Iron(II) and copper(II) phthalocyanine-catalyzed synthesis of 2-nitro-4-methylsulfonylbenzoic acid under mild conditions

Article abstract of DOI:10.1007/s12039-017-1373-4

Abstract : A novel method was?developed?to produce 2-nitro-4-methylsulfonylbenzoic acid (NMSBA) from the oxidation of 2-nitro-4-methylsulfonyltoluene (NMST) by oxygen catalyzed by iron(II) phthalocyanine (FePc) and copper(II) phthalocyanine (CuPc). The order of activity for oxidation of NMST was found to be: FePc > CuPc. Meanwhile, major reaction parameters such as concentrations of catalyst and NaOH, reaction temperature and oxygen pressure have been investigated. Through optimization of the reaction parameters, the highest yield of NMSBA and conversion of NMST (up to 53%, 89.3%, respectively) were achieved with oxygen (2.0 MPa), FePc (1.0×10-4molL-1), NaOH (0.6molL-1), in methanol at 55^°C for 8 h. A plausible mechanism for this catalytic process is proposed which involved deprotonation and radical pathways. Graphical Abstract: A novel method to produce 2-nitro-4-methylsulfonylbenzoic acid (NMSBA) from the oxidation of 2-nitro-4-methylsulfonyltoluene (NMST) by oxygen catalyzed by iron(II) phthalocyanine (FePc) and copper(II) phthalocyanine (CuPc) has been developed. The reaction parameters and mechanism were studied. The catalytic system will be readily applicable to large-scale production of NMSBA. [Figure not available: see fulltext.].

Full text of DOI:10.1007/s12039-017-1373-4

J. Chem. Sci.
© Indian Academy of Sciences
DOI 10.1007/s12039-017-1373-4
REGULAR ARTICLE
Special Issue on Recent Trends in the Design and Development of Catalysts and their Applications
Iron(II) and copper(II) phthalocyanine-catalyzed synthesis of
2-nitro-4-methylsulfonylbenzoic acid under mild conditions
CHENG HUANG, RUI LIU∗ , CAITING ZHANG, QIPENG CHENG and HONGJUN ZHU∗
Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Nanjing Tech University,
Nanjing 211816, China
E-mail: rui.liu@njtech.edu.cn; zhuhj@njtech.edu.cn
MS received 23 May 2017; revised 22 August 2017; accepted 30 August 2017
Abstract. A novel method was developed to produce 2-nitro-4-methylsulfonylbenzoic acid (NMSBA) from the
oxidation of 2-nitro-4-methylsulfonyltoluene (NMST) by oxygen catalyzed by iron(II) phthalocyanine (FePc)
and copper(II) phthalocyanine (CuPc). The order of activity for oxidation of NMST was found to be: FePc >
CuPc. Meanwhile, major reaction parameters such as concentrations of catalyst and NaOH, reaction temperature
and oxygen pressure have been investigated. Through optimization of the reaction parameters, the highest yield
of NMSBA and conversion of NMST (up to 53%, 89.3%, respectively) were achieved with oxygen (2.0 MPa),
FePc (1.0 × 10−4 mol L−1), NaOH (0.6 mol L−1), in methanol at 55◦C for 8 h. A plausible mechanism for this
catalytic process is proposed which involved deprotonation and radical pathways.
Keywords. 2-Nitro-4-methylsulfonyltoluene; 2-nitro-4-methylsulfonylbenzoic acid; aero oxidation; metal
phthalocyanines.
1. Introduction
both an economic and environmental point of view, the
effective catalytic oxidation processes using clean and
inexpensive oxidants, such as molecular oxygen for con-
verting aryl methyl group to carboxylic acid group on
an industrial scale, is the most gratifying route. In recent
years, significant progress has been made in oxidizing
aryl methyl group with molecular oxygen as oxidant,
and Co2+/Mn2+ salts as catalysts.5,6 Due to the low oxi-
dizing capability of oxygen, the reaction condition in
these catalytic systems is very harsh, especially when
strong electron-withdrawing groups such as −NO2 and
−SO2CH3 are attached on the benzene, even under high
reaction temperature and high pressure of air/oxygen.
Meanwhile, the catalytic system may bring undesirable
side reactions, such as the decarboxylation of the result-
ing carboxylic acid and decomposition of acetic acid
solvent.7
To avoid these drawbacks, it is very challenging
and significant to explore an efficient and green cata-
lyst to activate molecular oxygen and produce reactive
oxygen species which induce oxidation of aryl methyl
groups. Recently, metal phthalocyanines (MPc) have
been widely used in the oxidation of aryl methyl
groups, which have attracted much interest due to
their high catalytic activity.8 The oxidation process is
attributed to the ability of MPcs, which could excite
Mesotrione, namely 2-(4-mesyl-2-nitrobenzoyl)-1,3-
cyclohexanedione, is a commercial cornfield herbicide
developed by Syngenta company. It is widely produced
and utilized due to its safety, low toxicity, strong mis-
cibility and high efficiency.1 Along with the global
market demand of mesotrione increased rapidly, it has
been widely researched in recent years. In the pro-
ductionofmesotrione, 2-nitro-4-methylsulfonylbenzoic
acid (NMSBA) as a key intermediate of mesotrione is
usually oxidized from 2-nitro-4-methylsulfonyltoluene
(NMST).
In the chemical structure of NMST, there are two
strong electron-withdrawing substituents (−NO2 and
−SO2CH3) attached to the benzene ring, which increase
the difficulty of oxidizing NMST to NMSBA. To oxidize
methyl group on benzene rings with strong electron-
withdrawing substituents attached, traditionally, stoi-
chiometric oxidants such as potassium permanganate,2
sodium hypochlorite,3 or organic peroxides4 are nec-
essary for effective conversion. However, these stoi-
chiometric oxidants are not only relatively expensive
but also generate significant amounts of waste. From
*
For correspondence

Cheng Huang et al.
CH3
COOH
NO2
oxidation of NMST to NMSBA. The synthetic routes is
NO2
shown in Scheme 1. This work was aimed at optimiz-
ing the synthesis of NMSBA to decrease the economic
cost and reduce the pollution. These results would pro-
vide an important guidance for industrial production of
mesotrione.
O2 MPcs
,
CH OH
NaOH,
3
SO2CH3
NMST
SO2CH3
NMSBA
Scheme 1. Catalytic oxidation of NMST
to NMSBA.
ground-state molecular oxygen (3O2) to highly active
singlet oxygen (1O2).9,10 Song and co-authors11 inves-
tigated the bromide-free and acetic acid-free aerobic
oxidation of p-nitrotoluene to p-nitrobenzoic acid in
high yield by using MPcs as catalysts under mild
conditions in alkali-methanol solution. Up to 88.8% iso-
lated yield of p-nitrobenzoic acid was obtained. Shan
and co-authors12 reported a method for oxidation of
2. Experimental
2.1 Materials
Copper(II) phthalocyanine and iron(II) phthalocyanine were
purchased from the J&K Chemical Co., Ltd. (Beijing, China).
NMSBA was purchased from Energy Chemicals Co., Ltd.
(Shanghai, China). NMST was obtained from Changqing
Agrochemical Co., Ltd. (Jiangsu, China). Spectrophotmet-
nitrotoluenes to nitrobenzoic acids with molecular oxy- ric grade solvents were used for spectroscopic studies and
were purchased from Alfa Aesar Company. All the other
reagents and solvents were purchased from Sinopharm Chem-
ical Reagent Co., Ltd. (Shanghai, China) and used without
further purification.
gen and ionic liquid [omim][BF4] containing the iron
phthalocyanine complex with the isolated yield of 93%.
All these previous studies indicate that MPcs exhibited
excellent performance in catalytic oxidation by oxygen
under mild condition. In particular, iron(II) phthalo-
cyanine (FePc) and copper(II) phthalocyanine (CuPc)
showed high selective catalytic oxidation of aryl methyl
group to carboxylic acid group, which are preferred cat-
alysts in the catalytic oxidation reaction.
2.2 General procedure for the oxidation
The aerobic oxidation was carried out in the following steps.
Firstly, 1000 mL autoclave was charged with mixtures of sub-
strate (20 mmol) and methanol (200 mL), proposed amount of
NaOH and MPc. Secondly, high purity O2 was introduced into
the autoclave to replace the air for three times and then raise
the pressure in the reactor to proposed pressure and the stirring
was started. The agitation speed was maintained at 250 rpm.
Thirdly, the reactor was heated by an electric heating jacket.
At last, the temperature reached to the scheduled values and
reacted for 8 h. The reaction solution was diluted with the
solvent to a certain volume to be quantitatively analyzed the
NMST conversion and NMSBA yield by high-performance
liquid chromatography (HPLC).
On the other hand, oxidants are also major contrib-
utors in NMST selective oxidation to NMSBA. For
oxidation of NMST as a critical procedure in mesotrione
production, many ways have been tried. According to
the literature,13,14 some researchers reported the oxida-
tion of NSMT with concentrated nitric acid. However,
there were some disadvantages which included more
acid-containing waste water and nitro-compounds. Jary
and Cheng15,16 put forward that NMST could be oxi-
dized to NMSBA by hydrogen peroxide and different
metal catalysts in sulfuric acid solution. However, using
hydrogen peroxide as oxidant, the actual dosage of
hydrogen peroxide will be relatively large. As a well-
known green oxidant, oxygen is preferred to make
the oxidation of NMST to NMSBA, inexpensively
and efficiently.17 Long and co-authors18 developed a
method to oxidize NMST to afford NMSBA with
oxygen coupled with a homogeneous catalyst system
consisting of H3PW12O40/Co(II)/Mn(II)/Br. This tech-
nology used bromine as a promoter and it had a serious
impact on the environment, but compared with the
previous methods, this process demonstrated that oxy-
gen can be used as a green oxidant in oxidation of
NMST.
2.3 Analytical methods
The concentration of NMST and NMSBA was analyzed by
HPLC (Dionex, USA) equipped with Hypersil C18 ODS2
(4.6×150 mm, 5 µm) column and UV-detector set at λ =
254 nm. A solution of 60% deionized water and 40% ace-
tonitrile was used as a mobile phase with a flow rate of
1.0 mL min−1. The column temperature and injection vol-
ume were set at 25◦C and 20 μL, respectively. The calibration
curves of NMST and NMSBA were obtained by using stan-
dard solutions. Least-squares fits to the data yield standard
equations shown in Table 1. A stands for peak area and C
means the concentration with NMST or NMSBA.
In this work, we report a catalytic system with
TheNMSTconversionandNMSBAyieldweredetermined
efficient MPc catalysts and green oxidant oxygen for by Eqs. (1) and (2) respectively.18

Iron(II) and copper(II) phthalocyanine-catalyzed synthesis
Table 1. Standard equations of NMST and
derivatives with an electron-withdrawing substitute. As
an example, nitrobenzoic acid was obtained in a high
yield under oxygen using a very low concentration of
catalyst. However, the conversion of nitrotoluene and
yield of nitrobenzoic acids are very low when the air was
used as the oxidant. The reason for the low conversion
is that air exhibits lower oxidation efficiency than pure
oxygen. Additionally, the position of a substituent on the
toluene has also great influence on the conversion. Since
toluene is substituted by an electron-withdrawing group
at its para position, the C-H bond length of the methyl
group on the benzene ring will be longer than that at its
ortho position.19 Generally, a longer bond length rep-
resents a weaker bond.20 In other words, the oxidation
of o-nitrotoluene is difficult than p-nitrotoluene. Oxi-
dation of chloro-substituted toluene was also examined,
and the acid yield reached nearly 62%. The results of the
model reaction indicate the MPcs possess high activity
in the oxidation of toluene derivatives with electron-
NMSBA.
Sample
Linear Equation
R2
NMST CNMST = 0.369ANMST + 0.02
0.996
NMSBA CNMSBA = 0.479ANMSBA + 0.025 0.997
CN0MST − CNe MST
CN0MST
αNMST(%) =
× 100
(1)
where αNMST stands for the conversion of NMST, CN0MST for
the initial NMST concentration and CNe MST for the NMST
concentration (g mL−1) at the end of reaction in the solution.
CNe MSBA
Y
NMSBA(%) =
× 100
(2)
MNMSBA
MNMST
CN0MST
where YNMSBA is the yield of NMSBA, MNMST and MNMSBA
are the molecular weights of NMST and NMSBA, CNe MSBA
is the NMSBA concentration (g mL−1) at the end of reaction withdrawing substitute and can be used in the oxidation
in the solution.
of NMST.
3.1b Effect of MPc concentration on NMST oxida-
tion: It is necessary to measure the effect of MPcs
on the oxidation of NMSBA from NMST because it
is an important component of this catalytic system. A
3. Results and Discussion
3.1 Catalytic studies
3.1a Oxidation of different toluene derivatives with series of experiments were conducted with CuPc and
MPcs: In order to test the catalytic activities of FePc for a concentration of 1.0 × 10−5, 5.0 × 10−5,
MPcs, the oxidation oftoluene derivatives with electron- 1.0×10−4, 5.0×10−4 mol L−1, respectively. The results
withdrawing substitute was utilized for model reaction. are shown in Table 3, which indicate that as the con-
The reactions were performed in the presence of the centration of CuPc/FePc increased from 1.0 × 10−5 to
Cu(II) and Fe(II) phthalocyanine catalysts (Table 2).
The results in Table 2 show that MPc catalysts pos- from 10.8 to 89.3% and the yield of NMSBA also
sess high catalytic efficiency in the oxidation of toluene increased from 5 to 53%. Upon further increasing
1.0 × 10−4 mol L−1, the conversion of NMST increased
Table 2. Oxidation of different toluene derivatives with MPc catalystsa.
Entry
Substrate
Catalyst Oxidant Time (h) Conversionb (%) Yield of acidb (%)
1c
2c
3
4
5
6
7
8
9
p-Nitrotoluene CuPc
p-Nitrotoluene FePc
p-Nitrotoluene CuPc
Air
Air
O2
O2
O2
O2
O2
O2
O2
O2
24
24
8
8
8
3.4
3.8
3
3
91.3
92.6
85.3
89.3
70.9
71.3
81.4
87.0
65
73
55
61
53
58
58
62
p-Nitrotoluene
o-Nitrotoluene
o-Nitrotoluene
FePc
CuPc
FePc
8
2-Chlorotoluene CuPc
2-Chlorotoluene FePc
4-Chlorotoluene CuPc
4-Chlorotoluene FePc
12
12
12
12
10
aReaction conditions: MNaOH = 0.6 mol L−1, MMPc = 1.0 × 10−4 mol L−1, 55◦C, O2 (2.0
MPa).
bDetermined by high performance liquid chromatography (HPLC).
cAir bubbling.

Cheng Huang et al.
Table 3. Effect of MPcs concentration on NMST oxidation with CuPc and FePca.
Entry Catalyst
M
MPc (molL−1
)
Conversionb (%) Yieldb (%) TONc TOFd (h−1
)
1
2
3
4
5
6
7
8
CuPc
FePc
CuPc
FePc
CuPc
FePc
CuPc
FePc
1.0×10−5
1.0×10−5
5.0×10−5
5.0×10−5
1.0×10−4
1.0×10−4
5.0×10−4
5.0×10−4
10.8
12.4
43.3
73.5
81.6
89.3
51.8
53.7
5
8
581
877
526
839
495
604
62
73
110
66
105
62
75
8
23
37
43
53
27
30
68
9
aReaction conditions: MNaOH = 0.6 mol L−1, 55◦C, 8 h, O2 (2.0 MPa).
bDetermined by high-performance liquid chromatography (HPLC).
c
TON = mole of product/mole of catalyst.
TOF = mole of product/mole of catalyst × time (h).
d
the concentration of CuPc/FePc (5.0 × 10−4 mol L−1), general, for more electron-withdrawing groups on the
the yield of NMSBA and the conversion of NMST benzene ring, the methyl becomes more difficult to oxi-
decreased. This is due to the tendency of MPcs molecule dize. These might explain why the yield of NMSBA
to aggregate, leading to decrease in the active sites for is lower than those of monosubstituted corresponding
oxidation of NMST to NMSBA, which has also been acids.
proven previously.21 Meanwhile, it was observed that
the final products contained some other products such as
the corresponding alcohol and aldehyde.11 In addition,
3.1c Effect of NaOH concentration on NMST oxida-
taking reaction pathway and conditions into consid-
eration, the byproducts may also contain the dimeric
product.22 The results in Table 3 indicate that the opti-
mal MPc concent−ra1tion in this catalytic system may be
1.0 × 10−4 mol L .
tion: The above reaction occurs only when a large
amount of strong alkali such as NaOH-methanol exists
in the reaction system. The hydrogen of methyl is
abstracted by NaOH-methanol, and then completes the
catalytic oxidation of NMST to NMSBA. Experiments
were performed with the NaOH concentration of 1.0,
1.5, 2.0, 2.5 mol L−1 respectively, while keeping all
other conditions the same. The results are summarized
in Table 4.
From Table 4, it can be observed that the conversion
of NMST and yield of NMSBA increase with NaOH
concentration from 1.0 to 2.5 mol L−1. For example,
when the NaOH concentration increases from 1.0 to
2.5 mol L−1, the conversion of NMST increases from
44.8% to 90.4%. It is also noted that trace conversion
of NMST was obtained if NaOH was not utilized in the
oxidation, which indicates that NaOH played an impor-
tant role in the formation of NMSBA. The NMSBA
yield increases from 20% to 54% when the concen-
tration of NaOH increases from 1.0 to 2.5 mol L−1.
These results suggested that the increase of NaOH con-
centration could promote the oxidation of NMST.11
However, when the concentration of NaOH increased
above 2.0 mol L−1, the conversion of NMST, as well as
the yield of NMSBA, increased only slightly. In terms
of cost in post-processing and waste water discharge,
the best NaOH concentration is 2.0 mol L−1.
We found for the same concentration of MPc, the con-
version of NMST and yield of NMSBA for FePc were
higher than the corresponding results for CuPc. Mean-
while, the higher turnover number (TON) and turnover
frequency (TOF) obtained for FePc suggested that the
catalytic activity of FePc in NMST oxidation reaction
is better than that of CuPc. Catalytic oxidation of MPcs
may be assumed to be accompanied by coordination
of the oxygen to the central metal of the catalyst, in
a similar manner to porphyrins.23 MPcs with differ-
ent central metal ions may have varying coordination
abilities, which could result in varying modes and rates
of cleavage of the oxygen-MPc bonds hence resulting
in different product yields.24 For this reason, Fe(II)Pc
could easily react with oxygen to give PcFe(III)O-O
complex to improve the catalytic activity of FePc. How-
ever, invariant valence CuPc has a lower capacity for the
activation of oxygen, which leads to its catalytic activity
lower than that of FePc.25
Compared with the model reaction (Table 2), the
yield of NMSBA is lower than these correspond-
ing acids under identical experimental conditions. In

Iron(II) and copper(II) phthalocyanine-catalyzed synthesis
Table 4. Effect of NaOH concentration on NMST oxidationa.
Entry
Catalyst
M
NaOH (mol L−1
)
Conversionb (%) Yieldb (%) TONc TOFd (h−1
)
1
2
3
4
5
6
7
8
9
CuPc
FePc
CuPc
FePc
CuPc
FePc
CuPc
FePc
1.0
1.0
1.5
1.5
2.0
2.0
2.5
2.5
0
44.8
70.2
67.6
84.2
81.6
89.3
83.3
90.4
Trace
20
36
32
42
43
53
45
54
Trace
232
406
368
478
495
604
515
619
-
29
51
46
60
62
75
64
77
-
CuPc/FePc
Reaction conditions: MMPc = 1.0 × 10−4 mol L−1, 55◦C, 8 h, O2 (2.0 MPa).
bDetermined by high-performance liquid chromatography (HPLC).
c TON = mole of product/mole of catalyst.
TOF = mole of product/mole of catalyst × time (h).
a
d
Table 5. Temperature effect of NMST oxidation with CuPc and FePca.
Entry Catalyst T(◦C) Conversionb(%) Yieldb(%) TONc TOFd (h−1
)
1
2
3
4
5
6
7
8
CuPc
FePc
CuPc
FePc
CuPc
FePc
CuPc
FePc
35
35
45
45
55
55
65
65
28.5
28.3
54.3
63.3
81.6
89.3
84.6
91.2
14
13
33
39
43
53
42
50
160
150
376
441
495
604
475
573
20
19
47
55
62
75
59
72
aReaction conditions: MNaOH = 0.6 mol L−1, MMPc = 1.0 × 10−4 mol L−1, 8
h, O2 (2.0 MPa).
bDetermined by high performance liquid chromatography (HPLC).
c
TON = mole of product/mole of catalyst.
TOF = mole of product/mole of catalyst × time (h).
d
3.2 Effect of other reaction parameters on the NMST
oxidation
increasingtemperature, whichsuggeststhatthecatalysts
may facilitate the reaction toward an undesired pathway
by higher reaction temperatures.
3.2a Effect of reaction temperature and time on NMST
oxidation: To examine the influence of temperature on
the oxidation of NMST, the reactions were carried out
at a range of temperatures by using CuPc and FePc as
catalysts. Table 5 shows a significant effect of the tem-
perature on the conversion of NMST. The conversion of
NMST increases from 28.3% to 89.3% with the increase
of reaction temperature from 35 to 55◦C. Meanwhile,
the yield of NMSBA increases from 13% to 53% in
this temperature range. The highest yield reaches 43%
for CuPc and 53% for FePc at 55◦C with the highest
TON and TOF values (495, 62 for CuPc and 604, 75 for
FePc). However, further increase of reaction tempera-
ture to 65◦C, results in slight decreasing of the yield of
NMSBA 50% while NMST conversion remains nearly
quantitative. This yield loss of NMSBA at 65◦C may
be due to the decrease of selectivity to NMSBA with
Meanwhile, the conversion of NMST and the yield
of NMSBA increased with the reaction processing and
reached the highest point after 8 h by using FePc (Fig-
ure 1). However, only a slight decrease in yield of
NMSBA when the reaction time was prolonged to 10
h. Based on the results of conversion and yield, 8 h was
selected as the optimal reaction time.
3.2b Effect of oxygen pressure on NMST oxidation:
The influence of the pressure of oxygen was also inves-
tigated, as illustrated in Table 6. A series of experiments
were carried out with the initial oxygen partial pressure
of 0.1 MPa, 1.0 MPa, 2.0 MPa, 2.5 MPa and 3.0 MPa,
respectively.
Table 6 indicates the yield of NMSBA increases
sharply from 13% to 53% with the increase of oxy-
gen pressure from 0.1 MPa to 2.0 MPa. These results

Cheng Huang et al.
Table 6. Effect of oxygen pressure on NMST oxidationa.
Entry Catalyst Pressure(MPa) Conversionb (%) Yieldb (%)
1
2
3
4
5
6
7
8
9
CuPc
FePc
CuPc
FePc
CuPc
FePc
CuPc
FePc
CuPc
FePc
0.1
0.1
1.0
1.0
2.0
2.0
2.5
2.5
3.0
3.0
3.2
3.9
2
3
67.4
61.9
81.6
89.3
82.2
90.1
82.6
89.8
35
31
43
53
44
53
44
54
10
aReaction conditions: MNaOH = 0.6 mol L−1, MMPc = 1.0 ×
10−4 mol L−1, 55◦C, 8 h.
bDetermined by high performance liquid chromatography (HPLC).
in the reaction solvent which almost reaches the max-
100
80
60
40
20
0
conversion of NMST
yield of NMSBA
imum and the extra increase in the pressure of oxygen
has less influence on the increase in the concentration
of oxygen.12 Hence, the yields of NMSBA and con-
versions of NMST were almost unchanged, when the
oxygen pressure is above 2.5 MPa (Table 6, entries 9–
10).
3.3 Plausible mechanism
According to results of this work and the literature
reports previously,27,28 we propose a possible mecha-
nism of the catalytic oxidation (Scheme 2). The catalytic
procedure is confirmed to be initiated in a two-step pro-
cess. In the first step, NaOH-methanol could capture an
H+ from the methyl on the NMST and forms interme-
diate 4. This inference is verified by the experimental
result that trace NMSBA was detected without NaOH
in the oxidation system (Table 4, entry 9). In contrast, the
yield of NMSBA was rapidly increased with increasing
NaOH concentration (Table 4, entries 1–6).
0
2
4
6
8
10
Time (h)
Figure 1. Time-dependent curve of conversion and yield
under the optimum conditions, Reaction conditions:
MNaOH = 0.6 mol L−1, MFePc = 1.0 × 10−4 mol L−1, O2
(2.0 MPa), 55◦C, 8 h.
The second step of the catalytic procedure is adsorp-
suggest that the yield and oxygen pressure are closely tion of oxygen to the FePc to fully form the iron-oxygen
correlated. Adsorption of oxygen to the phthalocyanine complex. Fe(II) phthalocyanine (FePc) would be con-
would be very fast, and 0.1 MPa of oxygen would be verted into the starting Fe(III) species with generation
sufficient to fully form the metal-oxygen complex.26 of radical (Fe(III)PcOO ·),29 and then the radical reacts
However, only 0.1 MPa of oxygen could not provide with4togivebenzylicradical5. Then, oxygenistrapped
enough molecular oxygen into the solvent. The yield of by benzylic radical to afford benzylperoxy radical 6.
NMSBA increases with the oxygen pressure increase, in This radical could abstract a hydrogen from NMST to
that oxygen concentration increases in the solvent and obtain an intermediate 5 and give benzylic hydroperox-
eventually the increase of reaction rate. When the oxy- ide 7. The homolysis of hydroperoxide can promote the
gen pressure further increases to 2.5 MPa, the yield of generation of key intermediate alcohol. Finally, alcohol
NMSBA only slightly increased with an oxygen pres- 9 is further oxidized to give the product NMSBA. Thus,
sure increasing of 0.5 MPa. This phenomenon could be the pathway of NMST to NMSBA can be assigned to a
mainly attributed to the dissolved amount of the oxygen deprotonation and radical mechanism.

Iron(II) and copper(II) phthalocyanine-catalyzed synthesis
CH2
CH2
CH2
CH3
O
OH
O
O
O
N
N
N
N
CH
NaOH
3OH,
-H+
O
O
O
SO2CH3
SO2CH3
SO2CH3
SO2CH3
1
2
3
4
CH2
CH2O OH
NO2
CH2OO
NO2
NO2
O2
1
-5
PcFe(III)OO
CH2O
PcFe(II)
SO2CH3
SO2CH3
SO2CH3
5
6
7
O2
COOH
NO2
CHO
CH2OH
NO2
NO2
NO2
1
[O]
[O]
-5
-OH
SO2CH3
SO2CH3
SO2CH3
SO2CH3
11
8
10
9
Scheme 2. Plausible mechanism for the oxidation of NMST to NMSBA.
4. Conclusions
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In conclusion, we have developed a practical method for
the preparation of NMSBA by aerobic oxidation. The
reaction is catalyzed by metal phthalocyanine, which
is inexpensive and non-toxic. Furthermore, the oxida-
tion system was economical, environmentally friendly
compared with the conventional oxidation methods.
The catalytic performance was greatly affected by the
metal ions of phthalocyanine, reaction time, reaction
temperature, oxygen pressure and the concentration of
phthalocyanines and NaOH. It is found that the iron(II)
phthalocyanine exhibits better catalytic activity, which
efficiently converted NMST into NMSBA in yields up
to 53%. The results of the present study will be read-
ily applicable to large-scale oxidation of NMST in the
chemical industry in the future.
Acknowledgements
We greatly acknowledge the financial support in part by the
National Key Research and Development Program of China
(2016YFB0301703), National Natural Science Foundation of
China (21602106), Natural Science Foundation of Jiangsu
Province-Outstanding Youth Foundation (BK20170104) and
Industry-Academy-Research Prospective Joint Project of
Jiangsu Province (BY2016005-06).
10. Bata P, Demjén A, Notheisz F and Zsigmond Á 2012
Comparative study of immobilized phthalocyanines in
oxidative degradation Open Catal. J. 5 50
11. Song X F, She Y B, Ji H B and Zhang Y H 2005 Highly
efficient, mild, bromide-free and acetic acid-free dioxy-
gen oxidation of p-nitrotoluene to p-nitrobenzoic acid
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