48
X. Hu et al. / Journal of Molecular Catalysis A: Chemical 342–343 (2011) 41–49
OH
O
O
O
O
OH
Br
OH
[Et3NH][OAc]
LFe(IV)=O
Br
and
Fe(III)
LFe(III), L=OAc-
BrCCl3
and
H2O2
LFe(III)-OOH
LFe(IV)=O
Scheme 2. Mechanism of PhBrOH formation from phenoxy radical.
reported that the donation of electron density to the Fe(III) center
by ligand promoted homolytic cleavage of the O–O bond [55,56].
Scheme 3. Mechanism for hydroxylation of benzene in [Et3NH][OAc] system.
3.6. Reaction mechanism for hydroxylation of benzene
4. Conclusions
In order to examine whether the reaction mechanism for
hydroxylation of benzene included a hydrogen atom abstraction
step [57], BrCCl3, a famous carbon radical scavenger, was added
to detect the intermediate of phenyl radical. In the products, PhBr
was never detected, while large amount of PhBrOH was obtained
with a sharp decrease of the yield of phenol. The absence of PhBr
indicated that there was no phenyl radical formed, thus a hydro-
gen abstraction mechanism for hydroxylation of benzene by the
iron(IV)-oxo species could not work. In the reported studies, a
hydrogen abstraction mechanism was ruled out by a low kinetic
isotope effect (KIE) value obtained in the hydroxylation of aromatic
compounds by non-heme iron(IV)–O complexes, and a large nega-
tive Hammett ꢁ implied that the aromatic ring oxidation occurred
via electrophilic attack on the aromatic ring [58]. Thus, in our sys-
tem, it was suggested that the hydroxylation of benzene should also
proceed via electrophilic attack on the aromatic ring, but not via a
hydrogen atom abstraction mechanism. The electrophilic nature of
the corresponding iron(IV)-oxo species is still under investigation.
When phenol was used for hydroxylation reaction in the pres-
ence of BrCCl3, most of phenol was also transformed to PhBrOH.
Thus, it was deduced that the produced phenol during hydroxy-
lation of benzene could be further transformed to PhBrOH in the
presence of BrCCl3. The formation of PhBrOH implied the gener-
ation of the phenoxy radical. As shown in Scheme 2, the carbon
radical of the resonance structure of the phenoxy radical [59] is
mon intermediate in the oxidation of phenol by oxidant [59], and
tion of H from O–H of phenol by iron(IV)-oxo species, as it had
been proposed in the literature [60]. There was also a quite ordi-
nary reaction path for the hydroxylation of phenol by hydroxyl
radical to form di-phenol [59,61,62]. In our experiment, as shown
in Table 1, there were little amounts of di-phenols produced in
nol seldom occurred. On the contrary, di-phenols were the main
products in aqueous solution. In addition, the amount of di-phenols
increased when excessive water was added to [Et3NH][OAc] sys-
tem, as shown in Fig. 1. The results again proved the formation of
hydroxyl radical was enhanced by introduction of excessive water.
Although further oxidation of the produced phenol by the oxidizing
species also occurred in the [Et3NH][OAc] system, the reaction was
much prohibited than that in water. Hydrogen-bonding interac-
tion between the phenol (hydrogen-bonding donor, HBD) and the
acetate anion (hydrogen-bonding accepter, HBA) should be respon-
sible for it. As it was reported by MacFaul [63], the hydrogen-bond
role between PhOH and HBA could decline the rate constant for
species. Therefore, it was deduced that this hydrogen-bonding role
protected phenol from its further oxidation. The reaction mecha-
nism of the hydroxylation of benzene by Fenton reagent, as well
as the possible further oxidation in [Et3NH][OAc] system was then
outlined (Scheme 3).
Hydroxyl radical, the widely accepted oxidizing species in an
aqueous Fenton system, was not the main oxidizing species in
[Et3NH][OAc] system because of the reduction of the redox poten-
tial of the Fe(III)/Fe(II) couple. With the addition of excessive
water to [Et3NH][OAc] medium, the reductive ability of Fe(II) was
decreased, leading to an enhanced production of hydroxyl radical.
That is why over-oxidation of benzene was promoted by water. It
was revealed that the main oxidizing active species was the high-
valent iron(IV)-oxo species formed from the O–O bond homolysis
of a Fe(III)–OOH intermediate. The mechanism for hydroxylation
of benzene by the corresponding iron(IV) oxo species was mostly
via the electrophilic attack on benzene ring, rather than via hydro-
gen abstraction to form phenyl radical. Further oxidation of phenol
through H-abstraction from O–H of phenol by the iron(IV)-oxo
species was partly prohibited by the hydrogen-bond interaction
between phenol and acetate anion. Considering the diversity of ILs,
tuning the reaction mechanism for better efficiency and selectivity
could be greatly expected.
The financial support from the National Natural Science Foun-
dation of China (Nos. 20502017, 21021001, 20872102), PCSIRT
(IRT0846) and characterization of the catalyst from Analytical and
Testing Center of Sichuan University are greatly appreciated.
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