Porous Ligand Creates New Reaction Route: Bifunctional Single-Atom Palladium Catalyst for Selective Distannylation of Terminal Alkynes

Article abstract of DOI:10.1016/j.chempr.2020.06.020

We proposed a unique research concept of “mechanism-oriented catalyst design”: The structural elements of single-atom catalyst are designed according to the requirements of organic synthesis mechanism. This concept is totally different from the previous “electrocatalytic” single-atom-site research concept. This work suggests that single-atom-site catalysts not only afford an efficient platform for transforming homogeneous reactions into heterogeneous reactions, but also possess many interesting potentials in developing new synthetic reactions and solving homogeneous reaction problems. Single-atom-site (SAS) catalyst, as a highly reactive heterogeneous catalytic system, is one of the effective tools for complex organic synthesis. However, excessive attention on the catalysis and regulation of metal atoms has led to the neglect of the role of supports and ligands. Here, we employed a P-doped porous organic polymer as a support, as well as a ligand in the developed Pd SAS catalysts. The enrichment of the substrates by the pores, ligand action of the support, and high chemoselectivity and anti-agglomeration of SAS catalysts were the most striking features of this SAS catalyst. A highly selective distannylation of terminal alkynes with a new mechanism was achieved by using the multiple properties of SAS catalyst. This catalytic system offers an effective strategy to utilize the enrichment of the substrate by the pores to regulate the reaction mechanism, which opens a new frontier to use pores, ligands, and SASs for synergistic controlling reaction paths. Single-atom-site catalysts have many applications in organic synthesis. However, most of the research models related to organic synthesis still stay in the mode of “pursuing higher catalytic performance,” mainly based on electrocatalysis. This article provides a reasonable design of the catalyst in accordance with the characteristics and reaction requirements of organic synthesis and finally uses it for the development of a new reaction route. The distannylation of alkynes is very useful in the construction of complex molecules. In the past, only a few terminal alkynes could be achieved by selective distannylation. Based on the mechanism of the reaction, the porous organic polymer catalyst with triarylphosphonate as the backbone was designed and prepared with advantages including pore enrichment, ligand regulation, and single-atom selective regulation. This catalyst solved all the problems in this reaction, such as narrow substrate scope, poor selectivity, and low catalytic efficiency.

Full text of DOI:10.1016/j.chempr.2020.06.020

ll
Article
Porous Ligand Creates New Reaction Route:
Bifunctional Single-Atom Palladium Catalyst
for Selective Distannylation of Terminal
Alkynes
Wen-Yong Huang, Guo-Qing
Wang, Wen-Hao Li, ..., Hai-Tao
Tang, Ying-Ming Pan, Yun-Jie
Ding
httang@gxnu.edu.cn (H.-T.T.)
panym@mailbox.gxnu.edu.cn (Y.-M.P.)
dyj@dicp.ac.cn (Y.-J.D.)
HIGHLIGHTS
Design single-atom-site catalyst
based on organic synthesis
mechanism
Utilizing pores, ligands, and SAS
for synergistic controlling reaction
paths
A highly selective distannylation
of terminal alkynes was achieved
We proposed a unique research concept of ‘‘mechanism-oriented catalyst
design’’: The structural elements of single-atom catalyst are designed according to
the requirements of organic synthesis mechanism. This concept is totally different
from the previous ‘‘electrocatalytic’’ single-atom-site research concept. This work
suggests that single-atom-site catalysts not only afford an efficient platform for
transforming homogeneous reactions into heterogeneous reactions, but also
possess many interesting potentials in developing new synthetic reactions and
solving homogeneous reaction problems.
Huang et al., Chem 6, 1–14
September 10, 2020 ª 2020 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2020.06.020

Please cite this article in press as: Huang et al., Porous Ligand Creates New Reaction Route: Bifunctional Single-Atom Palladium Catalyst for
Selective Distannylation of Terminal Alkynes, Chem (2020), https://doi.org/10.1016/j.chempr.2020.06.020
ll
Article
Porous Ligand Creates New Reaction Route:
Bifunctional Single-Atom Palladium Catalyst
for Selective Distannylation of Terminal Alkynes
Wen-Yong Huang,1,3 Guo-Qing Wang,2,3 Wen-Hao Li,1,3 Ting-Ting Li,1 Guang-Jun Ji,2
1,4,
2,
Shi-Cheng Ren,1 Miao Jiang,2 Li Yan,2 Hai-Tao Tang,1, Ying-Ming Pan,
and Yun-Jie Ding
*
*
*
SUMMARY
The Bigger Picture
Single-atom-site catalysts have
many applications in organic
synthesis. However, most of the
research models related to
organic synthesis still stay in the
mode of ‘‘pursuing higher
catalytic performance,’’ mainly
based on electrocatalysis. This
article provides a reasonable
design of the catalyst in
Single-atom-site (SAS) catalyst, as a highly reactive heterogeneous
catalytic system, is one of the effective tools for complex organic
synthesis. However, excessive attention on the catalysis and regu-
lation of metal atoms has led to the neglect of the role of sup-
ports and ligands. Here, we employed a P-doped porous organic
polymer as a support, as well as a ligand in the developed Pd
SAS catalysts. The enrichment of the substrates by the pores,
ligand action of the support, and high chemoselectivity and
anti-agglomeration of SAS catalysts were the most striking fea-
tures of this SAS catalyst. A highly selective distannylation of ter-
minal alkynes with a new mechanism was achieved by using the
multiple properties of SAS catalyst. This catalytic system offers
an effective strategy to utilize the enrichment of the substrate
by the pores to regulate the reaction mechanism, which opens
a new frontier to use pores, ligands, and SASs for synergistic con-
trolling reaction paths.
accordance with the
characteristics and reaction
requirements of organic synthesis
and finally uses it for the
development of a new reaction
route.
The distannylation of alkynes is
very useful in the construction of
complex molecules. In the past,
only a few terminal alkynes could
be achieved by selective
INTRODUCTION
Controllable synthesis of structurally specific compounds has become one of
the research frontiers in contemporary synthetic chemistry.1 To achieve this goal,
a metal-ligand catalytic system is usually used to control reaction selectivity in ho-
mogeneous catalysis.2 In some competitive reactions, ligand screening is compli-
cated, because it often involves structurally complex and expensive homogeneous
organic ligands that are challenging to separate and recycle.3–6 Thus, converting
homogeneous catalysts for value-added reaction into a heterogeneous counter-
part is an increasingly popular strategy.7 The conventional strategy is to synthesize
novel heterogeneous catalysts via organic or inorganic reactions and to
apply them in homogeneous organic reactions to achieve high catalytic perfor-
mance.8–10 This strategy has almost completely dominated the field of heteroge-
neous catalysis and has been successfully extended to the field of single-atom
catalysis,11–27 which considerably improved the reaction results, such as yield,
catalytic efficiency, chemo-, and regioselectivity. However, to date, single-atom-
sites (SASs) catalysts can only achieve some reactions that have been realized
under homogeneous or heterogeneous catalysis, but their application in the
development of more complex and selective new reactions has not been reported
yet. Therefore, according to the characteristics of SASs, it is an attractive challenge
to design and synthesize catalytic systems with universal prospect applications
in the development of new reaction routes that cannot be accomplished by homo-
geneous catalysis.
distannylation. Based on the
mechanism of the reaction, the
porous organic polymer catalyst
with triarylphosphonate as the
backbone was designed and
prepared with advantages
including pore enrichment, ligand
regulation, and single-atom
selective regulation. This catalyst
solved all the problems in this
reaction, such as narrow substrate
scope, poor selectivity, and low
catalytic efficiency.
Chem 6, 1–14, September 10, 2020 ª 2020 Elsevier Inc.
1

Please cite this article in press as: Huang et al., Porous Ligand Creates New Reaction Route: Bifunctional Single-Atom Palladium Catalyst for
Selective Distannylation of Terminal Alkynes, Chem (2020), https://doi.org/10.1016/j.chempr.2020.06.020
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Article
Previous Work:
Pd/PPh3
Sn
a)
b)
c)
Sn
Sn
H
H
+
+
R
and
R
R
Sn
homogeneous
conditions
Pd0 Aggregation inactivation
[Pd]
+
Sn
Sn
• 9 examples
• TON up to 34
• Active isonitrile
Sn
[Pd]/t-BuNC
R
Sn
Sn
+
Sn
R
Our idea:
Sn
[Pd]
[Pd]
R
+
Sn
H
+
R
Sn
Sn
Sn
R
If a suitable functionalized palladium single-atom catalyst is created,
can it be used to accomplish this special transform in one-step?
Our tactic:
• Ligand-functional polymer support
• Single-atom site catalyst
• Enrichment with porous structure of catalyst
This Work: Single-atom-site catalyst for distannation
Pd1
O
O
P
Sn
Pd1@POL-5
O
+
Sn
H
R
Sn
THF, 24 h, r.t.
R
*
• Recayclable catalyst
• 20 examples, up to 89% yield • High Reactivity: TON up to 770
• Cheaper Sn source • low Pd metal loading
• High Selectivity: Z only, r.r up to 100:1
*
Pd1@POL-Ar3P
Scheme 1. Metal-Catalyzed Selective Distannylation of Terminal Alkynes
1.2-Ditin-substituted olefins can be easily converted into poly-functionally diverse
olefins via the Migita-Kosugi-Stille cross-coupling reaction,28 which are often used
as important synthetic blocks in organic synthesis.29 The distannylation of alkyne is
the only route for this special compound. Most of the existing distannylation of
alkynes uses alkylditin as substrates; these reactions suffer from inevitable short-
coming, such as high metal loading, low yields, poor stereo- and chemoselectivity,
narrow substrate scope, and requirement of stoichiometric amount of base (Scheme
1, path c).30–33 The use of alkyl tin hydrides as a source of tin is ideal for achieving
highly selective distannylation, due to their high reactivity and low cost (at only
one-third price of alkylditin). Kazmaier group reported the only example of W-cata-
lyzed highly regioselective distannylation of terminal alkynes by using alkyl tin hy-
dride as a tin source with a narrow substrate scope (only six species have been suc-
cessfully converted).34 In general, direct synthesis of a 1.2-ditin-substituted olefin
using alkyl tin hydride under palladium (Pd) catalysis is unfavored under homoge-
neous catalysis, because Sn-Pd-H and Sn-Pd-Sn intermediates are readily formed.
Sn-Pd-H is favorable in the homogeneous catalytic system and is readily converted
into a tin hydrogenation product (Scheme 1, path a).35–37 The Sn-Pd-Sn is the ideal
intermediate for 1.2-ditin-substituted olefins, but it rapidly agglomerates into large
Pd nanoparticles after a catalytic cycle, which in turn leads to loss of catalytic activity
(Scheme 1, path b).
1State Key Laboratory for Chemistry and
Molecular Engineering of Medicinal Resources,
School of Chemistry and Pharmaceutical Sciences
of Guangxi Normal University, Guilin 541004,
China
2Dalian National Laboratory for Clean Energy,
and State Key Laboratory of Catalysis, Dalian
Institute of Chemical Physics, Chinese Academy
of Sciences, Dalian 116023, China
3These authors contributed equally
4Lead Contact
*Correspondence: httang@gxnu.edu.cn (H.-T.T.),
panym@mailbox.gxnu.edu.cn (Y.-M.P.),
dyj@dicp.ac.cn (Y.-J.D.)
On the other hand, SAS catalysts with silicon- or carbon-based materials as supports
are also difficult to achieve this reaction because these materials do not play a role of
ligands. Our group has been working on the development of atomically dispersed
https://doi.org/10.1016/j.chempr.2020.06.020
2
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Please cite this article in press as: Huang et al., Porous Ligand Creates New Reaction Route: Bifunctional Single-Atom Palladium Catalyst for
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Article
R
R3Sn Pd2+ SnR3
PPh3
POL
POL
Pd2+ SnR3
R
POL PPh3
Pd2+
R3Sn
SnR3
SnR3
SnR3
T4
SnR3
R
T3
PPh3
H2
Detected by GC
PPh3
POL Pd0 PPh3
PPh3
Pd4+
POL
T2
H
R3Sn
R3Sn
H
PPh3
POL Pd0 PPh3
PPh3
POL
R3Sn
H
Pd2+
SnR3
R
R3Sn
H
PPh3
POL Pd2+
R3Sn
T1
PPh3
R3Sn
H
H
R
R3Sn Pd2+
POL
H
Scheme 2. The Mechanisms of Distannylation
metal catalysts that are supported on a porous organic ligand polymer (POLs).38–40
These POL-supported atomically dispersed catalysts integrate the advantages of
SAS (mononuclear) catalysts and homogeneous metal-ligand catalysts for known re-
actions. The high stability and dispersion of the single-atom metal active center
ensure its catalytic activity and chemoselectivity. The special steric hindrance and
electronic effects of the ligands, as well as the overall synergistic catalytic center,
improve the chemo- and stereoselectivity.41–49 However, different from the previous
metal-POL-catalyzed known reactions, the development of a new reaction mecha-
nism, which is controlled by the pore enrichment of the support and high dispersion
and anti-agglomeration nature of the SAS has not been explored to date. It has been
difficult to achieve so far, because it requires a complete and rational adjustment of
various factors, such as finding suitable reaction templates, designing reaction
mechanisms, controlling metal particle size, and selecting potential carriers.
RESULTS
Key Challenges and Their Origins
Challenge 1, for Ligand Support
As mentioned earlier, when isonitrile is used as a ligand, its instability leads to a
lower yield and narrow substrate scope.33 Triphenylphosphine (PPh3) can improve
the yield with a poor selectivity. On the other hand, as shown in Scheme 2, the
key point to realize distannylation is to control the selective formation of Pd-Sn
and Sn-Pd-Sn intermediates. For alkyl terminal alkynes with weak chelation and
inductive effects, it is hard to achieve the high selectivity with a simple homogeneous
ligand. Thus, a suitable heterogeneous ligand polymer is needed.
Challenge 2, for Metal
The heterogeneous catalyst needs to maintain dispersibility to restrain the compe-
tition of hydrostannylation. The SAS catalyst with single catalytic particle size and
atomic orbital can effectively regulate the chemoselectivity of the reaction when
the catalyst faces competitive reactions with different mechanisms.
Challenge 3, for the Catalytic System
The stability of Pd SASs is another important issue. Since the metal and the ligand are
free under homogeneous conditions, the catalytically active Pd can readily form a
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Article
stable structure of tetracoordinate with 18 electrons. During the reaction process, Pd
dissociates from the ligand and forms a transient intermediate state. In this process,
the homogeneous catalyst begins to slowly agglomerate. Therefore, if the support
introduced into the heterogeneous catalyst is not properly coordinated to the metal,
it is difficult to obtain atomically dispersed palladium, and the catalyst is usually
agglomerated into nanoparticles during the loading preparation stage. In particular,
when a catalytic cycle has been completed, the support should provide a corre-
sponding force to prevent Pd(0) aggregation and inactivation.
Solutions
(1) Using a POL with a triarylphosphine-skeleton as support for the heteroge-
neous catalyst. The enrichment of the pores of the polymer can effectively
enrich the Sn–H compounds, make the Sn–H concentration around the
metal active center higher, and promote the formation of Sn-Pd-Sn interme-
diates. Compared with homogeneous isonitrile and PPh3 ligands, the ligand
polymer not only improves the yield and substrate scope, but it also im-
proves the selectivity. This kind of vinyl polymer material has excellent
swelling properties. This special property can make it quickly absorb a large
amount of organic mixture during the reaction process, and then simulate
the reaction conditions closer to homogeneous catalysis. A special ‘‘semi-
homogeneous’’ catalytic environment can effectively enhance catalytic
activity.
(2) Using air-stable tetra-coordinated bistriphenylphosphine palladium dichlor-
ide as the metal precursor, through the exchange of the ligand, the atomic
level dispersion of the metal active sites in the polymer can be effectively
realized. Please note that for metal active centers, a stable coordination
environment of 18 electrons is the key to the formation of monoatomic
discrete catalysts. The nature of the Pd metal-catalyzed reaction is the disso-
ciation and re-coordination of the ligand. If Pd(OAc)2 is used, after polymer-
ization, the ligand polymer will often lead to the formation of a rigid struc-
ture with large steric hindrance. The distance between the coordination
points becomes longer, so Pd(PPh3)2(OAc)2, which can only form a lively
16-electron structure, often causes the catalyst to agglomerate during the
preparation process.
(3) The preparation of the catalyst center as a discrete SAS catalyst cannot only
effectively prevent side reactions caused by the difference in catalyst size,
but also improve the utilization rate of the atoms in the catalyst, further pro-
moting the chemoselectivity of the reaction.
With these basic ideas, we have achieved a Pd-catalyzed distannylation of terminal
alkynes with high selectivity and broad substrate scope, through rational design and
synergistic control of ligand polymer, porous, and SAS catalyst. It is noteworthy that
the distannylation reaction we developed under SAS catalysis affords a completely
different reaction result than that under the corresponding homogeneous condi-
tions (see Table S1).
Synthesis of Pd1@POL-PAr3 Samples
Vinyl functionalized PPh3 derivative was prepared by employing the Witing reaction
and substitution reaction. The Pd1@POL-PAr3 was prepared following the literature
procedure via a simple polymerization-coordination method (Scheme 5). A series of
PPh3-derived ligand polymers containing different substituent groups were success-
fully synthesized (see Supplemental Information for experimental details).
4
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Article
Pd1@POL-Ar3
P
+
Sn(n-Bu)3
Sn(n-Bu)3
Sn(n-Bu)3
(n-Bu)3SnH
H2
N
N
N
P
P
P
P
Me
Me
OMe
POL-1
47% yield, r.r=30:1
POL-2
60% yield, r.r=3:1
POL-3
41% yield, r.r=20:1
POL-4
26% yield, r.r=13:1
Pd
R
P
P
P
P
P
OMe
MeO
F
Me
R
R
POL-5
POL-6
POL-7
POL-8
89% yield, r.r>100:1
87% yield, r.r>100:1d
47% yield, r.r=20:1c
83% yield, r.r=50:1e
Pd1@POL-Ar3P
17% yield, r.r=7:1
17% yield, r.r=3:1
62% yield, r.r=18:1
P
P
P
P
Me
F
Me
F
POL-9
POL-10
POL-12
POL-11
55% yield, r.r=2:1
42% yield, r.r=7:1
60% yield, r.r=8:1
39% yield, r.r=1:4
Scheme 3. Reaction Optimization
aStandard conditions: alkyne (0.5 mmol), (n-Bu)3SnH (0.5 mmol), Pd1@POL-Ar3P (0.5 mol %), and
THF (2 mL), 25 ꢀC, 36 h. Isolated yield, the yields are based on a half amount of HSnBu3.
bRatio of distannylation product:other isomers as determined by 1H NMR. c0.25 mmol of (n-Bu)3SnH
was used. d0.75 mmol of (n-Bu)3SnH was used. e1.0 mmol of (n-Bu)3SnH was used.
Distannylation of Weak Electronically Terminal Alkynes
Weak electronically terminal alkynes are generally the most quintessential and chal-
lenging substrates for regio-, chemo-, and stereoselective transformations, due to
the lack of inductive and chelation effects. Thus, 1-(prop-2-yn-1-yl)-1H-indole and
tributyltin hydride were selected as substrates. Phosphine ligands can promote
the dehydrogenation coupling of Pd-catalyzed tributyltin hydride under homoge-
neous conditions with poor selectivity (see Table S1). After trying a series of
mono- or bi-dentate phosphine ligands, it was found that the typical mono- or bi-
dentate phosphine ligands exhibited low reactivity or selectivity under the homoge-
neous reaction conditions. Based on the previous results, we synthesized a simple
vinyl functionalized PPh3 polymer POL-1 and investigated its catalytic performance.
The results showed that the PPh3 polymer exhibited better reactivity than the homo-
geneous ligand, but the chemoselectivity of the reaction was not satisfactory. There-
fore, we synthesized a series of PPh3 polymers with different substituents and substi-
tution positions (Scheme 3, POL 2–12) and investigated their reactivity and
selectivity in the reaction. It turned out that the ligand polymer POL-5 was the
best ligand for the catalytic system, by which the Z-bis-tin-substituted olefin product
could be efficiently obtained in a short time, with a yield of 89% and a total selectivity
of 100:1. We think that this reaction has special requirements for the electronic prop-
erties of metal catalytic center. We propose that POL-5 has the most suitable elec-
tronic effect for a metal center. Surprisingly, under the catalysis of Pd1@POL-5,
the amount of (n-Bu)3SnH did not affect the selectivity of the reaction, which is in
stark contrast to the result of homogeneous Pd catalysis reported by the Lautens
group. In order to understand this reaction more deeply, we also analyzed hydrogen
and by-products by GC and LC (See Supplemental Information).
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Figure 1. Characterization of the Pd1@POL-5 Sample
(A) SEM image of Pd1@POL-5. Scale bar, 100 nm.
(B) HR-TEM image of a Pd1@POL-5. Scale bar, 10 nm.
(C) Representative AC HAADF-STEM image of the Pd1@POL-5. The green circles were drawn
around SAS Pd. Scale bar, 2 nm.
(D) EDX elemental mapping analysis of the Pd1@POL-5. Scale bar, 6 mm.
DISCUSSION
Characterization of Pd1@POL-5 Samples
To elucidate the structure-reactivity relationship and the synergistic effect of ligand
and porous structure, the prepared POL-5 and Pd1@POL-5 were characterized in
detail. Scanning electron microscopy (SEM, Figure 1A), transmission electron micro-
scopy (TEM, Figure 1B), energy-dispersive X-ray spectroscopy (EDS, Figure 1D),
elemental mapping images, and line scan analyses revealed that O and P were
distributed over the polymer sample, and Pd element was evenly distributed.
High-angle annular dark-field scanning TEM (HAADF-STEM, Figure 1C) image
further demonstrated that all of the Pd species in the polymer was uniformly
dispersed in the sample as a single atom. The HAADF-STEM image of the used
Pd1@POL-5 (see Figure S9) showed that the catalyst retained a highly dispersed
metal active site after use. This result confirmed that the highly stable and dispersed
metal catalyst center in the polymer could effectively ensure the reactivity of catalyst
after the completion of the Sn-H dehydrogenation coupling, which is different than
the metal agglomeration after the reaction under homogeneous conditions. The
support (POL-5) could effectively suppress the agglomeration tendency of metal
sites in the polymer, which facilitates the subsequent distannylation of terminal al-
kynes. The X-ray diffraction (XRD) pattern of Pd1@POL-5 (see Figure S16) showed
that Pd1@POL-5 was a highly polymerized and amorphous material. The Pd loadings
of Pd1@POL-5 was determined to be 1.7 wt % by inductively coupled plasma atomic
emission Spectrometry (ICP-AES).
The thermogravimetric (TG) curves of POL-5 and Pd1@POL-5 show that the
catalyst remains intact at temperatures up to 430ꢀC (see Figure S1). Nitrogen
6
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Figure 2. 1H NMR Spectroscopy Experiment for Chemical Adsorption of HSn(n-Bu)3
adsorption-desorption analysis (see Figure S2) of Pd1@POL-5 demonstrates that the
catalyst has large pore volume, high surface areas, and hierarchical porosity (BET
surface area and pore volume of Pd1@POL-5 were 810 m2/g and 0.759 cm3/g,
respectively). Based on non-local density functional theory (NLDFT), the pore sizes
of Pd1@POL-5 were primarily distributed between 0.6 and 4 nm (see Figure S3).
The NMR spectrum of the tributyltin hydride solution mixed with the catalyst
confirmed that the catalyst exhibited a strong adsorption effect on the compound
(Figures 2 and S24). Combined with the physical adsorption experiment on
Pd1@POL-5, it can be concluded that the porous structure of the catalyst could effec-
tively adsorb and concentrate tri-n-butyltin hydride to boost the concentration of the
reactants around the catalyst, thereby effectively promoting the formation of the Sn-
Pd-Sn intermediate. On the other hand, in situ FT-IR spectra in an acetylene atmo-
sphere were employed to investigate the formation and evolution of the active spe-
cies on the surface of Pd1@POL-5, which were recorded and are shown in Figure 3.
After acetylene was introduced into the system at 0 min, the FT-IR spectrum of acet-
ylene was scanned at about 3–5 min. The peak intensity at 2,070 cmÀ1 gradually
increased from almost zero, which hardly decreased after purging with argon for
at least half an hour. This result indicated that there was coordination between the
alkyne and the active Pd metal center during the reaction, which further promoted
the conversion of the reaction.
The X-ray photoelectron spectroscopy (XPS) spectrum of Pd1@POL-5 (see Fig-
ure S20) demonstrated the binding energies of P2p at 132.1 eV (PAr3 units). As
shown in the P spectrum fitting results of XPS, a small peak area at 132.1 eV was
1/14.4 (6.9%) of the main peak area (131 eV). This result indicates that only 6.9%
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Figure 3. The In Situ FT-IR Spectrum of Pd1@POL-5
(A) Under acetylene atmosphere
(B) Under an argon atmosphere.
of P atoms (0.161 mmol/g) coordinated with Pd metal, which corresponds to the
calculated value (0.160 mmol/g) measured by the ICP-AES. Thus, from these re-
sults, we can conclude that the P atoms in the polymer do coordinate with the
Pd metals. Solid-state 31P NMR analysis also corroborated the formation of the co-
ordination bonds (see Figure S13). Extended X-ray absorption fine structure (EX-
AFS) spectra were carried out to provide further insights into the coordination envi-
ronment of the Pd atoms in the Pd1@POL-5 catalyst. It can be seen that the Pd K-
edge XANES of Pd1@POL-5 was very different compared with the Pd foil (Figure 4),
suggesting that Pd was completely separated into isolated single Pd sites. The
quantitative coordination configuration of the Pd atom in the Pd1@POL-5 catalyst
can be obtained by EXAFS fitting (see Table S2). These characterizations (XPS, solid
state nuclear magnetic resonance, and EXAFS) effectively demonstrated that the
active ligand skeleton in the catalyst did not collapse and deteriorate during the
polymerization.
Substrate Scope and Applications
We further investigated the substrate scope of the distannylation to study the appli-
cability of the substrate and the influence of different substrates under the catalysis
of Pd1@POL-5. The Pd1@POL-5 catalytic system facilitated the generation of the
Z-dis-tin product and demonstrated to have broad application in synthesizing
Z-dis-tin products with various functional groups. For example, alkynes with oxy-
gen-containing functional groups (3a–3i), such as alcohol (3a and 3h), ether (3b,
3d–3f, and 3i), ester (3c), even sensitive epoxyethane (3g), were all readily converted
to the expected products in good to excellent yields. As shown in Scheme 4, the cat-
enarain alkynes bearing nitrogen-containing groups (3j–3o) also form the Z-dis-tin
product under the same conversion conditions in good yields. The aryl alkyne with
the electron-withdrawing group could also produce the target compounds success-
fully. Interestingly, para-nitro (3p), meta-nitro (3q), or ortho-nitro (3r) substituted
phenylacetylene completely converted into the target products without any reduc-
tion in the reaction rate, which indicated that substituent positions do not affect the
catalytic performance of Pd1@POL-5. When the substituents were aryl alkyne with
electron-donating group, however, an opposite reaction selectivity occurred to
afford completely different products. Under standard conditions, electron-deficient
ethynylpyridines were also converted into the corresponding products (3s).
8
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Article
Figure 4. Pd K-Edge Fourier Transform EXAFS Spectra and Corresponding Fitting Analysis for
Pd Foil
Encouragingly, the commercial herbicide, clodinafop-propargyl, afforded the corre-
sponding product (3t) in a yield of up to 83% under the standard reaction conditions,
which further demonstrated the application potential of this reaction in structural
modification of medicinal molecular. Moreover, the catalytic performance of
Pd1@POL-5 catalyst was not compromised in the scale-up reactions.
To date, the distannylation reaction of the alkyne is an optimal solution to synthesize nat-
ural products containing a (Z,Z)-configured chloro-1,3-diene (Scheme 6, 3hb) motif,
which can tolerate a variety of sensitive functional groups. In this regard, the distannyla-
tion product is superior to the similar 1,2-disubstituted boronization olefins. Forstner
group completed the first total synthesis of the natural product, putative chagosensine,
with a (Z,Z)-configured chloro-1,3-diene motif, by employing this distannylation strategy.
This work is an excellent example for demonstrating that the double tin reaction can
tolerate sensitive enol structures. The demonstrated substrate scope in our work
confirmed the broad applicability of this distannylation reaction. Unlike the boronation
reaction, the developed distannylation reaction can generally tolerate sensitive func-
tional groups, such as nitro, chlorine, cyano, and ethylene oxide under classical coupling
conditions. Interestingly, 3,3-diethoxyprop-1-yne was used as the starting material for
the first time to accomplish a one-step efficient and high chemo- and regioselective syn-
thesis of dis-tin-substituted propynal (Scheme 6, 4b) via a distannylation reaction. In
addition, to explore the practicality of the distannylation process, the distannylation re-
action of commercial herbicide, clodinafop-propargyl, was conducted with 0.2 mmol%
Pd1@POL-5 at 20 mmol scale, which readily afforded a good yield of 3t (Scheme 6, 71%).
Conclusions
In summary, we report
a series of POL-supported SAS Pd catalysts with
remarkable catalytic performance in distannylation reactions. The POL-n with
different electrical PPh3 structure were synthesized for the first time and success-
fully loaded with SAS Pd via a simple solvothermal polymerization-coordination
method. Different from common carbon-based material, the developed POL-n
support is abundant in phosphate coordination sites with special electrical prop-
erties that play a key role in stabilizing the Pd atoms at single atomic sites. Using
this catalyst, we have, for the first time, achieved
a highly chemo- and
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Article
SnBu3
SnBu3
SnBu3
Pd1@POL-PPh3
THF, r.t., 16 h
R
+
H
SnBu3
R
+
other isomers
R
+
4
1
2
3
O-containing terminal alkynes
O
SnBu3
SnBu3
SnBu3
EtOOC
EtOOC
SnBu3
SnBu3
SnBu3
SnBu3
SnBu3
O
O
S
n
HO
SnBu3
TBSO
B
u3
3a 72% (50:1)
3b 66% (100:1)
3c 65% (100:1)
OH
3d 84% (50:1)
3e 68% (7:1)b
O
SnBu3
SnBu3
SnBu3
SnBu3
O
O
SnBu3
SnBu3
O
O
SnBu3
SnBu3
3f 63% (5:1)b
3g 71% (7:1)
3i 60% (10:1)
3h 72% (10:1)
N-containing terminal alkynes
O
SnBu3
O
SnBu3
SnBu3
O
SnBu3
SnBu3
N
SnBu3
S
SnBu3
SnBu3
N
O
N
N
3
O
3j 89% (100:1)
3k 58% (25:1)
3l 61% (50:1)
3m 81% (50:1)
H
N
SnBu3
SnBu3
SnBu3
SnBu3
Boc
NC
3o 41% (14:1)b
3n 81% (100:1)
Heterocyclic and aromatic terminal alkynes
SnBu3
SnBu3
SnBu3
SnBu3
SnBu3
SnBu3
SnBu3
3r 67% (100:1)
O2N
N
SnBu3
NO2
O2N
3q 50% (20:1)b
3s 70% (20:1)
3p 82% (20:1)
Drug or useful structure post-modifications
O
Cl
F
O
O
SnBu3
SnBu3
N
O
3t 83% (50:1)
Scheme 4. Evaluation of Terminal Alkyne Scope
aStandard conditions: alkyne (0.5 mmol), (n-Bu)3SnH (0.6 mmol), Pd1@POL-Ar3P (0.5 mol %), and THF (2 mL), 25ꢀC, 36 h. Isolated yield, the yields are
based on a half amount of HSnBu3. The selectivity was determined by 1H NMR. We also isolated the excess alkynes, see Supplemental Information for
b
details. Pd-catalyst (1 mol %), 45ꢀC, 24 h.
10
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Article
Br
Br
Mg, I2
PCl3
1) polymerization
2) Pd(PPh3)2Cl2
Ph3PMeBr
P
Pd1@POL-Ar3P
CHO
n-BuLi
3
X
X
X
X = F, OMe, Me
Scheme 5. Synthesis of Pd1@POL-PAr3
stereoselective distannylation of terminal alkyne with tri-n-butyltin hydride as a
raw material. The as-fabricated Pd1@POL-5 catalyst displayed a superior catalytic
activity and selectivity in distannylation of various weak electronically terminal al-
kynes. The selectivity (Z only, r.r. up to 100:1) and catalytic performance were
much higher than those of any other heterogeneous or homogenous protocols
reported in the literature. More importantly, the catalytic results of this SAS Pd
catalyst were opposite to those hydrostannylation products obtained by conven-
tional homogeneous Pd catalysts for the same substrates. Finally, our work sug-
gests that SAS catalysts not only afford an efficient platform for transforming ho-
mogeneous reactions into heterogeneous reactions but also possess many
interesting potentials in developing new synthetic reactions and solving homoge-
neous reaction problems.
EXPERIMENTAL PROCEDURES
Full experimental procedures are provided in Supplemental Information.
Resource Availability
Lead Contact
Further information and requests for resources and reagents should be directed to and
will be fulfilled by the Lead Contact, Ying-Ming Pan (panym@mailbox.gxnu.edu.cn).
Materials Availability
Unique and stable reagents generated in this study will be made available on
request, but we may require a payment and/or a completed materials transfer agree-
ment if there is potential for commercial application.
The applications of distannylation in organic synthesis
I
OH
C5H11
(n-Bu)3SnH (1 equiv)
Pd1@POL-5 (0.5 mmol%)
COOEt
OH
C5H11
COOEt
OH
OH
C5H11
COOEt
a
b
(n-Bu)3Sn
C5
H11
THF, r.t., 8 h
Sn(n-Bu)3
Sn(n-Bu)3
Cl
3h
3ha
3hb
(n-Bu)3SnH (1 equiv)
Pd1@POL-5 (0.2 mmol%)
O
Sn(n-Bu)3
Sn(n-Bu)3
THF, r.t., 8 h
OHC
O
4a
4b, 65%, r.r.=20:1
Gram scale study: 20 mmol
O
O
(n-Bu)3SnH (20 mmol)
Pd1@POL-5 (0.1 mol%)
THF, 40 oC, 84 h
Cl
F
O
Cl
F
O
O
O
SnBu3
SnBu3
3t, 77%, 7.158g, TON=770
N
O
N
O
20 mmol
Scheme 6. Gram Scale and the Applications of Pd1@POL-5 in Organic Synthesis
aPd(PPh3)4 (20 mol%), CuTC, [Ph2PO2][NBu4], DMF, Ar, 52% yield.
bCuCl2, 2,6-lutidine, THF, 71% yield.
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Article
Data and Code Availability
This study did not generate or analyze datasets or code.
General Procedure for Distannylation of Terminal Alkynes
A flame-dried Schlenk tube was flushed under an atmosphere of nitrogen and
charged with 15.5 mg of Pd1@POL-5 (0.0025 mmol, 0.005 equiv-). The vial was
sealed with a rubber septum and 2 mL of distilled THF was added followed by addi-
tion of the alkyne (0.5 mmol). Tributyltin hydride (0.5 mmol, 1 equiv) was added
dropwise to the reaction vial. The reaction mixture was left to stir for 24–36 h at
room temperature. When the reaction was completed (monitored by TLC), the solu-
tion was filtered and washed with EtOAc. The filtered solution was evaporated under
vacuum. The crude product was purified directly by silica gel column chromatog-
raphy eluting with petroleum ether, ethyl acetate, and 1% triethylamine to afford
the corresponding product.
SUPPLEMENTAL INFORMATION
Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.
2020.06.020.
ACKNOWLEDGMENTS
We are grateful for the financial support from the National Natural Science Founda-
tion of China (nos. 21861006 and 91845101), Guangxi Natural Science Foundation
of China (nos. 2016GXNSFEA380001 and 2018GXNSFBA281151), State Key Labo-
ratory for Chemistry and Molecular Engineering of Medicinal Resources (no.
CMEMR2019-A03), and Innovation Project of Guangxi Graduate Education (no.
YCSW2019060) for financial support.
AUTHOR CONTRIBUTIONS
W.-H.L. and W.-Y.H. conceived the idea. W.-H. L., G.-Q.W., and H.-T.T. analyzed the
data and wrote the manuscript. W.-H.L., T.T.L., and S.-C.R. synthesized the catalysts.
W.-Y.H. and W.-H.L. carried out the catalysis experiments. G.-Q.W., M.J., and
G.-J.J. characterized the catalysts. L.Y. and Y.-J.D. designed the characterization
schemes. Y.-M.P. directed the project. All the authors have read the manuscript
and agree with its content.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: February 6, 2020
Revised: March 6, 2020
Accepted: June 15, 2020
Published: July 29, 2020
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