Photo-induced sonogashira C-C coupling reaction catalyzed by simple copper(I) chloride salt at room temperature

Article abstract of DOI:10.1002/adsc.201200683

The conventional thermal Sonogashira C-C coupling reaction requires the use of a palladium catalyst and a large amount of ligands. Although there were a few reports describing the use of inexpensive metal catalysts, such as, copper (Cu), iron (Fe), and nickel (Ni), for replacement of palladium (Pd) in the Sonogashira reactions, it was later questioned that the observed effects were due to ppb levels contamination of Pd present in the reagents used in the reactions. Herein, we report that simple copper(I) chloride (CuCl) salt, in the absence of Pd and ligands, can catalyze the Sonogashira reaction with high yields (80-99%) under blue LED light irradiation at room temperature. Control experiments show that no cross-coupling product was formed, when palladium(II) chloride (PdCl₂) was used to replace CuCl as a catalyst. A series of electron-rich and electron-poor substituted aryl halides (bromides and iodides) as well as aryl- and alkylacetylenes are examined and the reaction mechanism is discussed.

Full text of DOI:10.1002/adsc.201200683

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DOI: 10.1002/adsc.201200683
À
Photo-Induced Sonogashira C C Coupling Reaction Catalyzed by
Simple Copper(I) Chloride Salt at Room Temperature
Arunachalam Sagadevana and Kuo Chu Hwanga,*
a
Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan, R.O.C.
Fax : (+886)-3571-1082; e-mail: kchwang@mx.nthu.edu.tw
Received: August 1, 2012; Revised: October 25, 2012; Published online: December 4, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200683.
Abstract: The conventional thermal Sonogashira
C C coupling reaction requires the use of a palladi-
shira reaction,[2,3] Pd or Pd/Cu complexes are used as
catalysts. To avoid the use of expensive Pd catalysts,
ligands, and harsh reaction contiditons, efforts have
been devoted to search for cost-effective metal cata-
À
um catalyst and a large amount of ligands. Al-
though there were a few reports describing the use
of inexpensive metal catalysts, such as, copper (Cu),
iron (Fe), and nickel (Ni), for replacement of palla-
dium (Pd) in the Sonogashira reactions, it was later
questioned that the observed effects were due to
ppb levels contamination of Pd present in the re-
agents used in the reactions. Herein, we report that
simple copper(I) chloride (CuCl) salt, in the ab-
sence of Pd and ligands, can catalyze the Sonoga-
shira reaction with high yields (80–99%) under blue
LED light irradiation at room temperature. Control
experiments show that no cross-coupling product
was formed, when palladium(II) chloride (PdCl2)
was used to replace CuCl as a catalyst. A series of
electron-rich and electron-poor substituted aryl hal-
ides (bromides and iodides) as well as aryl- and
alkylacetylenes are examined and the reaction
mechanism is discussed.
lysts, such as, iron, and nickel to replace palladium for
[4–6]
À
catalysis of Sonogashira C C coupling reactions.
Previously, copper was also reported to catalyze Pd-
free Sonogashira reactions, in the presence of ligands,
such as, 1,10-phenanthroline, phosphane, DABCO,
1,3-diketone, etc.[4] It was also reported that a copper
cluster can catalyze the Sonogashira cross-coupling
reactions without the presence of any palladium
metal and ligands at 1108C.[5] However, these process-
es either do not work at room temperature, or require
the use of excessive amounts of ligands. The above
non-Pd catalyzed reactions were later questioned due
to the levels of Pd present in the reagents which were
used in the reactions.[6,7] It was determined by ICP-
MS that many common reagents used in Sonogashira
reactions contain ppb levels of Pd, for example,
K2CO3 (9.15 ppb), AgI (2.64 ppb), PPh3 (5.32 ppb),
DMF (0.32 ppb), etc.[6] In addition, it was demonstrat-
Keywords: carbon-carbon bond formation; copper;
palladium; photolysis; Sonogashira reaction
ed that a ppb level of Pd, in the presence of amino li-
gands, can also catalyze sp-sp C C coupling under
the same reaction conditions, albeit, the yields are
very low, in the range of 14–25%.[7] The demonstra-
tion that trace amounts of Pd, in the presence of li-
gands and at high temperatures, are able to catalyze
2
À
À
Formation of a new carbon-carbon bond, i.e., C C the Sonogashira reaction, however, cannot completely
coupling, is very important for the synthesis of phar- rule out the possibility of there being a truely Cu-cat-
maceutical molecules and many other industrially im- alyzed route.
À
portant products.[1] Emphasing the importance of C
Previously, it was also reported that cross-coupling
C coupling, the 2010 Nobel Prize in Chemistry was of electron-rich aryl chloride and alkynes could be
awarded to three chemists, namely, Heck, Negishi, achieved under longer (36–60 h) UV (254 or 310 nm)
and Suzuki, who have developed new methodologies irradiation times in the absence of any metal catalyst
for C C coupling between different functionalities. at room temperature.[8] In this case, UV irradiation of
The Sonogashira C C coupling reaction between aryl an aryl chloride leads to the formation of a phenyl
À
À
halides and alkynes is blooming as a popular method- cation, which further undergoes electrophilic attack to
ology for the construction of C
3
(sp2)/
N
À
bonds by cross-coupling of aryl/alkyl halides and ter- Sonogashira reaction has many limitations, for exam-
minal alkynes.[2] In the conventional thermal Sonoga- ple, the reaction will not be facilitated properly due
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Arunachalam Sagadevan and Kuo Chu Hwang
Table 1. Optimization of the reaction.[a]
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to UV light absorption by solvents, aromatic alkyne
reactants and products; low reaction yields were ob-
tained when the aromatic halides absorb 254/310 nm
poorly. In another study, it was reported that Pd-cata-
lyzed Sonogashira cross-coupling between aryl bro-
mide and arylalkynes can be achieved upon visible
light irradiation in the presence of a Ru-bipyridine
complex at room temperature.[9] In this case, the
photo-excited Ru-bipyridine complex serves as an
electron donor to reduce the Pd(II) precursor, with-
out forming Pd(0)-phenylacetylide to promote the
catalytic reaction. It was also reported that arylalk-
ACHTUNGTRENNUNGynes can be formed via Ni- or Cu-catalyzed alkynyla-
tion of electron-deficient polyfluoroarenes with termi-
nal alkynes in the presence of molecular oxygen at
room temperature.[10,11] This system requires the use
of 20–40 mol% of 1,10-phenathroline ligand, 15 mol%
of 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) and
a very strong base, LiO-t-Bu, to improve the reaction
yields (42–74%). This catalyst system does not work
for typical Sonogashira substrates, i.e., aryl halides
À
and terminal alkynes, which do not have acidic C H
bonds. It remains a great challenge to develop a facile
À
methodology to catalyze the Sonogashira C C cou-
pling under the following conditions: (a) at room tem-
perature, (b) without using the expensive Pd metal
and ligands, and (c) that is able to catalyze both elec-
tron-rich and electron-defficient substrates.
In this paper, we report that the simple CuCl salt,
in the absence of a Pd catalyst and ligands, can cata-
lyze the Sonogashira reaction with high yields (80–
99%) under blue LED light irradiation at room tem-
perature. Control experiments show that no cross-
coupling product was formed when PdCl2 was used to
replace CuCl as a catalyst, indicating that Pd is not
the catalyst responsible for the photo-induced Sono-
gashira coupling reaction. To the best of our knowl-
edge, a photo-initiated CuCl-catalyzed Sonogashira
cross-coupling reaction of aryl/alkyl halides (X: I, Br)
with terminal acetylene was never reported before.
In a typical experiment, a solution containing phe-
nylacetylene (1a) and 4-iodoanisole (2b) (0.5 mmol)
in acetonitrile and methanol was irradiated with blue
visible light (430–490 nm, 40 mW/cm2 at 460 nm) from
a blue LED array for 4–12 h in the presence of
K2CO3 (1.05 equiv.) and CuCl (5 mol%) at 258C. The
[a]
[b]
Reaction conditions: 0.5 mmol (2b) or (2d’), 0.6 mmol of
(1a), 1.05 (for X=I) or 1.5 equiv. (for X=Br) of base
and 5 mol% of CuCl.
Yields were determined by the 1H NMR integration
method using mesitylene as an internal standard.
1000 ppm (or 1.12 mmol%) of PdCl2 was used as catalyst.
1000 ppm (or 1.12 mmol%) of PdCl2 and 5 mol% of
CuCl were used as catalyst.
[c]
[d]
[e]
Optimization conditions for X=Br: 5 mol% of Cu
source, 1.5 equiv. of base and 2 mol% of AgOTf in 2–
4 mL solvent. purged with N2 gas for three times and the
reaction mixture was irradiated with blue LED light for
15 h under an N2 atmosphere.
1
product (3b) was confirmed by H/13C NMR spectros-
copy and mass spectrometry (see Supporting Informa-
1
tion, Figure S1 for the H and 13C NMR spectra of all
coupling products synthesized in this study). The reac- Cs2CO3. Although the use of these bases still can
tion of 4-iodoanisole (2b) and phenylacetylene (1a) result in product formation (with yields ranging from
was chosen as a model system to optimize the reac- 82 to 96%), they, however, require longer reaction
tion parameters. Among many organic and inorganic times to complete the reaction. In the catalytic
bases examined, K2CO3 was the most effective one fa- screening of cross-coupling reactions, copper(I) hal-
vorable for the formation of coupling product (3b) in ides (halide=bromide, iodide) gives the highest prod-
99% yield (Table 1, entry 2). Many other bases were uct yield (3b) of 99% (Table 1, entries 2, 4, and 5).
also examined, including amine, Et3N, Na2CO3 and Others, such as CuCl2, CuOAc, and CuSO4 result in
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Photo-Induced Sonogashira C C Coupling Reaction Catalyzed by Simple Copper(I) Chloride Salt
slightly lower (3b) yields of 95–20% (see Table 1, en- our reaction reagents show that CuCl, KOAc, and
tries 6–8). Interestingly, when copper phenylacetylide K2CO3 contain 43, <5, and <5 ppb of Pd impurities,
was isolated and used to replace CuCl as a catalyst, respectively. To further examine whether the ob-
a very high product (3b) yield of 99% was also ob- served high product yields in Table 1 are also due to
tained (see Table 1, entry 9). Since copper phenylace- trace amounts of Pd contamination, we deliberately
tylide is stable and can be easily isolated, the reaction added 1000 ppm of PdCl2 to the system (see Table 1,
intermediate (formed via coupling of CuCl and phe- entry 14), to replace CuCl as a catalyst, and photo-
nylacetylene in the presence of base) has a strong lyzed the Pd-containing solution with blue LED light
light absorption with an absorption maximun at under the same conditions. This control experiment,
476 nm [see see Supporting Information, Figure S2, however, did not result in any detectable amount of
(a)], the result in the entry 9 of Table 1, strongly sug- cross-coupling products, indicating that CuCl is the
gests that copper phenylacetylide might be the key key catalyst responsible for the observed photo-in-
catalyst responsible for the photo-induced Sonoga- duced Sonogashira cross-coupling reactions. Using
À
shira C C coupling reaction. When the blue LED 1000 ppm PdCl2 and 5 mol% CuCl as catalysts only
was replaced by a green LED (15 mW/cm2), or a red results in trace amount of products (see, entry 15 of
LED (7 mW/cm2), the product 3b (in the entry 2 of Table 1. Most obviously, upon addition of 1000 ppm
Table 1) was formed with lower yields of 93%, and PdCl2 to the solution, the solution changes from the
10%, respectively, after 24 h irradiation at room tem- original yellow transparent suspension to a black non-
perature. The results are qualitatively consistent with transparent solution. Most probably, the very poor
the visible light absorption profile of copper phenyl- light absorption of CuCl or the copper phenylacety-
ACHTUNGTRENNUNGacetylide [see the Supporting Information, Figure S2, lide, i.e., the true catalyst, in the presence of
(a)].
1000 ppm PdCl2 is responsible for the observed very
It is worth noting that the compatibility of solvent poor yield.
and base has a strong effect on the final product
Reactivities of substituted aryl iodides and bro-
yields. For example, the poor solubility of K2CO3 in mides: Under the optimized conditions, various sub-
acetonitrile-methanol mixed solvent leads to a poor stituted iodides and bromides were examined with
coupling yield of 35% for the reaction between phe- a wide range of alkynes (see results in Table 2 and
nylacetylene (1a) and 4-bromoanisole (2d’). Replace- Table 3). Gratifyingly, excellent yields were obtained
ment of the K2CO3 by an organic base KACTHNUGRTNEUNG(OAc) leads for most of the substrates. As shown in Table 2 and
to an improved yield of 60% (see Table 1, entry 16 vs. Table 3, aryl iodides or bromides bearing electron-do-
17). Furthermore, the yield of (3b) was improved to nating and electron-withdrawing substituents could be
92%, when 2 mol% of silver salt (AgBF4 or AgOTf) effectively coupled with phenylacetylene. However,
was presented (see Table 1, entry 19). This dramatic we found that the electron-rich group-substituted aryl
improvement in the yield could be attributed to the halides (X=I, Br) react significantly faster (or in
non-coordinating, halogen-abstracting nature of the shorter reaction times) than the electron-deficient
silver salt.[12] As compared to aryl iodides and bro- ones, in which the yields for electron-rich group-sub-
mides, aryl chloride and phenyl tosylate gave relative- stituted aryl halides have the following order in the
ly poor yields when coupled with phenylacetylene. It reaction rate: paraꢀortho>meta (see Table 2, entries,
is also important to note that the reaction is highly 2–11, and Table 3, entries 2–9) and the yields for the
sensitive to molecular oxygen. The presence of molec- electron-deficient substituted aryls are in the follow-
ular oxygen leads to formation of homocoupling prod- ing order, meta>ortho>para (Table 2, entries, 12–21
ucts (4a, 4b and 4c) exclusively via the Glaser cou- and Table 3, entries 10–15). Previously, electron-rich
pling reaction.[13]
group-substituted halides were found difficult to be
Control experiments show that removal of any one activated in the thermal Pd-catalyzed Sonogashira re-
of the three factors, i.e., light, CuCl and base, leads to actions.[14,15] In the current photo-induced Sonogashira
no product formation at room temperature (see the coupling reactions, electron-rich group-substituted
Supporting Information, Table I). Thermal heating, halides were found to give higher coupling yields than
instead of LED light irradiation, at 120–1308C for electron-withdrawing group-substituted halides (see
15 h under the same solution composition results in Table 2, entries, 2–11 vs. 12–21).
very low conversion efficiency (~5%), indicating the
Reactivities of aliphatic alkynes: In a similar fash-
necessity of the visible light and CuCl as a catalyst ion, we next examined aliphatic alkynes as a coupling
along with base in the observed cross-coupling reac- partner with aryl halides, in which we have observed
tions. As reported in the literature,[6,7] a ppb level of that aliphatic alkynes have lower reactivities in gener-
palladium impurities in the presence of ligands was al as compared to arylalkynes (see results in Table 4
responsible for the production of coupling products in vs. Table 2). The coupling reaction of 2-methyl-3-
so-called “Pd-free” Cu-, Ni-, or Fe-catalyzed Sonoga- butyn-2-ol (1c) and 4- iodoanisole (2b) gave a 94%
shira coupling reactions. ICP-MS measurements of yield (3a’) (see Table 4, entry 3) upon 12 h irradiation.
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Table 2. Reaction of phenylacetylene with aryl iodides.[a]
Table 3. Reaction of terminal acetylenes with aryl brom-
AHCTUNGTRENNUNG
ides.[a]
[a]
Reaction conditions: 0.5 mmol aryl bromide (2),
0.6 mmol of terminal acetylene (1), 1.5 equiv. of K(OAc)
ACHTUNGTRENNUNG
5 mol% of CuCl and 2 mol% AgOTf in 2 mL of metha-
nol, purged with nitrogen gas for three times and the re-
action mixture was irradiated with blue LED light for
15–27 h under an N2 atmosphere.
Isolated yield after silica column purification. Trace
amount of 4b product was obtained.
[b]
[a]
Reaction conditions: 0.5 mmol aryl or alkyl iodide (2),
0.6 mmol of phenylacetylene (1a), 1.05 equiv. of K2CO3
and 5 mol% of CuCl in 2 mL of ACN and 2 mL of meth-
anol, purged with nitrogen gas for three times and the re-
action mixture was irradiated with blue LED light for 4–
12 h under an N2 atmosphere.
Isolated yield after silica column purification. Trace
amount of 4a product was obtained.
Coupling between aliphatic halide (2x) and phenyla-
cetylene (1a), in general, gives a better yield (84%)
(see Table 2, entry 24). Following the first step of the
Sonogashira coupling between aryl halide and acety-
lene, an intramolecular cyclization can further occur if
there is electron-rich substitution present nearby the
acetylene moiety, that is, a domino Sonogashira reac-
tion can occur (see Table 5). In the literature, it was
reported that Au(I), Pt(II) and Rh(I) can catalyze in-
[b]
[c]
Heteroaromatic and aliphatic iodo substrates.
In contrast, the reaction of trimethylsilylacetylene tramolecular cyclization between an acetylene moiety
with aromatic halides was not very successful and and electron-rich moeities, such as, OH, COOH, and
only a poor yield of 26% was obtained (Table 4, C(=O)NHACTHNUTRGNEUNG
(OMe) groups.[16–19] As shown in Table 5,
entry 8). The reason for the low yield is most proba- when phenylacetylene (1a) was coupled with 2-iodo-
bly due to the poor blue light absorption of the inter- phenol (2z), the formation of benzofuran (3i’) with
mediate copper trimethylsilylacetylide [see the Sup- 95% yield was achieved (see Table 5, entry 1). Fur-
porting Information, Figure S2, (b) for the UV-visible thermore, 2-iodobenzoic acid (2a’) and 2-iodo-N-me-
absorption spectra of a few copper alkylacetylides]. thoxybenzamide (2b’) could be coupled with phenyl-
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Photo-Induced Sonogashira C C Coupling Reaction Catalyzed by Simple Copper(I) Chloride Salt
Table 4. Reaction of alkyl/arylacetylenes with aryl iodides.[a]
Table 5. Domino Sonogashira reaction of phenylacetylene
with substituted aryl iodides.[a]
[a]
Reaction conditions: 0.5 mmol aryl iodide (2), 0.6 mmol
of phenylacetylene (1a), 1.05 equiv. of K2CO3 and
5 mol% of CuCl in 2 mL of CH3CN and 2 mL of metha-
nol, purged with nitrogen gas for three times and the re-
action mixture was irradiated with blue LED light for 4–
12 h under an N2 atmosphere.
[b]
Isolated yield after silica column purification.
Mixture of CH3CN-DMF was used as a solvent.
[c]
[a]
Reaction conditions: 0.5 mmol aryl iodide (2), 0.6 mmol
of terminal acetylene (1), 1.05 equiv. of K2CO3 and
5 mol% of CuCl in 2 mL of ACN and 2 mL of methanol,
purged with nitrogen gas for three times and the reaction
mixture was irradiated with blue LED light for 12–20 h
under an N2 atmosphere.
Isolated yield after silica column purification. Trace
amount of 4c product was obtained.
Very recently, it was reported that the formation of
benzofuran 3i’ can be realized by UV light irradiation
of chlorophenol in the presence of alkylacetylene.[20]
The formation of benzofuran in ref.[20] occurs through
electrophilic attack of the phenyl cation to alkylacety-
lene, which is completely different from the mecha-
nism we propose in Scheme 1.
[b]
[c]
CH3CN and 1.5 equiv. of Et3N were used for the reac-
tion.
Reaction mechanism: The reaction mechanism for
the photo-initiated, CuCl-catalyzed Sonogashira reac-
ACHTUNGTRENNUNGacetylene, leading to the formation of 5-exo-dig cycli- tion is proposed in Scheme 1. The first step involves
zation products, with an array of different isomers, via the deprotonation by base and subsequent coordina-
the domino Sonogashira cross-coupling reaction with tion by CuCl, resulting in in-situ formation of copper
good yields, 94% and 90%, respectively (see Table 5, phenylacetylide (1). It is known that copper phenyla-
entries 2 and 3) [see the Supporting Information, Fig- ceylide can exist in a polymeric form.[21] Therefore,
ure S3, Table S1, and Table S2 for the crystalline the structure of (1) might be either a monomeric or
structures and data of isomers (Z)- and (E)-3-benzyli- a polymeric form or both. Residual CuCl might also
deneisobenzofuran-1(3H)-one]. The crystallographic coordinate onto the side of (1), since addition of
data for 3j’ (E and Z) have been deposited in the 0.1 mol% of CuCl to the copper phenylacetylide-cata-
data bank at the Cambridge Crystallographic Data lyzed reaction can accelerate the reaction rate (data
Centre with corresponding access numbers of CCDC not shown). In the second step, direct photoexcitation
900546 and CCDC 900547, respectively. The results in of copper phenylacetylide (labs =476 nm) leads to the
Table 5 indicate that similar to Au(I), Cu(I) can also formation of electron-deficient acetylene moiety via
act as a p-Lewis acid and catalyze the intramolecular ligand to metal charge transfer (LMCT),[22] which
cyclization of electron-rich moieties and acetylenes. generates a partially positive charge at the acetylene
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Arunachalam Sagadevan and Kuo Chu Hwang
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Experimental Section
General Procedure for the Aryl or Alkyl Iodides
A dry test tube (20 mL) with a rubber septum and a magnet-
ic stirrer bar was charged with aryl or alkyl iodides
(0.50 mmol, 1.0 equiv.), K2CO3 (0.52 mmol, 1.05 equiv.) and
5 mol% CuCl. The test tube was evacuated and purged with
dry N2 gas for three times and then dry CH3CN (2 mL) and
CH3OH (2 mL) were added via syringe, and finally the ter-
minal acetylene (0.60 mmol, 1.2 equiv.) using syringe. The
transparent suspension was irradiated with blue the light
output from a blue LED array (power density 40 mW/cm2 at
460 nm) at room temperature for 4–12 h until completion of
the cross-coupling reaction (as determined by thin layer
chromatography). The reaction mixture was diluted with
40% ethyl acetate in hexane and stirred for 10 min. The
mixture was filtered through celite and silica gel pads and
washed with ethyl acetate. The filtrate was concentrated and
the residue was purified by flash column chromatography
on silica gel to afford the desired cross-coupling product.
Scheme 1. Proposed mechanism for the visible light-mediat-
ed CuCl-catalyzed Sonogashira coupling reaction.
site, and thus favors nucleophilic attack of electron-
rich group-substituted phenyl halides to form transi-
tion state intermediate 3. Such a favorable coupling
reaction towards electron-rich group-substituted aryl
halides is opposite to the conventional Pd-catalyzed
thermal Sonogashira reactions. Subsequent dissocia-
tion of intermediate 3 leads to the formation of the
General Procedure for the Aryl Bromides
A dry test tube (20 mL) with a rubber septum and a magnet-
ic stirrer bar was charged with aryl bromide (0.50 mmol,
1.0 equiv.), KOAc (0.73 mmol, 1.5 equiv.), 5 mol% CuCl and
2 mol% of AgOTf. The test tube was evacuated and purged
with dry N2 gas for three times and then dry CH3OH (2 mL)
was added via syringe, and finally the terminal acetylene
(0.60 mmol, 1.2 equiv.) using a syringe. The transparent sus-
pension was irradiated with the blue light output from
a blue LED array (power density 40 mW/cm2 at 460 nm) at
room temperature for 15–27 h until completion of cross cou-
pling reaction (as determined by thin layer chromatogra-
phy). The further procedure is the same as the above-men-
tioned steps.
À
C C coupling products and regeneration of CuCl.
The proposed mechanism is supported by the experi-
mental results listed in Table 1 to Table 4, and the fact
that the isolated copper phenylacetylide alone, in the
absence of CuCl, can also catalyze the coupling reac-
tion of 4-iodoanisole (2b) and phenylacetylene (1a)
with 99% yield (Table 1, entry 9).
1
The H and 13C NMR spectra, IR, and HR-MS data of all
In summary, we have demonstrated a novel visible
light-mediated Sonogashira cross-coupling reaction
using the simple CuCl salt as a catalyst. The current
method works very well for coupling between a large
variety of aryl bromides/iodides and alkyl/arylalkynes
at room temperature, but works poorly for aryl chlor-
ides. Control experiments have completely ruled out
the possibility of Pd contamination being responsible
for the observed high yields. We have unambiguously
demonstrated that copper can indeed replace expen-
products, the UV-visible absorption spectra of copper acety-
lides, as well as the crystalline structures of products 3j’ E
and Z for this article are available in the Supporting Infor-
mation.
CCDC 900546 and CCDC 900547 contain the supplemen-
tary crystallographic data for products 3j’ E and Z. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
À
sive Pd to catalyze the Sonogashira C C coupling re-
action by visible light photo-irradiation, but not by
the thermal heating method. The mechanistic study
indicates that the copper acetylide (both alkyl and
aryl, labs =425–485 nm) is the key light absorbing spe-
cies, and is responsible for catalysis of the photo-in-
Acknowledgements
This work was supported by the National Science Council,
Taiwan.
À
duced Sonogashira C C coupling reactions. Mild con-
ditions, such as, Pd-free, low catalyst loading and high
reaction yields at room temperature, make the cur-
rent method a very promising, industrially scalable
green process as an alternative to the conventional
Pd-catalyzed, high-temperature Sonogashira cross-
coupling reactions for the synthesis of natural prod-
ucts and medicinal compounds.
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Photo-Induced Sonogashira C C Coupling Reaction Catalyzed by Simple Copper(I) Chloride Salt
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