A water-soluble palladium-salen catalyst modified by pyridinium salt showing higher reactivity and recoverability for Heck coupling reaction

Article abstract of DOI:10.1016/j.molcata.2014.09.031

A novel and water-soluble palladium-salen catalyst modified by pyridinium salt was synthesized and characterized by NMR, HRMS, FT-IR, ICP, elemental analysis and UVvis spectroscopy. The catalyst can efficiently catalyze the Heck reaction between aryl halides and olefins. The reaction using 0.5 mol% cat-alyst can achieve 98.3% conversion with 99% selectivity. The catalytic activity of catalyst modified bypyridinium salt is higher than that of un-modified one. Furthermore, the catalyst can be easily separatedfrom reaction system by water wash and subsequently recycled for six runs without significant loss ofactivity and selectivity.

Full text of DOI:10.1016/j.molcata.2014.09.031

Accepted Manuscript
Title: A water-soluble palladium-salen catalyst modified by
pyridinium salt showing higher reactivity and recoverability
for Heck coupling reaction
Author: Linghua Yuan Yun Xu Xingbang Hu Guoqiang Yang
Youting Wu
PII:
S1381-1169(14)00434-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.09.031
MOLCAA 9292
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
6-1-2014
20-9-2014
23-9-2014
Please cite this article as: L. Yuan, Y. Xu, X. Hu, G. Yang, Y. Wu, A water-soluble
palladium-salen catalyst modified by pyridinium salt showing higher reactivity and
recoverability for Heck coupling reaction, Journal of Molecular Catalysis A: Chemical
(2014), http://dx.doi.org/10.1016/j.molcata.2014.09.031
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A water-soluble palladium-salen catalyst modified by pyridinium
salt showing higher reactivity and recoverability for Heck coupling
reaction
Linghua Yuan, Yun Xu, Xingbang Hu∗, Guoqiang Yang, Youting Wu*
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093,
P. R. China
Abstract: A novel and water-soluble palladium-salen catalyst modified by pyridinium
salt was synthesized and characterized by NMR, HRMS, FT-IR, ICP, elemental
analysis and UV-Vis spectroscopy. The catalyst can efficiently catalyze the Heck
reaction between aryl halides and olefins. The reaction using 0.5% mol catalyst can
achieve 98.3% conversion with 99% selectivity. The catalytic activity of catalyst
modified by pyridinium salt is higher than that of un-modified one. Furthermore, the
catalyst can be easily separated from reaction system by water wash and subsequently
recycled for six runs without significant loss of activity and selectivity.
Keywords: Water-soluble palladium-catalyst; Ionic salt; Heck coupling; Recycle;
Ionic liquid
∗Corresponding authors:
Tel: +86 2583 596665; Fax: +86 2583 336599
E-mail Address:huxb@nju.edu.cn (X. B. Hu); ytwu@nju.edu.cn (Y. T. Wu)
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1. Introduction
Carbon-carbon bond construction is the basis of synthesizing many important
compounds. Catalysts containing transition metal represent the most useful tools to
achieve cross-coupling reactions under mild conditions with high yields [1, 2].
Palladium catalysts (Pd-catalysts) have been widely used in Heck reaction,
Suzuki-Miyaura coupling, and Sonogashira reaction [3-13]. In particular,
palladium-catalyzed cross-coupling reactions of aryl halides for the formation of new
carbon-carbon bonds have wide application in organic synthesis.
Homogeneous Pd-catalysts have been extensively studied in synthetic organic
chemistry because of their higher catalytic activity and selectivity [1, 2, 14]. However,
one drawback of this kind of reaction is the separation and recovering of the catalyst
when the reaction is completed. Reusing Pd is crucial because it is quite expensive,
especially for large industrial practice. Binding the active metal complexes to
insoluble or water-soluble carriers is an attractive strategy allowing separation by
filtration or extraction [1, 4, 5-9, 15-34]. Supporting Pd-catalysts on ionic liquid (IL)
or ionic salt is an effective method to recycle these expensive compounds [1, 23-27,
31]. A serious of palladium complexes have been supported on ionic salt, such as
triethylammonium ion-tagged diphenylphosphine palladium complex [23],
pyridyl-functionalized
imidazolium-based
Pd-catalyst
[24],
imidazolium
salt-coordinated palladium complex [25], sodium 4-(N-benzylideneamino)
-benzenesulfonate based Pd-catalyst [26], and ionic Porphyrin Pd-catalyst [27]. These
supported Pd-catalysts are homogeneous in reaction and can be reused by water wash
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[23-27]. Not all IL or ionic salt supported Pd-catalysts are water-soluble.
1-glycyl-3-methyl imidazolium chloride-palladium (II) complex was found to be a
heterogeneous catalyst for Heck reaction [31].
To reduce the amount of Pd consumed in the reaction, besides recycling it,
improving the reactivity of Pd-catalyst is also a crucial method. Though IL or ionic
salt modification have achieved great success on recycling Pd-catalyst [1, 17-31],
little research concerns how to achieve recoverability and higher reactivity by the
same IL or ionic salt modification. To achieve this aim, efforts should be made to
design new systems in which a cooperative effect between the carriers and the metal
center is well established. Basing on theoretic calculations, we had reported a strategy
to support homogeneous catalyst by IL or ionic salt, which can enhance the reactivity
of a catalyst [35-36].
Following the previously theoretic results [35-36], the focus of this paper is to bring
out a new palladium-salen catalyst modified by pyridinium salt. This catalyst is not
only recyclable, but also show higher reactivity compared with the un-modified one.
2 Experimental and computational methods
2.1. Materials and methods
The reagents and solvents were analytical grade and purchased from commercial
sources. They were used without further purification unless otherwise indicated.
The NMR spectra were detected by Bruker ARX 500NMR spectrometer, using
d6-DMSO or CDCl3 as solvents with TMS as the internal standard. Elemental analysis
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was performed on an Elementar vario EL II. ESI-MS were obtained on a Finnigan
LCQ mass spectrometer. FT-IR spectra were recorded on a Bruker APEX-III
spectrometer using KBr pellets in 400-4000 cm-1 region. ICP were recorded on
Optima 5300DV. The progress of reaction was monitored by GC5890C gas
chromatograph fitted with a SE-30 column (50 m, 0.025m diameters) and a hydrogen
flame ionization detector. The ultraviolet-visible light (UV-vis) spectra were recorded
on a UV-vis SPECORD 200 spectrophotometer.
2.2. Catalyst preparation
2.2.1. Synthesis of Pd-[Salen-Py][Br]2
The palladium-salen catalyst modified by pyridinium salt (Pd-[Salen-Py][Br]2) was
prepared as described in Fig. 1a. The [Salen-Py][Br]2 was synthesized according to
our previous procedure [37].
The Pd-[Salen-Py][Br]2 was prepared by mixing Pd(OAc)2 (0.1 mol),
[Salen-Py][Br]2 (0.1 mol) and 50 ml water in a round bottom flask. The bottom was
put in a 130°C oil bath to evaporate the water. During this period, fresh water was
added continuously to ensure that there was enough water for evaporating. This
process was persisted until the pH of the evaporating steam being neutral. After
filtered and dried in vacuum, brown solid Pd-[Salen-Py][Br]2 was obtained. Yield:
81.5%. 1H-NMR (500 MHz, d6-DMSO):δppm 9.15-9.16 (d, 4H, CHpyrid), 8.83-8.87 (t,
2H, CHpyrid), 8.33-8.35 (t, 4H, CHpyrid), 3.69 (t, 4H, CH2), 3.33 (s, 6H, H2O), 1.71 (s,
6H, CH3), 1.68 (s, 6H, CH3). 13C-NMR (126 MHz, d6-DMSO) δppm 174.52(N=C),
160.63(O-C), 150.24(Cpyrid), 148.10(Cpyrid), 129.82(Cpyrid), 121.39(=C-), 57.25(CH2),
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23.96(CH3), 17.86(CH3). Elemental analysis, Calculation for C22H26Br2N4O2Pd.3H2O:
C, 37.82; H, 4.58; N, 8.02. Found: C, 38.19; H, 4.84; N, 8.27. ICP, Calculated Pd:16.4,
Found Pd: 16.2. HRMS (m/z): [M-2Br]2+ Calculation for [C22H26N4O2Pd]2+,
242.0546; found, 242.0537. ESI-MS, Calculation for [M-2Br]2+ m/z=242.06 , Found
m/z= 242.17. FI-IR (cm-1): 3348, 3060, 2921, 1622, 1587, 1467, 1409, 1356, 1291,
1262, 1228, 1161, 1125, 1095, 1052, 1024, 964, 786, 743, 696, 649, 636, 592, 470.
2.2.2. Synthesis of Pd-Salen
Un-modified Pd-Salen was prepared as described in Fig. 1b. Salen-1 (0.1 mmol)
was added to the Pd(OAc)2 (0.1 mmol) in acetonitrile (100 ml), and then the mixture
was stirred at room temperature for 8h. The resulted mixture was filtered and washed
with dichloroethane, and the obtained light yellow solid was dried in vacuum.
1H-NMR (500 MHz, CDCl3):δppm 4.90 (s, 2H, CH); 3.44-3.45 (t, 4H, CH2); 2.05 (s,
6H, CH3); 2.00 (s, 6H, CH3). 13C-NMR (126 MHz,, CDCl3) δppm 177.81(N=C),
161.74(O-C), 98.92(=CH-), 55.83(CH2), 25.27(CH3), 20.54(CH3). Elemental analysis,
Calculation for C12H18N2O2Pd: C, 43.85; H, 5.52; N, 8.52. Found: C, 43.57; H, 5.62;
N, 8.50.
2.3. General procedures for the Heck reaction
In a typical catalytic reaction, 25 mL three-neck round bottom flask was charged
with Pd-[Salen-Py][Br]2 (0.05 mmol), aryl halide (10.0 mmol), olefin (12.5 mmol),
NEt3(10.0 mmol), and DMF (15 mL). The reaction mixture was heated to desired
temperature and stirred till completion of the reaction. The reaction was monitored
using a gas chromatograph. After completion of the reaction, a white precipitate
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appeared after water (10 mL) was added to the flask. The products was separated by
simple filtration. The products were identified by GC-MS analyses. The purity of the
product was determined by GC.
2.4. Recovery of the catalyst
After the reaction was completed, the reaction mixture was cooled down to room
temperature. Then, water was added and the organics was extracted with diethyl ether.
The aqueous solution was separated from the organic layer. The mixture of the
catalyst and the DMF was evaporated to dryness under reduced pressure.
Subsequently, the catalyst was used without further purification.
2.5. Computational methods
All the structures were fully optimized by the DFT based B3LYP method. 6-31G
basis set was used for these atoms except Pd and Br. It was necessary to take the
relativistic effect into consideration for the transition metal and heavy element [35,
36]. Hence, LANL2DZ basis set was used for Pd and Br atoms. Geometric
configuration optimization, energy calculation, frequency calculation, and zero-point
energy correction were performed by using the same basis set. Three different spin
state (singlet, triplet, and quintuplet) were taken into consideration. All calculations
were performed with the Gaussian 03 suite of programs [38].
3. Results and discussion
The catalytic activities of Pd-[Salen-Py][Br]2 were examined in the Heck reaction.
The coupling reaction of idodobenzene with styrene was used as a model system
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(Table 1). The amount of catalyst used in the reacton is an important issue for
coupling-reaction based on Pd compound due to its high price, especially for large
industrial practice [1, 2]. A control experiment indicated that the coupling reaction
did not occur in the absence of catalyst even after 10 h (Table 1, entry 1). Using 0.1%
mol Pd-[Salen-Py][Br]2, the desired products can be produced with a TOF value of
37.8 (Table 1, entry 2). Using 0.5% mol Pd-[Salen-Py][Br]2, the highest TOF can be
obtained (Table 1, entry 3).
The influence of reaction temperature, solvent, and additive were investigated
(Table 1). The results demonstrated that the temperature plays a significant role in the
reaction rate. TOF value significantly increased from 0.4 to 24.8 with only 1% mol
catalyst when the temperature increased from 70°C to 100°C (Table 1, entries
4-7).The obtained catalyst exhibited excellent solubility in water, DMF, DMSO, and
NMP, but slightly dissolved in p-xylene and acetonitrile. However, reaction gave
satisfied conversion only when DMF, DMSO, or DMF+H2O was used as solvent
(Table 1, entries 3, 9-14). About the base used in this reaction system, liquid organic
bases are better choice, such as Et3N and Bu3N (Table 1, entries 3 and 18). Inorganic
bases with poor solubility are not recommended because of the low conversions
(Table 1, entries 15 and 17).
For all the reactions investigated here, the trans:cis stereoselectivities range from
6.7:1 to 15.1:1 (Table 1). Using K2CO3 as base gives the highest trans:cis selectivity,
but the conversion is low in this case (Table 1, entry 15). In cases whose conversions
are larger than 90%, the trans:cis stereoselectivities range from 6.7:1 to 10.5:1. These
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selectivities are close to the same reactions catalyzed by Pd(OAc)2 supported by IL
(trans:cis=6.7) [39], PdCl2{MeSC6H4-2-(CH2PPh2)} (trans:cis=8.7) [40], palladium
polyether phosphinite (trans:cis=11.5) [41], and cyclopalladated imine complexes
(trans:cis=6.8-7.4) [42], but higher than these catalyzed by palladium/SBA-15
nanocomposites (trans:cis=5.3) [43] and Pd(OAc)2/1- (a-aminobenzyl)-2-naphthols
(trans:cis=0.6) [44].
Though IL or ionic salt modification have been widely used to recover the
homogeneous catalyst [1, 17-31, 45-60], little effort has been performed on how to
achieve recoverability and higher reactivity by the same modification. Basing on
theoretic researches [35-36], we proposed that a conjugated linkage between the ionic
salt and catalytic active center should be beneficial to enhance the reactivity of the
catalyst. Herein, the experimental results confirm this idea. A comparison between the
ionic salt modified catalyst (Pd-[Salen-Py][Br]2) and the un-modified one (Pd-Salen)
shows that this ionic modification remarkably enhance the catalytic activity of the
Pd-Salen without losing any reaction selectivity (Fig. 2). In the first hour of reaction,
the conversion with Pd-[Salen-Py][Br]2 reaches 95.1%, whereas that with Pd-Salen is
only 3.6% under the same condition. Though prolonging the reaction time to 8 hours,
the conversion with Pd-Salen is only 19.5%.
To understand the origin of the higher activity of Pd-[Salen-Py][Br]2, theoretical
calculations were performed for both Pd-Salen and Pd-[Salen-Py][Br]2 (Fig. 3). The
ground state of these two catalysts are singlet. It was found that ionic salt
modification shortens the Pd-O bond length by 0.011 Å. As a result, the Pd in
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Pd-[Salen-Py][Br]2 becomes more postive. The most important is that, comparing
with Pd-Salen, the ionic salt modification drastically lowers the energy level of the
lowest-unoccupied molecular orbitals (LUMO) by 1.56 eV and reduces the gap
between the LUMO and the highest-occupied molecular orbitals (HOMO) by 1.52 eV,
which will make it easier for electron transfer.
The catalytic activity of Pd-[Salen-Py][Br]2 is also slightly higher than that of
Pd(OAc)2 which is the most widely used catalyst for coupling reaction (Fig. 2) [1, 2].
At the same time, Pd-[Salen-Py][Br]2 can be easily reused whereas Pd(OAc)2 can not.
The reusability of the catalyst is a crucial issue especially when using expensive
palladium. The ionic salt modified catalyst Pd-[Salen-Py][Br]2 can be easily separated
from reaction system by simple extracted from diethyl ether/H2O system. The catalyst
can be reused at least 6 times without significantly loss in catalytic activity and
selectivity (Table 2).
Because the conjugated linkage between the ionic salt and catalytic active center,
the variation of the anion will influence the electron structure of metal center. The
anion of the catalyst was changed by the reaction between Pd-[Salen-Py][Br]2 and
-
-
-
-
AgX (X =NO3 , BF4 , CH3CO2 , PF6 ) in situ. The UV-Vis results of catalysts with
different anions show that the anion has influence on the electron structure of metal
center (Fig. 4). As a result, the reactivity of these catalysts change with the variation
-
of the anion (Table 3). Br- and NO3 were better choice for this target reaction.
Because changing the anion is quite easy, the reactivity of these ionic salt
modification catalysts are thus easily controllable.
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The reactivity of this new catalyst for a series of aryl halides and alkenes were
investigated (Table 4). Iodobenzene, 4-iodotoluene reacted with styrene at 100°C to
give satisfying conversion in 98.3 and 94.8% respectively (Table 4, entries 1 and 2).
However, 4-iodoanisole gave a lower conversion of 44.7% (Table 4, entry 3).
Bromobenzene and chlorobenzene gave almost no coupling products, which is in
agreement with the use of Pd(OAc)2 under similar reaction conditions (Table 4,
entries 4 and 5)[61]. For methyl acrylate, n-butylacrylate, and methyl methacrylate,
the TOF values ranged from 7.4 to 41.7 (Table 4, entries 6-8).
4. Conclusions
A novel and water-soluble palladium-salen catalyst modified by pyridinium salt
have been synthesized through a simple method. This catalyst can efficiently catalyze
the Heck reaction between aryl halides and olefins. The reactivity of catalyst modified
by ionic salt is higher than that of un-modified one. The types of counteranion in ionic
compounds could affect the catalytic activity of the catalysts. Furthermore, the
catalyst can be easily separated from reaction system by water wash and subsequently
recycled for six runs without significant loss of activity and selectivity.
Acknowledgement:
This work was supported by the National Natural Science Foundation of China
(No. 21176110) and Jiangsu Province Natural Science Foundation (BK20141311).
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15
Page 15 of 26

Figure Captions
Fig. 1 The synthesis of (a) Pd-[Salen-Py][Br]2 and (b) Pd-salen
Fig. 2 Heck coupling between iodobenzene and styrene catalyzed by 0.5% mol
Pd-[Salen-Py][Br]2, Pd-Salen or Pd(OAc)2
Fig. 3 (a) The optimized structure of Pd-[Salen-Py][Br]2 and Pd-salen. Values in
parentheses are charge carried by Pd. Other values are bond length in Å. (b) The
HOMO and LUMO of Pd-[Salen-Py][Br]2 and Pd-salen. The shown values are
relative energy of the orbits.
Fig. 4 The UV-Vis spectra of Pd-[Salen-Py][Br]2+AgX.
16
Page 16 of 26

Table 1 Effect of different parameters on the Heck coupling between iodobenzene and
styrene. a
catalyst
Ph
I
+
+
solvent,base
Ph
Ph
Trans
Cis
Entry
Temperature
Base
Solvent
Cat
Time
(h)
Con
(%)b
TOF
(h-1)c
Selectivity (%)d
(Trans:Cis) e
(mol%)
(℃)
1
2
100
100
100
100
70
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
K2CO3
DMF
DMF
0
10
4
1.1
—
>99 (2.9:1)
>99 (7.1:1)
>99 (9.4:1)
>99 (6.7:1)
>99 (10.9:1)
>99 (10.7:1)
>99 (8.5:1)
>99 (7.3:1)
>99 (10.3:1)
—
0.1
0.5
1
15.1
98.3
99.1
2.9
37.8
49.2
24.8
0.4
3
DMF
4
4
DMF
4
5
DMF
1
7
6
80
DMF
1
7
32.4
87.5
98.3
15.4
N.R
91.6
90.3
54.8
77.2
20.9
48.2
32.6
98.5
4.6
7
90
DMF
1
7
12.5
24.6
3.1
8
110
100
100
100
100
100
100
100
100
100
100
DMF
1
4
9
H2O
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
10
10
8
10
11
12
13
14
15
16
17
18
p-xylene
DMF+H2O
DMSO
CH3CN
NMP
—
22.9
30.1
13.7
38.6
4.2
>99 (8.9:1)
>99 (10.5:1)
>99 (8.8:1)
>99 (9.6:1)
>99 (15.1:1)
>99 (9.6:1)
>99 (8.9:1)
>99 (8.2:1)
6
8
4
DMF
10
10
10
10
C2H5ONa
NaHCO3
Bu3N
DMF
9.6
DMF
6.5
DMF
19.7
a
Reaction condition: iodobenzene (10mmol), styrene (12.5mmol), base (10mmol), 15ml solvent
and Pd-[Salen-Py][Br]2 as catalyst.
b Conversion of iodobenzene determined by GC.
c Turnover Frequency (TOF) = (mole of iodobenzene consumed)/[(mole of catalyst)×time (h)]
d Selectivity = (concentration of trans and cis product)/(total concentration of products)×100.
e The ratio of trans and cis products.
17
Page 17 of 26

Table 2 Reusing of catalysta
Run Times
Con (%)b
Selectivity (%)c
(Trans:Cis) d
>99 (9.4:1)
>99 (9.8:1)
>99 (9.9:1)
>99 (10.0:1)
>99 (9.9:1)
>99 (8.7:1)
Entry
1
2
3
4
5
6
Fresh
98.3
93.1
94.8
99.3
88.3
92.4
1
2
3
4
5
a
Reaction condition: iodobenzene (10mmol), styrene (12.5mmol), Et3N base (10mmol), 15ml
DMF and 0.05mmol Pd-[Salen-Py][Br]2 as catalyst.
b Conversion of iodobenzene determined by GC.
c Selectivity = (concentration of trans and cis product)/(total concentration of products)×100.
d The ratio of trans and cis products.
18
Page 18 of 26

Table 3 The effect of anions on the reactivity of the catalysts. a
Entry
Additive
Conversion b
TOF
(h-1)c
Selectivity (%)d
(Trans:Cis) e
1
2
3
4
5
None
AgCH3CO2
AgPF6
98.3
95.0
90.0
97.1
54.8
49.2
47.5
22.5
48.7
13.7
>99 (9.4:1)
>99 (9.0:1)
>99 (8.4:1)
>99 (8.5:1)
>99 (7.7:1)
AgNO3
AgBF4
a Reaction condition: iodobenzene (10mmol), styrene (12.5mmol), Et3N base (10mmol), 10ml
DMF, 0.05mmol Pd-[Salen-Py][Br]2 and 0.10 mmol AgX as additive, 100 for 4 hour.
b Conversion of iodobenzene determined by GC.
c Turnover Frequency (TOF) = (mole of iodobenzene consumed)/[(mole of catalyst)×time (h)]
d Selectivity = (concentration of trans and cis product)/(total concentration of products)×100.
e The ratio of trans and cis products.
19
Page 19 of 26

Table 4 Heck coupling reaction between different aryl halides and alkenes a.
Entry
R1
X
R2
Time
(h)
Con
(%)b
TOF
(h-1)d
Selectivity (%) e
(Trans:Cis) f
1
2
3
4
5
6
7
8
H
CH3
OCH3
H
I
I
Ph
4
4
98.3
94.8
44.7
N.R
0.21
58.8
83.4
57.2
49.2
47.4
22.4
—
>99 (9.4:1)
>99 (7.7:1)
>99 (5.7:1)
—
Ph
I
Ph
Ph
4
Cl
Br
I
10
10
16
4
H
Ph
0.03
7.4
>99 (72.8:1)
>99 (9.1:1)
>99
H
COOCH3
COO(CH2)3CH3
MMA c
H
I
41.7
28.6
H
I
4
>99 (4.6:1)
a
Reaction condition: aryl halides (10mmol), alkenes (12.5mmol), Et3N base (10mmol), 10ml
DMF, 0.05mmol Pd-[Salen-Py][Br]2, 100 .
b Conversion of halides determined by GC.
c Methyl methacrylate
d Turnover Frequency (TOF) = (mole of iodobenzene consumed)/[(mole of catalyst)×time (h)]
e Selectivity = (concentration of trans and cis product)/(total concentration of products)×100.
f The ratio of trans and cis products.
20
Page 20 of 26

ꢀ Highlights
ꢀ
A water-soluble palladium-salen catalyst modified by pyridinium salt was
synthesized.
ꢀ
ꢀ
This catalyst can efficiently catalyze the Heck reactions.
This catalyst is not only recyclable, but also show higher reactivity compared
with the un-modified one.
21
Page 21 of 26

*Graphical Abstract (for review)
Page 22 of 26

Figure 1
Page 23 of 26

Figure 2
Page 24 of 26

Figure 3
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Figure 4
Page 26 of 26