One-pot Synthesis of Aurones through Oxidation-cyclization Tandem Reaction Catalyzed by Copper Nanoparticles Catalyst

Article abstract of DOI:10.2174/1570178614666171110150853

Aurones were synthesized through an oxidation-cyclization tandem reaction of 2-(1- hydroxyprop-2-ynyl)phenols catalyzed by copper nanoparticles (Cu NPs) with bipyridine as the ligand. In the reaction, oxygen worked as a green and mild oxidant to give the best results. Cu NPs were dually activated by bipyridine ligand and water, and showed highly efficient catalytic activities to the oxygen oxidation and the cylization to give aurones and flavonoids.

Full text of DOI:10.2174/1570178614666171110150853

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70
Letters in Organic Chemistry, 2018, 15, 70-77
RESEARCH ARTICLE
ISSN: 1570-1786
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One-pot Synthesis of Aurones through Oxidation-cyclization Tandem Re-
action Catalyzed by Copper Nanoparticles Catalyst
BENTHAM
S
CIENCE
Min Yu1,2, Guangxiang Liu1, Chengyan Han2, Li Zhu3 and Xiaoquan Yao2,*
2
1School of Environmental Science, Nanjing Xiaozhuang University, Nanjing 211171, China; College of Material
3
Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China; School of
Pharmacy, Nanjing Medical University, Nanjing 210029, China
Abstract: Aurones were synthesized through an oxidation-cyclization tandem reaction of 2-(1-
hydroxyprop-2-ynyl)phenols catalyzed by copper nanoparticles (Cu NPs) with bipyridine as the ligand.
A R T I C L E H I S T O R Y
In the reaction, oxygen worked as a green and mild oxidant to give the best results. Cu NPs were dually
activated by bipyridine ligand and water, and showed highly efficient catalytic activities to the oxygen
oxidation and the cylization to give aurones and flavonoids.
Received: August 01, 2017
Revised: October 24, 2017
Accepted: October 24, 2017
DOI:
10.2174/1570178614666171110150853
Keywords: Aurones, bipyridine, oxidation-cyclization, tandem reaction, copper nanoparticles, oxygen oxidation.
1. INTRODUCTION
carbon bonds or carbon-hetero bonds by using copper
nanoparticles as catalyst.
Aurones have exhibited a wide range of biological ac-
tivities and have been used as antifungal agents, tyrosinase
inhibitors, antioxidants, and others [1]. Due to the impor-
tance of aurones, recently we focused on the catalytic synthe-
sis of aurones and developed different methods to aurones
construction: 1) an efficient alkynylation-cyclization ben-
zylidene-2, 3-dihydroben-zofuran-3-ol (4) by using Cy3PAgCl
as catalyst (Route A, Scheme 1) [2]; 2) the cyclization of
2-(1-hydroxy-3-arylprop-2-ynyl)phenols (1) catalyzed by sil-
ver-phosphine complexes [3] or Ag NPs [4] with phosphine
ligand to give 4 as product (Route B) [5].
On the other hand, although there are some examples
concerning the catalytic activity of Cu NPs and other coinage
metal nanopartices, there have been only a few reports con-
cerning ligand promotion in the construction of carbon-
carbon bonds or carbon-hetero bonds. Recently, Fernández et
al. developed a diboration of alkenes catalyzed by gold
nanoparticles with BINAP as ligand [9]. In previous re-
search, we also discovered a series of ligand-promoted coin-
age metal nanoparticles catalyzed reactions, such as Ag NPs-
phosphine ligand catalyzed cyclization of 2-(1-hydroxy-3-
arylprop-2-ynyl)phenols (1) [4] and addition of alkynes to
aldehydes [10], and Cu NPs-bpy catalyzed oxidation of
propargylic alcohols [11, 12].
However, in both of the methodologies, compound(4)
oxidized by stoichiometric MnO2 gave the final product
(2). To the best of our knowledge, no synthetic strategy
of aurones 2 through oxidation-cyclization tandem reaction
is known in the literature (Route C). In order to fill this
gap, we aimed to develop a new tandem reaction for the
synthesis and further diversification of aurones.
Herein, we hope to report a novel synthetic strategy
based on a Cu NPs-catalyzed oxidation/dehydrogenation-
cyclization tandem reaction of 1 to give aurones 2 di-
rectly (Route C). It is worthy to note that Cu NPs were
dually activated by bpy ligand and water.
Recently, the synthesis and application of metal nanopar-
ticles have received considerable attention in view of their
unique properties compared to the bulk metals [6, 7].
Copper nanoparticles (Cu NPs), being cheap, unusual
properties and potential applications are particularly at-
tractive and there are many excellent examples reported on
their utilization in organic reaction [8]. However, in most
cases Cu NPs were used as dehydrogenation catalysts, only a
few reports were concerned with the construction of carbon-
2. RESULTS AND DISCUSSION
As the preliminary attempt, with 2-(1-hydroxy-3-phenyl-
prop-2-yn-1-yl) phenol (1a) as a prototype, the catalysts were
screened for the tandem reaction following the basic condi-
tions we reported in the Cu NPs-bpy catalyzed oxidation of
propargylic alcohols [12], the same solvent was used as in our
previous report in which silver nanoparticle-catalyzed the cy-
clization of 2-(1-hydroxy-3-arylprop-2-ynyl)phenols.
*Address correspondence to this author at the Department of Applied
Chemistry, College of Material Science and Technology, Nanjing Univer-
sity of Aeronautics and Astronautics, Nanjing 210016, China; Fax: +86-25-
5211-2626; E-mail: yaoxq@nuaa.edu.cn
The results are summarized in Table 1.
1875-6255/18 $58.00+.00
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One-Pot Synthesis of Aurones through Oxidation-Cyclization Tandem Reaction Catalyzed
Letters in Organic Chemistry, 2018, Vol. 15, No. 1 71
Scheme (1). The methodologies for the synthesis of aurones.
Table 1. Catalytic oxidation/cyclization of 2-(1-hydroxy-3-phenylprop-2-yn-1-yl)phenol (1a)a.
Entry
Conditions
Yield of 2ab (%)
Yield of 3a b (%)
1c
2d
3
Cu NPs, H2O/Toluene (3:1), bpy (10 mol%), i-Pr2NEt, 2 h
Cu NPs, H2O/Toluene (3:1), i-Pr2NEt, 2 h
55
N. D.
70
20
Cu NPs, H2O/Toluene (3:1), bpy, i-Pr2NEt, 2 h
22
30
17
18
36
11
39
19
4
Cu NPs, H2O/Toluene (3:1), 1, 10-Phenanthroline, i-Pr2NEt, 2 h
CuSO4, H2O/Toluene (3:1), bpy, i-Pr2NEt, 7 h
56
5
43
Cu(AcO) , H2O/Toluene (3:1), bpy, i-Pr2NEt, 7 h
6
35
2
7
CuOTf, H2O/Toluene (3:1), bpy, i-Pr2NEt, overnight
CuCl, H2O/Toluene (3:1), bpy, i-Pr2NEt, 1 h
51
8
55
9
CuBr, H2O/Toluene (3:1), bpy, i-Pr2NEt, 1 h
52
10
11d
12
13
14d
15
16
17
18
19
20
21
CuI, H2O/Toluene (3:1), bpy, i-Pr2NEt, 1 h [Cu(bipy)2Cl] 2Cl2
52
H2O2Toluene (3:1), i-Pr N2t, overnight
N. D.
52
2
Cu NPs (20 mol% ), H2O/Toluene (3:1), bpy, i-Pr2NEt, 2 h
Cu NPs (5 mol% ), H2O/Toluene (3:1), bpy, i-Pr2NEt, 2 h
H2O/Toluene (3:1), bpy, i-Pr2NEt, 2 h
35
43
54
N. D.
Trace
Trace
Trace
Trace
47
Cu NPs, H2O/1, 4-Dioxane (3:1), bpy, i-Pr2NEt, Overnight
Cu NPs, H2O/CH2Cl2 (3:1), 40oC, bpy, i-Pr2NEt, Overnight
Cu NPs, H2O/DMF (3:1), bpy, i-Pr2NEt, Overnight
Cu NPs, toluene, bpy, i-Pr2NEt, Overnight
Cu NPs, H2O/Toluene (1:1), bpy, i-Pr2NEt, Overnight
Cu NPs, H2O/Toluene (1:3), bpy, i-Pr2NEt, 2 h
26
36
39
59
Cu NPs, H2O/Toluene (3:1), bpy, Et3N (10mol%), 2 h
59
(Table 1) Contd….

72 Letters in Organic Chemistry, 2018, Vol. 15, No. 1
Yu et al.
Yield of 3a b (%)
Entry
Conditions
Yield of 2ab (%)
22
23
24
25
26
27
28d
29
30e
31f
Cu NPs, H2O/Toluene (3:1), bpy, Na2CO3 (10mol%), 2 h
Cu NPs, H2O/Toluene (3:1), bpy, i-Pr2NEt, TEMPO, 5 h
Cu NPs, H2O/Toluene (3:1), bpy, i-Pr2NEt, NaNO2, 7 h
Cu NPs, H2O/Toluene (3:1), bpy, i-Pr2NEt, H2O2, 2 h
Cu NPs, H2O/Toluene (3:1), bpy, i-Pr2NEt, TBHP, 2 h
Cu NPs, H2O/Toluene (3:1), bpy, i-Pr2NEt, Overnight
Cu NPs, H2O/Toluene (3:1), bpy, i-Pr2NEt, r.t., 2h
Cu NPs, H2O/Toluene (3:1), bpy, i-Pr2NEt, 100oC, 2 h,
Cu NPs, H2O/Toluene (3:1), bpy, i-Pr2NEt, 2 h
53
49
40
38
39
18
24
27
-
50
50
51
53
N.D.
36
41
40
50
38
Cu NPs, H2O/Toluene (3:1), bpy, i-Pr2NEt, 2 h
45
a1a (22.4 mg, 0.1 mmol), catalyst (0.6 mg, 10 mol%), bipyridine (3.1 mg, 20 mol%), i-Pr2NEt (1.7 μL, 10 mol%) were mixed with water (1.5 mL) and toluene (0.5 mL), then
the mixture was heated at 65oC under oxygen atmosphere for 2 h; bIsolated yield; cBpy (1.6 mg, 10 mol%); dNot detected; eUnder nitrogen atmosphere; fIn air.
We are pleased to find that, a mixture of 2a [13] (isolated
55%) and 3a (isolated 20%) was obtained (entry 1). It is
worthy to note that the ligand proved to be the key role to
activate the copper catalyst in the reaction: in the absence of
ligand, there was no reaction at all (entry 2). While the load-
ing of the ligand was increased to 20 mol%, the regioselec-
tivity was increased to ca. 3.2:1 with 92% of total yield (en-
try 3). When 1, 10-Phenanthroline was used as a ligand, the
desired product was obtained with only 56% yield (entry 4).
Then, reaction time, temperature and the atmosphere
were also investigated (entries 27-31). The yield of the reac-
tion was decreased by prolonging the reaction time (entry
27). At room temperature, product could not be detected,
whereas increasing temperature to above 65°C resulted in the
reduction of the overall yield and/or regioselectivities (en-
tries 28 and 29). Interestingly, 78% of the total yield and ca.
1:1 of regioselectivity were also achieved under nitrogen
atmosphere at 65°C, which indicated that the formation of
carbonyl group in 2a or 3a might undergo a dehydrogenation
process (entry 30). It was also worthy to note that the tandem
reaction in air gave 95% of total yield but much lower re-
gioselectivity than the reaction under oxygen atmosphere
(entry 31).
Changing the catalyst to Cu(II) or Cu(I) salts reduce the
catalytic activities and selectivities (entries 5-10), in which
CuBr gave a 91% of total yield but with a much worse re-
gioselectivity (entry 9 vs. entry 3). On the other hand, when
[Cu(bipy)2Cl]2Cl2 was synthesized and used as a catalyst in
the reaction, there was no product observed and stoichiomet-
ric starting material was recovered (entry 11). Increasing or
decreasing the loading of the catalyst could not improve the
reaction (entries 12 and 13). Then, a blank reaction was also
carried out. The reaction could not be carried out success-
fully in the absence of Cu NPs (entry 14).
Thus, the combination of Cu NPs/bpy with 0.1 equiva-
lents of i-Pr2NEt in water/toluene (3:1) under oxygen atmos-
phere at 65oC was found to be the best choice.
Following this procedure, 70% of aurone (2a) and 22%
of flavonoid (3a) were achieved. Copper nanoparticles to-
gether with bipyridine worked as oxidation/dehydrogenation-
cyclolization catalyst in the tandem reaction.
With copper nanoparticles as catalyst, bpy as ligand,
other conditions, such as solvents, bases, additives and oxi-
dants were further optimized (entries 15-26). Study of the
solvent effects showed that the mixture of toluene and water
was the best choice of solvents to the reaction. The introduc-
tion of any other organic solvent led to the reduction in cata-
lytic activities. The desired product could not be obtained in
1, 4-dioxane, CH2Cl2 or DMF (entries 15-17). During our
previous studies, water proved to be a key role in group IB
metals-catalyzed construction of carbon-carbon bonds [2, 10]
or carbon-hetero bonds [3, 4]. The effect of water in current
catalytic reaction was also investigated: in the absence of
water, no reaction was observed (entry 18). Then, the ratio of
water and toluene was also examined: reducing the ratio
made the decrease in yields and/or regioselectivities (entries
19 and 20).
Under the optimized reaction conditions (10 mol% of Cu
NPs, 20 mol% of bipyridine and 10 mol% of i-Pr2NEt in
water-toluene mixed solvent under oxygen), the scope of this
reaction was investigated (Table 2). As shown in Table 2,
both electron-donating and electron-deficient substrates
gave aurones 2 in moderate to good yields, and compounds 3
was achieved in moderate yield.
2-(1-hydroxy-3-phenylprop-2-yn-1-yl)phenol(1a) provid-
ed the best reactivity in comparison with other substituted
substrates (entry 1 vs entries 2-10). When methoxyl group
was introduced on R3, the yield of the reaction was decreased
obviously, only moderate total yields were obtained (entries
3). However, for most reactions, only moderate regioselec-
tivities were observed.
In order to investigate the mechanism of the reaction,
intermediate 4a and 7a was prepared. Then the reactions
were set up under the standard condition. With 4a as the sub-
strate, no desired product was achieved and most of starting
material was recovered. This result suggested that the first
When other bases were utilized, lower yields and regiose-
lectivities were observed (entries 21 and 22). Introducing
additives could not promote the reaction either (entries 23
and 24). Other oxidants, such as TBHP and H2O2, were also
used, ca. 70% of yields were observed (entries 25 and 26).

One-Pot Synthesis of Aurones through Oxidation-Cyclization Tandem Reaction Catalyzed
Letters in Organic Chemistry, 2018, Vol. 15, No. 1 73
Table 2. Substrates scope for Cu NPs catalyzed cyclization/oxidation of substituted 2-(1-hydroxy-3-phenylprop-2-yn-1-yl) phenol
1.
O
O
R1
OH
R1
R1
Cu NPs, bipyridine, i-Pr2NEt
+
O
water/toluene(3:1), O2
O
R2
OH
R2
R2
R3
R3
R3
1
2
3
Yield (%)
Entry
Substrate
R1
R2
R3
2
3
1
2
1a
1b
H
H
H
H
H
2a, 70
2b, 45
3a, 22
3b, 41
CH
3
OCH
3
4
1c
1d
1e
1f
H
Br
Br
Cl
H
H
2c, 47
2d, 53
2e, 58
2f, 56
2g, 56
2h, 59
2i, 60
2j, 46
3c, 40
3d, 31
3e, 28
3f, 20
3g, 20
3h, 35
3i, 23
3j, 43
3
H
CH
5
H
3
6
H
H
CH
7
1g
1h
1i
Cl
H
3
8
Cl
Cl
Br
H
H
H
9
t-Bu
m-CH
10
1j
H
3
a1 (0.1 mmol), Cu nanoparticles (0.6 mg, 10 mol%), bipyridine (3.1 mg, 20 mol%), i-Pr2NEt (1.7 μL, 10 mol%) were mixed with water (1.5 mL) and toluene (0.5 mL), then the mix-
ture was heated at 65oC for 2 h under oxygen atmosphere; bisolated yield.
Scheme (2). Cu NPs catalyzed the reactions of 4a and 7a.
Pr NEt (1.7 μL, 10 mol%) were
2
(The reaction were carried out in standard condition: 4a or 7a (0.1 mmol), Cu nanoparticles (0.6 mg, 10 mol%), bipyridine (3.1 mg, 20 mol%), i-
o
mixed with water (1.5 mL) and toluene (0.5
mL), then the mixture was heated at 65 C for 2 h under oxygen atmosphere).
step of the tandem reaction did not include the cyclization of
the substrate and the catalyst was inefficient on the oxida-
tion/dehydrogenation of 4a (Scheme 2, reaction 1).
sult from the competition of intramolecular Michael addition
and Cu NPs-bpy catalyzed annulation [14].
Based on these experimental results, a tentative mecha-
nism is proposed in Scheme 3. Reaction of substituted 2-(1-
hydroxy-3-phenylprop-2-yn-1-yl) phenol 1 with Cu NPs
generates the species A [11, 15]. The acetyenic ketone then
reacts with base to give intermediate B, followed by trans-
attack of phenol to the triple bond to give the vinylcopper
intermediate C and D. The carbon-copper bond is then pro-
On the other hand, when 7a was utilized as the substrate,
the final product 2a was isolated in 45% of yield, while 3a
was isolated in 15% yield (Scheme 2, reaction 2). The simi-
lar ratio of 2a and 3a with the tandem reaction from 1a to
2a&3a might indicate that 7a should be one of the possible
intermediates in the reaction. The regioselectivity might re-

74 Letters in Organic Chemistry, 2018, Vol. 15, No. 1
Yu et al.
Scheme (3). Proposed Mechanism for the Reaction.
tonated to give the final product 2 and 3 with the generation
of the copper hydride species. The copper hydride species
reacts with a molecule of dioxygen to regenerate the catalyst.
(52.5 mg = 0.2 mmol, 10 mol%), were added into a 100 mL
Schlenk tube containing 5 mL of toluene and 15 mL of dis-
tilled water under nitrogen atmosphere. The mixture was
stirred at room temperature for 1 hour. The reaction was
stopped and extracted with ether (3 × 3 mL). The combined
organic phase was dried with Na2SO4 and concentrated under
reduced pressure. The residue was purified by flash column
chromatography (20:1 hexanes and ethyl acetate). Com-
pound 2a was obtained in 92 % yield.
3. EXPERIMENTAL SECTION
3.1. General
All commercial starting materials and reaction reagents
were used as received. In general, solvents were re- distilled
prior to use in the reactions, and deposited under an atmos-
phere of nitrogen. Flash column chromatography was per-
formed over silica gel 200-300 mesh. 1H NMR and 13C NMR
spectra were acquired by 400 MHz and 100 MHz, or 300
MHz and 75 MHz, respectively, and referenced to the inter-
nal solvent signals. The data for compounds 2a-2j, 3a-3j
were described here. Among them, compounds 2g and 3i
were unknown compounds.
3.4. General Procedure for the Preparation of 1-(2-
hydroxyphenyl)-3-phenylprop-2-yn-1-one (7a)
2-(1-hydroxy-3-phenylprop-2-yn-1-yl)phenol (1a, 448.5
mg = 2 mmol), MnO2 (869.4 mg = 10 mmol) and CH2Cl2 (5
mL) were added into 25 mL round flask in air, and then,
were stirred at room temperature for 12 h. The mixture was
concentrated by rotary evaporation. The residue was purified
by flash column (20:1 hexanes and ethyl acetate) to give 7a
with 85% of yield.
3.2. General Procedure for the Oxidation-Cyclization of
2- (1-hydroxy-3-phenylprop-2-yn-1-yl) phenol
3.5. (Z)-2-Benzylidenebenzofuran-3(2H)-one (2a) [2]
2-(1-hydroxy-3-phenylprop -2-yn-1-yl)phenol (1a, 22.4
mg = 0.1 mmol), Cu NPs [11] (0.6 mg = 0.01 mmol, 10
mol%), bipyridine (3.1 mg = 0.02 mmol, 20 mol%), i-Pr2NEt
(1.7 µL = 0.01 mmol, 10 mol%) and water/toluene (1.5 mL/
0.5 mL) were added into a 15 mL Schlenk tube under oxy-
gen, and then, were stirred at 65oC for 2 h. The mixture was
extracted by 3×3 mL of ether. The extraction was dried with
anhydrous sodium sulfate and concentrated by rotary evapo-
ration. The residue was purified by flash column (10:1 hex-
anes and ethyl acetate) to give 2a with 70% of yield. 3a was
achieved in 22% yield.
1H NMR (CDCl3, 400 MHz) 7.95 (d, J = 7.3 Hz, 2H, Ar-
H), 7.83 (d, J = 7.6 Hz, 1H, Ar-H), 7.68 (t, J = 8.4 Hz, 1H,
Ar-H), 7.51-7.41 (m, 3H, Ar-H), 7.36 (d, J = 8.3 Hz, 1H, Ar-
H), 7.25 (t, J = 7.5 Hz, 1H, Ar-H), 6.93 (s, 1H, ethylene); 13
C
NMR (CDCl3, 100 MHz) 184.8 (C=O), 166.2 (Cipso of Ph-O),
146.9 (C=C-Ph), 136.9ꢀCmeta of Ph-Oꢁ, 132.3 (Cipso of Ph-
C=C), 131.6, 129.9, 128.9 (5C, Ph), 124.7 (C ortho of Ph-CO),
123.5 (Cmeta of Ph-CO), 121.7 (Cipso of Ph-CO), 113.1 (C=C-
Ph), 113.0 (Cortho of Ph-O).
3.6. (Z)-2-(4-Methylbenzylidene)benzofuran-3(2H)-one
(2b) [3]
1H NMR (CDCl3, 300 MHz) 7.84-7.80 (m, 3H, Ar-H),
7.68-7.63 (m, 1H, Ar-H), 7.35-7.19 (m, 4H, Ar-H), 6.90 (s,
1H, ethylene), 2.41 (s, 3H, CH3); 13C NMR (CDCl3, 75
3.3. General Procedure for the Preparation of (Z)-2-
benzylidene-2, 3-dihydrobenzofuran-3-ol (4a) [4]
2-(1-Hydroxy-3-phenylprop-2-ynyl)phenol (1a, 448.5 mg
= 2 mmol), silver nanoparticles (21.6 mg = 0.2 mmol, 10
mol%), i-Pr2NEt (34 μL = 0.2 mmol, 10 mol %), and PPh3

One-Pot Synthesis of Aurones through Oxidation-Cyclization Tandem Reaction Catalyzed
Letters in Organic Chemistry, 2018, Vol. 15, No. 1 75
MHz) 185.1 (C=O), 166.4 (Cipso of Ph-O), 146.9 (C=C-Ph),
140.9(Cipso of Ph-CH3), 137.1 (Cmeta of Ph-O), 131.9 (Cipso of
Ph-C=C), 130.1, 129.9 (4C, Ph), 125.0 (Cortho of Ph-CO),
123.7 (Cmeta of Ph-CO), 122.1 (Cipso of Ph-CO), 113.8 (Cortho
of Ph-O), 113.3 (C=C-Ph), 22.0 (CH3).
2H, Ar-H), 6.95 (s, 1H, ethylene), 2.41 (s, 3H, CH3); 13C
NMR (CDCl3, 75 MHz) 181.9 (C=O), 164.5 (Cipso of Ph-O),
141.4 (C=C-Ph), 136.9 (Cipso of Ph-CH3), 132.1 (Cmeta of Ph-
O), 131.5 (Cortho of Ph-CO), 130.1, 129.7 (4C, Ph), 127.6
(Cipso of Ph-C=C), 124.5 (Cipso of Ph-Cl), 123.3 (Cipso of Ph-
CO), 114.9 (Cortho of Ph-O), 114.6 (C=C-Ph), 22.1 (CH3); GC/
MS m/z (%) 304(M+), 289(100), 144, 115, 103, 97, 89, 77;
HRMS calcd for C16H10Cl2O2: 304.0058; found: 304.0062.
3.7. (Z)-2-(4-Methoxybenzylidene)benzofuran-3(2H)-one
(2c) [5]
1H NMR (CDCl3, 400 MHz, ppm) δ 7.90 (d, J = 8.7 Hz,
2H, Ar-H), 7.81 (d, J = 7.5 Hz, 1H, Ar-H), 7.68-7.62 (m, 1H,
Ar-H), 7.33 (d, J = 8.4 Hz, 1H, Ar-H), 7.24-7.19 (m, 1H, Ar-
H), 7.01-6.98 (d, J = 9.0 Hz, 2H, Ar-H), 6.90 (s, 1H, ethyl-
ene), 3.88 (s, 3H,OCH3); 13C NMR (CDCl3, 75 MHz, ppm) δ
184.9 (C=O), 166.2 (Cipso of Ph-O), 161.4 (Cipso of Ph-OCH3),
146.2 (C=C-Ph), 136.9 (Cmeta of Ph-O), 133.8 (C ortho of Ph-
C=C), 131.1 (Cortho of Ph-CO), 124.9 (Cipso of Ph-C=C),
123.6 (Cmeta of Ph-CO), 122.3 (Cipso of Ph-CO), 114.8 (Cortho
of Ph-O), 113.8 (Cmeta of Ph-C=C), 113.2 (C=C-Ph), 55.7
(OCH3).
3.12. (Z)-2-Benzylidene-5,7-dichlorobenzofuran-3 (2-
H)-one (2h) [3]
1H NMR (CDCl3, 300 MHz) 7.97-7.94 (m, 2H, Ar-H),
7.68 (dd, J = 2.1, 8.7 Hz, 2H, Ar-H), 7.52-7.46 (m, 3H, Ar-
H), 6.99 (s, 1H, ethylene); 13C NMR (CDCl3, 75 MHz) 178.5
(C=O), 160.5 (Cipso of Ph-O), 136.3 (C=C-Ph), 135.4 (Cmeta of
Ph-O), 132.4 (Cipso of Ph-C=C), 131.5 (Cmeta of Ph-CO), 131.1
(Cortho of Ph-CO), 129.8, 129.5, 129.0 (5C, Ph), 125.7 (Cortho
of Ph-O), 123.0 (Cipso of Ph-CO), 116.0 (C=C-Ph).
3.13. (Z)-2-Benzylidene-7-bromo-5-(tert-butyl)benzo-
furan-3(2H)-one (2i) [3]
3.8. (Z)-2-Benzylidene-5-bromobenzofuran-3(2H)-one (2d)
[2]
1H NMR (CDCl3, 300 MHz) 7.97 (d, J = 6.9 Hz, 2H, Ar-
H), 7.85 (d, J = 1.8 Hz, 1H, Ar-H), 7.76 (d, J = 1.8 Hz, 1H,
Ar-H), 7.52-7.43 (m, 3H, Ar-H), 6.95 (s, 1H, ethylene), 1.36
(s, 9H, t-Bu); 13C NMR (CDCl3, 75 MHz) 184.9 (C=O),
155.7 (Cipso of Ph-O), 149.0 (C=C-Ph), 137.7 (Cipso of Ph-t-
Bu), 132.3, 132.2, 130.6 (5C, Ph), 129.4 (Cmeta of Ph-O),
124.6 (Cipso of Ph-C=C), 123.1 (Cortho of Ph-CO), 120.4 (Cipso
of Ph-CO), 114.6 (Cipso of Ph-Br), 105.8 (C=C-Ph), 35.3 (t-
Bu), 31.7 (CH3).
1H NMR (CDCl3, 400 MHz, ppm) δ 7.94-7.92 (m, 3H,
Ar-H), 7.77 (d, J = 8.7 Hz, 1H, Ar-H), 7.51-7.42 (m, 3H, Ar-
H), 7.27 (d, J = 7.1 Hz, 1H, Ar-H), 6.94 (s, 1H, ethylene);
13C NMR (CDCl3, 100 MHz, ppm) δ 183.3 (C=O), 164.7
(Cipso of Ph-O), 146.8 (C=C-Ph), 139.4 (Cmeta of Ph-O), 132.0
(Cortho of Ph-CO), 131.7 (Cipso of Ph-C=C), 130.3, 129.0,
127.4 (5C, Ph), 123.4 (Cipso of Ph-CO), 116.4 (Cipso of Ph-Br),
114.7 (Cortho of Ph-O), 114.2 (C=C-Ph).
3.9. (Z)-5-Bromo-2-(4-methylbenzylidene)benzofuran-3
(2H)- one (2e) [3]
3.14. (Z)-2-(3-methylbenzylidene)benzofuran-3(2H)-one
(2j) [16]
1H NMR (CDCl3, 300 MHz) 7.92 (d, J = 2.1 Hz, 1H, Ar-
H), 7.81 (d, J = 8.1 Hz, 2H, Ar-H), 7.74 (dd, J = 2.1, 8.7 Hz,
1H, Ar-H), 7.29-7.24 (m, 3H, Ar-H), 6.92 (s, 1H, ethylene),
2.42 (s, 3H,CH3); 13C NMR (CDCl3, 75 MHz) 180.7 (C=O),
164.9 (Cipso of Ph-O), 141.4 (C=C-Ph), 139.6 (Cmeta of Ph-O),
132.1 (Cipso of Ph-CH3), 131.5 (Cortho of Ph-CO), 130.1,
129.7 (4C, Ph), 127.6 (Cipso of Ph-C=C), 123.9 (Cipso of Ph-
CO), 116.6 (Cipso of Ph-Br), 115.0 (Cortho of Ph-O), 114.9
(C=C-Ph), 22.0 (CH3).
1H NMR (CDCl3, 400 MHz) 8.24 (d, J = 7.6 Hz, 1H, Ar-
H), 7.75-7.69 (m, 3H, Ar-H), 7.60-7.58 (d, J = 7.6 Hz, 1H,
Ar-H), 7.45-7.40 (m, 2H, Ar-H), 7.37-7.35 (m, 1H, Ar-H),
6.82 (s, 1H, ethylene), 2.47 (s, 3H, CH3); 13C NMR (CDCl3,
100 MHz) 178.5 (C=O), 163.7 (Cipso of Ph-O), 156.4 (C=C-
Ph), 138.9 (Cipso of Ph-CH3), 133.8 (Cmeta of Ph-O), 132.4
(Cipso of Ph-C=C), 131.8 (Cortho of Ph-CH3), 129.0 (Cmeta of
Ph-CH3), 126.9 (Cortho of Ph-C=C), 125.7 (C
of Ph-
ortho
CO), 125.2 (Cpara of Ph-CH3), 124.0 (Cmeta of Ph-CO),
123.6 (Cipso of Ph-CO), 118.1 (Cortho of Ph-O), 107.6 (C=C-
Ph), 21.5 (CH3).
3.10.
(2f) [3]
(Z)-2-Benzylidene-5-chlorobenzofuran-3(2H)-one
3.15. 2-Phenyl-4H-chromen-4-one (3a) [17]
1H NMR (CDCl3, 300 MHz) 7.93-7.91 (m, 2H, Ar-H),
7.78 (d, J = 2.4 Hz, 1H, Ar-H), 7.63-7.60 (m, 1H, Ar-H),
7.50-7.43 (m, 3H, Ar-H), 7.31 (d, J = 9.0 Hz, 1H, Ar-H),
6.93 (s, 1H, ethylene); 13C NMR (CDCl3, 75 MHz) 183.9
(C=O), 164.7 (Cipso of Ph-O), 147.3 (C=C-Ph), 137.0 (Cmeta of
Ph-O), 132.1 (Cipso of Ph), 131.3 (Cortho of Ph-CO), 130.6,
129.3, 128.9 (5C, Ph), 124.6 (Cipso of Ph-Cl), 123.2 (Cipso of
Ph-CO), 114.6 (Cortho of Ph-O), 114.5 (C=C-Ph).
1H NMR (CDCl3, 300 MHz) 8.17 (d, J = 7.5 Hz, 1H, Ar-
H), 7.86-7.83 (m, 2H, Ar-H), 7.66-7.61 (m, 1H, Ar-H), 7.51-
7.45 (m, 4H, Ar-H), 7.35 (m, J = 7.5 Hz, 1H, Ar-H), 6.75 (s,
1H, ethylene); 13C NMR (CDCl3, 75 MHz) 179.7 (C=O),
164.7 (C=C-Ph), 157.6 (Cipso of Ph-O), 135.1 (Cpara of Ph-
CO), 133.1 (Cipso of Ph-C=C), 133.0 (Cortho of Ph-CO), 130.4
(Cipso of Ph-CO), 127.6, 127.0, 126.6 (5C, Ph), 125.3 (Cpara of
Ph-O), 119.5 (Cortho of Ph-O), 108.9 (Cortho of Ph-O).
3.11. (Z)-5-chloro-2-(4-methylbenzylidene)benzofuran-3
(2H)-one(2g)
3.16. 2-(p-Tolyl)-4H-chromen-4-one (3b) [17]
IR: 3077, 2922, 1712, 1647, 1601, 1457, 1228, 1173,
1130, 813, 747; 1H NMR (CDCl3, 300 MHz) 7.81 (d, J = 8.1
Hz, 2H, Ar-H), 7.65-7.61 (m, 2H, Ar-H), 7.28 (d, J = 8.4 Hz,
1H NMR (CDCl3, 300 MHz) 8.22 (dd, J = 1.5, 7.8 Hz,
1H, Ar-H), 7.81 (d, J = 8.4 Hz, 2H, Ar-H), 7.71-7.66 (m, 1H,
Ar-H), 7.55 (d, J = 7.8 Hz, 1H, Ar-H), 7.43-7.38 (m, 1H, Ar-

76 Letters in Organic Chemistry, 2018, Vol. 15, No. 1
Yu et al.
H), 7.31(d, J = 8.1 Hz, 2H, Ar-H), 6.79 (s, 1H, ethylene),
2.43 (s, 3H, CH3); 13C NMR (CDCl3, 75 MHz) 179.9 (C=O),
165.0 (C=C-Ph), 157.6 (Cipso of Ph-O), 143.6 (Cipso of Ph-
CH3), 135.0 (Cmeta of Ph-O), 131.2 (Cortho of Ph-CH3), 130.4
(Cmeta of Ph-CH3), 127.6 (Cipso of Ph-C=C), 127.1 (Cortho of
Ph-CO), 126.5 (Cipso of Ph-CO), 125.4 (Cmeta of Ph-CO),
119.4 (Cortho of Ph-O), 108.4 (C=C-Ph), 22.9 (CH3).
(Cipso of Ph-Cl), 128.1 (Cortho of Ph-CH3), 126.4 (Cmeta of Ph-
CH3), 125.8 (Cipso of Ph-C=C), 124.5 (Cipso of Ph-CO), 124.0
(Cortho of Ph-O), 106.6 (C=C-Ph), 21.7 (CH3).
3.22.ꢂ6, 8-Dichloro-2-phenyl-4H-chromen-4-one (3h) [21]
1H NMR (CDCl3, 300 MHz) 8.10 (d, J = 2.4 Hz, 1H, Ar-
H), 8.00-7.97 (m, 2H, Ar-H), 7.74 (d, J = 2.4 Hz, 1H, Ar-H),
7.59-7.52 (m, 3H, Ar-H), 6.87 (s, 1H, ethylene); 13C NMR
(CDCl3, 75 MHz) 176.5 (C=O), 163.6 (C=C-Ph), 150.6 (Cipso
of Ph-O), 133.9 (Cpara of Ph-CO), 132.3 (Cpara of Ph-O),
131.0 (Cipso of Ph-C=C), 129.3 (Cpara of Ph-C=C), 129.3 (Cor-
3.17. 2-(4-Methoxyphenyl)-4H-chromen-4-one (3c) [17]
1H NMR (CDCl3, 300 MHz) 8.19 (d, J = 7.8 Hz, 1H, Ar-
H), 7.83 (d, J = 9.0 Hz, 2H, Ar-H), 7.67-7.62 (m, 1H, Ar-H),
7.51 (d, J = 8.4 Hz, 1H, Ar-H), 7.37 (t, J = 7.5 Hz, 1H, Ar-
H), 6.97 (d, J = 8.7 Hz, 2H, Ar-H), 6.70 (s, 1H, ethylene),
3.85 (s, 3H, CH3); 13C NMR (CDCl3, 75 MHz) 178.4 (C=O),
163.5 (C=C-Ph), 162.5 (Cipso of Ph-OCH3), 156.3 (4), 133.6
(Cmeta of Ph-O), 128.0 (Cmeta of Ph-OCH3), 125.7 (Cortho of
Ph-CO), 125.1 (Cipso of Ph-CO), 124.1(Cmeta of Ph-CO),
124.0 (Cipso of Ph-C=C), 118.0 (Cortho of Ph-O), 114.5 (Cmeta
of Ph-OCH3), 106.2 (C=C-Ph), 55.5 (OCH3).
of Ph-C=C), 126.5 (Cmeta of Ph-C=C), 125.8 (Cortho of Ph-
tho
CO), 124.6 (Cortho of Ph-O), 124.0 (Cipso of Ph-CO), 107.3
(C=C-Ph).
3.23. 8-Bromo-6-(tert-butyl)-2-phenyl-4H-chromen-4-one
(3i)
IR: 3437, 3058, 2958, 1650, 1603, 1457, 1356, 1261,
1089, 883, 771; 1H NMR (CDCl3, 300 MHz) 8.17 (d, J = 2.1
Hz, 1H, Ar-H), 8.04-8.01 (m, 2H, Ar-H), 7.96 (d, J = 2.4 Hz,
1H, Ar-H), 7.56-7.54 (m, 3H, Ar-H), 6.87 (s, 1H, ethylene),
1.39 (s, 9H, t-Bu); 13C NMR (CDCl3, 75 MHz) 179.5 (C=O),
164.6 (C=C-Ph), 152.2 (Cipso of Ph-O), 151.0 (t-Bu), 136.4
(Cmeta of Ph-O), 133.2 (Cipso of Ph-C=C), 132.7, 130.5, 127.8
(5C, Ph), 126.0 (Cortho of Ph-CO), 122.6 (Cipso of Ph-CO),
112.9 (Cipso of Ph-Br), 108.4 (C=C-Ph), 36.4 (t-Bu), 32.6
(CH3); GC/MS m/z (%) 356 (M+), 343(100), 241, 157, 103,
77, 69, 57, 43; HRMS calcd for C19H17BrO2: 356.0412;
found: 356.0410.
3.18. 6-Bromo-2-phenyl-4H-chromen-4-one (3d) [18]
1H NMR (CDCl3, 300 MHz) 8.36 (d, J = 2.4 Hz, 1H, Ar-
H), 7.93-7.89 (m, 2H, Ar-H), 7.78 (dd, J = 2.4, 9.0 Hz, 1H,
Ar-H), 7.55-7.46 (m, 4H, Ar-H), 6.83 (m, 1H, ethylene); 13
C
NMR (CDCl3, 75 MHz) 178.5 (C=O), 165.2 (C=C-Ph),
156.4 (Cipso of Ph-O), 138.2 (Cmeta of Ph-O), 133.3 (Cortho of
Ph-CO), 130.5, 129.8, 127.8 (5C, Ph), 121.4 (Cipso of Ph-
C=C), 120.1 (Cipso of Ph-CO), 114.2 (Cipso of Ph-Br), 109.0
(Cortho of Ph-O), 104.8 (C=C-Ph).
3.19.ꢂ6-bromo-2-(p-tolyl)-4H-chromen-4-one (3e) [19]
3.24. 2-(m-tolyl)-4H-chromen-4-one(3j) [22]
1H NMR (CDCl3, 300 MHz) 8.32 (d, J = 2.4 Hz, 1H, Ar-
H), 7.78-7.73 (m, 3H, Ar-H), 7.43 (d, J = 8.7 Hz, 1H, Ar-H),
7.30 (d, J = 8.1 Hz, 2H, Ar-H), 6.77 (s, 1H, ethylene), 2.43
(s, 3H, CH3); 13C NMR (CDCl3, 75 MHz) 178.4 (C=O),
165.3 (C=C-Ph), 156.3 (Cipso of Ph-O), 144.0 (Cmeta of Ph-
O), 138.0 (Cipso of Ph-CH3), 131.2 (Cortho of Ph-CO), 129.9,
129.7 (4C, Ph), 127.6 (Cipso of Ph-C=C), 126.6 (Cipso of Ph-
CO), 121.4 (Cipso of Ph-Br), 120.0 (Cortho of Ph-O), 108.2
(C=C-Ph), 23.0 (CH3).
1H NMR (CDCl3, 400 MHz) 7.79 (dd, J = 7.6 Hz, 20.8
Hz, 2H, Ar-H), 7.71 (s, 1H, Ar-H), 7.68-7.64 (m, 1H, Ar-H),
7.36-7.34 (m, 1H, Ar-H), 7.26-7.23 (m, 2H, Ar-H), 6.88 (s,
1H, ethylene), 2.43 (s, 3H, CH3); 13C NMR (CDCl3, 100
MHz) 184.8 (C=O), 166.2 (C=C-Ph), 146.9 (Cipso of Ph-O),
138.6 (Cipso of Ph-CH3), 136.8 (Cmeta of Ph-O), 132.3 (Cipso of
Ph-C=C), 132.2 (Cmeta of Ph-CH3), 130.9 (Cpara of Ph-C=C),
128.8 (Cortho of Ph-CH3), 128.7 (Cortho of Ph-CO), 124.7 (C
of Ph-CH3), 123.5 (Cipso of Ph-CO), 121.7 (Cmeta of Ph-
para
CO), 113.3 (Cortho of Ph-O), 113.0 (C=C-Ph), 21.5 (CH3).
3.20. 6-Chloro-2-phenyl-4H-chromen-4-one (3f) [17]
3.25.ꢂ(Z)-2-benzylidene-2,3-dihydrobenzofuran-3-ol(4a)
1H NMR (CDCl3, 300 MHz) 8.19 (d, J = 2.4 Hz, 1H, Ar-
H), 7.92-7.90 (m, 2H, Ar-H), 7.66-7.62 (m, 1H, Ar-H), 7.55-
7.52 (m, 4H, Ar-H), 6.83 (s, 1H, ethylene); 13C NMR
(CDCl3, 75 MHz), 178.6 (C=O), 165.1 (C=C-Ph), 156.0 (Cip-
[2]
1H NMR (CDCl3, 400 MHz) 7.70 (d, J = 7.6Hz, 2H),
7.46 (d, J = 7.2Hz, 1H), 7.38-7.36 (m, 3H), 7.24-7.21 (m,
1H), 7.07 (d, J = 6.8Hz, 2H), 5.97 (s, 1H, OH), 5.71 (s, 1H,
ethylene); 13C NMR (CDCl3, 100 MHz) 157.8 (Cipso of Ph-
O), 157.0 (C=C-Ph), 134.6 (Cipso of Ph-C=C), 130.6 (Cipso of
Ph-COH), 128.7 (Cmeta of Ph-C=C), 128.5 (Cortho of Ph-
C=C), 127.0 (Cmeta of Ph-O), 126.9 (Cpara of Ph-C=C), 125.7
(Cortho of Ph-COH), 122.9 (Cpara of Ph-O), 110.7 (C=C-Ph),
106.0 (Cortho of Ph-O), 72.5 (C-OH).
of Ph-O), 135.4 (Cpara of Ph-CO), 133.3, 132.8, 132.6(5C,
so
Ph), 130.5 (Cortho of Ph-CO), 130.1(Cipso of Ph-C=C), 127.7
(C-Cl), 126.6 (Cipso of Ph-CO), 121.2 (Cortho of Ph-O), 108.9
(C=C-Ph).
3.21.ꢂ 6, 8-Dichloro-2-(p-tolyl)-4H-chromen-4-one (3g)
[20]
1H NMR (CDCl3, 300 MHz) 8.09 (d, J = 2.7 Hz, 1H, Ar-
H), 7.86 (d, J = 8.1 Hz, 2H, Ar-H), 7.72 (d, J = 2.4 Hz, 1H,
Ar-H), 7.34 (d, J = 8.1 Hz, 2H, Ar-H), 6.82 (s, 1H, ethylene),
2.45 (s, 3H, CH3); 13C NMR (CDCl3, 75 MHz) 176.4 (C=O),
163.7 (C=C-Ph), 150.5 (Cipso of Ph-O), 143.1 (Cipso of Ph-
CH3), 133.7 (Cmeta of Ph-O), 130.8 (Cortho of Ph-CO), 130.0
3.26.ꢂ1-(2-hydroxyphenyl)-3-phenylprop-2-yn-1-one(7a)
[12]
1H NMR (CDCl3, 400 MHz) 11.75 (s, 1H, Ar-H), 8.13
(d, J = 8.8Hz, 1H, Ar-H), 7.70 (d, J = 7.2Hz, 2H, Ar-H),

One-Pot Synthesis of Aurones through Oxidation-Cyclization Tandem Reaction Catalyzed
Letters in Organic Chemistry, 2018, Vol. 15, No. 1 77
D.; Bonnet, S.; Mariotte, A.M.; Perrier, E.; Boumendjel, A. J. Med.
Chem., 2006, 49, 329-333.
7.55-7.50 (m, 2H, Ar-H), 7.46-7.43 (m, 2H, Ar-H), 7.02-6.98
(m, 2H, Ar-H); 13C NMR (CDCl3, 100 MHz) 181.2 (C=O),
[2]
Yu, M.; Skouta, R.; Zhou, L.; Jiang, H.; Yao, X.; Li, C.J. J. Org.
Chem., 2009, 74, 3378-3383.
161.8 (Cipso of Ph-OH), 136.1 (Cpara of Ph-CO), 132.1 (Cortho
of Ph-CꢀC), 132.0 (Cortho of Ph-CO), 130.1 (Cpara of Ph-
CꢀC), 127.7 (Cmeta of Ph-CꢀC), 119.8 (Cipso of Ph-CO),
118.7 (Cipso of Ph-CꢀC), 118.4 (Cpara of Ph-OH), 117.1 (Cor-
[3]
Lin, M.; Yu, M.; Han, C.; Li, C.J.; Yao, X. Synthetic Commun.,
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Yu, M.; Lin, M.; Han, C.; Zhu, L.; Li, C.J.; Yao, X. Tetrahedron
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[4]
[5]
A gold(I) salt catalyzed cyclization of 1, see: Harkat, H.; Blanc, A.;
Weibel, J.M.; Pale, P. J. Org. Chem., 2008, 73, 1620-1623.
(a) Schauermann, S.; Nilius, N.; Shaikhutdinov, S.; Freund, H.J.
Acc. Chem. Res., 2013, 46, 1673- 1681; (b) Yasukawa, T.; Miya-
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tho of Ph-OH), 95.0 (CꢀC-CO), 84.7 (CꢀC-CO).
[6]
CONCLUSION
In conclusion, copper nanoparticles together with bipyri-
dine were utilized as an effective catalyst in the oxidation/
cyclization of 1 and gave aurone and flavonoid in good
yields. In the reaction, oxygen worked as a green and mild
[7]
(a) Shekhar, M.; Wang, J.; Lee, W.S.; Williams, W.D.; Kim, S.M.;
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(a) Wang, F.; Shi, R.; Liu, Z.Q.; Shang, P.J.; Pang, X.; Shen, S.;
Feng, Z.; Li, C.; Shen, W. ACS Catal., 2013, 3, 890-894; (b)
Alonso, F.; Moglie, Y.; Radivoy, G. Yus, M. J. Org. Chem., 2013,
78, 5031-5037; (c) Honraedt, A.; Callonnec, F.L.; Grognec, E.L.;
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Alonso, F.; Moglie, Y.; Radivoy, G.; Yus, M. J. Org. Chem., 2011,
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oxidant to give the best results. Both water and bipyridine
prove to be the key factors to activate the Cu NPs catalyst.
The detailed mechanism, as well as the scope of the reaction
is currently under further investigation.
CONSENT FOR PUBLICATION
[8]
Not applicable.
CONFLICT OF INTEREST
The authors declare no conflict of interest, financial or
otherwise.
ACKNOWLEDGEMENTS
We are grateful to the financial support from National
Natural Science Foundation of China (21472092 to XY),
[9]
Ramírez, J.; Sanaú, M.; Fernández, E. Angew. Chem. Int. Ed.,
2008, 47, 5194-5197.
National Natural Science Foundation of China (21671107
to GL) and Natural Science Fund for Colleges and Univer-
sities in Jiangsu Province (15KJD150003 to MY). A pro-
ject funded by the Priority Academic Program Develop-
ment of Jiangsu higher education institutions.
[10]
Yu, M.; Wang, Y.; Sun, W.; Yao, X. Adv. Synth. Catal., 2012, 354,
71-76.
[11]
[12]
Han, C.; Yu, M.; Sun, W.; Yao, X. Synlett., 2011, 16, 2363-2368.
Lv, W.; Tian, J.; Deng, N.; Wang, Y.; Zhu, X.; Yao, X. Tetrahe-
dron Lett., 2015, 56, 1312-1316.
[13]
[14]
Only Z-isomer was observed.
In our pervious report on Ph3P-siliver NPs catalyzed cyclization of
1 to 4, aurones were found as the single product.
SUPPLEMENTARY MATERIAL
Supplementary material is available on the publisher’s
website along with the published article.
[15]
[16]
[17]
Liu, M.X.; Li, C.J. Angew. Chem. Int. Ed. 2016, 55, 10806-10810.
Weng, Y.; Chen, Q.; Su, W. J. Org. Chem., 2014, 79, 4218-4224.
Zhao, J.; Zhao, Y.F.; Fu, H. Angew. Chem. Int. Edit., 2011, 50,
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rial Department reserves the right to make minor modifications for further improvement of the manuscript.