An eco-friendly Co(OAc)2-catalyzed aerobic oxidation of 4-benzylphenols into 4-hydroxybenzophenones

Article abstract of DOI:10.1007/s11164-014-1801-8

An undecorated Co(OAc)₂-catalyzed aerobic oxidation system has been reported that enables direct transformation of 4-benzylphenols into the corresponding 4-hydroxybenzophenones. The procedure is especially suitable for electron-withdrawing group-containing substrates, which are commonly inefficient to conduct this category of oxidation. Based on well-defined p-benzoquinone methides and the confirmed ethereal intermediate, a plausible mechanism was depicted.

Full text of DOI:10.1007/s11164-014-1801-8

Res Chem Intermed
DOI 10.1007/s11164-014-1801-8
An eco-friendly Co(OAc)2-catalyzed aerobic oxidation
of 4-benzylphenols into 4-hydroxybenzophenones
•
Jian-Gang Huang Ying Guo Jian-An Jiang
•
•
•
Hong-Wei Liu Ya-Fei Ji
Received: 12 June 2014 / Accepted: 30 August 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract An undecorated Co(OAc)2-catalyzed aerobic oxidation system has been
reported that enables direct transformation of 4-benzylphenols into the corre-
sponding 4-hydroxybenzophenones. The procedure is especially suitable for elec-
tron-withdrawing group-containing substrates, which are commonly inefficient to
conduct this category of oxidation. Based on well-defined p-benzoquinone methides
and the confirmed ethereal intermediate, a plausible mechanism was depicted.
Keywords 4-Benzylphenols ꢀ 4-Hydroxybenzophenones ꢀ Aerobic oxidation ꢀ
Co(OAc)2ꢀ4H2O ꢀ Ethylene glycol
Introduction
As is well known, research on chemical intermediates has long been a point of
synthetic focus [1]. 4-Hydroxybenzophenones (4-HBPs) have served as significant
chemical intermediates and/or additives for the preparation of drugs, polymers, dyes
and perfumeries, as well as for academic researches [2–4]. For instance, these
compounds are indispensable to the synthesis of UV stabilizers, sunscreens, and
photoinitiators [5–7]. Also, 4-HBPs are widely used in the preparation of
therapeutic agents, such as estrogen receptor modulator Ospemifene [8] and
antineoplastic agent Tamoxifen [9].
Generally, Friedel–Crafts acylation and Fries rearrangement reactions provide
elementary routes to 4-HBPs with phenols as starting feedstocks [4, 10, 11].
Nevertheless, poor selectivity, tedious multistep reactions, and concomitant
pollution still urge chemists to explore an eco-friendly and green approach to
J.-G. Huang ꢀ Y. Guo ꢀ J.-A. Jiang ꢀ H.-W. Liu ꢀ Y.-F. Ji (&)
School of Pharmacy, East China University of Science and Technology, 130 Meilong Road,
Campus PO Box 363, Shanghai 200237, People’s Republic of China
e-mail: jyf@ecust.edu.cn
123

J.-G. Huang et al.
4-HBPs. Recently, the straightforward aerobic oxidation of the methyl group of
4-cresols into fascinating aldehyde group unfolds a greatly attractive organic
transformation [12–14]. More recently, we have reported a copper-catalyzed direct
oxidation of primary and secondary benzyl C–H bond into C=O bond under the
initiation of the hindered 4-hydroxyl group for electron-rich 4-alkylphenols [15]. In
continuation of our interest on aerobic oxidation with undecorated, inexpensive,
lowly toxic transition metal catalysts [16], herein we reported a step-economical
Co(OAc)2-catalyzed conversion of 4-benzylphenols into highly valuable 4-HBPs
with moderate yields (Scheme 1). In particular, the procedure is qualified to deal
with electron-withdrawing group (EWG)-containing substrates.
Results and discussion
Referring to the previous work involving aerobic oxidation of EWG-substituted
4-cresols into 4-hydroxybenzaldehydes [16], the bulky double benzyl group of
4-benzylphenols, although difficultly, could be also aerobically oxidized into
benzophenone framework under the optimized conditions of Co(OAc)2ꢀ4H2O
(8 mol %), NaOH (2.0 equiv) at 100 °C in ethylene glycol (EG). As shown in
Table 1, 2,6-dihalogenated 4-benzylphenols 1a and 1b were firstly converted into
the corresponding 4-HBPs 2a and 2b in moderate yields of 61 and 62 %,
respectively, with a long reaction time of 18 h (entries 1 and 2). Then, 2-EWG-6-
electron-donating group (EDG)-substituted substrate 1c reacted smoothly to give 2c
in 64 % yield with a shorten reaction time of 14 h (entry 3), displaying a facilitation
of ortho-EDG for the reaction. Afterwards, we further assessed the challenging
oxidation of unhindered 2-EWG-monosubstituted 4-benzylphenols, wherein chem-
ists were usually perplexed by an unhindered oxidative ortho, ortho-homocoupling
in the desired transformation (entries 4–6) [17, 18]. To our delight, all 2-EWG-
monosubstituted and unsubstituted electron-neutral substrates 1d–g were success-
fully converted into the target products 2d–g in moderate yields of 52–63 % (entries
4–7), in which 4.0 equiv of NaOH was surely able to inhibit the latent
homocoupling reaction. It is supposed that the formation of p-benzoquinone
methides and resulting benzyl ether is more rapid than the side-reaction involving a
radical coupling in this basic medium. As a result, the formation of homocoupling
byproducts would be suppressed [19]. Commonly, EWG-containing 4-HBPs are
inefficient to conduct this category of phenoxy radical-initiated oxidation [15].
On the other hand, the oxidation of the 2-EDG-substituted electron-rich substrate
1h provided 4-HBPs 2h with a relatively low yield of 45 %, due to an emergence of
coupling side-reactions (entry 8). Additionally, 4-benzyl-2,6-dimethoxyphenol 1i
favorably underwent the oxidation delivering the desired product 2i in a respected
yield of 66 % (entry 9), demonstrating a responsible feasibility for the electron-rich
substrates in the procedure.
Furthermore, the procedure was attempted to expand the scope to 4-alkylphenolic
substrates. The investigated results revealed that aerobic oxidation of the substrates
covering hindered, unhindered, electron-poor, or electron-rich 4-alkylphenols all
afforded low yields of 32–37 % (entries 10–12). In consideration of the existing
123

Eco-friendly Co(OAc)2-catalyzed aerobic oxidation
Ph
O
Ph
Co(OAc)2/NaOH
O2, EG
R1
R2
R1
R2
OH
OH
1
2
R1 = EWG; R2 = EWG, H or EDG
Scheme 1 Co(OAc)2-catalyzed aerobic oxidation of 1 into 2 in alkaline EG
Table 1 Scope of 1 for Co(OAc)2-catalyzed aerobic oxidation
R3
R3
O
.
Co(OAc)2 4H2O (8.0 mol%)
NaOH (2.0 equiv)
O2 (1.0 atm), EG
100 oC, t
R1
R2
R1
R2
OH
OH
1
2
Entry
1
Substrate
Product
O
t (h)
Yield (%)a
62
Br
Br
OH
18
18
14
OH
Br
a
1
Br
a
2
O
O
Cl
Cl
OH
1b
2
3
61
64
OH
Cl
Cl
2b
Br
Br
OH
OH
OMe
1c
OMe
2c
O
O
Br
Br
4
5
15
15
61b
60b
OH
OH
2d
1d
NO2
NO2
OH
1e
OH
2e
O
CN
CN
6
15
52b
OH
OH
1f
2f
123

J.-G. Huang et al.
Table 1 continued
Entry
Substrate
Product
t (h)
12
Yield (%)a
63b
O
7
8
9
O
H
OH
1g
2g
O
e
OM
OMe
12
12
45b
66
OH
OH
1h
2h
O
OMe
OMe
OH
OH
OMe
OMe
1i
2i
O
10
11
12
10
10
10
32
34b
37
Br
Br
1j
Br
Br
2j
OH
OH
O
Br
1k
Br
2k
OH
OH
O
OH
OH
2l
1l
Performed with 1 (2.0 mmol), Co(OAc)2ꢀ4H2O (0.16 mmol), NaOH (2.0 mmol) and EG (9.0 ml), O2
(1.0 atm) for a specified time
a
Isolated yield
b
Performed with NaOH (4.0 mmol)
side-reactions related to the active a-H of formed carbonyl group, these low
outcomes seemed to be acceptable.
Under the guide of the previous work [15, 16, 20], the control experiments
presented the ether 3a as a similar intermediate, whereas the expected thermody-
namically favored EG acetal 4 was not detected over the whole scenario under the
reaction conditions (Scheme 2). Based on the well-defined p-benzoquinone
methides [21] and the investigated results, a phenoxy radical-initiated oxidation
pathway is depicted in Scheme 3. Firstly, the reaction commences with single-
electron transfer from the phenolic anion of 1 to direct oxidant Co(III) species,
derived from Co(OAc)2ꢀ4H2O through aeration with molecular oxygen [22, 23], to
give the phenoxy radical. The resonance radicals III are disproportionated to
original 1 and transiently highly reactive p-benzoquinone methides A, to which the
nucleophilic additions of EG lead to ethers 3 [16, 21]. Subsequently, the
intermediate anions of 3 are analogously converted into the transitory p-
123

Eco-friendly Co(OAc)2-catalyzed aerobic oxidation
HO
O
O
Co(OAc)2.4H2O (8 mol%)
NaOH (2.0 equiv)
O
O
Br
Br
Br
Br
(a)
(b)
1a
+
+
+
OH
O
2 (1.0 atm), EG, 100 oC, 10 h
OH
OH
OH
Br
Br
Br
Br
(21%
recovered)
1a
2a (44%)
3a (25%)
4 (0%)
HO
O
O
Co(OAc)2.4H2O (8 mol%)
NaOH (2.0 equiv)
Br
Br
OH O2 (1.0 atm), EG, 100 oC, 10 h
OH
Br
Br
3a
2a (81%)
Scheme 2 Mechanistic control experiments
Ph
Ph
Ph
R
Ph
Ph
(1) OH-
EG
R
DP
R
R
R
O2
(2)
1
O
O
O
CoII
O
OH
CoIII
I
II
III
1
A
Ph
OCH2CH2OH
Ph
OCH2CH2OH
Ph
OCH2CH2OH
Ph
OCH2CH2OH
(1) OH-
R
R
R
R
O2
CoII
(2)
OH
O
I'
O
II'
O
III'
CoIII
3
Ph
Ph
OCH2CH2OH
Ph
OCH2CH2OH
H2O
Ph
O
HO
R
OCH2CH2OH
(1) -EG
(2) H+
DP
R
R
R
3
O
A'
O
OH
2
O
DP = disproportionation
Scheme 3 Plausible mechanism
benzoquinone methides A0, which give the end products 2 upon spontaneous
aromatization followed by a fast hydrolysis [24, 25].
Conclusions
In summary, we have developed a Co(OAc)2-catalyzed aerobic oxidation of
4-benzylphenols into significant 4-HBPs with atom, reaction step, and redox-
economy. The mechanism-based reaction achieves a direct construction of benzoyl
group from the bulky benzyl group. The procedure is especially suitable for EWG-
containing substrates, and compatible with unhindered and electron-rich ones, as
123

J.-G. Huang et al.
well as 4-alkylphenols. We believe that the investigations will promise a potential
value in oxidative functionalization chemistry.
Experimental
The NMR spectra were determined on a Bruker AV400 in CDCl3 or DMSO-d6 with
TMS as internal standard for 1H NMR (400 MHz) and 13C NMR (100 MHz),
respectively. HRMS were carried out on a QSTAR Pulsar I LC/TOF MS mass
spectrometer or Micromass GCTTM gas chromatograph-mass spectrometer. The
melting points were recorded on open capillaries and uncorrected. All solvents and
reagents were obtained from commercial sources and used without further
purification. Thin-layer chromatography (TLC) was performed on silica gel
HF254 and TLC visualizations were performed with ultraviolet (UV) light.
General procedure for preparation of 1
A mixture of substrate 1 (2.0 mmol), Co(OAc)2ꢀ4H2O (40 mg, 0.16 mmol) and
NaOH (160 mg, 4.0 mmol) in EG (9.0 ml) was stirred with molecular oxygen
(1 atm) being bubbled, under 100 °C for specified time. After completion of
reaction, diluted hydrochloric acid (15 ml, 2 %) and chloroform (15 ml) were
successively added to the reaction mixture. The organic layer was separated, and the
aqueous phase was extracted with chloroform (15 ml 9 2). The combined organic
phase was dried over anhydrous sodium sulfate and concentrated to give a residue,
which was purified by column chromatography on silica gel (eluents: petroleum
ether:ethyl acetate = 10:1) providing the desired products 2.
(3,5-dibromo-4-hydroxyphenyl)(phenyl)methanone (2a)
1
White solid, 442 mg (62 %), m.p. 139–141 °C; H NMR (400 MHz, CDCl3), d
(ppm): 7.96 (s, 2H, Ar), 7.75 (dd, J = 8.0, 1.2 Hz, 2H, Ar), 7.60–7.64 (m, 1H, Ar),
7.52 (t, J = 8.0 Hz, 2H, Ar), 6.31 (br s, 1H, OH); 13C NMR (100 MHz, DMSO-d6),
d (ppm): 197.4, 160.1, 141.9, 139.2 (2C), 137.9, 135.9, 134.6 (2C), 133.8 (2C),
116.7 (2C); HRMS (ESI): m/z [M–H]- calcd. for C13H7Br2O2: 354.8792; found:
354.8789.
(3,5-dichloro-4-hydroxyphenyl)(phenyl)methanone (2b)
1
White solid, 326 mg (61 %), m.p. 145–146 °C; H NMR (400 MHz, CDCl3), d
(ppm): 7.79 (s, 2H, Ar), 7.75 (dd, J = 8.0, 1.6 Hz, 2H, Ar), 7.62 (tt, J = 8.0,
1.2 Hz, 1H, Ar), 7.51 (t, J = 8.0, 2H, Ar), 6.31 (br s, 1H, OH); 13C NMR
(100 MHz, CDCl3), d (ppm): 193.3, 151.4, 136.9, 132.8, 130.9, 130.5 (2C), 129.8
(2C), 128.6 (2C), 121.3 (2C); HRMS (ESI): m/z [M–H]- calcd. for C13H7Cl2O2:
264.9823; found: 264.9824.
123

Eco-friendly Co(OAc)2-catalyzed aerobic oxidation
(3-bromo-4-hydroxy-5-methoxyphenyl)(phenyl)methanone (2c)
1
White solid, 393 mg (64 %), m.p. 110–112 °C; H NMR (400 MHz, CDCl3), d
(ppm): 7.75 (dd, J = 8.0, 1.2 Hz, 2H, Ar), 7.60 (t, J = 8.0 Hz, 1H, Ar), 7.55 (d,
J = 2.0 Hz, 1H, Ar), 7.50 (t, J = 8.0 Hz, 2H, Ar), 7.44 (d, J = 2.0 Hz, 1H, Ar),
6.36 (br s, 1H, OH), 3.98 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3), d (ppm):
194.5, 147.4, 147.2, 137.6, 132.3, 130.3, 129.7 (2C), 128.8, 128.4 (2C), 111.0,
107.4, 56.6; HRMS (ESI): m/z [M–H]- calcd. for C14H10BrO3: 304.9813; found:
304.9823.
(3-bromo-4-hydroxyphenyl)(phenyl)methanone (2d)
1
Yellow solid, 338 mg (61 %), m.p. 181–183 °C; H NMR (400 MHz, CDCl3), d
(ppm): 8.03 (d, J = 2.0 Hz, 1H, Ar), 7.72–7.76 (m, 3H, Ar), 7.60 (t, J = 8.0 Hz,
1H, Ar), 7.50 (t, J = 8.0 Hz, 2H, Ar), 7.10 (d, J = 8.4 Hz, 1H, Ar), 5.95 (br s, 1H,
OH); 13C NMR (100 MHz, DMSO-d6), d (ppm): 193.2, 158.5, 137.4, 134.8, 132.2,
131.4, 129.3, 129.2 (2C), 128.5 (2C), 115.9, 109.5; HRMS (ESI): m/z [M–H]-
calcd. for C13H8BrO2: 274.9708; found: 274.9707.
(4-hydroxy-3-nitrophenyl)(phenyl)methanone (2e)
1
Yellow solid, 292 mg (60 %), m.p. 89–91 °C; H NMR (400 MHz, CDCl3), d
(ppm): 10.93 (br s, 1H, OH), 8.60 (s, 1H, Ar), 8.14 (d, J = 8.8 Hz, 1H, Ar), 7.77 (d,
J = 7.6 Hz, 2H, Ar), 7.65 (t, J = 7.0 Hz, 1H, Ar), 7.53 (t, J = 7.0 Hz, 2H, Ar),
7.29 (dd, J = 8.4, 1.6 Hz, 1H, Ar), 13C NMR (100 MHz, CDCl3), d (ppm): 193.2,
157.9, 138.4, 136.7, 133.0, 132.9, 129.9, 129.7 (2C), 128.7 (2C), 127.9, 120.4;
HRMS (ESI): m/z [M–H]- calcd. for C13H8NO4: 242.0453; found: 242.0451.
5-benzoyl-2-hydroxybenzonitrile (2f)
1
Yellow solid, 232 mg (52 %), m.p. 187–189 °C; H NMR (400 MHz, CDCl3), d
(ppm): 8.03 (dd, J = 8.0, 2.0 Hz, 2H, Ar), 7.74 (d, J = 8.0 Hz, 2H, Ar), 7.63 (t,
J = 8.0 Hz, 1H, Ar), 7.52 (t, J = 6.8 Hz, 2H, Ar), 7.12 (d, J = 8.0 Hz, 1H, Ar),
6.56 (br s, 1H, OH); 13C NMR (100 MHz, DMSO-d6), d (ppm): 193.5, 164.2, 137.4,
136.9, 136.3, 133.0, 129.9 (2C), 129.1 (2C), 128.8, 116.8, 116.6, 99.7; HRMS (ESI):
m/z [M–H]- calcd. for C14H8NO2: 222.0555; found: 222.0553.
(4-hydroxyphenyl)(phenyl)methanone (2g)
1
White solid, 250 mg (63 %), m.p. 132–135 °C; H NMR (400 MHz, CDCl3), d
(ppm): 7.75–7.80 (m, 4H, Ar), 7.57 (tt, J = 7.2, 1.2 Hz, 1H, Ar), 7.48 (td, J = 7.2,
1.2 Hz, 2H, Ar), 6.90–6.93 (m, 2H, Ar), 5.63 (br s, 1H, OH); 13C NMR (100 MHz,
DMSO-d6), d (ppm): 194.7, 162.5, 138.6, 133.0 (2C), 132.2, 129.6 (2C), 128.8 (2C),
128.4, 115.7 (2C); HRMS (ESI): m/z [M–H]- calcd. for C13H9O2: 197.0603; found:
197.0603.
123

J.-G. Huang et al.
(4-hydroxy-3-methoxyphenyl)(phenyl)methanone (2h)
White solid, 205 mg (45 %), m.p. 97–98 °C; 1H NMR (400 MHz, CDCl3), d (ppm):
7.75 (dd, J = 7.6, 1.2 Hz, 2H, Ar), 7.57 (t, J = 7.6 Hz, 1H, Ar), 7.51 (d,
J = 1.6 Hz, 1H, Ar), 7.47 (t, J = 7.6 Hz, 2H, Ar), 7.34 (dd, J = 8.4, 1.6 Hz, 1H,
Ar), 6.95 (d, J = 8.4 Hz, 1H, Ar), 6.09 (br s, 1H, OH), 3.97 (s, 3H, CH3); 13C NMR
(100 MHz, CDCl3), d (ppm): 195.7, 150.3, 146.7, 138.3, 131.9, 129.8, 129.7 (2C),
128.2 (2C), 126.3, 113.6, 111.9, 56.1; HRMS (ESI): m/z [M–H]- calcd. for
C14H11O3: 227.0708; found: 227.0709.
(4-hydroxy-3,5-dimethoxyphenyl)(phenyl)methanone (2i)
1
White solid, 341 mg (66 %), m.p. 124–126 °C; H NMR (400 MHz, CDCl3), d
(ppm): 7.78 (d, J = 8.0 Hz, 2H, Ar), 7.58 (t, J = 7.2 Hz, 1H, Ar), 7.49 (t,
J = 7.2 Hz, 2H, Ar), 7.13 (s, 2H, Ar), 5.98 (br s, 1H, OH), 3.92 (s, 6H, CH3); 13C
NMR (100 MHz, CDCl3), d (ppm): 195.5, 146.6 (2C), 139.4, 138.2, 132.0, 129.7
(2C), 128.6, 128.2 (2C), 107.8 (2C), 56.5 (2C); HRMS (ESI): m/z [M?] calcd. for
C15H14O4: 258.0892; found 258.0896.
1-(3,5-dibromo-4-hydroxyphenyl)ethanone (2j)
1
White solid, 188 mg (32 %), m.p. 185–187 °C; H NMR (400 MHz, CDCl3), d
(ppm): 8.07 (s, 2H, Ar), 6.33 (br s, 1H, OH), 2.55 (s, 3H, CH3); 13C NMR
(100 MHz, CDCl3), d (ppm):194.3, 153.3, 132.6 (2C), 131.9, 110.1 (2C), 26.3;
HRMS (ESI): m/z [M ? H?] calcd. for C8H7Br2O2: 292.8813; found 292.8817.
1-(3-bromo-4-hydroxyphenyl)ethanone (2k)
1
Yellow solid, 146 mg (34 %), m.p. 112–114 °C; H NMR (400 MHz, CDCl3), d
(ppm): 8.13 (d, J = 2.0 Hz, 1H, Ar), 7.85 (dd, J = 8.4, 2.0 Hz, 1H, Ar), 7.07 (d,
J = 8.4 Hz, 1H, Ar), 6.04 (br s, 1H, OH), 2.55 (s, 3H, CH3); 13C NMR (100 MHz,
CDCl3), d (ppm): 196.3, 156.9, 133.3, 131.2, 130.0, 116.0, 110.6, 26.3; HRMS
(ESI): m/z [M–H]- calcd. for C8H6BrO2: 212.9551; found: 212.9569.
1-(4-hydroxy-3,5-dimethylphenyl)ethanone (2l)
1
Yellow solid, 122 mg (37 %), m.p. 157–159 °C; H NMR (400 MHz, CDCl3), d
(ppm): 7.64 (s, 2H, Ar), 5.43 (br s, 1H, OH), 2.54 (s, 3H, CH3), 2.29 (s, 6H, CH3);
13C NMR (100 MHz, CDCl3), d (ppm): 197.6, 157.0, 129.6 (3C), 123.1 (2C), 26.3,
16.0 (2C); HRMS (ESI): m/z [M?] calcd. for C10H12O2: 164.0837; found: 164.0838.
Procedure for Scheme 2a, preparation of 2,6-dibromo-4-((2-hydroxyethoxy)
(phenyl)- methyl)phenol (3a)
A mixture of substrate 1a (684 mg, 2.0 mmol), Co(OAc)2ꢀ4H2O (40 mg,
0.16 mmol) and NaOH (160 mg, 4.0 mmol) in EG (9.0 ml) was stirred at 100 °C
123

Eco-friendly Co(OAc)2-catalyzed aerobic oxidation
for 10 h with molecular oxygen (1 atm) being bubbled. The reaction mixture was
treated as the above workup process, affording the ethereal intermediate 3a (25 %),
target product 2a (44 %) and starting feedstock 1a (21 %).
2,6-dibromo-4-((2-hydroxyethoxy)(phenyl)methyl)phenol (3a)
201 mg (25 %); 1H NMR (400 MHz, CDCl3), d (ppm): 7.33 (s, 2H, Ar), 7.20–7.28
(m, 5H, Ar), 6.08 (br s, 1H, OH), 5.19 (s, 1H, CH), 3.70 (t, J = 5.2 Hz, 2H, CH2),
3.47 (t, J = 5.2 Hz, CH2); 13C NMR (100 MHz, CDCl3), d (ppm): 147.9, 139.8,
135.6, 129.4 (2C), 127.7 (2C), 127.0, 125.8 (2C), 109.1 (2C), 81.5, 69.4, 60.8;
HRMS (ESI): m/z [M–H]- calcd. for C15H13Br2O3: 398.9231; found: 398.9138.
Procedure for Scheme 2b
A mixture of substrate 3a (193 mg, 0.48 mmol), Co(OAc)2ꢀ4H2O (10 mg,
0.038 mmol) and NaOH (38 mg, 0.96 mmol) in EG (9.0 ml) was stirred at
100 °C for 10 h with molecular oxygen (1 atm) being bubbled. The reaction
mixture was treated as the above workup process, affording the target product 2a
(138 mg, 81 %).
Acknowledgments We gratefully thank the National Natural Science Foundation of China (Project
Nos. 21176074 and 21476074) and Research Fund for the Doctoral Program of Higher Education of
China (Project No. 20130074110009) for financial support.
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