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9,10-anthraquinone 4 was monitored by ESI-MS. Anthrone
1 formed during the reaction (192.8 [MꢁH]ꢁ and 194.9
[M+H]+) was found when the reaction lasted for 30 min
and 2 h, but disappeared after 6 h of reaction time.
5.2.2. 2-Methylanthracene (2b). 1H NMR (CDCl3,
500 MHz): d 8.36 (s, 1H), 8.31 (s, 1H), 7.90–7.99 (m, 3H),
7.75 (s, 1H), 7.42–7.44 (m, 2H), 7.32–7.35 (m, 1H), 2.55
(t, 3H). 13C NMR (CDCl3, 125 MHz): d 135.1, 132.1,
132.0, 131.4, 130.5, 128.4, 128.3, 128.1, 126.5, 126.1,
125.4, 125.3, 125.1, 22.1. HRMS (EI) calcd for C15H12:
192.0934 (M+), found: 192.0931.
Finally, the mechanism of InCl3-catalyzed reduction of
1,4-disubstituted anthraquinones (R¼H, C2H5) (Scheme 4)
was proposed (Scheme 7). Firstly, compound 4 was
hydrolyzed to 1,4-dihydroxy-9,10-anthraquinone 7. Then
compound 7 was reduced to intermediate 8 by two possible
routes. Finally, the product 5 was formed through the
intermediate 8.
5.2.3. 2-Ethylanthracene (2c). 1H NMR (CDCl3,
500 MHz): d 8.35 (s, 1H), 8.30 (s, 1H), 7.94–7.96 (m, 3H),
7.72 (s, 1H), 7.40–7.43 (m, 2H), 7.30–7.32 (m, 1H), 2.82
(q, 2H), 1.35 (t, 3H). 13C NMR (CDCl3, 125 MHz):
d 141.2, 132.2, 132.0, 131.4, 130.7, 128.3, 128.2, 127.4,
126.1, 125.5, 125.3, 125.1, 125.0, 29.3, 15.3. HRMS (EI)
calcd for C16H14: 206.1090 (M+), found: 206.1088.
4. Conclusion
5.2.4. 1,3-Dimethylanthracene (2d). 1H NMR (CDCl3,
500 MHz): d 8.46 (s, 1H), 8.28 (s, 1H), 7.94–8.02 (m, 2H),
7.60 (s, 1H), 7.41–7.44 (m, 2H), 7.14 (s, 1H), 2.84 (s, 3H),
2.56 (s, 3H). 13C NMR (CDCl3, 125 MHz): d 134.7, 134.1,
132.4, 131.7, 131.3, 130.2, 128.7, 128.7, 128.0, 125.0,
122.7, 22.0, 19.7. HRMS (EI) calcd for C16H14: 206.1090
(M+), found: 206.1090.
In conclusion, indium chloride, which was applied to the
reduction of anthrones and anthraquinone, showed good
catalytic activity. A simple, efficient, and environmentally
friendly synthetic method for anthracenes from anthrones
was proposed, which was also applicable for halogenated
anthrones. Although indium chloride catalyzed reductions
of most anthraquinones gave two products, compound 5
was obtained as the single product in this reaction. A reason-
able mechanism was proposed.
5.2.5. 1,4-Dimethylanthracene (2e). 1H NMR (CDCl3,
500 MHz): d 8.55 (s, 2H), 8.03–8.05 (m, 2H), 7.47–7.49
(m, 2H), 7.20 (s, 2H), 2.79 (s, 6H). 13C NMR (CDCl3,
125 MHz): d 136.4, 127.6, 127.2, 125.9, 125.0, 123.7,
121.5, 17.9. HRMS (EI) calcd for C16H14: 206.1090 (M+),
found: 206.1095.
5. Experimental
5.1. General
5.2.6. 2-Chloroanthracene (2f). 1H NMR (CDCl3,
500 MHz): d 8.39 (s, 1H), 8.31 (s, 1H), 7.92–7.99 (m, 4H),
7.46–7.50 (m, 2H), 7.36–7.38 (m, 1H). 13C NMR (CDCl3,
125 MHz): d 132.3, 131.9, 131.2, 130.0, 129.8, 128.4,
128.2, 126.7, 126.5, 126.2, 125.8, 125.5. HRMS (EI) calcd
for C14H9Cl: 212.0387 (M+), found: 212.0385.
1
The H NMR and 13C NMR spectra were obtained with
a Bruker AVANCE DRX-500 NMR spectrometer (1H,
500 MHz; 13C, 125 MHz) with TMS as internal standard
and CDCl3 as solvent. The ESI-MS spectra were taken on
a Bruker Esquire 3000 plus spectrometer. Precoated thin-
layer plates of silica gel 60 GF254 (Qingdao Haiyang Chem-
ical Co. Ltd., Qingdao, PR China.) were used for analytical
purpose. All the benzophenones, benzaldehydes, and aceto-
phenones were purchased from J&K chemical Co., or other
commercial suppliers and were used after appropriate puri-
fication (distillation).
5.2.7. 2-Bromoanthracene (2g). 1H NMR (CDCl3,
500 MHz): d 8.38 (s, 1H), 8.31 (s, 1H), 8.16 (s, 1H), 7.97–
8.01 (m, 1H), 7.45–7.50 (m, 3H). 13C NMR (CDCl3,
125 MHz): d 132.4, 132.3, 131.9, 130.0, 129.9, 129.0,
128.4, 128.3, 126.7, 126.2, 125.9, 125.5, 119.6. HRMS
(EI) calcd for C14H9Br: 255.9882 (M+), found: 255.9880.
5.2. General procedure for InCl3-catalyzed reduction
of anthrones by using Al powder
5.3. General procedure for InCl3-catalyzed reduction
of 9,10-anthraquinones by using Al powder
Anthrone (1 mmol), Indium chloride (0.2 mmol), Al powder
(1.5 mmol), and AcOH (1 mL) were added to 50% aqueous
alcohol solution. The mixture was stirred for the indicated
time at room temperature. Ethyl ether (10 mL) was added
to the mixture, stirred for another 5 min, and then filtered.
The filtrate was extracted by ethyl ether (10 mLꢂ3). The
combined organic layer was dried over anhydrous Na2SO4
and concentrated in vacuum. Purification by silica gel
column chromatography (200–300 mesh), using petroleum
ether (60–90 ꢀC) and ethyl acetate (200:1) as eluant,
afforded the corresponding product.
9,10-Anthraquinone (1 mmol), Indium chloride (1 mmol),
Al powder (indicated amount), and AcOH (1 mL) were
added to 50% aqueous alcohol solution. The mixture was
stirred for the indicated time under reflux. Ethyl ether
(10 mL) was added to the mixture, stirred for another
5 min, and then filtered. The filtrate was extracted by ethyl
ether (10 mLꢂ3). The combined organic layer was dried
over anhydrous Na2SO4 and concentrated in vacuum. Purifi-
cation by silica gel column chromatography (200–300
mesh), using petroleum ether (60–90 ꢀC) and ethyl acetate
(200:1) as eluant, afforded the corresponding product.
5.2.1. Anthracene (2a). 1H NMR (CDCl3, 500 MHz): d 8.43
(s, 2H), 8.00–8.02 (m, 4H), 7.46–7.48 (m, 4H). 13C NMR
(CDCl3, 125 MHz): d 132.2, 128.7, 126.6, 125.9. HRMS
(EI) calcd for C14H10: 178.0777 (M+), found: 178.0782.
5.3.1. 9,10-Dihydroxy-2,3-dihydroanthracene-1,4-dione
(5). H NMR (CDCl3, 500 MHz): d 13.5 (s, 2H), 8.36–
1
8.38 (m, 2H), 7.72–7.74 (m, 2H), 3.04 (s, 4H). 13C NMR