Replacing Pd(OAc)2 with supported palladium nanoparticles in ortho-directed CDC reactions of alkylbenzenes

Article abstract of DOI:10.1039/c5gc02554k

Supported palladium nanoparticles are used as efficient catalysts for the synthesis of aromatic ketones via cross dehydrogenative coupling reactions of 2-arylpyridines with alkylbenzenes. The catalyst can be reused for five cycles without significantly losing activity. Mechanism research showed that alkylbenzenes were oxidized to their corresponding aldehydes and subsequently coupled with 2-arylpyridines to generate aryl ketones through a Pd⁰/Pd^II/Pd^IV catalytic cycle.

Full text of DOI:10.1039/c5gc02554k

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Replacing Pd(OAc)2 with supported palladium nanoparticles in ortho-
directed CDC reaction of alkylbenzenes
Yong-Sheng Bao,* Dongling Zhang, Meilin Jia and Bao Zhaorigetu*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Supported palladium nanoparticles is used as an efficient
catalyst for the synthesis of aromatic ketones via cross
dehydrogenative coupling reactions of 2-arylpyridines with
alkylbenzenes. The catalyst can be reused for five cycles
10 without significantly losing activity. Mechanism research
showed that alkylbenzene were oxidized to their
corresponding aldehydes and subsequently coupled with 2-
arylpyridines to generate aryl ketones through
Pd0/PdII/PdIV catalytic cycle.
a
15 Introduction
Scheme 1. Various Reactions of Toluene through Being Oxidized to
Benzaldehyde Route
As significant materials and building blocks in the synthesis of
fine chemicals, aromatic ketones have a wide range applications
in many areas such as pharmaceuticals,1 agrochemicals,2 dyes and
fragrances.3 The direct introduction of carbonyl functional groups
20 onto the aromatic motifs via cross dehydrogenative coupling
50 moiety via directed aromatic C-H bond activation. The GC-MS
analysis of the model reaction solution showed no presence of
either benzaldehyde or benzyl alcohol which possibly form via a
radical oxidation of toluene. They proposed that the reaction
proceeded through the addition of benzyl radical to palladium
55 substrates and the benzylic C-H bond cleavage. Whereafter, P. P.
Sun et al.14 put forward another possible mechanism, that is,
toluene was oxidized to benzaldehyde and subsequently coupled
with 2-arylpyridines to give the corresponding aryl ketones. So
the acylation mechanism proceeding via whether the oxidative
60 dehydrogenation of benzyl intermediate or a consecutive process
that toluene is oxidized to benzaldehyde and subsequently
couples with 2-arylpyridines remains to be explored and verified.
Supported palladium nanoparticles (PdNPs) has been
successfully applied in many catalysis synthesis field in recent
65 years, such as Mizoroki-Heck cross coupling reaction,22
semihydrogenation of phenylacetylene,23 aerobic oxidation of
alcohols,24 and directed C-H activation reactions.25, 26 However,
to the best of our knowledge, there has been no report on the
supported PdNPs catalyzed ortho-directed CDC reaction using
70 alkylbenzene as acyl donors.
(CDC) reaction is
a
more environmentally-friendly,
regioselective alternative over traditional methods in aryl ketone
synthesis. Many successful strategies involve Pd-catalyzed ortho-
5
coupling reactions using aldehydes as acylation reagents.4,
25 Compared with aldehydes, alkylbenzenes have low toxicity and
are stable, commercially available, and easy to handle and thus
may potentially be used as ideal acylation reagents. It is now
commonly accepted that alkylbenzene could be oxidized in situ to
give benzaldehyde in the presence of peroxide and subsequently
30 complete various reactions, such as copper-catalyzed
esterifications6,7 and o-aroylations,8 tetrabutylammonium iodide
(TBAI) catalyzed synthesis of benzamides,9 iron-catalyzed
synthesis of thioesters10 and palladium-catalyzed ortho-
acylations11-20 (see Scheme 1).
35
In 2012, B. K. Patel et al.21 have reported Pd(OAc)2 catalyzed
synthesis of aromatic ketones using alkylbenzene as the aroyl
College of Chemistry and Environmental Science, Inner Mongolia Key
Laboratory of Green catalysis, Inner Mongolia Normal University,
Based on our research on the supported PdNPs catalyzed
ortho-directed C-C coupling reaction between 2-arylpyridines
and aldehydes,27 here we verify that supported PdNPs can be
used to drive the ortho-directed CDC reaction of alkylbenzene. In
75 a series of supported PdNPs, Pd/γ-Al2O3 catalyst with a PdNPs
mean diameter of 3.21 nm exhibited the best catalytic activity and
it could be used five times without significant loss in catalytic
40 Hohhot, 010022, China.
E-mail: sbbys197812@163.com; zrgt@imnu.edu.cn.
Tel: (+86)471-4392442
†
Electronic Supplementary Information (ESI) available: Figures S1−S2,
GC-MS analysis of reaction of 1a with 2a and 1a with 2e,
45 characterization data for the products, H NMR and 13C NMR spectra of
1
the products. See DOI: 10.1039/b000000x/
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Page 2 of 7
activity. A relatively clear mechanism was proposed based on the
Activity Test
experimental results and relative literatures.
In a typical reaction, a 25 mL oven-dried reaction tube was
55 charged with catalyst (35 mg), 2-phenylpyridine (0.2 mmol, 29
mg), tert-butyl peroxybenzoate (TBPB, 0.7 mmol, 136 mg) and
toluene (2 mL). The reaction tube was placed in an oil bath at 110
oC and the reaction mixture was stirred for 24 h. The products
were analyzed by GC (Shimadzu GC-2014) with a HP-5 capillary
60 column (50 m length, 0.25 mm internal diameter, and 0.25 μm
film thickness). The injector, column, and flame ionization
detector temperatures were kept constantly at 300 oC, 220 oC and
310 oC, respectively. All the products were confirmed by
comparison with previously reported 1H NMR and 13C NMR data.
Experimental
Catalyst Preparation
5
The PdNPs on γ-Al2O3 and other supports were prepared by an
impregnation-reduction method using the preparation of Au/γ-
Al2O3 as reference.28 For example, 3 wt% Pd/γ-Al2O3 catalyst
was prepared by the following procedure: 0.97 g of γ-Al2O3
powder was dispersed in 50 mL of distilled water. The two kinds
10 of aqueous solutions of PdCl2 (0.01 M, 28.2 mL) and L-Lysine
(0.03 M, 1 mL) were then added to the mixture consecutively
under vigorous stirring for 20 min. Subsequently, 0.1 M NaOH
aqueous solution was added into the mixture to adjust the pH to 7.
An aqueous solution of NaBH4 (0.35 M, 4.5 mL) was added
15 gradually in about 10 min to the suspension. The mixture was left
to stand for 24 h and the solid was separated, washed with
65 Catalysts recycle experiment
After each reaction cycle, the reactants, solvent, and products
were removed by centrifugation; the separated catalyst was
washed utterly with 0.1 M NaOH ethanol solution (once),
deionized water (thrice), and then washed twice with ethanol
70 followed by centrifugal separation and drying at 80 °C for 10 h.
The resultant catalyst was used for the next cycle.
o
distilled water (4 times) and ethanol (once), and dried at 80 C.
The dried solid was used directly in catalytic experiments.
Catalyst characterization
Results and discussion
20 The TEM images were recorded on a JEM-2100 transmission
electron microscope employing an accelerating voltage of 200 kV.
The samples were suspended in ethanol and dried on holey
carbon-coated Cu grids. The X-ray photoelectron spectroscopy
(XPS) was recorded on a Kratos Amicus of British equipped with
25 taper anode Mg Kα radiation. The C1s hydrocarbon peak at
284.60 eV was used as an internal standard for the correction of
binding energies. The X-ray diffraction (XRD) patterns of all
samples were tested on a Rigaku Ultimal IV X-ray diffractometer
using Cu Kα radiation (λ=1.5406 Å) from 2θ= 10o to 80o, at a
30 scan rate of 8o min−1, with the beam voltage and beam current of
40 kV and 40 mA, respectively. The specific surface areas of all
samples were obtained on a ASAP-2020 accelerated surface area
and porosity analyzer of American Micromeritics Company,
resulted from the nitrogen sorption data of the samples using the
35 Brunauer-Emmett-Teller (BET) model in a relative pressure
(P/P0) range between 0.06 and 0.30. The Pd content was
determined by the Z-8000-type polarized Zeeman atomic
absorption spectrophotometer (AAS) of Hitachi company. The Pd
sensitive wavelength is 244.8 nm. The products were analyzed by
40 GC (Shimadzu GC-2014) with a HP-5 capillary column (50 m
length, 0.25 mm internal diameter, and 0.25 μm film thickness).
Column, the injector and flame ionization detector temperatures
To have a good knowledge of the information for the catalyst,
the fresh and used (recovered after 5th cycle) PdNPs on γ-Al2O3
75 catalysts were studied by X-ray photoelectron spectroscopy
(XPS), X-ray diffraction (XRD), and transmission electron
microscopy (TEM) and so on. In order to understand the state of
PdNPs supported on γ-Al2O3, the catalysts before and after
reaction were tested by XPS analysis. The XPS results of the
80 catalysts confirm that palladium exists in the metallic state on γ-
Al2O3 supports before and after reaction. As shown in Figure 1a,
the binding energies of Pd 3d5/2 and Pd 3d3/2 electrons are 335.11
eV and 340.36 eV of fresh catalyst and 335.02 eV and 340.27 eV
of used catalyst respectively, which are identical to the bulk of
85 palladium metal. It is shown that Pd0 as the active center
completes the catalytic cycle.
Figure 1b depicts the XRD patterns of the fresh and used 3
wt% Pd/γ-Al2O3 catalysts. The XRD patterns of catalysts with
different loadings (1 wt%, 5 wt%) are shown in Figure S1 (see
90 ESI†). It is noted that no palladium peaks can be observed by
XRD pattern of all samples, probably indicate that the loaded
palladium did not form large particles, and was well dispersed in
the support structure.
o
o
o
were kept at 220 C, 300 C and 310 C for product analysis,
respectively. GC−MS analysis was detected on Thermo DSQ-II
45 with a DB-5 column. Thin layer chromatography (TLC) was
1
performed on pre-coated silica gel GF254 plates. The H NMR
and 13C NMR spectra were measured on a 500 MHz Bruker
Avance III nuclear magnetic resonance spectrometer, using
CDCl3 as the solvent with tetramethylsilane (TMS) as the internal
50 standard. Chemical shifts (δ) are expressed in ppm. The structures
of known compounds were further corroborated by comparing
their 1H NMR data with those of literature.
95 Figure 1. (a) XPS spectra of fresh and used 3 wt% Pd/γ-Al2O3; (b) XRD
patterns of fresh and used 3 wt% Pd/γ-Al2O3.
2
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The transmission electron microscopic (TEM) analysis of the 35 wt% Pd/γ-Al2O3, the conversion of 2-phenylpyridine was above
fresh and used catalysts are represented in Figure 2. The PdNPs
to 84% and the TON of this entry was 17, which was nearly 3-
fold of the acylation reaction in the presence of other oxidants
(entries 1-3). Besides, PdNPs supported on other oxide powders,
including CeO2 and Sm2O3, were prepared by the impregnation-
are distributed evenly on the γ-Al2O3 surface, and the mean
diameters of the PdNPs are 3.21 nm of fresh catalyst and 3.89 nm
of used catalyst, respectively. It does not cause obvious increase
5
in the average size of PdNPs after five runs. From the TEM 40 reduction method and applied to the CDC reaction of 2-
images of 1 wt% and 5 wt% Pd/γ-Al2O3, PdNPs supported on γ-
Al2O3 also exhibit small particles and narrow size distributions
(Figure S2, see ESI†).
phenylpyridine with toluene (entries 5 and 6). The PdNPs on
CeO2 and Sm2O3 exhibited much lower activity and selectivity.
The higher activity of PdNPs on γ-Al2O3 was for the reason that
γ-Al2O3 had a large surface area and open porosity which could
45 enable a high dispersion of a PdNPs catalyst.29 We also examined
the effect of different Pd loadings on the reaction. It was found
that the catalytic efficiency was significantly influenced by
palladium loading and that the catalyst with 3 wt % Pd exhibited
the best performance. Both a decrease and increase in the reaction
10
o
o
50 temperature (100 C and 120 C) reduced the conversion of 2-
phenylpyridine (entries 9 and 10). It can be explained that TBPB
might decompose at high temperature.30 Control experiment was
carried out in the absence of TBPB and it failed to give the
expected product 3aa, suggesting the importance of the oxidant
55 (entry 11). The results for various trial reactions are summarized
in Table 2.
Table 2. Optimization of reaction conditionsa
Figure 2. (a,b) TEM images of fresh 3 wt% Pd/γ-Al2O3 and used 3 wt%
Pd/γ-Al2O3, respectively; (c, d) PdNPs size distributions of fresh and used
15 catalysts, respectively.
T
Conv. Sel.
(%) (%)
Entry
1
Catalyst
Oxidant
TON
5
(oC)
TBHP
70% in water
CHP
TBHP ~5.5 M
in decane
TBPB
3 wt% Pd/γ-Al2O3
110
26
65
The BET analyses were conducted and specific surface areas
were given in Table 1. Comparing the γ-Al2O3 with the
corresponding Pd/γ-Al2O3 catalysts, it can be found that the
surface areas of all catalysts do not arise obvious change after
20 loading PdNPs. The amounts of Pd loading of the catalysts were
derived from atomic absorption spectrophotometer (AAS), and
the Pd content of fresh catalyst is approximately 3 wt %. We did
note a slight decrease in Pd content after being cycled 5 times
(2.86%, Table 1), which can further decrease the catalytic activity
25 on the basis of available Pd on the support surface.
2
3
3 wt% Pd/γ-Al2O3
3 wt% Pd/γ-Al2O3
110
110
23
32
49
36
5
6
4
5
6
7
8
9
10
11
3 wt% Pd/γ-Al2O3
3 wt% Pd/CeO2
110
110
110
110
110
100
120
110
84
46
69
68
56
70
36
-
97
40
38
17
9
14
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
-
3 wt% Pd/Sm2O3
1 wt% Pd/γ-Al2O3
5 wt% Pd/γ-Al2O3
3 wt% Pd/γ-Al2O3
3 wt% Pd/γ-Al2O3
3 wt% Pd/γ-Al2O3
30 14b
56 11c
78
17
-
14
7
-
a
Reaction conditions: 1a (0.2 mmol), 2a (2 mL), catalyst (35 mg),
Table 1. Characterization Results of BET, AAS of Catalysts
60 indicated oxidant ( 0.7 mmol), air, 24 h. Conversion and selectivity are
based on 1a, determined by GC. b catalyst (105 mg). c catalyst (21 mg).
Samples
SBET (m2/ g) Pd loading (wt %)
The directed aroylation of 2-arylpyridine with a set of
substituted alkylbenzenes was performed under the optimized
reaction conditions, and the results are presented in Table 3. The
65 initial investigations were focused on the CDC reactions of 2-
phenylpyridine 1a with various alkylbenzenes 2 (entries 1-9).
Various functional groups including methyl, methoxy, chloro,
bromo were compatible and the desired products were achieved
in moderate to good yields. For example, 4-methoxytoluene 2e
70 (which bears a strongly electron-donating group) afford the
desired product (4-methoxyphenyl)(2-(pyridin-2-yl)phenyl)meth
anone 3ae in a yield of 81% (entry 4). Gratifyingly, 4-
bromotoluene 2i and 3-bromotoluene 2j were also reactive, even
though only moderate yields of the acylation products 3ai and 3aj
γ-Al2O3
101
114
108
-
3 wt% Pd/γ-Al2O3(fresh)
3 wt% Pd/γ-Al2O3(used)
3.02
2.86
Our initial attempt was executed toward sp2 C-H bond
aroylation of 2-phenylpyridine 1a using toluene 2a as both the
30 aroyl reagent and solvent, 3 wt% Pd/γ-Al2O3 as the catalyst in the
presence of different oxidants such as tert-butyl hydroperoxide
(TBHP), cumene hydroperoxide (CHP) and tert-butyl
peroxybenzoate (TBPB). To our delight, TBPB gave the best
performance (entry 4): after 24 h of refluxing in the presence of 3
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products can undergo further modification facilely.4 Compared to
10 its para or meta isomers, o-xylene 2c and o-chlorotoluene 2g
delivered a lower yield, which resulted from the steric hindrance
effect. Encouragingly, ethylbenzene and n-propylbenzene kept to
perform this CDC reaction to give the same product as toluene
respectively in 31% and 44% yields (entries 10-11). This
15 indicated that the oxidation took place on the α-carbon of the
Table 3. Substrate Scope for the Synthesis of Aromatic Ketones via C-H
Functionalizationa
Yieldb (%)
80
ethylbenzene or n-propylbenzene and generated
a small
Entry 2-Arylpyridine Alkylbenzene
Product
percentage of benzaldehyde.31-33 The directly formation of benzyl
radical from ethylbenzene is almost impossible because C-C
homolytic cleavage is involved in the process. Therefore, the
1
2
1a
1a
2b
3ab
20 acylation
mechanism
proceeding
via
the
oxidative
64
76
dehydrogenation of benzyl intermediate is not accurate. In
addition, the optimized reaction conditions were implemented in
the coupling reactions between various phenyl-N-heteroarere 1
and toluene 2a (entries 12-14). It is noted that this acylation
25 reaction could be also applicable to heterocycle-substituted
pyridines such as benzo[h]quinolone, and the good yield (84%) of
the product benzo[h]quinolin-10-yl(phenyl)methanone 3ba was
obtained. The superior yield is probably due to the planar
structure of benzo[h]quinolone.5, 34 When 1-phenyl-1H-pyrazole
30 1c, five membered nitrogen containing heterocycles served as
directing- group, was employed instead of 2-phenylpyridine, the
acylation reaction also proceeded expediently to afford (2-(1H-
pyrazol-1-yl)phenyl) (phenyl)methanone 3ca in 70% yield.
Unfortunately, the highly electron-deficient 2-(2,4-difluoro-
35 phenyl)pyridine 1e failed to deliver our desired product under the
present reaction conditions (entry 15). The aroylation reaction
might be blocked by the electrophilic attack of the Pd(0) center to
the phenyl ring.4 It is important to point out that the reaction gave
the monoacylation products selectively in all cases.
2c
3ac
3
1a
2d
3ad
4
5
6
7
1a
1a
1a
1a
81
69
57
65
3ae
2e
2f
3af
2g
3ag
2h
3ah
8
9
1a
1a
63
68
40
The recyclability of catalyst was examined in the reaction of 2-
phenylpyridine 1a and toluene 2a as provided in the experimental
section. As shown in Figure 3, the catalyst can be reused for five
cycles with only 13% decline of activity after the 5th recycle.
Intrestingly, the average yield of five cycles of toluene was much
2i
3ai
2j
3aj
10
11
1a
1a
3aa
3aa
31
44
45 higher than benzylaldehyde in this heterogeneous catalyzed
system. From the TEM image of the used 3 wt% Pd/γ-Al2O3
catalyst (Figure 2b), the PdNPs still distribute evenly on the γ-
Al2O3 surface, no apparent agglomeration was observed. The
decreased catalytic activity of the PdNPs may result from the
50 slight increase in the average size of the particles recovered after
5th cycle (Figure 2d).
2k
2l
12
13
2a
84
70
1b
3ba
3ca
2a
1c
14
15
2a
2a
81
_
1d
3da
_
1e
a Reaction conditions: 1 (0.2 mmol), 2 (2 mL), 3 wt% Pd/γ-Al2O3 (35 mg),
5 TBPB (0.7 mmol), 110oC, 24 h. b isolated yield.
Figure 3. Recyclability of catalyst.
were obtained (entry 8 and 9). The notable results might be
explained that the aryl bromide substrates are usually very
reactive in Pd0/PdII catalytic cycles, and the bromo-substituent
To gain mechanistic insight, a radical capture experiment was
55 conducted. There was a great restraining of the reaction between
2-phenylpyridine 1a and toluene 2a in the presence of radical
4
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scavenger TEMPO (0.4 mmol) and no acylation product 3aa was
5th recycle. The practicality of this study may inspire further
studies on the heterogeneous catalyzed ortho-directed acylation
reactions. With the advantages of easy separation, recycling of
heterogeneous catalyst and high catalytic activity, the PdNPs
detected, suggesting a possible radical approach. This result
indicates that TBPB is potenially acting on both an oxidant and a
radical initiator. A GC-MS analysis of the reaction solution of 2-
5
phenylpyridine 1a and toluene 2a showed that both benzaldehyde 45 catalyst would be an alternative to Pd(OAc)2 in more catalytic
and benzyl alcohol were detected which could possibly result
from a radical oxidation of toluene 2a (see ESI†). In addition, the
similar result was also obtained for the reaction between 2-
phenylpyridine 1a and 4-methoxytoluene 2e (see ESI†).
Based on the previous reports35-41 and our own results, a
tentative mechanism is illustrated in Scheme 2. First step involves
the coordination of Pd0 with the nitrogen atom of the pyridine
group of 2-phenylpyridine 1 and the further oxidation of Pd0 to
PdII by air.42 Subsequently PdII activates the ortho C-H bond
organic reaction.
Acknowledgements
10
This research was financially supported by National Science
Foundation of China (21462031) and the Initial Special Research
50 for 973 Program (2014CB460609).
15 through chelate-directed effect to afford palladacycle
intermediate A. Secondly, the reaction of TBPB with
benzaldehyde which is resulted from the oxidation of toluene 2
provides a benzoyl radical by releasing the tert-butyl alcohol (or
benzoic acid). And intermediate A coordinated with benzoyl
20 radical and benzoate radical to form an PdIV complex B,43, 44
which fast releasing benzoic acid to form intermediate C. Finally,
the final product 3 is generated through the reductive elimination
of intermediate C and releases a Pd0 to continue catalytic cycle.
Notes and references
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Conclusions
In summary, it is found for the first time that metallic state
palladium, Pd0, can catalyze the ortho-directed CDC reaction of
alkylbenzenes including methyl, ethyl and propyl benzene for the
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30 synthesis of aromatic ketones via a Pd0/PdII/PdIV catalytic cycle. 90 24 J. Chen, Q. H. Zhang, Y. Wang, H. L. Wan, Adv. Synth. Catal. 2008,
350, 453-464.
Differed from B. K. Patel’s report, the acylation reaction might
undergo through another possible mechanism, that is, the
alkylbenzenes were oxidized to their corresponding aldehydes
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electron-withdrawing and electron-donating groups, can react
with 2-arylpyridine to synthesize the corresponding ketones in
yields up to 84%. The catalyst can be readily recovered and
40 reused for five cycles with only 13% decline of activity after the
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DOI: 10.1039/C5GC02554K
Pd(0) = Supported palladium nanoparticles
TEM images
XPS spectra
Recyclability of the
Catalyst