Visible-Light-Promoted Alkylation of Indoles with Tertiary Amines by the Oxidation of a sp3 C-H Bond

Article abstract of DOI:10.1002/adsc.201701131

A visible-light driven reaction for the synthesis of 3-arylmethyl indole derivatives using tertiary amines and indoles was first reported. Corresponding products were obtained with yields of up to 70%, and various functional groups on the indoles were well tolerated when Rose Bengal was used as a photosensitizer and air was used as a green oxidant under mild reaction conditions. (Figure presented.).

Full text of DOI:10.1002/adsc.201701131

Title: Visible-Light-Promoted Alkylation of Indoles with Tertiary Amines
by the Oxidation of a sp3 C-H Bond
Authors: Xuan Ding, Chun-Lin Dong, Zhi Guan, and Yan-Hong He
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201701131
Link to VoR: http://dx.doi.org/10.1002/adsc.201701131

10.1002/adsc.201701131
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Visible-Light-Promoted Alkylation of Indoles with Tertiary
Amines by the Oxidation of a sp3 C-H Bond
Xuan Ding, Chun-Lin Dong, Zhi Guan*, Yan-Hong He*
Key Laboratory of Applied Chemistry of Chongqing Municipality, School of Chemistry and Chemical Engineering,
Southwest University, Chongqing 400715, PR China
E-mails: guanzhi@swu.edu.cn (for Z. Guan); heyh@swu.edu.cn (for Y.-H. He)
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
Abstract. A visible-light driven reaction for the synthesis of 3-arylmethyl indole derivatives using tertiary amines and
indoles was first reported. Corresponding products were obtained with yields of up to 70%, and various functional groups
on the indoles were well tolerated when Rose Bengal was used as a photosensitizer and air was used as a green oxidant
under mild reaction conditions.
Keywords: Photocatalytic reaction; Visible-light; Rose Bengal; 3-Arylmethyl indoles; Indole alkylation
photocatalytic reactions such as cycloaddition,[6]
Introduction
decarboxylation,[7] alkylation,[8] -oxyamination,[9]
Visible-light is an environmentally friendly and
dehydrogenative coupling,[10] and oxidative C-H
sustainable energy that is considered to be an ideal
functionalization of tertiary amines.[11] Two main
reaction condition in green chemical synthesis.[1]
mechanisms are considered for Rose Bengal
Visible-light photocatalysis has been regarded as a
photosensitized reactions that involve photoredox
useful tool for construction of various structures in
single electron transfer (SET) and energy transfer
the past few years.[2] Many elegant works on
(ET).[12]
photocatalysis have been reported using transition
The substituted 3-alkyl indole moiety is an
metal ruthenium or iridium polypyridyl complexes as
important structural skeleton in various naturally
efficient photosensitizers.[3] However, these transition
occurring products[13] and pharmaceuticals.[14] Thus,
metals are expensive and have potential toxicity that
3-alkyl indole derivatives have been attracting great
have limited their availability.[4] Recently, cheap
interest in organic synthesis.[15] The synthesis of 3-
metal-free organic dyes have emerged as effective
arylmethyl indoles was reported by the Kumar group
photocatalysts because of their low cost,
using silica-supported perchloric acid (HClO4-SiO2)
environmental friendliness and long excited state
as a catalyst in 2009,[16] which was carried out
lifetime under visible-light irradiation.[5] Rose Bengal,
through a three-component reaction of indoles,
as an organic dye, has been applied in different
formaldehyde and tertiary aromatic amines (Scheme
1
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10.1002/adsc.201701131
Advanced Synthesis & Catalysis
Results and Discussion
1). Recently, using amine as a carbon source to
construct the C-C bond for the synthesis of indole
derivatives has received attention. In this strategy, the
imine or iminium ion is an important intermediate,
which is generated by the selective oxidation of the
sp3 C-H bonds adjacent to the nitrogen atom in a
tertiary amine. The imine or iminium ion is
vulnerable to attack by a variety of nucleophiles to
achieve C-H functionalization.[17] In 2010, the Che
group reported the direct C-3 alkylation of indoles via
an iminium ion by using ruthenium porphyrins or
RuCl3 as a Lewis acid catalyst and tert-butyl
hydroperoxide (TBHP) as an oxidant at 110 °C
(Scheme 1).[18] To obtain an iminium ion, metals,[19]
peroxides,[20] or inorganic oxidants[21] are usually
necessary for promoting the oxidation of the tertiary
amine in the reaction. Inspired by the above
preceding work, we developed the first metal-free
visible-light driven reaction for the synthesis of 3-
arylmethyl indole derivatives from tertiary amines
In our initial study, indole (1a, 0.25 mmol) and
N,N-dimethylaniline (2a, 1.5 mmol) were selected as
the model substrates, and the model reaction was
carried out with the presence of 1 mol% Rose Bengal
in MeCN (1.0 mL) at room temperature. After 48 h of
irradiation with a 20 W compact fluorescent lamp
(CFL), we were delighted to find that the desired
product 3a was obtained in a 15% yield. Then, a
number of organic dye photocatalysts and the wattage
of the lamp were screened. The data showed that
Rose Bengal and the fluorescent lamp wattage of 20
W were the best choices among the tested conditions
(for details, please see Table S1 in the Supporting
information).
Table 1. Influence of the water content on the model
reactiona.
and indoles by using Rose Bengal as
a
Entry
MeCN/H2O (v/v)
100:0
Yield (%)b
1
2
3
4
5
6
44
60
52
45
42
26
photosensitizer and air as a green oxidant under mild
reaction conditions (Scheme 1). Herein, we report
this work.
100:1
100:2
100:3
100:4
100:5
a Reaction conditions: a mixture of 1a (0.25 mmol), 2a (1.5
mmol), and Rose Bengal (5 mol%) in MeCN (1 mL) and
water under irradiation of a 20 W CFL in air at rt for 48 h.
b
Yield of the isolated product after silica gel
chromatography.
It has been demonstrated that Rose Bengal is the
most popular sensitizer used in water solutions.[22]
And the Rose Bengal photosensitized oxidation of
N,N-diethylhydroxylamine has been investigated in
water and acetonitrile. The results indicated that in
water the photo-oxidation proceeds mainly by an
electron transfer mechanism,[23] while in acetonitrile
an energy transfer mechanism plays a dominant
Scheme 1. Previous protocols and this work for the
synthesis of 3-arylmethyl indoles
2
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10.1002/adsc.201701131
Advanced Synthesis & Catalysis
role.[24] On the basis of the previous research, we
speculated that water may also affect our reaction.
Thus, in consideration of the solubility of the
substrates, the mixed solvents (MeCN containing
different amounts of water) were examined as
reaction media (Table 1). The results confirmed that
water indeed influenced the model reaction. When
increasing the water content from 100:0 to 100:1
(MeCN:H2O, v/v), the yield of the product (3a)
increased significantly (Table 1, entries 1 and 2).
However, the yield decreased when the water content
was further increased (Table 1, entries 3-6). The
reaction in MeCN/H2O (100:1, v/v) gave the best
yield of 60% after 48 h. (Table 1, entry 2).
found that product 3a was obtained in a good yield of
60% by using a Rose Bengal loading of 5 mol% (for
details, please see Table S2 in the Supporting
information). The molar ratio of substrates was also
screened (Table 2, entries 1-5), and the best result
was obtained when molar ratio was 1:6 (1a:2a)
(Table 2, entry 1). Thus, 1a:2a = 1:6 was selected as
a suitable molar ratio for the reaction. Subsequently,
Table 3. Substrate scope a.
Entry
R1
R2
R3
Product Yield
( % )b
1
2
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
70
68
65
45
62
48
-
3a
3b
3c
3d
3e
3f
2-CH3
2-C6H5
4-CH3
5-CH3
5-OCH3
5-NO2
5-F
Table 2. Optimization studies a.
3
4
5
6
Entry
Solvent
Molar ratio Temperature Yield
(1a:2a)
(%)b
60
7
-
1
2
3
4
5
MeCN/H2O
(100:1)
MeCN/H2O
(100:1)
MeCN/H2O
(100:1)
MeCN/H2O
(100:1)
1:6
rt
rt
rt
rt
rt
8
54
45
49
57
47
36
59
58
35
29
3g
3h
3i
9
5-Br
1:1
1:2
1:4
1:8
11
25
34
48
10
11
12
13
14
15
16
17
5-I
6-CH3
6-F
3j
3k
3l
6-Br
MeCN/H2O
(100:1)
7-CH3
N-CH3
N-Bn
3m
3n
3o
3p
6
7
THF
THF/H2O
(100:1)
1:6
1:6
rt
rt
19
25
N-4-Me-
C6H4
N-4-OMe-
C6H4
8
9
DMSO
DMSO/H2O
(100:1)
1:6
1:6
rt
rt
trace
33
18
H
CH3
17
3q
10
11
MeOH
MeOH/H2O
(100:1)
1:6
1:6
rt
rt
trace
16
19
20
2-CH3
3-Br
H
CH3
28
38
3r
3s
H
C2H5
12
MeCN/H2O
(100:1)
1:6
50 °C
70
a Reaction conditions: a mixture of 1 (0.25 mmol), 2 (1.5
mmol) and Rose Bengal (5 mol%) in MeCN (1 mL) and
water (0.01 mL) under irradiation of 20 W CFL in air at
50 °C for 48 h.
a Reaction conditions: a mixture of 1a (0.25 mmol), 2a, and
Rose Bengal (5 mol%) in MeCN (1 mL) and water under
irradiation of a 20 W CFL in air for 48 h.
b
b
Yield of the isolated product after silica gel
Yield of the isolated product after silica gel
chromatography.
chromatography.
Next, the catalyst loading was screened. It was
3
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10.1002/adsc.201701131
Advanced Synthesis & Catalysis
the effect of the solvent on the reaction was explored
(Table 2, entries 6-11), but no better result was
obtained in the other tested solvents compared to
MeCN. Hence, MeCN/H2O (100:1, v/v) was selected
as a suitable solvent for the reaction. To our delight,
increasing the temperature to 50 °C could promote
the reaction yield giving product (3a) in a 70% yield
(Table 2, entry 12). Therefore, based on the results
above, a Rose Bengal loading of 5 mol%, a molar
ratio of 1:6 (1a:2a), a reaction medium of
MeCN/H2O (100:1, v/v) and a temperature of 50 °C
1a giving the corresponding product in a yield of
38% (Table 3, entry 20). Ten new indole derivatives
were obtained, and their structures were confirmed by
1
13
HRMS, H NMR, and C NMR (for details, please
see the Supporting Information).
To understand the reaction mechanism, some
control experiments were conducted (Table 4). In the
absence of Rose Bengal or visible-light irradiation,
no reaction was observed (Table 4, entries 2-4).
When the reaction was carried out under N2 instead
of air, only a trace amount of 3a was obtained (Table
were chosen as the optimal conditions for the reaction. 4, entry 5). These results indicated that visible-light,
With the optimized conditions in hand, to
discover the scope of this transformation, the
reactivities of different indoles and scope of
substituted anilines were investigated (Table 3). The
position and electron properties of substituents on
indole had an effect on the experimental results.
Indoles substituted with -CH3, -OCH3, -aryl, -Bn and
halogens participated in the reaction smoothly, giving
the corresponding products in yields of 17-70%
(Table 3, entries 1-6 and 8-18). However, the strong
electron-withdrawing property of a nitro group likely
limited the nucleophilicity of the 5-NO2 indole, and
thus, no obvious reaction was detected (Table 3,
entry 7). N-methyl indole performed well with N,N-
dimethylaniline to give product in a yield of 58%
(Table 3, entry 15). N-Bn and N-4-Me-C6H4 and N-
4-OMe-C6H4 indoles showed much lower reaction
efficiencies (Table 3, entries 16-18). However, when
N-C6H5 indole was used to react with N,N-
dimethylaniline, no desired product was detected. In
this case, N-C6H5 indole was nearly unreacted but
N,N-dimethylaniline was consumed giving some
complicated by-products. When 3-bromo N,N-
dimethylaniline was used, a low yield was obtained
(Table 3, entry 19). Additionally, N-ethyl-N-
methylaniline could participate in the reaction with
Rose Bengal and oxygen are indispensable for the
reaction. Rose Bengal exhibited a strong absorption
in the visible-light region at λ = 563 nm (for the UV-
visible absorption spectrum, please see Fig. S1. in the
Supporting information). To verify whether the
reaction was carried out by a radical pathway, the
reaction of indole (1a) with N,N-dimethylaniline (2a)
was conducted in the presence of a free-radical
Table 4. Control experimentsa.
Visible
light
(20 W
CFL)
Rose
Bengal
(5
mol%)
TEMPO DABCO
Yield
(%)b
Entry
(1.5
(1.5
mmol)
mmol)
1
2
3
4
5c
6
7
+
-
+
-
+
+
+
+
-
-
+
+
+
+
-
-
-
-
-
-
-
-
-
-
70
NR
NR
NR
Trace
Trace
NR
+
-
-
+
a Reaction conditions: 1a (0.25 mmol), 2a (1.5 mmol) in
MeCN (1 mL) and water (0.01 mL) at 50 °C for 48 h.
b
Yield of the isolated product after silica gel
chromatography.
c Under N2
4
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10.1002/adsc.201701131
Advanced Synthesis & Catalysis
Bengal, but was much weaker than N,N-
dimethylaniline and indole (for details, please see the
Supporting Information). We speculate that a SET
photoredox process may also be involved in the
reaction. No reaction was observed when using
DABCO, probably due to the dual inhibition of
DABCO on singlet oxygen and photoexcited Rose
Bengal. Additionally, an isotope labelling (13C)
experiment was carried out under the optimized
reaction conditions (Scheme 2), and the results
clearly demonstrated that the carbon of the CH2 group
in the final product originates from the N,N-
1.30
N,N-dimethylaniline
Indole
1.25
1.20
1.15
1.10
1.05
1.00
0
2
4
6
8
Quancher Concentration /10-3
M
Figure 1. Stern-Volmer profile of Rose Bengal by various
concentrations of N,N-dimethylaniline and indole.
scavenger
2,2,6,6-tetramethyl-1-piperidinyloxy
13
dimethylaniline (for the determination of C labelled
(TEMPO), which only gave product 3a in a trace
amount (Table 4, entry 6). This result indicated that
the reaction may occur through a radical pathway.
Moreover, it is well known that oxygen (O2) under
the excitation of visible-light can form singlet oxygen
(1O2), which can form a superoxide radical anion
product, please see the Supporting Information).
Scheme 2 The 13C isotope labelling experiment.
•−
(O2 ) by SET,[25] and Rose Bengal is a good
photosensitizer for singlet oxygen generation. Thus,
to confirm if singlet oxygen is involved in the
reaction, 1,4-diazabicyclo[2.2.2]octane (DABCO),[26]
a quencher of singlet oxygen, was added to the
reaction system, and it was found that the reaction
was completely inhibited (Table 4, entry 7). This
result suggested that singlet oxygen may be involved
in the reaction.
Finally, on the basis of our control experiments
and literatures,[11,23,24] a plausible reaction mechanism
was proposed (Scheme 3). We believe that both
energy transfer and photoredox catalysis are possibly
involved in the reaction. Upon the absorption of
visible-light, Rose Bengal (RB) accepts a photon to
generate excited state RB*, and oxygen (3O2) can
form singlet oxygen (1O2) by energy transfer from
RB*. Tertiary amines can be oxidized by singlet
oxygen.[29] A SET from N,N-dimethylaniline 2a to
1O2 generates the amine cation radical I and
superoxide anion radical (O2•−). Meanwhile, in
photoredox catalysis, a SET from the tertiary amine
2a to RB* leads to the amine cation radical I and
RB•−. Then, RB•− is oxidized back to the RB by the
transfer of an electron to O2 to form the superoxide
Furthermore, from the redox potentials N,N-
ox
dimethyl aniline (2a) (E1/2 = 0.71 V vs SCE in
MeCN)[27] can be oxidized by the excited states of
red
Rose Bengal [E1/2 (RB*/ RB˙−) = 0.99 V vs SCE in
MeCN].[28] It is possible for SET to occur. Thus,
Stern-Volmer quenching experiments of Rose Bengal
were performed, and the results revealed that both
N,N-dimethylaniline and indole can quench the
fluorescence of Rose Bengal, but N,N-
dimethylaniline is more efficient than indole (Figure
1). DABCO also can quench photoexcited Rose
•−
radical anion O2 . A proton is transferred from the
•−
amine cation radical I to O2 to obtain the α-
aminoalkyl radical II. Subsequently, II is further
5
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10.1002/adsc.201701131
Advanced Synthesis & Catalysis
oxidized to a highly reactive imine intermediate III
by a second SET. The imine intermediate could be
easily trapped by nucleophiles. According to by-
products IX and X, we proposed that III can undergo
nucleophilic attack by the indole (Path A) and N,N-
dimethylaniline (Path B). For Path A, nucleophilic
attack on III by the indole gives the corresponding
alkylated indole IV, and the lone pair of electrons on
the nitrogen in indole possibly generates azafulvalene
intermediate VIII.[30] Intermediate VIII accepts N,N-
dimethylaniline to generate the final product 3a.[31]
Meanwhile indole attacks VIII to give a small
amount of by-product X. For Path B, nucleophilic
attack on imine intermediate III by N,N-
dimethylaniline gives intermediate V, which can be
converted into intermediate VII. The addition of
indole to VII gives final product 3a, while the attack
of N,N-dimethylaniline on VII forms by-product IX.
Scheme 3. Possible mechanism.
6
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10.1002/adsc.201701131
Advanced Synthesis & Catalysis
The presence of intermediates IV and V was
confirmed by HRMS of the reaction mixture (for
details, please see the Supporting Information).
Research
Project
of
Chongqing
(cstc2015jcyjBX0106).
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irradiation of a 20 W CFL in air at 50 C for 48 h.
The reaction mixture was then evaporated under
reduced pressure and purified by flash
chromatography on silica gel (petroleum ether/ethyl
acetate = 10:1, v/v) to obtain product 3.
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10.1002/adsc.201701131
Advanced Synthesis & Catalysis
FULL PAPER
Visible-Light-Promoted Alkylation of Indoles with
Tertiary Amines by the Oxidation of a sp3 C-H
Bond
Adv. Synth. Catal. Year, Volume, Page – Page
Xuan Ding, Chun-lin Dong, Zhi Guan*, Yan-Hong He*
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