A highly efficient catalyst for Suzuki-Miyaura coupling reaction of benzyl chloride under mild conditions

Article abstract of DOI:10.1039/c4ra06551d

Diarylmethane derivatives, essential building blocks in organic synthesis, are usually synthesized through the traditional electrophilic substitution reaction i.e., Friedel-Crafts reaction, which suffers from the rearrangement and weak reactivity of aromatic compounds with deactivating/electron-withdrawing groups. The Suzuki-Miyaura coupling reaction of low-cost benzyl chloride as an alternative method overcomes these defects. Pd(ii) organometallic catalysts immobilized on the triphenylphosphine-functionalized microporous knitting aryl polymer (KAPs(Ph-PPh₃)-Pd) as a novel heterogeneous catalyst was employed in Suzuki-Miyaura coupling reaction of benzyl chloride and exhibited excellent catalytic activity under mild conditions with a turnover frequency (TOF) up to 76 min^-1 (4569 h^-1). This work reveals that the microporous structure of the catalyst can rapidly adsorb substrates, consequently facilitating their interaction, and eventually promoting the catalytic efficiency. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra06551d

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A highly efficient catalyst for Suzuki–Miyaura
coupling reaction of benzyl chloride under mild
Cite this: RSC Adv., 2014, 4, 36437
conditions†
a
b
a
b
a
b
Zhenhong Guan,‡ Buyi Li,‡ Guoliang Hai, Xinjia Yang, Tao Li* and Bien Tan*
Diarylmethane derivatives, essential building blocks in organic synthesis, are usually synthesized through the
traditional electrophilic substitution reaction i.e., Friedel–Crafts reaction, which suffers from the
rearrangement and weak reactivity of aromatic compounds with deactivating/electron-withdrawing
groups. The Suzuki–Miyaura coupling reaction of low-cost benzyl chloride as an alternative method
overcomes these defects. Pd(II) organometallic catalysts immobilized on the triphenylphosphine-
functionalized microporous knitting aryl polymer (KAPs(Ph-PPh
3
)-Pd) as a novel heterogeneous catalyst
Received 2nd July 2014
Accepted 11th August 2014
was employed in Suzuki–Miyaura coupling reaction of benzyl chloride and exhibited excellent catalytic
ꢀ1
ꢀ1
activity under mild conditions with a turnover frequency (TOF) up to 76 min (4569 h ). This work
reveals that the microporous structure of the catalyst can rapidly adsorb substrates, consequently
facilitating their interaction, and eventually promoting the catalytic efficiency.
DOI: 10.1039/c4ra06551d
www.rsc.org/advances
Up to now, many types of homogeneous catalysts have been
used for coupling reactions of benzyl halides, including palla-
Diarylmethane derivatives are fundamental building blocks in dium chloride and palladium complexes with phosphine,
Introduction
10
11–19
2
0–23
24–26
27
organic synthesis and their preparation is an important N-heterocyclic carbene,
industrial goal. They have been exploited to produce biologi- ligands
oxime,
acetonitrile, N-based
and nickel complex etc. These reactions were
cally active compounds, polybenzyls as well as building carried out in a mixture of an organic solvent and an aqueous
28–31
32
1,2
3,4
5
–7
ꢁ
blocks in organic synthesis, and usually can be obtained inorganic base at room temperature to 130 C, with Pd loading
through the traditional electrophilic substitution reaction, i.e., of 0.1–10 mol%, frequently in the presence of substoichiometric
Friedel–Cras reaction. But the weak reactivity of the aromatic amounts of TBAB and good to excellent yields were obtained.
12
compounds containing electron-withdrawing groups and the Monteiro and co-workers demonstrated that Pd(PPh
3 2
)(OAc)
tendency of alkyl/aryl halides to rearrange seriously limit showed high activity for the coupling of benzyl bromides and
6
,8,9
industrial applications.
Hence, the replacement of this chlorides with aryl boronic acids, at low catalyst loading (1
ꢁ
method by so-call Suzuki–Miyaura cross-coupling reaction mol%) and mild conditions (80 C) for 19 h (not optimized)
employing a benzyl halide could be highly benecial to over- furnishing diarylmethane derivatives in high yields (86–99%).
25
come these problems, benetting from the use of less toxic More recently, Lo and Lam developed a new strategy to
materials, boronic acids and esters, mild and operationally easy enhance the efficiency of coupling reaction by microwave irra-
reaction conditions, tolerance to many functional groups and diation using a uoro oxime-based palladacycle catalyst. The
its suitability for sterically hindered substrates, especially high catalyst gave 92% yield of diphenylmethane within 3.5 min at
ꢁ
efficiency and selectivity of Suzuki–Miyaura cross-coupling 140 C in the coupling of benzyl chloride and phenylboronic
reaction with palladium catalysts.
acid, and its polymer-supported analogue showed a low yield
ꢁ
(86%) at 100 C, thus necessitating longer time to get the high
product yield. The heterogeneous catalytic systems for this
a
Key Laboratory for Large-Format Battery Materials and System Ministry of Education,
12–16
reaction were, however, not much demonstrated yet.
Corma
anchored the oxime carbapalladacycle
2
, polystyrene (PS), poly ethylene glycol bis(me-
School of Chemistry and Chemical Engineering, Huazhong University of Science and
Technology, Luoyu Road No. 1037, Wuhan, 430074, China. E-mail: taoli@mail.
33
and co-workers
complex on SiO
hust.edu.cn; Tel: +86-27-87557350
b
Institute of Polymer Science and Engineering, School of Chemistry and Chemical thacrylate) (PEA) and MCM-41, and PdL@SiO
gave 99% yield of
2
Engineering, Huazhong University of Science and Technology, Luoyu Road No. 1037,
product in the presence of 5 mol% Pd, in water at reux
Wuhan, 430074, China. E-mail: bien.tan@mail.hust.edu.cn; Tel: +86-27-87558172
Electronic supplementary information (ESI) available: Nitrogen adsorption and
temperature. Reusability of the PdL@SiO catalyst was investi-
2
†
gated using 4-chloroacetophenone and phenylboronic acid as
desorption isotherms, pore size distributions, XPS spectra, catalyst recycling data
1
substrates without any signicant change in the reactivity and
and HNMR. See DOI: 10.1039/c4ra06551d
34
selectivity aer 7 cycles. Later, N ´a jera performed the reaction
‡
Both Zhenhong Guan and Buyi Li contributed equally to this work.
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56
using a polymer-bound palladacycle derived from Kaiser oxime positively affect the catalytic activity. Herein, aiming to obtain
resin as a pre-catalyst and the catalyst was recovered by ltra- highly reactive activity toward the Suzuki–Miyaura coupling of
tion and reused for 3–9 cycles with up to 5% of Pd leaching in the challenging benzyl chlorides and overcome the negative
each run. Yang synthesized bulky N-heterocyclic carbene (NHC) effect of electron-withdrawing groups, KAPs(Ph-PPh )-Pd was
3
precursor in the framework of several hybrid materials: SBA- employed as heterogeneous catalyst in this work. Strikingly, it
3
5
36
1
6, mesoporous ethane–silica and core–shell-structured enables cross-coupling reactions of benzyl chloride in an
37
ꢁ
nanoporous organosilica microspheres. Apparently, in these ethanol solution quickly (5 min) under mild conditions (80 C)
catalysis systems, to obtain a high yield, a relatively long time with a turnover frequency (TOF) of up to 76 min
and high temperature were required. The low dispersion and
ꢀ
1
.
protection for palladium by meso- or macro-pores were believed
to be one of the reasons for low reaction rate. For this reason,
there is still a need to nd a method and strategy to prepare a
new solid catalyst so as to overcome these limitations.
Experimental section
Materials
2 3
Zeolites, silicas, and activated carbon have been extensively Benzene, triphenylphosphine, PdCl , FeCl (anhydrous), meth-
employed as heterogeneous catalysts given their high surface anol 1,2-dichloroethane (DCE), and acetonitrile were obtained
areas and substantial porous structure, however, there still are from National Medicines Corporation Ltd. of China, all of
limits due to their inorganic chemical structure. Zeolites, which were of analytical grade and were used as received.
silicas, activated carbon with monotonous micropore (pore Formaldehyde dimethyl acetal (FDA, Alfa Aesar, 98%) was also
sizes <2 nm) are not suitable for free diffusion of reaction used as received.
38
substrate and product. Their inorganic crystal structures
rigidify the micropore size and pore volume, and stop the
solvent swell the materials. Although mesoporous or macro- Synthesis of KAPs(Ph-PPh
porous aluminosilicate and zeolites are developed to settle the
5
6
)
3
3
FeCl (anhydrous 9.75 g, 0.06 mol) was added to a solution of
diffusion puzzle of microporous analogue, they tend to
decompose in acidic and basic circumstance, especially for long
reaction time or under hydrothermal condition. The post-
modication of porous inorganic materials as universal
method to gra ligand tend to block pores and the low ligand
loading is another defect. Besides, most porous inorganic
materials are used in organic catalysis system, result in weak
affinity in the interface between catalyst and reaction
benzene (1.56 g, 0.02 mol), triphenylphosphine (5.25 g, 0.02
mol) and FDA (4.56 g, 0.06 mol) in 20 mL DCE. The resulting
mixture was efficiently stirred initially at room temperature and
ꢁ
then at 45 C for 5 h to form original network, and then heated
ꢁ
at 80 C for 67 h to react completely. The resulting precipitates
were washed 3 times with methanol, then washed with meth-
anol in a Soxhlet for 24 h, and nally dried under reduced
ꢁ
pressure at 60 C for 24 h. Elemental analysis: C, 82.75; H, 5.67;
39
substrate. Comparing to inorganic porous materials, porous
P, 1.90.
40,41
organic polymers (POPs)
with high surface area have several
merits. (i) Most of MOPs are stable in acidic and basic envi-
4
2,43
44,45
ronment.
(ii) The porous size of POPs is adjustable
and
56
3
Preparation of KAPs(Ph-PPh )-Pd catalyst
43,46
amorphous POPs can swell in solvent,
for mass transfer. (iii) The particular advantage of POPs is to
introduce a broad range of useful chemical functionalities
within the porous framework by one-step. The introduction of
Tr ¨o ger's base into conjugated microporous polymers (CMPs) for
heterogeneous catalysis is an early example in this regard. The
use of metal–organic CMPs has been demonstrated for hetero-
geneous catalysis, and highly stable and porous cross-linked
polymers are reported as efficient photocatalysts. In addi-
tion, microporous phthalocyanine polymer with metal ions
and covalent organic frameworks (COFs) with palladium cata-
which are favourable
The Pd(II) organometallic catalyst immobilized on the KAPs(Ph-
PPh
PPh
3
3
) support was prepared by coordinating Pd(II) ions with the
ligand. Briey, 1.0 g KAPs(Ph-PPh ) was added to 20 mL
acetonitrile solution containing 0.10 g PdCl and stirred under
reux at 80 C for 12 h. The solid was separated by centrifuga-
tion, washed repeatedly with acetone, followed by Soxhlet-
extraction with acetone to remove physisorbed Pd(II) species.
47
3
2
ꢁ
48
49
ꢁ
50
The nal catalyst was dried at 60 C under vacuum and the as-
prepared catalysts were denoted as KAPs(Ph-PPh )-Pd. Pd, 0.7
3
51–53
ꢀ
1
wt% (0.066 mmol g ).
54
lyst have also been studied for heterogeneous catalysis reac-
tions. Previously, we have proposed a low-cost strategy to
synthesize high surface area microporous knitting aryl network
57
Preparation of MCM-41-PPh
2
polymers (KAPs) via one-step ‘knitting’ of rigid aromatic The mesoporous material MCM-41 (0.5 g) was treated with
55
compounds with an external crosslinker. Successively, we have ligand (EtO) SiCH CH CH PPh (0.5 g) in toluene (20 mL) in the
3
2
2
2
2
knitted PPh with benzene to get functionalized KAPs(Ph-PPh ) presence of pyridine (0.5 mL) at reux under N atmosphere for
3
3
2
3 3
and bonded Pd with PPh groups to form KAPs(Ph-PPh )-Pd 24 h. The mixture was slowly cooled to room temperature, and
which exhibited excellent catalyst activity in Suzuki–Miyaura then the precipitated white solid was isolated by a ltration,
coupling reactions of aryl chlorides in aqueous media. It illu- washed with methanol and acetone successively, and dried at 80
ꢁ
minated that microporous polymers could not only play the role
C under vacuum for 8 h. The mesoporous material MCM-PPh
2
of support materials, but also protect the metal catalyst and was eventually obtained as white solid powders.
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5
7
Preparation of MCM-41-PPh
2
-Pd
micropore structure, a slight hysteresis loop implying a spot of
mesopore and a sharp rise at medium and high pressure
1
.0 g MCM-PPh2 was added to 20 mL acetonitrile solution
ꢁ
regions (P/P
0
¼ 0.8–1.0) indicating the presence of macropores
containing 0.10 g PdCl
2
and stirred under reux at 80 C for 12
in these materials. The pore size distribution of KAPs(Ph-PPh )
3
h. The solid was separated by centrifugation, washed repeatedly
with acetone, followed by Soxhlet-extraction with acetone to
remove physisorbed Pd(II) species. The nal catalyst was dried
at 60 C under vacuum and the as-prepared catalysts were
denoted as MCM-PPh -Pd. Pd, 0.5 wt% (0.047 mmol g ).
3
and KAPs(Ph-PPh )-Pd (Fig. S2 and S4†) also conrms the
presence of such heterogeneous porous structure which is
favourable for catalysis process, as the macroporous structure
accelerates the mass diffusion of reactants and products, and
micropore of suitable size is benecial for anchoring and pro-
tecting metal catalysts. XPS spectroscopy was employed to
analyze the elemental compositions of the surface and coordi-
ꢁ
ꢀ
1
2
Typical procedure for the coupling reaction
ꢁ
The coupling reactions were carried out at 80 C in a 10 mL nation states of Pd species. In Fig. S5,† an XPS study of KAPs(Ph-
reactor under air or nitrogen atmosphere. In a typical run, a PPh )-Pd derived from PdCl reveals that C, P, Cl and Pd
3
2
catalyst containing 0.26 mol% Pd, 0.5 mmol benzyl chloride, elements exist in the KAPs(Ph-PPh )-Pd. The XPS spectra in
3
0
.75 mmol phenylboronic acid and 1.5 mmol base were added Fig. S6† reveals that Pd species in the KAPs(Ph-PPh )-Pd are in
3
to 2 mL solvent and allowed to react for a denite time. Then the +2 oxidation state rather than in metallic state, corresponding
mixture was separated quickly by centrifugation. The solid was to the binding energy (B.E.) of 337.7 eV and 342.8 eV in the Pd
extracted with copious ethyl acetate and the liquid was 3d5/2 and 3d3/2 levels respectively. In comparison with the PdCl2
II
concentrated. The product was obtained by preparative TLC. (338.0 and 343.2 eV), the Pd binding energy in the KAPs(Ph-
Purity of products was checked by NMR and yields are based on PPh )-Pd shis negatively by 0.3 eV, because of the strong
3
benzyl chlorides.
electron-donation of the Ph-PPh network. These results
demonstrate that the Pd is successfully immobilized on the
3
II
Catalyst durability
KAPs(Ph-PPh
rather than by physical adsorption of PdCl
3
) by coordination to PPh
3
functional groups
on the surface.
2
3
The KAPs(Ph-PPh )-Pd catalyst was allowed to settle down aer
In initial catalytic reaction attempt, a set of experiments were
carried out in order to establish the best reaction conditions:
solvent, base and temperature for the Suzuki–Miyaura cross-
coupling of benzyl chloride and phenylboronic acid as a reac-
tion model (Table 1). Our preliminary screening of solvent
revealed that the presence of alcohol could efficiently promote
the reaction, and in our previous study in Suzuki–Miyaura cross-
coupling reaction of various aryl halides and arylboronic acids,
the addition of water to the solvent could speed up the
each run of reactions and the clear supernatant liquid was
decanted slowly. Aer washing with acetone as well as water,
ꢁ
and drying under vacuum at 60 C, the catalyst was re-used with
fresh charge of solvent and reactant for subsequent reaction
under the same conditions. The content of Pd species leached
off from the heterogeneous catalyst was determined by AAS
analysis.
Characterization
58
conversion of the substrate to some extent. Thus, we tried
adsorption isotherm (77.3 K) and mixture of water and ethanol in different ratio and ultimately,
pore size distributions were measured using Micromeritics the best optimal conditions were achieved using K CO as a
Polymer surface areas, N
2
2
3
ASAP 2020 M surface area and porosity analyzer. Before anal- base and ethanol as the reaction medium (Entries 1–10).
ꢁ
ysis, the samples were degassed at 110 C for 8 h under vacuum Further studies indicated that Cs CO , Na CO , NaOAc$3H O,
2
3
2
3
2
ꢀ
5
(
10 bar). Elemental analyses (EA) were performed on a Vario KOAc, NaOH, KOH and Na PO $12H O were poor bases for this
3 4 2
Micro cube Elemental Analyzer (Elementar, Germany). Pd reaction (Entries 11–17). NaOH and KOH gave extremely poor
content data were obtained on Perkin Elmer AA-800 (USA). The yields, because of the dehalogenation of benzyl chloride under
XPS analysis was carried out with a VG Multilab 2000 spec- basic condition. Remarkably, when K PO $3H O was used
3
4
2
trometer using Al Ka radiation at a power of 300 W. The pass (Entry 18), excellent yield (99%) of diphenylmethane was
ꢁ
energy was set at 100 eV, and C1s line at 284.6 eV was used as a obtained within 5 min at 80 C. Besides, when reducing the
ꢁ
reference. Adsorption capacities of benzyl chloride and phenyl reaction temperature to 60 C, yield (91%) was obtained, but the
boronic acid were calculated by UV data (SHIMADZU UV-2550 reaction took a bit longer i.e., 15 min (Entry 19). Lowering the
spectrophotometer, Japan).
reaction temperature to room temperature, the catalyst activity
was reduced accordingly leading to lesser products yield (33%),
even aer letting the reaction to proceed for 30 min (Entry 20). A
reaction in 10 mmol scale was also carried out, and it was found
Results and discussion
KAPs(Ph-PPh
3
) and KAPs(Ph-PPh
3
)-Pd were synthesized as that the reaction proceeded successfully, indicating the effec-
56
previous method. The BET surface area of KAPs(Ph-PPh
3
) is tiveness of this catalyst for large-scale synthesis (Entry 21).
2
ꢀ1
2
ꢀ1
1
036 m g , while that of KAPs(Ph-PPh )-Pd is 1025 m g . As Thus, reaction conditions (ethanol as solvent, K
3
PO
4
$3H
2
O as
3
ꢁ
shown in Fig. S1 and S3,† the adsorption and desorption base, 80 C, 5 min in air, without any additives) were dened as
isotherms of KAPs(Ph-PPh ) and KAPs(Ph-PPh )-Pd are similar. the optimal set of conditions for the reaction. In several mes-
3
3
35–37
The adsorption isotherms present a steep nitrogen gas uptake oporous materials supported NHC–Pd catalyst,
using iso-
at low relative pressure (P/P < 0.001) reecting abundant propyl alcohol as solvent and KOt-Bu as base, 75–88% yields
0
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a
Table 1 Optimization of reaction conditions
Table
KAPs(Ph-PPh
2
Suzuki–Miyaura cross-coupling reaction catalyzed by
a
3
)-Pd
Entry
1
Product
Entry
8
Product
b
Entry
Solvent
Base
Yield [%]
1
2
3
4
5
6
7
8
9
1,4-Dioxane
PhMe
H O
2
MeOH
i-PrOH
EtOH
K
K
K
K
K
K
K
K
K
K
2
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
3
22
29
49
74
62
90
76
85
78
74
30
45
12
21
46
67
74
>99c
91
2
3
9
EtOH–H
EtOH–H
EtOH–H
EtOH–H
EtOH
2
2
2
2
O ¼ 1 : 4
O ¼ 2 : 3
O ¼ 3 : 2
O ¼ 4 : 1
10
10
11
12
13
14
15
16
17
18
19
20
21
Cs
Na
NaOAc$3H
KOAc
NaOH
KOH
2
CO
CO
3
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
2
3
2
O
4
5
6
11
12
13
14
3 4 2
Na PO $12H O
K
3
K
3
K
3
K
3
PO
PO
PO
PO
4
4
4
4
$3H
$3H
$3H
$3H
2
2
2
2
O
O
O
O
d
EtOH
EtOH
33
92
e
a
Reaction conditions: 0.26 mol% of Pd, 0.5 mmol of benzyl chloride,
.75 mmol of phenyl boronic acid, 1 mmol of base, 2 mL of solvent
0
ꢁ
b
c
ꢁ
d
(v/v), 80 C, 5 min, in air. Isolated yield. 60 C, 15 min. Room
e
temperature, 30 min. 10 mmol scale.
were obtained within 8–12 h. Our catalyst, however, resulted in
rapid reaction under mild conditions.
b
Under optimized conditions, we investigated the activity of
7
KAPs(Ph-PPh )-Pd for various substrates. The corresponding
3
results are summarized in Table 2. The coupling of benzyl
chloride and arylboronic acid were rst examined. It is worth to
note that the turnover frequency (TOF) reached ca. 76 min in
a
Reaction conditions: 0.26 mol% of Pd, 0.5 mmol of benzylic chloride,
ꢀ1
0
.75 mmol of arylboronic acid, 1 mmol of K
3
PO
4
$3H
2
O, 2 mL of EtOH,
ꢁ
b
the case of benzyl chloride and phenyl boronic acid, which is 80 C, in air; Isolated yield; mol product per mol Pd min. Methanol as
ꢀ
1
the solvent.
much higher than the highest TOF (17 min ) for heteroge-
34
neous catalytic system reported so far. This solid catalyst
yielded 4-methyl (99%), 2-methyl (80%) and 4-methoxyl (93%)
ꢀ
1
3
PPh )-Pd had a good tolerance toward the substituents on
diphenylmethane within 10 min, with a TOF of 38 min , 31
ꢀ
1
ꢀ1
arylboronic acids under these conditions. We further tested the
catalytic performance for the couplings of various benzylic
chlorides with phenylboronic acid (Entries 11–14). For benzylic
chlorides bearing electron donating groups –CH (Entry 11) and
–
min and 36 min , respectively (Entries 2–4). To our delight,
even deactivated arylboronic acids i.e., 2,6-dimethylphenylbor-
onic acid and 2-naphthalenylboronic acid with sterically
hindered groups resulted in product yields of 73% and 80%
respectively (Entries 5 and 6). For arylboronic acid bearing an
electron-withdrawing group, good to excellent yields were
obtained. As high as about 90% yields were obtained for para
3
CH(C H ) (Entry 12), the catalyst afforded fast conversion in 10
2 6
min and 94% and 99% yields were obtained, respectively.
Otherwise, when using a weak electron-withdrawing group
substituted benzyl chloride (Entry 13), reactions proceeded
slowly (60 min), but very smoothly and 90% yield was obtained
under these conditions. Under the optimised conditions, the
recycling ability of the catalyst was also studied. As shown in
Table S1,† the catalyst showed a good recyclability in the rst
run to the third run, up to 95%. In the fourth run, the catalyst
–
COOCH and –OCF substituted phenylboronic acid (Entries 7
3 3
and 8). Remarkably, substrate with the strong electron-
withdrawing group, i.e., 4-triuoromethylphenylboronic acid
and 3,5-diuorophenylboronic acid also gave near quantitative
yield (90% and 92%) within 10 min and 20 min (Entries 9 and
10), which was impossible by Friedel–Cras reaction of
aromatic substrates with benzyl chloride. Obviously, KAPs(Ph-
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activity was reduced to 84% by increasing the reaction time to
20 min. 4.86% of Pd leaching was observed aer the fourth run.
For the substrate with a strong electron-withdrawing group
substituted 4-nitrobenzyl chloride (Entry 14), the catalyst gave
trace of the target product, which aroused our great interest. In Fig. 1 Suzuki–Miyaura cross-coupling under new conditions.
59–61
Suzuki–Miyaura coupling reaction,
the rate determining step
is the oxidative addition of benzyl chloride. Electron donating
groups in benzyl chlorides can promote the oxidative addition PPh )-Pd, as well as MCM-41 loaded PPh -Pd (MCM-41-PPh -Pd)
3
2
2
and consequently accelerate the reaction rate. When the benzyl for comparison (Fig. 2). The surface area of these two kinds of
chlorides contain electron-withdrawing groups, the rate of materials were similar, while material were different, for MCM-
2
ꢀ1
reaction become slower, which implies that the oxidative addi- 41-PPh -Pd (996 m g ) possessed regular mesoporous struc-
2
62,63
tion is no longer the rate determining step,
the dehalogenation reaction become dominant competitive Pd (1025 m
reaction. Ethanol as a hydrogen donor tends to aggravate the MCM-41-PPh -Pd which had excellent catalytic activity in
and meanwhile ture and uniform pore diameter (3.9 nm), and KAPs(Ph-PPh )-
3
2
ꢀ1
g
) was comprised of abundant micropores.
2
64,65
dehalogenation reaction under this situation.
Therefore, we Suzuki–Miyaura cross-coupling reaction of iodobenzene or
considered non-protic solvent instead of protic solvent to inhibit bromobenzene with phenyl boronic acid (yields are 99% and
the dehalogenation reaction. The solvents were screened for the 98%, respectively) afforded only 7% yields of diphenylmethane
model reaction of 4-nitrobenzyl chloride with phenylboronic under the optimised condition when employing benzyl chlo-
acid under inert atmosphere to avoid phenylboronic acid ride. Under the similar conditions, we found that benzyl chlo-
deboronation homocoupling reaction in presence of oxygen ride in the reaction can be rapidly adsorbed on KAPs(Ph-PPh3)-
66
under this condition, the results of which are shown in Table 3. Pd and adsorption content reached 2.8 g/g within 2 min because
Solvent effect on the activity of KAPs(Ph-PPh )-Pd was initially of its high surface area and greater affinity for the substrate.
3
examined with different ether solvents and similar results were Although MCM-41-PPh -Pd had similar surface area, the
2
obtained under the same conditions (Entries 1–4). Reactions in adsorption capacity was only 1.5 g/g. Evidently, in adsorption
anisole, 1,4-dioxane, furanidine and glycol dimethyl ether gave kinetics of benzyl chloride both the two catalysts gave the
3
5%, 77%, 46% and 27% yields respectively. Then the reaction similar adsorption rate and reached saturation within 1 min,
was conducted in acetonitrile and 50% yield was obtained (Entry but with different adsorption capacity. These two kinds of
). Toluene was found to be the best solvent and complete catalyst exhibited discrepancy in the Suzuki–Miyaura cross-
conversion to 4-nitrobenzylbenzene was achieved within 30 min coupling of benzyl chloride and phenyl boronic acid under
Entries 6 and 7). Impressively, substrate with stronger electron- the similar conditions, and the product yield was in accordance
5
(
67
withdrawing group –F which gave trace of the coupling product to their adsorption characters. Reactants in these coupling
also proceeded smoothly (Fig. 1, 94%), indicating the effective- reactions can be remarkably adsorbed on KAP(Ph-PPh )-Pd
3
ness of this strategy for such substrates.
because of their better affinity with the organic framework of
To understand the reason for the high activity of this catalyst the catalyst. The increase of the reactant concentration in the
in such reactions and the role of porous polymer supported in catalyst leads to an enhancement of the activity, which is in
catalysis, we measured the catalytic reaction activity, adsorption
kinetics of benzyl chloride and phenyl boronic acid on KAPs(Ph-
Table 3 The effect of solvents for nitrobenzyl chloride and phenyl
a
boronic acid
b
Entry
Solvent
Time [min]
Yield [%]
1
2
3
4
5
6
7
Anisole
60
60
60
60
60
60
30
35
77
46
27
50
1,4-Dioxane
Furanidine
Glycol dimethyl ether
Acetonitrile
Toluene
c
99(65)
99
Toluene
a
Reaction conditions: 0.26 mol% of Pd, 0.5 mmol of 4-nitrobenzyl
PO O,
chloride, 0.75 mmol of phenyl boronic acid, 1 mmol of K
2
3
4
$3cH
2
ꢁ
b
mL of solvent, 80 C, under N
2
atmosphere. Isolated yield. The Fig. 2 Adsorption kinetics of benzyl chloride (a) and phenyl boronic
result in air in parenthesis.
acid (b) on KAPs(Ph-PPh )-Pd and MCM-41-PPh -Pd.
3
2
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good agreement with the reactant enrichment and their 13 T. Z. Nichele and A. L. Monteiro, Tetrahedron Lett., 2007, 48,
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improved rate of reaction in carbon nanotube catalysts.
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14 K. Karami, C. Rizzoli and M. M. Salah, J. Organomet. Chem.,
2
011, 696, 940–945.
Conclusion
1
5 E. Paetzold and G. Oehme, J. Mol. Catal. A: Chem., 2000, 152,
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To summarise, we report Pd(II) organometallic catalysts
immobilized on the triphenylphosphine functionalized 16 M. J. Burns, I. J. S. Fairlamb, A. R. Kapdi, P. Sehnal and
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3
ꢀ
1
turnover frequency (TOF) of up to 76 min and highly tolerance
towards electrophilic substituted substrates. This work also 18 F. Zhao, Q. Tan, F. Xiao, S. Zhang and G.-J. Deng, Org. Lett.,
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affinity with organic reactants compared to traditional inor- 19 A. Yu, X. Li, D. Peng, Y. Wu and J. Chang, Appl. Organomet.
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activity. We expect that this low-cost heterogeneous catalyst can 20 O. Diebolt, V. c. Jur ˇc ´ı k, R. Correa da Costa, P. Braunstein,
40, 7599–7603.
be eventually useful for Suzuki–Miyaura cross-coupling reac-
tions of benzyl chloride at industrial scale.
L. Cavallo, S. P. Nolan, A. M. Z. Slawin and C. S. J. Cazin,
Organometallics, 2010, 29, 1443–1450.
21 C. Song, Y. Ma, Q. Chai, C. Ma, W. Jiang and M. B. Andrus,
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Acknowledgements
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