asc.wiley-vch.de
NaHCO3 and then a buffer solution (20 mol/L of K2HPO4
and KH2PO4 in water) was added.
General Procedure for Oxidation of 1 or 2 under
Pressure of Air (30 atm)
The reaction was carried out in 50-mL stainless steel auto-
clave. An acetic acid solution (3 mL) of substrate (2 mmol),
NHPI
(5 mol %,
0.1 mmol),
Co(OAc)2
(0.5 mol %,
0.01 mmol), and Mn(OAc)2 (0.5 mol %, 0.01 mmol) was
placed in an autoclave. The autoclave was pressurized to 30
atm with air. The mixture was stirred at 150 °C for 3 h. The
work-up was performed as described above.
Figure 3. Time-dependence curves for the aerobic oxida-
tion of 1 catalyzed by NAPI. Conditions: 1 (2 mmol) was al-
lowed to react with O2 (1 atm) in the presence of NAPI (10
mol %) and Co(OAc)2 (0.5 mol %) in acetic acid (2 mL) at
100 °C.
Procedure for the Oxidation of o-Xylene under
Pressure of Air (30 atm)
After the oxidation of o-xylene as described above, the sol-
vent was evaporated under reduced pressure. Then the re-
action mixture was treated with NaOH (15 mmol) in H2O
(30 mL) at 90 °C overnight in order to hydrolyse phthalic an-
hydride, which was formed as a by-product, giving phthalic
acid isolated in 73% yield.
In conclusion, a new strategy for the aerobic oxida-
tion of p-xylene to terephthalic acid has been exam-
ined by the use of a combined catalyst system of NHPI
(or NAPI) with Co(OAc)2 and Mn(OAc)2. The present
method provides the direct oxidation of 1 to 3 and
two-step production to 3 through 2 from 1. These oxi-
dations could be carried out under relatively mild
conditions without any halogen ion.
Acknowledgements
This work was partially supported by Research for the Future
program JSPS and DAICEL Chemical Industries, Ltd.
Experimental Section
General
References and Notes
All starting materials and catalysts were purchased from
commercial sources and used without further treatment.
The yield of 3 was isolated yield from the reaction mixtures.
The yields of 2 and 5 were estimated from the peak areas
based on the internal standard technique by using HPLC.
GC analysis was performed with a flame ionization detector
using a 0.2 mm ´ 25 m capillary column (OV-1). 1H and 13C
NMR spectra were measured at 270 MHz and 67.5 MHz, re-
spectively, in CDCl3 with Me4Si as the internal standard.
GC-MS were obtained at an ionization energy of 70 eV. All
products were identified by comparison of the isolated pro-
ducts with authentic samples.
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General Procedure for Oxidation of 1 or 2 under
Dioxygen Atmosphere
An acetic acid solution (5 mL) of substrate (2 mmol), NHPI
(20 mol %, 0.4 mmol), Co(OAc)2 (0.5 mol %, 0.01 mmol),
and Mn(OAc)2 (0.5 mol %, 0.01 mmol) was placed in a
three-necked flask equipped with a balloon filled with O2.
Caution: A balloon filled with oxygen can be hazardous, even
with a small-scale reaction. The mixture was stirred at 100 °C
for 14 h. After the reaction, a white precipitate including
terephthalic acid (3) was formed. The solid was filtered off
under vacuum, and washed with acetic acid several times
to give almost pure terephthalic acid. For analysis by HPLC,
the reaction mixture was added to ca. 150 mL of aqueous
224
Adv. Synth. Catal. 2001, 343, 220±225