Visible-Light-Enabled Construction of Thiocarbamates from Isocyanides, Thiols, and Water at Room Temperature

Article abstract of DOI:10.1021/acs.orglett.8b02231

A convenient visible-light-induced method for the synthesis of thiocarbamates from isocyanides, thiols, and water has been developed under mild reaction conditions. The present protocol offers a cost-effective and operationally straightforward approach to the various thiocarbamates in moderate to good yields by simple use of cheap Rose Bengal as the photocatalyst and water as the reaction reagent as well as an environmentally benign cosolvent.

Full text of DOI:10.1021/acs.orglett.8b02231

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Visible-Light-Enabled Construction of Thiocarbamates from
Isocyanides, Thiols, and Water at Room Temperature
,
†,§
§
,‡
§
§
§
Wei Wei,*
Pengli Bao, Huilan Yue,* Sitong Liu, Leilei Wang, Yudong Li,
and Daoshan Yang†
,§
†State Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of
Science and Technology, Qingdao 266042, P. R. China
‡
Key Laboratory of Tibetan Medicine Research, Northwest Institute of Plateau Biology, Chinese Academy of Sciences and Qinghai
Provincial Key Laboratory of Tibetan Medicine Research, Qinghai 810008, China
§
School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, Shandong, China
*
S Supporting Information
ABSTRACT: A convenient visible-light-induced method for the
synthesis of thiocarbamates from isocyanides, thiols, and water has
been developed under mild reaction conditions. The present
protocol offers a cost-effective and operationally straightforward
approach to the various thiocarbamates in moderate to good
yields by simple use of cheap Rose Bengal as the photocatalyst and
water as the reaction reagent as well as an environmentally benign
cosolvent.
s an important class of organosulfur compounds,
thiocarbamates are ubiquitously found in various
Scheme 1. Methods for the Synthesis of Thiocarbamates
A
pharmaceutical molecules that possess a broad spectrum of
1
2
biological properties such as antiviral, fungicidal, bacter-
3
4
5
6
icidal, anesthetic, herbicidal, and pesticidal activities.
Owing to their important biological activity and pharmaco-
logical value, thiocarbamates have attracted much attention
from chemists in both academic and industrial commun-
7
−17
ities.
Generally, thiocarbamates are prepared by the
reaction of carbamoyl chlorides with thiols in the presence
7
of base (Scheme 1a), the reaction of alkyl chlorothioformate
8
(
RSCOCl) with amines (Scheme 1a), the nucleophilic
9
addition of thiols to isocyanates, and the reactions of amines,
thiols with phosgene derived carbonyldiimidazoles, and di-
tert-butyl dicarbonate. Alternative methods for construction
10
11
of thiocarbamates through direct carbonylation of amines with
carbon monoxide (CO) and RSSR or S /RX have also been
8
12
developed (Scheme 1b). These reactions, although effective,
usually involve the use of toxic phosgene/triphosgene reagents
to prepare the starting materials or hazardous carbon
13
monoxide as a C -reagent. To avoid the use of hazardous
1
reagents, other methods have also been reported. For example,
Lee developed a one-pot synthesis of thiocarbamates from
14
CBZ(Boc)-protected amines and thiophenols. Mandal and
co-workers described Boc-Oxyma mediated synthesis of
thiocarbamates from hydroxamic acids via Lossen rearrange-
most of these methodologies still suffer some limitations, such
as the need for prefunctionalized starting materials, a narrow
substrate scope, relatively harsh or complex reaction
conditions, poor atom economy, or low yields. Therefore,
the development of simple, mild, straightforward, and
1
5
ment. Tang presented the thiocarbamation of imidazopyr-
idines with carbamoyl chloride and elemental sulfur leading to
1
6
imidazopyridines-thiocarbamates. Very recently, Maes’s
group also reported an elegant iodine-catalyzed method for
the synthesis of secondary thiocarbamates from thiosulfonates
Received: July 17, 2018
17
and isocyanides in isopropanol (Scheme 1c). Nevertheless,
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02231
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
environmentally friendly approach to access thiocarbamates is
still greatly desired.
Table 1. Optimization of the Reaction Conditions
In recent years, visible-light-initiated photoredox catalysis
has become a fascinating and practical tool to develop
sustainable synthetic processes under very mild conditions
18,19
due to its economic and environmentally benign features.
Organic dyes have proven to be efficient catalysts in visible-
light-mediated organic transformations and show the advan-
tages of nontoxicity and cheapness compared with metal-based
2
0,21
photoredox catalysts.
With our continued interest in the
22
construction of sulfur-containing compounds, herein, we
wish to report a facile and efficient visible-light-induced
protocol for the direct construction of thiocarbamates from
isocyanides, thiols, and water at room temperature (Scheme
1
d). This photocatalytic protocol, which utilized inexpensive
Rose Bengal as a photocatalyst and water as the reaction
reagent as well as an environmentally benign cosolvent,
provided a new strategy for the synthesis of thiocarbamates.
Initially, the model reaction of 4-methylbenzenethiol (1a)
with ethyl isocyanoacetate (2a) was performed in the presence
b
entry
photocatalyst (mol %)
solvent
yield (%)
1
2
3
4
5
6
7
8
Na -Eosin Y (2)
Eosin B (2)
Rhodamine B (2)
Eosin Y (2)
CH CN/H O (1/1)
62
42
58
43
39
trace
74
46
trace
51
53
55
70
83
65
60
50
69
59
21
2
3
2
of Na -Eosin Y (2 mol %) in CH CN/H O (1/1) at room
2
3
2
CH CN/H O (1/1)
3 2
temperature under air (Table 1, entry 1). The reaction was
conducted under irradiation with 3 W blue LEDs lamp.
Gratifyingly, the desired product 3a was isolated in 62% yield
after 3 h (Table 1, entry 1). Furthermore, a series of
photocatalysts including Eosin B, Rhodamine B, Acridine
Red, Eosin Y, Methylene blue, and Bengal Rose were explored
CH CN/H O (1/1)
3 2
CH CN/H O (1/1)
3 2
Acridine Red (2)
Methylene blue (2)
Rose Bengal (2)
Rose Bengal (2)
Rose Bengal (2)
Rose Bengal (2)
Rose Bengal (2)
Rose Bengal (2)
Rose Bengal (2)
Rose Bengal (2)
Rose Bengal (2)
Rose Bengal (2)
Rose Bengal (2)
Rose Bengal (1)
Rose Bengal (5)
−
CH CN/H O (1/1)
3 2
CH CN/H O (1/1)
3 2
CH CN/H O (1/1)
3 2
DMF/H O (1/1)
2
(
Table 1, entries 1−7). Among the above photocatalysts
9
DMSO/H O (1/1)
2
examined, Bengal Rose was found to be the most effective one
to give the desired product 3a in 74% yield (Table 1, entry 7).
Next, a range of mixed solvents were screened (Table 1, entries
10
11
12
13
14
15
16
17
18
19
20
21
1,4-dioxane/H O (1/1)
2
THF/H O (1/1)
2
DCE/H O (1/1)
2
8
−14), with EtOAc/H O (1:1) being superior for the
CH Cl /H O (1/1)
2
2
2
2
formation of product 3a in 83% yield (Table 1, entry 14). It
is noteworthy that the desired product could also be obtained
in a moderate yield when the reaction was carried out in water
EtOAc/H O (1/1)
2
EtOAc/H O (5/1)
2
EtOAc/H O (1/2)
2
(Table 1, entry 17). Increasing or decreasing the amount of
H O
2
Bengal Rose did not improve the reaction efficiency (Table 1,
entries 18−19). The desired product 3a was only obtained in
EtOAc/H O (1/1)
2
EtOAc/H O (1/1)
2
2
1% yield under visible-light irradiation in the absence of the
EtOAc/H O (1/1)
2
c
photocatalyst (Table 1, entry 20). In contrast, no product was
detected when the reaction was performed in the dark (Table
Rose Bengal (2)
EtOAc/H O (1/1)
0
2
a
Reaction conditions: 1a (0.2 mmol), 2a (0.5 mmol), photocatalyst
(1−5 mol %), solvent (1 mL), 3 W blue LED lamps, air, rt, 3 h.
Isolated yields based on 1. Without light irradiation.
1
, entry 21).
b
c
With the optimized reaction conditions in hand, the
generality of this reaction was further examined using different
thiols and isocyanides (Scheme 2). Generally, aryl thiols with a
wide range of substituents including electron-rich substituents
and electron-poor substituents all generated the desired
products in good yields (3a−3k). Halogen moieties were
well compatible with this reaction leading to the desired
products in satisfied yields (3f−3j), opening the door for
further structural transformation. It was found that the steric
effect has no obvious effect on the reaction efficiency. The
steric encumbered substrates such as ortho-methyl, chloro and
trifluoromethyl substituted aryl thiols still showed good
activities (3c, 3i, and 3k). Naphthalene-2-thiol could be used
in the reaction to produce the corresponding product 3l in
isocyanoacetate, aliphatic isocyanides such as methyl iso-
cyanoacetate, tert-butyl isocyanide, and cyclohexyl isocyanide
in our cases were also well compatible with this reaction
leading to the desired products in good yields (3o−3u). In
addition, aromatic isocyanide such as 2,6-dimethylphenyl
isocyanide could also serve as a suitable substrate in this
reaction system, providing the corresponding product 3v in
68% yield.
In order to elucidate the reaction mechanism, a series of
control experiments were performed. Initially, the present
transformation was completely inhibited when TEMPO
(2,2,6,6-tetramethyl-1-piperidinyloxy, a well-known radical-
capturing species) was added into reaction system under the
standard conditions and a TEMPO-trapped complex (4-
MePhS-TEMPO) was detected by LC-MS analysis (see
Supporting Information (SI)), thus suggesting that this
transformation might involve a radical process (Scheme 3a).
When the model reaction of 1a and 2a was carried out under
7
1% yield. Notably, aliphatic thiols such as 2-phenylethanethiol
and phenylmethanethiol were also well tolerated in the present
reaction system, affording the expected products 3m and 3n in
6
7% and 68% yields, respectively. Nevertheless, when
heterocyclic thiols such as pyridine-4-thiol and thiophene-2-
thiol were investigated in the present reaction system, none of
the desired product was detected. In addition to ethyl
N , only a trace amount of product 3a was detected. This result
2
B
DOI: 10.1021/acs.orglett.8b02231
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Visible-Light-Induced Method for the Synthesis
of Thiocarbamates from Isocyanides, Thiols, and Water
68% yield when the reaction of 1a and 2b was carried out in
a
,
b
18
the presence of EtOAc/H O , indicating that the oxygen atom
2
of thiocarbamate came from water (Scheme 3d).
In addition, to prove an energy transfer process between
Rose Bengal and 4-methylbenzenethiol 1a, fluorescence
quenching (Stern−Volmer) experiments were performed
under visible-light irradiation. As demonstrated in Figures 1
Figure 1. Quenching of Rose Bengal fluorescence emission in the
presence of 1a.
a
Reaction conditions: 1 (0.2 mmol), 2 (0.5 mmol), Rose Bengal (2
Figure 2. Stern−Volmer plots. I is the inherent fluorescence intensity
mol %), EtOAc/H O (1/1) (1 mL), 3 W blue LEDs, air, rt, 3 h.
Isolated yields based on 1. 1a (2 mmol), 2a (5 mmol), 6 h.
0
2
b
c
of Rose Bengal. I is the fluorescence intensity of Rose Bengal in the
presence of 1a.
Scheme 3. Control Experiments
and 2, the emission intensity of excited Rose Bengal was
gradually diminished with increasing concentration of 1a. In
contrast, when Rose Bengal was separately mixed with ethyl
isocyanoacetate (2a), such a phenomenon was not observed
(
see SI). The above results suggest that an electron transfer
reaction actually occurred between Rose Bengal and thiol
under visible-light irradiation.
On the basis of the above experimental results and previous
17,21−26
reports,
a plausible mechanism for this reaction was
proposed as shown in Scheme 4. Initially, the excited state RB*
was produced from Rose Bengal under visible-light irradi-
21
ation. A single-electron transfer (SET) from the excited state
•
−
of RB* to thiol 1 gave the radical cation 4 and RB radical
anion. The oxidation of RB• by oxygen (air) would lead to
−
the formation of the ground state Rose Bengal and O2•
−
.
22c
Subsequently, the deprotonation of the radical cation 4 by
O2 produced the thiyl radical 5 and hydroperoxide radical
•
−
indicated that air (O ) is essential for the present trans-
2
formation (Scheme 3b). Furthermore, only a trace amount of
product 3r was detected when the reaction of 1a with 2b was
conducted in dry EtOAc (Scheme 3c). Also, 18O-labeling
experiment was performed to determine the source of oxygen
species. The disproportionation of the hydroperoxide radical
24
would generate O and H O . Next, the addition of thiyl
2
2
2
25c
25
radical 5 to isocyanide 2 gave C-radical intermediate 6,
which was oxidized by the excited state RB* through a single-
electron transfer (SET) process leading to nitrilium
18
in thiocarbamates. The corresponding O -3r was isolated in
C
DOI: 10.1021/acs.orglett.8b02231
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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(
In summary, we have successfully developed a visible-light-
promoted protocol for the synthesis of thiocarbamates from
isocyanides, thiols, and water at room temperature. The
present protocol, which utilizes a simple and cheap photo-
catalyst, readily available starting materials, and environ-
mentally benign conditions, provides an attractive approach
to various thiocarbamates in moderate to good yields. Further
investigation of the detailed reaction mechanism and synthetic
application are currently being pursued in our laboratory.
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ASSOCIATED CONTENT
Supporting Information
■
*
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02231.
(
Experimental details and compound characterization
(PDF)
(18) Selective examples: (a) Zou, Y.-Q.; Chen, J.-R.; Liu, X.-P.; Lu,
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AUTHOR INFORMATION
Corresponding Authors
■
2
2
012, 51, 784. (b) Ravelli, D.; Fagnoni, M.; Albini, A. Chem. Soc. Rev.
013, 42, 97. (c) Keshari, T.; Yadav, V. K.; Srivastava, V. P.; Yadav, L.
*
E-mail: weiweiqfnu@163.com.
E-mail: hlyue@nwipb.cas.cn.
ORCID
D. S. Green Chem. 2014, 16, 3986. (d) Ortgies, S.; Depken, C.;
Breder, A. Org. Lett. 2016, 18, 2856. (e) Wei, W.; Cui, H.; Yang, D.;
Yue, H.; He, C.; Zhang, Y.; Wang, H. Green Chem. 2017, 19, 5608.
*
(f) Yang, S.; Li, P.; Wang, Z.; Wang, L. Org. Lett. 2017, 19, 3386.
Wei Wei: 0000-0002-0015-7636
Daoshan Yang: 0000-0002-3047-5416
Notes
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W. C. Chem. Rev. 2013, 113, 5322. (b) Skubi, K. L.; Blum, T. R.;
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The authors declare no competing financial interest.
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̈ ̈
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This work was supported by the Natural Science Foundation
of Shandong Province (ZR2018MB009 and ZR2016JL012),
the International Cooperation Project of Qinghai Province
9994.
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́
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DOI: 10.1021/acs.orglett.8b02231
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