A convenient route to 1,3-diynes using ligand-free Cadiot–Chodkiewicz coupling reaction at room temperature under aerobic conditions

Article abstract of DOI:10.1080/00397911.2018.1554142

In this study, we report a copper-catalyzed ligand-free Cadiot–Chodkiewicz coupling reaction. No additives or ligands are required for this Cu-catalyzed C(sp)–C(sp) coupling reaction of terminal alkynes with alkynyl halides. The low cost of copper catalyst, excellent yield of the products, suppression of side-products and mild reaction conditions are the major advantages of this protocol.

Full text of DOI:10.1080/00397911.2018.1554142

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
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A convenient route to 1,3-diynes using ligand-free
Cadiot–Chodkiewicz coupling reaction at room
temperature under aerobic conditions
Sujatha Asha, Sasidharan Anjana, Anns Maria Thomas, Minu Elizabeth
Thomas, K. R. Rohit, K. Keerthi Krishnan, Sankuviruthiyil M Ujwaldev &
Gopinathan Anilkumar
To cite this article: Sujatha Asha, Sasidharan Anjana, Anns Maria Thomas, Minu Elizabeth
Thomas, K. R. Rohit, K. Keerthi Krishnan, Sankuviruthiyil M Ujwaldev & Gopinathan Anilkumar
(2019): A convenient route to 1,3-diynes using ligand-free Cadiot–Chodkiewicz coupling
reaction at room temperature under aerobic conditions, Synthetic Communications, DOI:
10.1080/00397911.2018.1554142
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https://doi.org/10.1080/00397911.2018.1554142
A convenient route to 1,3-diynes using ligand-free
Cadiot–Chodkiewicz coupling reaction at room temperature
under aerobic conditions
Sujatha Ashaa,b, Sasidharan Anjanaa, Anns Maria Thomasa,c, Minu Elizabeth
Thomasa, K. R. Rohita, K. Keerthi Krishnana, Sankuviruthiyil M Ujwaldeva, and
Gopinathan Anilkumara
b
aSchool of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India; Department of
c
Chemistry, Sree Vidyadhiraja NSS college, Theerthapadapuram, Kottayam, Kerala, India; Department of
Chemistry, Nirmala College, Muvattupuzha, Ernakulam, Kerala, India
ABSTRACT
ARTICLE HISTORY
Received 8 May 2018
In this study, we report
a
copper-catalyzed ligand-free
Cadiot–Chodkiewicz coupling reaction. No additives or ligands are
required for this Cu-catalyzed C(sp)–C(sp) coupling reaction of ter-
minal alkynes with alkynyl halides. The low cost of copper catalyst,
excellent yield of the products, suppression of side-products and
mild reaction conditions are the major advantages of this protocol.
KEYWORDS
C–C coupling; Cadiot–
Chodkiewicz coupling;
copper catalysis; cross
coupling; 1,3-diynes
GRAPHICAL ABSTRACT
Introduction
The conjugated 1,3-diynes are of great importance4 due to their prevalence in many
bioactive naturally occurring and synthetic molecules. Many 1,3-diynes exhibit antibac-
terial, anticancer, or anti-HIV activities.[1] Furthermore, these diynes play an important
role in the properties of many functional materials, such as nonlinear optical materials,
electrically conductive plastics, fibers with high density and strength, and liquid crys-
tal.[2] More importantly, diynes have thermal and moisture stability. Although highly
unsaturated, the conjugation of the pi-bonds provides extra stability.[3] Therefore, the
synthetic methods to develop 1,3-diynes especially unsymmetrical diynes, have drawn
the attention of chemists for decades. Glaser coupling and Cadiot–Chodkiewicz cou-
pling have been the widely employed approaches for the construction of 1,3-diynes. The
CONTACT Gopinathan Anilkumar
anilgi1@yahoo.com
School of Chemical Sciences, Mahatma Gandhi University,
Priyadarsini Hills P O, Kottayam, Kerala 686560, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

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S. ASHA ET AL.
Glaser reaction, first reported in 1869, involves oxidative coupling of two terminal
alkynes catalyzed by copper salts is limited for the synthesis of symmetrical diynes
only.[4] In this context, for the synthesis of unsymmetrical 1,3-diynes,
Cadiot–Chodkiewicz reaction was developed which involves the coupling of haloalkyne
with a terminal alkyne in the presence of a transition metal catalyst. The first reaction
was reported in 1955 by Cadiot and Chodkiewicz using bromoalkyne and 2-methyl-but-
3-yn-2-ol as the substrates in presence of Cu catalyst and amine.[5]
Later, significant modifications have been introduced for the efficient and highly
selective cross-coupling of alkynes and haloalkynes utilizing various catalytic systems
and solvents. Among these, the addition of palladium co-catalysts, use of ligands and
environmentally benign solvents are the promising protocols. In 1990, examples of pal-
ladium-promoted Cadiot–Chodkiewicz reactions were reported by Wityak[6] and
Alami.[7] Subsequently, Kurth and co-workers established the polymer-supported sys-
tems for the Cadiot–Chodkiewicz coupling to prevent the homo-coupling of the haloal-
kynes.[8] However, low yields of the products were observed in this protocol. Another
improvement was made by Jiang, who used the supercritical CO2 as the reaction
medium and NaOAc as the base instead of the organic amine.[9] Moderate to high
yields were obtained, and the hydroxyl group in haloalkynes was found to be crucial in
this protocol. In a recent report, Wang et al. have used CuI and tris(o-tolyl)phosphine
as the catalyst-system and yielded the unsymmetric diynes in good yields.[10] A copper-
catalyzed decarboxylative coupling reaction of potassium alkynyl carboxylates with 1,1-
dibromo-1-alkenes was developed for the synthesis of unsymmetrical 1,3-diynes.[11]
A
Ni/Cu co-operative catalytic system has also been used for the synthesis of unsymmet-
rical 1,3-diynes in the absence of any ligand through Cadiot–Chodkiewicz type coupling
reaction.[12] Even though a number of modifications of Cadiot–Chodkiewicz coupling
reactions have emerged, it is noteworthy that most of the reactions require harsh reac-
tion conditions like high temperature, presence of an amine base, need for ligands,
necessity of inert atmosphere, presence of palladium co-catalyst, and the use of special
additives. Our research group has recently reported an efficient Zn-catalyzed
Cadiot–Chodkiewicz reaction using diethylzinc in presence of L-proline.[13]
Based on the previous reports of using Cu as an effective catalyst in coupling reac-
tions and our own ongoing research in transition metal catalyzed coupling reactions[14]
we were prompted to check whether a Cu-based amine free procedure can be developed
for Cadiot–Chodkiewicz coupling. Herein, we report a new protocol using amine-free
Cu-catalyzed room temperature Cadiot–Chodkiewicz coupling reactions in an alcoholic
solvent under aerobic conditions for the synthesis of 1,3-diynes.
Results and discussion
In order to identify the optimal reaction conditions, the reaction between 4-ethynylani-
sole (1a) and bromoethynyl benzene (2a) was chosen as the model reaction. Initially, all
reactions were performed using CuI (20 mol %) as the catalyst and K2CO3 (2 equiv.) as
ꢀ
the base in ethanol at 120 C under nitrogen atmosphere. The results are summarized
in Table 1. Since most of the coupling reactions reported in literature were in the pres-
ence of ligands, we first screened the effect of various ligands such as L-proline (La),

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Table 1. Ligand screening for the Cadiot–Chodkiewicz reaction with 4-ethynylanisole and bromoe-
thynylbenzene.a
Entry
Ligand
Yield(%)b
Entry
Ligand
Yield(%)b
1
2
3
4
5
6
La
Lb
Lc
Ld
Le
Lf
20
39
Traces
Traces
76
7
8
9
10
11c
12d
Lg
Lh
Li
–
–
–
65
56
82
86
86
85
Traces
aReaction conditions: bromoethynyl benzene (1 mmol), 4-ethynylanisole, (1.1 mmol), K2CO3 (2 equiv.), CuI (20 mol %),
EtOH (3 ml), 120oC, 20 h under N2 atmosphere; bIsolated yield; cUnder air; dThe reaction was performed at room
temperature.
DABCO (Lb), ethylenediamine (Lc), N,N’-dimethylethylenediamine (Ld), PPh3 (Le),
BINOL (Lf), Trans 1,2-cyclohexanediol (Lg), ethylene glycol (Lh), and 1,10-phenanthro-
line (Li). It was found that the ligand 1,10-phenanthroline was the most effective one
giving 82% yield of the product (Table 1, entry 9). Interestingly when the reaction was
carried out in the absence of ligand we got 86% yield of the product along with a small
amount of the homo-coupled (Glaser) product (<5%). Therefore, we decided to per-
form the reactions under ligand-free conditions. The reaction when carried out in air
afforded yields comparable to those obtained when carried out in a nitrogen atmos-
phere. Thus it is proven that the inert atmosphere is not essential for this reaction.
Furthermore, the reaction temperature was lowered to room temperature while main-
taining the yield at 86% at the end (Table 1, entries 11 & 12).
We further explored the effect of different solvents and bases and catalyst loading.
Analysis of the solvents revealed that the solvents used have a great influence on the
Cadiot–Chodkiewicz coupling reaction. The reaction did not proceed in both water and
water/TBAB mixture (Table 2, entries 2 & 3). 1,4-Dioxane and THF gave moderate

4
S. ASHA ET AL.
Table 2. Optimization studies of the Cadiot–Chodkiewicz reaction with 4-ethynylanisole and bromoe-
thynylbenzenea.
Entry
Catalyst
Base
Solvent
Yield (%)b
1
2
3
4
5
6
7
8
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Et3N
Toluene
Water
Water/TBAB
t-Butyl
1,4-Dioxane
EtOH
THF
THF/Water
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
Traces
NRC
NR
26
44
86
45
42
93
42
56
66
94
64
43
65
NR
26
9d
10
11
12
13e
14f
15g
16h
17
18
19
20
21
NaOtBu
K3PO4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaOH
–
CuBr
CuBr2
Cu(OAc)2
CuI
NR
NR
78
aReaction conditions: bromoethynyl benzene (1 mmol), 4-ethynylanisole (1.1 mmol), base (2 equiv.), CuI (20 mol %), solv-
c
ent (3 ml), room temp., 20 h under air atmosphere; bIsolated Yield; No reaction; dReaction time: 20 h; eReaction time:
f
35 h; Reaction time: 12 h; g5 mol% of CuI was used; h10 mol% of CuI was used.
yields of the product while t-butanol gave low yield (Table 2, entries 5, 7, and 4). When
methanol was used as the solvent, to our delight, the yield of the product increased to
93% (Table 2, entry 9). Thus methanol was found to be the most suitable solvent for
this coupling reaction. We then screened different bases such as K2CO3, Et3N, K3PO4,
and NaOtBu. Here, K2CO3 was found to be the best choice for high yield of the cross-
coupled product. As expected, shortening of reaction time also lowered the yield of the
product. Similarly, the yield of the reaction was significantly lowered when the catalyst
loading was changed from 10 mol% to 5 mol% (Table 2, entries 15 & 16). Thus the
maximum yield of the product was obtained when 20 mol% of CuI and 2 equiv. of
K2CO3 were used at room temperature for 20 h in methanol. Finally, control experi-
ments revealed that the reaction did not yield the product at all when CuI was not
added. (Table 2, entry 17). When we used CuBr as the catalyst, the required product
was formed in 26% yield. When cupric salts were used as catalysts, the reaction did not
proceed well (Table 2, entries 18, 19, and 20).
Finally, we extended the protocol to a variety of diversely substituted aromatic and
heteroaromaticalkynyl bromides with a number of terminal alkynes. The reactions of
bromoethynyl benzene with terminal alkynes containing electron donating groups such
as –Me and –OMe underwent clean reactions affording the coupled products in excel-
lent yields (Table 3, entries 1 and 2). Apart from aromatic alkynes, heteroaromatic
alkynes and aliphatic alkynes including cyclohexyl-1-propyne and 3-methoxy prop-1-
yne also underwent smooth coupling reaction affording the required product in good

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S. ASHA ET AL.

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Scheme 1. Proposed mechanism of Cu-catalyzed Cadiot–Chodkiewicz coupling.
yield (Table 3, entries 4 and 5). It is noteworthy that the new methodology worked well
for substituted alkynyl bromides. Gratifyingly, 2-bromo ethynyl pyridine coupled suc-
cessfully with different terminal alkynes in good yields of the target products.
Based on the above observations and previous reports we propose a probable mech-
anistic pathway for this reaction (Scheme 1).[15] In the first step, a copper acetylide spe-
cies A is formed in presence of a base from terminal alkyne which then adds to the
haloalkyne to form a bisalkynyl copper species B. This species then undergoes a reduc-
tive elimination to furnish the required product C and the copper catalyst is
regenerated.
Experimental
The chemicals CuI and alkynes were purchased from Sigma Aldrich chemical company,
USA. K2CO3 was purchased from NICE Chemicals Pvt. Ltd. India. All these reagents
were used as such. Alkynylbromides were prepared by the procedure given below. All
solvents used for the reaction were purchased from NICE Chemicals Pvt. Ltd. India and
distilled prior to use. The NMR spectra were recorded at 400 or 500 (1H) and 100 or
125 (13C) MHz respectively on a Bruker-400 or 500 MHz NMR spectrometer instru-
ment. The chemical shift values are reported relative to Me4Si (1H) and CDCl3 (13C) as
internal standards. The value of coupling constant (J) is stated in Hertz (Hz). FT-IR
spectra were recorded on a PerkinElmer Spectrum 400 FT-IR spectrometer and are
reported in frequency of absorption (cmÀ1). The HRMS data were recorded using
XEVO G2 Q-TOF (Waters) mass spectrometer. Melting points were recorded on
MPHT melting point apparatus and are uncorrected. Thin layer chromatography was
carried out using AluchrosepSilicaGel 60/UV254 purchased from S D Fine-Chem. Ltd,
India and visualized by using either UV Fluorescence or Iodine chamber. Column chro-
matography was performed using silica gel 100–200 mesh purchased from Merck
Specialties Pvt. Ltd., India and mixtures of hexane-ethylacetatewereused for elution.

8
S. ASHA ET AL.
General procedure for the synthesis of alkynylbromides
To a solution of phenylacetylene (400 mg, 3.92 mmol) in acetone (50 ml), NBS (837
mg, 4.70 mmol) and AgNO3 (66 mg, 0.39 mmol) were added. The resulting mixture
was stirred under nitrogen at room temperature for 2 h. The reaction mixture was
quenched with saturated aqueous NH4Cl solution and the organic layer was extracted
with hexane (2 Â 20 ml), dried over anhydrous MgSO4 and concentrated under reduced
pressure affording a colorless oil of bromoalkyne (631 mg, 89%).
General procedure for the synthesis of unsymmetrical 1,3-diynes
A mixture of alkynylbromide (1.1mmol), terminal alkyne (1.0 mmol), and K2CO3 (2.0
mmol) in methanol(3 ml) was stirred under air at room temperature in the presence of
CuI (20 mol%) for 20 h and the completion of the reaction was monitored by TLC.
The reaction mixture was washed with saturated aqueous NaCl solution (2 Â 10 ml)
and then extracted with ethyl acetate (3 Â 10 ml), and the organic layers were com-
bined, dried over anhydrous Na2SO4, filtered and concentrated under reduced pressure.
The residue was separated and purified by column chromatography using Hex:EtOAc as
the eluent system.
Conclusion
In summary, we have developed a versatile, simple and cost-effective method for the
synthesis of 1,3-diynes at room temperature under aerobic conditions. Since 1,3-diynes
are widely used for pharmaceutical and material applications, the new methodology
developed would be of great use in organic synthesis. Further efforts to elucidate the
mechanism and application of the reaction are still going on in our laboratories.
Funding
GA and SA(2) thank the Kerala State Council for Science, Technology and Environment
(KSCSTE), Trivandrum, India (Order No. 341/2013/KSCSTE dated 15.03.2013) for financial sup-
port. SA(1) and AMT thank the Council for Scientific and Industrial Research (CSIR), New
Delhi-India for junior research fellowships. KRR and SMU thank the University Grants
Commission (UGC), New Delhi-India for junior research fellowships. KKK thanks the Ministry
of social justice and empowerment, UGC, New Delhi for the award of Rajiv Gandhi
National Fellowship.
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