Palladium-catalyzed benzothieno[2,3-b]indole formation via dehydrative-dehydrogenative double C-H sulfuration using sulfur powder, indoles and cyclohexanones

Article abstract of DOI:10.1039/c4cc08370a

Palladium-catalyzed C-S bond formation via dehydrative-dehydrogenative double C-H sulfuration using sulfur powder is described. The dehydrogenated intermediates of cyclohexanones can be trapped to act as efficient aryl sources under an oxygen atmosphere. This procedure provides a novel approach for the preparation of benzothieno[2,3-b]indoles. This journal is

Full text of DOI:10.1039/c4cc08370a

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Palladium-Catalyzed Benzothieno[2,3-b]indole Formation via
Dehydrative-Dehydrogenative Double C-H Sulfuration with Sulfur
Powder, Indoles and Cyclohexanones
Yunfeng Liao,[a,c] Yi Peng,[a] Hongrui Qi,[a] Guo-Jun Deng,*[a] Hang Gong,[a,b] Chao-Jun Li*[b]
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A palladium-catalyzed C-S bond formation via dehydrative-
dehydrogenative double C-H sulfuration with sulfur powder
is described. The dehydrogenated intermediates of
50 Scheme 1 Examples of thieno-fused indole derivatives
10 cyclohexanones were trapped to act as efficient aryl source
under oxygen atmosphere. This procedure provided a novel
approach for the preparation of benzothieno[2,3-b]indoles.
Sulfur-containing organic compounds play important roles in
organic synthesis, pharmaceutical drugs and materials science.1
15 In recent years, organosulfur compounds have received
considerable attention and numerous methods for the construction
of C-S bonds have been developed.2 Among them, transition
metal-catalyzed cross-coupling reaction has become one of the
most important methods for the construction of C-S bonds.3 On
20 the basis of sulfur source, there are two general approaches for
the construction of C-S bonds via cross-coupling reactions. One
is the transition metal-catalyzed cross-coupling reaction of
organic halides with organosulfur substrates; among which thiols
have been the most popular reagents in these reactions.4 However,
25 most thiols have unpleasant odors, which impedes their
applications by these processes. The second general cross-
coupling approach is based on inorganic sulfur reagents; sulfur-
55 been isolated from Streptomyces albogriseolus and exhibits
plant-growth regulatory properties.15 Benzothieno[2,3-b]indole
(C) shows great potential applications in organic
electroluminescent devices.16 However, efficient methods for the
preparation of these indole-containing heterocycles in one-pot are
60 rare. Unsubstituted benzothieno[2,3-b]indole (C) can be
synthesized from indoline-2,3-diones, cyclohexanones and P4S10
via a five-step procedure.17 Very recently, Hotta et al. reported
that the reaction of 3-bromobenzo[b]thiophene with 1-iodo-2-
nitrobenzene could provide benzothieno[2,3-b]indole C by a
65 sequential reaction of replacement, coupling and cyclization oper-
ations.16b, 16c Similarly, N-substituted benzothieno[2,3- b]indoles
can also be synthesized from 3-bromobenzo[b]thiophene via a
six-step procedure.18 Flynn and co-workers reported a five-step
procedure to convert 2-iodoaniline to 3-iodothienoindole using
70 iodine as a key promoter.19 These methods all require highly
prefunctionalized substrates and multi-step operations, which
severely limited the substrate scope and their further applications.
7
containing inorganic compounds such as Na2S,5 K2S6, Na2S4
8
9
Na2S2O3 and K2CS3 are used as cleaner and more sustainable
30 inorganic sulfur sources. Recently, the cheap and abundant sulfur
powder (S8) has also been successfully applied for the
construction of C-S bonds.10 In most cases, active substrates R-X
(X = Cl, Br, I, OTf, and B(OH)2) with leaving groups were
coupled with inorganic sulfides or sulfur power in the presence of
35 transition metals. In recent years, the direct sulfuration of C-H
bonds has attracted considerable interest since this strategy can
provide a more atom-economic route for C-S bond formation by
omitting substrate pre-activation.11 However, C-S bond formation
via a double C-H sulfuration using inorganic sulfur sources is
40 very challenging due to the low activity of both C-H bond and
inorganic sulfur sources, and there are only a few reports on this
kind of transformation.12
75
Scheme 2 New method for benzothieno[2,3-b]indole formation
80
It is highly desirable to develop efficient methods for the
synthesis of benzothieno[2,3-b]indoles in one pot using readily
available raw materials. Recently, the Stahl group developed a
strategy to convert cyclohexanones into phenols or
Condensed indole derivatives are valued for their biological
activities and electronic properties.13 Thieno[2,3-b]indole (A)
45 shows potent applications in treating diseases of the human
central nervous system (Scheme 1).14 Thienodolin (B) has
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Page 2 of 4
cyclohexenones via a dehydrogenation-tautomerization process.20
provide 3a in 75% yield (entry 17). Slightly lower yield (70%)
was obtained when the reaction was carried out in air (entry 19).
We and others successfully trapped the dehydrogenated
intermediates of cyclohexanones for C-C21 and C-hetero22 bonds
formation. Herein, we report a novel strategy for one-pot
benzothieno[2,3-b]indole formation from readily available and
simple starting materials via dehydrative-dehydrogenative double
C-H sulfuration under oxygen atmosphere (Scheme 2).
View Article Online
a
DOI: 10.1039/C4CC08370A
Table 2 Reaction of indoles with cyclohexanone (2a)
5
Our study was initiated by using 1-methyl-1H-indole (1a),
cyclohexanone (2a) and sulfur powder (S8) as the starting
10 materials to determine the optimized reaction conditions (Table
1). No desired product was observed when the reaction was
carried out in the absence of any catalyst under an oxygen
o
atmosphere at 125 C (Table S1 in SI, entry 1). To our delight,
the desired product 6-methyl-6H-benzo[4,5]thieno[2,3-b]indole
15 (3a) was observed in 35% yield when 5 mol % iodine was used
as the catalyst (entry 2). Inspired by this observation, several
iodide-containing catalysts were examined, and among them PdI2
showed the best efficiency to give 3a in 42% yield (entry 6).
Several nitrogen containing ligands were investigated, and among
20 them
5H-cyclopenta[1,2-b:5,4-b']dipyridin-5-one
(CPDO)
showed the best efficiency (entries 7-9). The yield of 3a could be
improved to 80% when 2.0 equiv of cyclohexanone was used
(entry 13). Other palladium salts showed lower catalytic
Table 1 Reaction of cyclohexanones (2) with 1-methyl-1H-indole (1a)a
O
5 mol% PdI2
10 mol% CPDO
O2, 125 oC, 16 h
R1
+ S8
R1
+
N
S
N
o-dichlorobenzene
1a
2
3
Entry
Cyclohexanone
O
Product
Yield (%)b
R1
R1
S
N
R1 = H
R1 = Me
R1 = Et
1
2
2a
2b
3a
70 (68)c
85
3b
3c
3
2c
80
R1 = isopropyl
R1 = n-pentyl
R1 = tert-pentyl
R1 = Ph
4
2d
2e
2f
3d
3e
3f
73
87
85
81
74
30
With the optimized reaction conditions established, the
substrate scope with respect to cyclohexanones was explored
(Table 1). When the reaction scale was enlarged to 5.0 mmol, the
desired product 3a was obtained in 68% yield (entry 1).
Cyclohexanones bearing an alkyl substituent at the 4-position
5
6
7d
2g
2h
3g
3h
R1 = CO2Et
8
35 were able to smoothly couple with 1a and sulfur powder to give
the corresponding product in good to high yields (entries 2-6).
When 4-pentylcyclohexanone (2e) was employed, the desired
product 3e was obtained in 87% yield (entry 6). The presence of a
phenyl substituent at the 4-position did not significantly decrease
40 the reaction yield (entry 7). The ester functional group was well
tolerated to give the product 3h in 74% yields (entry 8). The
position of the methyl substituent on the cyclohexanone ring
Me
O
9d
2i
2j
3i
3j
60
S
N
O
10
trace
Me
S
N
profoundly
affected
the
reaction
yield;
when
3-
a Conditions: 1a (0.5 mmol), 2 (1.0 mmol), PdI2 (0.025 mmol), CPDO (0.05
mmol), sulfur (1.0 mmol), o-dichlorobenzene (2.0 mL), 125 oC, 16 h, under
oxygen. b Isolated yield based on 1a. c Reaction was performed on a 5.0
mmol scale. d 24 h.
methylcyclohexanone (2i) was used, the desired product 3i was
45 obtained in 60% yield (entry 9); however, no desired product 3j
was observed when 2-methylcyclohexanone (2j) was used (entry
10). The structure of 3b was further confirmed by X-ray
crystallography (scheme 3).
25 efficiency than PdI2 under similar reaction conditions (entries 14-
16). However, a combined use of Pd(OAc)2 and iodine could
Scheme 3 X-ray structure of 3b
2
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PdI2 (5 mol%)
CPDO (10 mol%)
125 oC, O2
S8
4a
(a)
5a
4a
+
+
3b
View Article Online
70%
0%
25%
2 h
DOI: 10.1039/C4CC08370A
90%
16 h
PdI2 (5 mol%)
CPDO (10 mol%)
125 oC, 16 h, O2
3b
(b)
94%
Based on these observations, a plausible mechanism to
45 rationalize this transformation is illustrated in Scheme 5.
To further examine the scope and limitation of the reaction, we
Treatment
of
1-methyl-1H-indole
(1a)
with
4-
tested various indoles for this kind of reaction (Table 2). Firstly,
the influence of various substituents at C-5 position was
evaluated (entries 1-7). In all cases, the reaction proceeded
smoothly to give the corresponding products in moderate to good
yields. Functional groups such as cyano, fluoro, bromo and even
iodo were all compatible under the optimized reaction conditions.
methylcyclohexanone (2b) in the presence of sulfur powder
affords intermediate 1-methyl-3-(4-methylcyclohex-1-en-1-yl)-
1H-indole (5a).21 C-H activation of 5a with Pd(0) generates a
50 Pd(II) complex 6a. A subsequent insertion reaction and reductive
elimination afford the cyclized intermediate 4a. In the meanwhile,
Pd(II) can be transformed into Pd(0) in the presence of S8 and
CPDO, closing the first catalytic cycle. Another Pd(II) complex
7a can be formed by palladation of intermediate 4a. Subsequent
55 β-hydride-elimination will liberate intermediate 8a, which can
be converted into the final product 3b via a further oxidative
5
10 Similar results were observed when the substituents were located
at the C-6 position of 1-methyl-1H-indole (entries 8-12). When an
ester group was present, the desired product 3s was obtained in
65% yield (entry 9). However, 3x was achieved in only 38%
yield when the same group was located at C-7 position (entry 14).
15 Only a trace amount of 3y was observed when the methyl
substituent was located at C-4 position (entry 15). In addition, 1-
benzyl-1H-indole (1q) could also react with 2a and sulfur powder
to give the corresponding product 3z in 73% yield (entry 16).
To have a better understanding of the reaction, some control
20 experiments were performed under various conditions. The
reaction of 1-methyl-1H-indole (1a) with 4-methylcyclohexanone
(2b) and sulfur powder gave 1-methyl-3-(4-methylcyclohex-1-en-
1-yl)-1H-indole (5a) in 27% yield in 1 h. Meanwhile, 3,6-
dimethyl-2,3,4,6-tetrahydro-1H-benzo[4,5]thieno[2,3-b]indole
25 (4a) and the desired product 3b were observed in 43% and 5%
yields , respectively (Table 3, entry 1). When the reaction time
was extended to 4 h, the yield of 3b increased to 50% while the
yields of 5a and 4a both decreased (entry 2). No desired product
could be obtained when the reaction was carried out under argon
30 atmosphere (entry 3). Interestingly, 5a could be observed even in
the absence of palladium catalyst and ligand by GC-MS (entries
4-5). Treating 5a with sulfur powder afforded 4a and 3b with 70%
and 25% yields (Scheme 4, a). 4a could be further converted into
3b in 94% yield under the catalytic conditions (Scheme 4, b).
dehydrogenation-tautomerization process.
A
metal-hydride
species 9a can be regenerated into the initial catalyst 10a in the
presence of oxygen, thus closing the second catalytic cycle.20a
60 Scheme 5 Proposed mechanism
1a
2b
+
Pd
N
N
S8
6a
5a
Pd(0)
Pd(II)
S8, CPDO
S
N
LnPd(II)I2
4a
HI
35
10a
H2O
Table 3 Control experiments Ia
PdLnI
S
N
7a
O2, HI
3nd cycle
LnPd(H)I
3b
9a
S
N
8a
Conclusions
65 In conclusion, we have developed an efficient approach for the
construction of C-S bonds via double C-H sulfuration using
sulfur powder as the sulfur source. Cyclohexanones acted as the
novel aryl source via dehydrogenation-tautomerization process
40
Scheme 4 Control experiments II
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65
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Excellent Talents in University from Ministry of Education of
China (NCET-11-0974) and the Research Fund for the Education
Department of Hunan Province of China (13C192).
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