Exploration of the mechanism and scope of the CuI/DABCO catalysed C–S coupling reaction

Article abstract of DOI:10.1016/j.poly.2019.114269

A cost effective and easily available CuI/DABCO catalytic system has been developed for the C–S cross-coupling reaction. This method is extremely useful for the thioetherification of aryl and heteroaryl halides, providing excellent yields and good chemoselectivity. We have also explored the mechanism of the reaction using DFT studies.

Full text of DOI:10.1016/j.poly.2019.114269

Journal Pre-proofs
Exploration of the mechanism and scope of the CuI/DABCO catalysed C-S
coupling reaction
Anns Maria Thomas, D.R. Sherin, Sujatha Asha, T.K. Manojkumar, Gopinathan
Anilkumar
PII:
S0277-5387(19)30714-4
DOI:
https://doi.org/10.1016/j.poly.2019.114269
POLY 114269
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
14 August 2019
25 November 2019
25 November 2019
Please cite this article as: A. Maria Thomas, D.R. Sherin, S. Asha, T.K. Manojkumar, G. Anilkumar, Exploration
of the mechanism and scope of the CuI/DABCO catalysed C-S coupling reaction, Polyhedron (2019), doi: https://
doi.org/10.1016/j.poly.2019.114269
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Graphical Abstract
.
Exploration of the mechanism and scope of the CuI/DABCO
catalysed C-S coupling reaction
Anns Maria Thomas, Sherin D. R., Sujatha Asha,
Manojkumar T. K.* and Gopinathan Anilkumar*

1
Polyhedron
Exploration of the mechanism and scope of the CuI/DABCO catalysed C-S coupling
reaction
a,b
c
a,d
c
a,e
Anns Maria Thomas, Sherin D. R., Sujatha Asha, Manojkumar T. K.* and Gopinathan Anilkumar*
a
School of Chemical Sciences, Mahatma Gandhi University, Priyadarsini Hills P O, Kottayam, Kerala, INDIA 686560.
Nirmala College, Muvattupuzha P.O, Ernakulam, INDIA, 686661.
Indian Institute of Information Technology and Management-Kerala, Trivandrum, INDIA.
Govt. Polytechnic College, Thannurkara P.O, Chelakkara, Thrissur, Kerala, INDIA.
b
c
d
e
Advanced Molecular Materials Research Center(AMMRC), Mahatma Gandhi University, Priyadarsini Hills P O, Kottayam, Kerala, INDIA 686560.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A cost effective and easily available CuI/DABCO catalytic system has been developed for the
C–S cross-coupling reaction. This method is extremely useful for the thioetherification of aryl
and heteroaryl halides, providing excellent yields and good chemoselectivity. We have also
explored the mechanism of the reaction using DFT studies.
Available online
2
009 Elsevier Ltd. All rights reserved.
Keywords:
C-S coupling
Catalysis
DFT
Copper
good yields. Furthermore, the excellent chemoselectivity of this
method enabled the synthesis of 4-(4-
Introduction
nitrophenylsulfanyl)phenylamine, which is otherwise difficult to
obtain using metal-catalysed coupling reactions. 4-(4-
Nitrophenylsulfanyl)phenylamine can be used as a precursor for
Transition metal catalysed C–S bond forming reactions are
widely applied in organic synthesis for the production of key
moieties in biological, pharmaceutical or material fields. The
6
1
the synthesis of Dapsone, a drug for the treatment of leprosy.
traditional methods, which require high temperature, polar
solvents or strong reducing agents, can be replaced by this
Results and Discussions
2
protocol. Pioneering work on transition metal catalysed C-S
The reaction between 4-iodoanisole and thiophenol was
chosen as the model reaction to examine the scope of the Cu-
catalysed reaction. It is known that addition of a ligand to a metal
catalyst increases the solubility and enhances the catalytic action.
Inspired by this idea, we screened different N,N- and N,O-
bidentate ligands for the C-S coupling reaction. The results are
shown in Table 1.
3
coupling reactions was reported by Migita and co-workers.
Unfortunately Pd catalysts are expensive and have low turnover
numbers. In addition to this, Pd catalysts are usually combined
with phosphine ligands, requiring tedious procedures for their
preparation, and they are moisture sensitive too. As a result of
these constraints, improved alternatives were sought for large
scale applications and eventually transition metal catalysed C–S
Table 1: Ligand screening a
4
coupling reactions emerged. We have recently reported Zn and
d,4i
Fe-catalysed C–S coupling reactions.4 In continuation of our
efforts to develop a more affordable catalyst with better yield and
5
functional group tolerance, we focused on Cu catalysts. Copper
has certain desirable features, such as low toxicity, less cost and
is it compatible with many readily available and cheap ligands.
Moreover, it has many stable variable oxidation states suitable
for a catalytic cycle. Driven by these facts, we focussed on the C-
S coupling reaction employing Cu as the catalytic system. To our
delight we found that the combination of CuI and DABCO is
well suited for the C-S coupling reaction, with wide scope and

2
Polyhedron
Yield (%)b
temperature of the reaction reduced the yield of the desired
Entry
Ligand
Temperature
o
(
C)
product.
1
2
3
4
5
6
Ethylene diamine (L
1
)
80
80
80
80
120
RT
8
Table 3: Control experiments
BINOL (L
2
)
32
12
34
98
-
Entry
CuI
DABCO
K
2
CO
3
N
2
Yieldb
98
-
Trace
69
Trace
Picolinic acid (L
3
)
1
2
3
4
5










DABCO (L
DABCO (L
DABCO (L
4
)
4
)
4
)





a) Reaction conditions: 1a (0.6 mmol), 2a (0.5 mmol), CuI (10
mol%), ligand (20 mol%), base - K CO (2 equiv.), solvent -
DME, N2, b): isolated yield, RT - room temperature

2
3
a) Reaction conditions: ArX (0.6 mmol), thiol (0.5 mmol), CuI (5
mol%), ligand (10 mol%), base (2 equiv.), 120 C, N2,. b) isolated
o
On examining different ligands, we found that the N,N-
bidentate ligand DABCO was the most effective one for this
coupling reaction, giving 34% yield and on increasing the
temperature to 120 °C, we got a maximum yield of 98%.
yield.
On screening different substrates, we found that both electron
rich and electron deficient substrates gave excellent yields for the
reaction. First, we coupled aryl halides and aryl thiols. (Scheme
1). More importantly heterocyclic halides also gave very good
Table 2: Base and solvent optimization
yields. Sterically hindered substrates usually provide
a
challenging task to synthetic chemists in coupling reactions. To
our delight, sterically hindered reactants like 2-iodoanisole also
gave good yields for the reaction.
Entry
Solvent
CH CN
Water
Toluene
DMF
Base
Temperature
Yield
o
(%)b
(
C)
1
2
3
4
5
6
7
8
9
3
K
K
K
K
K
K
K
K
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
3
3
3
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
93
31
45
53
93
59
65
98
85
80
81
96
96
THF
DMSO
t-BuOH
DME
DME
DME
DME
DME
DME
DME
2
Na CO
3
1
1
1
1
1
1
0
1
2
3
4
5
K
3
PO
4
t
NaO Bu
KO Bu
t
TEA
_
c
69
DME
K
2
CO
3
85
a) Reaction conditions: 1a (0.6 mmol), 2a (0.5 mmol), CuI (10
mol%), ligand (20 mol%), base (2 equiv.), 120 °C, 12 h, N2, b):
isolated yield, c): 2.2 equiv. of DABCO was used
We then examined the influence of different solvents and
found that DME was the most effective one (Table 2). Non-polar
solvents were found to decrease the yield of the reaction. Next
we studied the effect of different bases on the outcome of the
a) Reaction conditions: ArX (0.6 mmol), thiol (0.5 mmol), CuI
5 mol%), ligand (10 mol%), base (2 equiv.), 120 °C, N2,. b)
2 3
reaction. The cheap inorganic base K CO was found to be the
(
most effective one for this reaction. Since the efficiency of the
metal catalysed reaction greatly depends upon the catalyst
loading, next we set out to optimize the catalyst loading.
Different ratios of ligand and catalysts were tried and we found
that a combination of 5 mol% of CuI and 10 mol% of DABCO
was the most efficient catalyst loading for this reaction.
Isolated yield, c) 1.6 equiv. of thiophenol was used and the
reaction was carried out for 24 h. d) The reaction was carried out
for 17 h.
Scheme.1: Substrate scope of Cu-catalysed C-S cross-
coupling between aryl iodides and aryl thiolsa
We conducted several control experiments to find the different
factors which limit the reaction (Table 3). First, we have carried
out the reaction in the absence of CuI and found that no product
was formed. This proves that presence of the Cu catalyst is
essential for this reaction. In the absence of the ligand DABCO
we got only a trace amount of the product. Next we checked
whether the ligand DABCO, which is an unhindered nucleophile,
could act as the ligand by conducting the reaction in the absence
Another significant feature of this reaction is that the product
-((4-nitrophenyl)thio)aniline can be used as a precursor of the
drug dapsone (Scheme 2).
4
2 3
of K CO and we found that the yield of the product was reduced
to 69%. Carrying out the reaction under an aerobic atmosphere
enhanced the homocoupling of the thiols and the required
product was obtained in small amounts. Lowering the time or

3
state (unique imaginary frequency). All geometry optimizations
and frequency calculations were performed using the SDD basis
set. Solvent effects were taken into consideration using the
polarizable continuum (PCM) model, taking DME as the
9
solvent. The Gibbs free energies of all the optimized structures
were calculated.
Results and discussion
Geometry optimizations were done using density functional
theory to locate the stationary points in the potential energy curve
of reaction profiles and their nature was confirmed using
computed vibrational frequencies.
Scheme 2: Synthetic route to the drug Dapsone
We then applied this rich chemistry for the coupling of aryl
halides with alkyl thiols, giving excellent yields of the products
(Scheme 3).
Figure 1: Structures of intermediates im-1 and im-2
a) Reaction conditions: ArX (0.6 mmol), alkyl thiol (0.5
mmol), CuI (5 mol%), ligand (10 mol%), base (2 equiv.), 120 °C,
2, b) isolated yield.
The possible steps involved in the catalytic cycle are as
follows:
N
1
). Formation of the Cu-DABCO complex (im-1)
Scheme 3: Cu-catalysed coupling of aryl halides with alkyl
thiols
According to the computational studies, the first step is CuI
salt coordinate formation with the ligand, forming a Cu(I)
DABCO complex. The optimized geometry shows that a linear
complex exists with a coordination bond between CuI and the
ligand. This complex may exist as part of an equilibrium
between the biligated and monoligated metal complexes. This is
supposed to be the active catalytic species in the reaction
pathway. In addition to Cu(I) complexes, other copper sources
like Cu(0), Cu(II) have also been reported for C-heteroatom
coupling reactions. However, it has been experimentally
confirmed and reported that from all these complexes, the active
catalytic species generated that provides the highest reaction rate
is Cu(I) based. The geometry of the Cu(I) DABCO complex
was optimized and the result is shown in Figure 1 (im-1). This
type of DABCO-Cu-I polymeric complex has already been
DFT Studies to explore the mechanism
To explore the mechanism, the reaction between 4-iodoanisole
and thiophenol was modelled (Scheme 4).
4
b
a): Reaction conditions: 4-iodoanisole (0.6 mmol), thiophenol
(0.5 mmol), CuI (5 mol%), DAcBCO (10 mol%), base (2 equiv.),
1
0
N
2.
Scheme 4: Cu-catalysed C-S coupling reaction
1
1
reported. The Cu-N bond length in the complex was computed
to be 1.954 Å and the Cu-I bond length was 2.455 Å.
The computational calculations were performed using DFT
theory using the hybrid B3LYP functional and the Gaussian-09
7
software package. Chemcraft visualization software was used to
2). Formation of the Cu-thiolate complex
8
construct the input molecular structure. In geometry
The nucleophilic species in this reaction is the thiolate anion
which is generated by the abstraction of a proton by the base
K CO . The generated thiolate anion adds to the Cu complex and
2 3
forms a Cu(I)-thiolate complex. The optimized geometry is
shown in Figure 1 (im-2). These types of copper-thiolate
complexes have already been isolated. The Cu-S bond length in
this complex was computed to be 2.193 Å and the Cu-N bond
length was slightly increased to 1.956 Å.
optimization studies, the stationary point may be a minimum,
transition state or a higher order saddle point. Locating the
stationary point and calculating its geometry and energy is called
geometry optimization. Determining the minimum is called
minimization and determining the transition state is referred to as
transition state optimization. Once a transition point is found,
vibrational analysis is carried out to find whether the transition
point is a minimum (zero imaginary frequency) or a transition
1
2

4
Polyhedron
3
). Addition of aryl iodide to the Cu(I)-thiolate complex
For the coupling to occur, the generated Cu(I)-thiolate
complex should add to the aryl iodide. This addition generates a
transition state (Figure 2, ts-1). The structure was first optimized
using the SDD basis set and on frequency analysis we got a
single imaginary frequency confirming that it is actually a
transition state. It was found that the nature of the halide ion
greatly affects the oxidative addition step and the rate of the
reaction. It is worth noting that in the intermediate im-2, the Cu-S
bond length was 2.193 Å and the C
iodoanisole was 2.156 Å, whilst in the transition state the Cu-S
bond distance was increased to 2.221 Å and the C -I distance
was increased to 2.646 Å. This clearly indicates that the Cu-S
and C -I bonds start dissociating in the transition state and the
-S (bond distance 2.478 Å) and Cu-I (bond distance 3.344 Å)
bonds start forming.
1 1
-I bond length in the
1
1
Figure 4: Structure of the transition state
1
1
Finally, dissociation of the Cu-S bond from this transition
C
1
1
state yields the required product. The
1
C -S bond length is
reduced to 1.84 Å, confirming the formation of a C-S bond.
Based on the above calculations and experimental results, a
plausible complete catalytic cycle is depicted in Scheme 5.
Figure 2: The structure of transition state ts-1
The mode of vibrations of the transition state clearly indicate
that bond formation occurs between the C atom labelled as C
and the S atom. From this transition state, an intermediate is
generated (Figure 3, im-3). The Cu-I bond length is reduced to
.584 Å and the S-C distance to 1.858 Å, confirming the
1
1
2
1
indication of initiation of bond formation.
Scheme 5: The complete catalytic cycle based on DFT
calculations
The calculated energies of the transition states and
intermediates are shown in Table 4, while the energy profile
diagram of the C-S coupling reaction is shown in Figure 5.
Table 4: Energies of the transition states and intermediates
Structure
Energy (kcal)/mol
-208.8286
-345.2708
-630.3728
-357.5347
-554.1640
-1172.4995
-12.0106
CuI
Dabco
PhSH
Iodoanisole
DABCO-Cu-I
DABCI-Cu-SPh
HI
Ts-1
Im-3
Ts-2
Product
-1529.9773
-1530.0603
-1530.0596
-975.8986
Figure 3: Optimized structure of intermediate 3, im-3
4
). Formation of the product and recovery of the catalyst
The energy of all the reactants is arbitrarily fixed as zero. The
complete catalytic cycle is shown below. The addition of 4-
iodoanisole to the copper(I) thiolate complex is found to be the
rate determining step and the activation energy barrier for this
step is found to be 35.6 kcal/mol. From this complex, a copper(I)
From this intermediate the Cu-S bond starts dissociating to
facilitate the product formation and another transition state is
obtained, which is shown in Figure 4 (ts-2).

5
intermediate (im-3) is formed. This is then transformed into a
transition state (ts-2) in which the Cu-S bond starts dissociating.
Finally, the required product is formed from this transition state
and the active catalyst is regenerated.
Conclusion
We have developed a mild and easily accessible method for
the C-S coupling reaction using the mild and easily accessible
CuI/DABCO catalytic system. The excellent functional group
tolerance of this method enabled the synthesis of a number of
sulfides and also the precursor for the drug, Dapsone. To get an
insight into the mechanism of the reaction, we have carried out
DFT studies and found that the possible reaction pathway
involves three intermediates and two transition states.
Acknowledgment
AMT and SA thank the Council of Scientific and Industrial
Research (CSIR, India) for junior research fellowships. GA
thanks the Kerala State Council for Science, Technology, and
Environment
41/2013/KSCSTE dated 15.03.2013) for financial support. We
thank the Institute for Intensive Research in Basic Sciences
IIRBS) of the Mahatma Gandhi University for the NMR facility.
(KSCSTE),
Trivandrum
(Order
no.
3
(
Figure 5: Energy profile diagram of the reaction
References and notes
Manojkumar T K: Formal analysis, data curation
Gopinathan Anilkumar: Suoervision, project
administration, Fund acquisition
Credit Author Statement
Anns
Maria
Thomas:
Conceptualisation,
methodology, investigation
Sherin D R: Software
Sujatha Asha: Methodology
Graphical Abstract
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