Syndiotactic Poly(aminostyrene)-Supported Palladium Catalyst for Ketone Methylation with Methanol

Article abstract of DOI:10.1002/cctc.201700907

Palladium nanoparticles immobilized on an amino-functionalized syndiotactic polystyrene (sPS-N) served as a novel recyclable catalyst for the dimethylation and cross methyl alkylation of a wide range of ketones with methanol as the methylation agent. This heterogeneous catalyst (Pd@sPS-N) was highly robust and showed excellent thermal stability and chemical resistance. It not only showed remarkably high activity, but it could also be easily recovered by filtration without loss of activity.

Full text of DOI:10.1002/cctc.201700907

Accepted Article
Title: Syndiotactic Poly(aminostyrene)-Supported Palladium Catalyst
for Ketone Methylation with Methanol
Authors: Zhaomin Hou, Lei Jiang, Fang Guo, Yang Li, and Zhenghai
Shi
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10.1002/cctc.201700907
ChemCatChem
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Syndiotactic Poly(aminostyrene)-Supported Palladium Catalyst
for Ketone Methylation with Methanol
Lei Jiang,[a] Fang Guo,*[a] Zhenghai Shi,[a] Yang Li,*[a] and Zhaomin Hou*[a,b]
Abstract: Palladium nanoparticles immobilized in an amino-
functionalized syndiotactic polystyrene (sPS-N) served as a novel
recyclable catalyst for the dimethylation and cross methyl-alkylation
of a wide range of ketones with methanol as the methylation agent.
This heterogeneous catalyst (Pd@sPS-N) was highly robust,
showing excellent thermal stability and chemical resistance. It not
only showed remarkably high activity, but it could also be easily
recovered by filtration without loss of activity.
Scheme 1. Dimethylation and methyl-alkylation of ketones with methanol as
the methylation agent by a novel polymerimmobilized palladium nanoparticle
catalyst (Pd@sPS-N).
The development of easily recyclable catalysts for chemical
synthesis is of great importance. In this endeavor, organic
polymer-supported metal nanoparticles have attracted much
recent attention as high-performance heterogeneous catalysts
because of their high activity, recoverability, tailorability, etc.[1]
resistance originated from the unique syndiotactic
However, compared with inorganic supports, organic polymers
polystyrene structure, but it also showed promising
usually show low thermal stability and poor solvent compatibility,
capability for the immobilization of palladium nanoparticles
which may therefore cause serious catalyst leaching and
because of the presence of the functional amino groups.[11]
deactivation at high temperatures or corrosive reaction
These results prompted us to examine whether the
conditions. The use of crosslinked polymers[2-4] or self-assembly
palladium nanoparticles immobilized in sPS-N can serve as
of heteroatom-containing polymers[5-7] with transition metals has
a recoverable catalyst for organic synthesis.
been reported to give the polymer network-supported metal
To demonstrate the potential of the sPS
palladium nanoparticles (Pd@sPS N) as a useful recyclable
catalyst, we selected the α-methylation of ketones with
methanol because this reaction is atom economical and
environmentally benign, which generates water as the only
byproduct. The C-alkylation with alcohols involving
hydrogen auto-transfer reaction has received increasing
attention for the construction of carbon−carbon bonds.[12] In
particular, methanol is an abundant, renewable
fundamental chemical feedstock, and the use of methanol
as a methylation agent is of great interest and importance.
-N-immobilized
nanoparticles that could show improved thermal stability and
chemical (solvent) resistance. Nevertheless, Despite these
recent advances, the development of new polymer-supported,
heat- and chemical- resistant metal nanoparticle catalysts
remains an attractive and important research subject.
Syndiotactic polystyrene (sPS) is known to show high
melting point, good crystallinity, and excellent resistance to
heat and chemicals.[8] The use of sPS for the intercalation
of functional molecules[9] and gold nanoparticles[10] in the
semi-crystalline polymer nanopores has been previously
reported. However, because of the lack of polar functional
groups, the capability of sPS for the immobilization of
recoverable active metal catalysts has been limited. We
have recently achieved the synthesis of amino-
functionalized syndiotactic polystyrene (sPS-N) by the
-
-
However, the dehydrogenation energy of methanol (∆H=
84 kJ mol-1) is higher than those of longer alcohols such as
ethanol (∆H =
68 kJ mol-1),[13] and therefore, the use of
﹢
﹢
methanol as a C1 alkylation agent is more difficult. In 2011,
Krische and coworkers reported the pioneering direct C−C
coupling of methanol with allenes using a homogeneous
iridium catalyst.[14] More recently, the groups of Obora,[15]
Donohoe,[16] Andersson[17] and Li[18] achieved the α-
methylation of ketones with methanol by using
homogeneous iridium and rhodium catalysts. Despite these
recent advances, a recyclable heterogeneous catalyst for
the methylation of ketones with methanol has remained
unknown to date, although several heterogeneous catalysts
such as Ru/hydrotalcite,[19] Pd/AlO(OH),[20] Pd/C,[21]
Ag/Al2O3,[22] and Pd/viologen-polymer[23] have been
reported for C-alkylation reactions with benzyl alcohols or
long chain aliphatic alcohols.
syndiospecific
polymerization
of
para-N,N-
dimethylaminostyrene using a half-sandwich scandium
catalyst.[8c,11] This polymer not only exhibited good thermal
stability (melting point 252 C), crystallinity, and chemical
[a]
[b]
L. Jiang, Dr. F. Guo, Dr. Z. Shi, Prof. Dr. Y. Li, Prof. Dr. Z. Hou
State Key Laboratory of Fine Chemicals, Department of Polymer
Science and Engineering, School of Chemical Engineering, Dalian
University of Technology
Dalian 116012 (China)
E-mail: houz@riken.jp, guofang@dlut.edu.cn, liyang@dlut.edu.cn
Prof. Dr. Z. Hou
We report here our studies on the α-dimethylation and
cross methyl-alkylation of ketones with methanol as the
methylation agent using the sPS-N-immobilized palladium
catalyst (Pd@sPS-N) (Scheme 1). We found that the
palladium nanoparticle catalyst Pd@sPS-N not only
Organometallic Chemistry Laboratory and Center for Sustainable
Resource Science, RIKEN
2-1 Hirosawa, Wako, Saitama 351-0198 (Japan)
Supporting information for this article is given via a link at the
end of the document.((Please delete this text if not appropriate))
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10.1002/cctc.201700907
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(Table 1, Run 1). When the reaction was carried out at
120 °C, the dimethylation product 2a was obtained in 90%
yield together with 6% of the monomethylation product 3a
(Table 1, Run 2). The use of 2 equiv. of KOH further
promoted the reaction, affording 2a in 94% yield in 12 h
(Table 1, Run 3). The reaction at further higher temperature
(150 °C) led to hydrogenation of the carbonyl group and
gave the dimethylated alcohol product 4a as a major
product (75%) (Table 1, Run 4). For comparison, various
related palladium catalysts were also examined. In sharp
Table 1. Methylation of acetophenone with methanol by Pd catalysts[a]
Yield [%][b]
T
KOH
T
Run
[Pd]
[mmol]
[h]
[C]
2a
4
3a
17
6
4a
0
contrast,
the
analogous
amino-free
syndiotactic
1
2
3
4
5
6
7
8
9
Pd@sPS-N[c]
Pd@sPS-N[c]
Pd@sPS-N[c]
Pd@sPS-N[c]
Pd@sPS[d]
Pd@aPS-N[e]
Pd@A21-1[f]
Pd/C
1
1
2
2
1
1
1
1
1
90
24
24
12
9
polystyrene- (Pd@sPS, Table 1, Run 5), atactic poly(para-
N,N-dimethylaminostyrene)- (Pd@aPS-N, Table 1, Run 6),
or commercially available Amberlyst A-21 (crosslinked
polystyrene containing dimethylamino group)- (Pd@A21,
Table 1, Run 7) supported palladium catalyst showed much
lower activity under the same conditions. The commercially
available heterogeneous Pd/C catalyst did not work well for
the present methylation of acetophenone with methanol
(Table 1, Run 8), though it showed good activity for the
alkylation of ketones with higher alcohols.[21] The typical
homogeneous palladium catalyst Pd(PPh3)4 gave the
monomethylation product 3a in 7% yield, while the
dimethylation product 2a was not observed under the same
conditions (Table 1, Run 9); note that precipitation of
palladium black occurred in a few hours. These results
clearly demonstrate that the syndiotactic poly(N,N-
120
120
150
120
120
120
120
120
90
94
15
2
0
2
0
2
75
0
24
24
24
24
24
17
10
30
17
7
1
0
24
2
0
0
Pd(PPh3)4
0
0
[a] Reaction conditions: acetophenone (1 mmol), methanol (2 mL), [Pd] (0.005
mmol), KOH (1 or 2 mmol). [b] GC yield based on acetophenone. [c] Palladium
(3 wt%) supported on syndiotactic poly(para-N,N-dimethylaminostyrene). [d]
Palladium (3 wt%) supported on syndiotactic polystyrene. [e] Palladium (3
wt%) supported on atactic poly(para-N,N-dimethylaminostyrene). [f] Palladium
(3 wt%) supported on Amberlyst A-21 (macroporous crosslinked polystyrene).
dimethylaminostyrene) is
a unique platform for the
immobilization of highly active palladium nanoparticle
catalyst.
The transmission electron microscope (TEM) analysis of
Pd@sPS
with a diameter of 2-3 nm were uniformly dispersed in the
sPS N matrix (Figs. 1a and 1b). In contrast, the Pd
nanoparticles in Pd@sPS, and Pd@aPS-N were
significantly aggregated (Figs. 1e and 1f). These results
suggest that both the amino functional group and the
syndiotactic structure of sPS-N should play an important
role in preventing the in-situ generated palladium
nanoparticles from aggregation. At room temperature, the
Pd morphology and distribution in Pd@sPS-N and Pd@A-
-
N revealed that spherical palladium nanoparticles
-
showed unusually high activity but it could also be easily
recovered and reused, thus constituting the first example of
a recyclable catalyst for C-alkylation with methanol.
At first, we examined the methylation of acetophenone
with methanol as a model reaction in the presence of
Pd@sPS-N (0.5mol %) and KOH (1 equiv.) under air (Table
1). At 90 °C the reaction was not efficient, which gave a
mixture of the dimethylation and monomethylation products
2a and 2b in 4% and 17% yields, respectively in 24 h
a) Pd@sPS-N
b) Pd@sPS-N
c) Pd@sPS-N(180C)
d) Pd@sPS-N(230C)
f) Pd@aPS-N
e) Pd@sPS
h) Pd@A-21(120C)
g) Pd@A-21
Figure. 1 TEM images.
a
) and
b
) Pd@sPS-N (original),
c
) and
d
) Pd@sPS-N after being heated at 180 C and 230 C, respectively in toluene for 2
) Pd@A-21 after being heated at 120 C in toluene for 1 day.
days,
e
) Pd@sPS (original),
f
) Pd@aPS-N (original),
g
) Pd@A-21 (original), h
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Table 2. Methylation of various ketones with methanol by Pd@sPS-N[a]
Table 3. Pd@sPS-N-catalyzed three component methyl-alkylation reaction of
various ketones with methanol and higher alcohols[a]
T
[C]
KOH
[mmol]
t
Yield
[%][b]
Run
1
Ketone
Product
[h]
T
[h]
Yield
[%][b]
Run
1
Ketone
Alcohol
Product
120
150
2
3
16
11
86
89
8
86
91
2
3
4
2
10
3
10
91
180
120
3
2
10
3
75
68
4
5
8
6
87
95
5
6
7
120
120
180
2
1
3
8
20
6
90
96
97
6
7
10
88
92
10
8
OH
8
91
69
57
8
9
120
120
2
2
2
62
80
2,
15
9[c]
48
3,
48
10[d]
10
11
12
13
150
150
120
120
3
3
2
2
18
30
10
16
76
78
94
87
[a] Reaction conditions: ketone (1.25 mmol), methanol (2 mL), alcohol (1.0
mmol), [Pd] (0.005 mmol), KOH (2.0 mmol), 120 °C, unless otherwise noted.
[b] All yields are isolated yields. [c] After the reaction of ketone (1.0 mmol) with
propanol (5.0 mmol) and KOH (1.0 mmol) was carried out for 2 h, another
portion of KOH (1.0 mmol) and methanol (2 mL) were added, further reacted
for 15 h. [d] After the reaction of ketone (1.0 mmol) with propanol (10.0 mmol)
and KOH (2.0 mmol) was carried out for 3 h, another portion of KOH (2.0
mmol) and methanol (2 mL) were added, further reacted for 48 h.
heated at 180 °C for two days (Fig. 1c). At 230 °C, which is
close to the melting point of sPS-N (252 °C), the Pd
nanoparticles in Pd@sPS-N began to aggregate (Fig. 1d).
The unusual robustness of the Pd nanoparticles in
Pd@sPS-N may result from the unique interplay of the
dimethylamino functional groups and the syndiotactic
polystyrene structures[10] as well as the excellent thermal
stability and chemical resistance of the sPS-N polymer
which may therefore effectively prevent the palladium
nanoparticles from aggregation or leaching. This could
[a] Reaction conditions: ketone (1 mmol), methanol (2 mL), [Pd] (0.005
mmol), KOH. [b] All yields are isolated yields.
21 were similar to each other (Figs. 1a and 1g). On heating
at 120 °C, the size of the Pd nanoparticles in Pd@A-21
significantly increased (Fig. 1h). In sharp contrast, the size
and the uniform distribution of the palladium nanoparticles
in Pd@sPS-N remained almost unchanged even after being
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10.1002/cctc.201700907
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account for the unusually high activity of Pd@sPS-N
compared to other related catalysts under the present
reaction conditions (Table 1).
Table 4. Pd@sPS-N recycling for the dimethylation of acetophenone with
methanol[a]
The dimethylation of various ketones with methanol
was then examined by using the Pd@sPS-N catalyst. Some
representative results are shown in Table 2.
Acetophenones having either electron-donating (such as
OMe and NMe2) or electron-withdrawing (such as CF3)
groups at the phenyl ring were all suitable for this reaction,
selectively affording the dimethylation products in high
yields (Table 2, Runs 1−4). Substituents at the ortho
positions of the carbonyl group did not hamper the high
reaction efficiency (Table 2, Runs 5−7). In the case of 4-
phenyl-2-butanone (1i), the reaction at 120 C for 2 h gave
selectively the methylene monomethylation product 2i in
62% yield (Table 2, Run 8), while the reaction for a longer
time (48 h) led to methylation at both the methylene unit
and the methyl group, affording the trimethylation product
2i' in 80% yield (Table 2, Run 9). These results suggest that
the methylene unit is more reactive than the methyl group.
Methyl isopropyl ketone (1j) and methyl cyclohexyl ketone
Use
1
2
3
4
5
2a yield [%][b]
95
93
97
92
91
[a] Reaction conditions: acetophenone (3.0 mmol), methanol (6.0 mL), [Pd]
(0.015 mmol), KOH (6.0 mmol). [b] GC yield based on acetophenone.
(
1k) exclusively underwent dimethylation at the methyl
group, affording 2j and 2k, respectively in high yields (Table
2, Runs 10 and 11). The methylation of
benzocyclohexanone (1l) and cyclohexanone (1m) was
also efficiently achieved, giving 2l and 2m, respectively in
high yields (Table 2, Runs 12 and 13). It is also worth
noting that all of these reactions could be carried out under
air with a catalyst loading as low as 0.5 mol %.
Figure 2. TEM images of Pd@sPS-N: a) original (b) after fifth reuse.
The present methylation of ketones with methanol may
possibly proceed according to a reaction pathway reported
previously.[15,16a,17] The dehydrogenation of methanol by the
Pd nanoparticles in Pd@sPS-N could result in formation of
formaldehyde and a Pd–hydride species, which is a key
step in the process of ketone alkylation. Base-catalyzed
cross-aldol condensation of formaldehyde with a ketone
would produce an α,β-unsaturated ketone, which could be
quickly reduced by the Pd–hydride species to give the α-
methylated product and regenerate the Pd nanoparticles
(See Scheme S1, ESI†).
With the success of the methylation of a wide range of
ketones with methanol, we then examined the three-
component cross methyl-alkylation reactions of ketones
with methanol and higher alcohols. Some representative
results are given in Table 3. The cross methyl-benzylation
of various ketones in the coexistence of methanol and
benzyl alcohols selectively took place in the presence of
Pd@sPS-N (0.5 mol % Pd) under air, affording the mixed
multi-substituted ketone products 5a−h in 86−95% yields
(Table 3, Runs 1−8). The gas chromatography monitoring
confirmed that the reaction occurred sequentially first by
alkylation with a benzyl alcohol followed by methylation with
methanol, probably because of the higher activity of benzyl
alcohol than that of methanol (Table S1, ESI†). These
results also demonstrate that the Pd@sPS-N catalyst can
distinguish a benzyl alcohol from methanol under their
coexistence despite its high catalytic activity. In the reaction
of propanol and methanol with ketones, a stepwise feeding
of propanol and methanol was needed to selectively
achieve the mixed propyl-methylation, but the reaction
could still be run in a “one-pot” fashion (Table 3, Runs 9
and 10).
To examine the recyclability of the Pd@sPS-N catalyst,
a 3 mmol scale reaction of acetophenone (1a) with
methanol was carried out by using the original and the
recovered catalysts (Table 4). After the first reaction, which
gave the dimethylation product 2a in 95% yield, the catalyst
was recovered simply by filtration without need of
centrifugation. The recovered of catalyst was repeatedly
used in five cycles without significant the activity (91−97%
yields of 2a). Catalyst leaching or palladium metal
aggregation was almost negligible (see Figure 2 and Table
S2, ESI†). These results suggest that the palladium
nanoparticles in Pd@sPS-N are highly robust.
In summary, we have demonstrated that the palladium
nanoparticles supported in a syndiotactic poly(para-N,N-
dimethylaminostyrene) polymer (Pd@sPS-N) can serve as
an excellent catalyst for the α-alkylation of a wide range of
ketones with alcohols (including the less reactive methanol)
as the alkylation agents. The reaction can be carried out
under air without using an extra solvent. The immobilized
palladium nanoparticle catalyst Pd@sPS-N is highly robust,
showing excellent thermal stability and chemical resistance.
It not only exhibited the highest TON (TOF) ever reported
for ketone alkylation with methanol, but it can also be easily
recovered and reused. The unusually high performance of
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10.1002/cctc.201700907
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palladium nanoparticles. Studies on the application of the
palladium nanoparticle catalyst Pd@sPS-N to other
chemical transformations as well as on the use of
functionalized syndiotactic polystyrenes for immobilization
of other metal catalysts are currently under progress.
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This work was financially supported by National Natural Science
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Keywords: alkylation • ketone • methanol • palladium • polymer-
supported catalyst
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10.1002/cctc.201700907
ChemCatChem
COMMUNICATION
COMMUNICATION
Lei Jiang, Fang Guo,* Zhenghai Shi,
Yang Li*, and Zhaomin Hou*
Page No. – Page No.
Syndiotactic Poly(aminostyrene)-
Supported Palladium Catalyst for
Ketone Methylation with Methanol
A novel recyclable and highly active catalyst (Pd@sPS-N) for the methylation and
methyl-alkylation of a wide range of commercially accessible ketones with methanol
as the methylation agent was established by immobilization of palladium
nanoparticles on a unique amino-functionalized syndiotactic polystyrene.
This article is protected by copyright. All rights reserved.