N-heterocyclic carbene conjugated with poly(ethylene glycol) for palladium-catalyzed Suzuki-Miyaura coupling in aqueous solvents

Article abstract of DOI:10.1007/s10562-013-1126-z

Here we report a type of N-heterocyclic carbene (NHC)- and phosphine-chelated palladium catalysts with poly(ethylene glycol) (PEG) chain for Suzuki-Miyaura crosscoupling reactions. 1-(2-Diphenylphosphinoferrocenyl)- ethyl- 3-imidazolium iodides conjugated with MeO-PEG₄₀₀ and MeO-PEG₇₅₀, respectively, have been synthesized and characterized. It was demonstrated that the salts bearing with PEG chain could act as NHC precursors successfully under the catalytic condition to form NHC-supported palladium complexes joined by phosphine. The formed palladium complexes are highly efficient for Suzuki-Miyaura coupling of aryl bromides with phenylboronic acid at the palladium loading of 0.1 mol% in both organic and aqueous solvents.

Full text of DOI:10.1007/s10562-013-1126-z

Catal Lett (2014) 144:158–164
DOI 10.1007/s10562-013-1126-z
N-Heterocyclic Carbene Conjugated with Poly(ethylene glycol)
for Palladium-Catalyzed Suzuki–Miyaura Coupling in Aqueous
Solvents
•
Ji-cheng Shi Hongwei Yu Dahong Jiang
•
•
•
Mei Yu Yanxian Huang Lanping Nong
•
•
•
Qing Zhang Zilin Jin
Received: 6 September 2013 / Accepted: 30 September 2013 / Published online: 22 October 2013
Ó Springer Science+Business Media New York 2013
Abstract Here we report a type of N-heterocyclic carbene
(NHC)- and phosphine-chelated palladium catalysts with
poly(ethylene glycol) (PEG) chain for Suzuki–Miyaura cross-
coupling reactions. 1-(2-Diphenylphosphinoferrocenyl)-ethyl-
3-imidazolium iodides conjugated with MeO–PEG400 and
MeO–PEG750, respectively, have been synthesized and char-
acterized. It was demonstrated that the salts bearing with PEG
chain could act as NHC precursors successfully under the cat-
alytic condition to form NHC-supported palladium complexes
joined by phosphine. The formed palladium complexes are
highly efficient for Suzuki–Miyaura coupling of aryl bromides
with phenylboronic acid at the palladium loading of 0.1 mol%
in both organic and aqueous solvents.
been focused on using them as ancillary ligands for tran-
sition metals [3–8]; they are alternatives to phosphine,
however, with a lower p-acceptor ability, resulting in the
coordinated metal center being richer in electron density
[9]. Compared to other organic ligands, the dissociation
energy between transition metals and NHC ligands is
higher, and consequently those complexes with NHCs have
excellent stabilities to air and moisture [10, 11]. The
electron-rich characteristics have been shown significantly
benefits in NHC-supported transition metals, such as pal-
ladium [12–19], ruthenium [20–23], rhodium [24–26], and
nickel [27–29], as catalysts for organic transformation.
Generally, homogeneous transition metal catalysis can
provide best reaction condition via ligand-tuning to achieve
high activities, while heterogeneous catalysis offers the
advantages of a simplified product separation and the
recycling of catalyst. Therefore, immobilization of homo-
geneous catalysts, including by organic polymers [30–33]
and inorganic materials [34–36], has attracted much
attention to combine both the advantages of homo- and
hetero-geneous catalysis [37, 38]. In addition, intensive
efforts have also been paid to reduce or eliminate the use of
flammable, hazardous, and expensive organic solvents in
the Green chemistry [39–42].
Keywords N-Heterocyclic carbene ꢀ PEG ꢀ
Palladium ꢀ Suzuki–Miyaura reaction ꢀ Aqueous
catalysis
1 Introduction
Since the isolation of stable free N-heterocyclic carbene
(NHC) by Arduengo st al. [1, 2] increasing attention has
Catalyst leaching often occurs in immobilized homo-
geneous catalysts; the high dissociation energies between
NHCs and many transition metals look to offer possibilities
to avoid the leaching problem [43]. Merrifield and Wang
resins have been successfully applied as supports for NHC-
transition metal catalysts. Particularly, water-soluble tran-
sition-metal-NHC complexes for aqueous-phase catalysis
have attracted more and more attentions [44–49]. Some
NHC–palladium complexes have also been tethered with
poly(2-oxazoline)s and poly(norborn-2-ene) for C–C bond-
forming reactions [50, 51]. Recently, two of poly(ethylene
J. Shi (&) ꢀ D. Jiang ꢀ M. Yu ꢀ Y. Huang ꢀ L. Nong ꢀ Q. Zhang
School of Chemistry and Life Sciences, Guangdong University
of Petrochemical Technology, Maoming 525000, China
e-mail: jchshi_mmc@126.com
H. Yu (&)
School of Chemical Engineering, Shijiazhuang University,
Shijiazhuang 050035, China
e-mail: yhw0411@163.com
Z. Jin
State Key Laboratory of Fine Chemicals, Dalian University of
Technology, Dalian 116012, China
123

N-Heterocyclic Carbene Conjugated with Poly(ethylene glycol)
159
glycol) (PEG) conjugated NHC ligands have been reported
and used for ruthenium-catalyzed olefin metathesis [52,
53]. The PEG-tag will make the incorporated catalyst
soluble in water as well as in some of organic solvents,
such as toluene and dichloromethane, however insoluble in
hexane, diethyl ether, tert-butylmethyl ether, isopropyl
alcohol, and cold ethanol, etc. The solubility character of
PEG would be beneficial for the catalysis occurring in
homogeneous phase in a wide range of solvents and useful
for separating product from catalyst [54].
imidazole to afford ferrocenylphosphine–imidazolium salts
in high yields. Therefore, a target poly(ethylene glycol)-
bound imidazolium salt could be produced by reacting 2
with the PEG-bound imidazole.
According to the literature method [59], MeO–PEG400
and MeO–PEG750 were tosylated, respectively, to give 3a
and 3b (Scheme 1). Treating 3 with imidazole in the pre-
sence of sodium hydride produced the PEG-bound imida-
zoles [4a: n = 7 (average); 4b: n = 15 (average)] in
76–86 % yields. Usually, compound 2 is treated with
excess of 1-substituent-imidazole derivative to prepare
ferrocenyl–imidazolium salts. However, it was found to be
tedious to separate the excess of 4 from the product 5 by
chromatography. Instead, 2 was used in excess and the
ferrocenylphosphine imidazolium salts conjugated PEG
were isolated in the yields of 88 % for 5a and 67 % for 5b
based on 4a and 4b, respectively. The imidazolium salts 5
are stable under nitrogen and in the air to some extent,
which were actually purified in the air, and have been
N
PPh2
OAc
PPh2
1) Imidazole
N
Fe
I-
Fe
R
2) NaI
2
1
i
t
R = Me (1a), Pr (1b), Bu (1c), 1-Ad (1d), Cy (1e),
2,6-dimethylphenyl (1f), 2,4,6-trimethylphenyl (1g),
2,6-diisopropylphenyl (1h)
1
characterized by H and 31P NMR spectra and API–MS.
ð1Þ
1
In the H NMR spectra of 5 the carbenic protons were
A series of 1-(2-diphenylphosphinoferrocenyl)ethyl-3-sub-
stituted-imidazolium iodides (1, Eq. 1) were prepared [55] and
found to be efficient in the palladium-catalyzed Suzuki–
Miyaura cross-coupling reaction (Eq 2), the catalyst Pd(OAc)2/
1d showing up to 20,000 turnover numbers for coupling
4-bromotoluene with phenylboronic acid [56]. The system
Pd(OAc)2/1 is also effective for Hartwig–Buchwald amination
of aryl bromides and chlorides [57]. These results inspired us to
prepare the water-soluble analogues of the imidazolium salts 1
for aqueous catalysis. The imidazolium salt bridged by a short
PEG chain has been found to be useful as supporting ligand for
palladium-catalyzed Suzuki–Miyaura reactions [58], but the
solubility of the supported catalyst in water is low. Towards
taking the benefits of PEG, we report here the synthesis of 1-(2-
diphenylphosphinoferrocenyl)ethyl-3-substituent-imida-
zolium iodides conjugated with different length of PEG
chain and their application in the palladium-catalyzed
Suzuki–Miyaura reaction in both organic and aqueous
solvents.
observed in the range of d 9.0–9.3 due to the highly acidic
character of 2-proton of imidazolium ions. The length of PEG
was estimated by the comparison of the area of d 3.55–3.80
contributed from the CH2 groups of the PEG-linkage with that
at d 6.0 from CH linked to ferrocene, consisting with the
starting material MeO–PEG400 and MeO–PEG750 used for
preparing 5a and 5b, respectively. The consistence could also
be visualized in the API–MS of 5, showing a series of peaks
with the mass difference of 44 for each OCH2CH2-fragment
and the richest m/z ion peak at 787 for 5a relating to n = 7. A
singlet 31P{1H} NMR peak observed around d -26.6 for 5 is
expected for ferrocenyldiphenylphosphine and the value is
close to that of the known compound 1 [55, 56].
2.2 Generation of the Palladium Catalyst In Situ
Some of palladium(II) complexes ligated with ferrocenyl-
phosphine–imidazolium salt 1 have been isolated and
characterized [55, 56], and it was found that the activity of
the isolated palladium complexes is comparable to those of
the catalyst generated in situ for Suzuki–Miyaura and
Buchwald–Hartwig reactions [57]. Before taking the
Pd/L Cat.
X
ArB(OH)
2
+
Ar
R
R
base
2
N
imidazole, NaH
TsO
OMe
N
OMe
O
O
n
n
2 Results and Discussion
3
4
2.1 Preparation of the PEG-Bound
i) 2
N
+
N
OMe
O
Fe
Ferrocenylphosphine–Imidazoliun Salts
PPh2 I-
n
ii) NaI
5
Recently, it has been established that the readily available
ferrocenyl derivative 2 [55, 56] reacts with 1-substituent-
Scheme 1 Synthesis of NHC precursors tethered with PEG
123

160
J. Shi et al.
Table 1 Influence of PEG-linkage on palladium-catalyzed Suzuki–Miyaura reactions of aryl halides with phenylboronic acid in 1,4-dioxane
Entry
ArX
Product
Ligand Pd (mol%)
Time
(h)
Yield (%)a
1
2
3
4
5
6
7
8
5a
5b
1a
5a
5b
1a
5a
1a
0.1
0.1
0.1
0.1
0.1
0.1
2
3
95
90
93
95
85
95
47b
52c
Me
Br
Br
Br
Br
Br
Br
Cl
Me
3
Me
Me
3
Me
Me
3
MeO
MeO
MeO
F3C
MeO
MeO
MeO
F3C
3
3
21
21
2
F3C
Cl
F3C
Reaction conditions: Aryl halide (5.0 mmol), phenylboronic acid (914 mg, 7.5 mmol), K2CO3 (1,382 mg, 10 mmol), 1,4-dioxane (10 mL),
Pd(OAc)2:ligand = 1:1.1, 110 °C (oil bath)
a
Isolated yield
b
GC yield calculated using decane as an internal standard
Using NaOBut as base
c
2.3 Influence of PEG Chain on the Palladium-Catalyzed
Suzuki–Miyaura Reaction in Organic Solvent
advantage of generating catalyst in situ, however, it was
still a concern of coordination ability of 5 to palladium
center due to the PEG chain presented. Treating Pd(OAc)2
with an equivalent of 5a and excess of K2CO3 in 1,4-
dioxane at 60 °C for 0.5 h, the peak at d 9.28 for 2-proton
Before studying on the PEG-bound salts 5 in palladium-
catalyzed Suzuki–Miyaura reaction in water, the catalytic
activity of 5 in 1,4-dioxane was compared firstly with 1a
for evaluating the influence of the PEG chain. For com-
parison, the catalysts from both 1a and 5 were generated
by treating Pd(OAc)2 with an equivalent of ligand and
excess of K2CO3 in 1,4-dioxane at 60 °C for 0.5 h, by this
1
of imidazolium disappeared in H NMR spectrum and the
peak of 31P resonance shifted from d -26.6 for 5a down to
d -1.0 in the 31P NMR spectrum. These facts suggested
that a palladium complex coordinated by NHC and phos-
phine was formed.
123

N-Heterocyclic Carbene Conjugated with Poly(ethylene glycol)
161
2.5 Effect of Base for the PEG-Bound Catalyst System
way the coordination of NHC to palladium has been
suggested as stated in the above section and also by
comparing the catalytic activities with those previously
reported [56].
Base plays a crucial role in the palladium-catalyzed
Suzuki–Miyaura reaction and it is necessary to match bases
with different catalysts. A brief investigation of base
influence for the PEG-bound 5b/Pd(OAc)2 system in
aqueous solvents was carried out and the results are sum-
marized in Table 3. In the coupling of 4-bromotoluene with
phenylboronic acid in aqueous solvents, the system mat-
ches well with many bases, among which the best ones are
Cs2CO3 and K2CO3 (entries 1 and 2). KF, which is fre-
quently used in palladium-catalyzed Suzuki–Miyaura
coupling reaction, showed moderate activity with 70 %
conversion (entry 8). It should be noted that K3PO4 and
KOAc are also excellent bases for the PEG-bound 5b/
Pd(OAc)2 system with 89–97 % of conversions within 1 h
(entries 6 and 7).
As can be seen from Table 1, the PEG-bound ferroce-
nylphosphine–imidazolium salts 5 as ligand gave the cat-
alyst with activity comparable to that of 1a (entries 3, 6,
and 8); for 4-methyl- and 4-methoxy-phenylbromide cou-
pling with phenylboronic acid using K2CO3 as base, nearly
quantitative conversions were achieved at 0.1 mol% of Pd
loading at 110 °C after 3 h, and the desired biaryls were
isolated in excellent yields (entries 1,2, and 4,5). For aryl
chloride substrate with electron-withdrawing group, 47 %
of conversion was achieved using 5a as ligand at 2 mol%
loading of catalyst (entry 7), comparable to 52 % using 1a
(entry 8) within experimental errors. These results mean
that the PEG chain up to 15 of OCH2CH2 groups has not
been found negative effects on their palladium catalysts.
2.4 Suzuki–Miyaura Reaction Carried Out in Aqueous
Solvents
Table 2 Pd(OAc)2/5 catalyzed Suzuki–Miyaura coupling of 4-bro-
motoluene with phenylboronic acid in aqueous solvents
Entry Co-solvent
(%)
Ligand Pd
(mol%)
Time (h) Conv (%)a
After the palladium catalyst was formed in situ, 1,4-diox-
ane was removed under reduced pressure and replaced with
aqueous solvents to evaluate the catalytic performance. In
the coupling of 4-bromotoluene with phenylboronic acid in
the presence of 1 mol% catalyst in water, it was found that
42 % of conversion of 4-bromotoluene was shown for 5a
(Table 2, entry 1) and 67 % for 5b (entry 2), respectively.
The lower activity compared to those in 1,4-dioxane could
be ascribed to the low solubility of 4-bromotoluene in pure
water, which is consistent with the findings by Shaugh-
nessy et al. [40] using water-soluble alkylphosphines as
ligands for Suzuki–Miyaura coupling of aryl bromides. To
our delight, as PEG400 was added as co-solvent in 1:1 ratio
in weight, the activity was recovered to the level in 1,4-
dioxane and nearly quantitative conversion was achieved
for 5a (entry 5) and 5b (entry 6) at 0.1 mol% of catalyst
loading although longer reaction time was needed for 5a as
ligand.
1
2
3
4
5
6
–
–
5a
5b
1
6
6
1
6
6
3
42
1
67
PEG400 (50) 5b
PEG400 (50)
1
[99
61
–
1
PEG400 (50) 5a
PEG400 (50) 5b
0.1
0.1
[99
[99
Reaction conditions:4-Bromotoluene (855 mg, 5.0 mmol), phenyl-
boronic acid (914 mg, 7.5 mmol), K2CO3 (1,382 mg, 10 mmol),
water (15 mL), Pd(OAc)2:ligand = 1:1.1, 110 °C (oil bath)
a
GC conversion was calculated using decane as an internal standard
Table 3 Effect of the base on Pd(OAc)2/5b catalyzed Suzuki reac-
tion of 4-bromotoluene with phenylboronic acid in aqueous solvents
Entry
Base
Time (h)
Conv (%)a
1
2
3
4
5
6
7
8
Cs2CO3
K2CO3
Na2CO3
KOH
1
1
1
1
1
1
1
1
[99
[99
91
It is attractive to investigate PEGs as a reusable solvent
medium for organic reactions [60], including for cross-
coupling reactions [61, 62], because of its ready avail-
ability, low volatility, and potential for recycling catalyst.
Recently, it was found that ligand-free Heck reaction of
aryl bromides can be catalyzed with Pd(OAc)2 in PEG2000
at the catalyst loading of 1 mol% [63]. However, without 5
as supporting ligand only 61 % of conversion was achieved
for 4-bromotoluene at 1 mol% of Pd(OAc)2 loading in
PEG400/water (1:1) (entry 4). This demonstrated that the
NHC as supporting ligand is essential for the catalytic
efficiency performed in the aqueous solvents.
98
NaOH
K3PO4
KOAc
KF
66
97
89
70
Reaction conditions: 4-Bromotoluene (172 mg, 1.0 mmol), phenyl-
boronic acid (183 mg, 1.5 mmol), base (2.0 mmol), water (2.5 mL),
PEG400 (2.5 g), Pd(OAc)2 (2.2 mg, 0.01 mmol), 5b (15.6 mg,
0.011 mmol), 110 °C (oil bath)
a
GC conversion calibrated with decane as an internal standard
123

162
J. Shi et al.
2.6 Scope of the System 5b/Pd(OAc)2 in Aqueous
Solvents
using K2CO3 as base in aqueous solvents. For the deactivated
substrate 4-bromoanisole 94 % of the isolated yield for the
desired product was realized within 3 h at 110 °C (entry 8).
Excellent yields in the range of 92–95 % were achieved for
phenyl, 3-methylphenyl, and 4-methylphenyl bromides
(entries 2, 4-6). For the substrate with ortho-substituent
As shown in Table 4, the procedure employing 5b/Pd(OAc)2
as catalyst is effective in coupling a range of aryl bromides
with phenylboronic acidin the presence of 0.1 mol% catalyst
Table 4 5/Pd(OAc)2 catalyzed Suzuki–Miyaura reaction of aryl bromides with phenylboronic acid in aqueous solvents
Entry Aryl bromide
Product
Ligand Time (h) Yield (%)a
O
O
1
5b
5a
5b
3
6
3
95
95
90
Br
Me
Me
2
Me
Br
Me
3
4
5
6
F3C
Br
F3C
5b
5b
5b
3
3
3
92
92
95
Br
Me
Br
Me
Me
Me
Br
Me
Me
7
8
5b
5b
3
3
93
94
Br
MeO
Br
MeO
Reaction conditions: Aryl bromide (5.0 mmol), phenylboronic acid (914 mg, 7.5 mmol), K2CO3 (1.38 g, 10 mmol), water (10 mL), PEG-400
(10 g), Pd(OAc)2 (1.1 mg, 0.0050 mmol), 5 (0.0055 mmol), 110 °C (oil bath)
a
Isolated yield
123

N-Heterocyclic Carbene Conjugated with Poly(ethylene glycol)
163
(entry 7) excellent yield was observed as well. As expected,
the catalyst system is highly effective for aryl bromides with
electron-withdrawing substituents (entries 1 and 3).
d 3.38 (s, 3 H), 3.56–3.76 (m, 28 H), 4.12 (t, J = 5.2 Hz,
2 H), 7.01 (br, 1 H), 7.04 (br, 1 H), 7.55 (br, 1 H).
4.2.2 MeO–PEG750–imidazole (4b)
3 Conclusion
4b was prepared according to that for 4a. Yield 86 %,
1
yellow liquid. H NMR (CDCl3, 400 MHz, TMS): d 3.38
(s, 3 H), 3.55–3.76 (m, 64 H), 4.12 (t, J = 5.2 Hz, 2 H),
In summary, the ferrocenylphospine–imidazolium salts
conjugated with different length of PEG chains have been
synthesized, and their structures along with those of their
palladium catalysts have been characterized by H and 31P
7.01 (br, 1 H), 7.04 (br, 1 H), 7.58 (br, 1 H).
1
4.2.3 Synthesis of MeO–PEG400–Imidazolium Salt (5a)
NMR spectra and API–MS measurement. The palladium
catalysts supported by the PEG-bound phosphine–carbene
ligand are efficient for Suzuki–Miyaura coupling reaction
of aryl bromides with phenylboronic acid in both organic
and aqueous solvents, although PEG400 as co-solvent is
crucial for the catalyst activity in aqueous solvents through
increasing the solubility of aryl bromides used.
A mixture of 2 (1.5 g, 3.3 mmol) and 4a (1.1 g, 2.7 mmol) in
CH3CN (20 mL) and water (5 mL) was stirred at 50 °C for
48 h. Volatiles were evaporated and NaI (3.0 g, 20 mmol)
was added along with 20 mL of EtOH. The mixture was
stirred at 35 °C for 12 h. The solvents were evaporated and
the residue was redissolved in CH2Cl2 and filtered through
Celite. Flash column chromatography [dichoromethane–
methanol (10/1, V/V)] afforded 5ain 88 % yield as an orange
liquid. 1H NMR (CDCl3,400 MHz,TMS): d 2.07 (d,
J = 7.4 Hz, 3H), 3.47 (s, 3 H), 3.57–3.73 (m, 28 H),
3.92–4.97 (m, 8 H), 5.95–6.00 (m, 1 H), 6.85–7.45 (m, 12 H),
9.28 (m, 1 H); 31P {1H} NMR (CDCl3, 161 MHz): d -26.6.
4 Experimental
4.1 General Considerations
All aryl bromides and chlorides (Aldrich or Acros) were
used as received. Palladium acetate was purchased from
Strem Chemical Company. 1,4-Dioxane and THF were
distilled under nitrogen from sodium benzophenone ketyl
prior to use. Cs2CO3, K2CO3, Na2CO3, KF, NaOBut,
K3PO4 and NaOH were used as received. All reactions and
manipulations involving air- and/or moisture-sensitive
compounds were carried out using standard Schlenk tech-
nique under nitrogen. NMR spectra were recorded on a
BRUKER DRX 400 MHz or Varian INOVA 400 MHz
4.2.4 MeO–PEG750–Imidazolium Salt (5b)
5b was prepared according to that for 5a in 67 % yield as
1
an orange liquid. H NMR (CDCl3, 400 MHz, TMS): d
2.05 (d, J = 6.8 Hz, 3 H), 3.38 (s, 3 H), 3.57–3.65 (m, 64
H), 4.00 (t, 2 H, J = 6.4 Hz), 3.92–4.94 (m, 8 H),
5.95–6.05 (m, 1 H), 6.64–7.45 (m, 12 H), 9.05 (s, 1 H); 31
{1H} NMR (CDCl3, 161 MHz): d -26.7.
P
1
(400 MHz for H NMR; 162 MHz for 31P NMR) spec-
4.3 Suzuki–Miyaura Coupling Reaction
trometers. Mass spectroscopies (HRMS, API) were
obtained on a Micromass Q-Tof Micro (Micromass Inc.,
Manchester, England).
4.3.1 General Procedure
In air, imidazolium salt, Pd(OAc)2 and base (2 equiv of
based on aryl halide) were added to an oven-dried Schlenk
tube equipped with a stir bar. The Schlenk tube was fitted
with a rubber septum, evacuated, and then refilled with
nitrogen. 1,4-Dioxane (5.0 mL) was added via syringe. The
mixture was stirred for 30 min at 60 °C for generating
NHC-coordinated palladium complex. If the catalytic
reaction was carried out in aqueous solvents, 1,4-dioxane
was evaporated under reduced pressure. Then aryl halide,
phenylboronic acid (1.5 equiv), and decane were added.
The tube was sealed and immersed in a silicon oil bath at
110 °C and the course of the reaction was monitored by
GLC. After the reaction was complete, the product was
extracted with ether for three times, purified by column
4.2 Preparation of Ligand
4.2.1 Synthesis of MeO–PEG400–Imidazole (4a)
A mixture of sodium hydride (0.45 g, 13 mmol) and imid-
azole (0.60 g, 8.8 mmol) in 20 mL of freshly distilled THF
was stirred for 30 min at rt. To the mixture a solution of
PEG400–tosylate 3a (5.80 g, 11 mmol), which was synthe-
sized according to a procedure analogous to that previously
reported [64], was added and stirred at 60 °C for 6 h. The
reaction mixture was filtered and concentrated under reduced
pressure. The crude product was purified by column chro-
matography (acetone–methanol from 5:1 to 2:1, V/V). Yield
1
1
76 %, yellow liquid. H NMR (CDCl3, 400 MHz, TMS):
chromatography, and identified by H NMR.
123

164
J. Shi et al.
Acknowledgments We gratefully thank the financial support from
The National Natural Science Foundation of China (20971023), The
National Natural Science Foundation of Fujian Province (2011J01033),
and Guangdong University of Petrochemical Technology.
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