Copper-Catalyzed Thiolation of Terminal Alkynes Employing Thiocyanate as the Sulfur Source Leading to Enaminone-Based Alkynyl Sulfides under Ambient Conditions

Article abstract of DOI:10.1021/acs.orglett.0c02308

A highly efficient protocol for copper-catalyzed thio-alkynylation of enaminone-based thiocyanates with terminal alkynes under mild conditions has been developed. This scalable amino group-directed thio-alkynylation proceeds in the open air with a broad substrate scope and an excellent yield. The demonstrated synthetic transformation creates the opportunity for a wide variety of sulfur-containing useful materials. Gram-scale synthesis and further synthetic transformations of alkynyl sulfides highlight the potential utility of the method.

Full text of DOI:10.1021/acs.orglett.0c02308

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Letter
Copper-Catalyzed Thiolation of Terminal Alkynes Employing
Thiocyanate as the Sulfur Source Leading to Enaminone-Based
Alkynyl Sulfides under Ambient Conditions
Chandran R., Ashwini Pise, Suraj Kumar Shah, Rahul D., Suman V., and Keshri Nath Tiwari*
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ABSTRACT: A highly efficient protocol for copper-catalyzed thio-
alkynylation of enaminone-based thiocyanates with terminal alkynes
under mild conditions has been developed. This scalable amino
group-directed thio-alkynylation proceeds in the open air with a
broad substrate scope and an excellent yield. The demonstrated
synthetic transformation creates the opportunity for a wide variety of
sulfur-containing useful materials. Gram-scale synthesis and further
synthetic transformations of alkynyl sulfides highlight the potential
utility of the method.
of the literature methods employ either sensitive or harsh
lkynes represent a privileged structural motif in synthetic
Aand medicinal chemistry because of their enhanced utility experimental conditions, thus limiting the scope of thio-
in chemical biology and material science.1 The unique
alkynylation.
To develop an efficient protocol for alkynyl sulfides, we
focused our attention on the coupling of terminal alkynes with
thiocyanates as the sulfur source utilizing the leaving group
ability of CN under ambient reaction conditions (Figure 1F).
Organic thiocyanates are considered as highly valuable
building blocks leading to functional materials and pharma-
ceuticals.7 As a result, several elegant methods for the
thiocyanation of C(sp2)−H8 and C(sp3)−H9 bonds have
been studied extensively. In general, the inorganic thiocyanates
such as KSCN, NaSCN, and NH4SCN are primarily employed
to construct a C−SCN bond via thiocyanation.10 In addition,
enaminones play a key role as synthetic intermediates in many
conversions leading to the potential pharmaceutical candi-
dates11 and fused heterocyclic compounds.12 Thiocyanated
enaminones, preferably, offer tremendous synthetic potential,
and hence, functionalization of the C(sp2)−H bond of
enaminone to the C(sp2)−S bond has been successfully
developed.13 We became interested in exploring the synthetic
ability of underutilized thiocyanated enaminones based on our
experience with enaminone-based multicomponent reac-
tions.14 Herein, we report the α-thiocyanation of enaminones
and consequential copper-catalyzed C(sp)−S bond formation
reactivity of the C(sp)−C(sp) bond makes them valuable
precursors in the generation of carbo- and heterocyclic
scaffolds ubiquitous in several complex natural products.2
Among them, the alkynes directly bonded with an atom like N
or S are of particular interest due to their ability to influence
the chemical transformations. As a consequence, nitrogen-
attached alkynes like ynamines and ynamides have been
explored in modern synthetic chemistry.3 Conversely, the
synthetic potential of analogous S-alkynes4 remains untapped
despite the omnipresence of organosulfur compounds.5 This
has garnered considerable interest from researchers across the
world in recent times for the development of novel and
efficient synthetic strategies for alkynyl sulfides.
The classical strategies for constructing the C(sp)−S bond
are mainly focused on highly reactive acetylide intermediates
with prefunctionalized thiols or activated sulfur-containing
leaving groups (Figure 1).6 Recently developed routes for
alkynyl sulfides are Cu(I)-catalyzed coupling of terminal
alkynes with thiols in the presence of molecular oxygen
(Figure 1A)6a and electrophilic alkynylation using alkynyl
iodonium salt [ethynylbenziodoxolone (EBX)] as the alkyne
transfer reagents (Figure 1B).6b,c Selected metal-free or metal-
catalyzed coupling strategies for thio-alkynylation with sulfur-
containing leaving group include thiosulfonates as the sulfur
source with terminal alkynes using CuI as the catalyst and t-
BuOLi as the base (Figure 1C),6d the CuI/Xantphos catalytic
system for thiolation with thiosulfonates (Figure 1D),6e and
transition metal-free t-BuOK-mediated coupling of terminal
aryl alkynes with thiocyanates (Figure 1E).6f Practically, most
Received: July 13, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02308
© XXXX American Chemical Society
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yield of desired product 3a, while lower yields were observed
with solvents like DCE, DCM, and CH3CN.
The evaluation of other oxidants like oxone and K2S2O8 led
to desired product 3a in lower yields. With the optimal
reaction conditions in hand, a series of enaminones (2a−2f)
were successfully transformed to thiocyanated enaminones
3a−3f in good to excellent yields (78−82%) (Figure 2).
Interestingly, the thiocyanation of enaminone with a secondary
amine under optimal conditions proceeded to yield 2-
iminothiazole 3g, which is in accord with literature precedent
(see the Supporting Information).15 The attempted thiocya-
nation of enaminone with tertiary amine also failed to provide
any expected product.
With thiocyanated enaminones in hand, we then attempted
the thio-alkynylation of derivative 3a with phenylacetylene 4a
using 20 mol % CuCl and Cs2CO3 in CH3CN at room
temperature. Unfortunately, no desired product was observed
even after reaction for 3 h (Table 1, entry 1). However, the use
a
Table 1. Optimization of the Reaction Conditions
b
entry catalyst (mol %)
base
solvent
time (h) yield (%)
Figure 1. Synthetic strategies for (A−E) S-alkynylation and (F) our
c
1
2
3
4
5
6
7
8
CuCl (20)
Cu(OAc)2 (20)
CuSO4 (20)
CuBr (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
EtOH
MeOH
iPrOH
DMSO
DMF
3
1
3
2
1
24
24
24
0.5
0.5
24
0.5
24
np
65
approach.
45
60
87
c
of thiocyanate with terminal alkynes under ambient conditions
(Figure 1E). The developed protocol demonstrates an
expanded substrate scope, and mechanistic studies reveal the
directive role of the amino group in this thio-alkynylation.
We first investigated the regioselective α-thiocyanation of
enaminone 2a with NH4SCN and N-bromosuccinimide
(NBS), which successfully provided 3a in 81% yield (Figure
2). Enaminone 2a was synthesized from commercially available
1,3-cyclohexanedione 1a. The preliminary screening of
solvents indicated that EtOH could provide a maximum
np
np
c
43
62
67
62
68
54
38
30
72
56
44
55
50
9
10
11
12
13
14
15
16
17
18
19
20
dioxane
acetone
DCE
DCM
24
24
2
2
3
toluene
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Na2CO3
t-BuOK
DBU
0.5
2
DABCO
a
Reaction conditions (unless otherwise specified): 3a (0.59 mmol),
4a (0.89 mmol), catalyst (0.11 mmol), base (0.59 mmol), solvent (5
b
c
mL). Isolated yields. No product.
of 20 mol % Cu(OAc)2 as a catalyst led to desired alkynyl
sulfide 5a in 65% yield (Table 1, entry 2). With similar
reaction conditions, expected product 5a was isolated in 45%
and 60% yields with 20 mol % CuSO4 and CuBr, respectively
(Table 1, entries 3 and 4, respectively). To our delight, product
5a was observed in 87% yield with 20 mol % CuI in CH3CN
after reaction for 1 h (Table 1, entry 5). Several silver salts like
Ag2CO3, AgCl, AgNO3, and AgOCOCF3 failed to yield the
desired product. For the solvent, thio-alkynylation with CuI in
EtOH and MeOH did not yield any product (Table 1, entries
6 and 7, respectively); however, expected product 5a was
obtained in 43% yield in iPrOH (Table 1, entry 8). Alkynyl
Figure 2. α-Thiocyanation of enaminones. Enaminone (4.50 mmol),
NH4SCN (9.00 mmol), NBS (6.75 mmol), and EtOH (20 mL).
Yields refer to isolated yields.
B
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sulfide 5a was obtained with average yields ranging from 62%
to 68% in polar solvents like DMSO, DMF, dioxane, and
acetone (Table 1, entries 9−12, respectively) and with
decreased yields in DCM, DCE, and toluene of 54%, 38%,
and 30%, respectively (Table 1, entries 13−15, respectively).
Application of inorganic bases like K2CO3, Na2CO3, and t-
BuOK and organic bases like DABCO and DBU resulted in
lower conversions (Table 1, entries 16−20, respectively).
With the optimized reaction conditions, the substrate scope
for this thio-alkynylation was then explored. Both aryl and
aliphatic alkyne partners were found to be favorable substrates
with thiocyanated enaminones affording efficient creation of
alkynyl sulfides (Figure 3). Six-membered thiocyanated
Figure 4. Scope of thiocyanates with terminal alkyne. Conditions: 3
(0.59 mmol), alkyne 4 (0.89 mmol), CuI (0.11 mmol), Cs2CO3 (0.59
mmol), and CH3CN (4.00 mL).
a
Scheme 1. Reaction of Thiocyanates without CuI
Figure 3. Scope of thiocyanates with terminal alkyne. Conditions: 3
(0.59 mmol), alkyne 4 (0.89 mmol), CuI (0.11 mmol), Cs2CO3 (0.59
mmol) and CH3CN (4.00 mL).
a
enaminones 3a−3d were readily amenable to subjected
conditions delivering S-alkynylated products 5a−5m in high
to excellent yields (78−87%). For alkynes, thiolations with
phenylacetylene 4a, 1-ethynylcyclohex-1-ne 4b, 1-heptyne 4c,
1-hexyne 4d, and 1-pentyne 4e proceeded smoothly to furnish
alkynyl sulfides 5a−5e, respectively, in excellent yields (85−
88%). The 5,5-dimethyl- substituted 3c readily reacted with
alkyne 4a and 4b yielding 5f and 5g in 80% and 85% yields,
respectively. Similarly, 5-methyl derivative 3b and 4,4-dimethyl
derivative 3d underwent smooth reaction with alkyne 4a, 4b,
and 4c to provide 5h−5m in very high yields (78−89%). In
addition, to extend the scope of the reaction, the thiolation was
tested with 3-amino-2-thiocyanatocyclopent-2-enone 3e with
alkynes like phenylacetylene 4a, 1-ethynylcyclohex-1-ne 4b, 1-
heptyne 4c, 1-hexyne 4d, and 1-pentyne 4e, which afforded
alkynyl sulfides 5n−5r, respectively, in good to excellent yields
(75−80%) (Figure 4). The alkynyl sulfides 5s−5y of the
acyclic analogue ethyl 3-amino-2-thiocyanatobut-2-enoate 3f
were also prepared with various alkynes 4a−4f in high yields
(77−91%) (Figure 4).
Reaction conditions: 3a (0.59 mmol), Cs2CO3 (0.59 mmol), and
CH3CN (5.0 mL).
compound 3f was also converted to ethyl 2-aminomethylth-
iazole-5-carboxylate 6b in quantitative yield with standard
reaction conditions. This cyclization could happen only in the
absence of copper, which otherwise generates acetylide with
terminal alkyne leading to thio-alkynylation. Products 6a and
6b appear to be a medicinally relevant scaffold that could be
readily utilized as a potential drug precursor. These valuable
intermediates and their derivatives could easily be obtained in
high yields with the current approach.
To probe the reaction mechanism of thio-alkynylation,
several control experiments were conducted (Scheme 2. First,
the reaction of thiocyanate 3a was performed with internal
alkyne 1,4-diphenylbuta-1,3-diyne and diethyl acetylenedicar-
boxylate (Scheme 2G), which resulted in no reaction
indicating the role of acetylide with terminal alkyne during
product formation. To investigate the directive effect of the
NH2 group in alkynylation, 2-thiocyanatocyclohexane-1,3-
dione 7 was treated with phenylacetylene 4a under standard
conditions, which also failed to provide any expected product
(Scheme 2H). Similarly, the reaction of 4-thiocyanatoaniline 8
with phenylacetylene 4a remained unfruitful (Scheme 2I). The
failure of these substrates to undergo reaction with terminal
To examine the role of copper in this thio-alkynylation, an
experiment was conducted with thiocyanated enaminone 3a
and alkyne 4a under standard conditions without CuI as a
catalyst. Interestingly, 2-amino-5,6-dihydrobenzo[d]thiazol-
7(4H)-one 6a was obtained in quantitative yield after reaction
for 1.5 h at room temperature (Scheme 1). Similarly,
C
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Scheme 2. Control Experiments
Figure 5. Plausible mechanism.
directive coordination of acetylide I with an amino group of 3a
subsequently follows σ-bond metathesis via SN2 type attack on
thiocyanate in structure II knocking out CN to yield product
5a. Without a copper catalyst, the amino group of 3a
nucleophilically attacks SCN in intramolecular fashion leading
to formation of product 6a apparently.
To evaluate the efficacy of the reaction, a gram-scale
synthesis of 5a was conducted with 1.0 g (5.00 mmol) of
thiocyanate 3a and phenylacetylene 4a furnishing 1.15 g of
thioalkyne 5a with 80% yield (Scheme 4a). Advantageously,
alkyne underlines the possible directive role of NH2 in copper-
catalyzed thio-alkynylation. To further understand this thio-
alkynylation, we tested the reaction of 2-amino-5,6-
dihydrobenzo[d]thiazol-7(4H)-one 6a with phenylacetylene
4a, which again failed to provide any product (Scheme 2J).
Finally, we set out to perform thio-alkynylation with 2-
thiocyanatoaniline to assess the directive effect. However,
during the thiocyanation of 3,4-dimethoxyaniline 9 with
bromine and KSCN, 5,6-dimethoxybenzo[d]thiazol-2-amine
10 was isolated rather than the expected product. In addition,
the treatment of compound 10 with 1-heptyne 4c provided
unexpected dialkynylated product 11 in 55% yield after
reaction for 1.5 h (Scheme 3). The use of even equimolar
Scheme 4. Gram-Scale Synthesis and Further
Transformations
a
Scheme 3. Alkynylation of Benzo[d]thiazol-2-amine 10
the unpleasant odor generally observed as part of the thiolation
reaction is not realized with the current protocol. In addition,
few synthetic transformations of alkynyl sulfides were also
performed (Scheme 4b). The treatment of compound 5a with
1.0 equiv of AgNO3 provided nitro compound 12 in 65% yield
by annulative nitration.16 Similarly, substrate 5a was easily
oxidized to corresponding sulfones 13 in 60% yield upon
treatment with m-CPBA at 0 °C for 1 h. These simple
transformations outline the versatility and usefulness of the
methods reported herein.
In summary, we have successfully developed α-thiocyanation
of enaminone and consequential copper-catalyzed alkynylation
using thiocyanates with terminal alkynes. The method utilizes
ambient reaction conditions with a wide substrate scope and
gives ready access to biologically relevant sulfur-containing
privileged scaffolds that are useful for developing new drug
candidates and materials. Further exploration of studies for the
synthesis of sulfur-containing heterocyclic scaffolds and
detailed mechanistic studies of reactions are currently
underway.
a
Reaction conditions: 9 (3.26 mmol), Br2 (3.26 mmol), and KSCN
(6.53 mmol); 10 (0.47 mmol), 4c (0.71 mmol), CuI (0.11 mmol),
Cs2CO3 (0.47 mmol), and CH3CN (5.0 mL).
alkyne also led to the same product. To the best of our
knowledge, this kind of dialkynylation has not been
documented and needs to be subjected to further mechanistic
studies. The fact that alkynylation was observed with only
benzo[d]thiazol-2-amine 10 and not with 2-amino-5,6-
dihydrobenzo[d]thiazol-7(4H)-one 6a points to the nature of
the difference in reactivity between the two substrates.
We propose a speculative reaction mechanism for the
formation of alkynyl sulfide based on the outcome of reactions
and control experiments (Figure 5). Two different reactions
could be operative on the basis of the presence or absence of
copper salt in the reaction. In the presence of copper, the
reaction likely proceeds via the initial generation of copper
acetylide I as indicated in the control experiment. Then, the
D
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(5) For selected reviews of bioactive sulfur-containing compounds,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02308.
■
́
see: (a) Pluta, K.; Morak-Młodawska, B.; Jelen, M. Eur. J. Med. Chem.
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Experimental procedures, analytical data, and NMR
spectra of products (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Keshri Nath Tiwari − National Institute of Pharmaceutical
Education and Research (NIPER), Raebareli, Lucknow
226002, India; orcid.org/0000-0001-5079-9453;
Email: keshri.tiwari@niperraebareli.edu.in
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2018, 20, 5464−5468.
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Angew. Chem., Int. Ed. 2015, 54, 5753−5756. (b) Guo, L.-N.; Gu, Y.-
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9312−9320.
Chandran R. − National Institute of Pharmaceutical Education
and Research (NIPER), Raebareli, Lucknow 226002, India;
orcid.org/0000-0003-2289-532X
Ashwini Pise − National Institute of Pharmaceutical Education
and Research (NIPER), Raebareli, Lucknow 226002, India
Suraj Kumar Shah − National Institute of Pharmaceutical
Education and Research (NIPER), Raebareli, Lucknow
226002, India
Rahul D. − National Institute of Pharmaceutical Education and
Research (NIPER), Raebareli, Lucknow 226002, India
Suman V. − National Institute of Pharmaceutical Education and
Research (NIPER), Raebareli, Lucknow 226002, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02308
(11) (a) Heinbockel, T.; Wang, Z. J.; Jackson-Ayotunde, P. L.
Pharmaceuticals 2014, 7, 1069−1090. (b) Edafiogho, I. O.;
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Kombian, S. B. Eur. J. Med. Chem. 2014, 76, 20−30.
Notes
The authors declare no competing financial interest.
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Chem., Int. Ed. 2009, 48, 8078−8081. (b) Zoller, J.; Fabry, D. C.;
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(b) Gao, Y.; Liu, Y.; Wan, J.-P. J. Org. Chem. 2019, 84, 2243−2251.
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ACKNOWLEDGMENTS
■
The authors gratefully acknowledge Dr. S. J. S. Flora, Director,
NIPER, Raebareli, for his support of this research program.
C.R., A.P., S.K.S., R.D., and S.V. thank the Department of
Pharmaceuticals, Ministry of Chemicals and Fertilizers,
Government of India, for providing fellowships. The authors
are thankful to Central Instrumentation Facility (CIF),
NIPER, Raebareli, for spectral data. NIPER, Raebareli,
communication NIPER-R/Communication/117.
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