Chemoselective transfer hydrogenation of Α,Β-unsaturated carbonyls catalyzed by a reusable supported Pd nanoparticles on biomass-derived carbon

Article abstract of DOI:10.1016/j.catcom.2018.11.017

We herein report highly chemoselective transfer hydrogenation of α,β-unsaturated carbonyl compounds to saturated carbonyls with formic acid as a hydrogen donor over a stable and recyclable heterogeneous Pd nanoparticles (NPs) on N,O-dual doped hierarchical porous biomass-derived carbon. The synergistic effect between Pd NPs and incorporated heteroatoms on carbon plays a critical role on promoting the reaction efficiency. A series of α,β-aromatic and aliphatic unsaturated carbonyl compounds was selectively reduced to their corresponding saturated carbonyls in up to 97% isolated yields with good tolerance of various functional groups. In addition, the catalyst can be successively reused for at least 6 times without significant loss in reaction efficiency.

Full text of DOI:10.1016/j.catcom.2018.11.017

CatalysisCommunications120(2019)80–85
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Chemoselective transfer hydrogenation of α,β-unsaturated carbonyls
catalyzed by a reusable supported Pd nanoparticles on biomass-derived
carbon
Tao Song, Yanan Duan, Yong Yang⁎
CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
We herein report highly chemoselective transfer hydrogenation of α,β-unsaturated carbonyl compounds to sa-
turated carbonyls with formic acid as a hydrogen donor over a stable and recyclable heterogeneous Pd nano-
particles (NPs) on N,O-dual doped hierarchical porous biomass-derived carbon. The synergistic effect between
Pd NPs and incorporated heteroatoms on carbon plays a critical role on promoting the reaction efficiency. A
series of α,β-aromatic and aliphatic unsaturated carbonyl compounds was selectively reduced to their corre-
sponding saturated carbonyls in up to 97% isolated yields with good tolerance of various functional groups. In
addition, the catalyst can be successively reused for at least 6 times without significant loss in reaction efficiency.
Pd nanoparticles
Biomass-derived carbon
Transfer hydrogenation
α,β-unsaturated ketones
Formic acid
1. Introduction
which can effectively boost the reaction to achieve high activity and/or
selectivity. However, due to the high surface energy of nanoscale par-
The development of an appropriate catalyst with the goal of facil-
itating organic transformations in a highly selective fashion remains a
challenge. In this context, selective hydrogenation reactions are one of
most important and useful reactions in organic synthesis. Among all the
reduction reactions, the chemoselective hydrogenation of α,β-un-
saturated carbonyl compounds into saturated ketones is a practical and
industrially important process, because the corresponding reduced
products are widely employed as intermediates for the synthesis of fine
chemicals, pharmaceuticals and functional materials [1–3].
In the past decades, great achievements have been made in the se-
lective reduction of C]C bonds in α,β-unsaturated carbonyl com-
pounds [4–6]. Such reactions are frequently performed with molecular
hydrogen as a reducing reagent but along with two serious drawbacks:
(1) H2 as highly flammable gas is readily to form explosive mixtures
with air; and (2) the difficult to control the stoichiometry of H2 often
results in over-reduction of substrates. To this end, catalytic transfer
hydrogenation as a viable alternative was accordingly developed to
replace the more classical reduction methods using molecular hy-
drogen. A number of metal catalysts, such as Pd [7–9], Ru [10–12], Rh,
[13,14], Ir [15,16], and other metals [17–22], have been developed for
the reduction with various hydrogen donors. Among these metal cata-
lysts investigated, palladium nanoparticles (NPs) have attracted con-
siderable attention due to its powerful dissociation capability of H2,
ticles, aggregation and Ostwald ripening of metal nanoparticles may
lead to much larger particles, thereby rendering their deactivation.
Therefore, a prerequisite to stabilize and disperse such Pd NPs on an
inert support is frequently necessary to achieve high activity with good
stability. Consequently, a variety of supported Pd NPs catalyst with
different preparation strategies have been developed [7–9,23–25].
Among them, a typical and most frequently employed strategies are to
modulate the interactions between Pd NPs and support by loading Pd
NPs onto the surface-functionalized supports with organic functional
groups or even ionic liquids, polymers (including porous organic
polymer) and heteroatoms-modified supports.
Regarding the modulation of metal and support interactions, in-
corporation of heteroatom into support lattices, as the most effective
and straightforward method, allows for precisely tuning chemical and
electronic properties of supports and in turn significantly affecting the
metal-and-support interactions to synergistically boost the activity and
selectivity. In this context, heteroatom-doped porous carbon materials
derived from biomass have recently attracted rising attention because
of their unique properties such as high electrical conductivity, large
specific surface area, high porosity, and easy availability [24–28].
Very recently, we have developed an operationally simple,
straightforward and environmentally friendly procedure to fabricate
N,O-dual doped hierarchical porous carbon (N,O-Carbon) starting from
⁎ Corresponding author.
E-mail address: yangyong@qibebt.ac.cn (Y. Yang).
https://doi.org/10.1016/j.catcom.2018.11.017
Received 27 September 2018; Received in revised form 31 October 2018; Accepted 27 November 2018
Availableonline11December2018
1566-7367/©2018PublishedbyElsevierB.V.

T. Song et al.
CatalysisCommunications120(2019)80–85
Table 1
Conditions Optimizationa.
Entry
x
y (equiv.)
z
(°C)
Solvent
Conversion
Selectivity
(%)b
(mg)
(%)b
1
2
3
5
5
4
6
6
6
6
6
6
6
6
6
6
6
120
120
120
120
100
80
120
120
120
120
120
120
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
H2O
CH3CN
THF
Me-THF
CH3OH
Toluene
42
> 99
> 99
> 99
> 99
> 99
> 99
> 99
-
> 99
> 99
> 99
-
100
100
80
65
20
10
NR
15
10
11
10
10
10
10
5
5
5
5
5
4c
5
6
7
8
9
10
11
12
0
NR
a
Reaction conditions: chalcone (0.2 mmol), HCOOH (y equiv.), solvent (2 mL), 20 h under argon atmosphere. bDetermined by GC-FID using dodecane as an
internal standard. c15 h.
Table 2
Comparison of reaction efficiency over different supported Pd NPs catalysta.
Entry
Catalyst
Conversion (%)b
Selectivity (%)b
1
2
3
4
5
6
Pd/Al2O3
Pd/ZrO2
Pd/MgO
Pd/C
Pd/AC
Pd/N,O-Carbon
51
46
61
57
61
100
> 99
> 99
> 99
> 99
> 99
> 99
a
Reaction conditions: chalcone (0.2 mmol), HCOOH (6 equiv.), supported Pd NPs catalyst (0.375 mol% of Pd), Toluene (2 mL), 20 h under argon at-
mosphere. bDetermined by GC-FID using dodecane as an internal standard.
naturally renewable and readily available bamboo shoot without using
extra heteroatoms sources or tedious and multi-step synthesis of sacri-
ficial templates. The resultant N,O-Carbon was employed as a support
to load Pd NPs to prepare a stable catalyst Pd/N,O-Carbon, which en-
ables highly efficient alkynes functionalizations [29–31]. Intrigued by
the catalytic utility of the catalyst Pd/N,O-Carbon, we became more
interesting in the development of innovative catalytic processes that
allows for chemoselective reduction of α,β-unsaturated carbonyl com-
pounds. Herein, we report the application of such catalyst Pd/N,O-
Carbon for the successful chemoselective transfer hydrogenation reac-
tion of α,β-unsaturated carbonyl compounds with formic acid (FA) as a
reducing reagent to synthesize saturated carbonyls. The present cata-
lytic system shows outstanding catalytic activity with exclusive se-
lectivity to C]C bond reduction and broad substrate scope of various
α,β-unsaturated carbonyl compounds with good tolerance of functional
group. In addition, the catalyst Pd/N,O-Carbon can be easily recycled
for successive uses without significant loss in activity and selectivity.
donor. To our delight, the reaction proceeded smoothly to convert 1a to
1,3-diphenylpropan-1-one (2a) in 42% GC yield with perfect selectivity
after 20 h, maintaining C]O bond intact (Table 1, entry 1). Encouraged
by this result, a set of factors, including the loading of the catalyst Pd/
N,O-Carbon and FA, reaction times and temperature, and solvents, were
inspected and the results are compiled in Table 1. Upon rapid in-
vestigation of several factors, the optimal reaction conditions involve 6
equivalent of FA as hydrogen donor, toluene as solvent, in the presence
of 0.375 mol% of catalyst Pd/N,O-Carbon at 120 °C reaction tempera-
ture (Table 1, entry 2).
For comparison, commercially available Pd/C (10 wt% of Pd) and
the catalysts prepared from commonly used activated carbon (AC with
surface area of 1580 m2 g−1) and oxides supports such as Al2O3, TiO2,
CeO2 and ZrO2 with roughly equal Pd loading and same preparation
procedure were employed for the reaction under otherwise identical
conditions (Table 2 and Table S1). None of them showed a comparable
activity to that of the catalyst Pd/N,O-Carbon. Remarkably, the reaction
efficiency of either the commercial catalyst Pd/C or Pd/AC without any
heteroatom-doping on carbon was significantly lower (Table 2, entries
4 & 5). In addition, we also compared the present system with previous
reported results (Table S3). It is obvious to find that our catalytic
system exhibited higher yields to the desired product with lower cata-
lyst loading and broader substrate scopes.
2. Results and discussion
From the outset, the hydride transfer hydrogenation of 1,3-diphe-
nylprop-2-en-1-one (chalcone, 1a) was chosen as a model substrate to
assess whether the catalyst Pd/N,O-Carbon could enable selective re-
duction of C]C or C]O bond using formic acid (FA) as a hydrogen
In order to shed light on the key influence factors on the catalytic
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CatalysisCommunications120(2019)80–85
Table 3
Substrate scopea.
a
Reaction conditions: α,β-unsaturated carbonyl (0.2 mmol), HCOOH (6 equiv.), Pd/N,O-Carbon (5 mg, 0.375 mol% of Pd), toluene (2 mL), 120 °C, 20 h under
argon atmosphere. b10 mg Pd/N,O-Carbon, HCOOH (8 equiv.). c10 h. Yield of isolated products.
activity among the commercial Pd/C, Pd/AC, and Pd/N,O-Carbon,
characterizations of XRD, BET, HR-TEM, and XPS were subsequently
performed. The catalysts Pd/C and Pd/AC had a greatly larger surface
area than that of Pd/N,O-Carbon (Table S2) without noticeable differ-
ence in particle size (Figs. S1), which indicate that the particle size of
Pd NPs and surface area of the support have a negligible impact on the
reaction efficiency. However, Pd 3d XPS spectra (Fig. S4 (B)) showed an
obvious downward shift for the catalyst Pd/N,O-Carbon compared with
that of Pd/AC with no N-doping, indicating that the interaction be-
tween Pd NPs and N on the carbon. We believe that such interaction not
only is beneficial for uniformly dispersing and stabilizing Pd NPs, but
also is helpful for activating the acidic molecule of formic acid as the
basic coordination sites and thereby facilitating the process of transfer
hydrogenation. Consequently, these results confirm the intrinsic nature
of N,O-dual-doped hierarchical porous carbon played a synergistic role
in catalysis, which is in line with our previous findings [29,30,31].
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CatalysisCommunications120(2019)80–85
Fig. 1. (A) Hot filtration test; (B) recyclability of the catalyst Pd/N,O-Carbon for the transfer hydrogenation of chalcone under the standard conditions.
Fig. 2. 1H NMR experiments on the reduction of chalcone in the presence of the catalyst Pd/N,O-Carbon in toluene-d8 at 120 °C at different time intervals.
To demonstrate the general applicability of the protocol, various
α,β-unsaturated carbonyls were subjected to the optimized reaction
conditions (Table 3). Both electron-donating and -withdrawing aro-
matic substituted α,β-enones (1a-1h) were reduced efficiently to pro-
vide good to excellent yields of the desired products. The position of the
substituted groups did not significantly affect reactivity, especially
ortho substituents positioned to either the –OH or -Me (1c, 1e, and 1h)
had marginal effect on the reaction efficiency. Halogen-substituted
chalcones (1h–1k) were well tolerated with the conditions, affording
the desired products in excellent yields and no dehalogenation was
observed in these cases. It is worthy to pointing out that the retaining of
halogen group on the phenyl ring makes them feasible to further
functionalize into some versatile pharmaceutical molecules through
palladium-catalyzed couplings. Heteroaromatic chalcone-type deriva-
tives (1m-1o), were also reduced to give the corresponding saturated
ketones in excellent yields with this protocol. The two conjugated C]C
bonds in diphenylpenta-1,4-dien-3-one (1l) were also selectively satu-
rated in 84% yield with 0.75 mol% Pd (10 mg of Pd/N,O-Carbon) and
more excess of FA (8 equiv.). More interestingly, a set of α,β-un-
saturated aldehyde (1p), ketone (1q), ester (1r), acid (1s), nitrile (1p),
and amide (1u) could be efficiently reduced with keeping C]O bond
intact in 80–91% isolated yields. In addition, cyclic enones, such as
benzoquinone (1v) and cyclohex-2-en-1-one (1w), were also converted
to the saturated ketones in 57% and 77% yields, respectively.
The practical utility of this protocol was further highlighted by its
application for synthesis of several natural and/or biologically active
compounds, such as Davidigenin, Rheosmin, Menthone, and
Testosterone. These compounds could be efficiently and chemoselec-
tively synthesized from their respective α,β-unsaturated carbonyls in
good to high yields under the standard reaction conditions.
Remarkably, Testosterone was synthesized with exclusive diastereos-
electivity affording cis-isomer as the sole product in the present
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CatalysisCommunications120(2019)80–85
chalcone (1a), which indicates that the catalytic transfer hydrogenation
reaction is preferable when chalcone (1a) is involved. Control experi-
ments of addition of the catalyst Pd/N,O-Carbon into formic acid in
toluene-d8 shows that the peak of aldehyde proton in formic acid also
shifted to a higher field together with the appearance of a peak at
4.3 ppm after 5 h (Fig. 2a & f), strongly indicating that the chemical
interaction between Pd/N,O-Carbon and formic acid and dehy-
drogenation reaction occur to release molecular hydrogen and CO2. GC-
TCD analysis of the headspace gas after reaction further confirms the
generation of H2 and CO2 originating from FA dehydrogenation in the
presence of Pd/N,O-Carbon as shown in Fig. S7.
Taken these observations into consideration, we proposed the re-
action mechanism for chemoselective transfer hydrogenation of α,β-
unsaturated carbonyls as shown in Scheme 1. Initially, N atoms on the
catalyst, as basic sites with remarkable role in facilitating proton
transfer, capture H+ from the formic acid to generate NH+ and form
the Pd-formate intermedidate (A). The Pd-formate intermediate can
further release a CO2 molecule to generate PdH− (B). Hydrogen atoms
from NH+ and PdH− (B) could combine to generate molecular hy-
drogen, while hydrogen production rate is very low due to the need of a
sufficient energy to drive the combination of hydrogen (Path B). As
such, when α,β-unsaturated carbonyl is added in the catalytic system,
the C]C bond of α,β-unsaturated carbonyl may be adsorbed almost
parallel to the metal surface, which the adsorption properties of metal
surface could be modified by the N,O-doped hierarchically structured
porous materials, then two parts of the hydrogen in NH+ and PdH−
prefer to react with the C]C bond to produce saturated ketones
without evolution of H2, thereby completing the entire catalytic cycle
(Path A) [32,33,35]. Accordingly, in the absence of α,β-unsaturated
carbonyls, dehydrogenation of formic acid takes place to produce mo-
lecular H2 and CO2 (Path B).
Scheme 1. Plausible mechanism of selective transfer hydrogenation of α,β-
unsaturated carbonyls.
catalysis system. Note that the trisubstituted unsaturated ketone was
hydrogenated as effectively and efficiently as the disubstituted ones,
exemplified as synthesis of Menthone.
Hot filtration test was implemented to verify whether the observed
catalysis was owing to the heterogeneous catalyst Pd/N,O-Carbon or a
leached Pd species in solution. We observed that conversion was
changeless when the catalyst was removed from the reaction mixture by
filtration at approximately 40% conversion of 1a (Fig. 1A). It was
confirmed by ICP-AES analysis that no Pd species could be detected in
the filtrate (below the detection limit). Meanwhile the catalyst Pd/N,O-
Carbon could be recovered by centrifugation and reused at least 5 times
for the reduction of 1a and FA with a slight decrease of conversions but
with keeping perfect selectivity (Fig. 1B). We suspect the decrease of
reaction efficiency is most likely due to the loss of catalyst during the
recycle process, which was further verified by addition of a lost-amount
of fresh catalyst into the reaction and full conversion of 1a was re-
covered in this case (Fig. 1B). Considering all these factors, we ruled out
any contribution to the observed catalysis from a homogeneous Pd
species, demonstrating that the observed catalysis was intrinsically
heterogeneous.
To ascertain a mechanistic understanding of the reaction, several
control experiments were carried out. Firstly, we tracked the reaction
progress as a function of reaction times using in-situ NMR spectroscopy,
as shown in Fig. 2. Note that the reaction efficiency is slightly slower
than that in the real reaction system because in-situ NMR experiment
was performed without stirring. HCOOH was added to an NMR tube
charged with the catalyst Pd/N,O-Carbon and chalcone (1a) in toluene-
d8, and the progress of the reaction was monitored by 1H NMR spec-
troscopy at various time intervals. As shown in Fig. 5b-e, two sets of
multiplet peaks at 2.63 and 2.50 ppm appear, assignable to the protons
of CH2-CH2 bond of the generated product 2a, respectively, and the
peak intensity gradually increases with the elapse of the reaction time.
Accordingly, the intensity of the characteristic peaks at around 7.5 ppm
corresponding to the protons of the phenyl ring in 1a decreases gra-
dually, indicating the consumption of 1a to form 2a. Meanwhile, an
intense single peak at 10.4 ppm is observed and gradually shifts to a
higher field (has a smaller chemical shift) with the progress of the re-
action. More importantly, a new peak at 4.3 ppm, assignable to in-situ
released H2, [31, 34] started to appear when 1a was approaching
complete conversion, while no such peak was observed if 1a still pre-
sented. It is obvious that the reaction rate of FA catalytic transfer hy-
drogenation is much higher than the rate of dehydrogenation of formic
acid. What's more, hydrogen was not observed in the presence of
3. Conclusions
In summary, we discovered that the catalyst Pd/N,O-Carbon shows
distinctly high catalytic activity in the chemoselective transfer hydro-
genation of α,β-unsaturated carbonyls to saturated carbonyl com-
pounds using formic acid as a reducing reagent. The synergistic effect
between Pd NPs and heteroatoms could effectively promoted the
transfer hydrogenation reaction. A broad set of α,β-unsaturated car-
bonyls could be efficiently reduced to their corresponding saturated
carbonyl compounds with good tolerance of various functional groups.
The catalyst is also applicable for expedient synthesis of a number of
natural and biologically active saturated ketone compounds. In addi-
tion, the catalyst could be reused for several times without significant
loss of catalytic activity and selectivity.
Acknowledgements
We gratefully acknowledge the start-up financial support from
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese
Academy of Sciences (NO. Y6710619KL).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.catcom.2018.11.017.
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