Metal-free difunctionalization of alkynes leading to alkenyl dithiocyanates and alkenyl diselenocyanates at room temperature

Article abstract of DOI:10.1039/c8ob02368a

A simple and practical method for the synthesis of alkenyl dithiocyanates and alkenyl diselenocyanates has been developed via stereoselective difunctionalization of alkynes with NaSCN or KSeCN at room temperature. Through this methodology, a series of alkenyl dithiocyanates and alkenyl diselenocyanates could be efficiently and conveniently obtained in moderate to good yields under mild and metal-free conditions by the simple use of oxone and PhI(OAc)₂ as the oxidants.

Full text of DOI:10.1039/c8ob02368a

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Biomolecular Chemistry
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Metal-free difunctionalization of alkynes leading
to alkenyl dithiocyanates and alkenyl
diselenocyanates at room temperature†
Cite this: Org. Biomol. Chem., 2018,
16, 9064
Ling-Hui Lu,a Si-Jia Zhou,a Wei-Bao He,a Wen Xia,b Ping Chen,b Xianyong Yu,b
a,b
Xinhua Xu *a and Wei-Min He
*
A simple and practical method for the synthesis of alkenyl dithiocyanates and alkenyl diselenocyanates
has been developed via stereoselective difunctionalization of alkynes with NaSCN or KSeCN at room
temperature. Through this methodology, a series of alkenyl dithiocyanates and alkenyl diselenocyanates
could be efficiently and conveniently obtained in moderate to good yields under mild and metal-free
conditions by the simple use of oxone and PhI(OAc)2 as the oxidants.
Received 23rd September 2018,
Accepted 13th November 2018
DOI: 10.1039/c8ob02368a
rsc.li/obc
through iodothiocyanation of allenes with KSCN and mole-
Introduction
cular iodine.10d In 2017, Sridhar Reddy et al. described the
As a key structural motif, the thiocyanato group is widely addition reaction of electron-deficient alkynes with KSCN in
present in a variety of natural products, biologically active com- acetic acid for the synthesis of alkenyl thiocyanates.10e Very
pounds and synthetic intermediates.1 Thus, the introduction recently, our group also reported ultrasound-promoted Brønsted
of the thiocyanato group into organic molecules has drawn acid ionic liquid catalyzed hydrothiocyanation of activated
much attention from chemists in terms of their important bio- alkynes with KSCN to access Z-alkenyl thiocyanates.10f However,
logical properties2 and widespread synthetic applications for all of these methods are limited to the hydrothiocyanation of
the construction of diverse valuable sulfur-containing com- alkynes leading to mono-alkenyl thiocyanates. To the best of
pounds such as disulfides,3 thioethers,4 thiocarbamates5 and our knowledge, only one example has been reported on the con-
sulfonylcyanides.6 Although many methods have been estab- struction of alkenyl dithiocyanates from alkynes, (dichloro-
lished to synthesize alkyl7 and aryl thiocyanates,8 the construc- iodo)benzene and lead(II) thiocyanate through an iodirenium
tion of alkenyl thiocyanates has not been fully developed, in ion intermediate11 (Scheme 1a). Obviously, the utilization of
spite of their versatile transformation abilities in organic syn- stoichiometric amounts of lead(II) reagent would increase the
thesis. Generally, alkenyl thiocyanates are prepared through risk for traces of toxic metals in the products and limit their
the nucleophilic substitution of vinyl halides with thiocyanate wide applications in the field of synthetic and pharmaceutical
salts.9 Other elegant methods for the synthesis of alkenyl thio- chemistry. Therefore, the development of a facile, efficient,
cyanates have also been developed.10 Among them, the
functionalization of alkynes with [SCN] reagents represents
one of the most powerful and straightforward methods for the
construction of alkenyl thiocyanates due to their high atom
economy and reaction efficiency. For example, Jiang and co-
workers presented silver catalyzed hydrothiocyanation of
haloalkynes with KSCN in AcOH at 100 °C leading to Z-alkenyl
thiocyanates.10c In the same year, Yan and coworkers devel-
oped a facile and mild protocol to obtain allyl isothiocyanates
aState Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University,
Changsha410082, China. E-mail: weiminhe2016@yeah.net
bKey Laboratory of Theoretical Organic Chemistry and Functional Molecule of
Ministry of Education, Hunan University of Science and Technology,
Xiangtan 411201, China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 Methods for the synthesis of alkenyl dithiocyanates and
c8ob02368a
alkenyl diselenocyanates.
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practical and eco-friendly protocol for the construction of various oxidants including Na2S2O8, (NH4)2S2O8, tert-butyl
alkenyl dithiocyanates is still in high demand. hydroperoxide (TBHP), di-tert-butyl peroxide (DTBP), PhI
With our continued interest in green synthesis,12 we have (OAc)2, oxone, H2O2, tert-butyl peroxybenzoate (TBPB), benzoyl
reported the eco-friendly functionalization of alkynes.13 In this peroxide (BPO), dicumyl peroxide (DCP), PhICl2 and dioxygen
paper, we wish to report a facile, efficient and metal-free were explored by using NaSCN as the [SCN] source (entries
method for the construction of various alkenyl dithiocyanates 4–15). Among various oxidants examined, oxone turned out to
via stereoselective difunctionalization of alkynes14 with NaSCN be the best choice in terms of high reaction efficiency and low-
at room temperature by simply using cheap oxone as the cost (entry 9). No thiocyanation reaction was observed when an
oxidant (Scheme 1b). In contrast to the known PhICl2/Pd acid promoter was used instead of Oxone (entries 16 and 17).
(SCN)2 mediated dithiocyanation of alkynes, the present reac- Further optimization of solvents suggested that DCM was
tion proceeds through an alternative radical process. the best reaction medium. When the reaction was performed
Moreover, this protocol could also be expanded to construct in DCE, PhCl or toluene, the desired product 2a was also
the corresponding alkenyl diselenocyanates through PhI(AcO)2 obtained in moderate to good yields (entries 18–20). In con-
mediated 1,2-diselenocayanation of alkynes with KSeCN in trast, the reaction did not occur in CH3CN, DMF, DMSO, H2O,
water (Scheme 1b).
EtOAc, or THF (entries 21–26). The reaction efficiency was not
obviously improved by increasing the amount of oxidant or
reaction temperature (entries 27–29). In addition, treatment of
1a with 1.1 equiv. of KSCN could not deliver any monothio-
cyanate product (entry 30). After an extensive screening of the
reaction parameters, the best yield of product 2a (80%) was
obtained by employing NaSCN (2.2 equiv.) and oxone
(1.1 equiv.) in DCM at room temperature.
Upon optimization of reaction conditions (Table 1, entry 9),
the substrate scope for the 1,2-dithiocyanation of alkynes with
KSCN was surveyed. As shown in Table 2, this difunctionaliza-
tion reaction proceeded with excellent stereoselectivity and
alkenyl dithiocyanates could be effectively obtained in moder-
ate to good yields. A series of phenylacetylene derivatives
bearing electron donating groups (Me and OMe) and electron
withdrawing groups (F, Cl, and Br) produced the corres-
Results and discussion
Initially, the reactions of phenylacetylene (1a) with [SCN]
sources such as KSCN, NaSCN and NH4SCN were investigated
in the presence of K2S2O8 (1.1 equiv.) in DCM at room tem-
perature under air (Table 1, entries 1–3). Among these tested
[SCN] sources, NaSCN was found to be the most effective one
to produce the product 2a in 76% yield (entry 2).15 Next,
Table 1 Optimization of reaction conditionsa
Table 2 Scope of alkenyl dithiocyanatesa,b
Entry
[SCN] (X equiv.)
Oxidant (equiv.)
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
KSCN (2.2)
K2S2O8 (1.1)
K2S2O8 (1.1)
K2S2O8 (1.1)
Na2S2O8 (1.1)
(NH4)2S2O8 (1.1)
TBHP (1.1)
DTBP (1.1)
PhI(OAc)2 (1.1)
Oxone (1.1)
H2O2 (1.1)
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCE
31
76
22
Trace
26
ND
ND
33
NaSCN (2.2)
NH4SCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
NaSCN (2.2)
9
80
10
11
12
13
14
15
16
17
18
19
20
21
22
23
ND
ND
ND
ND
31
ND
ND
ND
32
54
71
ND
ND
ND
TBPB (1.1)
BPO (1.1)
DCP (1.1)
PhICl2 (1.1)
O2 balloon
HOAc (1.1)
BAIL (1.1)
Oxone (1.1)
Oxone (1.1)
Oxone (1.1)
Oxone (1.1)
Oxone (1.1)
Oxone (1.1)
PhCl
Toluene
CH3CN
DMF
DMSO
a Conditions: 1a (0.2 mmol), [SCN] source, oxidant, solvent (1 mL), air,
rt, 2 h. b Isolated yields based on 1a. BAIL: 1-(3 sulfopropyl)pyridin- a Reaction conditions: 1 (0.2 mmol), NaSCN (0.44 mmol), oxone
1-ium hydrosulfate.
(0.22 mmol), DCM (1 mL), rt, 2 h. b Isolated yields based on 1.
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ponding products in good yields (2b–2h). However, when
1-ethynyl-4-(trifluoromethyl)benzene was used as the substrate,
only a trace amount of the dithiocyanation product was
observed. Heterocycle alkynes such as 3-ethynylthiophene
would participate in the reaction to give the corresponding
product 2i in 86% yield. Only a trace amount of (E)-3-(1,2-
dithiocyanatovinyl)pyridine was observed when 3-ethynylpyri-
dine was subjected under the standard conditions. Various
alkyl alkynes with hexatomic and triatomic rings as well as
with long aliphatic chains were also effectively reacted with
NaSCN to achieve products (2j–2p) in good yields. Notably, a
series of functional groups such –Br, –Cl, –OH, and –CO2H
groups could well be compatible with the reactions, thereby
facilitating possible further modifications. Moreover, the
corresponding alkenyl dithiocyanates (2q–2s) could also be
produced in good yields when internal aromatic and aliphatic
alkynes were used in the present reaction system.
Scheme 2 Control experiments.
Organic selenocyanates are also versatile building blocks
for the construction of structurally diverse seleno-containing
compounds known to possess potent biological activities.16
Subsequently, we turned our attention to explore the reaction
of 1,2-diselenocyanation of alkynes. After an extensive screen-
ing of the reaction parameters for the model reaction between
4-methylphenylacetylene 1a and KSeCN (see ESI, Table S1†),
the highest yield (81%) of the desired alkenyl diselenocyanate
3a was obtained when the reaction was carried out using
PhI(OAc)2 (1.1 equiv.) as the oxidant in H2O at room tempera-
ture. Upon optimization of reaction conditions, the substrate
scope for this transformation was surveyed. As shown in
Table 3, the reaction could proceed well by using diverse
aromatic alkynes with an electron-donating group or an
electron-withdrawing group on the aromatic ring so as to form
the corresponding products in moderate to good yields.
Heterocyclic alkynes such as 3-ethynylthiophene and internal
alkynes such as prop-1-ynylbenzene were also suitable for this
reaction, with the desired products 3i and 3j obtained in 78%
and 83% yields, respectively.
Scheme 3 Proposed mechanism.
might also proceed through a radical process. As shown in
Schemes 2a and b, when TEMPO (2,2,6,6-tetramethyl-1-piperi-
dinyloxy) and BHT (2,6-di-tert-butyl-4-methylphenol), well-
known radical-capturing species, were added into the reaction
system under the standard conditions, this reaction was sig-
nificantly inhibited. When (1-cyclopropylvinyl)benzene (4a)
was used as a radical-clock substrate, the ring-opened product
(5a) was obtained (Scheme 2c). The above results suggested
that the present difunctionalization of alkynes might involve a
radical process.
It is known that SCN radicals can be easily generated from
the [SCN] source such NaSCN and KSCN in the presence of
an oxidant,17 which indicates that the present transformation
Based on the above results and previous reports,17,18 a poss-
ible reaction pathway of the radical difunctionalization of
alkynes is proposed in Scheme 3. Initially, the SCN radical17
A
or the SeCN radical C was generated from NaSCN or KSeCN in
the presence of an oxidant. Subsequently, the anti-
Markovnikov addition of the SCN radical A or the SeCN radical
C to alkyne 1 produced the alkenyl radical B or D. Finally, the
SCN radical A or the SeCN radical C attacked more favorably
the sterically less hindered side of the alkenyl radical B or D
which would lead to the formation of desired products 2
and 3, respectively.
Table 3 Scope of alkenyl diselenocyanatesa,b
Conclusions
In conclusion, we have successfully developed a facile and
practical method for the synthesis of various alkenyl dithio-
cyanates and alkenyl diselenocyanates via stereoselective
a Reaction conditions: 1 (0.2 mmol), KSeCN (0.44 mmol), oxone
(0.22 mmol), DCM (1 mL), rt, 2 h. b Isolated yields based on 1.
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1,2-dithio/selenocyanation of alkynes with inexpensive NaSCN/
KSeCN at room temperature. The present reaction features a
relatively broad substrate scope, excellent functional group
compatibilities, mild conditions, and cheap and less-toxic oxi-
dants, and offers various alkenyl dithiocyanates and alkenyl
diselenocyanates in moderate to good yields. Preliminary
mechanistic studies revealed that the reaction might involve a
radical process. Further studies of the detailed reaction mecha-
nism and the synthetic applications are currently ongoing in
our laboratory.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We are grateful for the financial support from the National
Natural Science Foundation of China (No. 21545010).
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