Ionic liquid-coordinated ytterbium(III) sulfonate catalyzed michael addition of indoles to electron-deficient nitroolefins

Article abstract of DOI:10.1002/cjoc.201090094

The Michael addition of indoles to electron-deficient nitroolefins was effectively catalyzed by an ionic liquid-coordinated ytterbium(III) sulfonate catalyst. The recycling procedure of the catalyst was very simple without extraction with water, and the c

Full text of DOI:10.1002/cjoc.201090094

FULL PAPER
Ionic Liquid-coordinated Ytterbium(III) Sulfonate Catalyzed
Michael Addition of Indoles to Electron-deficient Nitroolefins
Shen, Wei(沈威)
Wang, Limin*(王利民)
Tang, Jun(唐俊)
Qian, Zhenhua(钱振华)
Tong, Xiaofeng(童晓峰)
Laboratory for Advanced Materials, Institute of Fine Chemicals, East China University of Science and Technology,
Shanghai 200237, China
The Michael addition of indoles to electron-deficient nitroolefins was effectively catalyzed by an ionic liq-
uid-coordinated ytterbium(III) sulfonate catalyst. The recycling procedure of the catalyst was very simple without
extraction with water, and the catalyst was reused for five times without any loss of its catalytic activity. Further-
more, to demonstrate the application of this methodology, the Pictet-Spengler reaction was chosen and successfully
carried out in the mixture of Brønsted-acidic ionic liquid and [bmim]BF4.
Keywords ionic liquid-coordinated ytterbium(III) sulfonate catalyst, Michael addition, Pictet-Spengler reaction
Introduction
catalytic activity in Michael addition of indoles to elec-
tron-deficient nitroolefins.6 And a point worthy empha-
sizing is that the methodology could be applied to the
Pictet-Spengler reactions and revealed fine results.
The concept of recoverable and recyclable catalysts
has become extremely important from both environ-
mental and economical points of view. In the past dec-
ades, this concept has been partly realized based on
various strategies. One of them is to develop rare earth
metal catalysts as effective Lewis acids to be recovered
and reused without any loss of activity. However, there
are still some problems in these fields. For example,
water is needed to extract rare earth metal triflate, which
is one of the most commonly used Lewis acids,1 from
organic solvent in some catalytic systems. Thus much
energy would be consumed to recover catalyst by re-
moving water. Solid-coordinated rare earth metal cata-
lyst is an alternative solution.2 However, solid-phase
synthesis would suffer a series of problems due to the
heterogeneous reaction conditions. Fluorous tails are
also introduced into rare earth metal catalysts for fluor-
ous biphase synthesis,3 but perfluorinated solvents are
expensive and troublesome.
Results and discussion
The preparation of ionic liquid-coordinated ytter-
bium(III) sulfonate 1 is very simple. Yb(BF4)3 was pre-
pared from a mixture of NaBF4 and anhydrous YbCl3 in
absolute ethanol. Then stirring the suspension of
Yb(BF4)3 and zwitterion 27 in THF afforded the ionic
iquid-coordinated catalyst 1 (Figure 1).
Figure 1 Ionic liquid-coordinated ytterbium(III) sulfonate cata-
lyst 1.
Recently ionic liquids (IL) as soluble catalyst sup-
port for organic synthesis have attracted considerable
attention.4 An attractive feature of ionic liquids is that
their solubility can be readily tuned by introducing dif-
ferent cations or anions. Then they can be phase sepa-
rated from organic as well as aqueous media.5 It indi-
cated that IL-coordinated catalyst would be readily re-
covered if IL was well-defined designed and introduced
into ligand part. Inspired by the successful examples of
ionic liquid-coordinated catalysis,4 herein we would like
to report a readily-available ionic-liquid-coordinated
ytterbium(III) sulfonate catalyst 1, which showed high
With the ionic liquid-coordinated ytterbium(III) sul-
fonate catalyst 1 in hand, ICP-AES was used to determine
the content of Yb (15.3%), and the loading amount of Yb
for 1 (0.87 mmol/g) was also determined by titration.8
Afterwards, the solubility of the ionic-liquid-
coordinated ytterbium(III) sulfonate catalyst 1 was
tested in various solvents (Table 1). It was found that
the compound 1 was favorable to dissolve in polar or-
ganic solvent such as MeCN, EtOH, but not in less polar
organic solvent such as DCM, EtOAc and ether. And
the catalyst 1 also had good solubility in water.
*
E-mail: wanglimin@ecust.edu.cn; Tel.: 0086-21-64253881; Fax: 0086-21-64253881
Received June 20, 2009; revised September 13, 2009; accepted December 1, 2009.
Project supported by the National Natural Science Foundation of China (No. 20672035) and Key Laboratory of Organofluorine Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences.
Chin. J. Chem. 2010, 28, 443— 448
© 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Shen et al.
FULL PAPER
Scheme 1 Synthesis of IL-coordinated ytterbium(III) sulfonate 1
Table 1 Catalyst for the reactions of Michael addition of indoles to nitroolefins in various mediaa
Entry
Catalyst
Solvent
MeCN
CH2Cl2
EtOH
Temperature/℃
Time/h
Yieldb/%
1
2
3
4
1
1
1
1
70
Reflux
70
2
12
2
84
21
94
EtOH
70
2
94, 95, 93, 94, 93c
b
c
a Reaction conditions: indole 1.1 mmol, β-nitrostyrene 1 mmol, catalyst 5 mol%, solvent 8 mL. Isolated yield. Catalyst was reused for
five times.
The Michael reaction of indole to β-nitrostyrene was
selected as the model for subsequent screening. The re-
action was conducted in the presence of 0.05 equiv. of
catalyst 1 (Scheme 2). As shown in Table 2, the
ionic-liquid-coordinated ytterbium(III) sulfonate catalyst
worked well for the desired Michael addition and was
more efficient in polar solvents than in less polar sol-
vents apparently (Entries 1—3). The reason may be that
the catalyst had poor solubility in less polar solvents. Of
the solvents examined, optimal yield was achieved in
EtOH medium. It could be concluded that EtOH was the
most appropriate solvent. Further inspection revealed
that increased reaction temperature would shorten the
reaction time and make the reaction finished in 2 h. Af-
ter completion of the reaction as checked by TLC, the
solvent was evaporated in a rotatory evaporator, and
then CH2Cl2 was added to dissolve the organic product.
IL tailed catalyst was collected by filtration and washed
with CH2Cl2, then dried and reused directly for the next
run. The reactions using the recycled catalyst were con-
ducted in a similar manner. It could be seen form Entry
4 that the IL-coordinated ytterbium (III) sulfonate 1 was
reused for five times without any loss of its activity.
Thus, we selected the optimized reaction condition to
examine the scope and utility of this catalyst. Various
electron-deficient nitroolefins were selected to undergo
the Michael addition in the presence of 5 mol% of
IL-coordinated ytterbium(III) sulfonate 1 in ethanol. The
results are summarized in Table 2, which showed that
the nitroolefins gave good to excellent yields.
Scheme 2 Reactions of Michael addition of indoles to nitroole-
fins
product 4 could be directly subjected to Pictet-Spengler
cyclization with benzaldehyde to furnish 1,4-diaryl-
substituted 1,2,3,4-tetrahydro-β-carboline.
Here we chose the mixture of Brønsted acidic ionic
liquid 69 and the conventional IL [bmim]BF4 (m/m＝1/5)
as the dual solvent-catalyst to carry out Pictet-Spengler
cyclization, since the sole Brønsted acidic ionic liquid 6
was so viscous that it was hard to handle. Compared
with the Pictet-Spengler reaction catalyzed by TFA re-
ported in the literature,10 the Brønsted acidic ionic liquid
6 was much less corrosive.
Further more, the Pictet-Spengler reaction proceed-
ing in Brønsted acidic ionic liquid was much faster than
in organic solvent. The reaction finished in 8 h, and
gave trans-isomer 5 as major product, which was de-
termined by 1H NMR and NOE-NMR experiments. The
detail was outlined in Figure 2 and Table 4.
In summary, an ionic liquid-coordinated ytterbium
(III) sulfonate catalyst 1 was prepared and found
catalyze Michael addition of indoles to electron-
deficient nitroolefins effectively. The catalyst could be
easily recovered and reused for five times without any
loss of its activity. In addition, the Michael addition
products were transformed to valuable compounds
tryptamines, which were subjected to Pictet-Spengler
cyclization using Brønsted acidic ionic liquid as dual
solvent-catalyst.
The high synthetic potential of the present transforma-
tion was then demonstrated by the straightforward con-
version of the product 3 into highly valuable compounds
tryptamine 4. As shown in Scheme 3 and Table 3, reduc-
tion of the nitro group of 3 proceeded under mild reaction
conditions giving tryptamine 4. Alternatively, crude
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Chin. J. Chem. 2010, 28, 443— 448

Ionic Liquid-coordinated Ytterbium(III) Sulfonate Catalyzed Michael Addition of Indoles
Table 2 Michael addition of indoles to nitroolefins catalyzed by IL-coordinated ytterbium(III) sulfonate catalyst 1a
Entry
Indole
Olefin
Product
Time/h
Yieldb/%
1
2
94
2
3
80
3
2
91
4
5
6
7
3
3
2
2
89
87
92
86
a Reaction conditions: indole 1.1 mmol, nitroolefins 1 mmol, catalyst 5 mol%, EtOH 8 mL, 70 ℃. b Isolated yield.
Chin. J. Chem. 2010, 28, 443— 448
© 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Shen et al.
FULL PAPER
Scheme 3 Pictet-Spengler reactions in Brønsted IL
Table 3 Reactions of Pictet-Spengler cyclization in Brønsted IL
Entry
Ar
C6H5
Product 4
Yield a of 4/%
Product 5
Yield a of 5/%
1
4a
4b
4c
4d
84
79
84
87
5a
5b
5c
5d
85
78
84
88
2
p-MeOC6H4
p-ClC6H4
2-Furyl
3
4
a Isolated yield.
Table 4 Configuration and ratio of product 5a— 5d
Entry
Product
5a
Yielda/%
trans/cisb
1
2
3
4
85
78
84
88
91∶9
5b
92∶8
5c
92∶8
5d
77∶23
a Isolated yield. b The ratio determined by 1H NMR of crude reac-
tion mixture.
spectrometer at 400 MHz for 1H NMR and 125 MHz for
13C NMR. 19F NMR spectra were recorded on a Bruker
WP-500SY (376 MHz) spectrometer with CFCl3 as an
internal standard. Coupling constants (J) are given in Hz.
ICP-AES data were recorded on a Varian 710 ES in-
strument. Elemental analysis was carried out by using a
Perkin-Elmer 2400 CHNS analyzer. All solvents were
distilled and dried according to the standard procedures.
All the chemicals were of reagent grade and used with-
out further purification.
Synthesis of the IL-coordinated catalyst 1
YbCl3 (4.83 g, 17.3 mmol) and NaBF4 (1.90 g, 17.3
mmol) were dissolved in absolute ethanol (25 mL), then
heated to reflux for 1 h and subsequently filtered to re-
move NaCl. The filtrate was concentrated to give
YbBF4. Then YbBF4 was treated with the same equiva-
lent of zwitterion 2 in dried THF for 3 h. The reaction
mixture was filtered, and the solids were washed with
THF, and dried under reduced pressure to give an air
stable white solid. Yield 95%.
Figure 2 NOE experiments for 5d.
Experimental
1H NMR (400 MHz, D2O) δ: 8.61 (s, 1H), 7.39 (t,
J＝2.0 Hz, 1H), 7.32 (t, J＝2.0 Hz, 1H), 4.23 (t, J＝7.2
Chin. J. Chem. 2010, 28, 443— 448
NMR spectra were recorded on a Bruker AM 400
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Ionic Liquid-coordinated Ytterbium(III) Sulfonate Catalyzed Michael Addition of Indoles
Hz, 2H), 2.79 (t, J＝7.2 Hz, 2H), 2.19—2.23 (m,
2H); 19F NMR (376 MHz, D2O) δ: －152.3. Anal. calcd
for C21H45B3F12N6O9S3Yb: C 23.90, H 4.29, B 3.07, F
21.63, N 7.97, Yb 16.4; found C 23.95, H 4.32, B 3.11,
F 21.71, N 7.82, Yb 15.3.
NMR (125 MHz, CDCl3) δ: 156.1, 141.9, 136.4, 125.4,
122.7, 122.3, 119.7, 119.0, 114.8, 111.1, 110.2, 107.1,
46.7, 42.3. Anal. calcd for C14H14N2O: C 74.31, H 6.24,
N 12.38; found C 74.28, H 6.35, N 12.31.
General procedure for Pictet-Spengler reaction of 4
General procedure for Michael addition of indoles to
electron-deficient nitroolefins
To the component solution of BAIL/[bmim]BF4
(m/m＝1/5, 10 mL) were added tryptamine 4 (1.30
mmol) and benzaldehyde (1.55 mmol). After being
stirred at 60 ℃ for 8 h, the mixture was cooled and
poured into saturated NaHCO3 solution (50 mL). The
aqueous phase was extracted twice with CH2Cl2. The
combined organic phases were dried over Na2SO4, fil-
tered and concentrated under reduced pressure. Purifica-
tion through flash silica gel column chromatography
afforded compound 5. Data of the new compounds are
shown below.
Indole (1.1 mmol) was added to a suspension of ni-
troolefin (1 mmol) in dried ethanol (8 mL) together with
ionic liquid-coordinated ytterbium(III) sulfonate 1 (5
mmol%). The reaction mixture was heated to 70 ℃ for
2—3 h and then cooled to room temperature. The sol-
vent was removed under reduced pressure, and then
CH2Cl2 was added to dissolve the crude product while
the catalyst 1 was filtered and washed with CH2Cl2 for
next run. The crude product was subjected to flash silica
gel column chromatography to give the pure product.
4-(4-Methoxyphenyl)-1-phenyl-1,2,3,4-tetrahydro-
1
9H-pyrido[3,4-b]indole (5b) m.p. 180—182 ℃; H
General procedure for reduction of 3
NMR (400 MHz, CDCl3, major/minor＝92/8) δ: 7.68
(br s, 1H), 7.43—7.35 (m, 5H), 7.23—7.18 (m, 3H),
7.12—7.08 (m, 1H), 6.94—6.87 (m, 4H), 5.32 (d, J＝
2.0 Hz, 0.08H), 5.29 (d, J＝2.0 Hz, 0.92H), 4.37 (ddd,
J＝7.2, 5.2, 2.0 Hz, 1H), 3.83 (s, 3H), 3.54 (dd, J＝12.8,
5.2 Hz, 1H), 3.03 (dd, J＝12.8, 8.4 Hz, 1H), 1.94 (br s,
1H); 13C NMR (125 MHz, CDCl3) δ: 158.3, 141.6,
136.1, 135.8, 135.2, 129.4, 128.9, 128.8, 128.7, 128.3,
126.8, 121.5, 119.9, 119.2, 113.8, 112.8, 110.9, 60.5,
58.2, 55.3, 52.9, 40.8. Anal. calcd for C24H22N2O: C
81.33, H 6.26, N 7.90; found C 81.42, H 6.37, N 7.79.
4-(4-Chlorophenyl)-1-phenyl-1,2,3,4-tetrahydro-
Pd/C 10% (186 mg) and HCOONH4 (1.15 g) were
sequentially added to a stirred solution of compound 3
(3.56 mmol) in CH3OH (50 mL). The reaction mixture
was stirred at room temperature overnight, then filtered
and the solution was evaporated under reduced pressure.
The white residue was dissolved in EtOAc, washed with
saturated Na2CO3, and the aqueous phase was extracted
twice with EtOAc. The combined organic phases were
dried over Na2SO4, filtered and evaporated to afford
compound 4 as a white solid. The crude product was
used in the next step without further purification. Data
of the new compounds are shown below.
2-(1H-Indol-3-yl)-2-(4-methoxyphenyl)ethanamine
(4b) m.p. 146—147 ℃; 1H NMR (400 MHz, CDCl3) δ:
8.17 (br s, 1H), 7.49 (d, J＝8.0 Hz, 1H), 7.34—7.29 (m,
4H), 7.22—7.14 (m, 2H), 7.03 (t, J＝7.6 Hz, 2H), 4.25
(t, J＝7.2 Hz, 1H), 3.75 (s, 3H), 3.43 (t, J＝6.8 Hz, 1H),
3.28 (t, J＝6.8 Hz, 1H), 1.49 (br s, NH2); 13C NMR (125
MHz, CDCl3) δ: 158.9, 136.6, 131.2, 128.8, 126.2,
123.7, 121.4, 119.9, 119.0, 114.8, 114.3, 111.4, 55.3,
45.7, 41.8. Anal. calcd for C17H18N2O: C 76.66, H 6.81,
N 10.52; found C 76.72, H 6.93, N 10.61.
2-(4-Chlorophenyl)-2-(1H-indol-3-yl)ethanamine
(4c) m.p. 142—144 ℃; 1H NMR (400 MHz, CDCl3) δ:
8.15 (br s, 1H), 7.47 (d, J＝8.0 Hz, 1H), 7.35—7.29 (m,
4H), 7.22—7.15 (m, 2H), 7.03 (t, J＝7.6 Hz, 2H), 4.25
(t, J＝7.2 Hz, 1H), 3.44 (t, J＝6.8 Hz, 1H), 3.29 (t, J＝
6.8 Hz, 1H), 1.53 (br s, NH2); 13C NMR (125 MHz,
CDCl3) δ: 137.3, 136.3, 132.1, 128.9, 127.1, 124.0,
122.1, 120.4, 119.8, 115.5, 113.6, 111.3, 47.1, 43.3.
Anal. calcd for C16H15ClN2: C 70.98, H 5.58, N 10.35;
found C 71.06, H 5.51, N 10.27.
1
9H-pyrido[3,4-b]indole (5c) m.p. 168—171 ℃; H
NMR (400 MHz, CDCl3, major/minor＝92/8) δ: 7.65
(br s, 1H), 7.39—7.34 (m, 5H), 7.32—7.28 (m, 2H),
7.24—7.21 (m, 2H), 7.18 (d, J＝8.0 Hz, 1H), 7.08—
7.04 (m, 1H), 6.89—6.81 (m, 2H), 5.28 (d, J＝2.0 Hz,
0.92H), 5.21 (d, J＝2.0 Hz, 0.08H), 4.38 (ddd, J＝7.2,
5.2, 2.0 Hz, 1H), 3.54 (dd, J＝12.8, 5.2 Hz, 1H), 3.03
(dd, J＝12.8, 8.0 Hz, 1H), 2.24 (br s, 1H); 13C NMR
(125 MHz, CDCl3) δ: 138.3, 137.1, 136.2, 133.4, 131.5,
129.2, 128.8, 128.6, 128.0, 127.4, 127.1, 122.2, 120.1,
119.2, 112.8, 111.1, 64.4, 59.1, 47.8. Anal. calcd for
C23H19ClN2: C 76.98, H 5.34, N 7.81; found C 76.91, H
5.43, N 7.73.
4-(Furan-2-yl)-1-phenyl-1,2,3,4-tetrahydro-9H-
pyrido[3,4-b]indole (5d) m.p. 103—105 ℃; 1H NMR
(400 MHz, CDCl3, major/minor＝77/23) δ: 7.72 (br s,
1H), 7.40—7.31 (m, 6H), 7.28—7.24 (m, 2H), 7.18—
7.11 (m, 1H), 7.09—7.02 (m, 1H), 6.35—6.34 (m,
0.77H), 6.31—6.30 (m, 0.23H), 6.10 (d, J＝3.2 Hz,
0.77H), 6.03 (d, J＝3.2 Hz, 0.23H), 5.25 (s, 0.77H),
5.19 (s, 0.23H), 4.47 (ddd, J＝7.2, 5.2, 2.0 Hz, 0.77H),
4.35 (ddd, J＝7.2, 5.2, 2.0 Hz, 0.23H), 3.58 (dd, J＝
12.8, 5.2 Hz, 0.23H), 3.49 (dd, J＝12.8, 5.2 Hz, 0.77H),
3.39 (dd, J＝12.8, 8.4 Hz, 0.23H), 3.28 (dd, J＝12.8,
8.4 Hz, 0.77H); 13C NMR (125 MHz, CDCl3) δ: 156.5,
141.7, 141.6, 136.2, 135.3, 129.0, 128.9, 128.3, 126.8,
121.8, 119.5, 119.2, 111.3, 110.4, 110.3, 107.0, 60.7,
2-(Furan-2-yl)-2-(1H-indol-3-yl)ethanamine (4d)
1
m.p. 120—122 ℃; H NMR (400 MHz, CDCl3) δ: 8.16
(br s, 1H), 7.56 (d, J＝8.0 Hz, 1H), 7.36 (d, J＝8.4 Hz,
2H), 7.21—7.17 (m, 1H), 7.12—7.07 (m, 2H), 6.32—
6.30 (m, 1H), 6.13 (d, J＝7.2 Hz, 1H), 4.34 (t, J＝6.8
Hz, 1H), 3.40—3.29 (m, 2H), 1.45 (br s, NH2); 13C
Chin. J. Chem. 2010, 28, 443— 448
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Shen et al.
FULL PAPER
57.2, 47.3. Anal. calcd for C21H18N2O: C 80.23, H 5.77,
N 8.91; found C 80.18, H 5.84, N 8.97.
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