
of 4-bromoacetophenone and phenylacetylene (1.5
equiv.) was carried out with 0.2 mol% of catalyst 4 in
Et3N at 90°C (Eq. (3)). After 72 h, we could get only one
major product in more than 98% yield. Thus, the
reactivity of catalyst 4 in our different model reaction
conditions proved to be similar to Herrmann’s pallada-
cycle catalyst 1.
17 and 53 ppm is about 7:3). The peak at 53 ppm in 31
P
NMR spectrum was become the sole peak after six times
recycling of the catalyst 6. Thus, the catalyst 6 was
polymerized completely under the reaction conditions.
To test the applicability of PS-supported palladacycle
catalyst 4, we examined the Heck reaction of methyl
acrylate and various aryl halide in the presence of
NaOAc in DMA (N,N-dimethylacetamide) as shown in
Table 1. Under various reaction conditions, all the
desired products were isolated in more than 90% yields.
Thus, the PS-supported catalyst 4 can easily promote the
Heck reaction of aryl bromides- and iodides-containing
Table 1. Heck reaction using PS-supported catalyst 4
(1)
Entry
Haloarene
Cat. 4 (equiv.)
Temp. (°C)
Time (h)
Iso. yield (%)
1
2
3
4
5
6
4-Bromobenzonitrile
Bromobenzene
4-Bromotoluene
4-Bromoanisole
Iodobenzene
0.02
0.05
0.05
0.05
0.02
0.002
100
130
130
130
100
100
10
48
48
48
8
>99
97
95
92
>99
>99
2-Bromobenzaldehyde
10

C.-A. Lin, F.-T. Luo / Tetrahedron Letters 44 (2003) 7565–7568
7567
(2)
(3)
Figure 1. Recycling of catalysts in different reactions from selected solvent.
The PS-supported catalyst 4 was soluble in THF,
DMA, DMF, CHCl3, CH2Cl2 and not very soluble in
MeOH, MeCN, hexane, and ether. Ether and MeCN
were used here to precipitate our new catalyst. Salt
removal was skipped due to the palladacycle catalyst,
which can be deactivated easily by water. Probably, the
phosphine ligand may form phosphine oxide in the
presence of water. The Heck reaction of 4-bromoben-
zonitrile and methyl acrylate (1.5 equiv.) with 2 mol%
of catalyst 4 and NaOAc (1.5 equiv.) in DMA at 100°C
was proceeded as the model reaction. Figure 1 shows
that the yield of the product can be isolated more than
80% after being recycled four times with ether as the
solvent to precipitate our PS-supported catalyst 4.
Using MeCN as the solvent may drop the yield of the
product below 60% after the second run. Figure 1 also
shows that the reactivity of Herrmann’s catalyst 1
diminished thoroughly after being recycled two times in
the model reaction. The use of catalyst 6 in the model
reaction also diminished the yields to near 20% after
recycled four times in the model Heck reaction. We also
found that the yield can be kept more than 99% after
recycling six times, using Et3N in stead of NaOAc as
the base in the model Heck reaction. The yield can be
kept more than 95% after repeating 7 times Suzuki
cross-coupling reaction (Eq. (2)), using the initial 5
mol% catalyst 4. The conversion of 4-bromoacetophe-
none can be maintained more than 90% with 5 mol%
catalyst 4 after repeating Sonogashira reactions (Eq.
(3)) four times.
In conclusion, we have synthesized a new PS-supported
palladacycle catalyst 4 which was successfully employed
in the carbonꢀcarbon bond formation to give good
yields in the Heck–Mizoroki, Suzuki–Miyaura and
Sonogashira model study. Our catalyst exhibited a high
reactivity as Herrmann’s palladacycle catalyst 1 and, in
addition to its high reactivity, it gave us a promising
solution to lower the reaction cost by its good recy-
clabilities of using pertinent organic solvents.
Acknowledgements
The authors thank the National Science Council of the
Republic of China and Academia Sinica for financial
supports.
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