An efficient: T -BuOK promoted C3-chalcogenylation of indoles with dichalcogenides

Article abstract of DOI:10.1039/c8ob00948a

A versatile and efficient method for the synthesis of 3-chalcogenyl-indoles from indoles and dichalcogenides employing t-BuOK as a promoter at room temperature has been achieved. The present protocol exhibited a broad functional group tolerance. Diverse 3-sulfenyl- and 3-selenyl-indoles were rapidly obtained in good to excellent yields with high regioselectivities. It is noteworthy that this transformation was applicable to N-protected and N-unprotected indoles, allowing N-deprotection and C3-chalcogenylation of indoles in one step.

Full text of DOI:10.1039/c8ob00948a

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An Efficient t-BuOK Promoted C3-Chalcogenylation of Indoles with
Dichalcogenides
Received 00th January 20xx,
Accepted 00th January 20xx
Yuanzu Yu,‡ Yan Zhou,‡ Zengqiang Song* and Guang Liang
DOI: 10.1039/x0xx00000x
A versatile and efficient method for the synthesis of 3-chalcogenyl-indoles from indoles and dichalcogenides employing t-
BuOK as promoter has been achieved at room temperature. The present protocol exhibited a broad functional group
tolerance. Diverse 3-sulfenyl- and 3-selenyl-indoles were rapidly obtained in good to excellent yields with high
regioselectivity. It is noteworthy that this transformation was applicable to N-protected and N-unprotected indoles,
www.rsc.org/
allowing
N-deprotection
and
C3-chalcogenylation
of
indoles
in
one
step.
synthetic practice and friendly to the environment. Iodine,11
persulfate,12 potassium carbonate,3f,13 sodium hydroxide,14
selectflour,15 N-bromosuccinimide16 were used as efficient
promoters when dichalcogenides and thiols were employed as
Introduction
The substituted indole moiety is prevalent in numerous
bioactive natural products, pharmaceuticals and functional
materials.1 Among a wide range of indole derivatives known,
3-chalcogenyl-indoles are currently attracting considerable
interest since they show a broad spectrum of biological and
pharmaceutical activities, which have a great therapeutic value
in the treatment of cancer, HIV, obesity, bacterial infection,
allergies, and heart diseases.2,3 They are also used as potent
inhibitors in tubulin polymerization.4
chalcogenylation agents. Furthermore,
a
variety of
sulfenylation agents such as arylsulfonyl chlorides,17 sulfonyl
hydrazides,18 quinone mono-O,S-acetals,19 sulfinic acids,20 and
N-hydroxyl sulfonamides21 were successfully used with the C3-
sulfenylation of indoles, proceeding smoothly under metal-
free conditions. Nevertheless, many of these methods
frequently require high temperature, long reaction times, toxic
reagents or excess additives, suffer from a poor substrate
scope.11-21 Therefore, the development of convenient and
efficient routes to chalcogenylation of indoles at the C3-
position under a mild condition with wide substrate scope is
highly in demand.
On account of the important applications of 3-chalcogenyl-
indoles, a number of different synthetic methods have been
established for constructing this heterocyclic scaffold.
Generically, two major synthetic strategies are typically
employed. The one route for the preparation of 3-chalcogenyl-
indoles involves the cyclization reactions of 2-alkynylanilines or
N, N-dialkyl-2-iodoanilines by using organochalcogen
electrophilic species, which is mediated by transition-metal
catalysts5 or a stoichiometric amount of iodine reagents.6 The
alternative route is achieved by the direct chalcogenylation of
a pre-existing indole ring because of its nucleophilic nature,
which has been exhibited to be the more convenient method
for the preparation of 3-chalcogenyl-indoles and extensively
explored. In this regard, a number of different strategies were
developed for the C3-chalcogenylation of indoles using
copper,7 palladium,8 iron,9 and magnesium10 as catalysts.
Some intriguing approaches for direct chalcogenylation of
indoles at the C3-position were further accomplished under
metal-free conditions, which are highly desirable due to the
R3
X
R1
t-BuOK
R3
X
R1
+
X
R3
N
N
R2
DMF, rt
R2
X: S, Se
Scheme 1. t-BuOK promoted C3-chalcogenylation of indoles.
Inspired by the aforementioned background3f,7-21 and our
continuous studies on the synthesis of indole derivatives,22 we
herein wish to describe the t-BuOK promoted regioselective
chalcogenylation of indole derivatives at the C3-position with
dichalcogenides. Notably, N-protected or N-unprotected
indole derivatives were all suitable for this transformation,
furnishing related products in a short reaction time (Scheme
1).
Results and discussion
We started our exploration focusing on the reaction between
1H-indole (1a) and diphenyl disulfide (2a). Initially, reaction
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Table 1. Optimization of the reaction conditions.a
Table 2. Chalcogenylation of 1H-indoles and dichalcogenides.a
Entry
1
Solvent
DMSO
DCM
Toluene
THF
Base (equiv.)
t-BuOK (2)
t-BuOK (2)
t-BuOK (2)
t-BuOK (2)
t-BuOK (2)
t-BuOK (2)
t-BuOK (2)
t-BuOK (2)
K2CO3 (2)
Cs2CO3 (2)
K3PO4 (2)
LiOAc (2)
t-BuOLi (2)
NaOH (2)
KOH (2)
Time (h)
12
12
12
12
12
12
2
Yield (%)b
60
2
16
3
17
4
32
5
MeCN
EtOH
DMF
H2O
48
6
39
7
94
8
12
12
4
trace
trace
87
9
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
10
11
12
13
14
15
16
17
18
19
20
12
12
12
4
38
n.r.c
23
90
12
12
12
12
12
12
57
Et3N (2)
n.r. c
trace
62
DBU (2)
t-BuOK (1.1)
t-BuOK (0.5)
-
41
n.r. c
aReaction conditions: 1a (0.2 mmol), 2a (0.22 mmol), base (equiv.) in
solvent (2 mL) at room temperature. b Yield refers to isolated products after
column chromatography. c No reaction was occurred.
was carried out in DMSO at room temperature under air, using
t-BuOK (2 equiv.) as a promoter. To our delight, the desired
product 3a was obtained in 60% yield after 12 h (Table 1, entry
1). Encouraged by this result, the reaction was systematically
evaluated under different reaction conditions to improve the
yield of 3a. At first, several solvents were tested (entries 2–8).
It was found that DMF was the best solvent, affording the
desired product in 94% yield in 2 h (entry 7). Other solvents did
not improve the efficiency of the reaction. Furthermore, the
use of water as the solvent, only trace amount of the
corresponding product was detected (entry 8). Subsequently,
reaction was investigated by replacement of t-BuOK with other
common bases, including inorganic bases and organic bases
(entries 9–17). K2CO3 and DBU gave desired product with trace
amounts (entries 9, 17). No reactions occurred in the presence
of LiOAc and Et3N (entries 12, 16). Moreover, bases such as
CsCO3, K3PO4, t-BuOLi, NaOH, KOH, all decreased the yield of
reaction in various degrees (entries 10, 11, 13–15). These
results proved that t-BuOK was the best promoter for this
transformation. Afterward, the amount of t-BuOK was further
screened (entries 18–20). It was found that the yields of
product decreased with reducing the amount of t-BuOK
(entries 18, 19), no reaction was observed in the absence of t-
BuOK (entry 20). Sulfenylation of indole was also explored with
thiophenol, but the efficiency of reactions was lower
compared with diphenyl disulfide (see the ESI). Therefore, the
optimal conditions for the preparation of 3-sulfenyl-indoles
aReaction conditions:
(2 mL) at room temperature.
1
(0.2 mmol),
2
(0.22 mmol), t-BuOK (2 equiv.) in DMF
were obtained: 1,2-diphenyl disulfide (1.1 equiv.), t-BuOK (2
equiv.), in DMF (2 mL) at room temperature for 2 h (entry 7).
After optimization study on the model reaction, we explored
the substrate scope and generality of this transformation. First
of all, indole derivatives containing various electron-donating
groups or electron-withdrawing groups at different positions
of indole rings were examined with diphenyl disulfide under
standard conditions. Generally, all of reactions proceeded very
well, furnishing desired products with good to excellent yields
in 2 h (Table 2, 3b–3j). To our delight, substrates possessing
halogens or amino groups were tolerated in this process,
providing an opportunity for further elaboration of 3-sulfenyl-
indole products which were generated (3c, 3d, 3g, 3i).
Interestingly, reaction of 2-methyl indole gave the
corresponding 3-sulfenyl-indole with excellent yield in 1 h (3j).
Furthermore, no desired product was detected when 3-methyl
indole was subjected to this process. Subsequently, we
evaluated the scope with respect to the disulfide reactant (3k
–
3s). It was found that diphenyl disulfide substrates bearing
various substituents (F, CF3, NO2, NH2, Cl, Me, OMe) at
different positions of the phenyl ring worked well in the
reaction, affording the desired products with good to excellent
yields in 2 h (3k
–
3q). It is worth mentioning that the electron-
2 | J. Name., 2012, 00, 1-3
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deficient 1,2-bis(2-(trifluoromethyl)phenyl)disulfane also gave To investigate the synthetic utility of this process, gram-scale
the desired product 3l in 98% yield in 1 h. Additionally, reaction was performed under the standard conditions. 10
reactions of heterocyclic disulfides, such as pyridine, mmol indole 1a and 11.1 mmol diphenyl disulfide 2a were
thiophene, proceeded smoothly, furnishing the corresponding subjected to the reaction in the presence of 20 mmol t-BuOK
products with moderate to good yields (3r
,
3s).
in 50 mL DMF at room temperature. After 6 h, the desired
The developed protocol also can be applied for the product was obtained in 81% yield, which demonstrated the
preparation of 3-selenyl-indoles using various indole practical application of this protocol to prepare 3-chacogenyl-
derivatives and diselenides. In general, the desired products indoles on gram-scale.
were rapidly formed in moderate to excellent yields in 2 h,
SPh
which was more efficient than the generation of 3-sulfenyl-
t-BuOK (2 equiv.)
PhSSPh
+
indoles with regard to the yields and reaction times (3t
Indole derivatives or diselenides with diverse substitutents
were all compatible with this transformation 3u 3ae).
–
3ag).
N
H
N
H
DMF (50 mL), rt
1a
2a
3a
1.8 g
(
–
10 mmol
11.1 mmol
Scheme 2. Scale-up reaction between 1a and 2a
.
Additionally, investigation on diphenyl diselenide with ortho-
substituents such as methyl group, nitro group, cynide group
provided the products in good to excellent yields (3z–3ab).
We also explored the mechanism of this process. A radical
trapping experiment was conducted by addition of TEMPO
(2,2,6,6-tetramethyl-1-piperidinyloxy) to reaction of indole 1a
and diphenyl disulfide 2a under the optimized conditions
(Scheme 3). Almost no influence on the formation of the
desired product was observed, suggesting no radicals were
involved in this transformation. Based on the above
experimental results and previous relevant studies, 3f,7-21 it was
proposed that 3-chalcogenyl-indoles were generated by base
(t-BuOK) promoted nucleophilic attack of C3-position of
indoles to dichalcogenides.
Table 3. Selenation of N-protected indoles and diphenyl diselenide.a
Entry
Indole (R2)
Pivaloyl (1k)
Benzoyl (1l)
Acetyl (1m)
Ts (1n)
Time (h)
Product (R2)
H (3a)
Yield (%)b
1
2
3
4
5
6
7
8
9
1.5
1
85
88
73
51
36
38
71
-
H (3a)
H (3a)
H (3a)
H (3a)
H (3a)
Boc (3ah)
n.d.c
6
2
SPh
t-BuOK (2 equiv.)
Pyridinyl (1o)
Pyrimidinyl (1p)
Boc (1q)
12
12
8
TEMPO (2 equiv.)
PhSSPh
+
N
H
N
H
DMF (2 mL), rt, 2 h
1a
0.2 mmol
2a
3a
Methyl (1r)
Benzyl (1s)
12
12
1.1 equiv.
yield: 90%
n.r. d
-
Scheme 3. Control experiments.
aReaction conditions:
1 (0.2 mmol), 2x (0.22 mmol), t-BuOK (2 equiv.) in
DMF (2 mL) at room temperature. bYield refers to isolated products after
column chromatography. cNo product was detected. dNo reaction was
occurred.
Conclusions
In summary, we have developed an efficient and convenient
route for C3-chalcogenylation of indole derivatives with
dichalcogenides at room temperature. This transformation
features mild reaction conditions, short reaction times, good
compatability with different functional groups as well as broad
substrate scopes. Moreover, this protocol is also suitable for
chalcogenylation of N-protected indoles, affording the
corresponding products with or without protecting groups.
Next, we turned our attention to explore the reaction of N-
substituted indoles with diphenyl diselenide under optimized
conditions (Table 3). Indoles with different N-protected groups
were examined. To our delight, the reaction of indoles with N-
protected groups such as pivaloyl, benzoyl, acetyl, Ts (p-
toluenesulfonyl) afforded the 3-selenyl-indoles with the
removal of protecting groups in moderate to good yields
(Table 3, entries 1–4). Pyridinyl and pyrimidinyl protected
indoles also gave 3-selenyl-indoles without protecting groups,
the yields of the products were 36% and 38% respectively
(entries 5, 6). These results indicated that selenation was
triggered by deprotection in these cases. Therefore, the
substituent is more easily removed by t-BuOK, the selenation
is more efficient. Surprisingly, the reaction of Boc (tertbutoxyl
carbonyl) protected indole provided the product 3ah in 71%
yield (entry 7). It is worth mentioning that the Boc group
normally will suppress the C3-chalcogenylation of indoles due
to its electron-withdrawing property. Additionally, when
methyl and benzyl protected indoles were used as substrates
for this transformation, almost no reactions occurred (entries
8, 9).
Experimental
A mixture of indole
1 (0.2 mmol), dichalcogenide 2 (0.22
mmol), t-BuOK (0.4 mmol), and DMF (2 mL) were added in a 5
mL glass tube, which was stirred at room temperature for
0.5−12 h. When the reaction was completed, it was mixed with
water and ethyl acetate. The reaction mixture was extracted
three times with ethyl acetate or dichloromethane. The
combined organic layer was washed two times with a little
amount of water, dried over anhydrous magnesium sulfate
and filtered. The filtrate was evaporated under vacuum and
the residue was purified by flash column chromatography on
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Conflicts of interest
There are no conflicts to declare.
6
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(81622043), the Natural Science Foundation of Zhejiang
Province (LQ18BO20004) for financial support.
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Organic & Biomolecular Chemistry
View Article Online
DOI: 10.1039/C8OB00948A
A practical route to prepare 3-sulfenyl- and 3-selenyl-indoles has been achieved using t-BuOK as a
promoter at room temperature.