An Efficient, Stable and Reusable Palladium Nanocatalyst: Chemoselective Reduction of Aldehydes with Molecular Hydrogen in Water

Article abstract of DOI:10.1002/adsc.201501131

Palladium nanoparticles (Pd-BNP) stabilized by a binaphthyl-backbone can be efficiently used for the chemoselective reduction of aldehydes in the presence of hydrogen at room temperature in water. The Pd-BNP catalyst is easily recovered and reused for five catalytic cycles. (Figure presented.).

Full text of DOI:10.1002/adsc.201501131

UPDATES
DOI: 10.1002/adsc.201501131
An Efficient, Stable and Reusable Palladium Nanocatalyst:
Chemoselective Reduction of Aldehydes with Molecular
Hydrogen in Water
Surya Srinivas Kotha,a Nidhi Sharma,a and Govindasamy Sekara,*
a
Department of Chemistry, Indian Institute of Technology Madras, Chennai, Tamil Nadu – 600036, India
Fax: (+91)-44-2257-4202; e-mail: gsekar@iitm.ac.in
Received: December 11, 2015; Revised: March 3, 2016; Published online: May 3, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201501131.
Abstract: Palladium nanoparticles (Pd-BNP) stabi-
lized by a binaphthyl-backbone can be efficiently
used for the chemoselective reduction of aldehydes
in the presence of hydrogen at room temperature in
water. The Pd-BNP catalyst is easily recovered and
reused for five catalytic cycles.
Recently, metal nanoparticles (NPs) have received
increasing attention as catalysts in the academic and
industrial research.[12] Since metal NPs have a large
surface area to volume ratio and are endowed with
a large number of potentially active sites for the sub-
strates, they exhibit higher catalytic activity than the
traditional heterogeneous catalysts. At the same time,
these metal NPs also retain the advantages of homo-
geneous catalyst, thus behaving as a semi-heterogene-
ous catalyst. The selectivity, stability and recyclability
of the nanocatalysts are desperately dependent on the
stabilizer and the solvent employed. Recently, there
has been a great increase in the use of Pd NPs as cat-
alyst in the hydrogenation reactions.[13] The majority
Keywords: aldehydes; binaphthyl-stabilized palladi-
um nanocatalyst; chemoselective reduction; reusa-
ble catalyst; water as a solvent
Hydrogenation of functional groups is an indispensa- of the methods used for the preparation of Pd NPs in-
ble reaction in synthetic chemistry and the reduction volve special solid supports and multistep stabilized
of functional groups such as carbonyl groups, nitro preparation. Additionally, most of the methods used
groups, C-C and C-hetero atom multiple bond plays organic solvents.[14,15] The use of water rather than or-
a major role for the synthesis of numerous biological- ganic solvents is a growing demand in the chemical
ly important molecules, natural products, and industry. Water is a cheap, non-hazardous, safe, eco-
drugs.[1,2] Common methods used for the reduction of friendly and economically benign solvent. However,
aldehydes are electron transfer reduction, hydride to the best of our knowledge, no palladium nanocata-
transfer reduction and hydrogenation reactions.[3] lyst has been explored for the hydrogenation of alde-
Both heterogeneous and homogeneous catalysts are hydes in aqueous medium. As part of our ongoing re-
used for these reductions. Particularly, transition search towards transition metal nanocatalysts,[16] we
metals such as Rh,[4] Ru,[5] Ir,[6] Pd,[7] Fe[8] and Au[9] herein report a green and suitable strategy for the hy-
catalyze hydrogenations using molecular hydrogen as drogenation of aldehydes using a palladium nanocata-
reducing agent and represent the most preferred lyst in water as the solvent (Scheme 1).
methods due to their reaction efficiency and minimal
The initial study was carried out by treating 4-
side reactions. Palladium on charcoal (Pd/C) is one of methylbenzaldehyde 1 with molecular hydrogen (H2
the significant catalysts in the catalytic hydrogenation balloon) as reducing agent in the presence of very
reactions in the presence of molecular hydrogen.[10] stable Pd-BNP 4a as catalyst in water (Figure 1) and
The drawback of Pd/C are not only that it is expen- the product 2a was obtained in 83% (Table 1,
sive, highly flammable and difficult to recover and entry 1). Encouraged by this initial result, we started
reuse, particularly for large scale reactions. The major the investigation of other Pd-NPs as catalyst stabilized
disadvantages associated with the catalytic hydroge- by phenyl (4b) and 4-decylphenyl (4c)[16c] groups for
nation reaction are often the requirement of auto- this chemoselective hydrogenation of aromatic alde-
claves, special equipment such as Parr equipment, hydes and the results are summarized in Table 1 (en-
high pressure,[11] usage of organic solvents such as eth- tries 2 and 3). To further improve the yield of the re-
anol, methanol and ethyl acetate.
action, different amounts of the Pd-BNP 4a catalyst
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An Efficient, Stable and Reusable Palladium Nanocatalyst
(entry 4). Increasing the catalytic load increased the
reaction rate with more or less the same yield. So, it
is decided that 1 mol% of Pd-BNP 4a will be used as
optimized conditions (entry 4) as it gave a quantitative
yield. It is important to mention that in the absence
of Pd-BNP 4a, no hydrogenation product was ob-
served (entry 8).[18–20]
After achieving the best reaction conditions, the
scope of the hydrogenation of various aldehydes was
investigated and the results are summarized in
Table 2. Aldehydes containing electron-releasing and
electron-withdrawing groups on the aromatic ring
were well tolerated and gave the corresponding alco-
hols in good to excellent yield (entries 3–18). Interest-
ingly, highly labile halogen, cyano, acid, ester and
keto group containing aldehydes also underwent this
hydrogenation reaction smoothly and provided the
corresponding alcohols in good yield (entries 8 and
12–16). Even ortho-substitution containing aldehydes
gave the corresponding reduction products in good
yields (entries 4 and 7).
Scheme 1. Approaches to the chemoselective hydrogenation
of aldehydes.
When the reduction reaction was carried out with
para- and ortho-substituted bromo-/chlorobenzalde-
hyde, product formation was not observed. Aliphatic
aldehydes such as phenylacetaldehyde and cinnamal-
dehyde also remained unreactive in this hydrogena-
tion reaction. When heteroaromatic aldehydes such as
2-pyridinecarboxaldehyde and furfural were used in
the reduction reaction under the standard reaction
conditions as well as with 4 mol% of Pd-BNP 4a, not
even a trace amount of product formation took place.
To check whether this is a heterogeneous reaction
or the reaction is catalyzed by leached Pd from the
nanocatalyst, the reduction reaction was carried out
with 10 equivalents of Hg. The absence of product
formation in this reaction showed that the Pd-BNP is
a heterogeneous catalyst.[17]
Figure 1. Aromatic derivative-stabilized Pd NPs.
Table 1. Screening of catalytic loading of Pd nanocatalyst for
the hydrogenation of aldehydes.[a]
In a control experiment, we carried out the reaction
with deuterium oxide (D2O) as solvent and molecular
hydrogen as reducing agent. Using D2O as solvent,
the reaction did not provide any deuterated alcohol
3a (Scheme 2). With this experiment we conclude that
the hydrogen source for the hydrogenation reaction is
solely molecular hydrogen.
To study the scalability of this hydrogenation reac-
tion of aldehydes in the presence Pd-BNP with molec-
ular H2, the reaction was performed in a gram scale in
[a]
Reaction conditions: 0.5 mmol of 1.
Isolated yield.
[b]
were screened. It is observed that 1 mol% of the cata-
lyst afforded an excellent yield of 96% in 8 h Scheme 2. Reduction of 1 in D2O.
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Surya Srinivas Kotha et al.
Table 2. Substrate scope for alcohols with Pd-BNP catalyst.[a]
[a]
[b]
Reaction conditions: aldehyde 1 (0.5 mmol), 5.3 mg Pd-BNP (10 wt% by ICP-OES analysis) in water (2 mL) at room
temperature.
Isolated yield.
water and the reaction gave a 93% isolated yield fuged. The resulting solid was washed with ethanol
(Scheme 3). and dried under vacuum. The recovered Pd nanocata-
After the successful utilization of stable Pd-BNP lyst was used for further hydrogenation cycles. The
catalyst for the hydrogenation of aldehydes in water, Pd-BNP was successfully recovered and reused for
the recoverability and recyclability of the nanocatalyst five times without loss in catalytic activity (Figure 2).
were investigated on hydrogenation of aldehydes.
HR-TEM analysis of the recovered Pd-BNP
With completion of the hydrogenation of an aldehyde, showed that the recovered Pd-BNP had the same par-
the mother liquor was diluted with EtOH and centri- ticle size without any apparent agglomeration even
after the fifth catalytic cycle of the hydrogenation of
an aldehyde (Figure 3).
In conclusion, we have developed an efficient,
green and catalytic process for the hydrogenation of
aldehydes. Particularly, cheap, non-flammable, non-
toxic and eco-friendly water is used as a solvent. Hy-
drogenation with molecular hydrogen, water as a sol-
vent and recyclability of the stable Pd-BNP nanocata-
lyst makes this methodology novel and environmen-
Scheme 3. Gram scale reaction.
tally benign.
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Acknowledgements
We thank DST nano mission (SR/NM/NS-1034/2012(G)) for
financial support. KSS and NS thank CSIR, New Delhi,
India for senior research fellowships.
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