A facile and highly efficient transfer hydrogenation of ketones and aldehydes catalyzed by palladium nanoparticles supported on mesoporous graphitic carbon nitride

Article abstract of DOI:10.1177/1747519819883792

A novel transfer hydrogenation methodology for the reduction of ketones (14 examples) and benzaldehyde derivatives (12 examples) to the corresponding alcohols using Pd nanoparticles supported on mesoporous graphitic carbon nitride (mpg-C₃N₄/Pd) as a reusable catalyst and ammonia borane as a safe hydrogen source in an aqueous solution MeOH/H₂O (v/v = 1/1) is described. The catalytic hydrogenation reactions were conducted in a commercially available high-pressure glass tube at room temperature, and the corresponding alcohols were obtained in high yields in 2–5 min. Moreover, the presented transfer hydrogenation protocol shows partial halogen selectivity with bromo-, fluoro-, and chloro-substituted carbonyl analogs. In addition, the present catalyst can be reused up to five times without losing its efficiency, and scaling-up the reaction enables α-methylbenzyl alcohol to be produced in 90% isolated yield.

Full text of DOI:10.1177/1747519819883792

8
research-arti
8cle2019 3792CHL0010.1177/1747519819883792Journal of Chemical ResearchNişancı and Dağalan
Research Paper
Journal of Chemical Research
–6
1
A facile and highly efficient transfer
hydrogenation of ketones and aldehydes
catalyzed by palladium nanoparticles
supported on mesoporous graphitic
carbon nitride
© The Author(s) 2019
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Bilal Nişancı1,2 and Ziya Dağalan1
Abstract
A novel transfer hydrogenation methodology for the reduction of ketones (14 examples) and benzaldehyde derivatives
(
(
(
12 examples) to the corresponding alcohols using Pd nanoparticles supported on mesoporous graphitic carbon nitride
mpg-C N /Pd) as a reusable catalyst and ammonia borane as a safe hydrogen source in an aqueous solution MeOH/H O
v/v=1/1) is described. The catalytic hydrogenation reactions were conducted in a commercially available high-pressure
3
4
2
glass tube at room temperature, and the corresponding alcohols were obtained in high yields in 2–5min. Moreover,
the presented transfer hydrogenation protocol shows partial halogen selectivity with bromo-, fluoro-, and chloro-
substituted carbonyl analogs. In addition, the present catalyst can be reused up to five times without losing its efficiency,
and scaling-up the reaction enables α-methylbenzyl alcohol to be produced in 90% isolated yield.
Keywords
graphitic carbon nitride, Pd nanoparticles, reusable catalyst, transfer hydrogenation
Date received: 7 June 2019; accepted: 30 September 2019
classical hydrogenation (direct hydrogenation) methods
Introduction
used in industrial and research laboratories usually
The synthesis of alcohols from ketones and benzaldehyde
involve high pressure/temperature conditions in the pres-
moieties still attracts great attention for applications in
ence of a noble metal catalyst (Ni, Pd, etc.) with hydro-
drugs, polymers, pharmaceuticals, fine chemicals, and
industrial manufacturing. An example of the selective
hydrogenation of acetophenone (AP) is 1-phenylethanol,
which is widely used in the pharmaceutical industry.
Several methodologies have been developed for these
valuable conversions. Typical methods for the reduc-
tion of carbonyl compounds use hydrides or molecular
hydrogen to generate a reducing agent. The transfer
9
gen gas. Due to their importance, researchers have
1
,2
3
1Department of Chemistry, Faculty of Science, Atatürk University,
Erzurum, Turkey
4
–6
2
Narman Vocational High School, Atatürk University, Erzurum, Turkey
Corresponding author:
Bilal Nişancı, Department of Chemistry, Faculty of Science, Atatürk
University, 25240 Erzurum, Turkey.
7
hydrogenation (TH) methodology is one of the most
8
effective methods for the synthesis of alcohols. The Email: bnisanci@atauni.edu.tr

2
Journal of Chemical Research 00(0)
Figure 1. (a) A representative TEM image, and (b) the XRD pattern of mpg-C N @Pd nanocatalysts.
3
4
focused on developing sustainable and environmentally Table 1. Optimization of the conditions for the
mpg-C N /Pd-catalyzed transfer hydrogenation of
1
0
friendly strategies for the synthesis of alcohols.
3
4
a
4
-methoxyacetophenone (1).
In recent years, the generation of hydrogen in the reac-
tion medium has been used to gain selectivity and safety.
For that reason, a range of hydrogen-containing regents,
11
12
13
such as isopropanol, glycerol, sodium hydride, hydra-
14
15
16
zine hydrate, dimethylamine borane, and silanes, have
been tested. These methods have some disadvantages, such
as the weakness of the hydrogen donor group, toxicity, and
the need for high temperature. One of the best hydrogen-
containing reagents is ammonia borane (AB). AB is a solid
material with a higher hydrogen content compared to other
hydrogen donors. Furthermore, it can transfer hydrogen rap-
idly with the help of transition metals in aqueous solvents.
By utilizing AB as a hydrogen reservoir, several efficient
Yieldb
Entry
Solvent
Time (min)
c
1
2
3
4
5
6
7
8
H O
5
5
5
5
3
5
5
5
44
76
2
c
H O/MeOH
2
H O
65
>99
90
2
H O/MeOH
2
H O/MeOH
2
d
H O/MeOH
50
17–19
2
and mild conditions have been reported.
In our previous
e
H O/MeOH
72
2
studies, we have presented some protocols using AB as a
hydrogen source with reduced graphene oxide–supported
f
H O/MeOH
–
2
20
bimetallic NiPd alloy nanoparticles (NPs), graphene Bold: best result.
21
a
Reaction conditions: 4-methoxyacetophenone (0.35mmol),AB
hydrogel–supported Pd NPs, and CuPt alloy NPs assem-
22
(0.70mmol), mpg-C
3
N
4
/Pd (4mg), water/methanol (2mL (1:1)).
bled on reduced graphene oxide as a catalyst, respectively.
b
GC-MS yields.
In a recent study, we have also demonstrated that in situ syn-
thesized Pd NPs supported on mesoporous graphitic carbon
nitride (mpg-C N /Pd) are highly efficient catalysts in the
c
mpg-C N /Pd (2mg) was used.
3
4
d
e
AB (0.35mmol) was used.
H O (1.5mL) and MeOH (0.5mL) were used.
2
3
4
f
No catalyst.
TH of nitroarenes to the corresponding aniline derivatives at
23
room temperature.
In order to clarify the efficiency of the newly developed mpg-C N @Pd nanocatalysts. As seen in a representative
3
4
mpg-C N /Pd catalyst, we describe herein an efficient, selec- TEM image given in Figure 1(a), Pd NPs with an average
3
4
tive, and fast reduction chemistry method for the synthesis of particle size of 2.5nm are dispersed over mpg-C N
3
4
alcohols using AB as a hydrogen source with mpg-C N /Pd nanosheets without any agglomeration. The XRD pattern of
3
4
in a water/methanol mixture (v/v=1/1). In addition, it should powdered mpg-C N @Pd nanocatalysts (Figure 1(b)) shows
3
4
be noted that less AB was used for the hydrogenation reac- five major reflections at 2θ=27.5°, 39.5°, 46.2°, 68.1°, and
20–23
tions compared to our previous studies,
making our new 82.3°, which are attributed to the (002) plane of mpg-C N ,
3 4
methodology cost efficient and more practical.
and the (111), (200), (220), and (311) planes of face-centered
cubic (fcc) palladium, respectively. The BET surface area
and the Pd loading ratio of the mpg-C N @Pd nanocatalysts
were found to be 190m /g and 6.1wt%, respectively.
Based on our earlier attempts at hydrogenation and reduc-
3
4
Results and discussion
2
The mpg-C N @Pd nanocatalysts were purchased from
3
4
20–25
NANOKAT R&D company, Turkey, and characterized by tive amination,
transmission electron microscopy (TEM), X-ray diffraction conditions by experimenting with different parameters, such
XRD), and Brunauer–Emmett–Teller (BET) surface area as solvent, temperature, time, the catalyst, and the amount of
analysis. Figure 1 presents the characterization data of the AB. 4′-Methoxyacetophenone was selected as the model
we tried to optimize the hydrogenation
(

Nişancı and Dağalan
3
a
Table 2. The substrate scope of the mpg-C N -catalyzed TH of ketones.
3
4
a
Reaction conditions: ketone (0.35mmol),AB (0.7mmol), mpg-C N /Pd (4mg), MeOH/H O (2mL, 1:1), room temperature, 2min.
3
4
2
GC-MS yields.
reagent for the synthesis of the corresponding alcohol under in Table 2. As can be clearly seen from Table 2, all the ketone
optimized conditions. The data in Table 1 demonstrate some derivatives used were converted into alcohols in high yields
of the key features of the TH chemistry. Primarily, it was in 5min at room temperature. Not only electron-donating
aimed to carry out TH reactions with the smallest amount of ketones but electron-withdrawing analogs were also quanti-
the catalyst possible (Table 1, entries 1 and 2). With the use of tatively converted into the corresponding alcohols. Fluorine-
water as a solvent, a 44% yield was obtained with 2mg of substituted ketones, on the other hand, were found to be
mpg-C N /Pd (Table 1, entry 1), which was increased to 77% reduced selectively without dehalogenation (Table 2, 2d, 2l,
3
4
with the addition of methanol (Table 1, entry 2). By increas- 2m). When chlorine- and bromine-substituted reagents were
ing the amount of the catalyst used, a significant improve- examined, the selectivity was determined to be high enough
ment in the yield of the TH reaction in water was observed to give only a small amount of dehalogenation side-products
(Table 1, entry 3), while a quantitative conversion was (Table 2, 2c, 2e, 2k). As a result, this method allows for the
obtained in the water/methanol mixture (Table 1, entry 4). synthesis of bromo-, chloro-, and fluoro-alcohols in a very
Reducing the time from 5–3min resulted in a relatively small short time. For compound 2n, the chemoselective reduction
decrease in the yield (Table 1, entry 5). Using 1 equiv. product was observed in excellent conversion (Table 2, 2n).
(0.35mmol) of AB provides moderate performance (Table 1,
In the second part of the study, aldehydes were hydrogen-
entry 6). Decreasing the amount of methanol led to a moder- ated in order to extend the scope of the TH process (Table 3).
ate yield (Table 1, entry 7). Furthermore, no reaction was Many aldehyde derivatives with a variety of substituents (12
achieved in the catalyst-free medium (Table 1, entry 8). examples) were successfully reduced by mpg-C N /Pd NPs.
3
4
Following these optimization experiments, it can be con- On account of the high reactivity of aldehydes, the reactions
cluded that the best results are obtained using 0.7mmol ofAB were complete in only 2min. As with ketones, fluoro-,
as a hydrogen source and 4mg of mpg-C N /Pd as a nano- bromo-, and chloro-substituted products were synthesized in
3
4
catalyst with a reaction time of 5min using 2mL of methanol/ excellent yields (Table 3, 4b, 4c, 4l). For iodine-substituted
water (1:1) as the solvent (Table 1, entry 4). carbonyl analogs, the hydrogenation reaction of 4-iodoben-
Following these optimization studies, TH of different zaldehyde (5) was tested, and the dehalogenation product
ketones (14 examples) was carried out to test the viability of was obtained as expected (Scheme 1). The reduction of both
this fast, selective methodology, and the results are outlined the nitro and aldehyde group enables the synthesis of 4f. In

4
Journal of Chemical Research 00(0)
a
Table 3. Hydrogenation of aldehydes with the commercially available catalyst mpg-C N /Pd.
3
4
a
Reaction conditions: ketone (0.35mmol),AB (0.7mmol), mpg-C N /Pd (4mg), MeOH/H O (2mL, 1:1), room temperature, 2min.
3
4
2
GC-MS yields.
Scheme 1. The TH reaction of 4-iodobenzaldehyde.
Scheme 2. Synthesis of α-methylbenzyl alcohol via the TH
addition to benzaldehyde derivatives (Table 3, 4a–4g), pyri-
dine-based (Table 3, 4h, 4i), naphthalene-based (Table 3, 4j),
and thiophene-based (Table 3, 4k) aldehydes were hydro-
genated under aqueous conditions selectively.
scale-up.
effective measurement, the first experiment was carried out
using mpg-C N /Pd (4mg) over 3min, during which the
3
4
Hydrogenation was carried out on a larger scale to verify
the practicality of the presented method (Scheme 2). Asam-
ple experiment with the ketone derivative acetophenone
was performed on a 3.5mmol scale (10-fold increase), and
α-methylbenzyl alcohol (2g) was isolated in 90% yield. It
was observed that increasing the amount of the substrate in
the reaction caused a slight decrease in the obtained yield.
For the scale-up of conversion, mpg-C N (10mg) with AB
conversion was 90%. In subsequent reactions, the reaction
time was kept at 3min to demonstrate the actual impact of
the catalyst on the TH. At the end of the sequential experi-
ments, it was found that the catalyst had not lost its effec-
tiveness after five consecutive runs.
3
4
Conclusion
(
210mg) was used in 4mL of MeOH/H O (1:1).
2
Finally, a recycling experiment, which is one of the most The interest in the development of new, efficient, selective,
important criteria required to highlight the effectiveness of and rapid catalytic methods in organic chemistry is increas-
heterogeneous catalysts, was performed. Scheme 3 shows ing every day because of stringent environmental policies
the recycling experiment of 4-methoxyacetophenone in the and financial considerations. In this study, we synthesized
presence of the mpg-C N /Pd catalyst. In order to make an the corresponding alcohols from ketones and aldehydes in
3
4

Nişancı and Dağalan
5
1
All H NMR data of the alcohols were in agreement with
5
,26–34
the reported spectra.
-(4-Methoxyphenyl)ethan-1-ol (2a): 1H NMR
400MHz, CDCl ): δ=7.28 (d, J=6.7Hz, 2H), 6.87 (d,
1
(
3
J=6.7Hz, 2H), 4.83 (q, J=6.4Hz, 1H), 3.79 (s, 3H), 1.47
3
0,34
(d, J=6.4Hz, 3H).
1
1
-(m-tolyl)ethan-1-ol (2b): H NMR (400MHz, CDCl ):
3
δ=7.26–7.08 (m, 4H), 4.85 (q, J=6.4Hz, 1H), 2.35 (s, 3H),
2
8
1
.48 (d, J=6.4Hz, 3H).
-(4-Bromophenyl)ethan-1-ol (2c): H NMR (400MHz,
CDCl ): δ=7.37 (d, J=8.2Hz, 2H), 7.25 (d, J=8.2Hz, 2H),
1
1
3
2
6
4
.90 (q, J=6.4Hz, 1H), 1.50 (d, J=6.4Hz, 3H).
1
1
-(4-Fluorophenyl)ethan-1-ol (2d): H NMR (400MHz,
Scheme 3. The recycling experiment of
CDCl ): δ=7.36–7.32 (m, 2H), 7.06–7.01 (m, 2H), 4.88 (q,
3
4-methoxyacetophenone catalyzed by mpg-C N /Pd.
3 4
29
J=6.4Hz, 1H), 1.48 (d, J=6.4Hz, 3H).
1
1
-(4-Chlorophenyl)ethan-1-ol (2e): H NMR (400MHz,
the presence of mpg-C N /Pd as a highly efficient and CDCl ): δ=7.36–7.26 (m, 4H), 4.87 (q, J=6.4Hz, 1H),
3
4
3
2
8
reusable nanocatalyst. The methodology presented herein 1.46 (d, J=6.4Hz, 3H).
has the following advantages over the available alterna-
4-(1-Hydroxyethyl)phenol (2f): H NMR (400MHz,
1
tives: (1) it is one of the fastest and selective TH method- D O): δ=7.22–7.19 (m, 2H), 6.83–6.79 (m, 2H), 4.76 (q,
2
3
2
ologies in the literature, (2) it enables the synthesis of J=6.4Hz, 1H), 1.37 (d, J=6.4Hz, 3H).
1
bromo-, chloro-, and fluoro-substituted alcohols, (3) the
1-Phenylethan-1-ol (2g): H NMR (400MHz, CDCl ):
3
reactions occur in aqueous systems, (4) the amount of AB δ=7.40–7.28 (m, 5H), 4.90 (q, J=6.4Hz, 1H), 1.50 (d,
2
9
for the TH reaction is less compared to known studies, (5) J=6.4Hz, 3H).
1-(3,4-Dimethoxyphenyl)ethan-1-ol (2h): 1H NMR
the reactions can be easily scaled up, (6) the catalyst is
recyclable, and (7) the methodology does not need any (400MHz, CDCl ): δ=6.95–6.81 (m, 3H), 4.85 (q, J=6.4Hz,
3
28
purification step. Therefore, we believe that this study will 1H), 3.89 (s, 3H), 3.88 (s, 3H), 1.48 (d, J=6.4Hz, 3H).
1
play an important role in the synthesis of alcohols and also
Cycloheptanol (2i): H NMR (400MHz, CDCl3):
other TH applications.
δ=3.87–3.83 (m, 1H), 1.93–1.84 (m, 2H), 1.62–1.47 (m,
5
9
H), 1.42–1.34 (m, 2H).
1
Diphenylmethanol (2j): H NMR (400MHz, CDCl ):
3
Experimental
5
δ=7.36–7.23 (m, 10H), 5.80 (s, 1H).
(
4-Bromophenyl)(phenyl)methanol (2k): 1H NMR
Material and general methods
3
1
(
400MHz, CDCl ): δ=7.45–7.22 (m, 9H), 5,70 (s, 1H).
3
Pd NPs supported on mesoporous graphitic carbon nitride was
purchased from NANOKAT R&D, Turkey. Aldehydes,
ketones, AB (90%), CDCl (99.8%), and D O (99.9%) were
obtained from Sigma-Aldrich and were used without purifica-
tion. The reactions were followed using thin-layer chromatog-
raphy (TLC) by utilizing aluminum-backed Merck Silica-Gel
1
(
4-Fluorophenyl)(phenyl)methanol (2l):
H
NMR
(
400MHz, CDCl ): δ=7.36–7.26 (m, 9H), 7.02–6.99 (m,
3
31
3
2
2H), 5.83 (s, 1H).
bis(4-Fluorophenyl)methanol (2m): 1H NMR (400
MHz, CDCl ): δ=7.82–7.79 (m, 2H), 7.33–7.31 (m, 2H),
3
31
7
.31–7.29 (m, 2H), 7.29–6.99 (m, 2H), 5,80 (s, 1H).
60 F254 plates. Preparative TLC was performed using Merck
-(1-Hydroxyethyl)phenyl acetate (2n): 1H NMR
4
silica gel 60 HF254+366. GC-MS was carried out using a
SHIMADZU GCMS QP2010 system (70eV electron impact
ionization) with an Rxi-5Sil MS Column (RESTEK, 30
meter). The characterization of the organic products was per-
formed using a 400 MHz Bruker NMR instrument.
(
(
1
400MHz, CDCl ): δ=7.29 (d, J=8.7 Hz, 2H), 6.83–6.79
3
d, J=8.7Hz, 2H), 4.85 (q, J=6.4Hz, 1H), 3.80 (s, 3H),
32
.47 (d, J=6.4Hz, 3H).
1
Phenylmethanol (4a): H NMR (400MHz, CDCl ):
3
15
δ=7.40–7.25 (m, 5H), 4.67 (s, 2H).
1
(
4-Chlorophenyl)methanol (4b): H NMR (400MHz,
3
3
CDCl ): δ=7.40–7.15 (m, 4H), 4.66 (s, 2H).
3
Typical procedure for the hydrogenation of
ketones and aldehydes
1
(
4-Fluorophenyl)methanol (4c): H NMR (400MHz,
CDCl ): δ=7.36–7.21 (m, 2H), 7.06–7.00 (m, 2H), 4.65
3
mpg-C N /Pd (4mg) and the ketone or aldehyde (0.35mmol)
29
3
4
(s, 2H).
4-Methoxyphenyl)methanol (4d): H NMR (400MHz,
CDCl ): δ=7.28 (d, J=8.5Hz, 2H), 6.89 (d, J=8.5Hz, 2H),
were suspended in methanol/water mixture (2mL, 1:1) in a
pressure tube. Subsequently, AB (0.75mmol) was added
and the solution was magnetically stirred for 2 (for alde-
hydes) or 5min (for ketones) at room temperature. After
completion of the reaction, the catalyst was filtered and
washed with methanol for further use. The solvent was
removed under the reduced pressure. The yield of each alco-
hol was determined by gas chromatography–mass spec-
trometry (GC-MS).
1
(
3
33
4
.60 (s, 2H), 3.80 (s, 3H).
-(Hydroxymethyl)phenol (4e): H NMR (400MHz,
D O): δ=7.23 (d, J=8.4Hz, 2H), 6.84 (d, J=8.4Hz, 2H),
1
4
2
29
4
.48 (s, 2H).
4-Aminophenyl)methanol (4f): H NMR (400MHz,
CDCl ): δ=7.18–7.14 (m, 2H), 6.68–6.60 (m, 2H), 4.55 (s,
1
(
3
15
2
H).

6
Journal of Chemical Research 00(0)
(
4-(Dimethylamino)phenyl)methanol (4g): 1H NMR
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. Mummadi S, Brar A, Wang GQ, et al. Chem Eur J 2018; 24:
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400MHz, CDCl ): δ=7.24 (d, J=8.6Hz, 2H), 6,73 (d,
J=8.6Hz, 2H), 4.56 (s, 2H), 2.94 (s, 6H).
3
1
5
16526–16531.
1
6. Pinto M, Friaes S, Franco F, et al. ChemCatChem 2018; 10:
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Pyridine-3-ylmethanol (4h): H NMR (400MHz, CDCl ):
3
2
33
δ=7.85–7.81 (m, 3H), 7.49–7.47 (m, 1H), 4.86 (s, 2H).
7
8
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Pyridine-2-ylmethanol (4i):
H
NMR (400MHz,
CDCl ): δ=8.57 (d, 1H), 7.71–7.67 (m, 1H), 7.23–7.21 (m,
3
3
3
2
H), 4.77 (s, 2H).
Naphthalen-2-ylmethanol (4j): 1H NMR (400MHz,
CDCl ): δ=8.57–8.51 (m, 4 H), 7.50–7.47 (m, 3H), 4.74 (s,
1
209–1215.
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3
33
δ=7.29–7.26 (m, 1H), 7.01–6.97 (m, 2H), 4.82 (s, 2H).
1
(
4-Bromophenyl)methanol (4l): H NMR (400MHz,
3
0
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CDCl ): δ=7.42–7.34 (m, 4H), 4.70 (s, 2H).
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3
4
(
2 mL, 1:1), and 4-methoxyacetophenone (0.35 mmol)
was added to a pressure tube. Subsequently, AB
(
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3
min at room temperature. Following completion of the
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methanol for further use. Subsequently, a new experi-
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TH chemistry was repeated five times. The yields were
acquired by GC-MS.
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Declaration of conflicting interests
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094–5096.
Funding
The author(s) disclosed receipt of the following financial support
for the research, authorship, and/or publication of this article: We
are grateful to Ataturk University BAP (Project No. FBA-2018-
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827) for their financial support.
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ORCID iD
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Bilal Nişancı
https://orcid.org/0000-0003-4290-1539
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Supplemental material
The spectral characterization of the compounds can be found in the
Supplemental material for this article is available online.
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