DMSO/SOCl2-mediated C(sp2)-H amination: Switchable synthesis of 3-unsubstituted indole and 3-methylthioindole derivatives

Article abstract of DOI:10.1039/d0cc07453e

The reaction of 2-alkenylanilines with SOCl2 in DMSO was found to selectively afford 3-unsubstituted indoles and 3-methylthioindoles. This switchable approach was found to be temperature-dependent: at room temperature, the reaction afforded 3-unsubstituted indoles through intramolecular cyclization and elimination; while at higher temperature, the reaction gave 3-methylthioindoles via further electrophilic methylthiolation.

Full text of DOI:10.1039/d0cc07453e

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DOI: 10.1039/D0CC07453E
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DMSO/SOCl2-mediated C(sp2)−H amination: switchable
synthesis of 3-unsubstituted indole and 3-methylthioindole
derivatives
Received 00th January 20xx,
Accepted 00th January 20xx
Jingran Zhang,a Xiaoxian Li,a Xuemin Li,a Haofeng Shi,a Fengxia Sun,b and Yunfei Du*a
DOI: 10.1039/x0xx00000x
The reaction of 2-alkenylanilines with SOCl2 in DMSO was found to
selectively afford 3-unsubstituted indoles and 3-methylthioindoles.
This switchable approach was found to be temperature-
dependent: at room temperature, the reaction afforded 3-
unsubstituted indoles through intramolecular cyclization and
elimination. While at higher temperature, the reaction gave 3-
methylthioindoles via further electrophilic methylthiolation.
Scheme 1 Metal-free synthesis of indoles from 2-alkenylanilines
Indole skeleton is one of the most abundant and important
motif which widely exists in natural products, such as
serotonin and reserpine.1 In addition, many best-selling small-
molecule drugs including tadalafil, rizatriptan, fluvastain and
arbidol, all possess the indole framework in their respective
structures.2 Over the past several decades, the synthesis of
indole and its derivatives has been a topic of great interests
because of their ever-expanding application in synthesis of
natural products and pharmaceutical agents.3 Although
numerous methods for their synthesis have been developed,
there is still a need to develop efficient and economic
strategies that can synthesize functionalized indoles bearing
some unique substituents.4
corresponding indoles by using different oxidative systems. In
this communication, we reported an alternative oxidative
approach for synthesis of indoles by treating 2-alkenylanilines
solely with SOCl2 in DMSO. Differing from the above methods
which only realize the direct oxidative C(sp2)-N bond formation
for the assemblage of indole framework, this approach can also
realize the further functionalization of the indole skeleton with
an unique methylthio group at its 3 position.
It is well known that DMSO is not only a widely used organic
solvent, but also an oxidant in some classical reactions, including
Swern oxidation,14 Pfitzner-Moffatt oxidation,15 and some other
newly discovered reactions.16 Literature survey indicated that
DMSO has been used as sulfur source to introduce thiomethyl
group17 or methylsulfonyl group18 to olefins, leading to the
formation of adducts via addition reactions. However, there are
Intramolecular oxidative C(sp2)−H amination of 2-
alkenylanilines is one of the straightforward approaches for the
construction of indole skeleton. In the past decades, numerous
transition metal-mediated methods, by using Pd,5 Cu,6 Ru7 and
Ag8 as catalysts, have been developed for the synthesis of this
privileged heterocyclic framework. Most strikingly, the
conversion of 2-alkenylanilines to indole compounds via
intramolecular oxidative cyclization could also be realized under
metal-free conditions. For instances, DMTST,9 PhIO,10 DDQ,11
few
reports
describing
the
direct
functionalization/methylthiolation of olefins by using DMSO as
sulfur source.19 In this regard, it is of interest to develop a new
method for methylthiolation of olefin compounds.
In our previous work,20 we have realized the synthesis of 4-
(methylthio)isochromenones via treating 2-alkynylbenzoates with
DMSO/SOCl2. Encouraged by the results, we were interested to
investigate whether an alkene substrate can also react with
DMSO/SOCl2 to enable the similar oxidative cyclization, affording
the corresponding 3-methylthio substituted indoline 2a’. Alkene 1a
was then used as the modeling substrate to test the feasibility of
proposed transformation. Surprisingly, 3-unsubstituted indole 2a,
ranther than the expected indoline was formed by treating N-Ts-2-
alkenylaniline 1a (0.5 mmol) with DMSO (1 mL) and (COCl)2 (2.0
equiv) at room temperature. Then we came to optimize the
reaction conditions for this newly discovered method. First, we
12
NFSI/(PhSe)2 and NIS,13 have all been utilized to react with 2-
alkenylanilines for synthesis of indole products (Scheme 1a). All
the above methods have their respective merits in producing the
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Table 1 Optimization of the reaction conditionsa
room temperature, and the best conditions for synthesis of 3-
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methylthioindoles were concluded to be 0D.5OIm: 1m0.o10l3o9f/D10aCwC0it7h4543.0E
o
equiv of SOCl2 in DMSO (1 mL) at 70 C.
Table 2 DMSO/SOCl2-mediated synthesis of 3-unsubstituted
indoles 2a, b
yield (%)b
additive
time
(h)
6
T (℃)
entry
solvent
DMSO
DMSO
DMSO
DMSO
DMSO
DCE
CH3CN
THF
1,4-dioxane
toluene
DMF
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
(equiv)
(COCl)2 (2.0)
SOCl2 (2.0)
TsCl (2.0)
2a
3a
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
2
3
4
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
70
70
70
55
73
12
16
40
64
56
60
67
49
62
83
92
86
80
43
0
3
12
12
12
12
12
12
12
12
12
2
AcCl (2.0)
TFAA (2.0)
SOCl2 (2.0)
SOCl2 (2.0)
SOCl2 (2.0)
SOCl2 (2.0)
SOCl2 (2.0)
SOCl2 (2.0)
SOCl2 (2.5)
SOCl2 (3.0)
SOCl2 (3.5)
SOCl2 (3.0)
SOCl2 (3.5)
SOCl2 (4.0)
5
6c
7c
8c
9c
a Reaction conditions: 1 (0.5 mmol), SOCl2 (3.0 equiv), DMSO (1 mL),
rt, 30 min, unless otherwise stated. b Isolated yield. c The reaction
was carried out at 0 oC for 30 min.
10c
11c
12
13
14
15
16
17
0.5
0.5
0.5
1
With the optimal conditions in hand, the scope and limitation
of this protocol were studied in Table 2. First, we investigated
the effects of substituent R1 residing on the aromatic moiety of
N-Ts-2-styrylanilines. We found that when R1 was electron-
withdrawing group (F, Cl, Br, CO2Me), the corresponding starting
material could afford the desired products in good to excellent
yield (Table 2, 2b-f). However, when the substrates bearing the
electron-withdrawing trifluoromethyl or cyano group were
applied (Table 2, 2g-h), the reaction afforded the corresponding
product in a relatively lower yield. Furthermore, when R1 was a
methyl group, all the reactions afforded the target products in
excellent yield (Table 2, 2i-k). It is worth noting that when R1 was
a methoxy group, the corresponding substrate was converted to
10
35
55
1
a Reaction conditions: 1a (0.5 mmol) in solvent (1 mL), unless
otherwise stated. b Isolated yield. c Reaction conditions: 1a (0.5
mmol), DMSO (3.0 equiv) in solvent (1 mL).
began to evaluate the additives including (COCl)2, SOCl2, TsCl,
AcCl and TFAA, the results indicated that SOCl2 was the most
effective one (Table 1, entries 1-5). Next, solvent screening
showed that using DMSO as both reactant and solvent was the
most favorable choice, while the reaction led to a much lower
yield when using 3.0 equiv of DMSO as reactant and DCE, CH3CN,
THF, 1,4-dioxane, toluene or DMF as respective solvent (Table 1,
entries 6-11). Later on, when the dosage of SOCl2 was increased
to 3.0 equiv, the yield of 2a was improved obviously to 92% and
the time was greatly shortened. However, when the dosage of
SOCl2 was further increased to 3.5 equiv, the reaction did not
afford a better result (Table 1, entries 12-14). Next, the reaction
temperature was further investigated. When the reaction was
Table 3 DMSO/SOCl2-mediated synthesis of 3-unsubstituted
indoles 2a, b
o
carried out at 70 C, we were surprisingly to find that product 2a
was formed in a much lower yield due to the formation of a
predominant byproduct, which was confirmed to be 3-
methylthioindole 3a. This outcome inspired us to further
investigate whether this method could be applied to the
synthesis of indoles bearing a 3-methylthio substituent. The
results revealed that improving reaction temperature was
indispensable for the conversion of 2-alkenylaniline 1a to 3-
methylthioindole 3a (Table 1, entries 13-15). At 70 oC, when the
dosage of SOCl2 was increased to 3.5 equiv, the yield of 3a was
further improved to 35% (Table 1, entry 16). Then, we tried to
achieve the optimal conditions of 3a formation through altering
the reaction temperature and the dosage of SOCl2 (see the
Supporting Information for details). On the basis of the
screening results, the optimal conditions of DMSO/SOCl2-
mediated synthesis of 3-unsubstituted indoles were confirmed
to be 0.5 mmol of 1a with 3.0 equiv of SOCl2 in DMSO (1 mL) at
aReaction conditions: 1 (0.5 mmol), SOCl2 (3.0 equiv), DMSO (1
mL), rt, 30 mis. b Isolated yield.
3-unsubsitituted indole with concomitant formation of 3-
methylthioindole under the standard condition. In order to avoid
the formation of 3-methylthioindole byproduct, we carried out
the reaction at 0 oC, and the 3-unsubstited indole derivative 2l
was obtained selectively in 93% yield (Table 2, 2l).
Subsequently, we proceeded to explore the substituent effect
at the alkene moiety in the substrates. As shown in Table 3, the
2 | J. Name., 2012, 00, 1-3
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majority of N-Ts-2-styrylanilines (R2 = aryl) could afford the
In the past decades, some deuterated pharmaceutical agents
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corresponding 3-unsubsitituted indoles in good yield irrespective have exhibited good biological activity,DOwI:h1i0c.h103h9a/sD0gCrCa0d7u4a5l3lEy
of the substituent type on phenyl ring (Table 3, 2m-y). To our received extensive attention.22 It is worth noting that DMSO
delight, it was proved that a thiophene ring or naphthalene could also be replaced with its deuterated counterpart in our
linked to the alkene moiety in the substrates was also well method. As shown in Scheme 2, treating substrate 2v (0.5 mmol)
tolerated under the standard conditions (Table 3, 2z, 2aa). with 3.5 equiv of SOCl2 in 1 mL of DMSO-d6 can conveniently
Unfortunately, when N-Ts-2-styrylaniline bearing a non-aromatic afford the desired 3-(d3-methylthio) indole 3’ in 78% yield.
R2 substituent (R2 = H, n-Pr or Bn) was applied, no desired
product was obtained in each case (not shown). Furthermore,
the N-Ts group in the substrates could also be replaced with N-
Ms, N-p-ClC6H4SO2 and N-Boc substituent, and the
Scheme
methylthio) indole 3’
2
DMSO-d6/SOCl2-mediated synthesis of 3-(d3-
corresponding substrates were converted to the desired
products in excellent yield (Table 3, 2ab-ad). Disappointingly,
this method was not applicable to the substrates bearing N-Me,
N-Bn or OH moieties, as the reaction in each case gave a
complex mixture under the standard conditions (not shown).
Table 4 DMSO/SOCl2-mediated synthesis of 3-methylthioindoles
3a,b
In order to authenticate the reaction mechanism for this
transformation, we carried out some control experiments
(Scheme 3). First, treating 2-alkenylanilines 1a with SOCl2 (2.0
equiv) in DMSO under room temperature afforded 3-
unsubstituted indole 2a in 72% yield and 3-methylthioindoline
2a’ in 20% yield at the same time (Scheme 3a). Next, treatment
of indoline 2a’ with SOCl2 (3.0 equiv) in DMSO under the
standard conditions generated the 3-unsubstituted indole 2a
(Scheme 3b), thus providing further support for the postulate
that 2a was formed through the formation of 3-
methylthioindoline 2a’ via intramolecular cyclization, followed
a Reaction conditions: 1 (0.5 mmol), SOCl2 (4.0 equiv), DMSO (1
mL), 70 oC, 50 min. b Isolated yield. c Reaction conditions: 1 (0.5
o
mmol), SOCl2 (3.5 equiv), DMSO (1 mL), 70 C, for 40 min.
Literature survey indicated that 3-methylthioindole
derivatives were widely studied in medicinal chemistry and drug
development due to their interesting biological activities.21
Having this in mind, we came to investigate substrate scope for
the synthesis of the biologically interesting 3-methylthioindole 3.
Scheme 3 Control experiments
by an elimination process. When 3-unsubstituted indole 2a was
further treated with SOCl2 (3 equiv) in DMSO under 70 oC, it
underwent
a
complete methylthiolation to afford 3-
The results listed in Table 4 showed that all substrates
1
methylthioindole 3a. However, when the same experiment was
carried out at room temperature, no desired product 3a was
obtained (Scheme 3c). The results of these control experiments
showed that substrates 1 were first converted to the 3-
unsubstituted indoles 2, which could further undergo
electrophilic methylthiolation to give 3-methylthioindole 3 at
higher temperature.
On the basis of outcomes from these control experiments as
well as previous reports,20 a plausible mechanism for this
switchable synthesis of 3-unsubstituted indole and 3-
methylthioindole derivatives was purposed in Scheme 4. First,
employed in this reaction were smoothly converted to the
corresponding products 3 in moderate to good yield. When N-
Ts-2-styrylaniline 1a was applied, the reaction afforded the
desired 3-methylthioindole 3a in 55% yield, together with the
formation of some unidentified byproducts. Specifically,
substrates 1 bearing either an electron-donating methyl group,
or an electron-withdrawing fluoro or bromo substutent on the
left phenyl ring could form the corresponding products 3b-d in
moderate yield. Furthermore, substrates bearing fluoro, methyl
or methoxy group on the right phenyl ring, were all conveniently
transformed into the corresponding products 3e-j, with 3-
methylthioindole 3e obtained in relatively lower yield.
Meanwhile, the reaction of 2-thienyl-substituted substrate also
gave the corresponding product 3k in satisfactory yield. To our
delight, it was found that when Ms, Boc or Ac group were used
to take the place of Ts substituents on the N-atom, the
corresponding indole products 3l-n could also be achieved under
the standard conditions.
MeSCl
B
was generated in situ via intermediate
dimethylsulfochlorine cation.20, 23 Then, the alkene double bond
in the substrate reacted with the reactive species MeSCl B,
through electrophilic addition to afford the sulfonium ion
intermediate C.24 Next, 5-exo-tet cyclization occurred in
intermediate C to give D, which was converted to indoline E by
the abstraction of a proton. Later on, indoline E underwent
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J. Name., 2013, 00, 1-3 | 3
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M. Hirama, Chem. Commun., 1998, 1399-14V0ie0w; A(retic)le RO.nlinCe.
elimination to give F, which was converted to 3-unsubstituted
indole 2a via deprotonative aromatization. When the reaction
was operated at 70 ℃, indole 2a reacted further with MeSCl B
via electrophilic methylthiolation, via intermediate G, to afford
3-methylthioindole 3a.
Larock, T. R. Hightower, L. A. Hasvold and K. P. Peterson, J.
DOI: 10.1039/D0CC07453E
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Scheme
4 Proposed mechanism for the formation of 3-
methylthioindoline 2a and 3-methylthioindole 3a
10 L. Fra, A. Millán, J. A. Souto and K. Muñiz, Angew. Chem. Int.
Ed., 2014, 53, 7349-7353.
In conclusion, we have developed a metal-free oxidative
protocol for the switchable synthesis of 3-unsubstituted indoles
and 3-methylthioindoles mediated by DMSO/SOCl2. The 3-
unsubstituted indoles derivatives could be prepared at room
temperature, which was proved to involve intramolecular
cyclization followed by elimination process. Besides, 3-
methylthioindoles were obtained at higher temperature,
through further electrophilic methylthiolation of the obtained 3-
unsubstituted indoles. Further investigations on the substrate
patterns, e.g., 2-(1-phenylvinyl)anilines and 2-alkynylanilines, as
well as reaction mechanism are still in progress in our lab..
We acknowledge the National Natural Science Foundation of
China (#22071175) for financial support.
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Conflicts of interest
There are no conflicts to declare.
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