Highly Selective Semihydrogenation of Alkynes to Alkenes by Using an Unsupported Nanoporous Palladium Catalyst: No Leaching of Palladium into the Reaction Mixture

Article abstract of DOI:10.1021/acscatal.7b02915

We report the highly chemoselective and stereoselective semihydrogenation of alkynes to Z-internal and terminal alkenes by using unsupported nanoporous palladium (PdNPore) as a heterogeneous catalyst under mild reaction conditions (room temperature and 1 atm of H₂). The semihydrogenation of various terminal/internal and aromatic/aliphatic alkynes afforded the corresponding alkenes in good chemical yields with high selectivities. PdNPore further showed high chemoselectivity toward terminal alkynes in the presence of internal alkynes, which has not yet been achieved using supported palladium nanoparticle catalysts. H-H heterolysis of H₂ on the surface of PdNPore was strongly suggested by deuterium labeling experiments. No Pd leached from PdNPore during the reaction, and the catalyst was easily recovered and reused without a loss of activity.

Full text of DOI:10.1021/acscatal.7b02915

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Article
Highly Selective Semihydrogenation of Alkynes to
Alkenes by Using an Unsupported Nanoporous Palladium
Catalyst: No Leaching of Palladium into Reaction Mixture
Ye Lu, Xiujuan Feng, Balaram S. Takale, Yoshinori Yamamoto, Wei Zhang, and Ming Bao
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02915 • Publication Date (Web): 25 Oct 2017
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Highly Selective Semihydrogenation of Alkynes to
Alkenes by Using an Unsupported Nanoporous
Palladium Catalyst: No Leaching of Palladium into
Reaction Mixture
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Ye Lu,†,ǁ Xiujuan Feng,†,ǁ Balaram S. Takale,†,# Yoshinori Yamamoto,†,§ Wei Zhang,┴ and Ming
Bao*,†,‡
†State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116023,
China.
‡School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin
124221, China.
§WPIꢀAdvanced Institute for Materials Research, Tohoku University, Sendai 980ꢀ8577, Japan
and Research Organization of Science and Technology, Ritsumeikan University, Kusatsu, Shiga
525ꢀ8577, Japan.
┴School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023,
China.
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ABSTRACT: We report the highly chemoselective and stereoselective semihydrogenation of
alkynes to Zꢀinternal and terminal alkenes by using unsupported nanoporous palladium
(PdNPore) as a heterogeneous catalyst under mild reaction conditions (room temperature and 1
atm H2). The semihydrogenation of various terminal/internal and aromatic/aliphatic alkynes
afforded the corresponding alkenes in good chemical yields with high selectivities. PdNPore
further showed high chemoselectivity toward terminalꢀalkynes in the presence of internal
alkynes, which has not yet been achieved using supported palladium nanoparticle catalysts. H–H
heterolysis of H2 on the surface of PdNPore was strongly suggested by deuterium labeling
experiments. No Pd leached from PdNPore during the reaction, and the catalyst was easily
recovered and reused without activity loss.
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KEYWORDS: alkyne, semihydrogenation, heterogeneous catalysis, nanoporous palladium, no
leaching
INTRODUCTION
The selective hydrogenation of alkynes to (Z)ꢀalkenes is highly important for the construction of
numerous valuable compounds, such as bioactive molecules, natural products, flavors, and
industrial materials.1 Various catalysts are available for the conversion of alkynes into (Z)ꢀ
alkenes. In previous catalytic approaches, this transformation is accomplished in the presence of
either a homogeneous2 (e.g., Pd, Ni, Ru, Rh, and Co complexes) or a heterogeneous3 (e.g., Pd
catalysts, Ni catalysts, and Au nanoparticles) catalyst. Among the known catalytic methods,
hydrogenation using Lindlar catalyst (Pd/CaCO3 treated by Pb salts) is the most popular and
widely used.4 Unfortunately, Lindlar catalyst has the following serious drawbacks: the
requirement of a toxic lead salt, partial isomerization of Zꢀalkene to Eꢀalkene, doubleꢀbond
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shifting, and lack of highly reproducible results.1a Furthermore, Lindlar catalyst has a limited
substrate scope, i.e., only internal alkynes are selectively hydrogenated into desired alkenes,
whereas terminal alkynes often suffer from the rapid overhydrogenation of resulting alkenes to
alkanes. From the perspective of green chemistry and other practical concerns, the development
of more environmentally benign and efficient catalyst systems for the semihydrogenation of
alkynes is highly desirable.
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Catalysis using unsupported nanoporous metal materials is attracting increased interest given
their potentially green and sustainable catalytic properties.5 The interesting features of
nanoporous metals such as nontoxicity, robustness, high recyclability, and simple recovery
render them attractive as heterogeneous catalysts. Over the past few years, we have demonstrated
that nanoporous metals are promising green heterogeneous catalysts for liquidꢀphase organic
synthesis.6 We have recently reported that nanoporous gold (AuNPore) is effective for the
transfer semihydrogenation of alkynes (Scheme 1, Eq. 1).6d,6o However, this method requires
excess amounts of formic acid/triethylamine or costly organosilanes as a hydrogen source.
Therefore, the formation of stoichiometric amounts of byproduct is inevitable. The use of
hydrogen gas has wellꢀaccepted advantages over Hꢀtransfer reagents because no waste is
generated after the reaction using hydrogen.7 We have further demonstrated that AuNPore can
serve as a robust catalyst for the semihydrogenation of alkynes to alkenes with very high Zꢀ
selectivity by using H2 as hydrogen source (Scheme 1, Eq. 2).6p Although we have achieved high
yields and selectivities, a slightly high hydrogen pressure (8 atm) and temperature (90 °C)
required for the reaction hinders industrial application. Hence, we became curious if alkynes can
be reduced using other metal nanoporous materials such as PdNPore under mild conditions.
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Another important and longꢀstanding concern for both heterogeneous and homogeneous
palladium catalysis is the leaching problem.8 The topic of palladium leaching from nanoparticle
form is debatable. The degree of leaching depends on palladium nanoparticle size, support
material, and reaction media and conditions. Palladium leaching in liquidꢀphase reactions using
PdNPore such as crossꢀcoupling reactions has been investigated, and data clearly show much less
palladium leaching from PdNPore than palladium black and Pd/C (i.e., 1.14 ppm for Heck
reaction and 0.69 ppm for Suzuki coupling).9 To the best of our knowledge, reports on Pd noꢀ
leaching in reactions using heterogeneous palladium catalysts are very few.10
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In this paper, we report the highly stereoselective and chemoselective semihydrogenation of
alkynes to Zꢀalkenes achieved by using unsupported nanoporous palladium under 1 atm H2 at
room temperature (Scheme 1, Eq. 3). In this hydrogenation, no palladium leaching from
PdNPore was observed.
Scheme 1. Semihydrogenation of Alkynes Catalyzed by AuNPore or PdNPore
RESULTS AND DISCUSSION
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Preparation and Characterization of PdNPore Catalyst. Pd nanoporous catalyst was prepared
according to previous literature.11 First, pure palladium (99.99%) and aluminum (99.99%) were
melted in a quartz crucible using a highꢀfrequency induction furnace to form Pd20Al80 ingots.
Then, a singleꢀroller melt spinning apparatus was used to remelt the prealloyed ingots (about 2–4
g) by highꢀfrequency induction heating in a quartz tube and then meltspun onto a 0.35 mꢀ
diameter copper roller at 1000 or 1500 rpm in controlled argon atmosphere. The ribbons
obtained were typically 20–50 µm thick, 2–4 mm wide, and several centimeters long. Finally, the
Pd20Al80 alloy ribbons were dealloyed in 20 wt% NaOH aqueous solution under corrosionꢀfree
conditions. Using 20 wt% NaOH solution, Pd20Al80 was dealloyed at room temperature until no
obvious bubbles emerged. Dealloying was continuously performed in alkaline solution at 90 °C
to further leach out residual Al in the samples. Dealloying was generally complete within <2 h.
The dealloyed samples were removed from the solution and then rinsed with deionized water and
dehydrated alcohol several times before being dried in a vacuum.
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Ligaments and nanopore channels formed uniformly across the entire PdNPore, as shown by
scanning electron microscopy (SEM) image (Fig. 1a). PdNPore clearly showed the structure of
PdNPore with bright nanopores and darkꢀcontrast ligaments, as revealed by the transmission
electron microscopy (TEM) image (Fig. 1b). The specific surface area of PdNPore was measured
to be 41 m2/g by Brunauer–Emmett–Teller method. Poreꢀsize distribution was also determined to
be 3.6 nm from the adsorption branch of isotherms based on density functional theory (DFT).12
The dealloyed material was found to comprise Pd88Al12 based on energyꢀdispersive Xꢀray
spectroscopy (EDX) analysis (Fig. 1c). SEM and TEM images as well as EDX analysis spectrum
of PdNPore after six runs are shown in Fig 1d, e, and f, respectively. An essentially same result
was observed for the fresh ones and those after six runs.
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Figure 1. (a) SEM image of fresh PdNPore, (b) TEM image of fresh PdNPore, (c) EDX analysis
of fresh PdNPore, (d) SEM image of PdNPore after six runs, (e) TEM image of PdNPore after
six runs, (f) EDX analysis of PdNPore after six runs.
To gain further insight into the catalytic property of PdNPore, Xꢀray photoelectron spectroscopy
(XPS) analysis was performed (Fig. 2). Fig. 2a shows that the coreꢀlevel spectrum of Pd 3d in
PdNPore had a doublet signal with binding energies of 335.55 and 340.80 eV for Pd 3d5/2 and Pd
3d3/2, respectively, which shifted to a binding energy higher than 335.30 eV for Pd 3d5/2 of
metallic palladium.13 Fig. 2b presents the XPS spectra of Al 2p in metallic aluminum and
PdNPore. Two kinds of Al 2p signal located at 75.55 and 72.85 eV, respectively, were observed
when metallic aluminum was subjected to XPS determination; the former was assigned to
aluminum oxide, and the latter was assigned to metallic aluminum.14 This result indicated that
some aluminum atoms of metallic aluminum were oxidized during the determination. The coreꢀ
level spectrum of Al 2p in PdNPore showed a peak with 74.50 eV binding energy, which was
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assigned to aluminum oxide. The binding energy of Al 2p in PdNPore shifted to a lower binding
energy than that of normal aluminum oxide. No peak of Al 2p at around 72.85 eV was observed
in the XPS spectrum of PdNPore, indicating that the residual aluminum in PdNPore transformed
to aluminum oxide during dealloying and drying. The upshift of the binding energy for Pd 3d in
PdNPore can be ascribed to the electron transfer from palladium atom to aluminum oxide
derived from residual aluminum.
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(a)
Metallic Pd
PdNPore
Pd3d5/2
Pd3d3/2
346
344
342
340
338
336
334
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330
Binding Energy / eV
Metallic Al
PdNPore
(b)
Al2p
Al2O3
84
82
80
78
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72
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68
66
Binding Energy (eV)
Figure 2. XPS spectra of Pd 3d (a) and Al 2p (b) of fresh PdNPore
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Semihydrogenation of Alkynes Catalyzed by PdNPore. The semihydrogenation of
diphenylacetylene (1a) in the presence of the catalyst PdNPore (5 mol%) was chosen as a model
in our preliminary studies to screen the base and solvent. Results are shown in Table 1. Our
previously obtained results indicated that a basic environment favored the formation of highly
selective Zꢀalkenes.6 Accordingly, diethylamine (Et2NH) and triethylamine (Et3N) were initially
examined as basic solvent under 1 atm H2 at room temperature. As expected, good conversion of
1a and excellent selectivity of Zꢀstilbene (2a) were observed (entries 1 and 2). The bases
Na2CO3, NaOH, EtONa, and NaOtBu were then tested in ethanol (EtOH) under the same
pressure and temperature (entries 3–6). Among the bases tested, NaOH proved to be the best
base, leading to 95% conversion of 1a and 97% selectivity of Zꢀstilbene (2a). The solvents,
EtOH, isopropanol (IPA), toluene, and tetrahydrofuran (THF), were subsequently screened using
NaOH as base (entries 4 and 7–9). The use of EtOH as solvent yielded the best results; a possible
reason is that NaOH has the best solubility in ethanol among the solvents tested. Decreased
conversion (88%) and selectivity (95%) were observed when the amount of NaOH was
decreased to 0.5 equivalents (entry 4 vs. entry 10). Further decreased conversion (80%) and
selectivity (88%) were observed in the absence of base (entry 11).15 The reaction conversion
decreased with both decreased catalyst amount and reaction time (entries 12 and 13). No reaction
was observed in the absence of catalyst PdNPore or by the use of the precursor of PdNPore,
Pd20Al80 alloy, in the semihydrogenation of 1a (entries 14 and 15). Therefore, the subsequent
semihydrogenation of various alkynes was performed in the presence of PdNPore (5 mol%) as
catalyst and using NaOH (1 equiv.) as base under 1 atm H2 at room temperature in EtOH.
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Table 1. Screening of Reaction Conditionsa
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sel. (%)b
2a/3a/4a
conv.
(%)b
entry base
solvent
1
none
none
Et2NH
Et3N
84
79
86
95
73
92
85
91/2/7
2
89/2/9
80/3/17
97/1/2
87/1/12
92/2/6
94/2/4
51/5/44
87/6/7
95/1/4
88/2/10
97/1/2
97/1/2
NRh
3
Na2CO3 EtOH
4
NaOH
EtOH
5
EtONa
EtOH
6
NaOtBu EtOH
7
NaOH
NaOH
NaOH
NaOH
none
IPA
8
toluene 89
9
THF
60
10c
11
12d
13e
14f
15g
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
88
80
NaOH
NaOH
NaOH
NaOH
70
68
NRh
NRh
NRh
aReaction conditions: 1,2ꢀdiphenylethyne (1a, 0.5 mmol), PdNPore (5 mol%), H2 (1 atm), base (1
equiv.), and solvent (3 mL) at room temperature for 18 h. bConversion of 1a and selectivity of 2a
1
c
d
were determined by H NMR using CH2Br2 as internal standard. NaOH (0.5 equiv.). 2 mol%
e
f
PdNPore was used. The reaction was performed for 12 h. The reaction was performed in the
g
h
absence of PdNPore. Pd20Al80 alloy was used. No reaction; the starting material 1a was
recovered.
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After determining the optimum conditions, we explored the reaction scope and limitation.
Scheme 2 shows the results of the PdNPoreꢀcatalyzed semihydrogenation of internal alkynes.
Reactions of symmetric alkynes 1b and 1c bearing an electronꢀdonating group methyl (Me) or
methoxy (MeO) on paraꢀ or metaꢀposition, respectively, completed within a relatively long time
compared with symmetric alkyne 1a. The desired Zꢀalkenes 2b and 2c were obtained in 75% and
70% yields, respectively, with very high selectivities. The alkyne substrates 1d–1f bearing an
electronꢀwithdrawing group fluorine (F) or trifluoromethyl (CF3) on paraꢀ or metaꢀposition,
respectively, exhibited relatively high reactivity to complete semihydrogenation within a short
time compared with alkyne 1a, and products 2d–2f were obtained in high yields (82%–88%)
with very high selectivities. However, only a trace amount of product 2g was obtained when the
alkyne 1g bearing an electronꢀwithdrawing group CF3 on orthoꢀposition was treated under the
optimum conditions despite prolonging the reaction time to 24 h. This negative result was
probably due to the large steric hindrance caused by the orthoꢀsubstituent CF3, which inhibited
the coordination of C–C triple bond to the surface of catalyst PdNPore, thereby preventing the
smooth semihydrogenation of 1g. These results described above indicated that the reactivity of
aromatic alkynes was influenced by the electronic property and steric effect of substituents. The
semihydrogenation of alkyne 1h containing two thiophene rings also proceeded smoothly to
produce the corresponding Zꢀalkene 2h in 87% yield with 99% selectivity, and no catalyst
poisoning caused by sulfur atoms was observed. Notably, the Zꢀalkene 2h was obtained in only
8% yield when the commercially available Lindlar palladium catalyst was used instead of
PdNPore catalyst.
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Scheme 2. PdNPoreꢀCatalyzed Semihydrogenation of Internal Alkynesa,b
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MeO
OMe
F
F
2c
2d, 82%, 16 h
, 70%, 20 h
2a
2b
, 90%, 18 h
, 75%, 20 h
2d 3d 4d
2c 3c 4c
:
:
= 100:0:0
:
:
= 100:0:0
2a 3a 4a
2b 3b 4b
:
:
= 97:1:2
:
:
= 99:0:1
F3C
CF3
CF3
F3C
F3C
CF3
2f
, 88%, 16 h
2f 3f 4f
= 94:1:5
2e
, 86%, 16 h
2e 3e 4e
= 97:0:3
c
2g,
trace, 24 h
:
:
:
:
S
S
O
2k
:
, 85%, 12 h
2k 3k 4k
= 99:0:1
2j
, 77%, 18 h
:
2h
2i, 95%, 20 h
, 87%, 14 h
:
2j 3j 4j
= 99:0:1
2h 3h 4h
2i 3i 4i
:
:
:
:
= 99:0:1
:
= 100:0:0
COOH
Cl
d
O2N
2m
2n
, 72%, 20 h
c
2l
, 81%, 18 h
, 0%, 24 h
2n 3n 4n
= 93:1:6
:
:
2l 3l 4l
:
:
= 95:0:5
aReaction conditions: PdNPore (5 mol%), alkyne (1, 0.5 mmol), and H2 (1 atm), NaOH (1
b
c
equiv.) in EtOH at room temperature. Isolated yield. Starting material was recovered.
dQuinoline (2 equiv.) was used instead of NaOH.
Subsequent examination demonstrated that symmetric aliphatic alkyne 1i was also a suitable
substrate in the target semihydrogenation reaction, producing only the aliphatic Zꢀalkene 2i in
95% yield. The asymmetric internal alkynes 1j–1n were then examined under the same
conditions. The Zꢀalkenes 2j–2l were obtained in 77%, 85%, and 81% yields, respectively, with
very high selectivities, and the functional groups such as acetyl (MeCO) and Cl were tolerated.
However, no reaction was observed upon examination of the alkyne 1m bearing a nitro (NO2)
group on paraꢀposition, and the reason was unclear. Interestingly, 3ꢀphenylpropiolic acid (1n)
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was also selectively reduced to Zꢀalkene 2n (72% yield with 93% selectivity) by changing only
the base from NaOH to quinoline. The semihydrogenation of terminal alkynes 5a–5j were finally
examined under the same conditions (Scheme 3). Both aromatic and aliphatic terminal alkynes
also smoothly underwent semihydrogenation, even when a substituent linked to orthoꢀposition,
to produce the corresponding alkenes 6a–6j at 75%–88% yields, and no detectable overꢀreduced
products were observed. Cl and Br atoms linked to the benzene ring were notably retained in the
structures of products 6e–6g, suggesting that further manipulation may produce more useful
compounds. This successful selective reduction of terminal alkynes to terminal alkenes indicated
that PdNPore was an attractive catalyst because terminal alkynes can be easily overꢀreduced to
alkanes on other heterogeneous palladium catalysts,16 such as Lindlar palladium catalyst and
palladium/carbon catalyst. Such high selectivity exhibited by PdNPore is most probably due to
3D bicontinuous netꢀwork structure of pore shape, which is not available in the case of the
ordinary heterogeneous catalysts.
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Scheme 3. PdNPoreꢀCatalyzed Semihydrogenation of Terminal Alkynesa,b
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aReaction conditions: PdNPore (5 mol%), alkyne (5, 0.5 mmol), and H2 (1 atm), NaOH (1
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b
equiv.) in EtOH at room temperature. Yield determined by 1H NMR using dibromomethane as
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internal standard. cIsolated yield.
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Selectivity Studies on Internal vs. Terminal Alkynes. To explore the catalytic selectivity of
PdNPore, a mixture of terminal alkynes and internal alkynes was hydrogenated under the
conditions in Scheme 2. The results are summarized in Table 2. Initially, a 1:1 mixture of the
internal alkyne, diphenylacetylene (1a), and the terminal alkyne, phenylacetylene (5k), was
treated for 18 h. The alkyne 5k was selectively reduced to the terminal alkene styrene (6k) in
72% yield; the formation of (Z)ꢀstilbene (2a) from 1a and ethylbenzene (7k) from 6k was not
observed at all (entry 1). Other combinations of the internal alkynes and terminal alkynes
provided similar results as a combination of 1a and 5k (entries 2–6). Further studies revealed that
the internal alkyne can also be reduced to Zꢀalkene after the completion of terminalꢀalkyne
semihydrogenation. For example, 2a was obtained in 25% yield when a mixture of 1a and 5k
was treated for 26 h (entry 1). These results indicated that the less sterically hindered terminal
alkynes can preferentially be reduced to terminal alkenes in the presence of internal alkynes. The
reason for this behavior was attributed to the steric effect of alkyne and the nanopore effect of
PdNPore; 3D bicontinuous netꢀwork structure6c of nanoꢀpore is most probably responsible to this
high selectivity. Poor chemoselectivity and a large amount of overꢀreduced product 7k were
observed when Lindlar palladium catalyst and palladium/carbon catalyst were used instead of
PdNPore catalyst (entries 7 and 8).
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Table 2. Semihydrogenation of Terminal Alkynes in the Presence of Internal Alkynes Catalyzed
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by PdNPorea
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yield (%)b
internal alkyne
terminal alkyne
entry
1: R1/R2
5: R3
2
6
7
1
1a: Ph/Ph
1a: Ph/Ph
1a: Ph/Ph
1a: Ph/Ph
1a: Ph/Ph
5k: Ph
2a, 0c 6k, 72 7k, 0
2
5a: 4ꢀMeC6H4
5c: 4ꢀMeOC6H4
5e: 4ꢀClC6H4
5f: 4ꢀBrC6H4
2a, 0
2a, 0
2a, 0
2a, 0
2j, 0
6a, 62 7a, 0
6c, 60 7c, 0
6e, 76 7e, 0
6f, 78 7f, 0
6c, 64 7c, 0
3
4
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1j: Ph/4ꢀMeC6H4 5c: 4ꢀMeOC6H4
7d
8e
1a: Ph/Ph
5k: Ph
2a, 10 6k, 32 7k, 36
1a: Ph/Ph
5k: Ph
2a, 36 6k, 0 7k, 87
aReaction conditions: internal alkyne (0.5 mmol 1), terminal alkyne (0.5 mmol 5), H2 (1 atm),
b
PdNPore (5 mol%), and NaOH (1 equiv.) in EtOH at room temperature for 18 h. Yield
c
determined by 1H NMR using CH2Br2 as internal standard. (Z)ꢀstilbene was obtained in 25%
d
yield with prolonged reaction time to 26 h. Lindlar palladium catalyst (5 mol%) was used as a
catalyst instead of PdNPore. e10 wt% Pd/C (5 mol%) was used as a catalyst instead of PdNPore;
diphenylethane was obtained in 16% yield.
Leaching Studies of PdNPore Catalyst. To determine whether the present reaction proceeded
heterogeneously on the surface of PdNPore or homogeneously through dissolved palladium
species, we performed the following leaching experiments (Scheme 4). After the
semihydrogenation of alkyne 1a under standard conditions for 6 h, half of the reaction solution
was transferred to another reaction vessel, and the yield of product 2a was determined to be 35%
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at this time. No change in the yield of 2a was observed when half of the reaction solution was
continuously stirred under H2 (1 atm) in the absence of solid catalyst for 12 h. By contrast,
product 2a was obtained in 93% yield through the reaction of residual containing PdNPore
catalyst. These results indicated that no palladium atoms were leached into the reaction solution,
thereby confirming that catalysis proceeded heterogeneously.
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Scheme 4. Leaching Test of PdNPore Catalyst in the Semihydrogenation of 1a
To investigate the stability of PdNPore catalyst, inductively coupled plasma mass spectrometry
(ICPꢀMS) was used to determine trace amounts of palladium leaching. For comparison, the
leaching amounts of palladium from Lindlar palladium catalyst and palladium/carbon catalyst
were also determined. Results are shown in Table 3. After the semihydrogenation of alkyne 1a
was completed, PdNPore catalyst was removed, and the remaining solution was used to
determinate palladium leaching by ICPꢀMS. As predicted, no palladium leached from PdNPore
catalyst (detection limit = 0.01 ppm). However, large amounts of palladium leaching were
observed for aboveꢀmentioned commercial available catalysts. These results clearly indicated
that PdNPore catalyst was more stable than the other two heterogeneous palladium catalysts
tested.
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Table 3. Comparison of Palladium Leaching Amount with PdNPore Catalyst, Lindlar Palladium
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Catalyst, and Pd/C Catalysta
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ratio of products
leaching amount
(%)b
Pd cat.
conv. (%)b
(ppm)
2a:3a:4a
93:2:5
73:6:21
13:4:83
PdNPore
Lindlar Pdc
Pd/Cd
92
91
95
<0.01
0.161
0.281
aReaction conditions: 1a (0.5 mmol), PdNPore (5 mol%), H2 (1 atm), and NaOH (1 equiv.) in
b
1
EtOH (3 mL) at room temperature for 18 h. Determined by H NMR using CH2Br2 as internal
c
d
standard. Lindlar palladium catalyst (5 mol%) was used as a catalyst instead of PdNPore. 10
wt% Pd/C (5 mol%) was used as a catalyst instead of PdNPore.
Reusability and Recovery of PdNPore Catalyst. The stability of PdNPore catalyst was further
investigated, and the results of reusability for the semihydrogenation of alkyne 1a are
summarized in Table 4. Using fresh catalyst, 94% conversion of 1a and 97% selectivity of Zꢀ
alkene 2a were obtained (run 1). The use of recovered catalyst also produced the same excellent
conversion and selectivity as in run 1 (run 2, 94% and 97%, respectively). The catalyst was
recovered again and used repeatedly. Even when the catalyst was reused for the sixth time, the
semihydrogenation of alkyne 1a proceeded smoothly to offer excellent conversion and
selectivity (run 6, 93% and 97%, respectively), and the unchanged catalyst nanostructure was
confirmed by SEM and TEM analysis (Fig. 1d and 1e), and the composition of the catalyst has
not changed determined by EDX analysis (Fig. 1f). These results clearly indicated that PdNPore
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catalyst was very stable and can be used repeatedly without loss of activity. Notably, PdNPore
catalyst can be easily recovered using tweezers and purified by just washing with acetone.
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Table 4. Reusability of PdNPore for Semihydrogenation of 1aa
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sel. (%)b
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conv. (%)b
2a
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EtOH at room temperature for 18 h. bDetermined by 1H NMR using CH2Br2 as internal standard.
Deuterium Labeling and Proposed Mechanism. To explore the mechanism underlying the
excellent selectivity in alkyne semihydrogenation conferred by PdNPore catalyst, deuterium
labeling was conducted. We were interested in whether or not hydrogenation involved Pd–H
species generated through the heterolytic cleavage of H2 gas on PdNPore assisted by a base,
which might act as a weak nucleophile (Scheme 5). A mixture of deuteriumꢀlabeled products 2kꢀ
dα and 2kꢀdβ (1:1) was obtained in 30% yield when the alkyne 1k was semihydrogenated under
optimum reaction conditions in the presence of D2. Formation of double deuterated product 2kꢀ
d2 was not observed at all. It is very interesting that only monoꢀdeuterated alkenes were obtained
without formation of double deuterated alkene,17 since text book chemistry tells us that
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hydrogenation of alkynes and alkenes using Pd catalysts proceeds through H–H addition to
unsaturated C–C bonds. The above observations suggested that the semihydrogenation might
involve Pd–H species. The electronꢀdeficient characteristics of palladium in PdNPore confirmed
by XPS experiments may have facilitated the formation of Pd–H species. To the best of our
knowledge, heterolytic cleavage of H–H bond in Pdꢀcatalyzed hydrogenation has not been
reported until now. Very high chemoselectivity between alkynes and alkenes is able to be
explained based on a weak nucleophilic characteristics of Pd–H species. Alkenes are known to
easily undergo electrophilic addition rather than alkynes, while alkynes undergo nucleophilic
addition more easily compared to alkenes.18 Accordingly, very high chemoselectivity was
observed in the present semihydrogenation of alkynes.
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Scheme 5. Deuterium Labeling
Based on our experimental outcomes and previous reports,6 a plausible catalytic cycle is
proposed to account for the present semihydrogenation reaction (Scheme 6). H2 molecule is
adsorbed onto the surface of PdNPore catalyst, and then the heterolytic cleavage of H–H bond
occurs through interaction of hydroxide ion (OH−) to one of hydrogen atoms to produce Pd–H
species. The coordination of alkyne to PdNPore catalyst and the nucleophilic addition of Pd–H
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species to the carbon–carbon triple bond successively occurs to produce alkenyl palladium
intermediate. The cleavage of Pd–C bond of alkenyl palladium intermediate with H2O finally
provides the corresponding Zꢀalkene and regenerates PdNPore catalyst.
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Scheme 6. Proposed Mechanism for Alkyne Semihydrogenation
CONCLUSIONS
We investigated the catalytic activity and stability of unsupported PdNPore in alkyne
semihydrogenation. Good to very high yields of Zꢀinternal and terminal alkenes were obtained
with very high selectivities under ambient pressure at room temperature. PdNPore catalyst could
be reused several times without any loss of activity. Mechanism studies revealed that Pd–H
species generated in situ must play an important role in achieving very high selectivity.
Regarding this mechanistic study, experimental results strongly suggested the heterolytic
cleavage of H–H bond on PdNPore. The unique features of PdNPore catalyst (mild reaction
conditions, high chemoselectivity and stereoselectivity, good functionalꢀgroup tolerance, high
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stability, and absence of leaching) renders this catalyst more useful for laboratory and industrial
applications than the commercially available Lindlar palladium catalyst.
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ASSOCIATED CONTENT
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Supporting Information.
Experimental procedure and characterization data. This material is available free of charge via
the Internet at http://pubs.acs.org.”
AUTHOR INFORMATION
Corresponding Author
*mingbao@dlut.edu.cn
Present Address
#Department of Chemistry & Biochemistry University of California, Santa Barbara, California
93106, United States.
Author Contributions
ǁThese authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the National Natural Science Foundation of China (Nos. 21373041, 21372035,
21573032, 21773021, and NSFCꢀIUPAC program, No. 21361140375) for their financial support.
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This work was also supported by the Fundamental Research Funds for the Central Universities
(DUT17ZD212).
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(15) A reviewer pointed out that in entry 11 even without external base, NaOH, a decent yield
and selectivity were observed, and asked why this result was obtained. It is known that EtOH is a
typical Lewis base solvent; for example see, Pan, D.; Zhang, J.; Li, Z.; Wu, C.; Yan, X.; Wu, M.
Chem. Commun. 2010, 46, 3681ꢀ3683. This Lewis base solvent may help break HꢀH bond
similarly as NaOH, but its efficiency is a bit weaker than that of NaOH.
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(17) A referee asked “why alkeneꢀd2 is not obtained and why only alkeneꢀd1 is obtained”. If
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D–D is used, DOH is produced after OH－ coordination to D–D and Pd–D is formed, according
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to the proposed mechanism shown in Scheme 5. EtOH is used as a solvent, and D–H exchange
of DOH readily occurs in the presence of EtOH, and therefore the protonolysis of Pd–C bond
takes place with H2O. This is a reason why alkeneꢀd2 is not obtained.
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