Multiwalled Carbon Nanotubes Supported Pd(II)-Salen Complex: An Effective, Phosphorous-Free, and Reusable Heterogeneous Precatalyst for the Synthesis of Diaryl Ketones

Article abstract of DOI:10.1002/hlca.201600161

The Suzuki–Miyaura cross-coupling reaction of aroyl chlorides and arylboronic acids has been carried out efficiently in the presence of Pd(II)-salen@MWCNTs as an air-moisture stable precatalyst. The influence of various parameters, such as solvent, temperature, and base on the reaction system, was studied. Furthermore, the catalyst can be easily recovered quantitatively by a simple filtration and reused for three consecutive runs without significant loss of its activity.

Full text of DOI:10.1002/hlca.201600161

Received Date: 01-Jun-2016
Revised Date: 04-Jul-2016
Accepted Date: 12-Jul-2016
Article Type: Full Paper
Multi-walled carbon nanotubes supported Pd(II)-salen
complex: An effective, phosphorous-free, and reusable
heterogeneous precatalyst for the synthesis of diaryl
ketones
*
Elmira Mohammadi, Zahra Hajilou, and Barahman Movassagh
Department of Chemistry, K. N. Toosi University of Technology, P.O. Box 16315-1618, Tehran, Iran
(
phone: +98-21-22853650; e-mail : bmovass1178@yahoo.com)
Abstract: Suzuki-Miyaura coupling of aroyl chlorides and arylboronic acids has been carried out efficiently in the
presence of Pd(II)-salen@MWCNTs as an air-moisture stable precatalyst. The influence of various parameters such as
solvent, temperature, and base on the reaction system were studied. Furthermore, the catalyst can be easily recovered
quantitatively by simple filtration and reused for three consecutive runs without significant loss of its activity.
Keywords: Palladium complex • Multi-walled carbon nanotubes • Suzuki-Miyaura coupling • Aromatic ketones
Supporting information for this article is given via a link at the end of the document.
Introduction
The synthesis of diaryl ketones is an active area in organic synthesis since they are ubiquitous in natural products [1a],
pharmaceuticals [2] and materials science [3]. For example, fenofibric acid is an aromatic ketone, which is a lipid
regulating agent and treats high cholesterol and triglyceride levels in the blood [1b].
The traditional approach for the introduction of ketone moiety into the aromatic skeletons is the Friedel-Crafts acylation
[
4]. However, low functional group tolerance, poor regioselectivity, and formation of undesired side products have
limited these reactions to be employed to their full potential [5]. The nucleophilic addition of organometallic reagents like
organozinc [6], cadmium [7], and tin [8] to carboxylic acid derivatives is an alternative method that addresses the
problems of drastic reaction conditions and low product yields due to the formation of tertiary alcohols. Thus, the
development of a mild as well as highly efficient method for the preparation of diaryl ketones has been receiving
increasing interest.
To date, Pd-mediated acylation reactions are the reaction of choice for the industrial scale formation of costly fine
chemicals due to the simplicity, environmentally friendly nature, and economical aspects [9]. These efficient
methodologies mainly include carbonylative Suzuki-Miyaura reactions [10] and direct Suzuki-Miyaura acylation of
organoboronic acids [11]. The latter approach was discovered firstly by Bumagin and co-workers who could successfully
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doi: 10.1002/hlca.201600161
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2
cross-couple acyl chlorides with aryl boronic acids in the presence of PdCl in aqueous acetone [12]. Since then,
several improvements on Pd-catalyzed coupling reactions of carboxylic acids [13] and their derivatives, including acid
chlorides [11], anhydrides [14], thioesters [15], and 2-pyridyl esters [16] with boronic acids have been widely described.
Most of the above protocols are homogeneous in nature suffering from several drawbacks such as contamination of
both metal and ligand residues in the final products, recovery of the catalyst, and destruction of the catalyst during the
course of the reaction. These factors, have been a major driving force for designing heterogeneous catalysts by
encapsulation or immobilization of catalytically active metal complexes on solid supports such as carbon nanotubes
(
CNTs) [17], polymers [18], and mesoporous silica [19]. In spite of the significant advances in this field, only a few
reports have employed heterogeneous palladium complexes as a catalyst for the acylation reaction of boronic acids. For
instance, silica-supported phosphine palladium(0) complex [20], Pd/C catalytic system [21], and palladium catalyst
anchored to silica gel functionalized with ≡Si(CH ) NHCH SMe [22] have been applied to the phenylation of acyl
2 3 2
chlorides. However, disadvantages including, moderate yields, and formation of homocoupling adducts have limited the
scope of these strategies.
In connection with our insights about the use of heterogeneous catalysts in the organic transformations [23], we
describe herein a mild, and efficient copper- and phosphine-free approach for the preparation of diaryl ketones
employing our introduced multi-walled carbon nanotubes functionalized with a palladium(II)-schiff base complex (Pd(II)-
salen@MWCNTs) as a precatalyst [23a].
Results and Discussion
In the first instance, the reaction conditions such as solvent, catalyst concentration, base, and temperature were
optimized for the Suzuki-Miyuara cross-coupling reaction of phenylboronic acid and benzoyl chloride as the test
substrates. The best molar ratios of benzoyl chloride (1a): phenylboronic acid (2a): K CO was found to be 2:1:2.5. In
2 3
order to find out the best reaction medium, the influence of various solvents was primarily investigated. While moderate
yield of the coupling product (79%) was obtained when neat acetone was used as the solvent (Entry 1, Table 1), the
addition of water increased the rate as well as the yield of the reaction (Entries 2-4, Table 1); among the ratios tested,
the (1:1) ratio of acetone: water was found to be the most effective (Entry 2, Table 1). Also, the yield dropped
significantly to 31% in neat water (Entry 5, Table 1). In addition, no satisfactory result was obtained under the solvent-
free conditions and trace amount of product was formed (Entry 6, Table 1). Moderate yields of the coupling product
were obtained in other solvents such as toluene and EtOH (Entries 7-9, Table 1). Next, the role of the catalyst
concentration in the yield and the rate of the reaction was screened; using 0.9 mol% of the catalyst led to 91% of
benzophenone in 1h (Entry 11, Table 1), whereas the yield dramatically decreased when 0.6 mol% of the catalyst was
used for the coupling reaction (Entry 10, Table 1). Moreover, applying 1.05 mol% of the heterogeneous Pd(II)-
salen@MWCNTs had no marked effect on the rate of the reaction (Entry 12, Table 1). Due to the important role
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a
Table 1. Optimizatin of the reaction conditions for the cross-coupling reaction of benzoyl chloride and phenylboronic acid )
O
O
Pd(II)-salen@MWCNTs
PhB(OH)2
+
Ph
Cl
Base, Solvent
Temp °C
Solvent [ratios]
Ph
Ph
1
a
2a
3a
Cat.
mol-%]
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.6
0.9
1.05
0.9
0.9
0.9
Time
[h]
2
2
2
2
2
2
2
2
2
2
1
b
Entry
Base
T [º]
Yield [%] )
[
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
acetone
K
K
K
K
K
K
K
K
K
K
K
K
Na
K
Cs
Et
K
K
2
2
2
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
3
3
3
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
40
25
79
87
83
80
31
trace
76
73
61
70
91
92
86
70
62
trace
82
acetone:H
acetone:H
acetone:H
2
2
2
O [1:1]
O [1:2]
O [1:3]
H
2
O
̶
toluene
toluene (dry)
EtOH
1
1
1
1
1
1
1
1
1
acetone:H
acetone:H
acetone:H
2
2
2
O [1:1]
O [1:1]
O [1:1]
O (1:1)
1
acetone:H
2
2
CO
PO
CO
3
2.7
1.2
1.5
24
1
acetone:H
acetone:H
acetone:H
acetone:H
acetone:H
2
O [1:1]
O [1:1]
O [1:1]
O [1:1]
O [1:1]
3
4
2
2
2
2
2
3
3
N
0.9
0.9
0.9
2
CO
CO
3
2
3
1
57
a
b
)
Reaction conditions: benzoyl chloride (2.0 mmol), phenylboronic acid (1.0 mmol), base (2.5 mmol), solvent (4.0 ml).
)
Isolated yields.
of the base in the Pd-catalyzed coupling reactions, the influence of various inorganic and organic bases were
studied. It was observed that K CO was the most effective base, while other bases including Na CO , K PO , Cs CO
and Et N exhibited weaker performances (Entries 13-16, Table 1). On the other hand, reducing the temperature to 40
and 25 ºC resulted in the formation of the target product in 82% and 57% yields, respectively (Entries 17 and 18, Table
). To determine the scope of the substrates, further experiments with diverse acyl chlorides and arylboronic acids
2
3
2
3
3
4
2
3
,
3
1
were carried out under the optimized reaction conditions (Scheme 1) and results are listed in Table 2. The coupling
reaction of either 4-methyl-
Scheme 1. Reaction of aroyl chloride (1) and arylboronic acid (2) in the presence of Pd(II)-salen@MWCNTs.
O
O
Pd(II)-salen@MWCNTs
2
Ar B(OH)2
Ar1
+
Cl
K CO , aetone:H O
Ar1
Ar2
2
3
2
5
0 °C
1
2
3
O
O
O
O
O
O
N
O
=
Pd
O
O
O
N
O
O
O
O
Pd(II)-salen@MWCNTs
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4-methyl- or 4-methoxybenzoyl chloride as electron-rich acyl chlorides with phenylboronic acid afforded the
corresponding products in 86% and 83% yields, respectively (Entries 2 and 3, Table 2). Furthermore, aroyl chlorides
bearing electron-deficient substituents such as 4-nitro- and 4-chlorobenzoyl chloride gave higher yields while
significantly accelerated the rate of the coupling reaction (Entries 4 and 5, Table 2). Upon treatment of the sterically
demanding 2-fluorobenzoyl chloride with phenylboronic acid, the desired fluorinated benzophenone was formed in 71%
yield (Entry 7, Table 2). It should be pointed out that heteroaroyl chlorides such as 2-furoyl- and 2-thenoyl chloride were
effectively cross-coupled with phenylboronic acid in short reaction times (87% and 81% respectively (Entries 8 and 9,
Table 2). Subsequently, the effect of varying substituents on arylboronic acids was also screened and no spectacular
electronic effect was observed in the cross-coupling reaction (Entries 1, 10 and 11, Table 2).
a
Table 2. Diaryl ketones from Suzuki-Miyaura coupling of aroyl chlorides and arylboronic acids )
1
2
Time
Yield
[%] )
TON
Entry
Ar
Ar
b
[h]
1
2
3
4
5
6
7
8
9
C
6
H
5
C
C
C
C
C
C
C
C
C
6
6
6
6
6
6
6
6
6
H
H
H
H
H
H
H
H
H
5
5
5
5
5
5
5
5
5
1
1.16
1.5
imdtly
imdtly
0.83
2
0.17
0.17
1
91
86
83
94
92
75
71
87
81
91
88
101
96
92
104
102
83
79
97
90
101
98
4-MeC
4-MeOC
4-NO
4-ClC
3-ClC
2-FC
2-furoyl
2-thenoyl
C
C
6
4
4
2
C
6
4
6
H
H
H
6
6
10
1
6
H
H
5
4-MeC
4-MeOC
6
4
1
6
5
4
1
a
)
2 3
Reaction conditions: aroyl chlroide (2.0 mmol), arylboronic acid (1.0 mmol), K CO (2.5 mmol), Pd(II)-Salen@MWCNTs (0.9 mol%) in
2
acetone/H O (v/v = 1:1) (4.0 ml) at 50 º.
b
)
Isolated yields.
The reusability of the Pd(II)-salen@MWCNTs as the heterogeneous catalyst in the reaction of benzoyl chloride and
phenylboronic acid was also investigated. The capability of recovery of the catalyst was confirmed after three
successive runs with minimal loss of activity (Figure 1). Furthermore, leaching of transition metal to solution during the
course of a reaction is always an important issue for the heterogeneous catalysts, which directly affects the catalytic
activity of these species. Therefore, ICP analysis was performed to determine the total leaching of Pd metal, and
we found that 1.2% of Pd was
95
90
85
80
75
70
65
60
55
50
91
90
90
84
1
2
3
4
No. of cycles
Figure 1. Reusability chart of the cross-coupling reaction of benzoyl chloride and phenylboronic acid.
th
Lost after four reaction cycles in the above Suzuki coupling. So, lower yield of the coupling product in the 4 cycle can
be attributed to the leaching of the palladium metal.
A plausible catalytic mechanism for this cross-coupling reaction involves the following steps [14] (Scheme 2): (i)
n
preliminary reduction of Pd(II) precatalyst to active L Pd(0) by phenylboronic acid [24]; (ii) oxidative addition of acyl
chloride to the L Pd(0) species to generate acyl palladium(II) chloride complex; (iii) transmetallation to transfer aryl
n
group from boron to the palladium center to produce palladium(II) complex which contain both the acyl and aryl
substituents to be coupled; (iv) reductive elimination to yield the corresponding diaryl ketone.
Finally, we compared the performance of our reusable catalyst with other reported catalytic systems in the literature for
the cross-coupling reaction of benzoyl chloride (1a) and phenylboronic acid (2a; Table 3). As shown in Table 3, the Pd-
salen@MWCNTs exhibited superior catalytic activity in this transformation.
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Table 3. Comparison of different catalytic systems with the present catalyst in the reaction between benzoyl chloride and phenylboronic acid
a
Entry
Catalyst
Pd(dba)
5 mol%)
PdCl (PPh
2 mol%)
Solvent
Et
Time [h]/T [°]
Yield [%]
TON
16
2
1
2
O
3, 100
75 [11a]
(
2
3 2
)
2
3
toluene
4, 110
12, 60
91 [25]
93 [20]
45
26
(
“Si”-P-Pd(0) (3mol%)
Cyclopalladated ferrocenyl-
imines
acetone:H
2
O(3:1)
4
toluene
12, 60
1, 50
92 [26]
196
101
(0.5 mol%)
Pd(II)-salen@MWCNTs (0.9
mol%)
5
acetone:H
2
O(1:1)
91 [present work]
a
)
References are given in brackets.
Conclusion
In summary, we have demonstrated that Pd(II)-salen complex anchored to multi-walled carbon nanotubes could be
conveniently used as a heterogeneous precatalyst for the efficient, general, phosphorous-free, and eco- friendly
synthesis of aromatic ketones in aqueous media. In addition to the high catalytic activity, this precatalyst could be easily
separated and reused for three consecutive cycles with consistent catalytic activity.
Scheme 2. Plausible mechanism for the cross-coupling reaction of aroyl chlorides and arylboronic acids.
Pd(II) precatalyst
O
O
activation
Ar
Cl
Ar
Ar'
O
LnPd(0)
reductive
elimination
oxidative
addition
O
Ar
Ar
Cl
Pd(II)Ln
Pd(II)Ln
Ar'
transmetallation
HO
Cl
B
Ar'B(OH)2
OH
Acknowledgment
The authors are grateful to K. N. Toosi University of Technology Research Council for financial support of this work.
Experimental Part
All chemicals were purchased from Merck and Aldrich chemical companies. Melting points were determined on a
1
13
Büchi melting point B-540 apparatus. NMR spectra were recorded at 300 ( H), and 75.5 ( C) MHz, on a commercial
Bruker DMX-300, respectively using CDCl
Spectrometer.
3
as solvent. IR spectra were recorded on an ABB Bomem Model FTLA 2000
The heterogeneous Pd(II)-salen@MWCNTs was characterized by transmission electron microscopy (TEM), attenuated
total reflection infrared spectroscopy (ATR), Raman spectroscopy, inductively coupled plasma (ICP), thermogravimetric-
differential thermal analysis (TG-DTA), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) techniques
-1
[
23a]. The ATR spectrum of the catalyst showed a band at 1708 cm associated with C=O stretching of the ester
linkage between the CNT and the Schiff base complex. Raman spectrum showed four bands including tangential
-
1
-1
-1
stretching G-band (1567 cm ), D-band (1331 cm ), second harmonic D*-band (2678 cm ) and radial breathing band
(
-1
RBM, 288 cm ) which are characteristic of CNTs. Moreover, the metal content of the complex was found to be 16.20
ppm using ICP, and the ratio of Pd/N in the catalyst was obtained to be 1.82%. Further evidence for the modification of
MWCNTs came from the XPS of the complex; the peaks at 290.8, 344.6 and 539.2 eV are attributed to carbon, nitrogen
and oxygen atom, respectively. Pd 3d3⁄2 and 3d5⁄2 peaks appear at 349.8 and 344.4 eV, respectively. Also, comparison
2
between the TEM images of the purchased CO H-MWCNTs and Pd(II)-salen@MWCNTs confirmed the presence of the
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Schiff base complex. The XRD pattern of the catalyst confirmed the presence of graphite at around 26° and also the
palladium metal at 39.9°, 47.4° and 82.0°.
General Procedure for the Suzuki-Miyaura cross-coupling of aroyl chlorides
A test tube was charged with Pd(II)-salen@MWCNTs (6 mg, 0.9 mol%), acyl chloride (2.0 mmol), arylboronic acid (1.0
mmol), K
0 ºC under air for appropriate time. The progress of the reaction was monitored using TLC. After completion of the
reaction, the solution was filtrated; the catalyst was thoroughly washed with ethyl acetate (2 × 8 mL) and subjected to
the next run. The combined organic phase was washed with 5% aqueous NaHCO (2 × 5 mL), and dried over sodium
2 3 2
CO (2.5 mmol, 345.0 mg), acetone (2.0 ml), H O (2.0 ml), and a magnetic stirr bar. The mixture was stirred at
5
3
sulfate. The solvent was then removed under reduced pressure. The product was further purified using preparative
TLC (silica gel) with hexane:EtOAc as eluent. Selected spectral data: 3b: White solid; mp 50-53 ºC (lit. [14] mp 56.5-57
1
°
2
C); H NMR (300 MHz, CDCl
3
): δ 2.45 (s, 3H), 7.27-7.31 (m, 2H), 7.46-7.51 (m, 2H), 7.56-7.59 (m, 1H), 7.72-7.75 (m,
): δ 21.6, 128.2, 129, 129.9, 130.3, 132.2, 134.8, 137.9, 143.2, 196.5.
13
H), 7.78-7.81 (m, 2H); C NMR (75 Hz, CDCl
IR(C=O) 1657. 3c: Pale yellow solid; mp 56-59 ºC (lit. [14] mp 58-60 °C); H NMR (300 MHz, CDCl ): δ 3.88 (s, 3H),
3
.95-6.98 (m, 2H), 7.45-7.5 (m, 2H), 7.54-7.57 (m, 1H), 7.75-7.78 (m, 2H), 7.82-7.85 (m, 2H); C NMR (75 MHz,
3
CDCl ): δ 55.5, 113.5, 128.2, 129.7, 130.1, 131.9, 132.5, 138.2 , 163.2 , 195.4 . IR (C=O) 1650. 3g: Yellow oil; H NMR
3
1
13
6
1
1
3
(
300 MHz, CDCl
3
): δ 7.15-7.20 (m, 1H), 7.23-7.31 (m, 1H), 7.45-7.65 (m, 5H), 7.80-7.87 (m, 2H). C NMR (75 MHz,
3
): δ 116.24 (d, J(C-F) = 21.7 Hz), 124.24 (d, J(C-F) = 3.5 Hz), 127.00 (d, J(C-F) = 14.9 Hz), 128.43, 129.8, 130.85 (d,
CDCl
J
(C-F) = 2.93 Hz), 133.03 (d, J(C-F) = 8.2 Hz), 133.4, 137.3, 160.04 (d, J(C-F)= 50.8 Hz), 193.5. IR(C=O) 1668. 3h: Yellow
1
3
oil; H NMR (CDCl , 300 MHz): δ 6.59 (dd, J = 3.5, 1.7 Hz, 1H), 7.23-7.27 (m,1H), 7.47–7.52 (m, 2H), 7.57–7.60 (m,
1
1
13
H), 7.71 (d, J = 1 Hz, 1H), 7.95–7.98 (m, 2H); C NMR (CDCl
47.1, 152.2, 182.6. IR (C=O) 1647.
3
, 75 MHz):δ 112.2, 120.6, 128.4, 129.2, 132.6, 137.2,
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Entry for the Table of Contents
O
O
Pd(II)-salen@MWCNTs
2
Ar B(OH)
Ar1
Cl
+
2
K CO , acetone:H O Ar1
Ar2
2
3
2
50 °C
1
Ar = Ph, 4-MeOC H , 4-MeC H ,
6
4
6 4
4
-ClC H , 2-FC H , 2-furoyl, 2-thenoyl
6 4 6 4
2
Ar = Ph, 4-MeOC H , 4-MeC H
6 4
6
4
This article is protected by copyright. All rights reserved.