NHPI and ferric nitrate: A mild and selective system for aerobic oxidation of benzylic methylenes

Article abstract of DOI:10.1039/c5cy01245g

A mild and selective system comprising N-hydroxyphthalimide (NHPI) and Fe(NO₃)₃·9H₂O was developed for the oxidation of benzylic methylenes with an atmospheric pressure of molecular oxygen at 25°C. The influences of reaction conditions such as solvent, different metal catalysts and catalyst loading were studied, as well as the kinetics of the oxidation reaction. Various benzylic methylene substrates could be oxidized to the corresponding carbonyl compounds in satisfactory yields with this catalytic system. Hammett analysis suggested that the substrates with electron-donating groups would have higher oxidation rates. Isotopic (¹⁸O) labeling experiments provided evidence of the participation of the nitrate anion in the catalytic cycle. In addition, a possible radical mechanism involving hydrogen atom abstraction by PINO (phthalimide-N-oxyl) and nitrate participation for the oxidation of benzylic methylenes in the Fe(NO₃)₃·9H₂O/NHPI/O₂ system was proposed.

Full text of DOI:10.1039/c5cy01245g

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NHPI and ferric nitrate: a mild and selective
system for aerobic oxidation of benzylic
Cite this: DOI: 10.1039/c5cy01245g
methylenes†
a
b
b
a
a
Chengxia Miao,* Hanqing Zhao, Quanyi Zhao, Chungu Xia and Wei Sun*
3 3 2
A mild and selective system comprising N-hydroxyphthalimide (NHPI) and FeIJNO ) ·9H O was developed
for the oxidation of benzylic methylenes with an atmospheric pressure of molecular oxygen at 25 °C. The
influences of reaction conditions such as solvent, different metal catalysts and catalyst loading were
studied, as well as the kinetics of the oxidation reaction. Various benzylic methylene substrates could be
oxidized to the corresponding carbonyl compounds in satisfactory yields with this catalytic system.
Hammett analysis suggested that the substrates with electron-donating groups would have higher oxida-
Received 2nd August 2015,
Accepted 23rd September 2015
18
tion rates. Isotopic ( O) labeling experiments provided evidence of the participation of the nitrate anion in
the catalytic cycle. In addition, a possible radical mechanism involving hydrogen atom abstraction by PINO
DOI: 10.1039/c5cy01245g
(
phthalimide-N-oxyl) and nitrate participation for the oxidation of benzylic methylenes in the FeIJNO
3 3
)
www.rsc.org/catalysis
·9H O/NHPI/O system was proposed.
2
2
precursor NHPI can be achieved by using various mediators,
including anthraquinone derivatives, 2,2′-azobis-
Introduction
The direct functionalization of C–H bonds has been a long-
lasting challenge in modern organic chemistry. Among the
isobutyronitrile (AIBN), oximes, aldehydes, HNO , quaternary
3
1
ammonium salts, bromine, transition metal salts and so on,
8
–14
techniques employed, oxidation of C–H bonds is one of the
with oxygen as the oxidant.
In spite of the impressive
2
most efficient methods to construct carbonyl compounds.
progress made in this area, oxidation using molecular oxygen
as the oxidant under mild conditions is still difficult and the
reaction usually occurs at high temperature to overcome the
high activation barriers. In addition, oxygen at higher temper-
ature, which exists as a diradical in the ground state, takes
part preferably in non-selective radical reactions. Thus, a low
temperature may be beneficial for the selectivity of the reac-
tion. However, there are only few reports on achieving the oxi-
dation of C–H bonds involving NHPI as the catalyst at low
temperature. Initially, Ishii and co-workers successfully
converted alkylbenzenes into the corresponding carboxylic
acids under atmospheric dioxygen at ambient temperature
Traditionally, a stoichiometric amount of a hazardous oxidant
such as manganese dioxide, potassium dichromate or chro-
mic acid is employed for these transformations, generally
generating large amounts of waste and unwanted by-prod-
3
ucts. Molecular oxygen is a convenient and green oxidant for
catalytic chemistry due to its economic and environmentally
4
benign features. However, O
2
is a relatively unreactive oxi-
dant toward the oxidation of inert C–H bonds. Therefore, the
direct oxidation of C–H bonds with molecular oxygen as the
5
oxidant is still regarded as one of the main challenges. To
overcome this impediment, various catalysts have been
1
5
applied to oxygen activation and subsequent C–H bond oxida-
using an NHPI and CoIJOAc) system in 1997. In addition,
2
6
tion. In recent years, NHPI (N-hydroxyphthalimide) has been
Einhorn also reported an efficient system for the aerobic oxi-
recognized as a valuable catalyst for the activation of C–H
bonds via hydrogen abstraction by the intermediate PINO rad-
ical. Also, the formation of the PINO radical from its
dation of benzylic compounds catalyzed by N-hydroxy-3,4,5,6-
1
6
tetraphenylphthalimide (NHTPPI) and CuCl at 35 °C. On the
other hand, iron is relatively non-toxic, cheap and environ-
mentally friendly and iron-based catalyst systems have been
7
1
7
a
applied in a variety of organic transformations. Although
NHPI combined with iron salts has also been used in various
oxidations, such as oxidation of alcohols, oxidative coupling
of NHPI with toluene, oxidation of indane in aqueous media
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000,
People's Republic of China. E-mail: chengxiamiao@163.com, wsun@licp.cas.cn
b
School of Pharmacy, Lanzhou University, Lanzhou 730000, People's Republic of
China
1
8
2 2
and so on, H O was used as the oxidant in some systems.
Detailed research on the substrate scope, kinetics and mecha-
†
Electronic supplementary information (ESI) available: Experiment of O isoto-
18
pic labeling with
2
O for the oxidation of diphenylmethane, data and NMR
spectra of part products. See DOI: 10.1039/c5cy01245g
nism of aerobic oxidation for benzylic C–H bonds has not
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1
8
been reported. Our continued efforts in oxidation motivated
us to study in detail the use of iron salts as mediators for the
transformation of NHPI to PINO, which further achieves the
aerobic oxidation of benzylic C–H bonds under mild condi-
NHPI to 12 mol% (Table 2, entry 4). Therefore, 10 mol% of
NHPI is considered as the desired catalyst loading. Further
studies indicated that good results were obtained with
8 mol% FeIJNO ) ·9H O (Table 2, entry 5).
3
3
2
1
9
tions. Delightedly, by combining NHPI with FeIJNO ) ·9H O,
With a set of optimized conditions in hand, the scope of
the methylene substrates was investigated and the results are
tabulated in Table 3. First, a variety of ethylbenzene deriva-
tives containing different substituted groups on the aromatic
ring were subjected to the oxidation system with NHPI and
FeIJNO ) ·9H O as the catalysts. The results demonstrated that
3
3
2
the aerobic oxidation of benzylic methylenes to the corre-
sponding carbonyl compounds at 25 °C with an atmospheric
pressure of molecular oxygen was successfully realized, giving
moderate to high yields (Scheme 1).
3
3
2
all the tested ethylbenzene derivatives could be oxidized to
the desired ketones in moderate to good yields (Table 3,
entries 1–8). For example, the oxidation of 4-ethylbiphenyl
provided 83% yield under the optimized conditions (Table 3,
entry 7). However, ethylbenzene derivatives bearing OAc or
Ph on the methyl group only gave 23% and 24% yields,
Results and discussion
Our
study
commenced
with
the
oxidation
of
diphenylmethane as the model substrate in the presence of
NHPI (5 mol%) at 25 °C with an atmospheric pressure of
molecular oxygen, and the screening of different parameters
is summarized in Table 1. Initially, the activities of a variety
of iron salts were evaluated (Table 1, entries 1–5). Only
FeIJNO ) ·9H O as the co-catalyst gave moderate conversion
respectively (Table 3, entries
9
and 10). Notably,
diphenylmethane, 9H-xanthene and 9H-fluorene demon-
strated excellent reactivities, providing 84% to 97% yields
(Table 3, entries 11–13). In the case of oxidation of
neopentylbenzene, the reaction did not take place, perhaps
due to the steric hindrance of tertiary butyl (Table 3, entry
14). In addition, tetrahydronaphthalene was oxidized to the
corresponding ketone in 50% yield (Table 3, entry 15). A non-
activated substrate, cyclooctane, was also tested, and 14%
yield of cyclooctanone was observed for the oxidation of
3
3
2
and good selectivity (Table 1, entry 5). Further experiments
indicated that FeIJNO ·9H O and NHPI were essential to the
)
3 3
2
aerobic oxidation of diphenylmethane (Table 1, entries 6 and 7).
The results also suggested that the anions of iron salts had a
great effect on the oxidation of diphenylmethane. Subse-
quently, a series of nitrates were also investigated to test
whether there was a certain relationship between the metal
cation and the reaction activity (Table 1, entries 8–15). Unfor-
tunately, most of the other nitrates were inert for the oxida-
tion reaction and the substrates were almost recovered
cyclooctane catalyzed by the FeIJNO
(Table 3, entry 16). Additionally, we compared the reaction
outcomes catalyzed by the FeIJNO ·9H O/NHPI system with
those of the NHPI/CoIJOAc) system developed by Ishii in
1997. Obviously, the dosage of CoIJOAc)2 in Ishii's system
was very low. The present FeIJNO ·9H O/NHPI system
3 3 2
) ·9H O/NHPI system
)
3 3
2
(Table 1, entries 9–15). Only in the case of CuIJNO
3 2
) ·3H
2
O as
2
1
5
the co-catalyst resulted in a 31% yield of the ketone (Table 1,
entry 8). Besides, the effect of the solvents on the oxidation
was also evaluated (Table 1, entries 5 and 16–20). The results
for screening of the solvents indicated that acetonitrile as the
solvent exhibited better catalytic activity compared to the
other solvents such as DMSO, dichloromethane, toluene and
so on (Table 1, entry 5), perhaps for the retard of the side
)
3 3
2
showed higher selectivity for the production of ketones
(Ishii's system, 12% and 17% alcohols as by-products with
47% and 31% 1-(p-tolyl)ethan-1-one, respectively).
Furthermore, we studied the kinetics of the oxidation to
get some hints for the possible mechanism. Fig. 1 presents
the kinetic curves of benzophenone accumulation over reac-
tion time under the optimized experimental conditions. The
results also indicated that the rate was fast during the initial
7 h, then reduced gradually.
2
0
reaction, such as nitration.
Taking all these results
and NHPI that is
together, it is the combination of FeIJNO )
3 3
necessary to efficiently catalyze benzylic methylene oxidation.
Additionally, the effect of the dosage of FeIJNO ) ·9H O and
3
3
2
NHPI was further examined at 25 °C with an atmospheric
pressure of molecular oxygen as the oxidant, as listed in
Table 2. In the presence of 5 mol% FeIJNO ) ·9H O and 2
mol% NHPI, only 23% yield was obtained (Table 2, entry 1).
The yield reached 72% when the amount of NHPI was
increased to 10 mol% (Table 2, entry 3), but almost no
change was observed by further increasing the amount of
On the other hand, we determined the dependence of the
initial rate of benzophenone formation (W ) on the concen-
0
tration of each reagent. The dependence of the oxidation rate
on the concentration of NHPI is linear in the range of con-
3
3
2
−
2
centration from 0 to 6.67 × 10 M (Fig. 2), similar to the
result of FeIJNO ·9H O (Fig. 3).
)
3 3
2
The effect of para-substituents of ethylbenzene derivatives
on the oxidation has also been investigated. The experiments
were carried out in CH CN (1.5 mL) at 25 °C under an atmo-
3
spheric pressure of molecular oxygen by mixing equimolar
amounts (1 mmol) of para-substituted ethylbenzene (p-Y-
C
8
H
10, Y = MeO, Me, H, Cl, Br or NO
2
) and ethylbenzene cata-
·9H O (8 mol%) for 1
lyzed by NHPI (10 mol%) and FeIJNO
)
3 3
2
hour. The relative rates for these para-substituted ethylben-
zene derivatives, krel, were evaluated by monitoring the
Scheme 1 Aerobic oxidation of benzylic methylenes.
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Table 1 Screening of metal salts and solvents for the aerobic oxidation of diphenylmethanea
b
b
Entry
Metal salt
ijFeIJCN)
Solvent
Conv. (%)
Yield (%)
c
1
2
3
4
5
6
K
3
6
]
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
NR
—
Fe IJSO ·9H
2
)
4 3
2
O
3
NR
Trace
—
c
FeCl
FeCl
2
·4H
·6H
2
O
O
3
2
28
24
73
—
—
31
3
4
3 3
FeIJNO ) ·9H
—
2
O
O
74 c
NR
NR
d
c
7
3 3
FeIJNO ) ·9H
2
8
9
CuIJNO
ZrIJNO
AlIJNO
CoIJNO
MgIJNO
NiIJNO
ZnIJNO
3
)
2
·3H
·5H
·9H
·6H
·6H
·6H
·6H
·5H
·9H
·9H
·9H
·9H
·9H
2
O
34
6
7
5
4
4
3
4
9
3
)
4
2
O
O
10
11
12
13
14
15
16
17
18
19
20
3
)
3
2
3
)
2
2
O
Trace
Trace
Trace
Trace
Trace
4
3
)
2
2
O
3
)
2
2
O
O
O
O
O
O
O
O
3
)
2
2
InIJNO
FeIJNO
FeIJNO
FeIJNO
FeIJNO
FeIJNO
3
)
3
2
3
)
)
)
)
)
3
3
3
3
3
2
2
2
2
2
CH Cl
2 2
3
3
3
3
Ethyl acetate
Dimethyl sulphoxide
25 c
22
—
—
—
NR
c
2
H O
NRc
Toluene
NR
a
b
Reaction conditions: diphenylmethane (1 mmol), metal salt (5 mol%), NHPI (5 mol%), acetonitrile (1.5 mL), O
Determined by GC using biphenyl as an internal standard. NR = no reaction. Without NHPI.
2
(1 atm), 25 °C, 40 h.
c
d
Table 2 The influence of the dosage of FeIJNO
3
)
3
·9H
O and NHPI on the aerobic oxidation of diphenylmethanea
2
b
b
Entry
3 3 2
FeIJNO ) ·9H O (mol%)
NHPI (mol%)
Conv. (%)
Yield (%)
1
2
3
4
5
6
5
5
5
5
8
2
8
10
12
10
10
27
63
74
77
84
86
23
60
72
73
84
85
10
a
b
Reaction conditions: diphenylmethane (1 mmol), O
2
(1 atm), acetonitrile (1.5 mL), 25 °C, 40 h. Determined by GC using biphenyl as an
internal standard.
reactions using gas chromatography (Table 4). When the log
krel values determined for each para-substituted ethylbenzene
were plotted against the substituent constants σ, Hammett
correlations were obtained (Fig. 4). A good linear correlation
between log krel and σ [with a negative ρ of −1.3 indicating an
electron-demanding transition state] was found, demonstrat-
ing the more electron-rich substrates with higher oxidation
rates.
indicated that only FeIJNO ) ·9H O also had a certain oxida-
3 3 2
tion ability and the partial oxygen of the product should
derive from nitrate. Moreover, Hulshof et al. reported that
changes of the valence state of iron and nitrogen oxides
2
0
existed in the oxidation of benzyl alcohol by ironIJIII) nitrate.
Besides, we supplemented control experiment using
diphenylmethane (1 mmol) catalyzed by FeCl ·6H O (8
a
3
2
mol%), NaNO (24 mol%) and NHPI (10 mol%) in 1.5 mL of
3
In order to further understand the mechanism of the reac-
tion, we have therefore studied the oxidation of
2
acetonitrile under O at 25 °C for 40 h. GC analysis showed
that the yield of benzophenone was 70%. Then, we replaced
NaNO3 with NaNO2 and found that the yield of benzophe-
1
8
diphenylmethane with FeIJNO ) ·9H O/NHPI under an O2
3
3
2
1
8
atmosphere. The incorporation of O into the products has
none decreased to 51%. The results indicate that the nitrate
−
also been examined (Fig. S1†). The result showed that the
anion plays an important role in the reaction, and NO and
3
1
8
−
percentage of the products containing O was 53%, which
indicated that O is not the only oxygen source for the pro-
NO2 could be converted into the real intermediate in the
2
process of the reaction.
duction of benzophenone. Then, we supplemented a blank
experiment using diphenylmethane (1 mmol) catalyzed by
FeIJNO ) ·9H O (8 mol%) and NHPI (10 mol%) in 1.5 mL of ace-
3 3 2
tonitrile under argon at 25 °C for 40 h. GC analysis showed
that the yield of benzophenone was 30%. The results
Accordingly, the iron cation and the nitrate anion were
necessary for the reaction. However, the change of the iron
cation and the transformation of nitrate with the help of the
iron cation have not been clear up to now. According to our
experimental results and previous report, we proposed a
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a
Reaction conditions: substrate (1 mmol), FeIJNO
NHPI (10 mol%), acetonitrile (1.5 mL), O (1 atm), 25 °C, 40 h.
Isolated yield. Determined by GC using biphenyl as an internal
3 3 2
) ·9H O (8 mol%),
Table 3 Selective aerobic oxidation of C–H bonds of various methylenes
a
2
3 3 2
by NHPI and FeIJNO ) ·9H O
b
c
d
b
standard. t = 20 h.
Entry
1
Substrate
Product
Yield (%)
c
70
possible mechanism as shown in Scheme 2. Based on our
previous report on the oxidation of alcohols catalyzed by
3
+
NHPI and FeIJNO ) ·9H O, Fe could be transformed into
3
3
2
2
+
c
3 6
Fe during the reaction detected by addition of K ijFeIJCN) ]
2
3
79
d
c
19c
57
solution. After the reaction, we tested the pH of the system
which was indicated to be a weakly acidic environment, prob-
ably due to the hydrolysis of hydrated ferric ions (Scheme 2,
Fe has a strong oxidizing ability and Fe could be
oxidized to Fe with the help of O or NO under acidic con-
2
0a,21
3+
2+
(
1)).
c
4
5
6
63
3
+
−
2
3
2
2
ditions (Scheme 2, (2) and (3)). The reaction may be initi-
ated by hydrogen atom abstraction from the hydroxyimide
group of NHPI by the iron compound and N O under oxygen
c
x
y
60
1
2b,22–24
atmosphere (Scheme 2, (4)).
Then, PINO abstracts the
hydrogen atom from diphenylmethane to produce the corre-
sponding radical 1, which subsequently reacts with molecular
c
25
54
2
oxygen or NO producing peroxy radical 2 or nitrite 2′.
Then, intermediate 2 accepts hydrogen to provide 3, which
then loses water and is transformed into the desire ketone
product. The hydroperoxide intermediate 3 may also be trans-
formed into the benzyl alcohol derivative 4, which under fur-
ther oxidation provides the corresponding ketone. Besides,
the desired product could be also produced by the elimina-
7
83
51
2
0a
tion or arrangement of 2′.
8
9
Experimental
General remarks
23
24
All starting materials and catalysts were purchased from com-
18
1
1
0
1
mercial suppliers without further purification. O was pur-
2
chased from Sigma-Aldrich. Column chromatography was
generally performed on silica gel (200–300 mesh) and TLC
inspections were carried out on silica gel GF254 plates.
GC analysis was performed on an Agilent Technologies
c
84
6
820 with a flame ionization detector and an HP-5MS column
1
2
3
97
90
(
0.25 mm × 30 m, film: 0.25 μm). GC-MS spectra were
recorded on an Agilent Technologies 7890A GC system with
an Agilent 5975 inert mass selective detector (EI) and an HP-
1
1
5MS column (0.25 mm × 30 m, film: 0.25 μm). H NMR and
1
3
C NMR spectra were recorded on a Bruker 400 MHz
1
1
1
4
5
6
Trace
50
c
14
Fig.
1
Benzophenone
accumulation
in
the
oxidation
of
−
2
diphenylmethane catalyzed by NHPI (6.67 × 10 M) and FeIJNO )
3 3
−
2
·
2 2
9H O (5.33 × 10 M) in the presence of O (1 atm) in acetonitrile.
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Fig. 2 Dependence of benzophenone initial accumulation rate W
0
on the concentration of NHPI (reaction conditions: diphenylmethane
1 mmol), FeIJNO ·9H O (8 mol%) , O (1 atm), acetonitrile (1.5 mL), at
5 °C, 4 h).
(
3
)
3
2
2
2
Scheme
2
Proposed
mechanism
for
the
·9H
oxidation
O with O
of
diphenylmethane catalyzed by NHPI and FeIJNO
3
)
3
2
2
.
General procedure for oxidation of benzylic methylenes
The substrate (1 mmol) and the desired amounts of FeIJNO
9H O and NHPI were added to 1.5 mL of acetonitrile in a 15
3 3
)
·
2
mL test tube. The solution was stirred for 40 h under an
atmospheric pressure of O at 25 °C controlled using a water
Fig. 3 Dependence of benzophenone initial accumulation rate W
0
on
2
the
diphenylmethane (1 mmol), NHPI (10 mol%) , O
1.5 mL), at 25 °C, 4 h).
concentration
of
FeIJNO
3
)
3
·9H
2
O
(reaction
conditions:
(1 atm), acetonitrile
bath. After the reaction, 30 mg of biphenyl or nitrobenzene
as an internal standard was added to the reaction system.
The solution was centrifuged and the supernatant was
diluted with diethyl ether and dried with anhydrous Na SO
2
(
2
4
for 30 minutes. The products were analyzed by GC and fur-
ther confirmed by GC-MS or NMR. The isolated yield was
obtained through column chromatography generally
performed on silica gel (200–300 mesh).
Table 4 List of krel and σ values for the oxidation of para-substituted
ethylbenzene derivatives
Entry
Y
k
rel
σ
1
2
3
4
5
MeO
Me
H
0.64
0.31
0
−0.27
−0.17
0
0.23
0.78
Conclusions
Cl
−0.19
We have developed a system including NHPI and FeIJNO3)3
NO
2
−0.87
·
°
9H
2
O for the aerobic oxidation of benzylic methylenes at 25
. Various benzylic methylenes
C in the presence of 1 atm O
2
could be oxidized to the corresponding carbonyl compounds
in satisfactory yields in this catalytic system. Hammett analy-
sis suggested that the substrates with electron-donating
1
8
groups would have higher oxidation rates. The isotopic ( O)
labeling study provided evidence of the participation of the
nitrate anion in the catalytic cycle. In addition, a radical
mechanism involving abstraction of the hydrogen atom by
PINO as well as dioxygen and nitrate anion participation in
the catalytic cycle was proposed. Further investigations
toward the scope and mechanism are underway in our
laboratory.
Fig. 4 Hammett correlation studies (log krel vs. σ) for the oxidation of
2
para-substituted ethylbenzene derivatives catalyzed by FeIJNO ·9H O
and NHPI with an atmospheric pressure of molecular oxygen as the
oxidant.
3
)
3
Acknowledgements
We are grateful for the financial support from the National
Natural Science Foundation of China (21103207 and
21133011), the Western Light Program of the Chinese Acad-
spectrometer using CDCl3 as the solvent with TMS as an
internal reference.
emy of Sciences (2012) and the Youth Innovation Promotion
Association, CAS.
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