L-Proline-catalyzed multicomponent synthesis of 3-indole derivatives

Article abstract of DOI:10.1016/j.tet.2015.10.027

A green one-pot multicomponent reaction for the synthesis of 3-indole derivatives was developed via l-proline-catalyzed condensation between indoles, aldehydes, and malononitrile. The process was carried out in ethanol at room temperature, and the desired

Full text of DOI:10.1016/j.tet.2015.10.027

Tetrahedron xxx (2015) 1e8
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L
-Proline-catalyzed multicomponent synthesis of 3-indole
derivatives
*
*
Yan-Hong He , Jian-Fei Cao, Rui Li, Yang Xiang, Da-Cheng Yang, Zhi Guan
Key Laboratory of Applied Chemistry of Chongqing Municipality, School of Chemistry and Chemical Engineering, Southwest University, Chongqing
400715, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A green one-pot multicomponent reaction for the synthesis of 3-indole derivatives was developed via L-
Received 5 August 2015
Received in revised form 8 October 2015
Accepted 10 October 2015
Available online xxx
proline-catalyzed condensation between indoles, aldehydes, and malononitrile. The process was carried
out in ethanol at room temperature, and the desired products were obtained in good to excellent yields
of up to 98%. Pyrrole as the nucleophile and 2,2,2-trifluoroacetophenone as the electrophile can also
work in this transformation. The salient features of this protocol were environmental friendliness,
simplicity of the procedure, ready accessibility of the catalyst, high yield, and mild reaction conditions.
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
L-Proline
Catalysis
Green
Multicomponent reaction
3-Indole derivatives
1. Introduction
precursors.7 And Zhou group reported a method to obtain 3-
substituted indole frameworks by a chiral thiourea catalyst.8 3-
Multicomponent reactions (MCRs), owing to the obvious ad-
vantages of atom economy, automatable procedure, time-saving,
inherent exploratory power and minimization of chemical waste,
have gained much attention in synthetic organic chemistry.1
Moreover, this strategy has proved to be a particularly valuable
and efficient methodology in the section of synthetic chemistry.2
The indole is probably one of the most ubiquitous moieties in
nature.3 A vast number of natural and synthetic indole derivatives
have been found a venerable value in pharmaceutical and medical
applications since they are able to bind with high affinity to many
receptors.4 Among them, 3-substituted indole derivatives have
attracted much attention because they exist in many natural
products, and their simple structure with broad scope of potent
biological activities.5
Due to the inherent advantages, MCRs have proved to some
traditional reactions that require two or three steps to synthesize
the structurally complex molecules, such as carbon-carbon bond
formation reaction in the field of alkylation of indoles. In 1978,
Yonemitsu et al. first reported the successful multicomponent-
based synthesis of 3-substituted indoles with Meldrum’s acid, al-
dehydes, and different indoles.6 Up to now, the Yonemitsu reaction
has been largely applied to the synthesis of complex indole alkaloid
Substituted indole derivatives have also been obtained via L-pro-
line catalyzed similar reactions based on dimedone,9aed Meldrum’s
acid9e and even ketones.9f In other instances, this structure is also
effectively obtained by the reaction between indoles, aldehydes
and active methylene compounds when different kinds of catalyst
are applied, like Ti(IV)/Et3N,10aec Sc(OTf)3,10d and molecular sie-
ves10e etc. And Perumal group reported InCl3-catalyzed three-
component reaction between salicylaldehydes, malononitrile and
indoles in 2007.11 It is even extended to cascade reactions recently
to afford 3-substituted indoles.12
The three-component reaction between indoles, aldehydes and
malononitrile is a unique type of reaction for the synthesis of 3-
substituted indole derivatives. There are some reports about this
reaction. Recently Zhou and co-workers reported this type of re-
action catalyzed by a copper(II) sulfonato Salen complex with
KH2PO4 as an additive in water at 60 ꢀC.13 Arjan and co-workers
reported an extensive investigation using Zn(II)/Salen complexes
as mediators in the presence of base.14 However, the Salen ligand
needs to be synthesized through several additional steps. And in
the same year, Wan and co-workers used polyethylene glycol (PEG-
200)-promoted this reaction in the presence of KH2PO4.15 After
ꢀ
that, Gree and co-workers reported that this three-component re-
action could be conducted using Cu(OAc)2 as a ligand-free catalyst
and KH2PO4 as an additive in PEG-400 at 70 ꢀC.16 Also, Singh group
used tetrabutyl ammonium fluoride trihydrate (TBAF.3H2O) to
catalyze this reaction in 2013.17
* Corresponding authors. Fax: þ86 23 68254091; e-mail addresses: heyh@swu.
edu.cn (Y.-H. He), guanzhi@swu.edu.cn (Z. Guan).
http://dx.doi.org/10.1016/j.tet.2015.10.027
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Y.-H. He et al. / Tetrahedron xxx (2015) 1e8
In consideration of the importance of this three-component
yields (Table 1, entries 12 and 13). Some natural amino acids were
reaction, there still remains a necessity to develop new environ-
mentally friendly methodology to realize this important trans-
formation for the preparation of potential medicinal scaffolds. And
also tested as catalysts (Table 1, entries 14e20). The desired product
4a was obtained in low yields when
serine, -leucine, and -threonine were used (Table 1, entries
14e18). A moderate yield was obtained with -cysteine (Table 1,
entry 19). Moreover, to our delight, when -proline was applied as
L-methionine, L-histidine, L-
L
L
L-proline has been effectively used as a versatile organocatalyst for
L
carbonecarbon bond formation in various organic trans-
formations.18 So we wish report the synthesis of 3-substituted-
alkylated indoles via the three-component reaction of indole (or
pyrrole), aldehyde (or 2,2,2-trifluoroacetophenone) and malono-
L
a catalyst, the desired product 4a was isolated in a good yield of 91%
with a trace amount of BIM 5 as a by-product (Table 1, entry 20). As
a
comparison, protected
ycarbonyl)pyrrolidine-2-carboxylic acid (N-Boc-
used as a catalyst, which gave 4a in a low yield of 28% (Table 1, entry
21), indicating that imino group was necessary for -proline to
display its catalytic activity in the reaction. We hypothesized that
L
-proline derivative, 1-(tert-butox-
nitrile in ethanol at room temperature, using
L
-proline as an in-
L-proline) was also
expensive and environmentally benign catalyst.
L
L-
2. Results and discussion
proline catalyzes the reaction by the formation of an iminium ion
intermediate with benzaldehyde, which facilitates the attack of
nucleophile malononitrile to form the Knoevenagel adduct. And
then Michael addition of indole to the adduct furnishes the desired
product 4a. Although pyrrolidine possesses the imino group similar
During the course of our initial investigation, the three-
component reaction of indole, benzaldehyde and malononitrile
was used as a model reaction for the catalyst screening in ethanol
(Table 1). The control experiment in the absence of catalyst was first
carried out, and the desired product 4a was only obtained in a yield
of 8% (Table 1, entry 1). When some Brønsted acids such as tri-
fluoroacetic acid (TFA), acetic acid, decanoic acid (C9H19COOH) and
p-toluenesulfonic acid were tested as catalysts, no desired product
4a was obtained, but bis(indolyl)methane (BIM) 5 was received
instead (Table 1, entries 2e5). Some organic and inorganic bases
were also screened, but none of them showed to be efficient to
catalyze the formation of 4a in a satisfied yield (Table 1, entries
6e11). When pyrrolidine was used as a catalyst, the desired product
4a was only obtained in a very low yield of 13% (Table 1, entry 6),
but a by-product was observed, which was formed by pyrrolidine
itself with benzaldehyde and indole via a Mannich-type reaction as
we described in the previous research.19 The model reaction with
salt KH2PO4 or NaOAc only gave the desired product 4a in low
to
act as a catalyst. It seems that carboxyl group gives
properties so that it can work as a catalyst instead of a reactant like
L
-proline, pyrrolidine itself participates in the reaction failing to
L-proline unique
pyrrolidine in the model reaction. Therefore,
as an appropriate catalyst.
L-proline was chosen
Next, the catalyst loading was investigated (Table 1, entries 20,
22 and 23), and it was found that 10 mol % of -proline was sufficient
for the model reaction to obtain the desired product 4a in a good
yield of 91% (Table 1, entry 22). Thus, 10 mol % of -proline was se-
lected as an optimal catalyst loading for further investigation.
Next, several solvents were tested for the -proline-catalyzed
L
L
L
reaction of benzaldehyde, indole and malononitrile. As shown in
Table 2, when the reaction was carried out in solvents like MTBE,
isopropyl ether, THF, 1,4-dioxane, CH2Cl2, CHCl3, n-butyl acetate,
Table 1
Catalyst screening for the multicomponent synthesis of 3-alkylated indolea
Entry
Catalyst
Yield of 4ab (%)
1
None
8
2
3
Trifluoroacetic acid
Acetic acid
0
0
4
Decanoic acid
Trace
5
6
7
8
p-Toluenesulfonic acid
Pyrrolidine
Piperidine
Dimethylamine
Triethylamine
NaOH
NaHCO3
KH2PO4
NaOAc
0
13
11
17
23
19
15
16
15
26
18
19
23
24
48
91
28
91
85
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
L
L
L
L
L
L
L
-Methionine
-Histidine
-Serine
-Leucine
-Threonine
-Cysteine
-Proline
N-Boc-
L
L
-proline
-Proline (10 mol %)
-Proline (5 mol %)
L
a
Unless otherwise noted, reaction conditions: benzaldehyde (1.0 mmol), indole (1.0 mmol), malononitrile (1.0 mmol), catalyst (20 mol %) in
ethanol (1.0 mL) at rt for 48 h.
b
Isolated yield after silica gel column chromatography.
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Y.-H. He et al. / Tetrahedron xxx (2015) 1e8
3
Table 2
DMSO and H2O at rt for 72 h, the desired product 4a was only
obtained in yields of 39e52 % (Table 2, entries 1e9). The model
reaction under solvent-free conditions gave product in a moderate
yield of 51% (Table 2, entry 10). Some alcohols such as MeOH, EtOH,
n-propanol and isopropanol were also tested as solvents. The re-
action in alcohol solvents gave better yields than in other tested
solvents. The best yield of 91% was received in EtOH at rt after 48 h
(Table 2, entry 12). Lower yields were obtained in other alcohols
even after 72 h (Table 2, entries 11, 13 and 14). Thus, EtOH was
selected as an optimal solvent for the reaction.
The effects of temperature were also investigated (Table 2, en-
tries 15e17). Rising temperature to 50 ꢀC did not lead to an increase
of the yield, but on the contrary a lower yield was obtained (Table 2,
entry 14) due to the formation of the by-product bis(indolyl)
methane (BIM) 5 at a higher temperature. Therefore, rt (approxi-
mately 25 ꢀC) was chosen as a suitable temperature for the reaction.
Finally, a number of structurally different aldehydes and indoles
were tested to explore the scope and generality of this methodol-
ogy under the optimized reaction conditions. As shown in Table 3,
aromatic aldehydes with electron-withdrawing and neutral groups
afforded the respective addition products in good yields of 89e97%
(Table 3, entries 1e9), while the aromatic aldehydes containing an
electron-donating groups gave the products in a moderate yield
after longer reaction time (Table 3, entries 10e13). And aliphatic
aldehydes could also react with indole to afford the desired prod-
ucts with moderate yields (Table 3, entries 14, 15). Moreover, in-
doles with either electron-withdrawing or electron-donating group
could react with various aromatic aldehydes smoothly to give the
Optimization of reaction conditionsa
Entry
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
MTBE
Isopropyl ether
THF
1,4-Dioxane
CH2Cl2
CHCl3
n-Butyl acetate
DMSO
H2O
Solvent-free
MeOH
EtOH
n-Propanol
Isopropanol
EtOH (15 ꢀC)
EtOH (35 ꢀC)
EtOH (50 ꢀC)
72
72
72
72
72
72
72
72
72
72
72
48
72
72
48
48
48
21
45
39
38
50
30
41
39
52
51
85
91
75
82
83
90
76
9
10
11
12
13
14
15
16
17
a
Unless otherwise noted, reaction conditions: benzaldehyde (1.0 mmol), indole
-proline (10 mol %) in solvent (1.0 mL) at rt.
Isolated yield after silica gel column chromatography.
(1.0 mmol), malononitrile (1.0 mmol),
L
b
Table 3
Scope of the
L
-proline-catalyzed three-component reactiona
Yield
(%)b
Entry
Aldehyde
Indole
Product
Time (h)
1
48
90
42
72
86
89
2
3
36
94
(continued on next page)
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Y.-H. He et al. / Tetrahedron xxx (2015) 1e8
Table 3 (continued)
4
5
6
7
8
62
48
36
32
48
92
93
92
95
97
48
72
83
89
9
10
72
79
11
72
78
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Y.-H. He et al. / Tetrahedron xxx (2015) 1e8
5
Table 3 (continued)
12
72
73
13
72
65
14
15
16
17
18
72
70
72
72
48
63
58
85
79
96
19
30
98
(continued on next page)
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Y.-H. He et al. / Tetrahedron xxx (2015) 1e8
Table 3 (continued)
20
21
22
42
95
38
93
38
72
82
83
a Reaction conditions: aldehyde (1.0 mmol), indole (1.0 mmol), malononitrile (1.0 mmol), L-proline (10 mol%) in
ethanol (1.0 mL) at r.t.. b Isolated yield after silica gel column chromatography.
corresponding products in good yields of 79e98% (Table 3, entries
16e22). When 2,2,2-trifluoroacetophenone instead of benzalde-
hyde was used as an electrophile to the reaction, the yield of 47%
was obtained after 62 h (Scheme 1). Moreover, electron-rich het-
erocycles, such as pyrrole, furan, thiophene and trimethox-
ybenzene (1, 2, 3-, 1, 3, 5-, and 1, 2, 3-), also were used to react with
benzaldehyde and malononitrile, but only pyrrole could work for
this transformation, affording the desired product in 38% yield
(Scheme 2). All the reactions were monitored by thin layer chro-
matography (TLC). When judged as completion or no more in ob-
vious progress, the reaction was terminated and the product was
isolated by silica gel column chromatography. For several examples
(Table 3, entries 2, 9 and 22), relatively lower yields were obtained
when the reactions were terminated according to the TLC moni-
toring (less than 48 h). As a comparison, the reaction time for these
examples was then prolonged to 72 h, but no obvious increase in
yield was observed. In total, nine new compounds were obtained,
and characterized by 1H NMR, 13C NMR, IR and HRMS. Un-
fortunately, there was no obvious enantiomeric excess of the
products observed by the chiral HPLC analysis.
In order to explore the mechanism of this reaction, we per-
formed several control experiments (Scheme 3). In the first series
experiments, benzaldehyde treated with malononitrile in the
presence of
excellent yield of 81% after less than 3 min. The same reaction in the
absence of -proline gave the Knoevenagel adduct I with a yield of
6% after 2 h. The results revealed that -proline greatly accelerated
the formation of Knoevenagel adduct I. Then benzaldehyde was
treated with indole in the absence of -proline under the same
reaction conditions, and almost no new material was generated
after 2 h. Also no reaction was observed when -proline (10 mol %)
L-proline (10 mol %) gave Knoevenagel adduct I in an
L
L
L
L
was added to the mixture of benzaldehyde and indole for 2 h. These
results suggested that the malononitrile instead of indole first re-
acts with benzaldehyde under the catalysis of
to verify if -proline catalyzes the Michael addition of indole with
Knoevenagel adduct I to form the final products, some more control
experiments were performed (Scheme 3). When treated with
L-proline. Moreover,
L
L-
proline (10 mol %), the reaction of indole and Knoevenagel adduct I
gave the desired product 4a in a good yield of 93% in EtOH for 48 h.
Scheme 1. L-Proline catalyzed reaction using 2,2,2-trifluoroacetophenone as the
electrophile.
Meanwhile, the same reaction carried out in the absence of L-pro-
line gave product 4a in a lower yield of 70%. The results indicated
that the Michael addition of indole with Knoevenagel adduct I can
take place without catalyst, however the reaction can still be pro-
moted by
L-proline.
Thus a plausible pathway for
L-proline-catalyzed alkylation of 3-
indole was proposed (Scheme 4). Firstly, the reaction commences
with the formation of an iminium ion intermediate from benzal-
Scheme 2. L-Proline catalyzed reaction using pyrrole as the nucleophile.
dehyde and -proline. Then, adduct I forms through Knoevenagel
L
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Y.-H. He et al. / Tetrahedron xxx (2015) 1e8
7
Scheme 3. Control experiments.
Scheme 4. A possible pathway for L-proline-catalyzed alkylation of indole.
condensation between imine intermediate and nucleophilic
malononitrile. Finally, Michael addition of indole with adduct I
furnishes the desired product.
(10 mol %) and ethanol (1.0 mL) were added sequentially into
a round-bottom flask to form a clear yellow solution. The resultant
mixture was stirred at rt for a specific time. After completion of the
reaction, the solvent was removed under reduced pressure, and the
residue was purified by silica gel flash column chromatography
with petroleum ether/ethyl acetate as eluent to give the desired
product.
3. Conclusion
In summary, we developed an L-proline-catalyzed multicom-
ponent reaction of aldehydes (or 2,2,2-trifluoroacetophenone),
malononitrile and indoles (or pyrrole) for the synthesis of 3-
alkylated indole derivatives in EtOH at room temperature. The
desired products were obtained in good to excellent yields of up to
Acknowledgements
This work was financially supported by the National Natural
98%.
L-proline was used as a readily available, inexpensive and
Science Foundation of China (No. 21276211 and No. 21472152).
metal-free catalyst. The procedure was simple, and the reaction
conditions were mild. Moreover, this multicomponent reaction was
highly atom-economic. 3-Alkylated indole derivatives could be
prepared in high yields using 1:1:1 M ratio of reactants by loss of
a molecule of water in total. These advantages make the method-
ology a rather attractive process for the synthesis of 3-alkylated
indole derivatives.
Supplementary data
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2015.10.027.
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