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T. Lu et al., N-hydroxyphthalimide-catalyzed Aerobic Selective Oxidation of Alkylaromatics in PEG1000–DAIL
Figure 1. Time-dependence curve for the aerobic oxidation of
toluene(a)
(a) Reaction condition: 0.5 mol toluene, 0.5 mol % Co(OAc)2,
10 mol % NHPI, 2.8 × 10−4 mol PEG1000 −DAIL, 80 °C.
Figure 2. Temperature-dependence curve for the aerobic
oxidation of toluene(a)
(a)
Reaction condition:0.5 mol toluene, 0.5 mol% Co(OAc)2,
Co(OAc)2 (0.5 mol %) gave benzyl alcohol (BOL),
benzoic acid (BAC) in 0.3 % and 99.5 % selectivity,
respectively, at 99.9 % conversion (Table 1, Run 1).
This shows that toluene is completely oxidized with O2
by NHPI to give selectively BAC. When the amount of
NHPI was halved, the conversion was sharply decreased
and a considerable amount of BOL and benzaldehyde
(BAl) was formed (Run 2). Removing Co(OAc)2 from
the oxidation system resulted in lower conversion with
the concomitant gradual increase of BAl and the de-
crease of BAC in the selectivities (Run 3). However, no
oxidation was induced when PEG1000 –DAIL was used
alone (Run 4). Substitution of Co(OAc)2 by Co(acac)2
(Ref. 15) or MnO2 (Ref. 3) led to a slightly lower con-
version with comparable selectivity toward the for-
mation of BAC (Run 5 and 6). When both Co(OAc)2 and
Mn(OAc)2 were used,15 the conversion increased but the
selectivity for BAC was slightly decreased (Run 7).
In the previous papers, it was shown that some
10 mol% NHPI, 2.8 × 10−4 mol PEG1000 −DAIL, 10 h.
most appropriate solvent (Run 12), the employment of
PEG1000 –DAIL led to a higher conversion and selectivi-
ty of BAC and lower selectivity of side product BAl
(Run 1). It was indicated PEG1000 –DAIL increased the
efficient catalytic ability of NHPI. In presence of ace-
tonitrile and dichloromethane, the conversions and
selectivities of BAC were relatively low (Run 13 and
14). Traditional ionic liquids [Hex-mim]BF4 and
11
[Bmim]PF6 were also tested, however, poor conver-
sions were observed (Run 15 and 16). The reaction in
PEG1000-DIL (PEG 1000-based dicationic ionic liquid)
resulted in slightly lower conversion and selectivity due
to its lack of acidity (Run 17).
In order to obtain information on the reaction
course of the present reaction, the oxidation of toluene
with O2 (1 atm) under the influence of NHPI and
Co(OAc)2 was monitored as a function of time and
temperature (Figure 1 and 2).
non-metallic compounds such as azobisisobutyronitrile
(ABIN)16 or HNO3 also can abstract the hydrogen
12
atom from the hydroxyimide moiety of the NHPI to
generate phthalimide N-oxyl (PINO) which abstracts the
hydrogen atom from the methyl moiety of toluene.
Thus, the oxidation of toluene was examined by non-
metallic radical initiator. Unsatisfactorily, when
dimethylglyoxime6 and anthraquinone17,18 were em-
ployed, the conversion with selectivity evidently de-
clined (Run 8 and 9). And in the presence of the ABIN
(Run 10), the reaction proceeded in much low conver-
sion to result in lots of undesired products (BAl). To our
disappointment, the conversion reached only 14.0 %
and no benzoic acid appeared when HNO3 was used
instead (Run 11).
The changes of product distribution of toluene oxi-
dation with time are shown in Figure 1. As illustrated,
the conversion of toluene increased with prolonging the
reaction time and the selectivity of BOL increased rapid-
ly in the initial 2.0 h. In the same period of time, the
selectivity of BAC increased slowly. With the further
increase of time, the selectivity of BAC continuously in-
creased, but the selectivity of BOL decreased due to the
continuous oxidation of BOL to BAC. However, after 10
h the conversion was changed slowly and became stag-
nated near 99.9 %. As a result, in a 10 h reaction course,
99.9 % toluene was oxidized with 99.5 % selectivity of
BAC. The change of the selectivity of BAl was similar
to BOL except the concentrate was always low.
Subsequently, several solvents were evaluated. In
contrast to the traditional NHPI/Co(OAc)2 catalyzed
aerobic oxidation of toluene where acetic acid8 was the
The effect of temperature on oxidation of toluene
was investigated and shown in Figure 2. Since the solu-
Croat. Chem. Acta 85 (2012) 277.