An efficient ligand free chemoselective transfer hydrogenation of olefinic bonds by palladium nanoparticles in an aqueous reaction medium

Article abstract of DOI:10.1039/c4ra04777j

A highly efficient ligand free catalytic system was developed for chemoselective transfer hydrogenation of α,β-unsaturated carbonyl compounds by palladium nanoparticles (PdNPs) in an aqueous reaction medium. The developed methodology offers chemoselective 1,4-reduction of various α,β-unsaturated carbonyl compounds such as carbonyls, amides, esters and nitriles with excellent chemoselectivity and effective catalyst recyclability. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra04777j

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An efficient ligand free chemoselective transfer
hydrogenation of olefinic bonds by palladium
nanoparticles in an aqueous reaction medium†
Cite this: RSC Adv., 2014, 4, 32834
Received 21st May 2014
Accepted 22nd July 2014
*
Dattatraya B. Bagal and Bhalchandra M. Bhanage
DOI: 10.1039/c4ra04777j
www.rsc.org/advances
A highly efficient ligand free catalytic system was developed for che- APTES functionalized SBA-15 for hydrogenation reaction using
moselective transfer hydrogenation of a,b-unsaturated carbonyl molecular hydrogen.13 Moreover, Zhang et al. used pincer Pd
compounds by palladium nanoparticles (PdNPs) in an aqueous reac- complexes for transfer hydrogenation of a,b-unsaturated
tion medium. The developed methodology offers chemoselective 1,4- carbonyls with alcohol as a hydrogen source.14
reduction of various a,b-unsaturated carbonyl compounds such as
However, the reported methods have disadvantages such as
carbonyls, amides, esters and nitriles with excellent chemoselectivity use of expensive metal and ligands, use of molecular hydrogen,
and effective catalyst recyclability.
low chemoselectivity, and use of hazardous organic solvents.
Furthermore, the majority of earlier reported protocols are
homogeneous which creates difficulty in recycling the precious
metal catalyst thus limits their applications. In view of this,
development of improved methodology with greener and
economical catalytic system which facilitates the reuse of
Continuously growing interest in the hydrogenation reaction is
dictated by a wide range of application in ne chemicals,
agrochemicals and various valuable materials.1–3 A selective
hydrogenation of the olenic bond of a,b-unsaturated carbonyl
compounds to the saturated carbonyl compounds is a chal-
lenging task since saturated carbonyl compounds are widely
used as intermediates for the synthesis of various biologically
active compounds.4 In contrast, the selective reduction of the
carbonyl group of a,b-unsaturated carbonyls to allylic alcohols
have been achieved with relative ease.5 In particular, conven-
tional hydrogenation reaction uses hazardous molecular
hydrogen and requires sophisticated instrument (autoclave),
whereas transfer hydrogenation6 process circumvents use of
hydrogen gas and autoclave hence is easy to execute.
Scheme 1 Chemoselective transfer hydrogenation of a,b-unsaturated
carbonyl compounds.
Not surprisingly, hydrogenation is one of the most broadly
studied area of the organometallic chemistry. Various transition
metal complexes such as palladium,7 iridium,8 rhodium,9
ruthenium,10 and other metal complexes11 were used for che-
moselective hydrogenation of olenic double bond of a,b-
unsaturated carbonyl compounds. More recently use of palla-
dium/magnesium–lanthanum mixed oxide catalysts were
reported by Kantam and co-workers for chemoselective reduc-
tion of olenic bond using molecular hydrogen.12 Burri and co-
workers explored palladium nanoparticles immobilized on
Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai-400 019,
India. E-mail: bm.bhanage@gmail.com; bm.bhanage@ictmumbai.edu.in; Fax: +91 22
2414 5614; Tel: +91 22 3361 1111/2222
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra04777j
Fig. 1 FEG-SEM image of synthesized palladium nanoparticles.
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expensive metal catalyst which can be efficiently catalyse che-
moselective transfer hydrogenation of a,b-unsaturated
carbonyls is still challenging.
Nowadays, synthesis of metal nanoparticles (NPs) have
received great attention due to their potential application in
catalysis.15 The high volume to mass ratio reveal high surface
area results into high catalytic activity and hence required less
catalyst concentration compared to bulk. In addition to this,
nanocatalysts could be reused for further reactions. Also
nanoparticles show high selectivity at high conversion rate
under mild reaction conditions.16 In continuation to the appli-
cation of nanocatalyst in various organic transformations, the
use of palladium nanoparticles (PdNPs) in catalysis has fasci-
nated much interest.17 In this regard, an efforts were taken to
utilize PdNPs for chemoselective transfer hydrogenation reac-
tion (Scheme 1).
Fig. 2 TEM image of synthesized palladium nanoparticles.
Continuing the exploration of the facile protocol for the
hydrogenation reactions with transition metal catalysts and
being interested in transfer hydrogenation reactions,18 we have
tested the feasibility of PdNPs for the reduction of the a,b-
unsaturated carbonyl compounds. Herein, we report highly
chemoselective ligand free protocol for transfer hydrogenation
of a,b-unsaturated carbonyl compounds catalysed by PdNPs in
aqueous reaction media.
Results and discussion
The synthesis of palladium nanoparticles (PdNPs) were carried
out using PdCl2 as a precursor, citric acid as a greener reducing
agent and polyvinyl pyrrolidone (PVP) as a capping agent by
concentrated solar energy in the aqueous reaction medium (see
Fig. 3 EDS pattern of synthesized palladium nanoparticles.
Table 1 Effect of reaction parameters on transfer hydrogenation of benzylideneacetophenonea
Entry
H2 source
H2 source (mmol)
Temp. (ꢀC)
Time (h)
Conv.b (%)
Selectivityc (2a : 3a)
Effect of hydrogen donors
1
2
3
4
5
6
HCOONa
HCOOK
HCOONH4
HCOOH + Et3N
HCOONa
3
3
3
3
1.5
5
100
100
100
100
100
100
8
8
8
8
8
8
99
78
67
95
63
99
99 : 1
98 : 2
97 : 3
98 : 2
97 : 3
93 : 7
HCOONa
Effect of temperature
7
8
9
HCOONa
HCOONa
HCOONa
3
3
3
25
50
70
8
8
8
36
57
78
99 : 1
99 : 1
99 : 1
Effect of time
10
11
HCOONa
HCOONa
3
3
100
100
4
6
53
87
99 : 1
99 : 1
a
Reaction conditions: benzylideneacetophenone (1 mmol), PdNPs (1 mol% stock solution in 5 mL deionised water), temperature (100 ꢀC).
Conversion based on GC analysis. c Selectivity based on GCMS analysis.
b
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Table 2 Chemoselective transfer hydrogenation of a,b-unsaturated carbonyl compoundsa
Entry
Substrate
Product
Conversionb (%)
Selectivityc (2a : 3a)
99 : 1
1
2
99
99
98 : 2
3
4
5
99
99
91
98 : 2
97 : 3
92 : 08
6
87
97
94
95
98
89 : 11
98 : 2
97 : 3
98 : 2
97 : 2
7
8
9
10
11
99
97 : 2
12
13
84
89
100 : 0
93 : 7
14
15
79
98
100 : 0
100 : 0
16d
99
98 : 2
a
Reaction conditions: substrate (1.0 mmol), PdNPs (1 mol% in 5 mL deionised water), HCOONa (3 mmol), temperature (100 ꢀC), time (8 h).
Conversion based on GC analysis. c Selectivity based on GCMS analysis. d Required 6 mmol of HCOONa.
b
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the ESI†).19 The characterization of prepared Pd (0) nano-
particles was carried out by using various analytical techniques
such as eld emission gun-scanning electron microscopy (FEG-
SEM), transmission electron microscopy (TEM), and electron
dispersive X-ray spectral (EDAX) analysis (Fig. 1–3). The size and
morphology of synthesized PdNPs were determined by FEG-
SEM and TEM analysis (Fig. 1 and 2). The Transmission Elec-
tron Microscopy (TEM) image conrmed that the obtained
particles are in the nano region with uniform size ranging from
25 nm to 40 nm. The EDS spectrum shows that the prepared
PdNPs contains palladium metal only (Fig. 3).
The prepared palladium nanoparticles solution was then
diluted up to 100 mL with deionised water to make concentra-
tion of 1 mol% and used as a stock solution for the transfer
hydrogenation of a,b-unsaturated carbonyls.
In order to optimize the reaction conditions, initial studies
were conducted using PdNPs as a choice of catalyst and
HCOONa as a hydrogen source at reux reaction temperature
for chemoselective transfer hydrogenation of the benzylide-
neacetophenone as a model reaction. A series of experiments
were performed to optimize various reaction parameters such
as effect of hydrogen donor, reaction temperature, time and the
results obtained are summarized in Table 1.
Initially, we screened the effect of various hydrogen donors
on transfer hydrogenation reaction (Table 1, entries 1–4), it was
observed that 3 mmol of HCOONa gave 99% conversion with
excellent chemoselectivity of the desired product (Table 1, entry
1), while HCOOK and HCOONH4 (Table 1, entries 2–3) gave
slightly lower conversion, a mixture of HCOOH + Et3N (1 : 1)
(Table 1, entry 4) as a hydrogen source provided 95% conversion
of the starting material. We found that using 1.5 mmol of
HCOONa provided 63% conversion with 97% selectivity of the
desired product (Table 2, entry 5), while further increase in the
amount of HCOONa, the selectivity decreases slightly with
further reduction of carbonyl group (Table 1, entry 6).
Fig. 4 Recyclability of PdNPs in aqueous medium.
observed dehalogenation of these substrates under the opti-
mised reaction condition. Furthermore, benzylacetone was also
provided excellent conversion and selectivity towards desired
product (Table 2, entry 7). It is noteworthy to mention that
heterocyclic enones such as furan and thiophene in the arene
ring showed excellent yield and selectivity of the desired
product (Table 2, entries 8–9).
Developed protocol was successfully applied to aliphatic
cyclic enones like 5,5-dimethylcyclohex-2-enone and cyclo-
hexenone providing desired yield and selectivity (Table 2,
entries 10–11). Further, substrate scope extension were
demonstrated on employing various functional groups such as
esters, aldehydes, amides and a,b-unsaturated nitrile for che-
moselective hydrogenation of olenic bond (Table 2, entries 12–
15). It is notable that aromatic enoate, amide and nitrile
provided excellent yield of corresponding products with 100%
selectivity. Furthermore, the multiple bonds were also selec-
tively reduced to the corresponding 1,4 adduct by adding excess
of hydrogen source (Table 2, entry 16).
Our next step led us to engage PdNPs for recyclability study
since nanoparticles offer reuse of precious palladium metal. It
was observed that PdNPs were successfully recycled up to ve
conjugative cycles giving moderate to excellent conversion and
chemoselectivity of the desired product (Fig. 4). Further we have
checked the morphology of reused (4th run) palladium nano-
particles by FEG-SEM analysis and we observed that the
morphology of PdNPs remains same (FEG-SEM image of 4th
run, see the ESI†) (Fig. 4).
Reaction conditions: benzylideneacetophenone (1 mmol),
HCOONa (3 mmol), PdNPs (1 mol% in 5 mL deionised water),
temperature (100 ꢀC). Conversion and selectivity determined by
GC and GC-MS analysis.
Nextly, we examined the effect of temperature on reaction
outcome; reactions were carried out at different temperatures
ranging from room temperature to reux (Table 1, entries 1,
7–9). It was observed that at 25 ꢀC the yield of the desired
product was low (36%) whereas, with increasing reaction
temperature gradually from room temperature to reux
temperature, 99% conversion with 99% selectivity of the desired
product was observed within 8 h (Table 1, entries 10–11). Hence,
the best optimised reaction conditions for chemoselective
transfer hydrogenation of 1 mmol of benzylideneacetophenone
were: PdNP: 1 mol% (5 mL stock solution in deionised water),
ꢀ
HCOONa: 3 mmol, temperature: 100 C, time: 8 h.
With these optimized reaction conditions in hand, we
examined the reaction scope for the chemoselective reduction
Conclusions
of a,b-unsaturated carbonyl compounds (Table 2). Introduction In summary, we have developed a highly chemoselective ligand
of the electron donating substituents on the aromatic ring of free catalytic system for transfer hydrogenation of a,b-unsatu-
the model benzylideneacetophenone did not have any inuence rated carbonyl compounds by PdNPs in aqueous reaction
on the reaction course providing good yields with excellent medium. The palladium nanoparticles offered reduction of
selectivity towards olenic bond (Table 2, entries 1–4) whereas, wide variety of a,b-unsaturated carbonyl compounds such as
employing electron withdrawing groups such as chloro on carbonyls, esters, amides and nitriles with good to excellent
aromatic ring gave good conversion of the desired product with conversion and chemoselectivity. The PdNPs were effectively
slightly lower selectivity (Table 2, entries 5–6). We have not recycled up to ve consecutive cycles with appreciable
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¨
conversion and selectivity. The use of PdNPs as a highly
efficient catalyst for 1,4-reduction of a,b-unsaturated carbonyl
compound under transfer hydrogenation condition in aqueous
media adds credit towards advance of greener methodology for
hydrogenation reaction.
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Experimental section
Experimental procedure for chemoselective conjugate
reduction of a,b-unsaturated carbonyl
In a 5 mL stock solution of 1 mol% PdNPs were suspended in
water, 1 mmol of corresponding a,b-unsaturated carbonyl
compound and 3 mmol of HCOONa were added and the reac-
ꢀ
tion mixture was stirred at 100 C for 8 h. The progress of the
reaction was monitored using thin layer chromatography (TLC)
and GC analysis (Perkin Elmer, Clarus 400) (BP-10 GC column,
30 m  0.32 mm ID, lm thickness 0.25 mm). On completion,
the products were extracted with ethyl acetate, dried over
Na2SO4 and the solvent was evaporated under vacuum. The
obtained crude product was then puried by column chroma-
tography using silica gel, (100–200 mesh size) with petroleum
ether/ethyl acetate (PE–EtOAc, 95 : 05) as eluent to afford pure
product. All the products are well known in literature and were
1
conrmed by H NMR (Varian Mercury, 400 MHz NMR Spec-
trometer), 13C NMR spectra (101 MHz), GC-MS (Shimadzu GC-
MS QP 2010) (Rtx-17, 30 m  25mmID, lm thickness 0.25
mm df) (column ow-2 mL minÀ1, 80 ꢀC to 240 ꢀC at 10 ꢀC minÀ1
rise.) and IR (Perkin-Elmer FT-IR) spectroscopic techniques.
Recyclability study
The reaction was carried out as mentioned above in typical
experimental procedure. Aer completion of reaction, the
reaction mixture was cooled to room temperature and the
product was extracted in ethyl acetate. The aqueous layer con-
taining PdNPs was then used for further catalyst recyclability
experiment and it was observed that the PdNPs could be reused
up to ve consecutive cycles affording good conversion with
appreciable chemoselectivity.
Acknowledgements
The authors (D. B. Bagal) express gratitude towards University
Grant Commission, India (UGC-SAP) for providing Research
Fellowship. We also thank Department of Science and Tech-
nology (DST), Govt of India for nancial support under Nano
Mission Project no. SR/NM/NS-1097/2011.
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