Sc(OTf)3: A highly efficient and renewable catalyst for michael addition of indoles to nitroolefins in water

Article abstract of DOI:10.1080/00397910903398676

A catalytic amount of scandium trifluoromethanesulfonate [Sc(OTf) ₃] (2.5mol%) was used to catalyze the Michael addition of indoles to nitroolefins in water to afford the corresponding 3-alkylated indoles in good to excellent yields. The short

Full text of DOI:10.1080/00397910903398676

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Sc(OTf)3: A Highly Efficient and
Renewable Catalyst for Michael Addition
of Indoles to Nitroolefins in Water
Jianwei Xie a , Xinhai Zhu a , Manna Huang a , Fei Meng a , Man Wang
a & Yiqian Wan a
a School of Chemistry and Chemical Engineering, Sun Yat-Sen
University, Guangzhou, China
Published online: 04 Oct 2010.
To cite this article: Jianwei Xie , Xinhai Zhu , Manna Huang , Fei Meng , Man Wang & Yiqian Wan
(2010) Sc(OTf)3: A Highly Efficient and Renewable Catalyst for Michael Addition of Indoles to
Nitroolefins in Water, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 40:21, 3259-3267, DOI: 10.1080/00397910903398676
To link to this article: http://dx.doi.org/10.1080/00397910903398676
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Synthetic Communications1, 40: 3259–3267, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903398676
Sc(OTf)3: A HIGHLY EFFICIENT AND RENEWABLE
CATALYST FOR MICHAEL ADDITION OF INDOLES
TO NITROOLEFINS IN WATER
Jianwei Xie, Xinhai Zhu, Manna Huang, Fei Meng,
Man Wang, and Yiqian Wan
School of Chemistry and Chemical Engineering, Sun Yat-Sen University,
Guangzhou, China
A catalytic amount of scandium trifluoromethanesulfonate [Sc(OTf)3] (2.5 mol%) was
used to catalyze the Michael addition of indoles to nitroolefins in water to afford the
corresponding 3-alkylated indoles in good to excellent yields. The short reaction times,
excellent yields, and renewability of the catalyst are noteworthy.
Keywords: Lewis acid; Michael addition; renewable catalyst; scandium trifluoromethanesulfonate; water
INTRODUCTION
In recent years, significant efforts have been dedicated to carrying out catalytic
organic reactions in water, because water is cheap, safe, and environmentally
benign.[1] In addition, when using water as solvent, many reactions illustrate unique
reactivities and selectivities that are not usually observed in organic media.[2] Thus,
the development of organic reactions in water has received increasing attention.
Indole and many of its derivatives are present in many natural products and
synthetic materials that possess biological and pharmacological activities.[3] Because
3-alkylated indoles are versatile intermediates for the synthesis a wide range of
indoles derivatives,[4] several methods for the preparation of indole derivatives were
established by means of easy electrophilic replacement reactions of the 3-position of
indoles. Lewis acid–catalyzed[5] Michael addition of indoles to enones or nitroolefins
synthesizes 3-alkylated indoles; however, a large amount of the catalyst (10 mol%)
and long reaction times (several hours) are often required. Furthermore, most of
the reactions are carried out in organic solvents. In recent years, although other cat-
alysts such as [Al(DS)3] ꢀ 3H2O (DS ¼ dodecyl sulfate),[6] heteropoly acid,[7]
carbohydrate-based tolylsulfonyl hydrazines,[8] and b-cycoldextrin[9] were used for
these reactions in water, the main drawbacks (high catalyst loading and long reac-
tion times) still remained. Very recently, Yao reported catalyst-free conjugative
addition of indoles to b-nitrostyrenes in water,[10] but high temperature (100 ꢁC)
Received August 7, 2009.
Address correspondence to Yiqian Wan, School of Chemistry and Chemical Engineering, Sun
Yat-Sen University, Guangzhou 510275, China. E-mail: ceswyq@mail.sysu.edu.cn
3259

3260
J. XIE ET AL.
and several hours were still required to reach full conversion of b-nitrostyrenes.
Therefore, the development of a highly efficient and environmentally friendly
catalyst to perform the reaction under mild conditions, in particular, at room
temperature and in a short time, would be valuable.
Sc(OTf)3, as a commercially available, water-tolerant, and reusable Lewis acid,
was reported to successfully catalyze many synthetic organic reactions in both
organic solvents and aqueous media, even pure water.[11] It was also reported to cat-
alyze Friedel–Crafts conjugate addition of indoles, by Mannabe and Kawatsura
groups, respectively.[12,13] However, Sc(OTf)3 showed less catalytic activity than
Sc(DS)3 in water. While Kawatsura and coworkers reported that Sc(OTf)3 as well
as Hf(OTf)4 can effectively catalyze this reaction in CH3CN, they mainly focused
on the addition reactions of indoles to carbonyls. In this article, we report the
remarkable catalytic activity of Sc(OTf)3 for the conjugate addition of indoles to
nitroolefins in water.
RESULTS AND DISCUSSION
Initially, the conjugate addition of indole to b-nitrostyrene was carried out in
the presence of Sc(OTf)3 (10 mol%) in water at 30 ꢁC. Fortunately, the reaction was
completed in only 10 min. When the catalyst loading was decreased to 2.5 mol%, the
reaction also finished in the same time, and excellent yield was afforded. However,
when the amount of the catalyst was reduced to 2 mol%, the b-nitrostyrene could
not be completely consumed within 10 min and the yield decreased to 90%. In con-
trast, the reaction was also carried out in the absence of catalyst, but the yield was
only a trace, even with prolonging reaction time. These results are summarized in
Table 1.
To test the chemoselectivity of indoles reacting with nitrostyrenes over enones,
we performed a reaction mixing indole (1 mmol), b-nitrostyrene (0.5 mmol), and
2-cyclopenten-1-one (0.5 mmol) under almost the same conditions as mentioned
previously except time was prolonged to 100 min. The gas chromatography=mass
Table 1. Effect of the catalyst loading on the Michael addition of indole to b-nitrostyrenea
Entry
Sc(OTf)3 (mol%)
Time (min)
Yield (%)b
1
2
3
4
5
6
10.0
5.0
2.5
2.5
2.0
–
10
10
97
95
10
95
100
10
80c
90
1440
Trace
aAll reactions were performed in water at 30 ꢁC.
bIsolated yield.
c1 Equiv 2-cyclopenten-1-one based on b-nitrostyrene was added.

Sc(OTf)3: CATALYST FOR MICHAEL ADDITION
3261
Table 2. Michael addition of indoles to nitroolefins catalyzed by Sc(OTf)3 in watera
Entry
1
Indoles
Nitroolefins
Product
Time (min)
10
Yield (%)b
95
2
3
60
97
91
120
4
5
10
70
92
93
6
30
90
7
8
15
92
87
1200
9
15
94
91
10
1440
11
10
93
(Continued )

3262
J. XIE ET AL.
Table 2. Continued
Entry
12
Indoles
Nitroolefins
Product
Time (min)
Yield (%)b
10
94
13
70
91
aAll reactions were carried out with 2.5 mol% Sc(OTf)3 at 30 ꢁC, and were stopped when nitroolefins
were consumed.
bIsolated yield.
spectrometry (GC=MS) analysis of the reaction mixture showed 4.21% of
b-nitrostyrene, 22.8% of 2-cyclopenten-1-one, and 72.24% and 0.75% of their corre-
sponding 3-alkylated indole products. Moreover, only 3-(2-nitro-1-phenylethyl)-
1H-indole (3a) was isolated in 80% yield (Table 1, entry 4). Hence, Sc(OTf)3 is a
highly chemoselective catalyst in this reaction system.
A variety of indoles and nitroolefins were synthesized using Sc(OTf)3
(2.5 mol%) as the catalyst in water at 30 ꢁC to investigate the scope of the substrates
(Table 2). As demonstrated in Table 2, all the conjugate adducts were obtained in
good to excellent yields, but the reaction times were varied according to the substi-
tuents on the olefins and indoles. For the olefins, the electron-donating substitutes
such as methyl and methoxy led to the formation of the corresponding products with
longer reaction times (Table 2, entries 2 and 3). On the other hand, the nitrostyrenes
bearing electron-withdrawing groups except the bromo group reacted smoothly in
short reaction times (within 30 min) to afford their corresponding products
(Table 2, entries 4–7). In the case of 1-nitro-2-(2-nitrovinyl)-benzene, the yield was
slightly less and needed a longer reaction time, which might result from the steric
hindrance (Table 2, entry 8). Furthermore, the nitroolefin containing an
acid-sensitive moiety could also be transformed to the desired product in excellent
yield under the present reaction conditions (Table 2, entry 9). The indoles, such as
2-methylindole and 5-methoxyindole, could react with b-nitrostyrenes to afford
the expected products in excellent yield only within 10 min, probably because of
the electron-rich groups, which can increase the electron density of the indole ring
to accelerate the reactions (Table 2, entries 11 and 12), whereas the indole rings bear-
ing electron-withdrawing groups such as 5-bromoindole needed a longer time to
complete the reaction (Table 2, entry 13). Interestingly, N-methylindole required
longer time than 2-methyl-indole to obtain the corresponding product. This could
be attributed to the fact that the free N-H of indole, which played an essential role
in activating the indole ring by Sc(OTf)3, was occupied (Table 2, entries 10 vs. 11).
Because Sc(OTf)3 is more soluble in water than in organic solvents, and
because the catalyst could be recovered almost quantitatively in the aqueous layer,[7]
we also carried out consecutive Michael addition reactions between indole and

Sc(OTf)3: CATALYST FOR MICHAEL ADDITION
3263
Table 3. Recycling Michael addition of indole to nitroolefins catalyzed by the
a
amount of 2.5 mol% Sc(OTf)3
Run
1
2
3
4
5
Time (min)
Yield (%)b
10
95
10
93
10
92
10
92
10
88
aAll reactions were carried out in water at 30 ꢁC.
bIsolated yield.
b-nitrostyrene to test the renewability of the catalyst (Table 3). After the initial
experiment, the organic compounds were carefully extracted with Et2O, and the resi-
due Et2O in the water phase was removed in vacuum. The resulting mixture was then
used in the next run. The catalytic activity of the catalyst was so remarkable that the
yield had only a little decrease even after five runs.
CONCLUSIONS
In summary, we have developed a simple and efficient method for Michael
addition of indoles to nitroolefins using catalytic amount of Sc(OTf)3 (2.5 mol%).
The reactions were performed well at 30 ꢁC in less than 100 min, with several excep-
tions, to provide the desired products in good to excellent yields without any
by-products. Moreover, the catalyst can be renewed without significantly decreasing
activities.
EXPERIMENTAL
Sc(OTf)3 ꢀ xH2O, indoles, and nitroolefins were commercially available and
used as received. Column chromatography was performed with silica gel (200–300
mesh) purchased from Qingdao Haiyang Chemical Co. Ltd. Thin-layer chromato-
graphy (TLC) was carried out with Merck silica-gel GF254 plates. All products were
1
characterized by MS, H NMR, and 13C NMR, which were compared with the
1
previously reported data. H NMR and 13C NMR spectra were recorded at room
1
temperature on a Varian Mercury Plus 300 instrument at 300 MHz for H NMR
and 75 MHz for 13C NMR with solvent as reference. Liquid chromatography mass
spectra (LCMS) were recorded on a Shimadzu LCMS-2010A instrument. Electron
impact–mass (EI-MS) spectra were recorded on a Thermo Trace DSQ El mass
spectrometer.
General Procedure for the Michael Addition of Indoles to
Nitroolefins in Water
Indole (1 mmol) and b-nitrostyrene (0.5 mmol) were added to a solution of
Sc(OTf)3 ꢀ xH2O (2.5 mol%) in 1 ml water. The mixture was stirred at 30 ꢁC, and
the reaction was monitored by TLC until the starting material was consumed. The
mixture was then diluted with water and extracted with EtOAc (3 ꢂ 10 mL). The
combined organic layers were washed with brine and dried over anhydrous Na2SO4.

3264
J. XIE ET AL.
The solvent was concentrated in vacuum, and the residue was purified by column
chromatography on silica gel (ethyl acetate–petroleum ether) to give the target
products.
General Procedure for the Recycling of the Catalyst
The Michael addition of indole to b-nitrostyrene was carried out as described
in the general procedure. After 10 min, the reaction mixture was extracted with
diethyl ester (4 ꢂ 2 mL), and the residual diethyl ester in water phase was removed
in a vacuum. Then indole and b-nitrostyrene were added to the aqueous layer for
the second run. This process was then repeated.
Data
3-(2-Nitro-1-phenylethyl)-1H-indole (3a). Mp 98–99 ꢁC (lit.[14] mp
1
97–99 ꢁC). H NMR (300 MHz, CDCl3) d 4.95 (dd, J ¼ 12.3, 8.4 Hz, 1H), 5.08 (dd,
J ¼ 12.3, 7.5 Hz, 1H), 5.21 (t, J ¼ 7.8 Hz, 1H), 7.03 (d, J ¼ 2.4 Hz, 1H), 7.09 (t,
J ¼ 7.5 Hz, 1H), 7.21 (t, J ¼ 7.5 Hz, 1H), 7.28–7.38 (m, 5H), 7.45 (d, J ¼ 7.8 Hz,
1H), 8.07 (s, 1H). 13C NMR (75 MHz, CDCl3) d 41.5, 79.5, 111.3, 114.1, 118.7,
119.8, 121.5, 122.5, 125.9, 127.4, 127.6, 128.8, 136.3, 139.0. ESI-MS: m=z ¼ 265
[M ꢃ H]ꢃ.
3-(2-Nitro-1-p-tolylethyl)-1H-indole (3b). Oil. 1H NMR (300 MHz, CDCl3)
d 2.34 (s, 3H), 4.93 (dd, J ¼ 12.3, 8.4 Hz, 1H), 5.06 (dd, J ¼ 12.0, 7.5 Hz, 1H), 5.17 (t,
J ¼ 8.1 Hz, 1H), 7.01–7.24 (m, 7H), 7.35 (d, J ¼ 8.4 Hz, 1H), 7.46 (d, J ¼ 8.1 Hz, 1H),
8.07 (s, 1H). 13C NMR (75 MHz, CDCl3) d 21.0, 41.2, 79.5, 111.3, 114.3, 118.7,
119.7, 121.4, 122.4, 125.9, 127.4, 129.4, 136.0, 136.3, 136.9. ESI-MS: m=z ¼ 279
[M ꢃ H]ꢃ.
3-(1-(4-Methoxyphenyl)-2-nitroethyl)-1H-indole (3c). Mp 149–150 ꢁC (lit.[15]
1
mp 149–150 ꢁC). H NMR (300 MHz, CDCl3) d 3.79 (s, 3H), 4.90 (dd, J ¼ 12.0,
8.4 Hz, 1H), 5.05 (dd, J ¼ 12.0, 7.2 Hz, 1H), 5.15 (t, J ¼ 8.1 Hz, 1H), 6.85 (d,
J ¼ 8.7 Hz, 2H), 7.02–7.11 (m, 2H), 7.17–7.26 (m, 3H), 7.36 (d, J ¼ 8.1 Hz, 1H),
7.43 (d, J ¼ 8.1 Hz, 1H), 8.07 (s, 1H). 13C NMR (75 MHz, CDCl3) d 40.9, 55.2,
79.7, 111.3, 114.1, 114.5, 118.8, 119.7, 121.3, 122.5, 125.9, 128.7, 131.1, 136.3,
158.6. ESI-MS: m=z ¼ 295 [M ꢃ H]ꢃ.
3-(1-(4-Chlorophenyl)-2-nitroethyl)-1H-indole (3d). Mp 107–108 ꢁC. 1H NMR
(300 MHz, CDCl3) d 4.91 (dd, J ¼ 12.6, 8.4 Hz, 1H), 5.06 (dd, J ¼ 12.6, 7.2 Hz, 1H),
5.18 (t, J ¼ 8.1 Hz, 1H), 7.02 (d, J ¼ 2.1 Hz, 1H), 7.09 (t, J ¼ 7.5 Hz, 1H), 7.22 (t,
J ¼ 8.1 Hz, 1H), 7.28–7.42 (m, 5H), 8.10 (s, 1H). 13C NMR (75 MHz, CDCl3) d
41.0, 79.2, 111.4, 113.7, 118.6, 119.9, 121.4, 122.7, 125.7, 128.9, 129.0, 133.2,
136.3, 137.6. ESI-MS: m=z ¼ 299 [M ꢃ H]ꢃ.
3-(1-(4-Bromophenyl)-2-nitroethyl)-1H-indole (3e). Mp 121–122 ꢁC. 1H NMR
(300 MHz, CDCl3) d 4.91 (dd, J ¼ 12.3, 8.7 Hz, 1H), 5.06 (dd, J ¼ 12.3, 7.2 Hz, 1H),
5.16 (t, J ¼ 7.8 Hz, 1H), 7.02–7.11 (m, 2H), 7.20–7.46 (m, 7H), 8.11 (s, 1H). 13C
NMR (75 MHz, CDCl3) d 40.9, 79.1, 111.4, 113.5, 118.6, 119.9, 121.3, 121.4,
122.6, 125.7, 129.3, 131.8, 136.3, 138.1. ESI-MS: m=z ¼ 343 [M ꢃ H]ꢃ.

Sc(OTf)3: CATALYST FOR MICHAEL ADDITION
3265
3-(2-Nitro-1-(4-nitrophenyl)ethyl)-1H-indole (3f). Mp 142–143 ꢁC (lit.[16]
1
mp 142 ꢁC). H NMR (300 MHz, CDCl3) d 5.00 (dd, J ¼ 12.9, 9.0 Hz, 1H), 5.11
(dd, J ¼ 12.9, 6.9 Hz, 1H), 5.28–5.34 (m, 1H), 7.06–7.12 (m, 2H), 7.21–7.24 (m,
1H), 7.36–7.41 (m, 2H), 7.52 (d, J ¼ 8.4 Hz, 2H), 8.18 (d, J ¼ 8.4 Hz, 2H), 8.25 (s,
1H). 13C NMR (75 MHz, CDCl3) d 41.2, 78.7, 111.6, 112.7, 118.3, 120.1, 121.5,
122.9, 124.0, 125.5, 128.6, 136.3, 146.6, 147.1. ESI-MS: m=z ¼ 310 [M ꢃ H]ꢃ.
3-(2-Nitro-1-(3-nitrophenyl)ethyl)-1H-indole (3g). Oil. 1H NMR (300 MHz,
CDCl3) d 5.01 (dd, J ¼ 12.9, 8.7 Hz, 1H), 5.12 (dd, J ¼ 12.6, 6.9 Hz, 1H), 5.29–5.24
(m, 1H), 7.07–7.12 (m, 2H), 7.20–7.25 (m, 1H), 7.28 (d, J ¼ 8.7 Hz, 2H), 7.51 (t,
J ¼ 7.8 Hz, 1H), 7.71 (d, J ¼ 7.8 Hz, 1H), 8.11–8.15 (m, 1H), 8.20 (s, 1H), 8.24 (s,
1H). 13C NMR (75 MHz, CDCl3) d 41.0, 78.7, 111.6, 112.6, 118.2, 120.0, 121.5,
122.4, 122.5, 122.8, 125.4, 129.7, 133.9, 136.2, 141.4, 148.3. ESI-MS: m=z ¼ 310
[M ꢃ H]ꢃ.
3-(2-Nitro-1-(2-nitrophenyl)ethyl)-1H-indole (3h). Mp 113–114 ꢁC. 1H
NMR (300 MHz, CDCl3) d 5.05–5.18 (m, 2H), 5.88 (t, J ¼ 7.8 Hz, 1H), 7.02–7.07
(m, 1H), 7.15–7.22 (m, 2H), 7.29–7.53 (m, 5H), 7.90 (d, J ¼ 7.8 Hz, 1H), 8.18 (s,
1H). 13C NMR (75 MHz, CDCl3) d 36.3, 78.1, 111.4, 112.4, 118.4, 119.9, 121.9,
122.7, 124.8, 125.7, 128.4, 129.7, 133.1, 133.5, 136.2, 149.3. ESI-MS: m=z ¼ 310
[M ꢃ H]ꢃ.
3-(1-(Furan-2-yl)-2-nitroethyl)-1-methyl-1H-indole (3i). Mp 68–69 ꢁC
1
(lit.[16] mp 69 ꢁC). H NMR (300 MHz, CDCl3) d 4.93 (dd, J ¼ 12.3, 7.8 Hz, 1H),
5.07 (dd, J ¼ 12.6, 8.1 Hz, 1H), 5.26 (t, J ¼ 7.8 Hz, 1H), 6.16–6.18 (m, 1H), 6.31–
6.33 (m, 1H), 7.11–7.25 (m, 3H), 7.37–7.39 (m, 2H), 7.56 (d, J ¼ 8.1 Hz, 1H), 8.17
(s, 1H). 13C NMR (75 MHz, CDCl3) d 35.7, 77.8, 107.2, 110.3, 111.3, 111.5, 118.5,
119.9, 122.4, 122.6, 125.5, 136.1, 142.1, 151.9. ESI-MS: m=z ¼ 255 [M ꢃ H]ꢃ.
1-Methyl-3-(2-nitro-1-phenylethyl)-1H-indole (3j). Mp 94–95 ꢁC (lit.[14]
mp 95–97 ꢁC). 1H NMR (300 MHz, CDCl3) d 3.76 (s, 3H), 4.95 (dd, J ¼ 12.3,
8.4 Hz, 1H), 5.07 (dd, J ¼ 12.3, 7.5 Hz, 1H), 5.20 (t, J ¼ 7.8 Hz, 1H), 6.87 (s, 1H),
7.06–7.11 (m, 1H), 7.21–7.37 (m, 7H), 7.46 (d, J ¼ 7.8 Hz, 1H). 13C NMR
(75 MHz, CDCl3) d 32.6, 41.4, 79.4, 109.4, 112.6, 118.9, 119.3, 122.1, 126.3, 126.4,
127.4, 127.6, 128.8, 137.2, 139.3. EI-MS: m=z ¼ 280 [M]þ.
2-Methyl-3-(2-nitro-1-phenylethyl)-1H-indole (3k). Mp 101–102 ꢁC (lit.[14]
1
mp 103–104 ꢁC). H NMR (300 MHz, CDCl3) d 2.42 (s, 3H), 5.12–5.26 (m, 3H),
7.01–7.14 (m, 2H), 7.25–7.39 (m, 7H), 7.89 (s, 1H). 13C NMR (75 MHz, CDCl3) d
11.8, 40.4, 78.5, 108.5, 110.6, 118.3, 119.5, 121.0, 126.6, 126.9, 127.1, 128.5, 132.8,
135.1, 139.3. EI-MS: m=z ¼ 280 [M]þ.
5-Methoxy-3-(2-nitro-1-phenylethyl)-1H-indole (3l). Mp 114–115 ꢁC. 1H
NMR (300 MHz, CDCl3) d 3.79 (s, 3H), 4.94 (dd, J ¼ 12.3, 8.4 Hz, 1H), 5.06 (dd,
J ¼ 12.3, 7.5 Hz, 1H), 5.15 (t, J ¼ 7.8 Hz, 1H), 6.84 (s, 1H), 6.87 (d, J ¼ 2.4 Hz,
1H), 7.01 (d, J ¼ 2.4 Hz, 1H), 7.23–7.34 (m, 5H), 7.99 (s, 1H). 13C NMR (75 MHz,
CDCl3) d 41.5, 55.8, 79.4, 100.7, 112.0, 112.5, 113.8, 122.2, 126.4, 127.4, 127.6,
128.7, 131.4, 139.0, 153.9. ESI-MS: m=z ¼ 295 [M ꢃ H]ꢃ.

3266
J. XIE ET AL.
5-Bromo-3-(2-nitro-1-phenylethyl)-1H-indole (3m). Mp 121–122 ꢁC (lit.[5e]
mp 116–120 ꢁC). 1H NMR (300 MHz, CDCl3) d 4.92 (dd, J ¼ 12.3, 8.1 Hz, 1H), 5.04
(dd, J ¼ 12.3, 8.1 Hz, 1H), 6.14 (t, J ¼ 8.1 Hz, 1H), 7.07 (d, J ¼ 2.7 Hz, 1H), 7.21–7.37
(m, 7H), 7.55 (s, 1H), 8.16 (s, 1H). 13C NMR (75 MHz, CDCl3) d 41.3, 79.3, 112.8,
113.1, 113.8, 121.2, 122.6, 125.4, 127.5, 127.6, 127.7, 128.9, 134.9, 138.5. ESI-MS:
m=z ¼ 343 [M ꢃ H]ꢃ.
ACKNOWLEDGMENT
We acknowledge the National High Technology Research and Development
Program of China (863 Program), No. 2006AA09Z446, for financial support of this
work.
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