Shuttling Catalyst: Facilitating C?C Bond Formation via Cross-Couplings with a Thermoresponsive Polymeric Ligand

Article abstract of DOI:10.1002/ijch.201900143

A poly(ethylene glycol) (PEG) linked ortho-MeO-phenyldicyclohexylphosphine (MeO-WePhos) ligand has been synthesized to promote Pd-catalyzed carbon-carbon bond formation by cross-couplings including Sonogashira, Heck, Hiyama and Stille reactions, providing corresponding (hetero)aryl substituted alkynes, alkenes and bi(hetero)aryls in good to excellent isolated yields with low Pd loadings. Facilitated by the lower critical solution temperature behaviour of the polymeric monophosphine ligand, the metal-complex could rapidly shuttle between organic and water phases as regulated by temperature, enabling highly efficient catalyst recycling via a simple phase separation. The chemical structure of ligand was determined by matrix-assisted laser desorption/ionization-time of flight mass spectrometry, nuclear magnetic resonance spectrometry and size-exclusion chromatography measurements. Notably, as demonstrated by the inductively coupled plasma-atomic emission spectrometry measurement, 98% Pd was kept in the water phase after 6 cycles of catalyst recycling experiments. Given the profound impact of transition-metal-catalyzed covalent bond formation and the increasing demand of sustainable chemistry, this work provides an alternative method to conduct cross-couplings with a polymeric shuttling catalyst.

Full text of DOI:10.1002/ijch.201900143

Communication
DOI: 10.1002/ijch.201900143
Shuttling Catalyst: Facilitating CÀ C Bond Formation via
Cross-couplings with a Thermoresponsive Polymeric Ligand
Erfei Wang,[a] Jiawei Zhang,[b] Zhuoran Zhong,[a] Kaixuan Chen,[a] and Mao Chen*[a]
Dedicated to Professors Stephen L. Buchwald and John F. Hartwig for celebration of the receipt of the 2019 Wolf Prize in
Chemistry.
Abstract: A poly(ethylene glycol) (PEG) linked ortho-MeOÀ good to excellent isolated yields with low Pd loadings. the
The chemical structure of ligand was determined by matrix- water phase after 6 cycles of catalyst recycling Facilitated by
phenyldicyclohexylphosphine (MeOÀ WePhos) ligand has as- the lower critical solution temperature experiments. Given
sisted laser desorption/ionization-time of flight mass been the profound impact of transition-metal- behaviour of the
synthesized to promote Pd-catalyzed carbon-carbon spec- polymeric monophosphine ligand, the metal- catalyzed
trometry, nuclear magnetic resonance spectrometry and covalent bond formation and the increasing complex could
bond formation by cross-couplings including Sonogashira, rapidly shuttle between organic and water demand of
size-exclusion chromatography measurements. Notably, as sustainable chemistry, this work provides an phases as
Heck, Hiyama and Stille reactions, providing corresponding regulated by temperature, enabling highly efficient alternative
demonstrated by the inductively coupled plasma-atomic method to conduct cross-couplings with a catalyst recycling
(hetero)aryl substituted alkynes, alkenes and bi(hetero)aryls via a simple phase separation. polymeric shuttling catalyst.
emission spectrometry measurement, 98% Pd was kept in in
Keywords: polymeric ligand · CÀ C bond formation · sustainable chemistry · biphasic catalysis · synthetic methods
1. Introduction
and/or reduce the amount of transition-metal catalysts.[12]
Consequently, the development of transition- metal catalysis
has been accompanied with the evolution and upgrading of
ligands,[13] which has enabled homogeneous catalysis with
decreased catalyst usages, improved activity and selectivity, as
well as expanded substrate scopes.[14] However, the application
of homogeneous catalytic processes could be influenced by
concerns of transition-metal contaminations. In this regard,
heterogeneous transition- metal-catalyzed cross-couplings
based on polymeric, inorganic and other carrying materials
have been developed to simplify the purification process and
reduce the catalyst cost.[15] A combination of the high activity
Transition-metal-catalyzed cross-coupling reaction has become
one of the most powerful transformations in organic synthesis
for the carbon-carbon bond formation. For example, named
reactions including Suzuki-Miyaura,[1] Sonogashira,[2] Heck,[3]
Hiyama[4] and Stille[5] reactions have revolutionized avenues to
access organic molecules. These homogeneous catalytic
processes have been extensively used in both academic and
industrial fields,[6] providing important compounds including
pharmaceuticals,[7] agrochemicals,[8] natural products,[9] poly-
mer materials[10] and so on.[11]
Given the increasing concerns of sustainable catalysis and
the ease in post-process of heterogenous catalysis would be
desirable for the cross-coupling reactions. Recently, we
demostrated a highly efficient and low- catalyst-loading Pd-
catalyzed SuzukiÀ Miyaura cross-coupling method with a
polymeric monophosphine ligand, “WePhos”, enabling rapid
catalsyt shuttling, simple catalyst seperation and continuous
catalyst-recycling synthesis under biphasic conditions
(Scheme 1).[16] However, when further efforts were taken to
extend the application scope of WePhos in other CÀ C bond
forming cross-couplings, lower reactivities were observed,
even at higher catalyst loadings. In this work, inspired by the
elegent design of biaryldialkyl phosphine ligands,[17] we
developed a poly(ethylene glycol) (PEG) development and
production costs (i.e., catalyst, purification), many efforts
have been devoted to improve the recyclability of catalysts
of
homogeneous
linked
ortho-MeOÀ phenyldicyclo-
hexylphosphine ligand, “MeOÀ WePhos”, which allows ex-
tending the efficient shuttling catalysis to Pd-catalyzed cross-
[a] E. Wang, Z. Zhong, K. Chen, Prof. Dr. M. Chen
State Key Laboratory of Molecular Engineering of Polymers,
Department of Macromolecular Science, Fudan University, Shang-
hai 200433 (China)
E-mail: chenmao@fudan.edu.cn
Homepage: http://polymaolab.com
[b] J. Zhang
Key Laboratory of Medicinal Chemistry for Natural Resource,
Ministry Education, School of Chemical Science and Technology,
Yunnan University, Kunming 650091 (China)
Supporting information for this article is available on the WWW
under https://doi.org/10.1002/ijch.201900143
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Scheme 2. Synthetic pathway of PEGylated phosphine ligands, Cy=
cyclohexyl group, 4a MeOÀ WePhos5000 4b WePhos5000 4c
PPh3À PEG5000
,
,
.
Scheme 1. Transition-metal-catalyzed cross-coupling reactions with a
thermoresponsive polymeric ligand.
couplings inlcuding Sonogashira, Heck, Hiyama and Stille
reactions as regulated by temperature.
2. Results and Discussion
The thermoresponsive polymeric ligand 4a MeOÀ WePhos5000
(number average molecular weight (Mn) of PEG is 5000 g/
mol) was synthesized at 79% overall yield following the
synthetic route summarized in Scheme 2. The structure of 4a
was analysed with matrix-assisted laser desorption/ionization-
time of flight (MALDI-TOF) mass spectrometry. As shown in
Figure 1a, a single set of peaks for phosphine ligand 4a is
observed in the MALDI-TOF mass spectrum, and the differ-
ence value of two neighbouring peaks equals the molar mass
of a single repeating unit (m/z=44.05, Figure 1b) in PEG. The
absolute value of m/z is in accord with the molecular weight
calculated by the MALDI-TOF result of 4a. In Figure 1c, the
y-intercept of m/z versus the corresponding number of
repeating units demonstrates that the molecular weight of the
chain-end group in 4a was consistent with the expected value.
Measurements with proton nuclear magnetic resonance (1H
NMR), 31P NMR (À 9.41 ppm) and size-exclusion chromatog-
raphy (SEC) further confirm the precise chemical structure of
the phosphine ligand MeOÀ WePhos5000 (Figures S1).
Figure 1. (a) MALDI-TOF mass spectrum of MeOÀ WePhos5000 4a.
(b) Magnified MALDI-TOF mass spectrum of the blue box in (a). (c)
m/z versus number of repeating units for MALDI-TOF mass
spectrum of 4a.
With the same synthetic route, the WePhos5000 4b[16] and
PPh3À PEG5000 4c[18] were prepared at 83% and 87% overall
yields, respectively. The structures of 4b and 4c were also
confirmed by MALDI-TOF mass spectra (Figures S2c–S2d),
NMR spectra and other measurements.
Sonogashira reaction is among the most useful methods to
synthesize conjugated (hetero)arynes via the cross-coupling of
terminal alkynes and aryl halides.[19] The polymeric ligands
from 4a to 4c were employed in the Pd-catalyzed Sonogashira
cross-coupling reaction of bromobenzene and phenylacetylene
°
using potassium carbonate (K2CO3) as the base at 90 C in
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water/toluene (v/v=4/1). As shown in Figure 2, the reaction
with 4a provided the highest yield (99%) in 6 h reaction time
as determined by gas chromatography (GC) (90% and 78%
for 4b and 4c, respectively). When the reaction was complete,
the reaction mixture was cooled to room temperature. The Pd
complex coordinated with MeOÀ WePhos5000 rapidly trans-
ferred into the aqueous layer, and the generated diphenylacety-
lene product was kept in the toluene layer. After a simple
phase separation, the aqueous layer was collected and reused
in the next reaction. The catalyst recycling process was
repeated for 6 times in a same way without clear decrease in
GC yields. The final aqueous layer was characterized with
inductively coupled plasma-atomic emission spectrometry
(ICP-AES) measurement, which confirmed that 98% Pd was
remained in the aqueous phase (Figure S3), highlighting that
the catalyst recycling process could be efficiently and easily
achieved using the catalyst shuttling approach.
We further investigated the Sonogashira reaction with a
variety of (hetero)aryl halides and aromatic alkynes to
generate substituted alkynes. In Scheme 3, all electrophiles
underwent complete conversions with MeOÀ WePhos5000 and
°
Pd at 90 C in 6 h, giving target products with isolated yields
of up to 99%. Electron- deficient, electron-rich and electron-
neutral aryl bromides were tolerant in this catalyst system, and
could be successfully conducted with ortho-, meta-, and para-
substituted (hetero)aryl compounds. Functional groups (i.e.,
acetyl (9)), and heterocycles (i.e., thiophene (10) and pyridine
(12)) could be compatible to generate the corresponding
products in good to excellent yields. In addition, the
MeOÀ WePhos5000 4a could also be employed to promote the
Sonogashira cross-coupling with (hetero)aryl chlorides (13,
14).
The vinylation or arylation of alkenes via the Pd- catalyzed
reaction was independently discovered by Heck and Mizoroki
around 1970,[3b,c] and was widely used in organic synthesis for
CÀ C bond formation.[7–8,20] As shown in Scheme 4,
MeOÀ WePhos5000 was investigated in the Pd-catalyzed Heck
reactions, providing a variety of substituted alkenes under
biphasic reaction conditions. Electrophiles of aryl and hetero-
aryl (i.e., thiophene (18) and pyridine (19, 22)) bromides
could be successfully transformed into corresponding products
in the presence of acrylates and styrene at high isolated yields
with 0.1 mol% Pd2(dba)3.
Figure 2. GC conversions and yields of Pd-catalyzed Sonogashira
reaction of bromobenzene and phenylacetylene with ligands from 4a
to 4c.
Scheme 3. Sonogashira reaction with MeOÀ WePhos5000 4a. Condi-
Scheme 4. Heck reaction with MeOÀ WePhos5000 4a. Conditions: 15
(0.5 mmol), 16 (0.6 mmol), K2CO3 (1.0 mmol), water (0.8 mL),
tions:
5
(0.5 mmol),
6
(0.6 mmol), K2CO3 (1.0 mmol), water
°
°
(0.8 mL), toluene (0.2 mL), 90 C, 6 h. Isolated yields are based on
aryl halides 5.
toluene (0.2 mL), 90 C, 6 h. Isolated yields are based on aryl
bromides 15.
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Organometallic reagents of organosilicon and organotin
were employed as nucleophiles, respectively, in Hiyama and
Stille cross-coupling reactions, delivering biaryl units in
pharmaceutical, agrochemical and other areas.[7–8,21] Next, the
polymeric ligand 4a was employed in the Hiyama and Stille
reactions as shown in Scheme 5. For both types of cross-
couplings, aryl, fused aryl and heteroaryl electrophiles were
investigated using either PhSi(OEt)3 or PhSn(Bu)3 as nucleo-
philes, providing bi(hetero)aryl products in 86–96% isolated
yields with 0.1 mol% Pd2(dba)3. For all reactions investigated
in this work, the reaction mixtures underwent rapid phase
separation at room temperature after reaction, affording
products in the organic phase and catalyst kept in the water
phase, further facilitating catalyst recycling and separation.
seperation. 98% Pd was kept in the water phase after recycling
the catalsyt solution for 6 times. Given the profound impact of
transition-metal-catalyzed covalent bond formation and the
increasing demand of sustainable chemistry, this work
provides an alternative method to conduct cross-couplings
with a shuttling catalyst. We believe that other innovative
ligands and new catalytic modes could be developed by
introducing the unique property of macromolecules into ligand
design.
Acknowledgements
This research was finally supported by the National Natural
Science Foundation of China (NSFC, no. 21704016), the start-
up funding from Fudan University, and the National Program
for Thousand Young Talents of China.
3. Conclusion
In conclusion, we have developed a novel thermoresponsive
polymeric ligand, MeOÀ WePhos, which promoted Pd-cata-
lyzed Sonogashira, Heck, Hiyama and Stille reactions under
biphasic conditions, facilitating the preparation of correspond-
ing alkynes, alkenes or bi(hetero)aryls from a variety of
electrophiles and nucleophiles. Importantly, the catalyst shut-
tling behavior enabled by the LCST property of PEG has
allowed convenient catalyst recycling by simply phase
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Manuscript received: November 8, 2019
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E. Wang, J. Zhang, Z. Zhong, K. Chen,
Prof. Dr. M. Chen*
1 – 6
Shuttling Catalyst: Facilitating CÀ C
Bond Formation via Cross-couplings
with a Thermoresponsive Polymeric
Ligand