Synthesis of 1: H -indole-3-sulfonates via palladium-catalyzed tandem reactions of 2-alkynyl arylazides with sulfonic acids

Article abstract of DOI:10.1039/c7ob01337j

An efficient method for the synthesis of 1H-indole-3-sulfonates via palladium-catalyzed tandem reactions of 2-alkynyl arylazides with sulfonic acids has been developed. The desired products were obtained in good to excellent yields under mild reaction con

Full text of DOI:10.1039/c7ob01337j

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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Synthesis of 1H-indole-3-sulfonates via Palladium-catalyzed
Tandem Reactions of 2-Alkynyl Arylazides with Sulfonic Acids
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaoxiang Zhang*, Ping Li, Chang Lyu, Wanxiong Yong, Jing Li, Xinbao Zhu and Weidong Rao*
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient method for the synthesis of 1H-indole-3-sulfonates via However, the desired 3-tosyloxyindole product was obtained
palladium-catalyzed tandem reactions of 2-alkynyl arylazides with in 10% yield. Hence, mild and efficient method for the
sulfonic acids has been developed. The desired products were preparation of 1H-indole-3-sulfonates is desired.
In recent years, gold catalysis have become an efficient
obtained in good to excellent yields under mild reaction
conditions. The reactions were shown to proceed very fast, in
most cases, within 10 min.
method for various novel organic transformation.11,12 In 2016,
we reported that
α
-imino gold carbene13 generated in situ
from 1-azido-2-(phenylethynyl)benzene 1a was attacked by
1,4-dioxane, which gave the unexpected ring-opening indole
derivative 4. (Scheme 1, a) Later, we repeated this reaction at
room temperature by using Pd(OAc)2 as catalyst and found
that 2-phenyl-1H-indol-3-yl methanesulfonate 3a was obtained
in 96% yield in 3 min (Scheme 1, b). The reason for the
competing addition of MsOH and 1,4-dioxane to metal
carbenes could be due to the concentration of nucleophiles
Aryl sulfonates are one of the most important cross-
coupling reagents in organic chemistry due to its relative
stability and low cost,1 which also can act as bridge structures
or ligands and be found in many bioactive compounds.2
Therefore, many methods for the synthesis of aryl sulfonates
have been established. Traditionally, the aryl sulfonates were
prepared through the tosylation of phenols in the presence of
TsCl/NEt3 or Py.3 Other methods mainly include the utilization
of organic salts prepared from sulfonyl chlorides and DMAP4 or
DBN5 and organometallic reagents such as organoindium6 and
organozinc7. In 2015, Yuan’s group reported the synthesis of
sulfonate esters via iodine-mediate radical reactions of sodium
and reactivity of
a mild and efficient method for the synthesis of 1H-indole-3-
sulfonates via palladium-catalyzed tandem reactions of 2-
alkynyl arylazides with sulfonic acids . The desired product
α -imino metal carbenes. Herein, we disclose
3
1
2
sulfinates with phenols.8 Recently, a copper-catalyzed C-H 1H-indole-3-sulfonates were obtained in moderate to excellent
sulfonyloxylation of electron-rich arenes with p-
toluenesulfonic acid9 has been developed by Xiang’s group.
However, most of these reported methods have some
drawbacks, such as the generation of hazard acids, harsh
reaction conditions or limited substrates scope. To our surprise,
yields and, in most cases, within 10 min.
Scheme
1
Palladium-catalyzed synthesis of 1H-indole-3-
sulfonates
3
.
the synthesis of 1H-indole-3-sulfonates
3 as an important
group of aryl sulfonates has not been well studied. Only two
examples were reported. In 1991, Piroelle’s group10a reported
the synthesis of 1H-indole-3-sulfonates through the direct
tosylation of 3-hydroxy indoles with TsCl, which provided the
desired products in 54-59% yields. Later, Somei's group10b
reported an example for the preparation of 3-tosyloxyindole
via a novel 3,3-sigmatropic rearrangement of 1-tosyloxyindole.
Jiangsu key lab of Biomass-based Green Fuels and Chemicals, College of Chemical
Engineering, Nanjing Forestry University, Nanjing 210037, China. E-mail:
zhangxiaoxiang@njfu.edu.cn (X. Zhang); weidong@njfu.edu.cn (W.Rao).
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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At the beginning, we chose to focus our attention on the desired 2-phenyl-1H-indol-3-yl methanesulfonates in 82-93%
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DOI: 10.1039/C7OB01337J
reaction of 1-azido-2-(phenylethynyl)benzene 1a and yields (Scheme 2, 3b-3e). Similarly, the present method was
methanesulfonic acid 2a as the probe substrates to establish shown to work very well for 2-alkynyl arylazides with different
the reaction conditions. This revealed treating a solution of 1a substituted groups on the aromatic ring (R1 = halide, CF3 or
(1 equiv) in 1,4-dioxane with 10 mol% of Pd(OAc)2 at room Me). In these reactions, the corresponding products 3f-3j were
temperature followed by 3 equiv of MsOH for 3 min gave the obtained in 90-95% yield. On the other hand, the analogous
best result (Scheme 1, entry 1). Under these conditions, 2- reactions involving 2-alkynyl arylazides bearing the aliphatic
phenyl-1H-indol-3-yl methanesulfonate 3a was provided in 96% substituted groups on the alkyne carbon were also examined
yield. As shown in entry 2, a comparable product yield was and found to afford the desired products in comparable yields
afforded on decreasing the catalyst loading from 10 to 5 mol % of 83-96% (Scheme 2, 3k-3o). However, we found reaction of
(Scheme 1, entry 2). However, lower product yields were 2-alkynyl arylazides containing a terminal alkyne group to be
found when the solvent system was changed from 1,4-dioxane less effective, affording a mixture of decomposition products
to (CH2Cl)2, PhCl, MeCN or MeNO2 (Scheme 1, entries 3-6). when that could not be identified by TLC and 1H NMR analysis of the
water was employed as the solvent, there was no reaction after crude mixture.
prolonging the reaction time to 24 h (Scheme 1, entry 7). Other
palladium catalysts were also examined. when PdCl2 was used as Scheme
2
Substrate scope to 2-phenyl-1H-indol-3-yl
methanesulfonates 3.a,b
catalyst, the reaction was found to proceed slowly and provide the
desired product in 50% yield (Scheme 1, entry 8). Slightly lower
yields of 80-88% were also obtained on repeating the reaction with
other palladium catalysts such as Pd (PPh3)4, PdCl2(PPh3)2, and
Pd(NO3)2·2H2O in place of Pd(OAc)2 (Scheme 1, entries 9-11). The
reaction was also tested without catalyst, and was found to give no
reaction (Scheme 1, entry 12). On the basis of these results,
reaction of 1a in the presence of 5 mol % of Pd(OAc)2 at room
temperature followed by 3 equiv of MsOH for 3 min was considered
to give the optimal conditions.
Scheme 1 Optimization of the reaction conditions.a
Entry
Catalyst
Solvent
Time
Yield/%b
1
2
3
4
5
6
7
8
9
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
1,4-dioxane
1,4-dioxane
(CH2Cl)2
PhCl
MeCN
MeNO2
H2O
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
3 min
3min
1 min
3 min
0.5 h
1 min
24 h
24 h
18 h
8 h
96%
96%c
70%
66%
58%
79%
n.r.
50%
88%
87%
80%
n.r.d
Pd (PPh3)4
PdCl2(PPh3)2
Pd(NO3)2·2H2O
-
a
All reactions were performed at r.t. in 0.4 mL 1,4-dioxane
10
11
12
with Pd(OAc)2: 1a : MsOH = 0.05: 1 : 3. bIsolated yield.
8 h
24 h
a
Aliphatic sulfonic acid MsOH 2a was shown to be efficient
sulfonylating reagent. Later, aromatic sulfonic acids were also
examined under the optimal reaction conditions. The reactions
Unless stated otherwise, all reactions were performed
at room temperature in 0.4 mL 1,4-dioxane with catalyst:
1a :
2a = 0.10: 1 : 2. Isolated yield. c 5mol% of catalyst
b
used. d Without catalyst. n.r. = no reaction was used.
of
found to give the corresponding products
(Scheme 3, 3q 3t). Other aromatic sulfonic acids such as
naphthalene-1-sulfonic acid 2c, 4-chlorobenzenesulfonic acid
2d and benzenesulfonic acid 2e were also performed at 60
1
with 4-methylbenzenesulfonic acid 2b were tested and
3
in 70-97% yields
With the optimal conditions in hand, we started to examine
the generality of the present procedure (Scheme 2). Reactions
of 2-alkynyl arylazides with aryl group containing a pendant
electron-donating or electron-withdrawing substitutes or
hetero aryl group on the alkyne carbon with MsOH gave the
-
℃,
2 | J. Name., 2012, 00, 1-3
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which provided the desired 1H-indole-3-sulfonates in 87-95 bearing electronic and sterically demanding substituted groups.
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results show the reaction tolerates a structurally diverse set of
yields (Scheme 3, 3u
-
3w).
Scheme
3
Substrates scope to 2-phenyl-1H-indol-3-yl aryl
sulfonates 3.a,b
2-alkynyl arylazides. The efficiency of current mild and simple
methodology was demonstrated by the good to excellent
yields and short reaction time, in most cases, within 10 min.
Efforts are currently underway to apply the method for the
transition metal-catalyzed cross-coupling reactions.
Acknowledgements
This project was supported by the National Natural Science
Foundation of China Grant (21302096, 21502093), Natural
Science Foundation of Jiangsu Province Grant (BK20130962,
BK20150871) and the Project Fund from the Priority Academic
Program Development of Jiangsu Higher Education Institutions
(PAPD).
Notes and references
a
All reactions were performed at r.t. in 0.4 mL 1,4-dioxane
with Pd(OAc)2: 1a : TsOH = 0.05: 1 : 3. bIsolated yield. cReaction
was conducted at 60
1
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Pd(OAc)2-catalyzed 2-phenyl-1H-indol-3-yl methanesulfonate
3a forming reaction to proceed by the mechanism outlined in
Scheme 4, although it is highly speculative. This could involve
the activation of the 1-azido-2-(phenylethynyl)benzene 1a via
coordination of Pd catalyst to triple bond and deliver the
complex
activated alkyne to give complex
imino palladium carbene species C14 was afforded, which was
then trapped by MsOH to deliver the complex . Subsequent
protondemetalation step led to the formation of desired
product 3a
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method to 1H-indole-3-sulfonates via Pd(OAc)2-catalyzed
tandem reactions of 2-alkynyl arylazides with sulfonic acids.
The reaction was shown to be applicable to various substrates
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Synthesis of 1H-indole-3-sulfonates via Palladium-catalyzed
tandem reactions of 2-alkynyl arylazides with sulfonic acids