Convenient deoxygenation of aromatic ketones by silica-supported chitosan schiff base palladium catalyst

Article abstract of DOI:10.1080/00397911.2010.527423

A novel and effective catalytic system, silica-supported chitosan Schiff base palladium complex (SiO₂-CS-Schiff base-Pd) was described for catalytic hydrogenation of aromatic ketones to corresponding hydrocarbons at ambient temperature and normal pressure of H₂. Taylor & Francis Group, LLC.

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Convenient Deoxygenation of Aromatic
Ketones by Silica-Supported Chitosan
Schiff Base Palladium Catalyst
S. W. Gong a , H. F. He a , Ch. Q. Zhao a , L. J. Liu a & Q. X. Cui a
a Department of Chemistry, Liaocheng University, Liaocheng,
Shandong, China
Accepted author version posted online: 04 Aug 2011.Published
online: 17 Oct 2011.
To cite this article: S. W. Gong , H. F. He , Ch. Q. Zhao , L. J. Liu & Q. X. Cui (2012): Convenient
Deoxygenation of Aromatic Ketones by Silica-Supported Chitosan Schiff Base Palladium Catalyst,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 42:4, 574-581
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Synthetic Communications1, 42: 574–581, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.527423
CONVENIENT DEOXYGENATION OF AROMATIC
KETONES BY SILICA-SUPPORTED CHITOSAN SCHIFF
BASE PALLADIUM CATALYST
S. W. Gong, H. F. He, Ch. Q. Zhao, L. J. Liu, and Q. X. Cui
Department of Chemistry, Liaocheng University, Liaocheng,
Shandong, China
GRAPHICAL ABSTRACT
Abstract A novel and effective catalytic system, silica-supported chitosan Schiff base pal-
ladium complex (SiO2–CS–Schiff base–Pd) was described for catalytic hydrogenation
of aromatic ketones to corresponding hydrocarbons at ambient temperature and normal
pressure of H2.
Keywords Aromatic ketones; chitosan support; hydrogenation; palladium catalyst
INTRODUCTION
=
The reduction of C O group to CH2, as famous Clemens or Wolff–Kishner
reactions, has been widely used and extensive studied since the past century.[1–3]
However, the reactions were either carried out in the presence of concentrated
HCl or employed a lot of hydrazine as reactant, both of which are not environmen-
tally benign. Many endeavors have been made to improve them, most of which were
[4]
[5]
introductions of new reductive agents. For example, LiAlH4 or NaBH4 have
been used to replace zinc=HCl or hydrazine for removing oxygen of carbonyl com-
pounds. Catalyzed by Cu-Cr, Fe, or Ni complex, hydrogen was also utilized as a
deoxygenating agent although the reaction proved less active and high temperature
(473–573 K) was essential.[6–8] More recently, palladium-based catalysts for hydroge-
native deoxygenation have been reported.[9–11] Avnir[9] once reported the catalytic
Received July 18, 2010.
Address correspondence to S. W. Gong, Department of Chemistry, Liaocheng University,
Liaocheng, 252059 Shandong, China. E-mail: gongshw@lcu.edu.cn
574

DEOXYGENATION OF AROMATIC KETONES
575
Table 1. Hydrogenation reaction of acetophenone over different catalystsa
Product (%)
Reaction time (h) Conversionb (%) Ethylbenzene 1-Phenylethanol
Entry
Catalyst
—
1
2
3
4
5
3
3
5
3
3
—
100
61
—
—
—
26
60
—
100
100
74
PdCl2
SiO2–CS–Pd
CS–Schiff base–Pd
SiO2–CS–Schiff base–Pd
100
100
40
aReaction conditions: 0.5 mL substrate, 0.2 g catalyst, 15 mL ethanol, 45 mL=min H2, 313 K.
bConversion was determined by GC analysis.
hydrogenolysis of aromatic ketones by
a
sol-gel entrapped combined
Pd-[Rh(cod)Cl]2 catalyst, but the selectivity of hydrogenation reaction toward
the methylene derivative was poor and the aromatic rings were fully hydrogenated.
In the case of palladium catalyst, the promotional effect of ionic liquids for
hydrogenolysis was also explored.[12]
Herein we disclosed a new, convenient method for deoxygenation of aromatic
ketones catalyzed by silica-supported chitosan Schiff base palladium complex (SiO2–
CS–Schiff base–Pd, which was abbreviated as SCSS-Pd). A commercially available
substance, chitosan is also a major naturally occurring biopolymer, as one of the
key constituents of the shells of crustaceans, and is a by-product of the fishing indus-
try. The flexibility of the material, insoluble in the vast majority of solvents, along
with its inherent chirality, make chitosan an excellent candidate as a support for cat-
alysts.[13–16] Chitosan-supported palladium catalysts have been used to hydrogenate
or degradate nitroaniline,[17] nitrotoluene,[18,19] nitrophenol,[20] nitrobenzene,[21]
chlorophenol,[22] octane,[23] etc.[24,25] However, the hydrogenation of aromatic
ketone catalyzed by SCSS-Pd has not been reported.
RESULTS AND DISCUSSION
As show in Table 1, different palladium catalysts for hydrogenation of aceto-
phenone were first studied, and the reaction equation is shown in Scheme 1. No reac-
tion took place in the absence of catalyst (entry 1). After 3 h of hydrogenation, PdCl2
exhibited high activity of hydrogenation but only 1-phenylethanol was formed as the
sole product (entry 2). For SiO2-CS-Pd catalyst, 61% acetophenone was hydroge-
nated into 1-phenylethanol within 5 h (entry 3), which was accordance with the
reported silica-supported CS-Pd catalyst that has high catalytic activity for hydro-
genation of ketones to form corresponding alcohols,[24,25] Yuan[24] once reported
the asymmetric transfer hydrogenation of acetophenone catalyzed by silica-
supported CS-Pd catalyst, and optical yields of R-1-phenylethanol could reach
99% after 8 h of reaction at 303 K under a hydrogen atmosphere. In the presence of
Scheme 1. Hydrogenolysis of acetophenone to ethylbenzene.

576
S. W. GONG ET AL.
Table 2. Hydrogenation reaction of acetophenone in different solventsa
Product (%)
Entry
Solvent
Conversionb (%)
Ethylbenzene
1-Phenylethanol
1
2
3
4
5
6
7
Ethanol
Methanol
Isopropanol
CH3CN
CCl4
100
100
100
60
86
—
—
—
—
—
37c
14
—
—
—
—
5
No reaction
No reaction
No reaction
No reaction
DMSO
Neat
aReaction conditions: 0.5 mL substrate, 0.2 g catalyst, 15 mL ethanol, 45 mL=min H2, 313 K, 5 h.
bConversion was determined by GC analysis.
cSome methyl ethers were observed.
unsupported CS–Schiff base–Pd catalyst, 100% acetophenone was converted and the
expected ethylbenzene was formed in 26% yields. In this sense, the modification of
chitosan with salicylaldehyde to produce Schiff base could improve the hydrogen-
ation selectivity of catalyst via the coordination between chitosan and palladium
atom. In other words, the insertion of a functional group of Schiff base in the chit-
osan matrix may improve its capacity to interact with metallic ions by complexa-
tion.[26] For supported catalyst, the SCSS-Pd, gave 100% conversion of
acetophenone and ethylbenzene formed in 60% yield after 3 h of reaction at 313 K,
and atmospheric hydrogen proved to be the most efficient for deoxygenation
(Table 1, entry 5). Therefore, silica seemed to be an excellent support to optimize
the catalytic activity and selectivity, which may be due to the high specific surface
area of silica to enhance the catalytic chance of substrates and active sites of catalyst,
=
so it was chosen as support for full reduction of C O to CH2.
Different solvents were studied for SSCS-Pd-catalyzed deoxygenation. As
shown in Table 2, proton solvents were essential to the reaction. In ethanol or meth-
anol, the reaction gave 100% conversion but by-product 1-phenylethanol formed in
37% yield when the reaction was carried out in methanol. In iso-prapanol, only a
trace of acetophenone was converted. In dimethylsulfoxide (DMSO), tetrachloride,
acetonitrile, or the absence of solvent, the reaction cannot take place.
The effect of reaction temperature on the hydrogenation of acetophenone
was shown in Table 3. Temperature was important for the reaction. For example,
Table 3. Hydrogenation reaction of acetophenone at different temperaturea
Product (%)
Entry
Reaction temperature (K)
Conversionb (%)
Ethylbenzene
1-Phenylethanol
1
2
3
4
rt
91
100
100
100
6
35
94
65
—
—
303
313
323
100
100
aReaction conditions: 0.5 mL substrate, 0.2 g catalyst, 15 mL ethanol, 45 mL=min H2, 5 h.
bConversion was determined by GC analysis.

DEOXYGENATION OF AROMATIC KETONES
577
Table 4. Effect of reaction time on hydrogenation of acetophenonea
Product (%)
Entry
Reaction time (h)
Conversionb (%)
Ethylbenzene
1-Phenylethanol
1
2
3
4
5
6
0.5
1
11
90
10
15
90
85
66
40
8
2
100
100
100
100
34
60
3
4
92
100
5
—
aReaction conditions: 0.5 mL substrate, 0.2 g catalyst, 15 mL ethanol, 45 mL=min H2, 313 K.
bConversion was determined by GC-MS analysis.
the hydrogenation of acetophenone in ethanol at room temperature gave 91%
conversion and only 6% of ethylbenzene (Table 3, entry 1). When temperature was
raised to 303 K, full conversion was detected, and the selectivity of ethylbenzene was
improved to 35%. The best result was obtained under 313 K, in which case 100% of
acetophenone was conversed to ethylbenzene after 5 h of reaction (Table 3, entry 3).
The effect of reaction time on the hydrogenation of acetophenone was shown in
Table 4, in which SCSS-Pd and ethanol were used as catalyst and solvent, respectively.
The results showed that hydrogenation of acetophenone catalyzed by SCSS-Pd gave
100% conversion within only 2 h, with the formation of 66% by-product of 1-pheny-
lethanol (Table 4, entry 3). When reaction time was prolonged to 5 h, only ethylben-
zene was detected, which indicated that the selectivity to 1-phenylethanol was
decreasing with increasing reaction time, or in other words, the CHOH functions
may be further reduced to CH2 over SCSS-Pd catalyst in the presence of hydrogen.
Different aromatic ketones 1a–1g were examined with these optimized con-
dition to study the scope and the limitations of this methodology. As shown in
Table 5, aromatic ketones afforded corresponding hydrogenation reduced 2a–2g
as major products, in some cases accompanied with trace products of hydrogenation
of aromatic rings (Table 5, entries 3 and 6). Hydrogenation reduction of 1a, 1b, and
1d formed only deoxygenation products. Diketone such as 1f could also be hydroge-
nated to remove both oxygen atoms (Table 5, entry 6). As a comparison, cyclohex-
anone (Table 5, entry 5) was hydrogenated under similar conditions to give
corresponding cyclohexanol in only 42% yield, which indicated that the SCSS-Pd
catalyst may be inactive to hydrogenation of aliphatic ketones.
As indicated, the hydrogenolysis of acetophenone into ethylbenzene could first
proceed via intermediate 1-phenylethanol, followed by further hydrogenolysis of
CHOH to form the final CH2 group. To confirm this point, 1-phenylethanol was
hydrotreated in ethanol solvent with SCSS-Pd as catalyst; after 5 h of reaction, no
1-phenylethanol was detected and the product was only ethylbenzene. Benzoin,
=
which contains an aromatic C O and a CHOH group, was also evaluated with same
=
hydrogenolysis condition; not only C O but also hydroxyl group was all transferred
into methylene (Table 5, entry 7), which indicated that CHOH was the intermediate
in the hydrogenolysis progress.
After the hydrogenation reaction, the SCSS-Pd catalyst was well isolated by
filtration and could be reused after drying, so acetophenone, 1-tetralone, and

578
S. W. GONG ET AL.
Table 5. Hydrogenation of some different carbonyl compounds in presence of the SiO2–CS–Schiff
base–Pd catalyst
Entry
Substrate
Conversion (%)a
Product (yield, %)
1
2
100
100
3
100
4
5
100
42
6
7
100
100
aConversion was determined by GC analysis.
benzophenone were selected as substrates to evaluate the recycling performance of
catalyst under corresponding reaction conditions. For all the reuse experiments,
all selected substrates retained almost 100% hydrogenation conversion over
SCSS-Pd catalyst for at least three runs. With additional runs, the hydrogenolysis
=
selectivity to methylene from the corresponding C O dicreased gradually. For
example, the hydrogenation of benzophenone over three runs with SCSS-Pd catalyst
gave 96% conversion and 87% selectivity to diphenylmethane, which indicated
that the SCSS-Pd catalyst was stable in this hydrogenolysis progress and could be
repetitively used.
The results clearly demonstrate the capacity of the SCSS-Pd catalyst system
for the hydrogenolysis of aromatic ketones to corresponding hydrocarbon com-
pounds at low temperature and hydrogen pressure. We have developed a practical
=
hydrogenolysis of C O group to CH2 catalyzed by the SCSS-Pd catalyst system,

DEOXYGENATION OF AROMATIC KETONES
579
which could be successfully applied to the reduction of aromatic ketones to corre-
sponding hydrocarbons. In contrast to classical reductions of carbonyl, an important
benefit of this method is that no pollution occurred in all the hydrogenolysis reaction
progress. Other advantages of this methodology include easy preparation of the
catalyst, good yields, mild reaction conditions, room temperature, and 1 atm H2.
We believe that this procedure will be a useful and attractive alternative to the exist-
ing methods for the reduction of aromatic ketones to synthesize linear alkylbenzenes.
EXPERIMENTAL
Preparation of SiO2–CS–Schiff Base–Pd Catalyst
According to Ref. 24, silica-supported chitosan (SiO2-CS) was first prepared,
and salicylaldehyde was used to prepare SiO2–CS–Schiff base. Then the prepared
SiO2–CS–Schiff base and PdCl2 were weighed in 10:1 mass ratio and were added
to ethanol solvent, respectively. After the mixture was stirred at 303 K for 72 h,
the solid product was filtered and washed with ethanol until the filtrate became
transparent and colorless, and then vacuum dried at 323 K to obtained brown cata-
lyst particles, namely SiO2–CS–Schiff base–Pd.
Characterization of Catalysts
The physicochemical characterization of the catalyst has been done by X-ray
diffraction (XRD), thermogravimetry, and Brunauer–Emmett–Teller (BET) surface
area measurements. The inductively coupled plasma atomic emission spectroscopy
(ICP-AES) method was adopted to estimate the content of Pd in the catalyst.[27]
Reaction Procedure
In a typical experiment, a mixture of aromatic ketones substrate (0.3 g for solid
substrates or 0.5 mL for liquid substrates) and catalyst (0.2 g) was charged in 15 ml
ethanol. The reaction vessel was flushed (three times) with pure hydrogen and then
retained the 45 mL=min current velocity of hydrogen. The reaction mixture was stir-
red magnetically at a rate of 150 rpm at 313 K for 5 h. Then the catalyst was filtered
1
and the identity of the reaction products was verified by H NMR, reference com-
pounds, and GC-MS using an Agilent 6890 gas chromatograph equipped with a
FID detector and a HP-5MS column, coupled to a 5973 MSD mass spectrometer.
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China (No. 20772055)
and the Doctor Foundation of Shandong Province (BS2010CL011) for financial sup-
port of this study.
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