Palladium oxide nanoparticles supported on graphene oxide: A convenient heterogeneous catalyst for reduction of various carbonyl compounds using triethylsilane

Article abstract of DOI:10.1002/aoc.4837

Palladium oxide nanoparticles supported on graphene oxide - triethylsilane was found to be an effective reductive system for a broad range of reduction processes, including the reduction of various carbonyl compounds such as aromatic aldehydes to their corresponding alcohols or methyl arene compounds, aromatic ketones to their respective alcohols or saturated compounds, aromatic acyl chlorides to their reduced compounds. The desired products were obtained in good to excellent yields under mild conditions. The heterogeneous environmentally friendly catalyst can be easily separated from the reaction mixture through a simple filtration, facilitating purification of the prepared compounds.

Full text of DOI:10.1002/aoc.4837

Received: 6 November 2018
DOI: 10.1002/aoc.4837
Revised: 24 December 2018
Accepted: 7 January 2019
F U L L P A P E R
Palladium oxide nanoparticles supported on graphene
oxide: A convenient heterogeneous catalyst for reduction
of various carbonyl compounds using triethylsilane
Maryam Mirza‐Aghayan1
| Meisam Kalantari1 | Rabah Boukherroub2
1 Chemistry and Chemical Engineering
Research Center of Iran (CCERCI), P. O.
BOX 14335‐186, Tehran, Iran
2 Univ. Lille, CNRS, Centrale Lille, ISEN,
Univ. Valenciennes, UMR 8520 ‐ IEMN,
F‐59000 Lille, France
Palladium oxide nanoparticles supported on graphene oxide ‐ triethylsilane
was found to be an effective reductive system for a broad range of reduction
processes, including the reduction of various carbonyl compounds such as aro-
matic aldehydes to their corresponding alcohols or methyl arene compounds,
aromatic ketones to their respective alcohols or saturated compounds, aro-
matic acyl chlorides to their reduced compounds. The desired products were
obtained in good to excellent yields under mild conditions. The heterogeneous
environmentally friendly catalyst can be easily separated from the reaction
mixture through a simple filtration, facilitating purification of the prepared
compounds.
Correspondence
Maryam Mirza‐Aghayan, Chemistry and
Chemical Engineering Research Center of
Iran (CCERCI), P. O. BOX 14335‐186,
Tehran, Iran.
Email: m.mirzaaghayan@ccerci.ac.ir
KEYWORDS
carbonyl compounds, graphene oxide, palladium oxide nanoparticles, reduction, triethylsilane
1 | INTRODUCTION
compounds with hydrosilanes in presence of Lewis or
Brønsted acids is well‐known.[13–17] The combination
Reducing agents represent an important and fundamen-
tal class of reagents to synthetic chemists for the transfor-
mation of organic substrates to desired organic
products.[1] Hydride reduction of aldehydes and ketones
can be achieved by common hydride‐bearing reducing
agents such as LiAlH4 and NaBH4 or di‐isobutyl alumin-
ium hydride (DIBAL) to give the corresponding alco-
hols.[2] Very different methods also investigated the
complete reductive conversion of a carbonyl group to a
methylene group. Complete removal of oxygen‐
containing groups or deoxygenation can be achieved by
Wolff‐Kischner,[3–5] Clemmensen,[6,7] and Mozingo or
thioketal reduction reactions.[8,9] It should be noted that
thioketal reduction is milder than either the Wolff‐
Kishner or Clemmensen reduction reactions, which
employ respectively strong basic or acidic conditions, that
can affect other functional groups.[10]
of a silane with a transition‐metal catalyst has been
established for several reactions such as the reduction
of Schiff bases,[18] selective dehydroxylation of second-
ary benzylic alcohols,[19] and direct catalytic deoxygen-
ation of propargyl alcohols.[20] In recent reports, we
have demonstrated the efficiency of Et3SiH/PdCl2 cata-
lytic system for various organic transformations,[21,22]
including the reduction of aromatic carbonyl
compounds.[23]
Graphite oxide, graphene oxide (GO), reduced
graphene oxide (rGO), graphene and their combination
with metal nanoparticles have been widely utilized as
effective heterogeneous catalysts for various organic con-
versions.[24–33] For example, Pd‐supported graphene
oxide under a hydrogen atmosphere (1 atm) has been
investigated as catalyst for the reduction of α,β‐
unsaturated
carbonyl
compounds.[30]
GO/PdO
Trialkylsilanes are known to be poor/mild reducing
agents,[11,12] even though reduction of carbonyl
nanohybrid was used as a heterogeneous catalyst for
reduction of 4‐nitrophenol using NaBH4,[31] and Pd/PdO
Appl Organometal Chem. 2019;e4837.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4837

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MIRZA‐AGHAYAN ET AL.
nanoparticles supported on porous graphene was utilized
as an electrocatalyst for methanol oxidation.[32]
2 | EXPERIMENTAL
2.1 | General information
Herein, we report on the preparation of palladium
oxide nanoparticles supported on graphene oxide (PdO
NPs/GO) and their investigation as a heterogeneous cata-
lyst for the reduction of various carbonyl compounds in
presence of triethylsilane (Et3SiH). Compared to other
reducing agents or catalysts utilized for related reduction,
PdO NPs/GO offers some benefits such as low cost, ease
of synthesis, and good reusability.
Ultrasonic homogenizer Bandelin Sonoplus HD 3100
(probe ultrasonic) and Elmasonic P ultrasonic cleaning
unit (bath ultrasonic) were used to prepare the PdO
NPs/GO nanocomposite samples. Palladium content of
the PdO NPs/GO catalyst was determined using
SPECTRO ARCOS ICP‐OES analyzer after sample
FIGURE 1 SEM images of (a) graphite oxide, (b) PdO NPs/GO hybrid, and the EDX spectra of (c) graphite oxide and (d) PdO NPs/GO
hybrid

MIRZA‐AGHAYAN ET AL.
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FIGURE 2 (a) AFM image and (b) three‐dimensional AFM image of PdO NPs/GO sheet; (c) shows the height profile at the indicated place
in red line in Figure 2a
dissolution in a mixture of HNO3/HCl (1/3 ratio). SEM
and EDX data were acquired using a VEGA3 LMU
TESCAN SEM. FT‐IR spectra were recorded from KBr
disks with a Bruker Vector 22 FT‐IR spectrometer. The
surface morphology of PdO NPs/GO catalyst was charac-
terized by atomic force microscopy (AFM). High resolu-
tion transmission electron microscopy (HRTEM) images
were recorded on a JEOL, JEM‐2100F, 200KV TEM.
UV–Vis absorption spectra were recorded using an
Agilent 8453 spectrophotometer. X‐ray photoelectron
spectroscopy (XPS) were collected on a Thermo Scien-
tific, ESCALAB 250Xi Mg X‐ray resource.
2.2 | Catalyst preparation
Graphite oxide powder (200 mg) in 200 ml of deionized
water were sonicated for 2 hr in an Elmasonic P
FIGURE 3 HRTEM images of the PdO NPs/GO nanocomposite

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MIRZA‐AGHAYAN ET AL.
FIGURE 4 (a) XPS wide‐scan spectrum and high resolution spectra of the (b) C1s, (c) O1s and (d) Pd3d of PdO NPs/GO composite
ultrasonic cleaning unit (ultrasonic bath) with a fre-
quency of 37 kHz and 100% output power at ambient
temperature and air atmosphere. Then 10 ml of the previ-
ously prepared GO dispersion in 100 ml of deionized
water were sonicated for 30 min using bath ultrasonic
with a frequency of 80 kHz at ambient temperature.
Afterwards, 4 ml of aqueous solution of H2PdCl4
(50 mM), prepared by dissolving 177.3 mg of palladium
chloride powder in 100 ml of 20 mM HCl solution, was
added to the previous mixture. The resulting mixture
was placed in an ice bath and sonicated for 40 min using
an ultrasonic homogenizer Bandelin Sonoplus HD 3100.
Next, the mixture was centrifuged at 3700 rpm for
20 min and washed three times with deionized water.
Finally, the residue was filtered followed by dehydration
on a rotary evaporator under vacuum and dried in an
oven at 80 °C for 2 hr.
TABLE 1 Different conditions for reduction of benzaldehyde in
ethanol
PdO NPs/
GO (mg)
Et3SiH
(mmol)
Product
A/B/C (%)a
Entry
Time
1
2
3
‐
2
180 min
24 hr
98/2/0
7.5
7.5
‐
100/0/0
0/76/24
0/10/90
0/0/100
2
10 min
30 min
30 min
+ 2
4
4
7.5
FIGURE 5 UV–Vis spectra of GO dispersion (a), PdCl2 (b), and
PdO NPs/GO nanocomposite (c)
aDetermined by GC analysis and p‐xylene was used as an internal standard.

MIRZA‐AGHAYAN ET AL.
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TABLE 2 Reduction of carbonyl compounds using PdO NPs/GO catalyst and triethylsilane
Entry
Substrate
Product
Et3SiH (mmol)
Time (min)
GC Yielda (%)
1
2
4
10
30
24
100
2
3
2
4
10
30
50
100
2
4
10
30
48
92
4
5
6
4
30
100
2
6
10
45
0b
95
6
45
68
7
2
10
20
30
40
4c
+2
+2
‐
35
70
80
8
9
6
45
85
6
120
54
10
11
6
45
100
2
10
20
30
45
0d
30
72
85
+2
+2
‐
12
13
14f
2
20
87e
‐
30
100
2
30
4g
+2
+2
90
27
46
120
15
16
17f
2
20
20
100
‐
100
2
30
28
+2
+2
90
100
100
180
aDetermined by GC analysis and p‐xylene or toluene for entry 4 was used as an internal standard.
b1‐Phenylethanol (83%) and acetophenone (17%) were obtained.
c1‐Phenylpropan‐1‐ol was obtained in 72% along with 24% of the starting material.
dAcetophenone was obtained in 100%.
eEthylbenzoate (87%) and toluene (13%).
fThe reaction was performed in THF as solvent.
gBenzaldehyde (81%) and benzyl alcohol (15%).

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TABLE 3 Reduction of carbonyl compounds to their corresponding alcohol derivatives using PdO NPs/GO catalyst
Entry
Substrate
Product
Et3SiH (mmol)
Time (min)
GC Yielda (%)
1
1.5
20
81
2
3
1.5
1.5
20
15
57
63
4
1.5
2
20
20
10
30
30
60
82
58
83
21
61
81
5
6b
7
2
2
8c
9b
3
6
10d
4
90
73
aDetermined by GC analysis and p‐xylene was used as an internal standard.
bThe reaction was performed with 7.5 mg of catalyst.
cThe reaction was performed with 5.4 mg of catalyst.
dTHF was used as solvent and 7.5 mg of catalyst was used.
on graphene oxide (PdO NPs/GO) for the reduction of ben-
zyl alcohols to the corresponding methylene compounds
under mild conditions.[34] Herein, we took benefit of the
intrinsic reactivity of PdO NPs/GO catalyst for the reduc-
tion of various aromatic carbonyl compounds. The pre-
pared PdO NPs/GO hybrid was characterized using
inductively coupled plasma optical emission spectroscopy
(ICP‐OES), scanning electron microscopy (SEM), atomic
force microscopy (AFM), high resolution transmission
electron microscopy (HRTEM), X‐ray photoelectron spec-
troscopy (XPS) and ultraviolet–visible (UV–Vis)
spectrophotometry.
2.3 | General procedure for reduction of
carbonyl compounds
To a solution of carbonyl compound (1 mmol) and
Et3SiH (amount indicated in Tables 2 and 3) in 5 ml
dry solvent (EtOH or THF) was added PdO NPs/GO
(amount indicated in Tables 2 and 3) under an inert
atmosphere. The resulting mixture was stirred for the
time indicated in Tables 2 and 3. The mixture was fil-
tered and washed with ethyl acetate for catalyst separa-
tion prior to GC analysis. All compounds were known
and their physical and spectroscopic data were compared
with those of authentic samples.
3.1 | Characterization of catalyst
3 | RESULTS AND DISCUSSION
ICP‐OES analysis of prepared catalyst was performed by
completely dissolving the catalyst in a mixture of
HNO3/HCl (1/3 ratio) and the results revealed palladium
loading of 0.85 wt% in the hybrid catalyst.
In continuation of our efforts on the use of Et3SiH/PdCl2
system[21–23] and graphite oxide,[25–27] we have studied
the efficiency of palladium oxide nanoparticles supported

MIRZA‐AGHAYAN ET AL.
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TABLE 4 Reusability of PdO NPs/GO catalyst for reduction of
benzaldehyde
distribution of PdO nanoparticles on the surface of GO
sheet is supported by AFM images except a few little
aggregates.
Run
1st
2nd
3rd
4th
5th
6th
The HRTEM images of the PdO NPs/GO nanocom-
posite revealed the presence of nanoparticles homoge-
neously distributed on the surface of GO sheets
(Figure 3). The nanoparticles display an average diameter
ranging from 2 to 5 nm.
Toluene (%)a
100
100
100
100
100
70b
aThe obtained results after 30 min.
bToluene (70% yield) in the presence of benzyl alcohol (30% yield) were
obtained by GC analysis.
The chemical composition of PdO NPs/GO nanocom-
posite was additionally characterized using X‐ray photo-
electron spectroscopy (XPS) measurements (Figure 4).
The XPS wide‐scan spectrum of PdO NPs/GO comprises
two peaks at about 287 and 533 eV due to C1s and O1s,
respectively and the presence of palladium in the hybrid
catalyst was established by two peaks of Pd3d at about
338 and 343 eV which are close to the reported values
of Pd (II) (Figure 4a).[35–37] The atomic composition of
PdO NPs/GO nanocomposite was obtained from the
XPS survey spectrum and consists of C (67.8%), O
(30.5%) and Pd (1.7%). As shown in Figure 4b, the XPS
high resolution of the C1s spectrum can be fitted with 3
different components assigned to C‐C (284.9 eV), C=O/
C‐O (287.5 eV), and O‐C=O (289.1 eV) bonds,[38] and
their atomic concentrations were determined to be as
49.99%, 44.91% and 5.10%, respectively. XPS high resolu-
tion of the O1s spectrum is deconvoluted into two compo-
nents attributed to C‐O‐C (535.1 eV) and C=O/C‐O
(532.78 eV) bonds. Their atomic concentration was
86.29% and 13.70%, respectively. As shown in Figure 4d,
the peaks at 338.1 and 343.4 eV are assigned to Pd3d3/2
and Pd3d5/2, respectively with a separation of 5.3 eV
between the d5/2 and d3/2 levels, which is very close to
the value reported in the literature.[37] XPS high resolu-
tion of the Pd3d spectrum is deconvoluted into two com-
ponents assigned to PdO (338.1 and 343.4 eV) and PdCl2
(345.5 eV). The XPS results suggest that the palladium is
mostly in form of PdO along with a small amount of
PdCl2, in full agreement with EDX analysis.
Figure 1a‐b shows the SEM images of graphite oxide
and PdO NPs/GO catalyst. The images display typical
wrinkled‐type morphology of GO for both materials.
The chemical composition was examined using
energy‐dispersive X‐ray (EDX) spectroscopy. Figure 2c
depicts the EDX spectrum of GO. It comprises peaks
due to carbon (C), oxygen (O) and sulfur (S). The pres-
ence of sulfur is most likely due to impurities from
H2SO4 used for the preparation of graphite oxide. EDX
spectrum of PdO NPs/GO reveals the presence of C, O,
S, Pd and Cl (Figure 1, d). The presence of Cl results most
likely from Cl adsorption on the oxygen‐containing
groups present on the GO surface.
The morphology of the samples was further examined
by atomic force microscopy (AFM). The sample was pre-
pared by depositing a droplet of PdO NPs/GO hybrid dis-
persion on a glass sheet and dried at room temperature.
Figure 2a‐b displays the AFM images and three‐
dimensional AFM image of PdO NPs/GO sheets, respec-
tively, and Figure 2c depicts the corresponding height
profile at the indicated place in red line in Figure 1a.
Figure 2a‐b reveals that PdO NPs/GO has an average
roughness of 39.7 nm. The average thickness of PdO
NPs/GO is in the range of 1.45 nm (Figure 2c) with an
average height of about 22.5 nm (Figure 2a‐b). As shown
in AFM images, the PdO NPs supported on the surface of
GO sheet appears as brighter spots. The homogeneous
The optical absorption properties of GO, PdCl2 and
PdO NPs/GO are depicted in Figure 5. The UV–Vis spec-
trum of graphite oxide dispersion in water possesses one
peak at ~230 nm due to π‐π* absorption of C=C bonds
and a weak peak at ~310 nm due to n‐π* absorption of
C=O bonds (Figure 5a).[39] The UV–Vis spectrum of
PdCl2 (Figure 5b) exhibits peaks at 207, 236, 307 and
418 nm. The UV–Vis spectrum of PdO NPs/GO nano-
composite dispersion (Figure 5c) displays a peak at
~228 nm attributed to GO and the typical n‐π* absorp-
tion of C=O bond at 310 nm was extinct, while the
C=C bond at 230 nm was shifted to 267 nm for the
PdO NPs/GO sheets, suggesting partial reduction of
graphite oxide sheets.
FIGURE 6 UV–Vis spectra of PdO NPs/GO nanocomposite
before (a), and after (b) five consecutive runs

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MIRZA‐AGHAYAN ET AL.
FIGURE 7 Proposed mechanism
(50%), 4‐methoxybenzyl alcohol (26%) and 4‐
methoxybenzaldehyde (24%) as starting material (entry 2,
Table 2). Following this result, we examined the effect of
Et3SiH concentration on the same reaction by using 4 eq.
of Et3SiH; the reduction of 4‐methoxybenzaldehyde gave
100% of 4‐methylanisole after 30 min (entry 2, Table 2).
The reduction of 2‐methoxybenzaldehyde by using 2 eq
of Et3SiH produced the corresponding 2‐methylanisole
in 48% yield (entry 3, Table 2), while in the presence of
4 eq of Et3SiH, 2‐methylanisole was obtained in 92% after
30 min (entry 3, Table 2). The catalytic reduction of 4‐
methylbenzaldehyde with 4 eq of Et3SiH gave p‐xylene
in 100% after 30 min (entry 4, Table 2).
In the next step, we investigated the reduction of
ketones such as acetophenone, 2‐methoxyacetophenone,
propiophenone, and benzophenone using similar condi-
tions. The reduction of acetophenone with 2 eq of Et3SiH
yielded 1‐phenylethanol in 83% after 10 min along with
17% of the starting material (entry 5, Table 2). By increas-
ing the amount of Et3SiH to 6 eq, ethylbenzene was
obtained in 95% after 45 min (entry 5, Table 2). Similarly,
1‐ethyl‐2‐methoxybenzene was prepared in 68% in the
presence of 25% of the corresponding alcohol along with
7% of the starting material by reduction of 2‐
methoxyacetophenone after 45 min (entry 6, Table 2).
The reduction of 1‐phenyl‐1‐propanone by using 2 eq of
Et3SiH afforded 4% of propylbenzene, 72% of the corre-
sponding alcohol and 24% of the starting carbonyl com-
pound, after 10 min (entry 7, Table 2). By addition of
2 eq. of Et3SiH to the previous mixture, we observed the
formation of 35% of propylbenzene, 63% of the corre-
sponding alcohol and 2% of the starting material. Subse-
quent addition of 2 eq. of triethylsilane (third time) to
the previous mixture gave propylbenzene in 70% along
with 30% of the corresponding alcohol after 30 min. The
GC analysis of the same mixture after 40 min indicated
the formation of propylbenzene (80%) and the corre-
sponding alcohol (20%) (entry 7, Table 2). The reduction
of propiophenone by using 6 eq. of Et3SiH in one step
3.2 | Catalytic performances of PdO
NPs/GO catalyst
For exploiting the intrinsic reactivity of PdO NPs/GO cat-
alyst for reduction of various carbonyl compounds, firstly
we studied the reduction of benzaldehyde (1 mmol)
under different conditions. The reaction was analyzed
by GC after the time indicated in Table 1. When the
reduction of benzaldehyde in ethanol as solvent was car-
ried out in presence of neat triethylsilane or PdO
NPs/GO, only the starting material was recovered, sug-
gesting that Et3SiH and PdO NPs/GO alone were not
active for this chemical transformation (entries 1 and 2,
Table 1). Similarly, the combination of GO and Et3SiH
was not active for this chemical reaction. In contrast,
the combination of PdO NPs/GO and Et3SiH was found
to be very efficient for reduction of benzaldehyde. The
reduction of benzaldehyde takes place using PdO
NPs/GO and Et3SiH to yield toluene after 30 min (entry 3,
Table 1). We observed
a
mixture of benzyl
alcohol/toluene in 76/24 ratio after 10 min using 7.5 mg
of catalyst in presence of 2 mmol of Et3SiH (entry 3,
Table 1). Subsequent addition of Et3SiH (2 mmol) to the
same mixture led to the formation of toluene in 90% yield
after 10 min (entry 3, Table 1). That is to say, by sequen-
tial addition (2 + 2) equivalents of triethylsilane, toluene
was obtained in 90% yield after a total of 30 min reaction
(entry 3, Table 1). When 4 eq. of triethylsilane was added
in one step, the GC yield of the crude mixture showed tol-
uene formation in 100% after 30 min (entry 4, Table 1).
Under optimized reaction conditions, various car-
bonyl compounds (1 mmol) such as aromatic aldehydes,
ketones and acyl chlorides reacted with Et3SiH in pres-
ence of PdO NPs/GO (7.5 mg) in dry solvent to give the
corresponding reduced compounds at room temperature
(Table 2).
Firstly, we examined the reduction of various alde-
hydes. The reaction of 4‐methoxybenzaldehyde with
2 eq. Et3SiH gave the corresponding 4‐methylanisole

MIRZA‐AGHAYAN ET AL.
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led to the formation of 85% of propylbenzene and 15% of
the corresponding alcohol after 45 min (entry 8, Table 2).
By using 6 eq. of Et3SiH for the reduction of benzophe-
none, diphenylmethane was obtained in 54% accompa-
nied by 46% of the corresponding alcohol after 120 min
using 7.5 mg of PdO NPs/GO as catalyst (entry 9,
Table 2).
elimination of esterification reaction, but the reduction
reaction was slow and a prolonged reaction time was nec-
essary. The reduction of benzoyl chloride in THF as sol-
vent led to the formation of benzaldehyde (81%), benzyl
alcohol (15%) and toluene (4%) after 30 min using 2 eq.
of triethylsilane and 7.5 mg of PdO NPs/GO catalyst
(entry 14, Table 2). After a second addition of 2 eq. of
Et3SiH, we observed the presence of 27% of toluene and
73% of benzyl alcohol after 90 min. In the next addition
of Et3SiH to the same mixture, toluene was obtained in
46% along with 54% of benzyl alcohol after 120 min (entry
14, Table 2). It should be noted that the reduction took
place in THF as solvent but increased reaction time and
triethylsilane concentration were necessary. The reduc-
tion of phenylacetyl chloride in ethanol as solvent pro-
duced ethyl phenylacetate (entry 15, Table 2). Probably
the first stage of the reaction involves a nucleophilic
attack of ethanol on the carbon atom of carbonyl group,
followed by elimination to yield ethyl phenylacetate. In
a control experiment, we performed the same reaction
in absence of Et3SiH in ethanol as solvent; ethyl
phenylacetate was obtained in 100% after 20 min using
PdO NPs/GO catalyst (entry 16, Table 2). Then we per-
formed the reduction of phenylacetyl chloride in THF.
The obtained results indicated that by using of 2 eq. of
Et3SiH, the reduction of phenylacetyl chloride gave 2‐
phenylacetaldehyde in 28% along with 72% of the starting
material after 30 min; the halogen group was replaced by
hydrogen which may account for the formation of a white
smoke (entry 17, Table 2). Subsequent addition of Et3SiH
in the same mixture yielded 2‐phenylacetaldehyde in
100% yield after 90 min. Further addition of Et3SiH in
the same mixture did not have any impact on the reaction
pathway; no reduction of phenylacetaldehyde occurred
even after 180 min (entry 17, Table 2).
Benzyl alcohol derivatives are valuable intermediates
for the synthesis of various compounds such as vitamins,
fragrances, pharmaceuticals, pesticides etc. Thus different
methods were investigated to produce benzyl alcohol
from benzaldehyde reduction.[41–45] We screened differ-
ent reaction conditions to drive this reaction. The
obtained results were summarized in Table 3. By decreas-
ing the amount of Et3SiH (1.5 eq) and PdO NPs/GO cata-
lyst (3.6 mg), the reduction of benzaldehyde produced
benzyl alcohol in 81% after 20 min (entry 1, Table 3). 4‐
Methoxybenzyl alcohol, 2‐methoxybenzyl alcohol and 4‐
methylbenzyl alcohol were obtained in 57%, 63% and
82%, respectively from the reduction of 4‐
methoxybenzaldehyde, 2‐methoxybenzaldehyde and 4‐
methylbenzaldehyde under similar conditions (entries
2–4, Table 3).
Further, we examined the reduction of phenacyl chlo-
ride as a substituted acetophenone. The reduction of
phenacyl chloride by using 6 eq. of Et3SiH led to the for-
mation of 100% of ethylbenzene after 45 min (entry 10,
Table 2). In this case, we observed spontaneous formation
of a white smoke after addition of Et3SiH in the reaction
mixture. So, we followed this reaction by GC analysis
after addition of Et3SiH step by step. Upon addition of
2 eq. of Et3SiH, the reduction of phenacyl chloride to
acetophenone was quantitative (100% yield) after
10 min; that is to say that the halogen group was replaced
by hydrogen, which can explain the formation of HCl as a
white smoke (entry 11, Table 2). It should be noted that
the reduction of organic halides using Et3SiH/PdCl2 sys-
tem was reported in the literature.[40] Following the sec-
ond addition of Et3SiH, we observed the formation of
30% ethylbenzene, 50% 1‐phenylethanol and 20%
acetophenone after 20 min by GC analysis (entry 11,
Table 2). Upon addition of 2 eq. of triethylsilane for a
third time, ethylbenzene and 1‐phenylethanol were
obtained in 72% and 28%, respectively after 30 min. The
GC analysis of the previous mixture after 40 min showed
the presence of 85% ethylbenzene and 15% 1‐
phenylethanol (entry 11, Table 2).
In the next experiment, we studied the reduction of
acyl chlorides such as benzoyl chloride and 2‐
phenylacetyl chloride. Initial experiments revealed that
the reduction yield was not satisfactory; for example,
reduction of benzoyl chloride gave 13% toluene and 87%
of ethylbenzoate by using 2 eq. of Et3SiH and 7.5 mg of
PdO NPs/GO catalyst in ethanol as solvent (entry 12,
Table 2). The result suggested that under our experimen-
tal conditions, benzoyl chloride reacted instantly with
ethanol to form ethylbenzoate; this most likely occurred
through a nucleophilic addition and an elimination reac-
tion to yield ethyl benzoate rather than the reduction
reaction to toluene (entry 12, Table 2). We also performed
the same reaction in absence of Et3SiH in ethanol as sol-
vent. Under these conditions, only ethyl benzoate was
formed after 30 min; PdO NPs/GO catalyzed nucleophilic
addition/elimination reaction for esterification, but the
reduction of benzoyl chloride to toluene did not occur
(entry 13, Table 2).
To avoid the esterification reaction, we replaced etha-
nol by tetrahydrofuran (THF) as solvent; the obtained
results showed that this modification was useful for
We also examined the reduction of ketone com-
pounds. Our experiment revealed that reduction of

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MIRZA‐AGHAYAN ET AL.
ketones required an increased amount of Et3SiH. For
example, 1‐phenylethanol was obtained in only 58% after
20 min using 2 eq. of Et3SiH (entry 5, Table 3). However,
by increasing the catalyst amount to 7.5 mg, 1‐
phenylethanol was formed in 83% after 10 min using
2 eq. of Et3SiH (entry 6, Table 3). 1‐Phenylpropan‐1‐ol
was obtained in 21% using 2 eq. of Et3SiH after 30 min
(entry 7, Table 3) and in 61% after 30 min by carrying
the reaction in the presence of 3 eq of Et3SiH and
5.4 mg of PdO NPs/GO catalyst (entry 8, Table 3).
Diphenyl methanol was produced in 81% by reduction
of benzophenone using 6 q. of Et3SiH and 7.5 mg of
PdO NPs/GO catalyst after 60 min (entry 9, Table 3).
Reduction of benzoyl chloride gave benzyl alcohol in
90% using 4 eq. of Et3SiH and 7.5 mg of PdO NPs/GO cat-
alyst in THF as solvent after 90 min (entry 10, Table 3).
In this mechanism, it is assumed that molecular hydro-
gen generated in situ by the reaction of Et3SiH and
PdCl2/GO is responsible of the reduction of the carbonyl
group. We believe that mechanism of hydrogenation cat-
alyzed by PdO NPs/GO is a stepwise process (Figure 7).
The first step relates to the reaction of PdO NPs/GO with
Et3SiH, leading to the formation of active metallic Pd(0)
NPs/GO entity. The oxidative addition of triethylsilane
to Pd(0)/GO leads to the formation of Et3SiPdH/GO com-
plex, which upon reaction with ethanol (solvent) pro-
duces molecular hydrogen and triethylsilyl ether
accompanied by regeneration of the Pd(0)/GO catalyst.
The generated molecular hydrogen serves in the hydroge-
nation reaction. In the case of THF as solvent, Et3SiPdH/
GO complex can react with triethysilanol to yield molec-
ular hydrogen and hexaethyldisiloxane by the regenera-
tion of the Pd(0)/GO catalyst. The advantages of Et3SiH/
PdO NPs/GO system over previously developed sys-
tems[21–23] is simple reusability of this catalyst.
3.3 | Recyclability of PdO NPs/GO catalyst
To evaluate the reusability of the catalyst after comple-
tion of the reduction, ethyl acetate was added and the
mixture was filtered through a sintered funnel to recover
the catalyst. The reaction of benzaldehyde in the presence
of the recovered catalyst and 4 eq. of Et3SiH was per-
formed for six consecutive cycles. The result indicated
that the catalyst was active for five consecutive runs.
However, we observed a loss of activity of catalyst after
the 5th run. Toluene was obtained in 70% along with
30% of benzyl alcohol in the 6th run (Table 4). The activity
loss of the catalyst after five consecutive runs was due to
palladium leaching, as revealed by ICP‐OES analysis.
Indeed, the palladium content decreased from 0.85 wt%
to 0.52 wt% after the 6th cycle. It should be noted that
UV–Vis spectra of recycled catalyst after five runs consec-
utive show a significant decrease of the peak at ~228 nm
which can be attributed to palladium nanoparticles
leaching (Figure 6).
4 | CONCLUSIONS
In conclusion, a simple procedure was developed for the
reduction of various carbonyl compounds such as aro-
matic aldehydes, ketones and acyl chlorides to the corre-
sponding methylene derivatives in presence of Et3SiH
using a catalytic amount of PdO NPs/GO catalyst as an
available, cheap, reusable and environmentally friendly
catalyst. We also demonstrated that a decrease of the cat-
alyst and Et3SiH amounts led to the reduction of carbonyl
compounds to their respective alcohols. The reduction
was carried out at room temperature in high yields and
very short reaction times using reusable catalyst for six
consecutive cycles with a very simple work‐up procedure.
The present method has many obvious advantages as
compared with many previous reports, including the sim-
plicity, the generality of the methodology without having
the use of gaseous hydrogen.
3.4 | Mechanism
In earlier studies by us on the isomerization[46] and
reduction of 1‐alkenes,[22] α,β‐unsaturated com-
pounds,[21] carbonyl compounds[23] using Et3SiH/PdCl2,
Pd/C or Pd (OAc)2 system, we have observed the forma-
tion of molecular hydrogen along with metallic Pd and
we have proposed a mechanism in which Pd species serve
as the active entity in the catalyzed isomerization or
reduction reaction. In this work, the characterization of
catlyst showed that the palladium is mostly in form of
PdO along with a small amount of PdCl2. When palla-
dium exists in form of PdCl2, the reduction reaction
occurs according to the mechanism proposed in ref 23.
ORCID
Maryam Mirza‐Aghayan
2214-5775
Rabah Boukherroub
9888
https://orcid.org/0000-0002-
https://orcid.org/0000-0002-9795-
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How to cite this article: Mirza‐Aghayan M,
Kalantari M, Boukherroub R. Palladium oxide
nanoparticles supported on graphene oxide: A
convenient heterogeneous catalyst for reduction of
various carbonyl compounds using triethylsilane.
Appl Organometal Chem. 2019;e4837. https://doi.
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