FeCl3-promoted alkylation of indoles by enamides

Article abstract of DOI:10.1039/c0ob00709a

An efficient iron-promoted alkylation of indoles with enamides has been accomplished under mild reaction conditions. The reaction proceeded with remarkable regioselectivity leading exclusively to substitution by indoles at α-position of enamides.

Full text of DOI:10.1039/c0ob00709a

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FeCl3-promoted alkylation of indoles by enamides†
Tianmin Niu, Lehao Huang, Tianxing Wu and Yuhong Zhang*
Received 13th September 2010, Accepted 7th October 2010
DOI: 10.1039/c0ob00709a
An efficient iron-promoted alkylation of indoles with enamides has been accomplished under mild
reaction conditions. The reaction proceeded with remarkable regioselectivity leading exclusively to
substitution by indoles at a-position of enamides.
Table 1 Optimization of reaction conditionsa
Introduction
The hydroarylation of alkenes is one of the most important
reactions for the functionalization of arenes and heteroarenes.1
One important application of this transformation is the alkylation
of indoles, which are key structural motifs of numerous natural
products and biologically active compounds.2 Although Friedel–
Crafts reactions are well-known transformations for alkylation,
the methods often suffer from drastic reaction conditions (high
reaction temperature, strong acidic/base conditions) and re-
gioselectivity problems.3 In recent years, the transition-metal
catalyzed process has emerged as an attractive alternative to
the conventional Friedel–Crafts reaction, and Cu(OTf)2,4 CeCl3,5
Entry
Catalyst
Solvent
T/◦C
Yield (%)b
1
2
3
4
5
6
7
8
none
ZnCl2
RuCl3
InBr3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DMF
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
r.t.
40
40
40
40
40
40
n.r
n.r
trace
n.r
42
41
90
NiCl2·6H2O
SnCl4
CuCl2
BF3·Et2O
FeCl3
89
Zn(OTf)2,6 InBr3,7 SmI3,8 Sc(OTf)3,9 PtCl2,10 or AuCl3 catalytic
11
9
99 (98)c
97
systems have been developed. These processes have been proved
remarkably effective under mild reaction conditions and enjoy a
broad application in the synthesis of alkylated indoles. However,
most of these investigations focus on the addition of indoles to
electron-neutral alkenes or electron-deficient alkenes, which are
activated by a conjugated electron-withdrawing group. There has
been only scant attention in developing a general methodology for
the addition of indoles to electron-rich alkenes such as enamides.12
To the best of our knowledge, the metal-catalyzed addition of
indoles to enamides has not been explored. The development of an
efficient procedure for the alkylation of indoles with electron-rich
olefins such as enamides under mild conditions is highly desired.
Recently, iron has been increasingly explored in organic
transformations as an inexpensive and environmentally benign
catalyst.13 There have been a series of reports concerning novel
iron-catalyzed reactions, which lead to efficient C(sp2)–C(sp3),14
C(sp2)–C(sp2),15 C(sp2)–C(sp),16 and C–N17 bond formations.
Herein, we report a highly efficient method for the addition of
indoles to enamides in the presence of an iron catalyst under mild
reaction conditions (Scheme 1). This facile catalytic system was
also applicable to indolizines.
10
11
12
13
14d
15
16
17
18
19
20
21
22
23
FeCl3·6H2O
Fe2O3
32
FeCl2
Fe(acac)3
HCl
TMSCl
HOAc
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
trace
n.r
77
74
n.r
53
n.r
86
78
70
80
CH3CN
THF
toluene
acetone
H2O
FeCl3
FeCl3
78
a Reaction conditions: indole (0.5 mmol), 1-vinyl-2-pyrrolidinone
(0.6 mmol), catalyst (0.05 mmol), solvent (5 ml), 40 ◦C, 30 min. b Isolated
yields based on indole. c The isolated yield in anhydrous CH2Cl2 under
nitrogen atmosphere is given in parentheses. d Concentrated hydrochloric
acid (wt%: 36.5%) was used.
Results and discussion
In our initial studies, we tested the effect of various metals on
the addition of indole (1a) toward enamide (2a) in CH2Cl2. As
shown in Table 1, no reaction was observed in the absence of
metal catalyst (Table 1, entry 1). Treatment of 1a and 2a with
ZnCl2, RuCl3 or InBr3 failed to give any product at 40 ◦C for
30 min (Table 1, entries 2–4). The desired addition product 1-
(1-(1H-indol-3-yl)ethyl)pyrrolidin-2-one (3a) was isolated when
NiCl2·6H2O or SnCl4 were applied as catalysts, but with very
Department of Chemistry, Zhejiang University, Hangzhou 310027, P.R.
China. E-mail: yhzhang@zju.edu.cn; Fax: 0086-571-87953244; Tel: 0086-
571-87952723
† Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for all new compounds along with
copies of 1H and 13C NMR spectral data. See DOI: 10.1039/c0ob00709a
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Table 2 Iron-catalyzed direct alkylation of indoles and indolizidines with
1-vinyl-2-pyrrolidinonea
Entry
1
Indole/indolizine
Product
Yield (%)b
99
2
3
82
78
Scheme 1 Iron-catalyzed alkylation of indoles and indolizines with
enamides.
low yields (Table 1, entries 5–6). A more promising result was
obtained by the use of 10 mol% CuCl2 (Table 1, entry 7). When
BF3·Et2O was applied, 89% yield was isolated (Table 1, entry 8).
Further exploration led to a discovery that a 99% isolated yield
was obtained by employing FeCl3 as catalyst (Table 1, entry 9).
Importantly, the reaction was carried out in open air without
exclusion of oxygen from the reaction flask, and FeCl3·6H2O
showed almost the same reactivity with FeCl3 (Table 1, entry 10).
In contrast, iron reagents such as Fe2O3, FeCl2 and Fe(acac)3 were
inactive (Table 1, entries 11–13), showing that the source of iron
species influenced the reaction significantly. Catalytic amounts
(10 mol%) of Brønsted acids delivered no or relatively lower
product yields (Table 1, entries 14–16). The reaction could be
performed at room temperature in lower yield (Table 1, entry
17). The solvents are crucial to the reaction. No reaction was
observed in DMF (Table 1, entry 18). The comparable results
were obtained by the use of CH3CN, toluene, acetone, and THF
as solvent (Table 1, entries 19–22). It should be noted that the
reaction could carried out in water to give a 78% yield in 40 ◦C for
30 min (Table 1, entry 23).
4
5
6
96c
95
88
7
8
96
90
Under the optimized reaction conditions, we examined the
reactivity of various indoles as summarized in Table 2. In general,
indoles with both electron-rich and electron-deficient substituents
are active to give the adducts in high yields. Indoles with an
electron-donating group were highly active to afford the alkylated
indoles in excellent yields at 40 ◦C within 30 min (Table 2, entries
1–6). The indoles with C2 substituents delivered the corresponding
alkylated indoles in high yields (Table 2, entries 7–8), illustrating
that steric hindrance played a poor role to the reaction. Indole
with moderate electron-withdrawing bromide group was active
also to afford the corresponding alkylated indole in 98% yield
(Table 2, entry 9). However, the strong electron-withdrawing
substituents in indoles led to the decrease of the reaction rate, and
the prolongation of the reaction time was needed to access the high
yields (Table 2, entries 10–12). N-Substituted indoles presented
equally high efficiency in respect to that of free indoles to give the
adducts in high yields (Table 2, entries 13–15). Furthermore, this
facile catalytic system was also applicable to various indolizines
(Table 2, entries 16–18).
9
98
10
99d
11
12
83d
86
13
14
98
97
The reactivity of various enamindes was examined and the
results are summarized in Table 3. Satisfying results were obtained
when 1-vinylpyrrolidin-2-one 2a was replaced with 1-vinylazepan-
2-one 2b (Table 3, entries 1–3). However, N-vinylformamide 2c
gave the corresponding products in low yields under the reaction
conditions. To our delight, when the ratio of the substrates was
274 | Org. Biomol. Chem., 2011, 9, 273–277
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Table 2 (Contd.)
Table 3 Iron-catalyzed direct alkylation of indoles and indolizines with
various enamidesa
Entry Enamide
1
Indole/indolizine Product
Yield (%)b
90
1a
Entry
15
Indole/indolizine
Product
Yield (%)b
88
2
3
2b
2b
1m
1q
91
99c
16
17
18
95
99
92
4d
5d
1a
97
67e
81f
2c
2c
1m
99
99
6d
1q
a Reaction conditions: indoles (0.5 mmol), 1-vinyl-2-pyrrolidinone
(0.6 mmol), FeCl3 (8 mg, 0.05 mmol), in CH2Cl2 (5 ml), 40 ◦C, 30 min.
b Isolated yields based on indole. c 6-Methyl-1H-indole (0.75 mmol),
1-vinyl-2-pyrrolidinone (0.5 mmol), isolated yields based on N-
vinylformamide, 3 h was used. d The reaction time was 6 h.
7
8
1a
79g
81
modified from indole : enamide = 1 : 1.2 to enamide : indole =
1 : 1.5, the reaction was dramatically improved to afford the
products in excellent yields (Table 3, entries 4–6). N-Methyl-N-
vinylacetamide 2d afforded low yield in the reaction with N-free
indole 1a possibly due to the decomposition of the product, and
a 79% yield was obtained when the reaction was performed at
room temperature (Table 3, entry 7). On the contrary, N-methyl
indole 1m participated in the reaction smoothly to afford 6b in
81% yield at 40 ◦C within 30 min (Table 3, entry 8). In the case of
indolizine 1q, longer reaction time and higher temperature were
required (Table 3, entries 3 and 9).
During the study of the reaction of indoles with N-methyl-
N-vinylformamide 2d, we found that the ratio of indoles to
enamides influenced the final products. An excess of indole led
to the substitution of indoles to 2d to give a mixture of alkylated
indole 6a and a second alkylation product of bis-indolylmethane
7a. Since bis-indolylmethane and its derivatives are key structural
units in many natural drugs used as cancer growth inhibitors,18 we
investigated the reaction conditions for selective formation of bis-
indolylmethane products as shown in Table 4. It was found that the
amount of indole played a key role for the second alkylation. When
3 equivalents of indole were used, only bis-indolylmethane 7a
was obtained in 91% yield (Table 4, entry 1). This transformation
was compatible with a broad of functional groups, including –
CH3, –Br, –CN, and N–CH3 (Table 4, entries 2–6). Importantly,
asymmetric bis-indolylmethanes could be prepared by the use of
6a as initial substrate (Table 4, entry 7–8). The reaction was also
applicable to indolizine (Table 4, entry 9). The enamides 2a, 2b,
and 2c failed to furnish the bis-indolylmethane under the reaction
conditions.
2d
2d
1m
9
1q
80c
a Reaction conditions: indoles (0.5 mmol), enamides (0.6 mmol), FeCl3
(8 mg, 0.05 mmol), in CH2Cl2 (5 ml), 40 ◦C, 30 min. b Isolated yields based
on indole. c 3 h, 60 ◦C, were used. d Indole (0.75 mmol), N-vinylformamide
enamide (0.5 mmol), isolated yields based on N-vinylformamide. e Isolated
yield when concentrated HCl (10 mol %) was used as catalyst. f Isolated
yield when TMSCl (10 mol%) used as catalyst. g The reaction was at room
temperature.
There are two possible pathways for the intermolecular C-3
indole alkylation as shown in Scheme 2. (1) With the aid of Lewis
acid FeCl3, enamide 2a transforms to the iminium species I, which
reacts with indole 1a via a Friedel–Crafts-type process to afford the
alkylation product 3a (Scheme 1, path A).19 (2) The protonation
of enamide 2a by the Brønsted acid hydrolyzed from FeCl3
generates the iminium II, which undergoes nucleophilic addition
with indoles to give the final product (Scheme 1, path B).12,20
We performed the reaction in anhydrous CH2Cl2 under nitrogen
atmosphere, and it was found that a high yield of desired product 1-
(1-(1H-indol-3-yl)ethyl)pyrrolidin-2-one3a was obtained (Table1,
entry 9). Furthermore, typical Lewis acid BF3·Et2O also worked
well in this transformation (Table 1, entry 8). On the other hand,
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Table 4 Synthesis of symmetric and unsymmetric bis-indolylmethanesa
Entry
1
Indole/indolizine
Product
Yield (%)b
91
2
89
Scheme 2 Proposed mechanism for alkylation of indoles from enamide.
The production of bis-indolylmethanes in Table 4 might take
place through the elimination of amine moiety of N-(1-(1H-
indol-3-yl)ethyl)-N-methylacetamide 6a with the assistance of iron
catalyst to form the intermediate III, which undergoes a Friedel–
Crafts-type process with indoles or indolizine to deliver the sym-
metrical or unsymmetrical bis-indolylmethanes (Scheme 3).18a,21
3
4
88c
67d
5
75
Scheme 3 Proposed mechanism for bis-indolylmethanes.
6
7
90
97
Conclusion
In summary, we have developed a highly efficient iron-catalyzed
process for the addition of indoles to enamides under mild
reaction conditions. The simple catalytic system worked well
with a broad range of indoles and allowed the facile synthesis
of alkylated indoles as well as symmetric and unsymmetric bis-
indolylmethanes.
8
9
6a
96
Experimental section
General
Unless otherwise stated, all reactions were carried out in an
88e
1
oven-dried flask in air. H NMR spectra were recorded at 400
or 500 MHz and the chemical shifts are reported in parts per
million (d) relative to internal standard TMS (0 ppm) for CDCl3
or DMSO-d6. The peak patterns are indicated as follows: s, singlet;
d, doublet; t, triplet; q, quartet; m, multiplet; dd, doublet of
doublet; dt, doublet of triplet. The coupling constants, J, are
reported in Hertz (Hz). 13C NMR spectra were recorded at 100
or 125 MHz and referenced to the internal solvent signals (center
peak is 77.00 ppm in CDCl3 or 39.90 ppm in DMSO-d6). Mass
spectroscopy data were collected on an HRMS-EI instrument.
Melting points were measured on a Yanaco MP-500 apparatus and
uncorrected. FT-IR spectra were recorded on a Nicolet Nexus 470
FT-IR spectrophotometer and the data were reported in reciprocal
centimetres (cm-1). Indoles and enamide materials were purchased
from common commercial sources and used without additional
purification.
a Reaction conditions: indoles (1.5 mmol), N-methyl-N-vinylacetamide
(0.5 mmol), FeCl3 (8 mg, 0.05 mmol), in CH2Cl2 (5 ml), 40 ◦C, 1 h.
b Isolated yields based on N-methyl-N-vinylacetamide. c The reaction time
prolongs to 5 h. d 12 h was used. e The reaction was in 60 ◦C for 12 h.
the employment of hydrochloric acid (10 mol%) and trimethyl
chlorosilane (TMSCl) (10 mol%) as catalysts resulted in the desired
products with enamide 2a and 2c in moderate yields (Table 1,
entries 14 and 15; Table 4, entry 4), and the reaction with enamides
2b and 2d failed. Thus, we postulate the reaction takes place mainly
through path A.
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Representative procedure for the preparation of 3-alkylindole
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To a mixture of indole (59 mg, 0.5 mmol), FeCl3 (8 mg, 0.05 mmol)
andCH2Cl2 (5 mL), 1-vinylpyrrolidin-2-one (67 mg, 0.6 mmol) was
added dropwise at room temperature. The resulting mixture was
stirred at 40 ◦C for 30 min. After the reaction, the reaction solution
was filtered through a pad of celite, and the solvent was removed
under reduced pressure. Purification by flash chromatography over
silica gel, eluting with acetate–dichloromethane (1 : 10), provided
1
the desired compound as a white solid (113 mg, 99%): H NMR
(400 MHz, CDCl3, TMS) d 8.89 (s, 1 H), 7.62 (d, J = 8.0 Hz, 1 H),
7.38 (d, J = 7.6 Hz, 1 H), 7.19 (t, J = 7.2 Hz, 1 H), 7.13 (s, 1 H),
7.08 (t, J = 7.2 Hz, 1 H), 5.80 (q, J = 6.8 Hz, 1 H), 3.26 (dt, J = 8.6,
5.4 Hz, 1 H), 2.86 (dt, J = 9.0, 5.6 Hz, 1 H), 2.51–2.38 (m, 2 H),
1.95–1.85 (m, 1 H), 1.82–1.71 (m, 1 H), 1.58 (d, J = 7.2 Hz, 3 H);
13C NMR (100 MHz, CDCl3) d 174.4, 136.6, 126.4, 122.3, 122.2,
119.7, 119.3, 115.7, 111.3, 42.7, 42.2, 31.8, 17.7, 16.7. HRMS (EI)
Calcd for C14H16N2O: [M]+ 228.1263; Found, 228.1260; IR n (KBr)
3244, 3058, 2973, 2932, 1659, 1493, 1455, 1428, 1342, 1287, 1248,
1198, 1116, 771, 745, 659. cm-1; mp: 147–149 ◦C.
Representative procedure for the preparation of
bis-indolylmethanes
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resulting mixture was stirred at 40 ◦C for 1 h. After the reaction,
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solvent was removed under reduced pressure. Purification by flash
chromatography over silica gel, eluting with acetate–petrol ether
(1 : 10), provided the desired compound as a white solid (118 mg,
91%): 1H NMR (400 MHz, CDCl3, TMS) d 7.75 (s, 2 H), 7.63 (d,
J = 7.2 Hz, 2 H), 7.34 (d, J = 8.0 Hz, 2 H), 7.22 (t, J = 7.4 Hz,
2 H), 7.10 (t, J = 7.6 Hz, 2 H), 6.84 (s, 2 H), 4.72 (q, J = 7.2 Hz,
1 H), 1.85 (d, J = 7.2 Hz, 3 H); 13C NMR (100 MHz, CDCl3) d
136.6, 126.9, 121.8, 121.6, 121.3, 119.7, 119.0, 111.5, 28.2, 21.8.
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Acknowledgements
Funding from Natural Science Foundation of China (No.
20872126) and Zhejiang Provincial Natural Science Foundation
of China (R407106) is acknowledged.
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