132
N.V. Tkachenko et al. / Journal of Organometallic Chemistry 871 (2018) 130e134
Table 2
2 2
Catalytic oxidation of aromatic substrates with H O
in the presence of complex 2.a
Entry
Substrate
Conversion (%)
Aliphatic oxidationb
Aromatic oxidationb
Aromatic/aliphaticb
Selectivity for aromatic oxidation (%)c
TNd
1
2
3
4
5
6
7
8
9
benzene
toluene
ethylbenzene
cumene
isobutylbenzene
o-xylene
m-xylene
p-xylene
2-ethyltoluene
3-ethyltoluene
4-ethyltoluene
13.8
28.8
19.5
13.9
19.7
35.8
98.3
43.5
34.0
24.8
45.5
e
11.1
18.2
11.1
9.2
12.1
20.4
51.6
27.1
15.2
14.6
30.3
e
e
22.0
43.9
29.6
19.7
29.7
56.3
151.9
65.3
44.5
36.4
67.7
0.3
0.8
0.6
0.1
0.1
e
4.7
3.8
1.1
3.3
8.4
26.2
6.4
7.3
3.8
2.6
99
95
94
99
99
>99
98
98
93
93
0.6
0.4
1.3
2.5
1
1
0
1
a
ꢁ
Conditions: 0 C; substrate (0.10 mmol), H
2
O
2
(0.40 mmol), EHA (1.0 mmol), catalyst (1.24
mmol), CH
3
CN (0.40 mL), syringe-pump addition of H
2
O
2
during 30 min, fol-
lowed by 2.5 h stirring.
b
Yields, moles of products/mole of Fe. Detailed compositions of the reaction mixtures are provided in Table S2, Supplementary data.
c
Calculated as 100%∙{“aromatic oxidation products” þ “aromatic/aliphatic oxidation products”}/overall amount of identified oxidation products.
d
TN defined as {moles of single oxidation products/mole of Fe}þ2{moles of double oxidation products/mole of Fe}. Ketones and aldehydes were considered as double
oxidation products.
system 2/AcOH/H
2
O
2
[20]; the substrate conversions demonstrated
system were either higher or similar to those by
system.
cyclohexane and substituted benzenes was evaluated [19,30].
by the 2/EHA/H
the 2/AcOH/H
2
O
2
Herein, we have evaluated the reactivity of intermediate 2aEHA,
2
O
2
generated in the catalyst system 2/EHA/H
at ꢀ80 C. As was shown previously, the catalyst systems 1/AA/
2
O
2
, toward benzene
ꢁ
CH
3
CO
and 1a
3
H and 1/EHA/CH
3
CO
3
H display maximum concentration of
3.2. EPR spectroscopic study of the iron-oxo intermediates
AA
EHA
ꢁ
1a
just after mixing the reagents at ꢀ70 C, and then
their concentration decreases [19]. In contrast, the concentration of
In order to get the insight into the reason of the high aromatic
oxidation selectivity of the catalyst system 2/EHA/H , formation
of the high-valent iron intermediates in this system was monitored
by EPR spectroscopy at low temperature.
EHA
2a
formed in system 2/EHA/H
O
2 2
is quasi-stationary during
2
O
2
ꢁ
5
e10 min after the reaction onset at ꢀ80 C, and then decreases
ꢀ3 ꢀ1
(Fig. 3A). The first order rate constant k
1
¼ (4 ± 1) ꢂ 10
s
of the
EHA
intermediate 2a
self-decay can be determined from the decrease
2 2
Previously, we showed that the catalyst systems 2/AA/H O and
of its concentration with time after the quasi-stationary period
Fig. 3C). In the presence of benzene (0.04 M), the quasi-stationary
AA
EHA
2
/EHA/H
2
O
2
, display highly unstable intermediates 2a and 2a
¼ 2.66, g ¼ 2.42,
(
(Table 3) with strictly different EPR spectra g
1
2
EHA
concentration of 2a
was 7 times lower (Fig. 3B and C). These
for
with benzene at ꢀ80 C from the equation
g
3
¼ 1.71 and g
1
¼ 2.069, g
2
¼ 2.007, g ¼ 1.963, respectively [7].
3
data allowed the evaluation of the second-order rate constant k
2
These intermediates are highly unstable and can be generated and
EHA
ꢁ
the reaction of 2a
þ k ∙[C ])/k
ꢁ
monitored only at temperatures as low as ꢀ80 … ꢀ70 C. The in-
ꢀ1 ꢀ1
s is at least
(k
1
2
6
H
6
1
¼7. The resulting k
2
¼ 0.6 ± 0.2 M
EHA
termediate 2a
was assigned to the oxoiron(V) species on the
2
one order of magnitude lower than the corresponding value of k >
basis of the close similarity of its EPR spectrum to that of well
spectroscopically characterized oxoiron(V) species [7,27,28],
whereas the structure of intermediate 2a remains debatable [29].
ꢀ1 ꢀ1
EHA
1
0 M
s
, evaluated for the reaction of 1a
with benzene
ꢁ
at ꢀ70 C [19].
AA
EHA
More electron-rich substrate, toluene, reacted with 2a
more
The catalyst systems 1/AA/H
2
O
2
and 1/EHA/H
2
O
2
, exhibit in-
readily, so that in the presence of toluene (0.04), the concentration
AA
EHA
ꢁ
termediates 1a and 1a
at ꢀ80 … ꢀ70 C with virtually iden-
EHA
of 2a
was below the detection limit. This indicates that the
tical EPR spectra (g
1
¼ 2.07, g
2
¼ 2.007, g
3
¼ 1.96), nearly coinciding
ꢀ1 ꢀ1
corresponding value of k
2
is noticeably larger than 1 M
s .
EHA
with that of 2a
(Table 3). The same intermediates are formed in
AA
Intriguingly, the self-decay of the intermediate 2a with large
the catalyst systems 1/AA/CH
However, in the systems with CH
tration is several times higher than in the systems with H O
2 2
3
CO
3
H and 1/EHA/CH CO H [9,19].
3
3
g-factor anisotropy in system 2/AA/H
2
O
2
did not accelerate in the
3
CO H their maximum concen-
. Very
3
ꢁ
presence of either benzene (0.04 M) at ꢀ80 C or more electron-
rich pseudocumene (0.04 M). This apparent contradiction to the
observation of direct reactivity of the intermediates with large g-
factor anisotropy in closely structurally related catalyst systems
toward organic substrates [31,32] is at the moment not well un-
derstood and requires further studies. Presumably, the different
AA
EHA
recently, the direct reactivity of intermediates 1a
formed in the catalyst systems 1/RCOOH/CH CO
and 1a
(
3
3
H) toward
Table 3
EPR spectroscopic data (CH
EHA
AA
3
CN/CH
2
Cl
2
, 77 K) for S ¼ 1/2 aminopyridine iron-oxo
reactivities of the key iron-oxygen species, 2a
and 2a may be
intermediates observed in the catalyst systems 1/RCOOH/H
RCOOH ¼ AA or EHA).
2
O
2
and 2/RCOOH/H O
2 2
connected with the different aromatic oxidation selectivities of the
corresponding catalyst systems 2/EHA/H O and 2/AA/H O .
2 2 2 2
(
No
Intermediate
g
1
g
2
g
3
ref.
V
2þ
1
2
3
4
[(PDP*)Fe ¼O(OC(O)R)] (1aEHA
)
2.070
2.071
2.069
2.66
2.008
2.008
2.007
2.42
1.958
1.960
1.963
1.71
7
21
7
4. Conclusions
V
2þ
AA
[(PDP*)Fe ¼O(OC(O)CH
3
)] (1a
)
[(PDP)Fe ¼O(OC(O)R)] (2aEHA
V
2þ
)
Iron complex [(PDP)Fe(OTf)
matic hydroxylation of alkylarenes (substituted benzenes) with
2
] (2) efficiently catalyzes the aro-
AA
2a
5, 7