Water-promoted regioselective hydrothiolation of alkynes

Article abstract of DOI:10.1139/V09-130

Water promotes hydrothiolation of unactivated alkynes efficiently without any catalyst or additive. The reaction at room temperature furnishes vinyl sulfides with high regioselectivity via anti-Markovnikov addition. The terminal al- kynes provide dithiola

Full text of DOI:10.1139/V09-130

1605
Water-promoted regioselective hydrothiolation of
alkynes
Sukalyan Bhadra and Brindaban C. Ranu
Abstract: Water promotes hydrothiolation of unactivated alkynes efficiently without any catalyst or additive. The reaction
at room temperature furnishes vinyl sulfides with high regioselectivity via anti-Markovnikov addition. The terminal al-
kynes provide dithiolanes at 80 8C by bis-addition. The reactions are very clean and high yielding.
Key words: alkynes, hydrothiolation, vinyl sulfides, regioselectivity, water.
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Resume : L’eau favorise l’hydrothiolation des alcynes non actives d’une fac¸on efficace, sans catalyseur ou additif. A la
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temperature ambiante, la reaction conduit a la formation de sulfures de vinyle, avec une regioselectivite par le biais d’une
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addition anti-Markovnikov. Les additions a 80 8C sur des alcynes terminaux conduisent a des dithiolanes resultant d’une
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bis-addition. Les reactions sont tres propres et les rendements sont eleves.
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Mots-cles : alcynes, hydrothiolation, sulfures de vinyle, regioselectivite, eau.
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[Traduit par la Redaction]
Introduction
Scheme 1. Reaction of alkynes with thiols in water.
The hydrothiolation of alkynes by thiols is a very interest-
ing and challenging reaction being associated with mono- and
bis-condensation, Markovnikov and (or) anti-Markovnikov
addition, and stereoselectivity. This reaction is also of great
importance as it provides an attractive method for the forma-
tion of vinyl sulfides, which are valuable synthetic intermedi-
ates in total synthesis1 and serve as precursors to a wide range
of functionalized molecules.2 The conventional procedures
for hydrothiolation of alkynes involves transition metal cata-
lysts3 or a base4 in organic solvents. Recently, a few proce-
dures using catalytic phenylselenyl bromide,5a Al2O3–KF,5b
nickel5c under solvent-free conditions, b-cyclodextrin in
water–acetone,5d and NaBH4–alumina under solvent-free
conditions5e,5f have been reported.
According to the basic principles of Green Chemistry the
use of organic solvents should be avoided wherever possible
and use of alternative green solvents such as water are en-
couraged.6a Thus, the reaction in water has attracted consid-
erable interest in the context of Green Chemistry.6 In
addition, water often exhibits unique reactivity and selectiv-
ity that cannot be attained in conventional organic solvents.7
Thus, development of an efficient procedure for an organic
reaction using water as a promoter as well as reaction
medium has received intense attention in the design of a
chemical process. Here, we report an efficient hydrothiola-
tion of alkynes in water without any catalysts or additives,
imparting good control on regioselectivity (Scheme 1).
Results and discussion
The experimental procedure is very simple. A mixture of
alkyne and thiol in water was stirred at room temperature
(or at 80 8C) for a required period of time (TLC). Standard
work-up provided the product.
A variety of terminal and internal alkynes underwent hy-
drothiolation with thiophenol and alkane thiols at room tem-
perature by this procedure to provide the corresponding
vinyl sulfides. The results are summarized in Table 1. Aryl-
as well as alkyl-substituted alkynes and both aromatic and
aliphatic thiols participated in this reaction uniformly. All
reactions proceeded through anti-Markovnikov additions.
Regarding the stereochemistry of the products, the reaction
of terminal alkynes and thiophenols (entries 1–3, 6–8, 10,
and 11 in Table 1) provided low to high selectivity with re-
spect to E-isomers, whereas alkane thiols (entries 13, 14, 16,
and 17 in Table 1) produced Z-isomers predominantly. The
internal alkynes (entries 5 and 9 in Table 1) provided Z-iso-
mers selectively. However, dimethyl acetylene dicarboxylate
(entries 4, 12, 15, and 18 in Table 1) furnished Z-isomers as
the sole product. As a whole, the stereoselectivity is excel-
lent in a few of the reactions, although, in some reactions,
no selectivity was observed. The steric distribution of prod-
Received 2 March 2009. Accepted 24 July 2009. Published on
the NRC Research Press Web site at canjchem.nrc.ca on
27 October 2009.
S. Bhadra and B.C. Ranu.1 Department of Organic Chemistry,
Indian Association for the Cultivation of Science, Jadavpur,
Kolkata 700 032, India.
1Corresponding author (e-mail: ocbcr@iacs.res.in).
Can. J. Chem. 87: 1605–1609 (2009)
doi:10.1139/V09-130
Published by NRC Research Press

1606
Can. J. Chem. Vol. 87, 2009
Table 1. Hydrothiolation of alkynes.
Entry
1
2
3
4
5
6
7
8
R
Ph
Ph
Ph
Ph
Ph
Ph
R1
Ph
R2
H
H
H
CO2Me
C2H5
H
H
H
C2H5
H
H
CO2Me
H
H
Z:Ea
38:62
Time (h) Yield (%)b Reference
3
96
87
88
94
86
89
95
94
84
92
90
90
89
93
90
85
87
86
87
5d
C4H9
47:53
47:53
100:0
75:25
42:58
0:100
33:67
83:17
23:77
12:88
100:0
69:31
70:30
100:0
85:15
70:30
100:0
89:11
3
3.2
3
4b
3c
8
C6H13
CO2Me
C2H5
p-ClC6H4
p-ClC6H4
Ph
3
9
2.5
2.5
2.7
2.5
2
2.8
2
2.5
3
2
3.4
3
2
3.2
5d
10
5d
11
12
13
14
3a
15
—
4b
—
16
—
p-MeC6H4
p-ClC6H4
p-MeC6H4
p-MeC6H4
p-ClC6H4
C6H4CH2
C6H4CH2
C4H9
C12H25
C12H25
C4H9
C4H9
HSC2H4
9
C2H5
10
11
12
13
14
15
16
17
18
19
p-MeC6H4
p-CICC6H4
CO2Me
Ph
Ph
CO2Me
Ph
p-ClC6H4
CO2Me
Ph
CO2Me
H
H
CO2Me
H
aE:Z ratio is based on NMR.
bIsolated yields.
Table 2. Bis-hydrothiolation of alkyne.
Sample No.
R
C4H9
n
2
2
2
2
2
3
3
Time (h)
4
8
4
5
5
5
4.5
Yield (%)a
Reference
11
17a
17a
—
—
—
18
1
2
3
4
5
6
7
72
67
81
76
73
74
80
C6H13
Ph
p-MeOC6H4
p-CIC6H4
p-CIC6H4
Ph
aIsolated yields.
ucts is probably controlled by the steric compulsion of the
bulky substituents.
The dithianes and dithiolanes are very useful com-
pounds.19 The standard methods17 employ aldehydes for
dithiolane–dithiane formation using an acid catalyst,
whereas this procedure provides an attractive and novel al-
ternative using alkynes through a simple operation in mild
and neutral conditions.
Water plays a very significant role in this reaction. With-
out water the reaction in neat is very slow and low yielding.
Possibly, water promotes the reaction through hydrogen
bond formation with the sulfhydryl hydrogen of the thiol
and thus enhances the nucleophilicity of the thiolate ion to
add to the alkyne at the less-hindered carbon atom, giving
anti-Markovinkov product. Although we do not have any
concrete evidence regarding the exact mechanism, it may be
speculated that addition of thiolate anion to the C:C bond
The terminal alkynes produced dithiolanes or dithianes
when treated with dithiols at 80 8C, based on the use of
ethane or propane dithiol. It has already been mentioned
that a similar reaction at room temperature produced only
vinyl sulfide (entry 19 in Table 1). The results are summar-
ized in Table 2. However, the use of 2 equiv. of monothiols
in the reaction of phenyl acetylene under identical condi-
tions produced only vinyl sulfides and no dithioacetal was
obtained. On the other hand, reaction of 0.5 equiv. of ethane
dithiol and phenyl acetylene at room temperature produced
the corresponding vinyl sulfide associated with the starting
material, whereas reaction at 80 8C provided the correspond-
ing dithiolane together with unreacted phenyl acetylene.
Published by NRC Research Press

Bhadra and Ranu
1607
Scheme 2. Speculative reaction pathway.
takes place either through a six- (A) or four-membered (B)
transition state,3f with steric factors controlling the regiose-
lectivity (Scheme 2).
lectivities providing a single isomer is not uniform in reac-
tions using other reagents.3,4,5a,5b,5c,5e,5f
In conclusion, we have developed a very simple and effi-
cient methodology for the addition of thiols to unactivated al-
kynes in water, producing vinyl sulfides and dithiolanes at
room temperature and 80 8C, respectively. The additions ex-
hibited high regioselectivity (exclusive anti-Markovnikov ad-
dition) and excellent stereoselectivity in several reactions
(products obtained as one stereoisomer). Other significant ad-
vantages are simple operation, mild conditions (room temper-
ature and neutral reaction media), general applicability to a
variety of substituted alkynes and aromatic as well as aliphatic
thiols, no catalyst or additive is required, high yields of iso-
lated products, environment-friendly experimental conditions,
and cost effectiveness. We believe, being endowed with so
many attractive features, that this procedure will make an im-
portant addition to the existing methods of hydrothiolation.
It was found that, with the progress of reaction the prod-
uct, vinyl sulfide isomerizes to the thermodynamically more
stable isomer over time. For example, during the reaction of
phenyl acetylene and thiophenol, the 32:68 E/Z mixture of
product after 45 min of reaction ultimately gave rise to a
68:32 E/Z mixture at the end of the reaction after 3 h. This
ratio for this mixture did not change even when the reaction
was allowed to proceed past the 3 h time point. To check for
the possibility of a radical process, the reaction was carried
out in the dark. The progress of the reaction remained undis-
turbed. It should be mentioned that all the reactions were
carried out in water that contained dissolved oxygen without
an interference. Thus, a radical pathway is unlikely.
In general, the reactions were very clean. The hydrothiola-
tions were accomplished in water at room temperature much
faster (2–3.5 h) and in higher yields (84%–96%) when com-
pared with many of the existing procedures.3–5 All the prod-
ucts were obtained in high purities. The ratio of stereoisomers
was determined by integration of the corresponding peaks in
1H NMR spectra. Although a report5d using b-cyclodextrin in
water stated that no reaction was observed in the absence of
b-cyclodextrin, very interestingly, all the reactions listed in
Tables 1 and 2 proceeded in water without any difficulty in
the absence of additive. In addition, our water-mediated pro-
cedure is more general. The cyclodextrin–water mediated
procedure5d failed to initiate reactions with alkane thiol, ter-
minal alkyl alkynes, and internal alkynes, whereas our proce-
dure was very successful for all these substrates. However,
the stereoselectivity is better in cyclodextrin–water than in
water alone by the present procedure. The reason for all these
differences is not very clear to us. In general, the transition
metal catalyzed reactions led to predominantly Markovnikov
products,3a,3b,3d,3e whereas nonmetal-catalyzed reactions,
including the present procedure, proceeded through anti-
Markovnikov addition.3f,4,5a,5b,5d,5e,5f Excellent stereoselectiv-
ity was observed producing E-isomers in all the reactions
using b-cyclodextrin,5d whereas attainment of high stereose-
Experimental
IR spectra were recorded on a Shimadzu 8300 FT IR
spectrometer in neat. H NMR and 13C NMR spectra were
1
run on a Bruker DPX-300 instrument at 300 and 75 MHz,
respectively. HR-MS were taken on a Microtek Qt of a Mi-
cro YA263 spectrometer. All commercial reagents were dis-
tilled before use.
General experimental procedure for the synthesis of vinyl
sulfides
Representative procedure for the reaction of phenyl
acetylene with thiophenol (Table 1, entry 1)
A mixture of phenyl acetylene (102 mg, 1 mmol) and thi-
ophenol (121 mg, 1.1 mmol) was stirred in H2O (2 mL) at
room temperature (30 8C) for 3 h (TLC). The reaction mix-
ture was extracted with Et2O (3 Â 15 mL). The ether extract
was washed with H2O, NaOH (5%) solution, and brine and
then dried over Na2SO4. Evaporation of solvent gave a crude
product that was purified by column chromatography over
silica gel (hexane) to provide (E)- and (Z)-phenyl styryl sul-
fane (204 mg, 96%) as a colourless liquid (E:Z = 62:38). IR
(neat, cm–1) n: 3057, 3022, 1598, 1581, 1477, 1438, 1024,
Published by NRC Research Press

1608
Can. J. Chem. Vol. 87, 2009
1
945, 738, 688. H NMR (300 MHz, CDCl3) d: 6.61 (d, 1H,
J = 10.8 Hz), 6.70 (d, 1H, J = 10.8 Hz), 6.85 (d, 1H, J =
15.4 Hz), 6.99 (d, 1H, J = 15.4 Hz), 7.34–7.54 (m, 20H).
13C NMR (75 MHz, CDCl3) d: 123.4, 126.0 (2C), 126.9,
127.2, 127.6, 128.3, 128.7 (2C), 128.8, 129.2 (2C), 129.8
(2C), 130.1, 131.8, 135.3, 136.2, 136.5 (2C). These data are
in good agreement with those reported.5d This procedure
was followed for all the reactions listed in Table 1. The
known compounds were identified by comparison of their
spectral data with those reported earlier (see refs. in Table 1).
The unknown compounds were characterized properly by
their spectroscopic data (IR, 1H NMR, 13C NMR, and
HRMS), which are reported as follows.
and evaporated to get the crude product, which was purified
by column chromatography over silica gel (hexane–ether,
95:5) to provide 2-benzyl-[1,3]-dithiolane (159 mg, 81%) as
a colorless oil. IR (neat, cm–1) 3060, 3026, 2930, 2894,
1
1604, 1498, 1453, 1427, 1270, 1240, 1172, 743, 688. H
NMR (300 MHz, CDCl3) d: 3.10 (d, 2H, J = 7.08 Hz),
3.17–3.24 (m, 4H), 4.72 (t, 1H, J = 7.08 Hz), 7.20–7.32 (m,
5H). 13C NMR (75 MHz, CDCl3) d: 38.5 (2C), 45.2, 54.8,
126.7, 127.9 (2C), 129.0 (2C), 139.0. These data are in
good agreement with those reported.17a This procedure was
followed for all the reactions listed in Table 2. The known
compounds were identified by comparison of their spectral
data with those reported earlier (see refs. in Table 2). The
unknown compounds were characterized properly by their
2-Dodecylsulfanyl-but-2-enedioic acid dimethyl ester
(Table 1, entry 15)
Pale yellow oil. IR (neat, cm–1) n: 2925, 2854, 1741,
1
spectroscopic data (IR, H NMR, 13C NMR, and HRMS),
which are reported as follows.
1
2-(3-Methoxy-benzyl)-[1,3]-dithiolane (Table 2, entry 4)
1716, 1585, 1434, 1338, 1247, 1164, 1035, 723. H NMR
Yellowish oil. IR (neat, cm–1) n: 2999, 2922, 2833, 1598,
(300 MHz, CDCl3) d: 0.87 (t, 3H, J = 6.02 Hz), 1.25–1.39
(m, 17H), 1.61–1.69 (m, 3H), 2.79 (t, 2H J = 7.34 Hz),
3.69 (s, 3H), 3.87 (s, 3H), 5.70 (s, 1H). 13C NMR (75 MHz,
CDCl3) d: 14.2, 22.8, 28.1, 28.9, 29.1, 29.4 (2C), 29.5, 29.6,
29.7, 31.9, 32.0, 51.9, 53.2, 112.4, 150.9, 164.2, 166.2.
HRMS m/z [M + Na]+ calcd. for C18H32O4SNa: 367.1919;
found: 367.1915.
1
1583, 1488, 1454, 1434, 1259, 1153, 1049, 769, 694. H
NMR (300 MHz, CDCl3) d: 3.09 (d, 2H, J = 7.2 Hz,),
3.12–3.22 (m, 4H), 3.79 (s, 3H), 4.72 (t, 1H, J = 7.2 Hz),
6.73–6.87 (m, 3H), 7.21 (t, 1H, J = 7.68 Hz). 13C NMR
(75 MHz, CDCl3) d: 28.0 (2C), 45.1, 54.6, 55.0, 112.0,
114.6, 121.2, 129.2, 140.5, 159.4. HRMS m/z [M + Na]+
calcd. for C11H14OS2Na: 249.0384; found: 249.0412.
1-(2-Butylsulfanyl-vinyl)-4-chloro-benzene (Table 1, entry
17; Z:E = 70:30)
Colourless oil. IR (neat, cm–1) n: 2957, 2928, 2871, 1558,
2-(4-Chloro-benzyl)-[1,3]-dithiolane (Table 2, entry 5)
Colourless oil. IR (neat, cm–1) n: 2923, 2250, 1712, 1490,
1
1
1520, 1489, 1397, 1091. H NMR (300 MHz, CDCl3) d:
1397, 1222, 909, 732. H NMR (300 MHz, CDCl3) d: 2.84–
0.69–0.76 (m, 6H), 1,17–1.29 (m, 4H), 1.41–1.50 (m, 4H),
2.55–2.60 (m, 4H), 6.06 (d, 1H, J = 10.9 Hz), 6.13–6.19
(m, 2H), 6.50 (d, 1H, J = 15.6 Hz), 6.96–7.04 (m, 2H), 7.09
(d, 2H, J = 8.5 Hz), 7.20 (d, 2H, J = 8.5 Hz). 13C NMR
(75 MHz, CDCl3) d: 13.7 (2C), 21.7 (2C), 32.3 (2C), 35.7
(2C), 124.0 (2C), 125.1, 126.6, 128.6, 128.7, 128.8, 128.9,
129.8 (2C), 130.1 (2C), 132.0 (2C), 135.6 (2C). HRMS m/z
[M + Na]+ calcd. for C12H15ClSNa: 249.0481; found:
249.1879.
2.93 (m, 4H), 2.97–3.04 (m, 2H), 4.59 (t, 1H, J = 6.9 Hz),
7.12–7.29 (m, 4H). 13C NMR (75 MHz, CDCl3) d: 5.0,
38.2, 38.9, 54.6, 129.2 (2C), 129.9 (2C), 132.8, 135.1.
HRMS m/z [M + H]+ calcd. for C10H11ClS2H: 231.0069;
found: 231.1078.
2-(4-Chloro-benzyl)-[1,3]-dithiane (Table 2, Entry 6)
Yellowish oil. IR (neat, cm–1) n: 3060, 3026, 2898, 2825,
1602, 1494, 1452, 1421, 1274, 1242, 1178, 906, 740, 698,
663. 1H NMR (300 MHz, CDCl3) d: 1.98–2.06 (m, 2H),
2.70–2.81 (m, 4H), 2.91 (d, 2H, J = 7.3 Hz), 4.13 (t, 1H,
J = 7.3 Hz), 7.08 (d, 2H, J = 7.4 Hz), 7.20 (d, 2H, J =
7.4 Hz). 13C NMR (75 MHz, CDCl3) d: 25.5, 30.4 (2C),
40.9, 48.3, 128.3 (2C), 130.5 (2C), 130.8, 135.6. HRMS
calcd. for [M+Na]+ C11H13ClS2Na: 267.0045; found:
267.010.
2-Styrylsulfanyl-ethanethiol (Table 1, entry 19; Z:E =
89:11)
Colorless oil. IR (neat, cm–1) n: 2980, 2930, 2865, 2223,
1487, 1462, 1272, 730. 1H NMR (300 MHz, CDCl3) d:
2.93–3.00 (m, 4H), 3.04–3.14 (m, 4H), 6.19 (d, 1H, J =
10.8 Hz), 6.48 (d, 1H, J = 10.8 Hz), 6.55 (d, 1H, J =
15.1 Hz), 6.67 (d, 1H, J = 15.1 Hz), 7.19–7.25 (m, 2H),
7.28–7.37 (m, 4H), 7.45 (d, 4H, J = 7.6 Hz). 13C NMR
(75 MHz, CDCl3) d: 35.4 (2C), 39.3 (2C), 126.3 (2C),
127.2 (2C), 127.3 (2C), 128.7 (4C), 129.0 (4C), 137.0 (2C).
Anal. calcd. for C10H12S2: C 61.18, H 6.16; found: C 61.57,
H 6.28.
Acknowledgements
We are pleased to acknowledge the financial support from
Department of Science and Technology (DST), New Delhi
(Grant No. SR/S5/GC-02/2006). S.B. is thankful to Council
of Scientific and Industrial Research (CSIR) for a fellow-
ship.
General procedure for the synthesis of dithiolanes–
dithianes
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