Cu2O Nanocrystals-Catalyzed Photoredox Sonogashira Coupling of Terminal Alkynes and Arylhalides Enhanced by CO2

Article abstract of DOI:10.1002/cssc.201901813

Herein the first visible-light-activated Sonogashira C?C coupling reaction at room temperature catalyzed by single-metal heterogeneous Cu₂O truncated nanocubes (Cu₂O TNCs) was developed. A wide variety of aryl halides and terminal alkynes worked well in this recyclable heterogeneous photochemical process to form the corresponding Sonogashira C?C coupling products in good yields. Mechanistic control studies indicated that CO₂ enhances the formation of light-absorbing heterogeneous surface-bound Cu^I-phenylacetylide (λ_max=472 nm), which further undergoes single-electron transfer with aryl iodides/bromides to enable Sonogashira C_sp² ?C_sp bond formation. In contrast to literature-reported bimetallic TiO₂-containing nanoparticles as photocatalyst, this work avoided the need of cocatalysis by TiO₂. Single-metal Cu^I in Cu₂O TNCs was solely responsible for the observed C_sp² ?C_sp coupling reactions under CO₂ atmosphere.

Full text of DOI:10.1002/cssc.201901813

DOI: 10.1002/cssc.201901813
Communications
Cu2O Nanocrystals-Catalyzed Photoredox Sonogashira
Coupling of Terminal Alkynes and Arylhalides Enhanced
by CO2
Munusamy Shanmugam, Arunachalam Sagadevan, Vaibhav Pramod Charpe,
V. Kishore Kumar Pampana, and Kuo Chu Hwang*[a]
Herein the first visible-light-activated Sonogashira CÀC cou-
pling reaction at room temperature catalyzed by single-metal
heterogeneous Cu2O truncated nanocubes (Cu2O TNCs) was
developed. A wide variety of aryl halides and terminal alkynes
worked well in this recyclable heterogeneous photochemical
process to form the corresponding Sonogashira CÀC coupling
products in good yields. Mechanistic control studies indicated
that CO2 enhances the formation of light-absorbing heteroge-
neous surface-bound CuI-phenylacetylide (lmax =472 nm),
which further undergoes single-electron transfer with aryl
iodides/bromides to enable Sonogashira Csp2ÀCsp bond forma-
tion. In contrast to literature-reported bimetallic TiO2-contain-
ing nanoparticles as photocatalyst, this work avoided the need
of cocatalysis by TiO2. Single-metal CuI in Cu2O TNCs was solely
responsible for the observed Csp2ÀCsp coupling reactions under
CO2 atmosphere.
shira cross-couplings are the predominant inexpensive proto-
cols for constructing conjugated alkynes,[3d,4,5] However, the
usage of excess ligands and the harsh thermal reaction condi-
tions diminish their merits.
To address the above challenges, we previously reported
visible-light-triggered CuCl-catalyzed Sonogashira cross-
coupling between aryl halides and terminal alkynes at room
temperature.[6] We envisioned that the CÀC coupling reactions
can be initiated by photoexcited copper(I)-phenylacetylide that
undergoes
a single-electron transfer (SET) process with
ground-state aryl halides via inner-coordination sphere to ini-
tiate the CÀC coupling reaction upon visible-light irradiation.
We questioned whether it is possible to replace the molecular
CuCl catalyst with recyclable solid copper-containing nanopar-
ticles (NPs) for the Sonogashira Csp2ÀCsp cross-coupling reac-
tions between aryl halides and terminal alkynes under visible-
The Sonogashira coupling reaction is one of the most
powerful and straightforward methods for construc-
tion of aryl alkynes/conjugated alkynes through Csp2
À
Csp cross-coupling of aryl/alkenyl halides and terminal
alkynes, which are the most substantial intermediates
in the synthesis of natural products as well as phar-
maceutical, polymeric, and organic materials.[1] In
such transformations, conventional methodologies
involve the use of palladium phosphine complexes
along with copper as a cocatalyst in the presence of
excess organic amine bases (Scheme 1a).[2] However,
these methods often require expensive Pd catalysts
and Cu salts as cocatalyst with excessive ligands/
amines or harsh thermal conditions. Recently, a
number of reports have demonstrated different
modifications in a catalytic fashion,[3] such as a) Cu-
free Pd-catalytic systems,[3a–d] b) Pd-free Fe-, Ni-, or
Scheme 1. Various methods for Sonogashira coupling.
Cu-complexes,[3e,f] and c) Cu-catalysts with ligands under harsh
conditions.[3g] Among them, Pd-free copper-catalyzed Sonoga-
light irradiation at room temperature. Such recyclable hetero-
geneous photocatalysts are highly desirable for Sonogashira
coupling reactions and remain unexplored in the literature.
Heterogeneous catalysts are important owing to their recy-
clability and easy separation/recovery, especially in industrial
applications. Recently, a few reports demonstrated the feasibili-
ty of photoinduced heterogeneous catalysis for Sonogashira
cross-coupling at room temperature, such as: a) using a supra-
molecular ensemble of a twisted intramolecular charge-transfer
states aggregation-induced emission enhancement (TICT-AIEE)
active pyrazine and CuO nanoparticles (Scheme 1b);[7] b) a sili-
[a] M. Shanmugam, Dr. A. Sagadevan, V. P. Charpe, V. K. K. Pampana,
Prof. K. C. Hwang
Department of Chemistry
National Tsing Hua University
Hsinchu (Taiwan) (P.R. China)
E-mail: kchwang@mx.nthu.edu.tw
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/cssc.201901813.
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con carbide-supported Pd–Cu alloy (Pd3Cu1/SiC) or TiO2-
supported palladium NPs (Pd@TiO2) (Scheme 1c);[8,9] and
c) TiO2-supported CuO bimetallic photocatalysts (Scheme 1d),
for which cocatalysis of Ti to Cu ions is required for the cou-
pling reactions to occur.[10] Despite increased attention, most
heterogeneous NP-based catalysts still have some limitations.
They require cocatalysis from a metal-oxide support (e.g., TiO2,
ZnO, Fe2O3, Nb2O5, CdS, BiTaO4), usually have poor photocata-
lytic activities owing to weak light absorption and an ineffi-
cient charge separation,[11,12] and have limited substrate scope
and poor recyclability. Thus, a single-metal-based heterogene-
ous catalyst would be more attractive for Sonogashira CÀC
coupling. CO2 is known to be a major contributor to global
warming and can promote or trigger some chemical reactions,
which has received significant attention in organic synthesis.[13]
It was reported in the literature that CO2 (a Lewis acid) can
physically/chemically absorb on various kinds of metal oxides
(Lewis bases), leading to formation of carbonates or undergo-
ing electrochemical reductive conversion to methanol on the
Cu2O electrode surface.[14a–d] Theoretical calculations also
showed that chemical adsorption of CO2 on the Cu2O solid sur-
face will release À56 to À233 kJmolÀ1 adsorption energy, de-
pending on the mode of adsorption.[14e,f] In other words, ad-
sorption of CO2 on the Cu2O surface is exothermic and will
occur spontaneously. Herein we report (Scheme 1e) single-
metal-based heterogeneous catalysis that avoids the require-
ment of cocatalysis by metal-oxide support (such as TiO2) and
achieves visible-light-activated Sonogashira CÀC coupling be-
tween aryl halides and terminal alkynes at room temperature
catalyzed by heterogeneous Cu2O truncated nanocubes
(TNCs). The notable key features of current method are as fol-
lows: (i) Cu2O TNCs are solely responsible for the generation of
the surface-bound light-absorbing CuI TNCs-phenylacetylide
complex; (ii) CO2 was shown to enhance the CÀC cross-
coupling product yields; (iii) moderate-to-good yields were
achieved with no significant loss in catalytic efficiency even
after five cycles; (iv) good recyclability of the catalyst was ach-
ieved, in which the heterogeneous Cu2O TNCs photocatalyst
can be recycled by simple centrifugation–filtration.
Table 1. Effects of reaction parameters.
Entry
Variation from standard conditions
Yield[a] [%]
1
2
3
under N2
none (1 atm CO2)
Au NCs
68[b]
89[c]
n.r.
n.r.
10
n.r.
n.r.
n.r.
82
4
Pd NPs
5
6
no light, 808C
no base
7
8
9
10
no Cu2O TNCs
CuO nanospheres
Cu2O octahedral NCs
Cu2O NCs
80
[a] Yield of the isolated product. [b] 22% homocoupling product, 1,4
diyne, was observed. [c] Trace amount of homocoupling products. n.r.=
no reaction.
pling byproduct (entry 2). The increase in the 3a yield under
CO2 atmosphere is probably owing to two factors. First, remov-
al of molecular O2 can suppress the homocoupling products
because an O2 atmosphere is known to promote formation of
Glaser homocoupling products. Second, coordination of CO2
onto the surface of Cu2O TNCs probably enhances the forma-
tion of surface-bound CuI-phenylacetylide and prolongs its ex-
cited-state lifetime, leading to more efficient SET to aryl halides
(see below). We also found that CuO NPs were not able to cat-
alyze the formation of 3a (entry 8) under the standard condi-
tions, whereas Cu2O NCs with different morphologies all could
catalyze formation of 3a with good yields (entries 9,10, see
Figure S3–S5 in the Supporting Information). These results un-
ambiguously indicate that only CuI- but not CuII-containing
NPs can catalyze the Sonogashira CÀC coupling reactions be-
tween aryl halides and terminal alkynes.
Under the optimized conditions (Table 1, entry 2), we then
investigated the scope of the aryl iodides (2) with 1a. Various
aryl iodides containing neutral, electron-rich, halogen-substi-
tuted, or electron-poor moieties all reacted well with 1a to
afford CÀC coupling products 3a–i in good yields (70–89%,
see Table 2). To our disappointment, we observed very low
yields of products 3 f and 3k in the case of aryl bromide
substrates.
We synthesized the highly stable Cu2O TNCs through a hy-
drothermal route,[15] further analyzed their morphologies by
SEM and TEM, and investigated their photophysical properties
by methods such as UV/Vis/near-IR (NIR), and XRD (for details
of synthesis and structural characterization, see Figure S1 in
the Supporting Information). Inspired our previous work,[6] we
intended to develop a single-metal heterogeneous Cu catalyst
for photoinduced CÀC cross-coupling reactions. We began our
investigations by using phenylacetylene (1a) and iodobenzene
(2a) as substrates under blue-light irradiation with different
NP-based catalysts, solvents, and bases. We were delighted to
find that the combination of Cu2O TNCs and K2CO3 in CH3CN/
CH3OH (1:1 v/v) as a cosolvent provided 3a in 68% yield along
with 22% of Glaser homocoupling product under visible-light
irradiation in N2 atmosphere at room temperature (Table 1,
entry 1). Further optimization of the reaction conditions
showed that the presence of CO2 could further boost the yield
of 3a to 89% accompanied by a trace amount of homocou-
Further, we explored the scope of alkyl/aryl terminal alkynes
with 4-iodoanisole (2 f) under the optimized conditions
(Table 3). A wide range of aliphatic terminal alkynes reacted
well with 2 f to form the corresponding CÀC cross-coupling
products 4a–f in 81–89% yield. In addition, the system worked
very well for aryl terminal alkynes substituted with neutral, hal-
ogen, and electron-withdrawing groups (tBu, Cl, CN, acyl) and
heterocyclic alkynes (e.g., carbazole), producing CÀC coupling
products 4g–p in good yields.
To gain mechanistic insights into the Cu2O TNCs-catalyzed
Sonogashira CÀC coupling reaction, several experiments were
performed (Scheme 2). Addition of Cu2O TNCs to a phenylace-
tylene/CH3CN/CH3OH solution (with or without CO2) resulted in
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Table 2. Substrate scope of aryl halides.[a]
Scheme 2. Mechanistic control studies of the Cu2O TNCs CuI-phenylacetylide
complex.
and a peak at 1930 cmÀ1, a characteristic feature for the pres-
ence of CꢀC-CuI.[16,17] Then, UV/Vis/NIR spectra of Cu2O TNCs,
the yellow suspension, and starting materials were measured,
respectively (Figure 1b). The maximum UV/Vis absorbance at
472 nm was observed for the yellow suspension, indicative of
formation of a Cu2O TNC-CuI-phenylacetylide complex, which
serves as a light-absorbing photocatalyst. Notably, this result
closely resembles our previous free molecular CuI-phenylacety-
lide complex.[6] Furthermore, sequential addition of different
amounts of 2 f to a solution containing the Cu2O-TNCs(I)-
phenylacetylide complex resulted in a progressive photolumi-
nescence quenching of the Cu2O TNCs-CuI-phenylacetylide.
Furthermore, we also explored the coupling reactions be-
tween electron-donating/-withdrawing aryl iodides with linear
aliphatic/electron-donating/-withdrawing aryl terminal alkynes
to obtain the desired CÀC cross-coupling products 4k–4n in
moderate-to-good yields (79–87%). The addition of 2 mm 2 f
reduced the excited-state lifetime of CuI-phenylacetylide com-
plex from 13.20 to 6.15 ms. This result also suggested that a
SET process is operative from surface-bound photoexcited CuI-
phenylacetylide complex to 2 f (see lifetime measurements in
Figure S6 in the Supporting Information).
[a] Yield of the isolated product. [b] Under 0.5 atm N2 +0.5 atm CO2.
[c] Under 1.5 atm CO2. [d] Instead of aryl iodide, aryl bromide substrates
were used for 18 h. [e] Reaction for 10 h.
Table 3. Substrate scope of terminal alkynes.[a]
Next, we performed a coupling reaction between 2 f and 1a
in the presence of a radical scavenger (2,2,6,6-tetramethylpi-
peridin-1-yl)oxyl (TEMPO) or butylated hydroxytoluene (BHT).
The presence of radical scavengers dramatically inhibited the
product formation, indicating the presence of a radical inter-
mediate during the reaction. Next, we measured electron para-
magnetic resonance (EPR) spectra under the standard reaction
conditions by using a spin-trapping agent, (3,3,5,5-tetramethyl-
1-pyrroline-N-oxide) to trap the proposed aryl radical. Fig-
ure 1d depicts the EPR signal of the aryl radical adduct, of
which the g value and splitting pattern match well with those
reported in the literature.[17]
To look for more experimental evidence to rationalize how
CO2 enhances the CÀC coupling product yields, we performed
(a) steady-state phosphorescence measurements, (b) phosphor-
escence-lifetime measurements of the surface-bound Cu2O-CuI-
phenylacetylide under a CO2 atmosphere, and (c) EPR measure-
ments of phenyl radical trapping by TEMPO in the presence/
absence of CO2. In the presence of CO2, the steady-state phos-
phorescence intensity of surface-bound Cu2O-CuI-phenylacety-
lide increases slightly (see Figure S7a in the Supporting Infor-
mation), indicating that CO2 can promote formation of surface-
[a] Yield of the isolated product. [b] Reaction for 10 h. [c] Reaction for
12 h.
the formation of a yellow Cu2O TNCs-phenylacetylide complex
suspension, of which the Fourier-transform (FT)IR spectrum
(Figure 1a) shows the characteristic CꢀC peak at 2200 cmÀ1
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Figure 1. Mechanistic studies. (a) FTIR-spectra of in situ-generated Cu2O-CuI-phenylacetylide. (b) UV/Vis/NIR spectra of in situ-generated Cu2O-CuI-phenylacety-
lide in CH3CN; PhAC=phenylacetylene. (c) Photoluminescence quenching of photoexcited Cu2O-CuI-phenylacetylide by addition of 2 f. (d) EPR spectrum of
the adduct of aryl radical with a spin-trapping agent (3,3,5,5-tetramethyl-1-pyrroline-N-oxide) in dark and under photoirradiation. The g value is 2.0053 with a
coupling constant aH =15.90 G.
bound Cu2O-CuI-phenylacetylide because the steady-state lu-
minescent intensity is in general proportional to the amount
of luminescent compound present in the solution. The phos-
phorescence lifetime of the surface-bound Cu2O-CuI-phenyla-
cetylide changes from 13.9 ms in the absence of CO2 to 15.3 ms
under a CO2 atmosphere (see Figure S7b in the Supporting In-
formation). A longer excited-state lifetime is more favorable to
a bimolecular electron transfer from surface-bound photoexcit-
ed Cu2O-CuI-phenylacetylide to aryl halide in the solution,
which should lead to higher amounts of aryl radical as well as
a higher yield of CÀC coupling product. Indeed, we observed a
slightly stronger aryl radical EPR signal under a CO2 atmos-
phere than under normal air conditions (see Figure S7c in the
Supporting Information). Overall, the presence of CO2 enhan-
ces the Sonogashira CÀC coupling product yield through en-
hanced formation of the surface-bound Cu2O-CuI-phenylacety-
lide intermediate and slightly prolonged excited-state lifetime.
Finally, we evaluated the recycling performance of the
single-metal heterogeneous Cu2O TNCs by using 1a and 2h
under the optimal conditions and blue-LED irradiation
(Figure 2). After completion of the reaction, the catalyst was
collected by centrifugation and washed with water several
times to remove K2CO3. The recycled Cu2O TNCs were further
examined for their ability to catalyze the Sonogashira coupling
reaction, which showed that the catalytic activity decreased
only slightly from 86% yield of 3a in the first run to 80% in
the fifth cycle. Notably, as shown in Figure 2b, SEM images of
the recycled catalyst displayed that the morphology and size
of Cu2O TNCs remain unchanged. The crystallinity of Cu2O
TNCs was stable even after five catalytic cycles (Figure 2c). Be-
sides Cu2O TNCs, other Cu2O NPs such as octahedral NPs and
cubes can all catalyze the Sonogashira CÀC coupling reaction,
but they are less stable and tend to form aggregates and
change morphologies after reactions (see Figures S4 and S5 in
the Supporting Information).
The reaction mechanism for the photoinduced Sonogashira
coupling reaction catalyzed by single-metal heterogeneous
Cu2O TNCs was proposed in Scheme 3. Based on the UV/Vis
and FTIR spectroscopy and other supporting mechanistic evi-
dence, CuI on Cu2O TNCs reacts with 1a to form surface-
bound CuI-phenylacetylide (1a’) on Cu2O TNCs. 1a’ has an ab-
sorption maximum at 472 nm, and its redox potential is proba-
bly close or similar to its molecular counterpart (Eredox
=
À2.048 V vs. saturated calomel electrode, SCE). Upon photoex-
citation by blue LEDs, a long-lived triplet photoexcited cop-
per(I)-phenylacetylide (5) was generated,[18] followed by SET to
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Figure 2. (a) Catalytic activities of Cu2O TNCs versus number of cycles. (b) SEM image and (c) XRD pattern of recycled Cu2O TNCs.
show noticeable degradation after five cycles. Moreover, our
visible-light photocatalyst can be used to explore other CuI-
mediated reactions under photoredox conditions and is a
good alternative for replacement of the existing expensive pre-
cious-metal catalysts in homogeneous and heterogeneous
catalysis.
Acknowledgements
This work was supported by the Ministry of Science & Technology,
Taiwan.
Conflict of interest
The authors declare no conflict of interest.
Scheme 3. Plausible reaction mechanism.
Keywords: CO2 · Cu2O truncated nanocubes · heterogeneous
catalysis · photocatalysis · Sonogashira reaction
aryl iodide 2a (Eredox =À1.20 V vs. SCE)[19] to produce an aryl
radical. The aryl radical can rapidly react with CuII-phenylacety-
lide (6) within the coordination sphere, regenerates the hetero-
geneous Cu2O TNCs, and eventually leads to the formation of
the CÀC cross-coupling product 3a. The presence of CO2 can
promote the formation of surface-bound CuI-phenylacetylide
and prolong its excited-state lifetime, leading to more efficient
bimolecular electron transfer to aryl halides and thus higher
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heterogeneous Cu2O TNCs are able to catalyze Sonogashira CÀ
C cross-coupling reactions under visible-light irradiation at
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Manuscript received: July 3, 2019
Revised manuscript received: August 31, 2019
Accepted manuscript online: September 2, 2019
Version of record online: && &&, 0000
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COMMUNICATIONS
M. Shanmugam, A. Sagadevan,
V. P. Charpe, V. K. K. Pampana,
K. C. Hwang*
&& – &&
Cu2O Nanocrystals-Catalyzed
Photoredox Sonogashira Coupling of
Terminal Alkynes and Arylhalides
Enhanced by CO2
Surface-bound: A Cu2O truncated
single-metal heterogeneous CuI-phenyl-
acetylide (lmax =472 nm) photocatalyst,
a CÀC coupling product yield enhanced
by CO2, and good recyclability of the
solid photoredox catalyst.
nanocubes-catalyzed Sonogashira CÀC
coupling reaction between aryl halides
and terminal alkynes is demonstrated at
room temperature. The features of the
reaction include in situ formation of a
ChemSusChem 2019, 12, 1 – 7
www.chemsuschem.org
7
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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