Iodine-catalyzed efficient 2-arylsulfanylphenol formation from thiols and cyclohexanones

Article abstract of DOI:10.1039/c3gc41671b

A novel method for the formation of 2-arylsulfanylphenols from thiols and cyclohexanones is described. Iodine was used as an effective catalyst for this kind of transformation. Cyclohexanones were used as a phenol source via a dehydrogenation and tautomerization reaction.

Full text of DOI:10.1039/c3gc41671b

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Iodine-catalyzed Efficient 2-Arylsulfanylphenol Formation from Thiols
and Cyclohexanones
Yunfeng Liao,a,b Pengcheng Jiang,a Shanping Chen,a Hongrui Qi a and Guo-Jun Deng*,a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
from readily available starting materials is highly desirable.
50 Cyclohexanones are cheap, stable, readily available and easy to
handle. Recently, the Stahl group found that these compounds can
be dehydrogenated under palladium catalyzed reaction conditions
to afford the corresponding phenols or cyclohexenones using
molecular oxygen as hydrogen acceptor.22 We and others
55 successfully used cyclohexanones as aryl sources for C-N23 and
C-O bonds24 formation. Based on this, we envisaged that 2-
arylsulfanylphenols might be prepared from commercially
available cyclohexanones and thiols via tandem transformation of
C-S coupling/dehydrogenation/tautomerization. Herein, we
60 describe an iodine-catalyzed efficient 2-arylsulfanylphenol
formation from thiols and cyclohexanones under metal-free
conditions using oxygen as the hydrogen acceptor (Scheme 1, eq
3).
A novel method for formation of 2-arylsulfanylphenols from
thiols and cyclohexanones is described. Iodine was used as
effective catalyst for this kind of transformation.
Cyclohexanones were used as phenol source via
10 dehydrogenation and tautomerization reaction.
Aryl thioethers are important organosulfur compounds and
widely exist in natural products, functional materials and
pharmaceuticals.1 Consequently, development of efficient
methods for facile construction of aryl thioethers has stimulated
15 considerable interest. In the past decade, the cross-coupling of
aryl halides (or triflates) with thiols has become one of the most
powerful methods for the construction of C-S bonds.2 Various
transition-metals such as palladium,3 copper,4 nickel,5 cobalt,6
indium7 and iron8 were used as efficient catalyst for aryl
20 thioethers (Scheme 1, eq 1). In recent years, the direct conversion
of C-H bonds into C-S bonds has arisen as an efficient alternative
approach. Aryl thioethers could be synthesized by electrophilic
substitution of arenes (especially electron-rich (hetero)arenes)
with sulfenylating agents such as thiols,9 disulfides,10 sulfonyl
25 chlorides11 and sulfonyl hydrazides.12
Cross-coupling reaction
S
X
SH
R2
(1)
catalyst
R1
R2
R1
+
X = I, Br, OTf, H
C-S coupling/C-H hydroxylation
As an important type of aryl thioethers, 2-phenylthiophenol
and its derivatives can be used as the glycine transporter-1 (GlyT-
1) inhibitors.13 The conventional methods for the synthesis of
these important compounds mainly involve the metal-catalyzed
30 cross-coupling of thiols with 2-halophenols. Copper,14 nickel,15
and nano-CuFe2O416 were successfully employed as catalysts for
this type of reaction. In addition, 1,2-diaryldisulfanes also could
be used as sulfur sources for preparation of 2-(aryl)thiophenols.17
Kumar and co-workers developed a transition-metal-free method
35 for the synthesis 2-(aryl)thiophenols from diaryl disulfides and
phenols using potassium persulfate in trifluoroacetic acid.18
Dehydrogenation of 2-(aryl)thio cyclohexanones could afford an
alternative approach to 2-arylthiophenols. However, 2-(aryl)thio
cyclohexanones are not commercially available and excess
40 bromine or strong base is required in this transformation, which
seriously limited its application.19 Deprotection of the
OH
OH
X
SH
S
S
CuI/L
R2
R1
R2
R1
+
(2)
(3)
Cs2CO3
DMSO
X = I, Br
This work
O
SH
I2
R2
R1
+
R1
R2
NMP, O2
Metal-free !
65 Scheme 1 Various methods for C-S bond formation.
Our study was initiated using equal amount of
cyclohexanone 1a and 4-methylbenzenethiol 2a as starting
materials to detemine the optimized reaction conditions (Table
70 1). The desired product 3a was observed in trace amount
when the reaction was carried out in NMP at 150 oC under
oxygen in the absence of any additives (entry 1).
Subsequently, various iodide-containing additives were
investigated under similar reaction conditions (entries 2-5).
75 Cyclohexanone 1a could smoothly react with 4-
methylbenzenethiol 2a to give the desired product in
corresponding methoxy compounds provided
a multi-step
procedure for 2-(aryl)thiophenols.20 In 2010, Pan and co-workers
reported an efficient protocol for the preparation of 2-
45 (arylthio)phenols via copper(I)-catalyzed tandem transformation
of C-S coupling/C-H functionalization (Scheme 1, eq 2).21
Although various methods have been discovered for 2-
(arylthio)phenols, development of general and efficient methods
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DOI: 10.1039/C3GC41671B
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observable yield. Among the addtives investigated, iodine
showed the best efficiency to give the corresponding product
the corresponding products in good yields (entries 2-5). When
4-phenylcyclohexanone (1f) was employed to react with 2a,
in 54% yield (entry 5). It’s necessary to use 20 mol% of 30 3-(p-tolylthio)-[1,1'-biphenyl]-4-ol (3f) was obtained in 80%
iodine to get satisfactory yield. The reaction yield decreased
to 40% when 10 mol % iodine was used (entry 6). Except
NMP, other solvents were also tested and failed to provide
more favorable outcomes (entries 7-10). No product was
yield (entry 6). To our delight, functional groups, such as
ester and acetamido were well tolerated in this reaction
(entries 7 and 8). The influence of the methyl substituent on
the cyclohexanone moiety was also evaluated. The position of
5
observed when the reaction was carried out in toluene or 35 the substituents on the cyclohexanone ring profoundly
diglyme. The ratio of 1a to 2a had an obvious influence on the
affected the reaction yield. When 3-methylcyclohexanone (1i)
was used, the desired product 3i was obtained in 67% yield
(entry 9). However, 3j was observed in only 40% yield when
2-methylcyclohexanone (1j) was used (entry 10).
10 reaction yield (entries 11 and 12). The reaction yield could be
improved to 70% when 2 equiv of cyclohexanone was used.
The reaction temperature played an important role in this kind
of transformation. The reaction yield could be further
improved to 80% by increasing the reaction temperature to
15 160 oC (entry 13). Lower yield was observed when the
reaction was carried out under air atmosphere (entry 14) and
no product was obtained under argon (entry 15). This means
molecular oxygen acted as an efficient oxidant to accept
hydrogen generated from cyclohexanone dehydrogenation
20 step.
40
Table
2
Reaction of 4-methylbenzenethiol (2a) with
cyclohexanonesa
OH
O
I2 (0.2 equiv.)
SH
S
+
R1
NMP, 160 o
O2
C
R1
1
2a
3
Yield (%)b
Cyclohexanone
O
Product
OH
Entry
Table 1 Optimization of the reaction conditionsa
S
OH
O
R1
R1
SH
S
catalyst
solvent
+
1
2
3
4
5
6
7
R1 = H
1 = Me
1a
1b
1c
1d
1e
1f
3a
75
70
72
75
71
80
62
3a
2a
3b
3c
3d
3e
3f
1a
Entry
R
R1 = Et
Temp. (oC)
Yield (%)b
Catalyst
Solvent
R1 = n-pentyl
R1 = tert-pentyl
1
2
3
4
5
6
7
8
9
NMP
150
150
150
150
150
150
150
150
150
trace
15
34
49
54
40
10
0
KI (0.2 eq)
NIS (0.2 eq)
ICl (0.2 eq)
I2 (0.2 eq)
I2 (0.1 eq)
I2 (0.2 eq)
I2 (0.2 eq)
I2 (0.2 eq)
NMP
R
R
1 = Ph
NMP
1 = CH3CONH
1g
3g
NMP
R
1 = CO2C2H5
61
1h
1i
3h
8
NMP
OH
OH
NMP
O
S
3i
3j
9
67
DMF
toluene
diglyme
O
S
0
40
10
1j
I2 (0.2 eq)
I2 (0.2 eq)
I2 (0.2 eq)
I2 (0.2 eq)
I2 (0.2eq)
I2 (0.2eq)
10
DMSO
150
150
150
160
160
160
19
59
70
80
67
0
a
11c
NMP
NMP
NMP
NMP
NMP
Conditions: 1 (1.0 mmol), 2a (0.5 mmol), I2 (0.1 mmol), NMP (2.0 mL), 160
oC, 20 h, under oxygen. b Isolated yield based on 2a.
12d
13d
To further examine the scope and limitation of the reaction, we
14d,e
45 tested various thiols for this kind of reaction (Table 3). The
results demonstrated that thiols bearing electron-donating groups
on the aryl ring could couple with cyclohexanone to give the
desired products in moderate to good yields (entries 1–3).
Functional groups such as acetamido, hydroxyl, fluoro, chloro
50 and bromo were well tolerated under the optimal reaction
conditions (entries 4–8). It should be noted that no cleavage of
the C-Br thiol bond was observed. When naphthalene-2-thiol (2j)
was reacted with 1a, the target product 3s was furnished in 67%
yield (entry 9). To our delight, aliphatic thiols such as
55 cyclohexanethiol (2k) also could react with 1a to give the product
15d,f
a conditions: 1a (0.5 mmol), 2a (0.5 mmol), solvent (2.0 mL) under
oxygen unless otherwise noted. b GC yield based on 2a using
dodecane as internal standard. c 2a (1.0 mmol). d 1a (1.0 mmol).
e Under air. f Under argon.
With the optimized reaction conditions in hand, the
25 substrate scope with respect to cyclohexanones was explored
(Table 2). Cyclohexanones bearing an alkyl substituent at the
para position were able to smoothly couple with 2a to give
2
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DOI: 10.1039/C3GC41P67a1gBe 2 of 4
3t in 50% yield (entry 10).
In summary, we have developed a novel iodine-promoted
To elucidate the mechanism, some control experiments were
carried out (Scheme 2). 1,2-di-p-Tolyldisulfane (4a) was formed
approach for the synthesis of 2-arylsulfanylphenols using
cyclohexanones as phenol source under metal-free conditions.
as the major products when the reaction of cyclohexanone (1a) 30 Catalytic amount of iodine was used as catalyst and molecule
5
with 4-methylbenzenethiol (2a) was stopped in 2 h under the
optimized reaction conditions. 2-(p-Tolylthio) cyclohexanone (5a)
and the desired product 3a were obtained in 14% and 8% yields
as determined by GC method (Scheme 2, a). In the absence of 1a
oxygen was used as hydrogen acceptor in this transformation.
Functional groups such as fluoro, chloro, bromo, hydroxyl,
acetamido and ester were all well tolerated under the
optimized reaction conditions. The C-S bond formation,
and iodine catalyst, 4a was formed in 80% yield in 0.5 h (Scheme 35 dehydrogenation and tautomerization were realized in one-pot
10 2, b). Treatment of 4a with 1a under the standard reaction
conditions in 2 h afforded the desired product 3a in 39% yield
together with 2-(p-tolylthio) cyclohexanone (5a) in 10% yield
(Scheme 2, c). 5a could further convert into the corresponding
without any metal-catalyst. Since cyclohexanones and thiols
are readily available starting materials, this method can afford
an efficient and environmentally benign approach for 2-
arylsulfanylphenols. The scope, mechanism and synthetic
product 3a in 78% yield via dehydrogenation and tautomerization 40 applications of this reaction are under investigation.
15 (Scheme 2, d).
O
S
S
I2, NMP
S
1a
2a
+
Table 3 Reaction of cyclohexanone (1a) with thiolsa
+
+
3a
a
160 oC, O2
2 h
OH
4a
5a
14%
O
SH
I2 (0.2 equiv.)
S
70%
8%
R2
+
R2
NMP, 160 oC
O2
SH
O2, NMP
160 oC, 0.5 h
S
1a
2
3
S
b
Product
Yield (%)b
Entry
Thiol
4a
2a
OH
80%
SH
S
R2
O
O
R2
S
I2, NMP
3a
c
4a
+
+
160 oC, O2
2 h
1
2
R = 2-Me
R = 3-Me
2b
2c
3k
3l
60
64
5a
1a
10%
39%
OH
3
R = 4-OCH3
2d
3m
52
O
S
S
I2, NMP
d
4
5
R = 4-NHCOCH3 2e
3n
3o
72
81
160 oC, O2
20 h
R = 4-OH
R = 4-F
2f
5a
3a
78%
6
7
8
2g
3p
3q
3r
70
62
71
Scheme 2 Control experiments.
R = 4-Cl
R = 4-Br
2h
2i
O
SI
SH
O2
4a
OH
S
SH
9
61
50
3s
3t
2j
2a
1a
OH
SH
S
2k
10
O
I2
HI
S
a
1a
2
O2
Conditions:
(1.0 mmol), (0.5 mmol), I2 (0.1 mmol), NMP (2.0 mL),
160 oC, 20 h, under oxygen. b Isolated yield based on
2
.
O2
H2O
5a
Based on these observations, a plausible mechanism was
proposed as outlined in Scheme 3. Initially, oxidation of 4-
20 methylbenzenethiol (2a) under oxygen affords disulfide 4a.
Reaction of 4a with I2 to form an electrophilic species 4-Me-
PhSI,25 which can couple with cyclohexanone (1a) to generate an
intermediate 2-thio cyclohexanone (5a) and HI. Subsequent
dehydrogenation and tautomerization of 5a provides the target
25 product 3a promoted by iodine catalyst.19a Meanwhile, HI can be
oxidized into I2 by oxygen, thus closing the catalytic cycle.
I2
HI
OH
S
3a
Scheme 3 Proposed mechanism.
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Acknowledgments
70
75
This work was supported by the National Natural Science
Foundation of China (21172185, 21372187), the Hunan
Provincial Natural Science Foundation of China (11JJ1003,
12JJ7002), the Research Fund for the Doctoral Program of
Higher Education of China, Ministry of Education of China
(20124301110005) and the Research Fund for the Education
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a Key Laboratory of Environmentally Friendly Chemistry and Application
of Ministry of Education, College of Chemistry, Xiangtan University,
Xiangtan 411105, China. Fax: (+86)0731-5829-2251; Tel: (+86)0731-
5829-8601; E-mail: gjdeng@xtu.edu.cn
15 b School of Chemistry & Chemical Engineering, Hunan Institute of
Engineering, Xiangtan 411104, China.
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