Palladium catalyzed synthesis of aryl thiols: Sodium thiosulfate as a cheap and nontoxic mercapto surrogate

Article abstract of DOI:10.1016/j.tetlet.2010.10.128

A Pd-catalyzed coupling reaction of ArBr/ArCl/ArOTf with sodium thiosulfate takes place in presence of Cs₂CO₃ at 80 °C. The reaction mixture is directly treated with Zn/HCl to afford aryl thiols in good to excellent yields.

Full text of DOI:10.1016/j.tetlet.2010.10.128

Tetrahedron Letters 52 (2011) 205–208
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Tetrahedron Letters
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Palladium catalyzed synthesis of aryl thiols: sodium thiosulfate as a cheap
and nontoxic mercapto surrogate
⇑
⇑
Jun Yi, Yao Fu , Bin Xiao, Wei-Chen Cui, Qing-Xiang Guo
Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A Pd-catalyzed coupling reaction of ArBr/ArCl/ArOTf with sodium thiosulfate takes place in presence of
Cs2CO3 at 80 °C. The reaction mixture is directly treated with Zn/HCl to afford aryl thiols in good to excel-
lent yields.
Received 30 July 2010
Revised 13 October 2010
Accepted 26 October 2010
Available online 30 October 2010
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
odorless white powder, which can be easily handled, its nontoxic
and environmentally friendly character and cheap price will impel
Aryl thiols are important building blocks in the synthesis of sul-
fur-containing natural products and pharmaceutical compounds.
Recently development in the transition metal catalyzed synthesis
of thioethers widely utilized aryl thiols as substrate.1 Unfortu-
nately few aryl thiols are commercially available. Classical
methods for the synthesis of aryl thiols are from phenols by New-
man-Kwart and Schonberg rearrangement,2 Grignard reagent react
with sulfur,3 and reduction of aryl sulfonyl chloride4 or aryl
disulfides.5 Although these methods are often used for aryl thiols
formation, but they often have limited substrate scope, and typi-
cally require elevated temperatures. More recently, transition me-
tal catalyzed methods also provide an alternative and convenient
way for the synthesis of aryl thiols. Ma’s group has developed a
CuI catalyzed coupling of aryl iodides with sulfur and subsequent
reduction protocol.6 Although it is a major breakthrough in aryl
thiols synthesis, but much more cheaper and easy accessible aryl
chlorides and phenols cannot be transferred into thiols. The Itoh7
and Hartwig8 groups utilized different hydrogen sulfide surrogates
in the palladium-catalyzed coupling with aryl halides, but the
atomic economy is poor or very few aryl thiols can be synthesized.
The last decade has seen the transition metal catalyzed C–N and
C–O bond-forming cross-coupling reactions of aryl halides emerge
as tremendously powerful synthetic tools.9 Especially promoted by
the recent progress in the synthesis of arylamines10 and phenols,11
we believe a much more versatile and easy handle protocol should
be developed.
its widel use.
4-Bromotoluene (1a) was chosen as a model substrate to react
with sodium thiosulfate (2), and then the reaction mixture was di-
rectly treated with Zn/HCl to afford 4-toluenethiol.12 The initial
optimization of the experimental conditions is shown in Table 1.
A blank test, which was conducted only in the present of Cs2CO3,
failed to give the desired product 3a, and the starting material
was recovery in 95% yield (entry 1). When 2 mol % Pd2(dba)3 and
8 mol % Xphos were added into the reaction system, the desired
4-toluenethiol was obtained in 8% yield (entry 3). Various solvents
were investigated for their effect on the catalysis system (entries
4–7). A mixed solvent was chosen for the reaction. Interestingly,
water could strongly gear up the chemical efficiency (entries 6
and 7), when 2 equiv water was added the quantity of the
Pd2(dba)3 could lessen to 1 mol % without obvious effect on the
yield (entry 8). Water in the system may improve the solubility
of the sodium thiosulfate. After we screened a series of the ligands,
we found Xphos was still the best choice (entries 9–13). Besides,
we found that the basicity of base has significant influence on
the reaction efficiency (entries 7, 14–16). Very low yields were ob-
tained when too strong base t-BuOK or too weak base K2CO3 were
used in the reactions. When the base was absent, no product was
formed. Very similar activity was obtained if Pd(OAc)2 was used
as catalyst (entry 15).
With the optimal catalyst in hand, we examined the scope
of this reaction with a series of functionalized aryl bromides
(Table 2). As can be seen, the presences of an ortho-hindrance
group diminish the efficiency of the reaction (Table 2, entry 2).
Functional groups that are compatible with this Pd-catalyzed
protocol include triflouromethyl, fluoro, phenyl, and methoxy
groups (entries 5–9). The substrates with those electron-
withdrawing groups (entries 5 and 9) afforded higher yield than
that with electron-donating groups (entries 7 and 8). Nearly
quantitative conversion was obtained from 2-bromonaphthalene
(1d) to desired naphthalene-2-thiol (3d).
We now report palladium catalyzed cross-coupling of aryl chlo-
rides, aryl bromides, and aryl trifluoromethanesulfonate with so-
dium thiosulfate. This method is attractive for several reasons:
the aryl chlorides and aryl bromides are cheaper and widely
accessible than aryl iodides, phenols can be easily transferred into
aryl thiols avoiding elevated temperature, sodium thiosulfate is
⇑
Corresponding authors. Tel.: +86 551 360 7476; fax: +86 551 360 6689 (Y.F.).
E-mail addresses: fuyao@ustc.edu.cn (Y. Fu), qxguo@ustc.edu.cn (Q.-X. Guo).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.10.128

206
J. Yi et al. / Tetrahedron Letters 52 (2011) 205–208
Table 1
Table 2
Reaction condition screening resultsa
Examination of the scope of the aryl bromidesa
Br
SH
Pd2(dba)3 1 mol%
Xphos 4 mol%
Br
SH
Zn/HCl
Pd, L
Zn/HCl
Na2S2O3
+
R1
R1
Na2S2O3
2
+
º
base, 80 C, Ar
Cs2CO3 2 eq, Ar
H2O 2 eq, 80 ºC,
2
1a
Entry
3a
1
3
Pd (mol %)
L (mol %)
Base (2 equiv)
Yieldb (%)
Entry
1
Aryl bromides
Product
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
—
—
—
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
t-BuOK
—
0
Pd2(dba)3 (2%)
Pd2(dba)3 (2%)
Pd2(dba)3 (2%)
Pd2(dba)3 (2%)
Pd2(dba)3 (2%)
Pd2(dba)3 (2%)
Pd2(dba)3 (2%)
Pd2(dba)3 (2%)
Pd2(dba)3 (2%)
Pd2(dba)3 (2%)
Pd2(dba)3 (2%)
Pd2(dba)3 (2%)
Pd2(dba)3 (2%)
Pd2(dba)3 (2%)
Pd2(dba)3 (2%)
Pd2(OAc)2 (2%)
Trace
8
20
28
62
Br
SH
Xphosc (8%)
Xphosd (8%)
Xphosd (8%)
Xphose (8%)
Xphosf (8%)
Xphosf (4%)
1a
81
3a
Br
Br
SH
2
3
75
67
1b
3b
3c
90
88(81)
2
f,g
SH
PCy3 (8%)
tBuXphosf,h (8%)
Davephosf,i (8%)
Ruphosf,j (8%)
IPrf,k (8%)
10
2
30
3
Trace
Trace
Trace
81
1c
Xphosf (8%)
Xphosf (8%)
Xphosf (8%)
Xphosf (8%)
Br
SH
1d
3d
4
5
95
80
Cs2CO3
Br
SH
a
1e
3e
3f
Reaction condition: 1a (0.5 mmol), 2 (2.5 equiv), Pd catalyst (shown in the Table
1), L (shown in the Table 1), base (2 equiv), 80 °C, 0.4 mL tBuOH and 0.6 mL toluene,
under argon, 24 h.
F3C
Ph
F3C
Ph
Br
SH
b
GC yield (isolated yield in the parentheses).
c
Xphos = 2-dicyclohexyl-phosphino-20,40,60-tri-i-propyl-1,10-
1f
6
90
1 mL
biphenyl.
toluene,
d
1 mL tBuOH.
1 equiv water.
2 equiv water.
Br
SH
e
f
1g
3g
7
8
9
78
82
73
g
h
i
PCy3 = 2-(dicyclohexyl-phosphino)-20-(N,N-dimethylamino)biphenyl.
tBuXphos = 2-di-tert-butyl-phosphino-20,40,60-tri-i-propyl-1,10-biphenyl.
Davephos = 2-(dicyclohexyl-phosphino)-20-(N,N-dimethylamino)biphenyl.
Ruphos = 2-dicyclohexyl-phosphino-20,60-diisopropoxybiphenyl.
IPr = 1,3-bis(2,4,6-tri-isopropyl-phenyl)imidazol-2-ylidene.
OMe
Br
OMe
SH
1h
1i
3h
3i
j
k
Br
SH
F
F
Although the aryl bromides are interesting substrates, we fo-
a
Reaction conditions: 1a (0.5 mmol), 2 (2.5 equiv), Pd2(dba)3 (1 mol %), Xphos
cused our attention on aryl chlorides, which are less reactive elec-
trophiles but of great interesting for industrial application.
Fortunately, pleased results were got with Buchwald’s preactiva-
tion strategy (Table 3).10 The substrates with both electron-
withdrawing and electron-donating groups afforded moderate to
excellent yields (entries 1–4). Aryl trifluoromethanesulfonates
were easily transferred from the phenols, which also could be com-
patible in our system (Table 3). Also the yields were satisfactory.
Although a detailed mechanistic investigation of the palladium
catalyzed cross-coupling reaction of aryl halides with sodium
thiosulfate awaits further experimentation, tentative proposals are
outlined in Scheme 1. Initial oxidative addition of aryl halide to
zerovalent palladium occurs to afford arylpalladium halide.11 Subse-
quent ion exchange between the palladium intermediate and thio-
sulfate in the presence of the base provides palladium thiosulfate.
Finally, reductive elimination from the intermediates gives 5 and
regenerates Pd(0). After treated with Zn/HCl, 5 turns into Aryl thiols.
In summary, our studies have led to the development of a new
process for the synthesis of aryl thiols from aryl halides cross-
coupling with sodium thiosulfate. Sodium thiosulfate is odorless
white powder, which can be easily handled, its nontoxic and environ-
mentally friendly character and cheap price will impel the widel use.
(4 mol %), Cs2CO3 (2 equiv), 80 °C, 0.4 mL tBuOH and 0.6 mL toluene, 2 equiv water,
under argon, 24 h.
b
Isolated yield after flash column chromatography.
2.1. General procedure for Tables 1 and 2
An oven-dried Schlenk tube was charged with aryl halides 1
(0.5 mmol), sodium thiosulfate
2 (100 mg), Cs2CO3 (1 mmol,
325 mg), appropriate amount of Pd2(dba)3/Pd(OAc)2 and Xphos,
equipped with a magnetic stir bar. The tube was evacuated and
backfilled with argon three times before the solvent was added.
The mixture was stirred until homodisperse at rt. Then the tube
was stirred at 80 °C for 24 h. The solid substance was separated
from the reaction mixture and washed with ether. Zn dust (0.5 g)
and HCl (10%, 5 mL) was added to the solid substance with cooling
by ice-water. After stirred for 1 h, the mixture was extracted with
ethyl acetate. The organic layer was washed with H2O and brine
and dried over Na2SO4. Removal of solvent in vacuo provided the
desired product with satisfactory purity. Some relatively stable
products were purified via column chromatography.
2.2. General procedure for Table 3
2. Experimental
1H NMR spectra were recorded at rt at 400 MHz on an ARX 400
Bruker spectrometer. Chemical shifts are reported in parts per mil-
lion referenced to the residual proton resonances of the solvents.
Coupling constants are expressed in Hertz.
An oven-dried Schlenk tube was charged with Pd(OAc)2
(11.2 mg) and Xphos (71.5 mg), equipped with a magnetic stir
bar. The tube was evacuated and backfilled with argon three times
before tBuOH (4 mL)/toluene (6 mL) and H2O (180 lL) was added.

J. Yi et al. / Tetrahedron Letters 52 (2011) 205–208
207
Table 3
ArSH
Examination of the scope of the aryl chlorides and aryl trifluomethanesulfonatesa
Pd(OAc)2 1 mol%
X
SH
Zn/HCl
ArS2O3M
Zn/HCl
Xphos
3 mol%
Cs2CO3 2 eq, Ar
H2O 2 eq, 80 C,
Ln
Pd(0)
R1
R1
Na2S2O3
2
+
Ar-X
1
4a
3a
º
5
Entry
2
Product
Yieldb (%)
Cl
SH
82(68)c
4a
3a
1
Cl
SH
4b
Cl
3j
2
3
72
85
SH
4c
3e
Ln
Ln
Pd(II) S2O3M
Ar
4
Pd(II)
X
F3C
F3C
Ar
Cl
SH
3
4d
OTf
3g
4
5
68
90
OMe
OMe
SH
4e
3d
Na2S2O3, Cs2CO3
CsX
2
OTf
SH
6
7
4f
3k
91
70
Scheme 1. Proposed mechanism.
OTf
SH
4g
3e
2.6. Naphthalene-2-thiol (3d)
F3C
F3C
1H NMR (400 MHz, CDCl3) d 7.70–7.07 (m, 7H). 3.55 (s, 1H).
2.7. 4-(Trifluoromethyl)benzenethiol (3e)
1H NMR (400 MHz, CDCl3) d 7.30–7.20 (M, 4H), 3.50 (s, 1H).
2.8. Biphenyl-4-thiol (3f)
a
Reaction conditions: 1a (0.5 mmol), 2 (2.5 equiv), Pd(OAc)2 (1 mol %), Xphos
(3 mol %), Cs2CO3 (2 equiv), 80 °C, 0.4 mL tBuOH and 0.6 mL toluene, 2 equiv water,
under argon, 24 h.
b
Isolated yield after flash column chromatography.
Pd2(dba)3 (1 mol %), Xphos (4 mol %).
c
The solution was heated to 110 °C for 1.5 min. One millilitre of the
solution was transferred to another oven-dried Schlenk tube,
which was charged with aryl halides 1 (0.5 mmol), sodium thiosul-
fate 2 (100 mg), Cs2CO3 (1 mmol, 325 mg) equipped with a mag-
netic stir bar. Then the tube was stirred at 80 °C for 24 h. The
solid substance was separated from the reaction mixture and
washed with ether. Zn dust (0.5 g) and HCl (10%, 5 mL) were added
to the solid substance with cooling by ice-water. After stirred for
1 h, the mixture was extracted with ethyl acetate. The organic
layer was washed with H2O and brine and dried over Na2SO4. Re-
moval of solvent in vacuo provided the desired product with satis-
factory purity. Some relatively stable products were purified via
column chromatography.
1H NMR (400 MHz, CDCl3) d 7.55 (d, J = 7.2 Hz, 2H), 7.49–7.41
(m, 4H), 7.36–7.33 (m, 3H), 3.50 (s, 1H).
2.9. 2-Methoxybenzenethiol (3g)
1H NMR (400 MHz, CDCl3) d 7.27–7.24 (m, 1H), 7.12 (t,
J = 7.8 Hz, 1H), 6.88–6.82 (m, 2H), 3.89 (s, 3H), 3.81 (s, 1H).
2.10. 3-Methylbenzenethiol (3h)
1H NMR (400 MHz, CDCl3) d 7.15–7.06 (m, 3H), 6.96 (d,
J = 7.5 Hz, 1H), 3.40 (s, 1H), 2.30 (s, 3H).
2.3. 4-Methylthiolphenol (3a)
2.11. 4-Fluorobenzenethiol (3i)
1H NMR (400 MHz, CDCl3) d 7.18 (d, J = 7.8 Hz, 2H), 7.05 (d,
J = 8.1 Hz, 2H), 3.38 (s, 1H), 2.29 (s, 3H).
1H NMR (400 MHz, CDCl3) d 7.30–7.25 (m, 2H), 6.80 (t,
J = 8.1 Hz, 2H), 3.43 (s, 1H).
2.4. 2-Methylthiophenol (3b)
2.12. Benzenethiol (3j)
1H NMR (400 MHz, CDCl3) d 7.28 (d, J = 7.3 Hz, 1H), 7.18–7.14
(m, 1H), 7.09–7.05 (m, 2H), 3.30 (s, 1H), 2.33 (s, 3H).
1H NMR (400 MHz, CDCl3) d 7.26–7.19 (m, 4H), 7.12–7.15 (m,
1H), 3.42 (s, 1H).
2.5. Naphthalene-1-thiol (3c)
2.13. 3,5-Dimethylbenzenethiol (3k)
1H NMR (400 MHz, CDCl3) d 8.24–8.20 (m, 1H), 7.75–7.72 (m,
1H), 7.66 (d, J = 7.9 Hz, 1H), 7.60 (d, J = 7.3 Hz, 1H), 7.40–7.35 (m,
2H), 7.29 (t, J = 7.7 Hz, 1H), 4.33 (s, 1H).
1H NMR (400 MHz, DMSO) d 6.89 (s, 2H), 6.77 (s, 1H), 3.34 (s,
1H), 2.24 (s, 6H).

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J. Yi et al. / Tetrahedron Letters 52 (2011) 205–208
Feng, Y. S.; Li, Y. Y.; Tang, L.; Wu, W.; Xu, H. J. Tetrahedron Lett. 2010, 51, 2489;
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