A Novel Three-Step Tandem Reaction for Efficient Syntheses of Bulky Anthracenyl Esters from 2-Benzylbenzoic Acids

Article abstract of DOI:10.1055/s-0036-1588384

Bulky anthracenyl esters could be efficiently synthesized from 2-benzylbenzoic acids via a novel three-step tandem reaction containing intramolecular Friedel-Crafts acylation, enolization, and esterification. A mechanism for the tandem reaction is proposed.

Full text of DOI:10.1055/s-0036-1588384

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SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2016, 48, A–E
paper
A
Y. Wang et al.
Paper
Synthesis
A Novel Three-Step Tandem Reaction for Efficient Syntheses of
Bulky Anthracenyl Esters from 2-Benzylbenzoic Acids
Yabai Wanga,b
Shiwei Yangb
R1
COOH
1. TCT, pyridine
CH2Cl2, reflux, 15 min
Guangling Bian*b
Ling Song*b
R2
R1
2. AlCl3, CH2Cl2, reflux, 1 h
O
O
R1 = H, Me, OMe
R2 = H, Me, OMe, Cl
7 examples, yield: 69~85%
a Department of Chemistry, Fuzhou University, Fuzhou 350108,
P. R. of China
b The Key Laboratory of Coal to Ethylene Glycol and Its Related
Technology, Fujian Institute of Research on the Structure of
Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002,
P. R. of China
R2
R2
R1
glb@fjirsm.ac.cn
songling@fjirsm.ac.cn
Received: 21.10.2016
Accepted after revision: 06.12.2016
Published online: 08.02.2017
boxylic acids to form ketones. Instead of 2a, a novel anthra-
cenyl ester 3a was obtained as the main product (Scheme
1), whose structure was confirmed by X-ray photoelectron
spectroscopy (XPS) as shown in Figure 1.
DOI: 10.1055/s-0036-1588384; Art ID: ss-2016-h0741-op
Abstract Bulky anthracenyl esters could be efficiently synthesized
from 2-benzylbenzoic acids via a novel three-step tandem reaction con-
taining intramolecular Friedel–Crafts acylation, enolization, and esteri-
fication. A mechanism for the tandem reaction is proposed.
O
proposed
COOH
2a
Key words tandem reaction, Friedel–Crafts acylation, enolization, es-
terification, anthracenyl esters
1. TCT, pyridine
minor
CH2Cl2, r.t., 5 min
2. AlCl3, CH2Cl2
r.t., 1 h
The preparation of complex organic molecules via mul-
tistep organic reactions is usually a tedious and laborious
work owing to the necessity of protection/deprotection of
functional groups and purification of intermediate prod-
ucts.1 In comparison with the traditional approach, tandem
reaction can overcome these drawbacks and is an efficient
modern organic synthetic methodology.2 Therefore, it is of
great practical significance to develop novel tandem reac-
tions.3 Anthracene derivatives are widely applied in organic
light-emitting diodes (OLEDs) as emitting and charge-
transporting materials.4 Besides, they are important struc-
tural units of anticancer agents such as azonafides.5 So far, a
large number of synthetic methods have been developed
for the syntheses of functional compounds containing an-
thracene skeleton, most of which are modified directly
from anthracene via multiple reactions.6 Herein, we report
a novel tandem reaction, which can be applied to prepare
anthracenyl esters in one pot from 2-benzylbenzoic acids.
This novel tandem reaction was discovered in our lab
during the preparation of anthrone (2a) from 2-benzylben-
zoic acid (1a) in the presence of cyanuric chloride (2,4,6-
trichloro-1,3,5-triazine, TCT), pyridine, and aluminum chlo-
ride using Kangani’s method,7 which had been reported to
be efficient for the Friedel–Crafts acylation8 of various car-
obtained
O
O
1a
3a
major
Scheme 1 The reaction of compound 1a with cyanuric chloride, pyri-
dine, and AlCl3
Figure 1 Ball-and-stick representation of 3a viewed down the b-axis
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Synthesis
Table 1 Optimization of the Reaction Conditions for the Tandem Reaction
Entry
Pyridine (equiv)
AlCl3 (equiv)
TCT (equiv)
Temp (°C )
Solvent
Yield (%)a
3a
2ab
1ab
1
2
1
1
1.6
1.6
1.6
1.6
1.6
1.6
2
r.t.
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
38
46
25
55
76
40
35
81
50
21
<5
<5
6
53
43
65
32
15
39
49
<5
36
64
83
1.2
1.5
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1
r.t.
3
1
r.t.
<5
8
4
1.2
2
r.t.
5
r.t.
<5
<5
10
<5
<5
<5
<5
6
2.5
2
r.t.
7
r.t.
8
2
1.6
1.6
1.6
1.6
reflux (40)
reflux (61)
reflux (81)
reflux (84)
9
2
10
11
2
MeCN
2
ClCH2CH2Cl
a Isolated yields.
b Recovered yields of starting materials 1a and 2a.
The initial reaction gave us only 38% yield of 3a (Table 1,
entry 1). To improve the yield of 3a, the experimental con-
ditions of the reaction were optimized. Investigation on the
loadings of TCT, pyridine, and AlCl3 at room temperature in
CH2Cl2 showed that the best ratio of pyridine/AlCl3/TCT was
1.2:2:1.6 giving 76% yield of 3a (entries 2–7). Running the
reaction in refluxing CH2Cl2 (40 °C) increased the yield to
81% (entry 8). By replacing CH2Cl2 with other solvents hav-
ing higher boiling points and running the corresponding re-
actions at their reflux temperatures, the product yields
dropped dramatically due to the instability of 2-benzylben-
zoyl chloride at high temperatures (entries 9–11, see also
the Supporting Information). The substrate scope of the re-
action system was explored following the optimized condi-
tions shown in entry 8 of Table 1. Products 3b, 3c, 3d, 3e, 3f,
and 3g were obtained in a yield of 70, 69, 85, 80, 81, and
84% from 1b, 1c, 1d, 1e, 1f, and 1g, respectively (Table 2, en-
tries 2–7), which proved the applicability of the tandem re-
action in the syntheses of bulky anthracenyl esters from
various 2-benzylbenzoic acids.
On the basis of our experimental results, we propose
that the formation of 3a involved a Friedel–Crafts acylation,
enolization, and esterification via three steps in one pot
(Scheme 2). 2-Benzylbenzoyl chloride was quickly generat-
ed from 1a in the presence of TCT and pyridine, and then
reacted with AlCl3 to form 2a via the intramolecular Frie-
del–Crafts acylation. Afterwards, enolization of 2a gave an-
thracen-9-ol, which further reacted with 2-benzylbenzoyl
chloride to afford the final product 3a. We hypothesized
that the Friedel–Crafts acylation was the rate-
determining step in the tandem reaction. To verify the hy-
pothesis, a competitive experiment was carried out by add-
ing 1-naphthol in the reaction system. In the presence of 1-
naphthol, the esterification product of 1-naphthol was the
main product instead of 3a, indicating that the esterifica-
tion reaction was a faster reaction than the Friedel–Crafts
acylation (see Supporting Information).
In summary, we have developed a three-step tandem
reaction involving intramolecular Friedel–Crafts acylation,
enolization, and esterification. Bulky anthracenyl esters
could be simply approached by this efficient reaction from
2-benzylbenzoic acids. In addition, a reasonable mecha-
nism is proposed. To the best of our knowledge, this is a
novel three-step tandem reaction and can be used for effi-
cient syntheses of anthracene derivatives.
Table 2 Scope of the Tandem Reaction
R1
O
OH
R2
O
O
R1
1. TCT, pyridine, CH2Cl2
2. AlCl3, reflux, 1 h
R2
R2
R1
1
3
Entry
Product
R1
R2
Yield (%)a
1
2
3
4
5
6
7
3a
3b
3c
3d
3e
3f
H
H
81
70
69
85
80
81
84
H
Me
Cl
H
H
OMe
H
Me
Me
OMe
OMe
H
3g
a Isolated yields.
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Y. Wang et al.
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Synthesis
3a
O
COOH
Cl
Yellow solid; yield: 157 mg (81%); mp 133.1–133.9 °C.
IR (KBr): 1739.25, 1240.68, 1127.22, 1072.92, 731.83 cm–1
1H NMR (400 MHz, CDCl3): δ = 8.64 (dd, J = 7.8, 1.3 Hz, 1 H), 8.43 (d, J =
11.6 Hz, 1 H), 8.05 (d, J = 8.5 Hz, 2 H), 7.84–7.77 (m, 2 H), 7.67 (m, 1
H), 7.58–7.38 (m, 6 H), 7.32–7.23 (m, 3 H), 7.22–7.16 (m, 2 H), 4.59 (s,
2 H).
13C NMR (100 MHz, CDCl3): δ = 165.72, 144.16, 142.16, 140.84,
133.28, 132.38, 131.90, 131.85, 129.07, 128.47, 128.38, 128.26,
126.84, 126.29, 126.07, 125.55, 124.76, 124.12, 121.47, 39.78.
O
N
Cl
pyridine
N
H
.
TCT
N
N
Ph
Ph
+
1a
Cl
O
Cl
N
Cl
N
AlCl3
N
H
O
N
Cl
Ph
2-benzylbenzoyl
chloride
HRMS (ESI): m/z [M + Na]+ calcd for C28H20O2Na: 411.1356; found:
411.1356.
O
O
Friedel–Crafts
acylation
enolization
pyridine
AlCl4
X-ray Crystal Structure Analysis
– H
Product 3a was recrystallized from EtOH to give yellow nubby crys-
tals stable in air. A single crystal suitable for X-ray analysis was select-
ed by hand. For details of X-ray crystal data, see the Supporting Infor-
mation.
slow reaction
2a
anthracen-9(10H)-one
O
Cl
OH
3b
O
O
Yellow solid; yield: 146 mg (70%); mp 105.0–106.1 °C.
IR (KBr): 1730.85, 1236.42, 1187.81, 1101.16, 1070.56, 729.56 cm–1
Ph
.
esterification
1H NMR (400 MHz, CDCl3): δ = 8.65 (dd, J = 7.8, 1.2 Hz, 1 H), 8.37 (s, 1
H), 8.03 (d, J = 8.4 Hz, 1 H), 7.96 (d, J = 8.7 Hz, 1 H), 7.79 (d, J = 8.8 Hz,
1 H), 7.70–7.62 (m, 2 H), 7.56–7.51 (m, 1 H), 7.49–7.42 (m, 2 H), 7.43–
7.37 (m, 1 H), 7.34 (dd, J = 8.7, 1.3 Hz, 1 H), 7.13–7.06 (m, 4 H), 4.56 (s,
2 H), 2.50 (s, 3 H), 2.35 (s, 3 H).
fast reaction
anthracen-9-ol
anthracen-9-yl
2-benzylbenzoate
3a
Scheme 2 Proposed mechanism of the tandem reaction
13C NMR (100 MHz, CDCl3): δ = 165.80, 144.53, 141.40, 137.77,
136.11, 135.43, 133.23, 132.24, 131.87, 131.37, 130.67, 129.15,
128.93, 128.50, 128.39, 128.27, 128.18, 126.71, 126.10, 125.11,
124.58, 124.39, 124.24, 121.42, 119.53, 39.30, 22.36, 21.07.
HRMS (ESI): m/z [M + Na]+ calcd for C30H24O2Na: 439.1669; found:
439.1664.
All commercial reagents were used as received without further puri-
fication unless otherwise stated. 1H NMR (400 MHz) and 13C NMR
(100 MHz) spectra were recorded in CDCl3 solution using a 400 MHz
spectrometer. Chemical shifts were reported in parts per million
(ppm) using CDCl3 (δH = 7.26, δC = 77.16) as internal standards. Stan-
dard abbreviations were used to indicate multiplicities of the signals.
High-resolution mass spectra (HRMS) were obtained by the ESI ion-
ization sources using TOF MS technique. Melting point was obtained
on a micro melting point apparatus. Substrates 1a and 1g are avail-
able from commercial sources. Substrates 1b,9 1c,10 1d,10 1e,11 and
1f11 were prepared according to literature procedures (for details, see
Supporting Information).
3c
Yellow solid; yield: 158 mg (69%); mp 130.2–131.4 °C.
IR (KBr): 1740.00, 1240.35, 1082.23, 1066.55, 1014.15, 750.19 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 8.64 (d, J = 7.8 Hz, 1 H), 8.39 (s, 1 H),
8.02 (dd, J = 16.6, 8.7 Hz, 2 H), 7.85 (s, 1 H), 7.69 (dd, J = 17.8, 8.4 Hz, 2
H), 7.58 (t, J = 7.6 Hz, 1 H), 7.54–7.49 (m, 1 H), 7.49–7.38 (m, 3 H), 7.23
(d, J = 8.3 Hz, 2 H), 7.10 (d, J = 8.3 Hz, 2 H), 4.52 (s, 2 H).
13C NMR (100 MHz, CDCl3): δ = 165.38, 143.86, 141.29, 139.19,
133.66, 132.46, 132.12, 131.99, 131.92, 130.24, 129.88, 128.57,
128.47, 127.67, 127.22, 126.97, 125.95, 125.02, 124.65, 124.40,
121.30, 119.93, 39.30.
HRMS (ESI) m/z [M + Na]+calcd for C28H18Cl2O2Na: 479.0576; found:
479.0576.
Bulky Anthracenyl Esters via Three-Step Tandem Reaction; Gener-
al Procedure
To a solution of 1 (1 mmol) in anhyd CH2Cl2 (5 mL) at r.t. was added
TCT (295 mg, 1.6 mmol), followed by the dropwise addition of pyri-
dine (95 mg, 1.2 mmol) within 5 min. After stirring the reaction mix-
ture at reflux temperature (40 °C) for 15 min, AlCl3 (267 mg, 2 mmol)
was added portionwise. The resulting reaction mixture was further
stirred at the reflux temperature for 1 h. Then, the mixture was
cooled to 0 °C and quenched with aq 1 M HCl (5 mL). The aqueous
layer was extracted with CH2Cl2. The organic layer was washed with
H2O and brine, and dried (anhyd Na2SO4). The solvent was removed
under reduced pressure and the resulting crude product was purified
by column chromatography (PE/EtOAc = 200:1) to afford the desired
product 3.
3d
Yellow solid; yield: 190 mg (85%); mp 125.8–126.7 °C.
IR (KBr): 3462.34, 1745.03, 1631.77, 1508.98, 1469.06, 1227.96,
1181.68, 1066.59, 1025.76 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 8.63 (d, J = 7.2 Hz, 1 H), 8.34 (s, 1 H),
8.02 (d, J = 7.5 Hz, 1 H), 7.95 (d, J = 9.2 Hz, 1 H), 7.73 (d, J = 8.0 Hz, 1 H),
7.66 (t, J = 7.0 Hz, 1 H), 7.53 (t, J = 7.5 Hz, 1 H), 7.50–7.37 (m, 3 H), 7.19
(dd, J = 9.2, 2.3 Hz, 1 H), 7.12 (d, J = 8.5 Hz, 2 H), 7.06 (d, J = 1.9 Hz, 1
H), 6.80 (d, J = 8.6 Hz, 2 H), 4.52 (s, 2 H), 3.80 (s, 6 H).
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–E

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Y. Wang et al.
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Synthesis
13C NMR (100 MHz, CDCl3): δ = 165.65, 157.97, 157.92, 144.74,
140.60, 133.20, 132.87, 132.21, 131.64, 130.42, 130.26, 130.03,
128.61, 128.46, 128.20, 126.72, 126.41, 125.12, 124.70, 124.64,
124.58, 120.98, 120.80, 113.81, 97.09, 55.21, 55.15, 38.81.
HRMS (ESI): m/z [M + H]+ calcd for C30H25O4: 449.1747; found:
449.1747.
Acknowledgment
The authors gratefully acknowledge financial support from the Stra-
tegic Priority Research Program of the Chinese Academy of Sciences
(XDB20000000) and the Natural Science Foundation of Fujian Prov-
ince, China (No. 2016J01086), the Key Laboratory of Coal to Ethylene
Glycol and Its Related Technology, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences. We thank Mingli
Liang and Wenwen Wang in FJIRSM for their help with the X-ray data
analysis.
3e
Yellow solid; yield: 166.5 mg (80%); mp 150.8–151.9 °C.
IR (KBr): 3053.22, 3027.03, 2920.46, 1746.22, 1247.19, 1229.32,
1132.62, 1070.69, 1012.34, 724.75 cm–1
.
Supporting Information
1H NMR (400 MHz, CDCl3): δ = 8.54 (d, J = 7.8 Hz, 1 H), 8.29 (s, 1 H),
8.01 (d, J = 8.4 Hz, 1 H), 7.79 (s, 2 H), 7.72 (d, J = 8.8 Hz, 1 H), 7.46 (t, J =
7.3 Hz, 1 H), 7.36 (dd, J = 19.7, 8.1 Hz, 2 H), 7.27 (m, 5 H), 7.20 (d, J =
7.5 Hz, 2 H), 4.56 (s, 2 H), 2.56 (s, 3 H), 2.50 (s, 3 H).
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0036-1588384.
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13C NMR (100 MHz, CDCl3): δ = 165.68, 144.18, 144.03, 142.25,
140.99, 135.14, 133.12, 132.28, 132.08, 132.05, 129.09, 129.00,
128.39, 128.31, 127.17, 126.51, 125.97, 125.75, 125.40, 125.35,
123.67, 123.56, 122.81, 121.54, 121.39, 39.72, 21.92, 21.74.
HRMS (ESI): m/z [M + Na]+ calcd for C30H24O2Na: 439.1669; found:
439.1669.
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3f
Yellow solid; yield: 193 mg (81%); mp 148.1–149.5 °C.
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1H NMR (400 MHz, CDCl3): δ = 8.53 (d, J = 7.9 Hz, 1 H), 8.22 (s, 1 H),
7.91 (d, J = 9.2 Hz, 1 H), 7.76 (s, 1 H), 7.64 (d, J = 8.7 Hz, 1 H), 7.33 (d,
J = 7.9 Hz, 1 H), 7.26 (t, J = 9.2 Hz, 2 H), 7.15 (dd, J = 16.2, 5.4 Hz, 3 H),
7.04 (s, 1 H), 6.80 (d, J = 8.5 Hz, 2 H), 4.49 (s, 2 H), 3.79 (d, J = 5.2 Hz, 6
H), 2.54 (s, 3 H), 2.49 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 165.60, 157.83, 157.58, 144.82,
143.98, 140.74, 134.12, 133.06, 132.98, 131.88, 130.79, 130.11,
129.98, 129.18, 128.77, 127.46, 126.65, 125.25, 124.57, 123.57,
123.23, 120.92, 120.60, 113.23, 97.18, 55.18, 55.11, 38.77, 21.80,
21.73.
HRMS (ESI): m/z [M + Na]+ calcd for C32H28O4Na: 499.1880; found:
499.1880.
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3g
Yellow solid; yield: 188 mg (84%); mp 165.2–166.6 °C.
IR (KBr): 1736.73, 1635.45, 1602.16, 1236.50, 1218.48, 1127.61,
1080.72 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 8.63 (d, J = 8.7 Hz, 1 H), 8.24 (s, 1 H),
7.98 (d, J = 8.5 Hz, 1 H), 7.81 (d, J = 8.6 Hz, 1 H), 7.69 (d, J = 9.4 Hz, 1 H),
7.46 (t, J = 7.4 Hz, 1 H), 7.42–7.34 (m, 1 H), 7.34–7.16 (m, 6 H), 7.11 (d,
J = 9.3 Hz, 1 H), 7.01 (d, J = 8.6 Hz, 1 H), 6.94 (s, 1 H), 4.57 (s, 2 H), 3.98
(s, 3 H), 3.93 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 165.16, 163.35, 157.26, 147.01,
142.63, 140.60, 134.36, 133.13, 132.61, 129.03, 128.44, 127.78,
126.08, 125.67, 125.19, 123.41, 122.93, 122.44, 121.67, 121.32,
120.80, 120.16, 118.00, 111.56, 103.75, 55.50, 55.32, 40.14.
HRMS (ESI): m/z [M + H]+ calcd for C30H25O4: 449.1747; found:
449.1747.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–E