Recyclable sulfonated amorphous carbon catalyzed friedel-crafts alkylation of indoles with α,β-unsaturated carbonyl compounds in water

Article abstract of DOI:10.1002/asia.200900726

Green practice: A sulfonated amorphous carbon-based solid acid derived from d-glucose was synthesized. It was explored for its catalytic efficiency in Friedel-Crafts reactions of indoles with various a,b-unsaturated carbonyl compounds in H2O or H2O/THF at

Full text of DOI:10.1002/asia.200900726

COMMUNICATION
DOI: 10.1002/asia.200900726
Recyclable Sulfonated Amorphous Carbon Catalyzed Friedel–Crafts
Alkylation of Indoles with a,b-Unsaturated Carbonyl Compounds in Water
[
a]
[a]
[a]
[a]
[b]
Jimei Ma, Simon Ng, Youjun Yong, Xiao-Zhou Luo, Xin Wang, and
[
a]
Xue-Wei Liu*
Indole derivatives are currently explored as privileged
structures for drug design and discovery owing to their high
the use of toxic transition metals and nonbenign solvents
still remains as an issue in many of these solid acid catalytic
systems. A few years ago, Kobayashi and his co-workers de-
veloped a neutral catalytic system by combining silica-sup-
[1]
binding affinity to many receptors. Among them, 3-substi-
tuted indoles, which are important precursors in the synthe-
sis of many biologically active compounds and natural prod-
[10]
ported sodium sulfonate with ionic liquid in water. While
the neutral system allowed the Michael reaction of acid-
labile substrates, the reaction time required was generally
longer than that of other solid acid catalysts. Despite the
recent advances, there is still a demand for other environ-
mentally friendly and efficient alternatives for the F-C reac-
tions of indoles with a,b-unsaturated carbonyl compounds.
Recently, a novel class of carbon-based solid acids derived
from carbohydrates has attracted considerable attention for
its potential as an inexpensive, environmentally benign and
[2]
ucts, have drawn the most attention. They are known to be
easily synthesized via Michael-type Friedel–Crafts (F-C) re-
actions of indoles with a,b-unsaturated carbonyl compounds,
[3]
in the presence of either Brønsted or Lewis acids. Howev-
er, owing to the homogeneous nature of these conventional
acids, large stoichiometric amounts and tedious isolation
procedures are required. In view of the increasing emphasis
on the use of environmentally friendly processes, solid acid
catalysts have been proposed as an attractive alternative to
conventional liquid acid catalysts owing to their simple
[11]
stable catalyst with high catalytic performance. Consisting
of flexible polycyclic carbon sheets which bear phenolic hy-
droxy (-OH), carboxylic acid (-COOH), and sulfonic acid
[4]
workup, ease of recovery, and reusability. To date, several
silica-supported solid acid catalysts have been developed
and employed in the F-C reaction of indoles with electron-
(-SO H) groups, the catalyst can be readily prepared by in-
3
[5]
deficient olefins. Other heterogeneous catalytic systems,
complete carbonization of natural carbohydrates, such as
glucose, cellulose, and starch, followed by sulfonation of the
[
6]
[7]
which include nanosized TiO , ZrOCl ·8H O, and hetero-
2
2
2
[8]
[12]
polyacids , have also been reported. Unfortunately, their
heterogeneous nature does not guarantee the consistency of
catalytic performance after their recovery.
resulting amorphous carbon. Initially used as a green and
sustainable catalyst in the esterification of fatty acids in bio-
[5b,8]
[11c]
In fact, many
diesel production,
it was soon reported to demonstrate
of the silica-supported catalysts are mechanically unstable in
water and consequently not reusable in reality. Moreover,
high catalytic efficiency in other reactions such as hydroly-
[9]
[12a]
[12b]
As is
H
U
G
R
N
N
and hydration.
Inspired by these reports, we envis-
aged the use of this carbon-based solid acid in catalyzing F-
C alkylation to synthesize 3-substituted indoles in an envi-
[
3g,12a,13]
[
a] J. Ma, S. Ng, Y. Yong, X.-Z. Luo, Prof. Dr. X.-W. Liu
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Science
Nanyang Technological University
Singapore 637371 (Singapore)
ronmentally benign fashion.
To the best of our
knowledge, this is the first demonstration of carbon-based
solid acid in catalyzing carbon–carbon bond formations.
The sulfonated carbon-based solid acid 1a was prepared
[
11c]
Fax : (+65)67911961
according to the literature method.
The content of 1a
E-mail: xuewei@ntu.edu.sg
was determined by elemental analysis, and the result ob-
tained for sulfur content was 1.13 wt%. Since all sulfur
[
b] Dr. X. Wang
Division of Chemical and Biomolecular Engineering
School of Chemical and Biomedical Engineering
Nanyang Technological University
Singapore 637457 (Singapore)
atoms in the carbon material were in the form of SO H
3
[
12b]
groups,
the density of SO H groups in 1a was thus esti-
mated to be 0.35 mmolg . The solid acid was further char-
acterized by X-ray diffraction (XRD). The XRD pattern is
3
À1
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900726.
[12a,b]
in good agreement with the literature,
showing that the
778
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 778 – 782

prepared solid acid is in the form of amorphous carbon (see
the Supporting Information).
exhibited the best yield in water, the use of solvent mixture
H O/THF in the composition of 4:1 was found to afford a
2
The catalytic performance of 1a was first examined using
the F-C reaction of indole with methyl vinyl ketone (MVK)
as a model reaction. To our delight, the reaction took place
comparably good yield as well (Table 1, entry 8). Other sol-
vents provided less satisfactory results, ranging from low to
moderate yields (Table 1, entries 9–13).
[3e,14]
smoothly in water
at ambient temperature to give isolat-
Treatment of indole and MVK with 5 mol% of sulfuric
acid as catalyst brought forth only a small amount of F-C
adducts (Table 1, entry 14). Although we raised the amount
of sulfuric acid up to one equivalent, limited increase in the
amount of product was observed, giving rise to only a mod-
erate yield of product (Table 1, entry 15). These sluggish re-
sults show that 1a is a superior acid catalyst to its conven-
tional homogeneous counterpart.
ed product in excellent yield (Table 1, entry 1). Investigation
[
a]
Table 1. Catalyst screening and optimization of reaction conditions.
Encouraged by the positive results from the preliminary
studies, we investigated the efficiency of 1a in the alkylation
of indole and its derivatives with various a,b-unsaturated
carbonyl compounds under the optimized conditions. Most
reactions proceeded smoothly in either H O or H O/THF
solvent mixture to furnish the corresponding Michael ad-
ducts in moderate to excellent yields (Table 2). Moreover,
the advantage of our catalytic system is that protection of
the indole NH functional group is unnecessary.
In view of its environmental and economical advantages,
water is the preferred solvent system in the course of our in-
vestigation. While some reactions were able to proceed
smoothly in water to afford excellent yields, others were
more sluggish as shown by their prolonged reaction times
and the unsatisfactory yields. Poor solubility of the sub-
strates and products in water, especially those of higher
complexity, is probably the reason behind these results. We
Entry Catalyst
Cat. loading
Solvent
T
[h]
Yield
[%]
[
b]
[mol%]
1
2
3
4
5
6
7
1a
1a
1a
1a
1b
1c
black
carbon
1a
1.0
2.0
5.0
10.0
29 mg
29 mg
29 mg
H
H
H
H
H
H
H
2
2
2
2
2
2
2
O
O
O
O
O
O
O
6
3
1
96
96
96
2
2
0.5 94
48
8
[
[
c]
c]
43
95
18
24
[
d]
8
5.0
H
2
O/
1
93
THF
THF
CH Cl
2 2
MeCN
MeOH
toluene
9
1
1
1
1
1
1
1a
1a
1a
1a
1a
5.0
5.0
5.0
5.0
5.0
5.0
1
1
1
1
1
1
1
84
79
77
66
19
40
63
0
1
2
3
4
5
H
H
2
SO
SO
4
4
H
H
2
O
O
2
100.0
2
[
(
a] Indole (0.2 mmol) was treated with MVK (0.4 mmol) in solvent
1 mL) at room temperature in the presence of the catalyst. [b] Yields of
isolated product after purification. [c] The densities of the phosphoric
would then opt for an alternative solvent system of H O/
2
THF in the composition of 4:1, which gave comparable
good results in our earlier screening of solvents. As antici-
pated, a remarkable improvement in both the reaction time
and yield of isolated product were observed when we
switched to this solvent system.
and perchloric acid groups were not determined. [d] H
ratio of 4:1 was used.
2
O/THF in the
shows that the reaction time required for complete conver-
sion is inversely proportional to the amount of 1a loaded
The reaction of indole and its derivatives with MVK pro-
ceeded smoothly at ambient temperature in the presence of
5 mol% of solid acid catalyst 1a to give 85–96% yield of
isolated product without side reactions of dimerization or
polymerization (Table 2, entries 1–5). It is noteworthy that
both electronic and architectural modification of the indole
aromatic ring could be accomplished without compromising
the good yield. The excellent yield (91%) could also be ach-
ieved in the Michael reaction of indole with 4-hexen-3-one
(Table 2, entry 6). After all, the Michael acceptor employed
is much more sterically hindered in comparison to MVK
owing to the elongation of the aliphatic chain. Reactions of
various indole derivatives with 4-hexene-3-one also gave sat-
isfactory results (Table 2, entries 7–10).
(
Table 1, entries 1–4). However, along with an accelerated
reaction rate, the yield remained consistent at about 94% to
9
1
6% as the amount of 1a employed was increased from 2 to
0 mol% (Table 1, entry 4). Thus, 5 mol% of 1a was deter-
mined to be the optimal amount of catalyst loading.
Next, phosphonated (1b) and perchlorated carbon solid
acid (1c) were synthesized by treating the black amorphous
carbon with concentrated phosphoric acid and perchloric
acid, respectively. They were investigated for their catalytic
efficiencies in the model reaction. Less satisfactory results
(
Table 1, entries 5 and 6) were obtained than that of sulfo-
nated 1a. This suggested that the presence of SO H groups
3
was crucial to the high catalytic performance of the solid
acid. To validate this hypothesis, a non-sulfonated black
carbon, the precursor of 1a, was used in a control experi-
ment. As anticipated, a very low yield of product was ob-
tained even after prolonged reaction time (Table 1, entry 7).
We also examined the solvent effect by screening different
solvent systems (Table 1, entries 8–13). While the reaction
To further validate the effectiveness of 1a in promoting
F-C reaction of indole with unreactive electron-deficient
olefins, several Michael acceptors which are notorious for
their poor reactivity to nucleophilic attack were employed
(Table 2, entries 11–15). Among them, chalcone and b-nitro-
styrene gave satisfactory yields of Michael adducts (Table 2,
entries 11 and 12). Other substrates such as a,b-unsaturated
Chem. Asian J. 2010, 5, 778 – 782
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779

X.-W. Liu et al.
COMMUNICATION
Table 2. Michael-type Friedel–Crafts reactions of indole and derivatives with a,b-unsaturated carbonyl com- esters, aldehydes, and nitriles
[
a]
pounds.
were found to be less appropri-
ate for Michael reaction with
indole using the same catalytic
system.
Cyclic enones, such as 2-cy-
clopentenone and 2-cyclohexe-
none, are generally less reactive
than acyclic enones. Therefore
they tend to proceed sluggishly,
affording low yields even under
[
b]
Entry
Michael donor
Michael acceptor
Product
T [h]
Yield [%]
96
[
[
[
c,d]
c,d]
c]
1
2
3
1
[5d]
prolonged reaction times.
Catalyst 1a was demonstrated
to be effective in promoting of
the F-C reaction of indole with
1
89
90
2
-cyclopentenone. The reaction
0.5
proceeded smoothly to give the
desired monoindolyl cyclopen-
tanone in a moderate yield of
[
[
c]
c]
4
5
6
0.5
0.5
2
85
88
91
72% (Table 2, entry 13).
A
complication was observed
when 2-cyclohexenone was em-
ployed as the Michael acceptor,
giving triindolylcyclohexane as
the major product (Table 2,
entry 14). It is prone to over-
react with indole under acidic
conditions, making it difficult to
obtain a monosubstituted prod-
uct with satisfactory yield. The
regioselectivity was affected
owing to the competition be-
tween 1,4-addition and 1,2-ad-
dition of the indole to 2-cyclo-
[
d]
7
8
4
2
81
86
[15]
hexenone.
The difference in
the formation of major product
when using 2-cyclohexenone
and 2-cyclopentenone as ac-
ceptors is accounted for by a
torsional strain effect of their
9
1
1
1
3
3
82
80
78
76
corresponding
monoindolyl
[16]
0
1
2
products.
Nevertheless, the
replacement of indole with 2-
methylindole gave an increased
regioselectivity in the Michael
addition to 2-cyclohexenone.
The corresponding 1,4-conju-
gated monoindoyl adduct was
obtained as the major product
in a moderate yield of 67%
[
[
e]
d]
48
48
(
Table 2, entry 15).
To validate the reusability of
1
a, 5 mol% catalyst was initial-
[
c]
1
3
48
72
ly employed in the F-C reaction
of indole (0.2 mmol) with MVK
(
0.4 mmol) in water (1 mL) for
1
h at room temperature. After
780
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Chem. Asian J. 2010, 5, 778 – 782

Friedel–Crafts Alkylation of Indoles
Table 2. (Continued)
reusable up to five times with-
out any significant loss in cata-
lytic performance. Thus, this
carbon-based solid acid catalyt-
ic system serves as an environ-
mentally benign tool for the ef-
ficient synthesis of 3-substituted
indole derivatives.
[
b]
Entry
Michael donor
Michael acceptor
Product
T [h]
Yield [%]
[f]
1
1
4
4
12+60
Experimental Section
[
c]
5
6
67
All reagents and solvents were ob-
tained from commercial suppliers and
used without further purification
unless otherwise stated. Methyl vinyl
ketone, 2-cyclopentenone, and 2-cyclo-
hexenone were distilled prior to use.
Evaporation of organic solutions was
achieved by rotary evaporation with a
water bath temperature below 408C.
Product purification by flash column
chromatography was accomplished
[
(
a] Unless specified, Michael donor (0.20 mmol) was reacted with Michael acceptor (0.22 mmol) in H
0.8 mL/0.2 mL) at room temperature in the presence of 1a (5 mol%). [b] Yields of isolated product after pu-
rification.[c] MVK (0.40 mmol) was used. [d] H O (1 mL) was used. [e] H O/THF (0.5 mL/0.5 mL) was used.
f] Conversion yield calculated based on indole.
2
O/THF
2
2
[
recovery through a simple filtration and removal of volatile
solvents under vacuum, the catalyst was reused in catalyzing
the same model reaction without further activation. The re-
action proceeded smoothly even after five runs, without any
extension of reaction time or marked loss in yield
using silica gel 60 (0.010–0.063 nm). Chromatograms were visualized by
fluorescence quenching with UV light at 254 nm or by staining using a
base solution of potassium permanganate. Technical-grade solvents were
used for chromatography and distilled prior to use. NMR spectra were
recorded at room temperature on 300 MHz Bruker ACF 300 and
4
00 MHz Bruker DPX 400 spectrometers. The residual solvent signals
(
Figure 1). These positive results indicated that the sulfonat-
1
were taken as the reference (7.26 ppm for H NMR spectra and 77.0 ppm
1
3
ed carbon-based solid acid 1a could be recycled up to five
for C NMR spectra in CDCl ). Chemical shift (d) is reported in ppm,
3
times with no noticeable loss in its catalytic efficiency.
coupling constants (J) are given in Hz. The following abbreviations classi-
fy the multiplicity: s=singlet, d=doublet, t=triplet, m=multiplet or un-
resolved. HR-MS (ESI) spectra were recorded on a Waters Q-Tof pre-
TM
mier
mass spectrometer. Elemental analysis was measured on a
Perkin–Elmer Series II CHNS/O Analyzer 2400. Powder X-ray diffrac-
tion (XRD) was collected from a Shimadzu 6000 diffractometer. Carbon-
ization of d-glucose was carried out on a Carbolite 12008C three-zone
tube furnace.
Typical procedure for Michael-type Friedel–Crafts reaction of indoles
with a,b-unsaturated carbonyl compounds: To a stirred mixture of indole
(
23.4 mg, 0.20 mmol) and 1a (29.0 mg, 0.01 mmol) in H
2
O (1 mL) or
H
2
O/THF (0.8 mL/0.2 mL), the Michael acceptor (0.22 mmol) was added
in one portion. The reaction mixture was stirred at room temperature
and the progress of the reaction was monitored by TLC. After comple-
tion of the reaction, the mixture was diluted with ethyl acetate (2 mL),
filtered, and washed successively with water (5 mL), ethyl acetate (3ꢁ
5
mL), and diethyl ether (5 mL). The filtrate was collected and the organ-
ic layer was separated. The aqueous layer was extracted with ethyl ace-
tate (3ꢁ5 mL). The combined organic layers were dried with anhydrous
Na SO , filtered, and concentrated under reduced pressure to afford the
2 4
Figure 1. Catalytic performance of recovered 1a in the F-C reaction of
indole with MVK.
crude product, which was purified by column chromatography on silica
gel to give the corresponding product. The remaining solid acid catalyst
was dried under reduced pressure to remove all the volatile components,
and then reused in the next run.
In conclusion, we have explored the efficiency of a
carbon-based solid acid in catalyzing F-C reactions of indole
and its derivatives with a variety of a,b-unsaturated carbon-
yl compounds in water or H O/THF solvents at ambient
2
temperature. It is worthy to highlight the water-tolerant
property of the carbon-based solid acid since it is capable of
exhibiting a consistent high catalytic activity without being
poisoned by water. Furthermore, the solid acid, derived
from d-glucose, which is accessible from nature in abun-
dance, is considerably inexpensive. Most importantly, easy
recovery of the catalyst upon completion of the reaction is
achievable by simple filtration, and the recovered catalyst is
Acknowledgements
Financial support from Nanyang Technological University (RG50/08) and
the Ministry of Education, Singapore (MOE 2009-T2-1-030) are grateful-
ly acknowledged. S.N. and X.-Z.L. are thankful for the grant from Na-
nyang Technological University under the Undergraduate Research Ex-
perience on Campus (URECA) programme.
[14]
Chem. Asian J. 2010, 5, 778 – 782
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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781

X.-W. Liu et al.
COMMUNICATION
Keywords: alkylation
recycling · Friedel–Crafts reaction · heterogeneous catalysis
·
amorphous carbon
·
catalyst
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4
Received: December 17, 2009
Revised: December 26, 2009
Published online: February 8, 2010
2
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Chem. Asian J. 2010, 5, 778 – 782