Angewandte
Communications
Chemie
À1
18
Table 1: Product analysis in oxidation of benzene and its derivatives
catalyzed by 1.
isotope shifts were 43 and 53 cm when H2 O was used (see
2
Figure S9). These values are similar to bands at 827 (44) and
À1
[
a]
[b]
[d]
[e]
847 (55) cm attributed to OÀO stretching bands for the
Entry
Substrate
Products [%]
TOF
TON
[
c]
À1
[15,16]
(o/p/m)
(h
)
Cu O2 and CuO H complexes, respectively.
Thus, the
2
2
data confirm that the second intermediate contains a peroxo
ligand and we hypothesize that it is a dihydroperoxodicopper-
1
2
3
Benzene
Benzene
Toluene
Phenol [95.2],
p-benzoquinone [4.8]
Phenol [93.2],
p-Benzoquinone [6.8]
Cresol [72]
1010
540
12550
6250
(II) (CuO H)2 complex, which is in equilibrium with the
2
Cu O complex and H O .
2
2
2
2
380
4320
Here, we propose a mechanism (Scheme 1) for the H O
2
2
(57:43:trace)
activation and benzene hydroxylation catalyzed by 1 and it is
based on the relative reactivity and regioselectivity in
oxidation of substituted benzenes, the spectroscopic studies
of reaction intermediates, and DFT calculations. The reaction
pathway is estimated by DFT calculations, and we confirmed
the interaction between the CuÀO moiety of the active
Benzaldehyde [26]
p-Benzoquinone [99]
Nitrophenol [99]
[
f]
4
5
Phenol
Nitrobenzene
350
72
1630
860
(91:4:5)
[
3
a] Reaction conditions: entry 1: benzene (60 mmol), H O (10 mL of
0% aqueous H O , 120 mmol), 1 (1.0 mmol), and TEA (5.0 mmol) in
2 2
MeCN (20 mL) at 508C under N . Entries 2, 3, 4, 5: 30 mmol of substrate
was added under the same reaction conditions as entry 1. [b] Product
yield based on substrate consumed after 40 h for Entries 1–3 and 5, and
after 10 h for Entry 4. [c] Product ratio of hydroxylation at ortho-, meta-,
and para-position in cresol and nitrophenol. [d] Turnover frequency of all
product after 1 h. [e] Turnover number of all product after 40 h.
2
2
species and benzene in the transition state. The details are
shown in Figures S11 and S12. The (CuO H) complex is
2
2
2
shown to be a precursor of the active species with an
electrophilic radical character shown in the oxidation of
substituted benzenes. The (CuO H) complex releases H O
2
2
2
reversibly with the assistant of H O to give a copper-bound
2
[
f] Turnover number of all product after 10 h.
oxyl and peroxyl radical which is stabilized by hydrogen-
bonding interactions with H O. Here, it reacts with benzene in
2
II
to Cu as shown in the crystal structure of 3, thus explaining
the relatively low TON. Selective p-benzoquinone production
in this reaction system suggests that the oxidation occurred at
the para-position of a copper-bonded phenoxide, which is
more reactive because of the radical character than free
phenol, where ortho-positions did not react as a result of the
the rate-limiting step. This reactivity is consistent with kinetic
results for the phenol production. It is also not a radical-chain
reaction as discussed previously. The active species may then
react with DMPO to undergo decomposition, but does not
form a stable radical, such as a CO H-adduct, and this may be
2
the reason why DMPO did not inhibit phenol production. The
present mechanism is one of many possible pathways, and
further studies are needed for clarification.
[
13]
steric hindrance of the 6-hpa. The relative reactivity order
estimated from the initial rate (TOF, see Table 1) is phenol ꢀ
toluene @ nitrobenzene, and increases with increasing
electron density of the aromatic ring, thus indicating an
electrophilic character of the active species. The regioselec-
tivity is ortho and para for toluene, ortho for nitrobenzene,
and para for phenol, therefore indicating a radical character
of the active species. Thus, based on these results, the active
species may have an electrophilic radical character such as
a metal-bonded oxyl radical, which was previously proposed
as the active species in the reactions of Co, Ni, Cu, and Ru
In a mechanism reported for benzene hydroxylation
[
6]
catalyzed by 2, it was shown that CO H was generated as
2
an active oxygen species by OÀO bond scission of the CuO H
2
[17]
species, a step which is energetically unfavorable.
In
contrast, in the case of 1, the active species is formed by
a more energetically favorable intramolecular dehydration of
the (CuO H) complex as shown by DFT calculations, where
2
2
the two CuO H moieties are encapsulated by 6-hpa. There-
2
fore, it is concluded that the dinuclear structure of 1 is
favorable for the formation of the active species to specifically
enhance the catalytic activity.
[
14]
complexes with various oxidants.
Two intermediates were spectroscopically detected in the
reaction of 1 with H O2 and provide insight into the
In summary, a new dicopper complex with the dinucleat-
ing ligand 6-hpa, which specifically stabilizes a dinuclear
2
mechanism of H O activation. When 1 equivalent of H O
2
2
2
2
was added to 1 in the presence of Et N in MeCN at À408C,
3
clear bands, which are typical of an end-on trans-
peroxodicopper(II) (Cu O ) complex, appeared at l =
2
2
[
15]
5
20 nm, and decayed with a half-life of 1200 s as shown in
Figure S7. In contrast, upon addition of excess amounts of
H O , the Cu O complex that once formed rapidly decayed
2
2
2
2
to give a new species that showed a band at l = 380 nm, close
to the band at l = 379 nm for the hydroperoxocopper(II)
[
16]
(
CuO H) complex.
The Cu O2 complex decayed with
2
2
a half-life of 10 seconds in the presence of 100 equivalents
of H O2 at À408C. The reaction rate showed saturation
2
kinetics with an increase of concentration of H O , as shown
2
2
in Figure S8. The resonance Raman spectra of the Cu O2
2
complex and the as-prepared new species showed clear bands
at 821 and 846 cm , respectively, and the corresponding
Scheme 1. Proposed mechanism of H O activation and benzene
hydroxylation catalyzed by 1.
2
2
À1
Angew. Chem. Int. Ed. 2017, 56, 1 – 5
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3
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