Q. Liu et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 71–77
77
Table 4
tive to activate the ring C–H bonds. The smaller steric hindrance
of 2,2ꢀ-bipyridine than 1,10-phenanthroline made bpV(bpy) the
most efficient catalyst for ring C–H bond activation of EB. The HOAc
molecule was involved not only in the ligand exchange around the
vanadium center, but also in the stabilization of the diperoxovana-
dium species in the media. Primary experimental results revealed
a polar intermediate mechanism by generating electrophilic cat-
alytic species. The bpV(bpy)–H2O2–HOAc system was found to be
an effective system for the preferential activation of the aromatic
ring C–H bond (S = 68.3%).
Effect of the addition of radical scavenger on the catalytic performance.a
Additive
Molar ratio of
additive to
substrate
Conversion of
ethylbenzene (%) activation (%)
Selectivity to ring
1/2
18.4
16.5
12.7
63.2
59.3
58.4
t-Butyl alcohol (TBA) 1/1
2/1
2,6-Di-t-butyl-4-
methylphenol
(BHT)
1/2
1/1
2/1
11.6
8.0
6.3
69.8
81.7
96.5
a
Reaction conditions: bpV(bpy), 0.20 mmol; EB, 1.2 mL; H2O2, 1.0 mL (dropped
in five runs); glacial HOAc, 15 mL; 50 ◦C; 4 h.
Acknowledgements
The financial support from the National Natural Science Foun-
dation of China (No. 20901053), PCSIRT (No. IRT0846), and
characterization of the catalyst from Analytic and Testing Center
of Sichuan University are greatly appreciated.
tert-butyl alcohol (TBA) or 2,6-di-tert-butyl-4-methylphenol (buty-
lated hydroxytoluene, BHT) were added into the reaction system
to scavenge the possible radical species. The results are shown in
Table 4.
A blank experiment without any additives revealed that the
selectivity to ring oxidation of EB over bpV(bpy) catalyst reached
68.3% under the optimal reaction condition (Table 3, in glacial
HOAc, 50 ◦C, 4 h). As expected, the conversion of EB was lowered
after the addition of TBA or BHT, suggesting that the reactive oxy-
gen species generated from H2O2 was consumed by TBA or BHT.
These reactive oxygen species included the free radicals such as
superoxide anion radicals (O2•−) or hydroxyl radicals (OH•), and
non-radical species such as H2O2. It was found that the selectivity
to ring oxidation was decreased when TBA, an effective hydroxyl
radical scavenger, was added. Because the presence of free OH• in
the oxidation mainly led to the preferential activation of the ben-
zylic C–H bond, the addition of TBA in the system would enhance
the selectivity to ring C–H bond activation if free OH• existed in
the system. The decreased selectivity to ring oxidation indicated
that OH• was rarely formed in the investigated reaction system. In
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4. Conclusions
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