Selective Transfer Semihydrogenation of Alkynes with H2O (D2O) as the H (D) Source over a Pd-P Cathode

Article abstract of DOI:10.1002/anie.202009757

We reported a selective semihydrogenation (deuteration) of numerous terminal and internal alkynes using H₂O (D₂O) as the H (D) source over a Pd-P alloy cathode at a lower potential. P-doping caused the enhanced specific adsorption of alkynes and the promoted intrinsic activity for producing adsorbed atomic hydrogen (H*_ads) from water electrolysis. The semihydrogenation of alkynes could be accomplished at a lower potential with up to 99 % selectivity and 78 % Faraday efficiency of alkene products, outperforming pure Pd and commercial Pd/C. This electrochemical semihydrogenation of alkynes might proceed via a H*_ads addition pathway rather than a proton-coupled electron transfer process. The decreased amount of H*_ads at a lower potential and the more preferential adsorption of the Pd-P to C≡C π bond than C=C moiety resulted in the excellent alkene selectivity. This method was capable of producing mono-, di-, and tri-deuterated alkenes with up to 99 % deuterium incorporation.

Full text of DOI:10.1002/anie.202009757

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Selective Transfer Semihydrogenation of Alkynes with H2O
(D2O) as the H (D) Source over a Pd-P Cathode
Authors: Yongmeng Wu, Cuibo Liu, Changhong Wang, Siyu Lu, and
Bin Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202009757
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10.1002/anie.202009757
Angewandte Chemie International Edition
RESEARCH ARTICLE
Selective Transfer Semihydrogenation of Alkynes with H2O (D2O)
as the H (D) Source over a Pd-P Cathode
Yongmeng Wu,+,[a] Cuibo Liu,+,[a] Changhong Wang,[a] Siyu Lu,[c] and Bin Zhang*,[a],[b]
[a]
Y. Wu, Dr. C. Liu, Dr. C. Wang, Prof. B. Zhang
Institute of Molecular Plus, Department of Chemistry, School of Science, Tianjin University
Tianjin 300072, China
E-mail: bzhang@tju.edu.cn
[b]
[c]
[+]
Prof. B. Zhang
Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Frontiers Science Center for Synthetic Biology (Ministry of Education), Tianjin University
Tianjin 300072, China
Dr. S. Lu
Green Catalysis Center, College of Chemistry, Zhengzhou University
Zhengzhou 450000, China
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Developing a facile controllable avenue to selective
semihydrogenation of alkynes to alkenes with defined
tedious modification procedures, safety risk, and environmental
concerns caused by using flammable hydrogen gas (H2) or other
toxic hydrogenating reagents, inevitable over-hydrogenation,
or/and low functional groups tolerance. Furthermore, present
strategies are in weak compatibility with the deuterated alkenes
owing to the hard access to suitable deuterium (D) sources.[3-7]
a
configuration using a cheap and safe hydrogen donor is highly
desirable, but remains a big challenge. Here, we reported a selective
semihydrogenation (deuteration) of numerous terminal and internal
alkynes using H2O (D2O) as the H (D) source over a Pd-P alloy
cathode at a lower potential. Density functional theory (DFT)
calculation revealed that the doping of P caused the enhanced
specific adsorption of alkynes and the promoted intrinsic activity for
producing adsorbed atomic hydrogen (H*ads) from water electrolysis.
The semihydrogenation of alkynes could be accomplished at a lower
potential with up to 99% selectivity and 78% Faraday efficiency of
alkene products, outperforming pure Pd and commercial Pd/C. This
electrochemical semihydrogenation of alkynes might proceed via a
H*ads addition pathway rather than a proton-coupled electron transfer
process. The decreased amount of Hads at lower potential and the
more preferential adsorption of the Pd-P to C≡C π bond than C＝C
moiety resulted in the excellent alkene selectivity. The
thermodynamically unstable but synthetically significant Z-alkenes
with > 99% Z selectivity were expediently accessed. This method is
capable of producing the synthetically more useful but highly
challenged mono-, di-, and tri-deuterated alkenes with up to 99%
deuterium incorporation. The paired production of crucial industrial
feedstock adiponitrile at anode and alkene at the cathode in a Pd-P
|| NiSe two-electrode system demonstrated its promising potential.
Therefore,
developing
a
facile
and
controllable
semihydrogenation (deuteration) method of alkynes and with
good functional group tolerance using cheap and safe hydrogen
(deuterium) donor is urgently demanded.
The electrochemical transformation represents a powerful,
environmentally benign, and easy-to-operate method in organic
synthesis.[8] The product selectivity and distribution can be tuned
by the external potential or current density via changing the
adsorption/desorption modes or the reaction intermediate
configurations.[9] Electrocatalytic water splitting provides an
efficient route to produce chemisorbed hydrogen on cathode
surface via Volmer process.[10] The amount of reactive H*ads is
highly dependent on the applied electricity. Therefore, using in-
situ generated H*ads (D*ads) from water electrolysis to develop an
electrochemical semihydrogenation (deuteration) of alkynes will
be highly interested in the remediation to traditional
hydrogenation (deuteration) processes. Although Atobe and
coworkers have recently displayed an advance in
electrocatalytic semihydrogenation of alkynes in a flow reactor[11]
some critical issues were required to be addressed, such as the
narrow substrate scopes, inadequate mechanism investigations,
untouched deuteration of alkynes. Thus, it is significant to
search appropriate cathode materials for achieving the high
activity and selectivity toward alkynes semihydrogenation by
efficiently generating H*ads and inhibiting the competing
hydrogen evolution at a low potential, especailly for pairing
oxidative production of value-added products to replace water
oxidation at the anode.
,
Introduction
Selective semihydrogenation of alkynes to alkenes, especially
the configuration defined Z-alkenes, is
a
significant
transformation due to the essential role of alkenes scaffolds in
the manufacture of natural products, pharmaceuticals, and
advanced materials.[1] Currently, Pd is a preferable catalyst
toward alkynes hydrogenation owing to the excellent affinity of
alkynes on Pd and efficient formation and storage of adsorbed
reactive hydrogen (H*ads).[2] Recently, the well-designed
modification of Pd catalysts by forming alloys or Lindlar-type
formulation,[3] employing ligands,[4] downsizing the Pd scale to
atomic level[5] or/and introducing the p-block elements[6] were
developed to change the adsorbed state of alkyne/alkene or
decrease the overall amount of H*ads for achieving high alkene
selectivity. But their full applications were retarded by the
Here, by employing H2O (D2O) as the H (D) source, a facile
and controllable electrocatalytic semihydrogenation (deuteration)
strategy of alkynes was reported to selectively synthesize
alkenes and deuterated alkenes over Pd-P alloy nanoparticle
networks (Pd-P NNs) cathode under ambient conditions (Figure
1). The incorporation of
P
was crucial for achieving
semihydrogenation of alkynes with accelerated reaction kinetics
and high yields and selectivity at lower potential. A variety of
terminal and internal alkynes bearing different functional groups
could be selectively hydrogenated and deuterated.
1
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10.1002/anie.202009757
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RESEARCH ARTICLE
cell consisting of a working electrode, a Pt plate counter
electrode, and a Hg/HgO reference electrode. The cathode cell
(10 mL) and anode cell (10 mL) containing 1 M KOH solution
(2.0 mL Diox and 5.0 mL H2O), respectively, were separated by
the membrane. 0.2 mmol of alkynes were added into the
cathode and sonicated to form a homogeneous solution. Then,
chronoamperometry was carried out at
a given constant
potential -0.8 V vs. Hg/HgO and stirred until the starting
materials disappeared. After that, the products at cathode were
extracted with DCM. The DCM phase was removed, and the
residuals were subjected to be separated either by flash column
chromatography or using thin-layer chromatography (TLC) plate
to give the isolated yields or was analyzed by GC to provide the
GC yields. The yields were calculated by dividing the amount of
the obtained desired product by the theoretical yield. The GC
yields were calculated according to standard calibration curves.
The procedure and reaction setup of electrochemical
semideuteration of alkynes to tri-deuterated alkenes and di-
deuterated alkenes were similar to the semihydrogenation
process except that the H2O and 1 M KOH were replaced by
Figure 1. Electrocatalytic transfer semihydrogenation of alkynes with H2O
(D2O) over a Pd-P cathode.
Experimental Section
Preparation of Pd-P NNs and Pd NNs. Pd-P NNs were
synthesized according to the reported literature with slight
modification.[12] In a typical synthesis of the Pd-P NNs, 0.01 g
NaH2PO2 and 0.29 g Brij 58 were dissolved in 8 mL of water. 0.1
mL HCHO and 4 mL Na2PdCl4 (20 mM) were added to the
solution with stirring, and the pH of the solution was adjusted to
10 by adding an appropriate amount of NaOH. Next, the mixed
solution was transferred into a Teflon-lined stainless-steel
autoclave and kept at 150 ℃ for 8 h. Then it was cooled down
naturally. The resulting product was washed by distilled water
and absolute ethanol several times, respectively, and dried in a
vacuum oven at 40 ℃ for 6 h. The Pd NNs were also prepared
by the same method without adding NaH2PO2 into solution.
Characterization. The scanning electron microscopy (SEM)
images were taken with a Hitachi S-4800 scanning electron
microscope (3 kV). The transmission electron microscopy (TEM)
images were obtained with the JEOL-2100F system equipped
with EDAX Genesis XM2. The X-ray diffraction (XRD) patterns
of the products were recorded with Bruker D8 Focus Diffraction
System using a Cu Kα source (λ = 0.15406 nm). X-ray
photoelectron spectroscopy (XPS) measurements were
D2O and 0.5
M K2CO3 respectively. To produce mono-
deuterated alkenes, 0.2 mmol alkynes were dispersed in 0.5 M
K2CO3 (Diox/D2O, 1:1 v/v, 4 ml) solution with stirred for 2 min.
Then the products were extracted with DCM and concentrated to
obtained monodeuterated alkynes precursors. After that, the
monodeuterated alkynes without purification were added directly
into the cathode for the electrochemical semideuteration of
alkynes to mono-deuterated alkenes using the similar procedure
with the electrochemical semihydrogenation of alkynes except
that the 1 M KOH was replaced by 1 M phosphate buffer
solution (pH = 5.8). The purification and quantification methods
for deuterated products were similar to those of hydrogenated
products.
The scavenge of high active *H with tertiary butanol (t-
BuOH). To ensure the reliability of the comparison experiment,
we cut a piece of Pd-P NNs electrode into two halves with the
same area, and used them in the electrocatalytic
semihydrogenation of alkynes with and without t-BuOH
respectively. 0.1 mmol of 1a were added into the electrolytic cell
for the following semihydrogenation. Chronoamperometry was
carried out at a given constant potential -0.8 V vs. Hg/HgO for
140 min. The content of 2a was detected every 15 minutes.
The procedure for paired reactions at both cathode and
anode. Typically, 0.6 mmol of 1a and 0.15 mmol of 4a were
added into the cathodic and anodic cell respectively,
performed on
a photoelectron spectrometer using Al Kα
radiation as the excitation source (PHI 5000 VersaProbe). All the
peaks were calibrated with the C1s spectrum at binding energy
of 284.8 eV. The NMR spectra were recorded on Varian Mercury
Plus 400 instruments at 400 MHz (1H NMR) and 101 MHz (13C
NMR). Chemical shifts were reported in parts per million (ppm)
down field from internal tetramethysilane. Multiplicity was
indicated as follows: s (singlet), d (doublet), t (triplet), m
(multiplet), br (broad). Coupling constants were reported in hertz
(Hz). The gas chromatograph-mass spectrometer (GC-MS) was
carried out with TRACE DSQ. The gas chromatograph (GC) was
measured on Agilent 7890A with thermal conductivity (TCD) and
flame ionization detector (FID). The injection temperature was
chronoamperometry was carried out at
a given constant
potential -1.8 V, after passing charge of about 140 C, the
conversions of 1a and 4a reached to 90% and 99% respectively,
and the selectivities of 2a and 5a were 92% and 99%
respectively.
o
set at 300 C. Nitrogen was used as the carrier gas at 1.5 mL
min-1. The high-performance liquid chromatography (HPLC) was
measured on Agilent 1220.
Results and Discussion
General procedure for electrochemical semihydrogenation
(deuteration) of alkynes. Typically, 2 mg of Pd-P was
dispersed in a solution containing 1 mL ethanol and 10 μL 5%
Nafion solution by sonicating for 20 min. Then the homogeneous
catalyst ink was spread uniformly on carbon paper (CP) as the
working electrode (the area was controlled to 1 cm2 with the
loading amount of ~ 2 mg cm-2). Electrochemical measurements
were carried out in a divided three-compartment electrochemical
Synthesis and Characterization of Pd-P NNs. Pd-P NNs were
prepared through our reported wet-chemical route with slight
modification.[12] Scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) images (Figure 2a)
suggested the three-dimensional (3D) network-like morphologies
of Pd-P nanoparticles. Such 3D structures were favorable for
electron/mass transfer and nanoparticle anti-aggregation. X-ray
2
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10.1002/anie.202009757
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RESEARCH ARTICLE
diffraction (XRD) patterns (Figure 2b) showed the face-centered
cubic (fcc) phase of Pd-P NNs (JCPDS no. 05-0681).[13] This
XRD pattern was similar to that of Pd nanoparticles, but the
peak shifted slightly to a higher angle, suggesting the successful
incorporation of P into Pd NNs by replacing palladium atomic
sites.[14] The clear fringes in the high-resolution transmission
electron microscopy (HRTEM) image with a lattice spacing of
0.23 nm also confirmed the fcc Pd (111) planes (Figure S1a).
The X-ray photoelectron spectroscopy (XPS) spectra of P 2p
indicated the successful doping of the P element into Pd. A
characteristic signal assigned to the Pd-P bond was observed at
around 130 eV (Figure 2c).[15] The prominent peaks at 341.2 (Pd
the theoretical value for ultimately converting 0.2 mmol of 1a to
2a). Either increasing or decreasing the applied potential
resulted in lower yields, selectivity, and Faradaic efficiencies
(FEs). Compared to Pd-P NNs, pure Pd nanoparticle networks
(Pt NNs) with a similar electrochemically active surface area
exhibited a higher onset potential and much lower current
density (Figures 3d and Figure S3). Note that this
electrochemical semihydrogenation only began to occur at -0.9
V vs. Hg/HgO for pure Pd, and bad yields, low selectivity, and
unacceptable FEs were delivered, showing the promotion effect
of P in this semihydrogenation process (Figure 3d). Replacing
Pd-P with Pt/C, Ni2P or CoP led to decreased efficiency even
though at more negative potential or with much more electricity
input (Table S1). The high yield and selectivity were maintained
during six cycles (Figure 3c), suggesting the good durability of
the Pd-P cathode for this electrocatalytic transformation.
Furthermore, no Pd species was detected in the solution by the
inductively coupled plasma mass spectrometry test, suggesting
the Pd-P driven electrocatalytic rection. The XRD spectra of the
recycled samples further reflected the robust stability of Pd-P
NNs (Figure S4). Control experiments demonstrated that this
semihydrogenation process was an electrochemically mediated
process with H2O as the sole hydrogen source (Scheme S1).
3d3/2
)
and
335.9
eV
(Pd
3d5/2
)
belonging
to
Pd(0)[12,15]demonstrated the positive shift of ca. 0.4 eV compared
to pure Pd (Figure 2d), suggesting the electronic interaction
between Pd and P.[12,15] This interaction caused the electron
transfer from Pd to P, leading a decrease in the 3d electron
density of Pd.[12,15]The energy dispersive X-ray (EDX) mapping
revealed a uniformly dispersed P throughout the entire Pd-P
nanoparticle networks (Figure S1b). These results indicated the
successful preparation of Pd-P NNs.
E/Vvs. RHE
-0.2
-0.4
0.0
0.2
a
0
-15
-30
-45
b
Conv.
Sel.(2a)
Sel.(3a)
FE(2a)
100
80
60
40
20
0
100
80
60
40
20
0
3a
1a
H2
Pd-P
without 1a
with 0.2 mmol 1a
2a
-1.4
-1.2
-1.0
-0.8
-0.7 -0.8 -0.9 -1.0 -1.1 -1.2 -1.3
E / V vs. Hg/HgO
E / V vs. Hg/HgO
c
d
Conv.
Sel.(2a)
Conv.
Sel.(2a)
Sel.(3a)
FE(2a)
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
Pd
1
2
3
4
5
6
-0.8 -0.9 -1.0 -1.1 -1.2 -1.3
Cycles
E / V vs. Hg/HgO
Figure 2. (a) SEM image of Pd-P NNs (inset: TEM image). (b) XRD patterns
of Pd–P NNs and Pd NNs. XPS spectra of (c) Pd 3d and (d) P 2p of Pd–P
NNs.
Figure 3. (a) LSV curves of Pd-P NNs at a scan rate of 10 mV s-1 in 1 M KOH
solution (Diox/H2O, 2:5 v/v) with and without 0.2 mmol of 1a, (b) Potential-
dependent selectivities and FEs of 2a over Pd-P NNs, (c) Cycle-dependent
conversions of 1a and selectivities of 2a, (d) Potential-dependent selectivities
and FEs of 2a over Pd NNs.
The Performance of Electrocatalytic Semihydrogenation of
Alkynes. The electrocatalytic semihydrogenation of alkynes
over Pd-P NNs was performed in a divided three-electrode
system (Figure S2). 4-ethynylaniline (1a) was selected as the
model substrate using 1.0 M KOH as the electrolyte and 1,4-
dioxane (Diox)/H2O as the co-solvents. A much lower onset
potential and a noticeable increase of current density were
observed from the linear sweep voltammetry (LSV) curves after
adding 0.2 mmol of 1a, indicative much easier reduction of 1a
than hydrogen evolution reaction (HER) (Figure 3a). But at
below -1.18 V vs. Hg/HgO, the current density decreased
probably due to the dominant HER, as observed in LSV with a
perceptible HER current in the absence of 1a (Figure 3a). The
potential-depended results (Figure 3b) showed that 92%
conversion yield of 1a, 98% selectivity, and 78% Faradaic
efficiency (FE) of alkene product 2a could be achieved at the
optimal potential (-0.8 V vs. Hg/HgO). Here, 45 C was adopted
for obtaining the biggest FE with low energy input (38.4 C was
Mechanism of Electrocatalytic Transfer Semihydrogenation.
To gain insight into the reaction mechanism, combining control
experiments with theoretical calculations were done. Firstly, no
changes in the LSV curves with and without 1a using pure
dioxane as the solvent, showing no electron transfer between
Pd-P cathode and 1a. The current density did only increase after
the addition of water, indicating this semihydrogenation of
alkynes might be induced by the electrocatalytic water
electrolysis (Figure 4a). Deuterated experiments confirmed that
water was the hydrogen origin for this hydrogenation reaction
(Scheme S1 and Table 2). And the kinetic isotope effect (KIE)
experiment with kH/kD = 2.2 indicated that the O−H cleavage of
water was the rate-determining step (Scheme 1a).[16] Secondly,
the semihydrogenation of alkynes was initiated at a much lower
3
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10.1002/anie.202009757
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0
without 1a
with 1a
excess alkene 2a, alkyne 1a could still be completely
hydrogenated to 2a with only negligible overhydrogenated
product. It further illustrated that the highly selective
semihydrogenation of alkynes was ascribed to the specific
adsorption/activation of alkynes on Pd-P over alkenes rather
than concentration relevance (Scheme 1b). Thus, a possible
reaction mechanism was proposed (1t was selected as the
model substrate.) (Figure 4f). Firstly, the active Pd-H*ads species
I was formed on the Pd-P surface at low potential. Then, it
coordinated with the substrate 1t to give complex II, which
experienced stereoselective Pd–H*ads addition across the C≡C
bond in a syn fashion to afford III (see the supporting information
for DFT details). Subsequent reductive elimination released the
stereo-defined Z-alkene 2t and regenerated the catalyst.
a
b
8
4
Habs
-1
-2
-3
without 1a and H2O
Hads
H2
with 1a
with 1a and H2O
0
-4
-2.7
-2.4 -2.1
E / V vs. Hg/HgO
-1.8
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2
E / V vs. Hg/HgO
Eads on Pd(111)
i
c
d
0.0
-0.2
-0.4
-0.6
ii
H+ + e-
Pd
Pd-P
1/2 H2
-2.53 eV
Eads on Pd-P(111)
iii
-1.41 eV
-1.55 eV
-0.49
-0.57
H*
iv
Reaction coordinate
-2.63 eV
f
e
6
5
4
3
2
1
0
without t-BuOH
with t-BuOH
0
20 40 60 80 100 120 140
Time / min
Figure 4. (a) LSV curves of Pd-P NNs in dioxane, (b) CV curves of Pd-P NNs
in 0.5 M Na2SO4 solution with and without 1a, (c) and (d) DFT calculations of
free energy diagram for hydrogen adsorption and adsorption energy of
acetylene and ethylene on Pd-P and Pd, respectively, (e) Time-dependent
concentration change of 2a with and without t-BuOH, (f) A proposed reaction
mechanism of semihydrogenation of alkynes over Pd-P NNs cathode.
Scheme 1. Kinetic isotope effect (KIE) experiments and demonstration of
alkyne priority on Pd-P alloy anode
The Universality of the Electrocatalytic Transfer
Semihydrogenation (deuteration). The applicability of this
electrochemical semihydrogenation over Pd-P NNs was
examined (Table 1). A series of functionalized terminal aromatic
alkynes, including the heterocyclic substrates and the S and N
moieties on aryl ring, were all efficiently hydrogenated to
produce the corresponding alkenes in good yields (2a-r).
Delightedly, the easily reducible C–Cl, C–Br, and C=O bonds
survived well during the electrocatalytic process (2i-k), providing
significant opportunities for downstream preparation of more
complex molecules. Moreover, this facile method was suitable
for aliphatic alkynes to deliver 2s and 2t in the yield of 92% and
91%, respectively. Impressively, the internal alkynes were
amenable to our strategy, leading to the thermodynamically
unfavorable Z-alkenes with satisfactory yields and excellent
stereoselectivity (up to 99:1 Z:E ratio) (2u-x). Note that, Z isomer
2u could not be formed when [(E)-2-Phenylethenyl] benzene
was subjected to the standard conditions (Scheme S2),
revealing that Z-alkenes was not originated from the
electrochemical conversion of E-alkenes. Thus, this highly
stereoselective semihydrogenation of internal alkyne was a
direct electrocatalytic process.
potential than H2 evolution due to the formation of surface
adsorbed H*ads (Figure 3a). Cyclic voltammograms (CVs) at the
neutral conditions displayed a characteristic H*ads peak at about
-0.2 V vs. Hg/HgO (Figure 4b), which decreased obviously after
the addition of 2a. This demonstrated that the H*ads might be the
key species for this electrochemical hydrogenation of alkynes,[17]
which was consistent with the result in Figure 4e by adding
tertiary butanol (t-BuOH) for the trap of the hydrogen atoms.[18]
Additionally, the decrease of the absorbed atomic hydrogen
(Habs) at the subsurface of Pd-P might be due to its out-migration
to the surface and then form Hads for the hydrogenation of
alkynes.[2b] Furthermore, the incorporation of P might decrease
the number of H atoms at the surface and subsurface regions to
lower undesirable over hydrogenation, thus improving the
selectivity of semihydrogenated products, as observed as B-
doped Pd nanoparticles for thermally catalytic hydrogenation
with 3 bar H2.[19] Thirdly, as discussed in XPS results, the
incorporation of P lowered the 3d electron density of Pd, which
was favorable for the adsorption of alkyne. This was also
supported by larger adsorption energy of acetylene (Acetylene
was usually selected as the model substrate) on Pd-P (Figure
4d). The stronger bonding of H* after incorporating P into Pd
facilitates the generation of active H*ads for promoting
hydrogenation of alkynes and suppresses the by the production
of H2 (Figure 4c). Additionally, the adsorption energy of the Z
product adsorbed on Pd-P was more negative than that of the
corresponding E isomer, providing further justification for the
predominant formation of Z-alkenes in our system (Figure S5).
These results proved the vital role of P in facilitating this
electrochemical transformation. Lastly, in the presence of
4
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Table 1. Substrate scope for electrocatalytic transfer semihydrogenation of
alkynes with H2O over a Pd-P cathode.
and deuterated ratio (Scheme 2a) in an undivided two-electrode
cell. Furthermore, by replacing kinetically sluggish anodic
oxygen evolution reaction[22] with the oxidation of 1,6-
Hexanediamine, adiponitrile (the important feedstock for nylon
production) and alkene 2a were simultaneously obtained with
high yields in a Pd-P||NiSe two-electrode electrolyzer (Scheme
2b). Nearly 297 mV voltage was saved to reach a benchmark
current density of -10 mA cm−2. These promising results
highlighted the potential utility of our method.
Table 2. Substrate scope for the electrocatalytic transfer semideuteration of
alkynes with D2O over a Pd-P cathode.
aReaction conditions: alkynes substrates (0.2 mmol), Pd-P NNs (working area:
1 cm2), 1.0 M KOH (Diox/H2O, 2:5 v/v, 7 mL), room temperature, -0.8 V vs.
Hg/HgO. Conversion yields were reported, and the data in brackets referred to
the ratio of yields of the semi-hydrogenated and full-hydrogenated products.
aReaction conditions: alkynes substrates (0.2 mmol), Pd-P NNs (working area:
1 cm2), 0.5 M K2CO3 (Diox/D2O, 2:5 v/v, 7 mL), room temperature, -0.8 V vs.
Hg/HgO. bPhospahte buffer saline 5 mL (pH ≈ 5.8), Diox 2 mL. Conversion
yields were reported, and the data in brackets referred to the ratio of yields of
semi-deuterated and full-deuterated products.
The easy and efficient access to deuterated alkenes is a
fascinating application of our strategy. Using D2O as the low-
cost and safe deuterium source could bypass the tedious
handling process of expensive and flammable D2 gas and other
toxic
deuterated
reagents.[20]
This
electrochemical
semideuteration of alkynes proceeded efficiently with D2O to
generate the desired deuterated alkenes in good yields with up
to 99% deuterium incorporation and 99% Z selectivity (for
internal alkenes) (Table 2, 3i and 3j). Here, the tri-deuterated (D3)
terminal alkenes, the important building blocks for the
construction of a variety of targeted deuterated compounds,
could be expediently synthesized by the electrocatalytic
semihydrogenation of unlabeled alkynes with D2O (3a-h). Our
semideuteration strategy could avoid the pre-deuteration of C−H
bonds in terminal alkynes, complex processes, over-deuterated
alkane byproducts and/or unsatisfied deuterated ratios that were
the main issues for traditional deuterated methods.[21] No H/D
exchange happened on both -NH2 and -OH groups (3a, 3h, and
3j), showing the good methodology reliability.[21c] Additionally,
mono-deuterated (D1) alkenes with high yields and moderate
deuterated ratios could also be synthesized by the designed
semihydrogenation of the deuterated alkynes with H2O (3k-n).
Impressively, this electrochemical deuteration of terminal alkyne
1b could enable the gram-scale synthesis with unaffected yield
0
-5
-10
297 mV
with 1a
with 1a and 4a
-15
-20
-25
-2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0
E / V
Scheme 2. Electrocatalytic gram-scale deuteration over a Pd-P alloy cathode
and its paired transformations with anodic amine oxidation
5
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10.1002/anie.202009757
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663–673; (d) Y Li, J. Yang, H. Yang, Q. Guan, L. Lu, H. Yu, L. Jiang,
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In conclusion, using H2O (D2O) as the H (D) source, Pd-P NNs
were demonstrated as an efficient cathode for electrocatalytic
semihydrogenation of various functionalized alkynes to
hydrogenated and deuterated alkenes with high yield and
selectivity. DFT calculations revealed that the incorporation of P
led to the enhanced specific adsorption of alkynes, which was
consistent with the results of XPS. In addition, the incorporated
P could also promote the generation of H*ads from water
electrolysis, and decrease the amount of the hydrogen atoms at
the surface and subsurface of Pd for suppressing undesirable
over-hydrogenation. In these regards, Pd-P NNs enabled the
semihydrogenation of alkynes at lower potential with higher
activity and selectivity than pure Pd NNs. Furthermore, the CV
tests and the trapping of H*ads experiments revealed that this
electrochemical semihydrogenation of alkynes occurred via a
H*ads addition to the C≡C bond pathway rather than a proton-
coupled electron transfer process. Interestingly, the mono-, di-,
and tri-deuterated alkenes with specific deuterated sites and >
99% deuterium ratios were readily synthesized. Gram-scale
preparation of tri-deuterated alkenes with high yield and
deuterated ratio in a two-electrode cell highlighted the potential
utility. This work not only offers a new platform by using in-situ
generated hydrogen from electrocatalytic water electrolysis for
selective semi- hydrogenation/deuteration reactions, but also
opens new paradigm for the development of rationally designed
electrocatalysts for efficient organic electrosynthesis.
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Keywords: electrocatalysis • semihydrogenation • deuteration •
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Entry for the Table of Contents
Pd-P nanostructures were efficient electrocatalysts for selective semihydrogenation (deuteration) of terminal and internal alkynes
using H2O (D2O) as the H (D) source at a lower potential via an adsorbed atomic hydrogen (H*ads) addition pathway. The doping of P
caused the enhanced specific adsorption of alkynes and the promoted intrinsic activity for producing H*ads from water electrolysis.
High Z selectivity, good substrate tolerance, gram-scale and paired synthesis highlighted the promising potential.
8
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