A magnetic nanoparticle-supported N-heterocyclic carbene-palladacycle: An efficient and recyclable solid molecular catalyst for Suzuki-Miyaura cross-coupling of 9-chloroacridine

Article abstract of DOI:10.1039/c7cc06958h

A robust magnetic nanoparticle-supported N-heterocyclic carbene-palladacycle has been readily synthesized by directly anchoring the structural defined acenaphthoimidazolylidene palladacycle with a long tail on magnetic nanoparticles (MNPs), and functioned as a solid molecular catalyst and exhibited extremely high catalytic activity towards the challenging Suzuki-Miyaura cross-coupling reactions between less-studied heterocyclic 9-chloroacridine and diverse boronic acids. Remarkably, the catalyst could be used 5 times without obvious loss of activity highlighting the efficiency of our strategy of immobilization of previledged catalysts.

Full text of DOI:10.1039/c7cc06958h

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A Magnetic Nanoparticle-Supported N-heterocyclic Carbene-
Palladacycle: An Efficient and Recyclable Solid Molecular Catalyst
for Suzuki-Miyaura Cross-Coupling of 9-Chloroacridine
Received 00th January 20xx,
Accepted 00th January 20xx
Qinyue Deng,a Yajing Shen,a Haibo Zhua and Tao Tu*a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
robust magnetic nanoparticle-supported N-heterocyclic (MNP@NHC-Pd) catalysts were therefore obtained.6,7 The
carbene-palladacycle has been readily synthesized by directly catalysts could also be fabricated directly from MNP, NHCs and
anchoring the structrual defined acenaphthoimidazolylidene palladium precursors in one pot manner.8 However, these
palladacycle with long tail on the magnetic nanoparticles (MNPs), conventional immobilization approaches always suffered from
which functioned as a solid molecular catalyst and exhibited various negative effects such as lower stability, reduced
extremely high catalytic activity towards the challenging Suzuki- catalytic activity and/or selectivity as a result of the poor
Miyaura cross-coupling reactions between less-studied accessibility, random anchoring, or disturbed geometry of the
heterocyclic 9-chloroacridine and diverse boronic acids. active sites in the solid matrix of these recyclable catalysts.
Remarkably, the catalyst could be used 5 times without obvious
Salt (3),
1) CBr4, PPh3, CH2Cl2;
loss of activity highlighing the efficiency of our strategy on the
immobilization of the previledge catalysts.
N
PdCl2,
2) p-Hydroxybenzaldehyde, K2CO3;
3) Et2NH, NaBH(OAc)3, CH2Cl2;
4) H2PtCl6 6H2O, SiH(OEt)3
OEt
Si
OEt
OH
EtO
K2CO3,
CH3CN
9
O
2
9
1
As strong σ-donors and weak π-acceptors, N-heterocyclic
carbenes (NHCs) represent one type of important robust
ligands.1 Numerous transition-metal NHC complexes have
been successfully developed and unveiled excellent catalytic
activity towards a broad number of transformations.2 Among
them, NHC-Pd complexes have been demonstrated extremely
high catalytic activity especially in the Suzuki-Miyaura cross-
coupling reactions, accessing bi-phenyl derivatives with
potential applications in pharmaceuticals and functional
organic materials.3 However, the consumption and leaching of
the noble metals of the catalysts lead to numerous waste and
environmental problems. Therefore, the immobilization of
privileged catalysts based on NHC-M complexes on various
organic matrixes or solid supports constitute one of potential
solutions.4 Amongst, magnetic nanoparticles (MNPs) have
drawn great attention because the supported catalysts could
be readily recovered by magnetically separation.5
Fe3O4
N
N
N
N
Toluene, Reflux,
ClPd N
ClPd N
Et2
Et2
OEt
Si
OEt
O
O
EtO
Fe3O4
O
Si
OEt
O
9
9
4a
5: SMNP@NHC-Pd
Scheme 1. Synthesis of MNPs-supported N-heterocyclic carbene-palladacycle
5
.
Very recently, we demonstrated novel NHC-based
palladacycles exhibited high catalytic activity towards
challenging amination of heteroaryl chlorides.9 Notably, the
extremely high thermal stability of the structure defined
palladacycles during the transformations make them as
suitable candidates for the direct immobilization on MNPs.10
Following our recent research interests in the design, synthesis
and application of various types of solid molecular catalysts,11
herein, we synthesized an acenaphthoimidazolyildene
palladacycle with long chain triethoxysilyl tail and successfully
immobilized it on the surface of MNPs. The resulting structural
defined recyclable MNPs-supported NHC-palladacycle
exhibited extremely high catalytic activity towards challenging
Suzuki-Miyaura cross-coupling reactions between heterocyclic
9-chloroacridine and diverse boronic acids.
The general approach for NHC-Pd catalyst immobilization is
anchoring firstly the imidazolium salts with terminal
triethoxysilyl groups on the MNPs’ surface via hydrosilation
reactions.6b After further metalation/coordination with
suitable palladium precursors, the MNPs supported NHC-Pd
As shown in Scheme 1, the key linker
2 was synthesized
from inexpensive and commercial available 10-undecene-1-ol
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(1) after simple bromination, etherification, ammoniation and heteroaryl chlorides with vairious boronic acids. Nowadays,
hydrosilation steps. Then the corresponding triethoxysilyl- acridine derivatives have been found various potentials in
functionalized NHC-palladacycle 4a was readily prepared from material sciences and pharmaceuticals.16,17 Despite the broad-
PdCl2, readily available acenaphthoimidazolium salt (
3) and the spectrum utility of this motif, only one approach had been
linker
2
in toluene in a good yield.12 Subsequently, the developed to synthetize aryl or alkyl substituted acridines with
palladacycle with long tail (4a) was straightforward anchored active organozinc reagents.17 Therefore, we would like to using
on the surface of the core-shell silica coated magnetic less-studied Suzuki-Miyaura coupling reactions of heterocyclic
nanoparticles (SMNPs) via hydrolysis13 to produce the desired 9-chloroacridine and phenylboronic acid as a model reaction to
supported NHC-palladacycle (5, SMNP@NHC-Pd), in which explore the catalytic activity and recyclability of newly
tertiary amine moiety may function as a hemilabile ligand14 developed SMNP@NHC-Pd.
and benefit the active Pd(0) species formation.15 The loading
Table 1. Optimization of reaction conditionsa
of Pd in SMNP@NHC-Pd is 0.01 mmol/g, which is quite
consistent from batch-to-batch and determined by inductively
Et
Et Et
Et
N
N
Cl
coupled plasma atomic emission spectrometry (ICP-AES).
N
N
Et
Et
[Cat.], K3PO4
+
Pd
N
Et
Et
Cl
B(OH)2
N
Cl
ClPd
N
Toluene, Temp
N
Et2
Cl
6
4b
4c
Entry
1
[
Cat.](mol%)
Temp (oC)
80
Yield (%)b
89
5
5
5
(1)
(1)
(1)
2
90
98
3
100
100
100
100
100
100
100
100
100
99
4
5
5
5
(0.5)
(0.4)
(0.2)
99
5
94
6
87
7
8c
9 c
10 c
11 c
4a (0.5)
99
Pd(OAc)2 (0.5)/PPh3 (1.0)
23
Fig. 1 TEM images of SMNP@NHC-Pd
recovered single particle; XPS spectra of (d) fresh prepared and (e) recovered
SMNP@NHC-Pd
5: (a) full image, (b) fresh prepared and (c)
Pd(OAc)2 (0.5)/Sphos (1.0)
79
5.
4b
4c
95
99
a Reaction was carried out with 0.15 mmol 9-chloroacridine, 3.0 equiv. K3PO4 in 4
mL toluene with catalyst under N2 at 100 oC for 24 h; Isolated yield; c Reaction
With the desired supported SMNP@NHC-Pd
5 in hand,
b
transmission electron microscopy was applied to investigate
their morphologies. As shown in Fig. 1, in comparison with
was carried out with 0.5 mmol scale.
SMNPs precursors, particles of SMNP@NHC-Pd
bigger with an average diameter of 150 nm (Fig 1a vs. Fig S1b,
†ESI). The thickness of the out layer of SMNP@NHC-Pd is ca.
5 are slightly
In the presence of 1 mol% SMNP@NHC-Pd (150 mg, 5), the
coupling reaction between 9-chloroacridine and phenylboronic
acid was carried out with K3PO4 in toluene at 80 oC, the desired
5
40 nm (Fig 1b), which is quite uniform with the observed
particles. In order to further confirm the immobilization of
palladacycles 4a on the surface of SMNPs, energy-dispersive X-
ray (EDX) study was also carried out. To our delighted, EDX
product 9-phenylacridine
6 was formed in 89% yield (Table 1,
entry 1). Further increasing the reaction temperature, the yield
was also increased (98% and 99% for 90 oC and 100 oC,
respectively, Table 1, entries 2 and 3). When other organic
solvents and bases were screened, no better results were
produced (Table S2, †ESI). To our delighted, even with 0.5
mol% catalyst loading, a quantitative yield was still obtained
(Table 1, entry 4). Especially, no difference was observed with
quantification of the SMNP@NHC-Pd particles
5 indicates the
presence of all expected elements including Fe, Pd, O and Si
(Fig. S2, †ESI). Notably, the formaꢀon of agglomerated
palladium black nanoparticles on SMNP was not observed. The
XPS studies also support this observation. Only Pd(II) signals
are found at 337.26 (3d5/2) and 342.57 (3d3/2), corresponding
to two oxidation states of palladium, which clearly indicates
the accomplishments of immobilization of palladcycles on the
surface of SMNPs. Finally, the differential scanning
calorimetry-thermogravimetric analysis (DSC-TGA) study
further highlights good thermal stability of the obtained
SMNP@NHC-Pd parꢀcles (Fig. S3, †ESI), which could be stored
under ambient condition for years.
yields for the SMNP@NHC-Pd
5 from batch-to-batch. Further
decreasing the catalyst loading, only slightly lower yields were
observed (94% and 87% for 0.4 mol% and 0.2 mol% catalyst
loadings, respectively, Table 1, entries 5 and 6), which further
confirmed the catalytic efficiency of the supported catalyst
In comparison, the palladcycle 4a the precursor for
immobilization, was also applied as a homogenous molecular
catalyst in the model reaction. Similar as SMNP@NHC-Pd , at
5.
,
5
least 0.5 mol% catalyst loading was required to achieve the
quantitative yield (Table 1, entry 7). In contrast, in the case of
homogenous catalysts generated in situ from selected
palladium precursors,16a such as Pd(OAc)2, PdCl2 and
Pd2(dba)3,and readily avaiable phosphine ligands (PPh3, BINAP
and dppf), only up to 23% isolated yield could be obtained
Due to palladacycle 4c exhibited very high catalytic activity
towards challenging amination reactions between heteroaryl
chlorides in our recent study.9 With the structure defined
supported palladacycle
5 (SMNP@NHC-Pd) in hand, we would
like to further investigate its catalytic activity and recyclability
toward challenging Suzuki-Miyaura coupling reactions of
2 | J. Name., 2012, 00, 1-3
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under the standard reaction conditions (Table 1, entry 8 and isolated yield of the corresponding acridine derivative (11) was
Table S2, entries 36-39).18 When more bulky viable catalysts still obtained. Furthermore, the naphthylboronic acids
including SPhos and 4b, which showed extremely high catalytic successfully coupled and delivered corresponding products (15
activities in a broad number of coupling reactions,18b-d were and 16) in quantitative yields, too. Heterocyclic aryl boronic
applied, the yields of 79% and 95% could be obtained, acid was also a suitable substrate; whereas, slightly low yield
respectively (Table 1, Entries 9 and 10). In contrast, when was observed (17). In general, alkyl boronic acids are regarded
homogenous palladacycle 4c was used instead, a compatible as rather poor partners in the previously homogeneous Suzuki-
isolated yield with supported catalyst
was produced (Table 1, Entry 11), further highlighting the alkenyl boronic acids resulted in excellent to quantitative
5
and its precursor 4a Miyaura studies.19 To our delighted, all selected alkyl and even
efficiency of our immobilization strategy.
yields (18-20). The protocol is readily extended to the double
Suzuki-Miyaura coupling of challenging 6,9-dichloro-2-
methoxyacridine, and a quantitative yield was obtained. In
consideration the catalytic activity of such type of NHC-
palladacycles has not been investigated in the routine Suzuki-
Miyaura coupling of chlorobenzene, a number of boronic acids
were involved. The relative positions
(S1a-c), electronic
properties (S2-S4) of substituents and heterocyclic properties
of boronic acids showed less impact on the coupling efficiency,
good to quantitative yields were achieved (72-99%, Table S6,
†ESI) which further highlighted the catalytic efficiency of
Fig. 2 Reusability of SMNP@NHC-Pd
5 in the Suzuki reaction of 9-chloroacridine
and phenylboronic acid (0.15 mmol scale)
With this good result in hand, the reusability of the catalyst
SMNP@NHC-Pd was then carried out. After the reaction
SMNP@NHC-Pd 5.
completion, the supported catalyst
5 in the model reaction
Table 2.Substrate scope
could be readily fully recovered by magnetic separation (99%,
confirmed by ICP-AES). After washing by ethanol and H2O
successively, the recovered supported catalyst could be reused
directly in the second run, by simply adding the substrates,
solvent and base. To our delighted, SMNP@NHC-Pd could be
recovered and reused for 5 consecutive trials without loss of
its catalytic activity, and almost quantitative yield was found
for each run (Fig. 2). The TEM image and XPS spectra of the
recovered SMNP@NHC-Pd also further support these
outcomes. The recovered SMNP@NHC-Pd particles after 5
runs still kept core-shell spherical morphology with ca. 35-40
nm out layer (Fig. 1c vs. 1b). The absorption peaks of Pd(II) of
the recovered SMNP@NHC-Pd particles basically remain
unchanged (Figure 1e vs. 1d). This is also agreed with the ICP-
AES study of the filtrates (Table S3). After each run, almost no
leaching palladium species was detected in the filtrate and
products. All these data clearly indicate the excellent stability
R'
Cl
N
SMNP@NHC-Pd
Toluene, K3PO4
N
Ar
+
R
B(OH)2
,
100 o
C
R'
6-21b
O
S
Me
N
N
N
N
7a: 99% (o)
: 99% (m)
: 99% (p)
6
8a: 93% (o)
9a: 67% (o)
: 97% (p)
: 99%
7b
7c
8b
9b
: 99% (p)
X
N
N
N
N
N
N
Cl
F
CF3
12a: 99% (X=H)
12b
13:
10
14
11: 53%
99%
:
99%
: 95% (X=F)
O
N
N
N
and reusability of the newly developed SMNP@NHC-Pd 5.
17: 46%c
16: 99%
15: 99%
: 99%
In order to further confirm the reaction was catalyzed by the
SMNP@NHC-Pd catalyst, but not Pd(0) nanoparticles formed in
situ, a set of mercury tests was performed under the optimal
conditions. In the presence of 0.5 mol % SMNP@NHC-Pd, one
drop of Hg was added after 0, 2, and 4 hours, corresponding
product could be obtained in 82 %, 83 %, and 91 % yields,
respectively (Table S5). These outcomes clearly indicated the
O
N
N
N
21: >99%c
19: 99%
18
: 99%
20: 93%
a Reaction was carried out with 0.15 mmol acridine, 0.5 mol%
Pd), 3.0 equiv. K3PO4 in 4 mL toluene under N2 at 100 oC for 24 h; Isolated yield;
5 (SMNP@NHC-
b
molecular catalyst property of SMNP@NHC-Pd 5.
c
With 1.0 mol%
5 (SMNP@NHC-Pd)
With the results obtained so far, the substrate scope on
various (hetero-)arylboronic acids was then explored (Table 2).
In summary, a robust magnetic nanoparticle-supported N-
Delightedly, the relative positions
properties 7-14 of substituents hardly affect coupling
efficiency. Even with ortho-methylthio substitute, usually toxic
to Pd catalysts, a moderate yield was still presented ( ). In the
case of 4-chlorophenylboronic acid, although there was a
(7a-c) and electronic
heterocyclic carbene-palladacycle has been readily developed
(
)
by
directly
anchoring
the
structural
defined
acenaphthoimidazolyildene palladacycle with long chain
triethoxysilyl tail on the magnetic nanoparticles (MNPs), which
9
functioned as
a solid molecular catalyst and exhibited
competition between the self-coupling reactions,
a 53%
extremely high catalytic activity towards challenging Suzuki-
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M. Moghadam, S. Tangestaninejad and V. Mirkhani, J. Mol.
Catal. A-Chem., 2014, 385, 78.
Miyaura coupling reactions between heterocyclic 9-
chloroacridine and diverse boronic acids. In comparison with
homogenous catalysts derived from viable ligands, the
protocol well tolerated a broad range of aryl, alkyl, alkenyl and
even heterocyclic boronic acids under mild reaction conditions
at 0.5 mol% catalyst loading and afforded the corresponding
functional acridine derivatives in excellent yields. Remarkably,
the catalyst could be used 5 times without obvious loss of
activity highlighting our strategy efficiency on the
immobilization of the privileged catalysts.
7
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Catal. Commun., 2016, 87, 78; (b) F. Martínez-Olid, R.
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9
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2334.
Financial support from the National Key R&D Program of China
(2016YFA0202902), National Natural Science Foundation of
China (No. 21572036 and 21172045), the External Cooperation
Program of Jiangxi Province (20151BDH80045), and
Department of Chemistry, Fudan University is gratefully
acknowledged.
,
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Conflicts of interest
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Lett., 2012, 14, 4250; (d) W. Fang, Q. Deng, M. Xu and T. Tu,
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