A visible-light-mediated oxidative C-N bond formation/aromatization cascade: Photocatalytic preparation of N-arylindoles

Article abstract of DOI:10.1002/anie.201205137

Just add light and air: Structurally diverse N-arylindoles can be prepared from readily prepared o-styryl anilines through visible-light photocatalysis. The reaction, which is conducted open to air, is mediated by [Ru(bpz) ₃](PF₆)₂ (bpz=2,2'-bipyrazine) and involves both C-N bond formation and aromatization (see scheme). Using suitably substituted substrates, a 1,2-carbon shift can be also incorporated into this cascade reaction. Copyright

Full text of DOI:10.1002/anie.201205137

Angewandte
Chemie
DOI: 10.1002/anie.201205137
Photocatalysis
ꢀ
AVisible-Light-Mediated Oxidative C N Bond Formation/
Aromatization Cascade: Photocatalytic Preparation of
N-Arylindoles**
Soumitra Maity and Nan Zheng*
Indoles are heterocyclic motifs that are embedded in a large
number of bioactive natural products and pharmaceuticals.[1]
As such, the search for sustainable and more efficient
methods for the preparation of indoles is of constant
conditions. Herein, we report that nitrogen-centered radical
cation 2, which is generated from styryl aniline 1, can undergo
electrophilic addition to the tethered alkene, thus triggering
a cascade reaction involving either aromatization (when
2
interest.[2,3] Amination of vinyl C H bonds of styryl anilines
R = H in 2) or C C bond migration followed by aromatiza-
ꢀ
ꢀ
would provide direct and potentially more efficient access to
indoles, particularly because styryl anilines are readily
prepared by the Buchwald–Hartwig amination[4] of 2-bromo-
styrene. When this approach was first reported by the
research group of Hegedus, it employed a palladium complex
under reaction conditions of high temperature.[5a,b] However,
its use in indole synthesis has been limited, although one
notable example, in the form of a palladium-catalyzed
cyclization of 2-chloro-N-(2-vinyl)aniline, was recently
reported by Tsvelikhovsky and Buchwald.[5c]
tion (when R2 ¼ H) to form indoles 3a and 3b (Scheme 1).
This new photocatalytic approach for preparing indoles is
especially attractive because mild aerobic oxidation condi-
Recently, visible-light photocatalysis has become the
subject of a flurry of activity in organic chemistry.[6] In
parallel with the efforts of other research groups in this
field,[7–11] our research group[12] has been engaged in exploring
new types of reactivity of nitrogen-centered radical cations
that are generated through direct oxidation of the corre-
sponding amines by using photoexcited ruthenium–poly-
pyridyl complexes.[13] Although this type of oxidation was
first discovered in the late 1970s,[14] its potential in organic
synthesis has not been extensively explored before recent
efforts. Under visible-light photoredox conditions, the fate of
nitrogen-centered radical cations has been shown to follow
one of three reaction pathways: conversion into an iminium
ion with concomitant release of a hydrogen radical,[15]
conversion into an a-amino radical by deprotonation,[16] or
coupling with an irreversible ring-opening process to form
a carbon-centered radical that is distal to the nitrogen
atom.[12b] We speculated that other reaction pathways that
nitrogen-centered radical cations are known to follow,
including electrophilic addition to alkenes, might be amena-
ble to visible-light photocatalysis under similar reaction
Scheme 1. Visible-light-mediated indole synthesis.
tions (visible light, open to air, and room temperature) are
employed.
Styryl aniline 5a was chosen as the model substrate for
initial investigation (Table 1). [Ru(bpz)3](PF6)2 (4a)[17] was
employed as the photocatalyst and an 18 W white-light LED
was used as the source of visible light. Using 2 mol% of 4a in
CH3CN and leaving the reaction mixture open to air, we were
pleased to find that the desired indole product 6a was formed
in 31% yield, although, even after 24hours, the reaction had
not proceeded to completion (Table 1, entry 1). Either the use
of O2 in the place of ambient atmosphere (Table 1, entry 2) or
the use of TFE[18] as the solvent (Table 1, entry 3) did not
improve the yield. Addition of silica gel[18] to the reaction
mixture significantly accelerated the reaction, which was
complete after 12 hours, thus providing indole 6a in 68%
yield (Table 1, entry 4). Doubling the catalyst loading further
shortened the time necessary for full conversion to 5hours
and led to an increase in the yield of indole 6a to 88%
(Table 1, entry 5). [Ru(bpy)3](PF6)2 (4b; bpy = 2,2’-bipyridi-
ne)[17b] was found to be inferior to 4a.[19] Control studies
showed that the catalyst, light, and air were all essential for
[*] Dr. S. Maity, Prof. Dr. N. Zheng
Department of Chemistry and Biochemistry, University of Arkansas
Fayetteville, AR 72701 (USA)
E-mail: nzheng@uark.edu
this transformation (Table 1, entries 7–9). Notably,
a
[**] We thank the University of Arkansas, the Arkansas Bioscience
Institute, and the NIH NCRR COBRE grant (P30 RR031154 and
P30 GM103450) for generous support of this research. We also
thank Prof. Bill Durham for insightful discussions on photochem-
istry.
p-alkoxyphenyl group on the nitrogen atom is also critical
for the reaction.[18] When the p-methoxyphenyl group of 5b
was replaced by a phenyl group (N-phenyl-2-vinylaniline,
5b’), no reaction was observed under the same reaction
conditions. We are currently investigating the role of the
p-alkoxyphenyl group in the reaction.[20]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205137.
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Table 1: Optimization of the catalytic system.
products 6 in moderate to good yields (Scheme 2). The
presence of electron-donating and electron-withdrawing
groups on the aryl ring A were tolerated. A number of
alkenes containing various functional groups such as alkanes,
alkenes, arenes, and furans were good substrates for this
reaction as long as the C2 position was monosubstituted.
Disubstituted (2,3- and 3,3-) and trisubstituted (2,3,3-)
alkenes were all converted into 2-substituted, 3-substituted,
and 2,3-disubstituted indoles, respectively. This method
represents a general method for the regioselective introduc-
tion of substituents at the C2 and C3 positions of indoles,[2b,e,f]
a need that is currently unmet in indole synthesis. Substrates
that are sterically hindered in the vicinity of the alkene moiety
are well tolerated (6h; Scheme 2). When the alkene was part
of a carbocycle, fused indoles were obtained as the products
(6n and 6o; Scheme 2); the yield of 6n was high, although the
reaction took a longer time to reach completion when it was
scaled up to 1 g.
We propose the catalytic cycle that is outlined in
Scheme 3. The key steps include: (i) conversion of amine 7
into nitrogen-centered radical cation 8 through oxidation
using a photoexcited [Ru(bpz)3](PF6)2 (4a), (ii) electrophilic
addition of nitrogen-centered radical cation 8 to a tethered
alkene to generate benzylic radical 9,[21] (iii) oxidation of
benzylic radical 10 to its corresponding benzylic cation 11, and
finally (iv) aromatization through deprotonation, thus form-
ing indole 12. A control study using tetraphenylporphyrin
(TPP)[22] in place of 4a did not lead to the desired indole. This
result strongly indicates that the reaction is not mediated by
singlet oxygen (see the Supporting Information for details).
Entry Reaction conditions[a]
t [h] Conv. of
Yield of
5a [%][b]
6a [%][b]
1
2
3
4
5
6
4a (2 mol%), CH3CN
4a (2 mol%), CH3CN, O2
4a (2 mol%), TFE
24
24
24
44
49
42
31
33
21
68
88
19
4a (2 mol%), silica gel, CH3CN 12 100
4a (4mol%), silica gel, CH3CN
4b (2 mol%), silica gel,
CH3CN
5
100
24 100
7
8
silica gel, CH3CN, no 4a, light 24
8
4
4
2
4a (2 mol%), silica gel,
CH3CN, no light
24
9
4a (2 mol%), silica gel,
CH3CN, degassed
24
8
0
[a] Reaction conditions: 5a (0.2 mmol), solvent, open to air, irradiation
with a white-light LED at RT. [b] Measured by GC using dodecane as an
internal standard. bpz=2,2’-bipyrazine, TFE=trifluoroethanol.
To examine the substrate scope of this method, a series of
styryl anilines 5 were prepared using the Buchwald–Hartwig
amination of a 2-bromostyrene derivative with anilines.
Under the optimized reaction conditions (Table 1, entry 5),
styryl anilines 5 were converted into the desired indole
ꢀ
Scheme 2. Substrate scope of the reaction involving C N bond formation and aromatization. Reaction conditions, unless otherwise noted,
ꢀ
4 mol% [Ru(bpz)3](PF6)2, silica gel, air, CH3CN, irradiation with a 18 W white-light LED. The newly formed C N bonds are shown in bold. Yields
of products isolated using column chromatography are given. [a] Using 6 mol% [Ru(bpz)3](PF6)2. [b] Gram-scale reaction.
2
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optimized reaction conditions (Table 2). Whether the sub-
stituents were independent (13b), part of a carbocycle (13c
and 13d), or part of an oxocycle (13e), aryl groups prefer-
ꢀ
Table 2: Scope of substrate in the C N bond formation/1,2-carbon shift/
aromatization cascade reaction.[a]
Entry
Styryl anilines
Indole product
t [h]
Yield [%][b]
1
4.5
62
Scheme 3. Proposed mechanistic model.
We suspect that silica gel might play several roles in the
reaction, including adsorbing oxygen, being a source of
protons, which ensures that the nitrogen-centered radical
cation 8 is protonated, and facilitating the oxidation through
a proton-coupled electron transfer process.[23] The presence of
silica gel alone, that is, in the absence of the ruthenium
catalyst, led to no reaction (Table 1, entry 7). Replacing silica
gel with either HCl or TsOH led to the decomposition of the
starting amine. The use of either AcOH or PPTS in place of
silica gel gave the desired indole, although they were not as
effective as silica gel (see the Supporting Information for the
details). More studies are needed to understand the role(s) of
silica gel.
2
6
52
3
6
60
Having proposed that a benzylic carbocation is an
intermediate, we suspected that substrates lacking a C2
ꢀ
C H bond might participate in a 1,2-carbon shift, thus
ꢀ
forming a new C C bond (Scheme 4). The research group
4
6
58
40
5[c]
16
[a] Reaction conditions, unless otherwise noted: 4 mol% [Ru(bpz)3]-
(PF6)2, silica gel, air, CH3CN, irradiation with a 18 W white-light LED. The
Scheme 4. Cyclization involving a 1,2-carbon shift.
ꢀ
ꢀ
newly formed C N and C C bonds are shown in bold. [b] Yields of
product isolated using column chromatography. [c] Using 6 mol%
[Ru(bpz)3](PF6)2.
of Driver has extensively explored the synthetic utility of this
1,2-carbon shift in their studies of rhodium-catalyzed decom-
position of styryl azides, a transformation that presumably
involves similar benzylic carbocations.[24] To test this hypoth-
esis, gem-diphenyl styryl aniline 13a was prepared and
subjected to the optimized visible-light photocatalytic con-
ditions (Scheme 4). To our delight, the expected 2,3-diphenyl
indole 15a was isolated in 60% yield. This result lent further
credence to the possible involvement of benzylic carbocation
14 in the visible-light-mediated reaction.
entially migrated over alkyl groups. However, the 1,2-carbon
shift is not limited to the migration of aryl groups only. When
the C2 position formed part of a cyclopentane ring, the
desired ring-expansion product 15 f was formed, although the
efficiency of the transformation was not as high as those
involving aryl shifts (Table 2, entry 5). However, when the C2
position bore two methyl groups, the desired methyl-migra-
tion product was not formed. Notably, when a tetralene
product was formed, further oxidation to a fully aromatized
product was observed (Table 2, entry 2).
To further explore the synthetic potential of this
1,2-carbon shift in indole synthesis, a series of gem-disubsti-
tuted styryl anilines were synthesized and tested under the
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Du, Nat. Chem. 2010, 2, 527; f) K. Zeitler, Angew. Chem. 2009,
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In summary, we have developed a visible-light-mediated
photocatalytic reaction for the preparation of N-arylindoles.
These reactions, which were catalyzed by 4 mol% [Ru(bpz)3]-
(PF6)2 (4a), were conducted at ambient temperature, and the
reaction mixtures were exposed to open air and were
illuminated by an 18 W white-light LED. The mild aerobic
oxidation conditions were compatible with the presence of
a variety of functional groups. More importantly, these studies
revealed, for the first time, that arylamines could participate
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Received: July 1, 2012
Published online: && &&, &&&&
Keywords: domino reactions · homogeneous catalysis ·
.
indoles · photochemistry · ruthenium
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[19] The effectiveness of the catalysts correlates with the redox
potentials of Ru2+*/Ru1+ (versus SCE): [Ru(bpz)3], 1.3 V;
[Ru(bpy)3], 0.78 V. The values are taken from Ref. [17a].
[24] a) B. J. Stokes, S. Liu, T. G. Driver, J. Am. Chem. Soc. 2011, 133,
4702; b) K. Sun, S. Liu, P. M. Bec, T. G. Driver, Angew. Chem.
2011, 123, 1740; Angew. Chem. Int. Ed. 2011, 50, 1702; c) M.
Shen, B. E. Leslie, T. G. Driver, Angew. Chem. 2008, 120, 5134;
Angew. Chem. Int. Ed. 2008, 47, 5056.
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
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Angewandte
Communications
Communications
Photocatalysis
Just add light and air: Structurally diverse
N-arylindoles can be prepared from
readily prepared o-styryl anilines through
visible-light photocatalysis. The reaction,
which is conducted open to air, is medi-
ated by [Ru(bpz)3](PF6)2 (bpz=2,2’-
S. Maity, N. Zheng*
&&&& — &&&&
ꢀ
A Visible-Light-Mediated Oxidative C N
Bond Formation/Aromatization Cascade:
Photocatalytic Preparation of N-
Arylindoles
ꢀ
bipyrazine) and involves both C N bond
formation and aromatization (see
scheme). Using suitably substituted
substrates, a 1,2-carbon shift can be also
incorporated into this cascade reaction.
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!