Synthesis of 2-Substituted Indoles through Visible Light-Induced Photocatalytic Cyclizations of Styryl Azides

Article abstract of DOI:10.1002/adsc.201400527

A visible light-induced photocatalytic intramolecular cyclization of styryl azides has been developed in the presence of the ruthenium complex Ru(bpy)₃Cl₂ (0.5 mol%) as photocatalyst at room temperature. The present photocatalytic strategy features operational simplicity as well as high functional group tolerance, and provides a facile access to various 2-substituted N-free indoles in good to excellent yields. Importantly, the present process can employ sunlight as the light source to afford the corresponding products without loss of reaction efficiency.

Full text of DOI:10.1002/adsc.201400527

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DOI: 10.1002/adsc.201400527
Synthesis of 2-Substituted Indoles through Visible Light-Induced
Photocatalytic Cyclizations of Styryl Azides
Xu-Dong Xia,+a Jun Xuan,+a Qiang Wang,a Liang-Qiu Lu,a Jia-Rong Chen,a,*
and Wen-Jing Xiaoa,b,*
a
Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal
University, 152 Luoyu Road, Wuhan, Hubei 430079, Peopleꢀs Republic of China
Fax : (+86)-27-6786-2041; phone: (+86)-27-67862041; e-mail: chenjiarong@mail.ccnu.edu.cn or wxiao@mail.ccnu.edu.cn
Collaborative Innovation Center of Chemical Science and Engineering, 92 Weijin Road, Tianjin 300072, Peopleꢀs
b
Republic of China
+
These authors contributed equally to this work.
Received: May 28, 2014; Revised: July 1, 2014; Published online: September 11, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400527.
Abstract: A visible light-induced photocatalytic in-
tramolecular cyclization of styryl azides has been
developed in the presence of the ruthenium com-
alternative protocols for the synthesis of 2-substituted
indoles. In this context, the Driver group has reported
a rhodium-catalyzed intramolecular C H amination
À
plex Ru
N
of aryl azides and azidoacrylates, which represents
one of the most significant and efficient methods for
the assembly of various indoles and polycyclic indole
derivatives.[5] Other practical methods include transi-
tion metal-catalyzed arylation reactions at the C-2 po-
sition of indoles with arylboronic acids,[6] aryltrifluoro-
borate salts,[7] and hypoiodide reagents,[8] as well as
room temperature. The present photocatalytic strat-
egy features operational simplicity as well as high
functional group tolerance, and provides a facile
access to various 2-substituted N-free indoles in
good to excellent yields. Importantly, the present
process can employ sunlight as the light source to
afford the corresponding products without loss of
reaction efficiency.
direct dual C H functionalization.[9] Despite these im-
À
pressive advances, the development of new and effi-
cient methodologies for the synthesis of 2-substituted
indoles under mild and environmentally friendly con-
ditions remains highly desirable.
Keywords: azides; heterocyclic compounds; indoles;
photoredox catalysis; visible light
Recently, visible light-induced photocatalysis has
been nicely applied to the construction of useful and
important carbocycles and heterocycles due to its sus-
tainable character.[10,11] For example, the group of
Indoles and their derivatives are a family of privileged Zheng in 2012 developed an elegant example of
heterocyclic structures commonly found in a number indole synthesis through a visible light photocatalytic
À
of natural products and many biologically active mol- oxidative C N bond formation/aromatization se-
ecules.[1] Among them, 2-substituted indoles represent quence of styrylanilines.[12] They found that only the
a subclass of biologically and synthetically useful scaf- para-alkoxyphenyl-protected styrylanilines proved to
folds.[2] Thus, the development of efficient methods to be suitable for this transformation (Scheme 1, a).
construct these molecules has evoked considerable at- Quite recently, Rueping and co-workers disclosed
tention from both academic and industrial communi- a visible light photocatalytic protocol to construct N-
ties.[3] Traditional methods for the preparation of 2- alkylindoles via an intramolecular cyclization of the
substituted indoles are mainly based on the Fischer a-amino radical to tethered alkenes (Scheme 1, b).[13]
indole synthesis,[4] which always requires the heating In addition, azides have been used as versatile re-
of phenylhydrazines with aldehydes or ketones under agents to participate in a variety of transformations
acid catalysis. However, harsh reaction conditions for the construction of diversely functionalized nitro-
sometimes limit the functional group tolerance. Over gen-containing molecules due to their sensitivity to
the past 10 years, transition metal-catalyzed C H thermolysis, photolysis or transition metals.[14] For ex-
À
functionalization processes have been established as ample, Liu and co-workers recently described a visible
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Table 1. Optimization of the reaction conditions.[a]
Entry
Solvent
Conc. [M]
Time [h]
Yield [%][b]
1
2
3
4
5
6
CH3CN
DMF
EtOH
acetone
THF
DMF
DMF
DMF
DMF
DMF
DMF
0.05
0.05
0.05
0.05
0.05
0.1
0.05
0.05
0.05
0.05
0.05
24
24
24
24
24
24
24
24
24
24
24
88
96
93
74
7
93
96
96
12
trace
63
7[c]
8[d]
9[e]
10[f]
11[g]
[a]
Scheme 1. Visible light photocatalysis in indole synthesis.
Unless otherwise noted, the reaction conditions are as
follows: 1a (0.3 mmol), Ru(bpy)3Cl2·6H2O (2 mol%), sol-
AHCTUNGTRENNUNG
vent (6 or 3 mL), 3 W white LEDs, at room temperature.
Yield of the isolated product.
[b]
[c]
[d]
[e]
[f]
light-induced reduction of azides to the corresponding
amines with high efficiency.[15] As part of our ongoing
research program on the visible light photoredox cat-
alysis[16] and indole synthesis,[17] we report herein
a highly efficient route to construct N-free 2-substitut-
ed indoles from the readily available styryl azides via
a visible light-induced photocatalysis (Scheme 1, c).
1 mol% Ru
ACHTUNGTRENNUNG
0.5 mol% RuACHTUNGTRENNNUG
Without the use of photoredox catalyst.
In the absence of light source.
Without degas.
[g]
Initially, the styryl azide 1a was selected as the Incorporation of an electron-donating group (e.g.,
model substrate to optimize the reaction conditions. methyl, tert-butyl, phenoxy) or an electron-withdraw-
As summarized in Table 1, indole 2a was isolated in ing group at the para position of the benzene ring
88% yield when the reaction was performed in (such as the fluoro, chloro, trifluoromethyl, cyano)
CH3CN with 2 mol% RuACHTUNGRTNEUNG(bpy)3Cl2·6H2O as the photo- was well tolerated, affording the corresponding in-
catalyst (Table 1, entry 1). Encouraged by this pre- doles in 69–99% yields (2b–g, 2i). Moreover, the dis-
liminary result, the effects of other reaction media ubstituted and trisubstituted arenes also proved to be
were then investigated. Notably, the reaction in DMF suitable (2h, 2j). Notably, it seems that the steric and
or EtOH occurred smoothly to give the desired electronic effects have some impact on the reaction
indole product in 96% and 93% yields, respectively efficiency. For instance, the substrate containing
(Table 1, entries 2 and 3). Other reaction media such a nitro group at the ortho-position of the benzene
as acetone and THF resulted in inferior results ring gave indole 2k in only 46% yield after 60 h. To
(Table 1, entries 4 and 5). It is worth noting that our delight, styryl azide 1l bearing a naphthalene ring
changing the concentration of 1a leads to no increase was also tolerated and provided 2-(naphthalen-1-yl)-
in the yield of 2a (Table 1, entry 6). Further investiga- 1H-indole 2l in 81% yield.
tion revealed that the catalyst loading could be re-
In addition, a variety of substituents on the indole
duced to 0.5 mol% without apparent erosion of the ring was also investigated to further extend the gener-
reaction efficiency (Table 1, entries 7 and 8). Impor- ality of this transformation (Table 3). Variation of the
tantly, when the reaction was carried out in the ab- electronic properties of the R1 group at the C-5 posi-
sence of the photocatalyst or visible light irradiation, tion of the indole ring could be achieved without loss
only 12% or a trace amount of 2a was obtained after of reaction efficiency. Thus, a range of 5-substituted
24 h (Table 1, entries 9 and 10). The reaction without indoles (e.g., fluoro, chloro, bromo, ester, and methyl)
the degas process only provided the corresponding was synthesized in good to excellent yields (2m–o, 2q,
product in moderate yield (Table 1, entry 11).
2s). A difluoro-substituted substrate also appeared to
With the optimal reaction conditions established, be suitable to give the corresponding product 2p in
different aryl-substituted styryl azides 1 were then 91% yield. Surprisingly, it was found that 4-methyl-
employed to examine the generality of the methodol- substituted indole (2r) was obtained in only moderate
ogy. As shown in Table 2, structural variations of the yield (68%), which was probably due to the unexpect-
aryl ring at the C-2 position of indole were possible. ed light-induced E to Z isomerazition of the starting
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Table 2. Visible light-induced indole synthesis: variation of Ar.[a]
[a]
Unless otherwise noted, the reaction conditions are as follows: 1 (0.3 mmol), RuACHTNUTRGNE(UNG bpy)3Cl2·6H2O
(0.5 mol%), DMF (6 mL), 3 W white LEDs, at room temperature. Yield of the isolated product is
given.
material. Also, the Z isomer is reluctant to undergo tuted indole (2w), which could be useful for further
the desired cyclization (see the mechanistic studies in transformations.[18]
Scheme 3). More importantly, the photocatalytic pro-
To further demonstrate the synthetic utility, a gram
tocol was successfully extended to the synthesis of 5- scale reaction with substrate 1a was also conducted
methoxy-2-vinyl-1H-indole (2v) and 2-benzoyl-substi- [Eq. (1)]. The designed reaction proceeded smoothly
to afford the desired product in comparable yield
after 30 h. More importantly, the sunlight-driven pro-
cess also proved feasible to afford the indole product
with the same reaction efficiency, which further high-
lighted the synthetic advantages of this methodolo-
gy.[19]
To probe the possible reaction route towards indole
formation, cyclic voltammetry (CV) data of substrate
1a (À1.8 V vs. SCE)[20] was firstly measured, which in-
dicated that [RuCAHTUNGTRENNUNG
give an electron to reduce the corresponding substrate
1a. Therefore, a plausible reaction mechanism for this
photocatalytic transformation via an energy trans-
Scheme 2. Proposed mechanism.
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fer[16d,21,24] was favored and is depicted in Scheme 2.
Initially, visible light excitation of [Ru
(bpy)3]2+ might
generate the excited state [Ru
(bpy)3]2+*. An energy
transfer process occurred between 1a and the excited
state [Ru
(bpy)3]2+* to give the nitrene intermediate A
Table 3. Visible light-induced indole synthesis: variation of
R.[a]
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
with release of N2. Note that it has been previously
documented that the nitrenes could easily be generat-
ed by photolysis or thermolysis of the azides.[22] Final-
ly, a concerted nitrene insertion reaction through
transition state B delivers the desired product. In
order to detect the possible nitrene insertion process,
a D-labeling experiment was also performed to gain
some insights [Scheme 3, Eq. (3)]. The kinetic isotope
effect (KIE) of the reaction was KH/KD =1.2 upon
51% conversion of the starting material, which sug-
gested that the concerted nitrene insertion process
was likely to be involved.[23] Moreover, the fact that
the reaction with substrate 1x led to no formation of
methyl migration product 2x further confirmed such
a possible mechanism [Eq. (4)], which also excluded
the nitrene addition pathway through transient inter-
mediate C. Additionally, it was found that the expo-
sure of a Z/E mixture of styryl azide substrates to the
standard conditions resulted in Z/E isomerization
based on the results highlighted in Eq. (5) and the
case of 1r in Table 3. Thus, it can be concluded that
the cis-configurated substrates could not directly gen-
erate the corresponding products due to the blockage
of the nitrene-insertion process.
[a]
Unless otherwise noted, the reaction conditions are as
follows: 1 (0.3 mmol), RuACHTUNGRTNEUNG(bpy)3Cl2·6H2O (0.5 mol%),
DMF (6 mL), 3 W white LEDs, at room temperature.
Yield of the isolated product is given.
The E/Z ratio of the starting material was 12.5:1.
The Z and E isomers existed in the starting material, the
ratio was 2.4:1.
In conclusion, a mild and efficient method for the
facile construction of various N-free 2-substituted in-
doles has been developed by a visible light-induced
intramolecular cyclization of styryl azides.[24] Impor-
tantly, the reaction occurs smoothly at room tempera-
ture in the presence of low catalyst loading [0.5 mol%
[b]
[c]
Scheme 3. Control experiments.
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R. M. Rao, S. Sandra, R. Kapavarapu, D. Rambabu, G.
RuACHTUNGTRENNUNG(bpy)3Cl2·6H2O] and provides the corresponding
products in good to excellent yields.[24] Further exten-
sive mechanistic studies and synthetic applications of
this transformation are currently underway in our lab-
oratory.
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Experimental Section
General Procedure
To a 25-mL Schlenk flask equipped with a magnetic stirrer
bar were added 1a (0.3 mmol), RuACHTNUTGRNEG(UN bpy)3Cl2·6H2O
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(0.0015 mmol) and 6 mL DMF. The resulting mixture was
vigorously degassed through a ꢂfreeze-pump-thawꢀ proce-
dure three times. Then the solution was stirred at room tem-
perature under irradiation of 3 W white LEDs at a distance
of approximately 5 cm. Upon completion of the reaction, as
monitored by TLC, the crude mixture was poured into
20 mL H2O and extracted with Et2O (30 mLꢃ3).The com-
bined organic layer was dried over Na2SO4 and the solvent
was removed under reduced pressure. The crude product
was subjected to flash chromatography on silica gel (silica:
200–300; eluent: petroleum ether/ethyl acetate=10:1) to
provide pure indole 2a as a light yellow solid; yield: 96%.
Acknowledgements
We are grateful to the National Science Foundation of China
(Nos. 21272087, 21232003, and 21202053) and the National
Basic Research Program of China (2011CB808603) for sup-
port of this research.
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