Selective Solvent-Free and Additive-Free Oxidation of Primary Benzylic C–H Bonds with O2 Catalyzed by the Combination of Metalloporphyrin with N-Hydroxyphthalimide

Article abstract of DOI:10.1007/s10562-020-03214-y

Abstract: A protocol for solvent-free and additive-free oxidation of primary benzylic C–H bonds with O₂ was presented through adjusting the combination of metalloporphyrins and NHPI as binary catalysts to overcome the deficiencies encountered in current oxidation systems. The effects of reaction temperature, porphyrin structure, central metal, catalyst loading and O₂ pressure were investigated systematically. For the optimized combination of T(2-OCH₃)PPCo and NHPI, all the primary benzylic C–H bonds could be functionalized efficiently and selectively at 120 °C and 1.0?MPa O₂ with aromatic acids as the primary products. The selectivity towards aromatic acids could reach up to 70–95% in the conversion of more than 30% for most of the substrates possessing primary benzylic C–H bonds in the metalloporphyrin loading of 0.012% (mol/mol). And the superior performance of T(2-OCH₃)PPCo among the metalloporphyrins investigated was mainly attributed to its high efficiency in charge transfer and fewer positive charges around central metal Co (II) which favored the adduction of O₂ to cobalt (II) forming the high-valence metal-oxo complex followed by the production of phthalimide N-oxyl radical (PINO) and the initiation of the catalytic oxidation cycle. This work would provide not only an efficient protocol in utilization of hydrocarbons containing primary benzylic C–H bonds, but also a significant reference in the construction of more efficient C–H bonds oxidation systems. Graphic Abstract: The solvent-free and additive-free oxidation of primary benzylic C–H bonds with O₂ was presented through adjusting the combination of metalloporphyrins and NHPI as binary catalysts, and the highest selectivity towards aromatic acid reached up to 95.1% with the conversion of 88.5% in the optimized combination of T(2-OCH₃)PPCo and NHPI.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-020-03214-y

Catalysis Letters
https://doi.org/10.1007/s10562-020-03214-y
Selective Solvent‑Free and Additive‑Free Oxidation of Primary
Benzylic C–H Bonds with O2 Catalyzed by the Combination
of Metalloporphyrin with N‑Hydroxyphthalimide
Hai‑Min Shen1 · Bei Qi1 · Meng‑Yun Hu1 · Lei Liu1 · Hong‑Liang Ye1 · Yuan‑Bin She1
Received: 13 February 2020 / Accepted: 4 April 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
A protocol for solvent-free and additive-free oxidation of primary benzylic C–H bonds with O2 was presented through
adjusting the combination of metalloporphyrins and NHPI as binary catalysts to overcome the defciencies encountered in
current oxidation systems. The efects of reaction temperature, porphyrin structure, central metal, catalyst loading and O2
pressure were investigated systematically. For the optimized combination of T(2-OCH3)PPCo and NHPI, all the primary
benzylic C–H bonds could be functionalized efciently and selectively at 120 °C and 1.0 MPa O2 with aromatic acids as the
primary products. The selectivity towards aromatic acids could reach up to 70–95% in the conversion of more than 30% for
most of the substrates possessing primary benzylic C–H bonds in the metalloporphyrin loading of 0.012% (mol/mol). And
the superior performance of T(2-OCH3)PPCo among the metalloporphyrins investigated was mainly attributed to its high
efciency in charge transfer and fewer positive charges around central metal Co (II) which favored the adduction of O2 to
cobalt (II) forming the high-valence metal-oxo complex followed by the production of phthalimide N-oxyl radical (PINO)
and the initiation of the catalytic oxidation cycle. This work would provide not only an efcient protocol in utilization of
hydrocarbons containing primary benzylic C–H bonds, but also a signifcant reference in the construction of more efcient
C–H bonds oxidation systems.
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-020-03214-y) contains
supplementary material, which is available to authorized users.
* Yuan-Bin She
shenhm2019@163.com
Hai-Min Shen
haimshen@zjut.edu.cn
Bei Qi
qibzjut17s@163.com
Meng-Yun Hu
humengyun111@163.com
Lei Liu
miscongcong@163.com
Hong-Liang Ye
1170272423@qq.com
1
College of Chemical Engineering, State Key Laboratory
Breeding Base of Green Chemistry-Synthesis Technology,
Zhejiang University of Technology, Hangzhou 310014,
China
Vol.:(0123456789)
1 3

H.-M. Shen et al.
Graphic Abstract
The solvent-free and additive-free oxidation of primary benzylic C–H bonds with O2 was presented through adjusting the
combination of metalloporphyrins and NHPI as binary catalysts, and the highest selectivity towards aromatic acid reached
up to 95.1% with the conversion of 88.5% in the optimized combination of T(2-OCH3)PPCo and NHPI.
Keywords Benzylic C–H bond · Oxidation · Molecular oxygen · Metalloporphyrin · Selectivity
22, 23], hydrogen peroxide [24–26], molecular oxygen [3,
27, 28] and so on [29, 30], but molecular oxygen (O2) has
been regarded as the frst choice for its readily availability,
high atom economy, cleanness and environmental-friend-
liness. Thus in the current industrial processes, the oxida-
tion of primary benzylic C–H bonds is usually carried out
with O2 as oxidant in the presence of homogeneous cobalt
(II) acetate or manganese (II) acetate as catalyst and bro-
mide as promoter in the solvent of acetic acid [18, 31–34].
For the high inertness of primary benzylic C–H bonds and
triplet oxygen molecule, high reaction temperature above
160 °C and high oxygen pressure must be employed [18,
32–34]. The existence of solvent acetic acid and promoter
bromide, not only would bring about some difculties to the
separation of oxidation products and catalysts, and increase
the waste discharged into the environment, but also would
lead to serious corrosion to the production equipment,
which make the current industrial processes not so compat-
ible with the requirements of green chemistry. In addition,
due to the high reaction temperature, the over-oxidation of
obtained oxygen-containing products to CO2 might occur,
hence the conversion of methylbenzene in current process
was controlled below 15% with less efciency [18, 31–34].
Therefore, considering the huge market demand and high
value of the products, and the defciencies encountered in
current oxidation process, it is a great urgency to develop a
1 Introduction
The direct, convenient, and selective functionalization of
C–H bonds in hydrocarbons through oxidation reactions has
always been an extremely useful, challenging, and fascinat-
ing research topic, not only in the industrial chemistry but
also in the academic research on chemistry [1–5]. In these
signifcant transformations, the widely available and inex-
pensive hydrocarbons are converted to high value-added
hydroperoxides, alcohols, aldehydes, ketones, acids and
esters, and all of these oxygen-containing compounds are
essential fne chemicals and chemical intermediates [1, 6–8].
Among these processes, one typical example is the oxidation
of primary benzylic C–H bonds, in which methylbenzene
and its derivatives are converted to valuable benzyl hydrop-
eroxide, benzyl alcohol, benzaldehyde, benzoic acid, and
their derivatives [9–12]. These oxidation products of pri-
mary benzylic C–H bonds have been applied widely as food
additives, favors, fragrances, medicines, and the irreplace-
able intermediates in the manufacture of pharmaceuticals,
perfumes, agricultural chemicals, dyes, resins, and synthetic
polymer fbers, possessing huge market demand [7, 13–18].
To realize the transformation from primary benzylic C–H
bonds to these oxygen-containing products, a series of oxi-
dants can be employed, such as iodosylbenzene [19, 20],
potassium persulfate [1, 21], tert-butyl hydroperoxide [11,
1 3

Selective Solvent-Free and Additive-Free Oxidation of Primary Benzylic C–H Bonds with O2…
mild route to realize the selective and efcient oxidation of
primary benzylic C–H with O2 under solvent-free conditions
in the absence of any additive. To achieve this purpose, suit-
able and convenient catalytic system is the key.
documented the oxidation of methylbenzene using O2 cata-
lyzed by the combination of NHPI and porphyrin manganese
(III) in acetic acid with methanol as additive or in the presence
of cetyl trimethyl ammonium bromide (CTAB) as additive,
and the conversion reached up to 41.0% with the selectivity of
70.6% towards benzoic acid and 25.0% towards benzaldehyde
[50–52]. Zhu et al. reported the oxidation of methylbenzene
using O2 catalyzed by the combination of NHPI with immo-
bilized phthalocyanine iron (II) in acetonitrile, and the conver-
sion was 35.3% with the selectivity of 71.8% to benzoic acid
and 20.1% to benzaldehyde [53]. Despite the satisfying con-
version and selectivity under mild conditions, the presence of
solvent and additive became the main obstacle in the extensive
industrial application of current route employing the combi-
nation of NHPI and transition metal complexes as catalysts.
Therefore, in the endeavor to achieve the efcient oxi-
dative functionalization of C–H bonds in hydrocarbons to
valuable oxygen-containing products in our group, we have
been focusing on the solvent-free and additive-free route
under mild conditions involving metalloporphyrins all the
time [54–58]. And in this work, we carried out the selective
solvent-free and additive-free oxidation of primary benzylic
C–H bonds using O2 as oxidant. Based on the systemati-
cal screen on the structure and performance of metallopor-
phyrins, primary benzylic C–H bonds were transformed to
their oxygen-containing products efciently without any
solvent and additive under mild conditions employing the
combination of NHPI with porphyrin cobalt (II) as cata-
lyst. For the representative compounds of primary benzylic
C–H bonds, the conversion of methylbenzene and 4-methyl
nitrobenzene reached up to 35.1% and 49.4% with aromatic
carboxylic acids as major products (selectivity of 92.3% and
64.5% respectively) at 120 °C and 1.0 MPa O2 pressure.
The major superiorities of this protocol are no solvent, no
additive, mild conditions, higher conversion and selectivity
compared with the current oxidation systems, which make
the route for primary benzylic C–H bonds oxidation reported
here possess not only huge potential in industrial application,
but also enormous reference value for other C–H bonds oxi-
dative functionalization. To our best knowledge, this work
is also a very convenient, practical and novel example in
the efcient utilization of widely available and inexpensive
hydrocarbons, and would act as a signifcant reference in
both academic research and industrial application for more
efcient utilization of hydrocarbons.
The catalytic systems combined by N-hydroxyphthal-
imide (NHPI) or its analogues with transition metal ions
such as cobalt (II) and manganese (II) have been applied
widely in the oxidation of C–H bonds in various hydrocar-
bons with O2[14, 18, 32, 35–40]. High efciency and mild
conditions usually could be obtained. Arzumanyan and co-
workers reported the selective oxidation of primary ben-
zylic C–H bonds in 4-tolylsiloxanes with O2 catalyzed by the
combination of N-hydroxysuccinimide (NHSI) and cobalt
(II) acetate at 40–60 °C under atmospheric pressure in ace-
tonitrile, and the yield of 4-carboxyphenylsiloxanes reached
up to 96%, in which the primary benzylic C–H bonds were
converted to carboxylic acids efficiently and smoothly
[40]. Shi and co-workers documented the efcient oxida-
tion of methylbenzene by O2 catalyzed by the combination
of N-hydroxyphthalimide (NHPI) and CoOx immobilized
on SiO2 at room temperature under atmospheric pressure
in hexafuoropropan-2-ol, and the conversion reached up to
91.2% with the selectivity of 68.8% towards benzaldehyde
and 20.3% towards benzoic acid, in which the utilization
of hexafuoropropan-2-ol as solvent was the key for the
outstanding conversion and selectivity [14]. Zhang et al.
reported the selective oxidation of primary benzylic C–H
bonds with O2 catalyzed by the combination of NHPI and
Co(OAc)2 at 100 °C under atmospheric pressure employing
imidazolium-based ionic liquid as additives, and for meth-
ylbenzene as substrate, the conversion reached up to 80%
with the selectivity of 97.5% towards benzoic acid [41].
Besides these, the combination of NHPI with copper nano-
particles on N-doped graphene in acetone [39], NHPI with
cobalt (II) acetate in hexafuoropropan-2-ol [38], NHPI and
its analogues with cobalt (II) acetate in acetonitrile [42],
immobilized NHPI with cobalt (II) in acetic acid [34],
immobilized N, N-dihydroxypyromellitimide (NDHPI) with
cobalt (II) in acetonitrile [32, 33], were also applied in the
oxidation of benzylic C–H bonds using O2 as oxidant under
mild conditions. Acceptable conversion and selectivity usu-
ally could be achieved.
Metalloporphyrins are a family of useful and important
transition metal complexes, and have been employed exten-
sively in the oxidative functionalization of C–H bonds in
hydrocarbons as catalysts [5, 43–47]. Owing to the high ef-
ciency of metalloporphyrins in the activation of O2, the cata-
lyst loading in one ten-thousandth of substrates would be ade-
quate to achieve the oxidative functionalization of C–H bonds,
thus the combination of NHPI with metalloporphyrins could
make the oxidation of C–H bonds in hydrocarbons consume
less transition metal catalysts and approach the requirement of
green chemistry [48, 49]. For instance, Guo and co-workers
2 Experimental Section
2.1 Materials
Benzaldehyde and its derivatives used as reactants in the
porphyrin syntheses were mainly purchased from Shanghai
1 3

H.-M. Shen et al.
Macklin Biochemical Co. Ltd., Adamas Reagent Co. Ltd.,
Xilong Chemical Reagent Co. Ltd., and Shanghai Energy
Chemical Co. Ltd., China. Pyrrole in the syntheses of por-
phyrins was purchased from Xilong Chemical Reagent Co.
Ltd., and was purifed before used through reduced pressure
distillation. The metal acetates such as cobalt (II) acetate,
manganese (II) acetate, copper (II) acetate, iron (II) acetate
in the metalloporphyrins syntheses were purchased from
Adamas Reagent Co. Ltd. and Shanghai Energy Chemical
Co. Ltd. China. Methylbenzene, substituted methylbenzene
and their oxidation products in oxidation of primary ben-
zylic C–H bonds were purchased from Shanghai Macklin
Biochemical Co. Ltd., Adamas Reagent Co. Ltd., Xilong
Chemical Reagent Co. Ltd., and Shanghai Energy Chemical
Co. Ltd., China. All the chemicals used in this work were
analytical reagents and were not purifed any more before
used unless otherwise noted.
in propionic acid (450 mL) under the nitrogen protection,
followed by the addition of redistilled pyrrole (150 mmol)
dropwise under refuxing conditions. After refuxing for
2.0 h in the shield from ambient light, the resultant mixture
was cooled in air to room temperature and kept standing
until a large amount of solid appeared. Then the solid was
collected through suction fltration and suspended in metha-
nol (200 mL) with stirring for 6.0 h. The collected purplish
red precipitate was washed successively using methanol
(2 × 100 mL) and purifed through silica column chroma-
tography separation with cyclohexane and dichloromethane
as eluent (10:1–4:1, volume/volume).
After dried under reduced pressure for 8.0 h at 80 °C,
the obtained porphyrin (0.20 mmol) was dissolved in
DMF (60 mL) with corresponding anhydrous metal ace-
tate (2.0 mmol), and the obtained solution was heated to
refuxing (150 °C) with stirring under N2 atmosphere. The
refuxing solution was kept stirring for 24.0 h, and cooled
to room temperature in air. Then DMF was removed under
reduced pressure through rotary evaporation, and the result-
ant solid was dissolved in dichloromethane (60 mL) to form
a purplish red solution, which was washed with water fve
times (5×200 mL) until the upper water layer became clear
and colorless, and dried over anhydrous Na2SO4. Follow-
ing rotary evaporation, obtained purple solid was further
purifed through silica column chromatography separation
with cyclohexane and dichloromethane as eluent (4:1–1:1,
volume/volume). All of the metalloporphyrins synthesized
herein were dried under reduced pressure for 8.0 h at 80 °C
before used. More details for the syntheses of metallopor-
phyrins have been demonstrated in the Electronic Supple-
mentary Material.
2.2 Characterizations
The qualitative characterizations of porphyrins and met-
1
alloporphyrins such as H NMR, 13C NMR, and ESI–MS
were carried out on the Bruker AVANCE III 500 MHz NMR
spectrometer, and Agilent 6210 LC/TOF mass spectrom-
eter respectively. In the NMR analyses, tetramethylsilane
was used as internal standard, and CDCl3 and DMSO-d6
as solvents. In the ESI–MS analyses, direct injection mode
was adopted with CH2Cl2 as solvent. The UV–Vis analy-
ses were performed on a HITACHI U-3900 spectrometer
in a quartz cuvette at room temperature, and the sample
was dissolved in DMF. The thermal gravimetric analyses
(TGA) were carried out on a PerkinElmer Diamond TG/
DTA instrument to study the thermal stability of metallopor-
phyrins, and all the TG analyses were conducted in the tem-
perature range of room temperature to 800 °C (10 °C/min)
under the atmosphere of air. The redox activity analyses of
metalloporphyrins used as catalysts were conducted on the
ZAHNER Zennium electrochemical workstation at 25 °C
under the atmosphere of nitrogen. In the electrochemical
measurements, three-electrode system was adopted contain-
ing working electrode (a glassy carbon), counter electrode
(a platinum wire), and reference electrode (Ag/AgCl), and
tetrabutylammonium hexafuorophosphate (TBAPF6) as sup-
porting electrolyte (0.025 mol/L in DMF).
2.4 Oxidation of Primary Benzylic C–H Bonds
All the oxidation of primary benzylic C–H bonds were con-
ducted in a 100 mL stainless steel autoclave reactor. In order
to eliminate the uncertain efect of the presence of metal ele-
ment, a Tefon liner was employed in all the oxidation reac-
tion. In a typical procedure, methylbenzene or substituted
methylbenzene (50 mmol) were sealed in the autoclave reac-
tor in the presence or absence of catalysts, and heated to the
set reaction temperature with stirring. After the temperature
reached, O2 was injected into the reactor to obtain the set
pressure, and the reaction mixture was kept heating and stir-
ring for 8.0 h. Then the reactor was cooled by cold water to
room temperature. When the autoclave reactor was opened,
acetone (50 mL) was added to dilute the reaction mixture
followed by the addition of triphenylphosphine (10 mmol)
to reduce the unconverted hydroperoxide to alcohol. After
stirring for another 30 min at room temperature, the obtained
reaction mixture was totally transferred into a volumetric
fask (100 mL) employing acetone as solvent. In succession,
2.3 Syntheses of Metalloporphyrins
The metalloporphyrins were mainly synthesized through the
condensation of benzaldehyde or its derivatives with pyrrole,
and then metallization of the obtained porphyrins according
to the procedure reported [59–66]. In the typical procedure
of porphyrin syntheses, benzaldehyde or its derivatives
(150 mmol) was heated to refuxing (145 °C) with stirring
1 3

Selective Solvent-Free and Additive-Free Oxidation of Primary Benzylic C–H Bonds with O2…
quantitative analyses were carried out employing GC and
HPLC to determine the conversion and selectivity in the
primary benzylic C–H bonds oxidation.
through the UV–Vis absorbance analyses. It was demon-
strated in Figs. 1 and S1 that, for all the porphyrins and
metalloporphyrins measured, the characteristic absorbance
peaks in about 425 nm were found, which were mainly
attributed to the Soret band absorbance of porphyrins and
metalloporphyrins, and indicated the correctness in molecu-
lar structure [51, 67–71]. And for the diference in molecular
structure and solubility, both of porphyrins and metallopor-
phyrins exhibited diverse intensity in UV–Vis absorbance
spectra. Figures 2 and S2 were the TGA plots of representa-
tive metalloporphyrins in the atmosphere of air, in which it
could be found that all of the metalloporphyrins measured
possessed well thermal stability before 300 °C, even in the
oxidative atmosphere. Thus, to some extent, it is feasible to
employ metalloporphyrins as catalysts in oxidation of pri-
mary benzylic C–H bonds at the temperature not too high
(below 150 °C). Then the cyclic voltammograms of metal-
loporphyrins were measured in a DMF solution of tetrabu-
tylammonium hexafuorophosphate (0.025 mol/L) to check
their redox behaviors employed as catalysts in oxidation of
primary benzylic C–H bonds. As illustrated in Fig. 3, obvi-
ous redox currents were detected under a suitable external
voltage. The good performance in electron transfer indicated
that the metalloporphyrins utilized in this work possessed
the possibility to perform as catalysts in oxidation of pri-
mary benzylic C–H bonds.
2.5 Product Analyses
The unreacted substrates possessing primary benzylic C–H
bonds, and oxidation products were confrmed qualitatively
through comparison with the authentic samples, especially
the retention times in GC and HPLC analyses. The quan-
titative analyses of the major products, aromatic carbox-
ylic acids, were conducted on a high performance liquid
chromatograph (HPLC) (Thermo Scientifc Ultimate 3000)
with 2-naphthalenecarboxylic acid as the internal standard.
And the model of chromatographic column was Amethyst
C18-H (250 mm×4.6 mm×0.25 µm), and the detector was
Ultimate 3000 Photodiode Array Detector. The quantitative
analyses of other oxidation products, such as alcohols, alde-
hydes and peroxides were carried out on a gas chromato-
graph (Thermo Scientifc Trace 1300) with naphthalene as
the internal standard. The model of chromatographic column
was TG-5MS capillary column (30 m×0.32 mm×0.25 μm),
and the detector was Flame Ionization Detector. The quan-
tity of peroxide was mainly obtained based on the triph-
enylphosphine oxide which was produced through reduction
of hydroperoxide by triphenylphosphine quantitatively.
3.2 Oxidation of Primary Benzylic C–H
Bonds Catalyzed by Metalloporphyrin
and N‑Hydroxyphthalimide
3 Results and Discussion
3.1 Characterizations
With the objective to realize the selective, solvent-free and
additive-free oxidation of primary benzylic C–H bonds, the
combination of metalloporphyrin and NHPI was employed
In addition to the NMR and ESI–MS spectra, the porphy-
rins and metalloporphyrins were characterized qualitatively
TPP
TPPCo
1.0
0.4
T(4-Cl)PP
T(4-Cl)PPCo
0.8
0.6
0.4
0.2
0.0
-0.2
T(2-Cl)PP
T(2-Cl)PPCo
0.3
T(2-NO2)PP
T(4-COOH)PPCo
T(2-CH3)PPCo
0.2
T(2-CH3)PP
T(2-OCH3)PPCo
T(4-OCH3)PP
0.1
0.0
-0.1
350
400
450
500
550
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
a
b
Fig. 1 UV–Vis spectra of porphyrins (a) and metalloporphyrins (b) in DMF (2.0×10–6 mol/L or saturated)
1 3

H.-M. Shen et al.
whole temperature range from 70 to 120 °C (Table S5). And
the conversion reached up to 16.6% with the selectivity of
61.1% towards 4-nitrobenzoic acid. Considering the mild
conditions, the reaction temperature was kept not higher than
120 °C. Although the catalytic transformation of primary
benzylic C–H bonds was gained employing NHPI as cata-
lyst, the conversion of 4-methyl nitrobenzene (16.6%) was
not so competitive and satisfying. Thus metalloporphyrins
were introduced to form binary catalysis system. For the
representative metalloporphyrin T(4-Cl)PPCo, only 0.012%
in molar ratio was employed to form binary catalysis system
with NHPI, the conversion of 4-methyl nitrobenzene being
increased dramatically from 16.6 to 31.3% with selectiv-
ity above 60% towards 4-nitrobenzene acid. And when the
molar ratio of NHPI was increased to 10% (mol/mol), the
conversion increased further to 40.4% with the selectivity
above 50% towards aromatic acid. The tremendous promo-
tion in catalytic activity was mainly attributed to the delicate
cooperation between metalloporphyrin and NHPI, in which
the metalloporphyrin acted as initiator, and the hydrogen
atom in the hydroxyl of NHPI molecule was abstracted by
the high-valence metal-oxo complex obtained from the inter-
action of metalloporphyrin and O2 frstly, then the formed
phthalimide N-oxyl radical (PINO) initiated the catalytic
oxidation cycle of primary benzylic C–H bonds [72, 73].
Compared with the employment of metalloporphyrin or
NHPI as unitary catalyst, the combination of metallopor-
phyrins and NHPI is a more efcient catalytic system for the
selective, solvent-free and additive-free oxidation of primary
benzylic C–H bonds, based on which the catalytic transfor-
mation was obtained under very mild conditions (120 °C)
with aromatic acids as primary products (selectivity above
50%).
TPPCo
100
80
60
40
20
0
TPFPPCo
T(4-CH3)PPCo
T(2-CH3)PPCo
T(4-COOH)PPCo
200
400
600
800
Temperature (oC)
Fig. 2 The TGA plots of representative metalloporphyrins
T(2-OCH3)PPCo
T(2-CH3)PPCo
6
4
T(2-Cl)PPCo
T(4-Cl)PPCo
T(4-COOH)PPCo
T(2,5-diCl)PPCo
2
0
-2
-4
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Potential (V)
Fig. 3 Typical cyclic voltammograms of metalloporphyrins in DMF
in scan rate of 100 mV/s
3.3 Efect of Porphyrin Structures on Oxidation
of Primary Benzylic C–H Bonds
as binary catalysts in this work. For the mild conditions, we
tried to carry out the catalytic transformation at lower tem-
perature to avoid the unselective autoxidation, and 4-methyl
nitrobenzene was employed as the representative substrate to
screen a suitable reaction temperature. As demonstrated in
Tables 1 and S1–S4, there was not any oxidation transforma-
tion of 4-methyl nitrobenzene observed before 140 °C, no
matter in the presence of metalloporphyrins as catalysts or
not. So it might not be a sensible choice to conduct the oxida-
tion of primary benzylic C–H bonds only using metallopor-
phyrins as catalysts. When N-hydroxyphthalimide (NHPI)
in molar ratio of 5% was employed as catalyst, the catalytic
oxidation of 4-methyl nitrobenzene could be obtained in the
The performance of metalloporphyrins in catalytic oxida-
tion was infuenced dramatically by the structure of ligands,
which resulted in the excellent adjustability in the catalytic
properties. To obtain the efficient oxidation of primary
benzylic C–H bonds under mild conditions, the efect of
the porphyrin ligands on the oxidation of primary ben-
zylic C–H bonds catalyzed by the combination of metal-
loporphyrins and NHPI was investigated systematically
employing 4-methyl nitrobenzene as representative sub-
strate at 120 °C and 10% NHPI (mol/mol). As illustrated
in Tables 2 and S7, for all the metalloporphyrins investi-
gated, the conversion could reach up to above 35.0% with
4-nitrobenzene acid as the main product, and 4-nitrobenzyl
1 3

Selective Solvent-Free and Additive-Free Oxidation of Primary Benzylic C–H Bonds with O2…
Table 1 Oxidation of 4-methyl nitrobenzene in the absence or presence of catalysts
No
Catalysts
Temp. (°C)
Conversion (%)
Selectivity (%)
R1-OOH
R1-OH
R2-CHO
R2-COOH
1
2
3
4
5
6
7
8
9
10
–
–
–
110
115
120
110
115
120
110
115
120
110
<1
<1
<1
<1
<1
N. D.a
N. D
N. D
N. D
N. D
N. D
18.1
16.6
15.8
11.8
N. D
N. D
N. D
N. D
N. D
N. D
12.9
14.3
15.7
20.4
N. D
N. D
N. D
N. D
N. D
N. D
12.4
10.1
7.4
N. D
N. D
N. D
N. D
N. D
N. D
56.6
59.0
61.1
61.7
T(4-Cl)PPCo
T(4-Cl)PPCo
T(4-Cl)PPCo
NHPI
NHPI
NHPI
<1
14.0
15.7
16.6
29.0
T(4-Cl)PPCo
6.1
&NHPI
11
T(4-Cl)PPCo
&NHPI
115
120
120
30.1
31.3
40.4
11.1
9.9
20.9
21.3
25.2
5.4
4.9
5.1
62.6
63.9
54.5
12
T(4-Cl)PPCo
&NHPI
13b
T(4-Cl)PPCo
&NHPI
15.2
Reaction conditions: 4-methyl nitrobenzene (50 mmol, 6.86 g), T(4-Cl)PPCo (0.012%, mol/mol, 0.0049 g), NHPI (5%, mol/mol, 0.41 g), O2
(1.0 MPa), 8.0 h, 800 rpm
aNo obvious product detected
bNHPI (10%, mol/mol, 0.82 g)
alcohol and 4-nitrobenzyl hydroperoxide as the primary by-
products. Especially for the T(2-OCH3)PPCo, the conversion
reached up to 49.4% with the selectivity of 64.5% towards
4-nitrobenzene acid, which was a very high level in both of
the conversion and selectivity for oxidation of primary ben-
zylic C–H bonds with O2. More importantly, the protocol in
our work was under the solvent-free and additive-free con-
ditions with very low metal catalyst loading, which would
facilitate the separation process after reaction and simplify
the industrial process for oxidation of primary benzylic C–H
bonds. Based on intensive study on the molecular structure
of metalloporphyrins investigated herein, the higher activ-
ity of T(2-OCH3)PPCo was mainly ascribed to the ligand
T(2-OCH3)PP decreased the positive charge density around
central metal cobalt (II) efciently as shown in Table S8
obtained from quantum chemical calculation [74]. The fewer
positive charges around cobalt (II) would favor the adduction
of O2 to cobalt (II) forming the high-valence metal-oxo com-
plex, followed by the production of phthalimide N-oxyl radi-
cal (PINO) and the initiation of the catalytic oxidation cycle
[72, 73]. The superior performance of T(2-OCH3)PPCo in
charge transfer was also demonstrated in the electrochemi-
cal measurement illustrated in Fig. 3, in which for the typi-
cal metalloporphyrins involved, when an external voltage
was exerted, the lowest oxidation potential was found for
T(2-OCH3)PPCo. Thus, as a brief summary, the lower posi-
tive charge density around central metal cobalt (II) would
accelerate the formation of high-valence metal-oxo complex,
1 3

H.-M. Shen et al.
Table 2 Efect of porphyrin structures on oxidation of primary benzylic C–H bonds catalyzed by metalloporphyrins and NHPI
Entry
Catalysts
Conversion (%)
Selectivity (%)
R1-OOH
R1-OH
R2-CHO
R2-COOH
1
2
3
4
5
6
7
8
T(2-OCH3)PPCo
T(3-OCH3)PPCo
T(4-OCH3)PPCo
TPPCo
TPFPPCo
T(2-Cl)PPCo
49.4
40.7
38.2
38.3
38.6
43.0
37.6
40.4
43.2
41.5
43.8
40.4
42.3
40.0
42.8
10.1
10.3
10.9
12.1
16.6
10.5
12.4
15.2
13.4
16.3
15.8
15.2
14.0
12.2
11.8
21.5
24.0
28.9
22.2
16.2
20.5
25.0
25.2
18.7
18.9
17.3
20.2
17.3
26.2
24.9
3.9
2.6
2.7
2.6
2.6
2.7
3.1
5.1
2.3
2.4
2.5
2.5
2.7
2.5
2.5
64.5
63.1
57.5
63.1
64.6
66.3
59.5
54.5
65.6
62.4
64.4
62.1
66.0
59.1
60.8
T(3-Cl)PPCo
T(4-Cl)PPCo
9
T(4-COOH)PPCo
T(2-CN)PPCo
T(3-CN)PPCo
T(4-CN)PPCo
T(2-NO2)PPCo
T(2,6-di Cl)PPCo
T(2,5-di Cl)PPCo
10
11
12
13
14
15
Reaction conditions: 4-methyl nitrobenzene (50 mmol, 6.86 g), metalloporphyrins (0.012%, mol/mol), NHPI (10%, mol/mol, 0.82 g), O2
(1.0 MPa), 8.0 h, 800 rpm
resulting in the more efcient catalytic oxidation cycles for
4-nitrobenzene acid in the presence of Co (II) complexes,
meanwhile the conversions were not higher than 30% for
other metal complexes without obvious advantage in selec-
tivity too. The superior performance of Co (II) complexes
was mainly attributed to their high efciency in charge
transfer and fewer positive charges around central metal
Co (II). In the cyclic voltammogram measurement shown
in Fig. 4, only the Co (II) complex and Mn (II) complex
exhibited obvious oxidation current peaks when an appropri-
ate external voltage was exerted, which meant the existence
of charge transfer. For other metal complexes, no apparent
current peak was observed, which was consistent with their
lower catalytic activity. The discrepancy in the catalytic
property between Co (II) and Mn (II) was primarily from
the diference in the positive charge density around them
(Table S10). The fewer positive charges around cobalt (II)
favored the adduction of O2 to cobalt (II) forming the high-
valence metal-oxo complex, followed by the production of
primary benzylic C–H bonds.
3.4 Efect of Central Metal on Catalytic Oxidation
of Primary Benzylic C–H Bonds
With the optimized porphyrin ligand in hands, the efect of
central metal on the catalytic oxidation of primary benzylic
C–H bonds was investigated systematically, and transition
metals such as Co (II), Mn (II), Ni (II), Fe (II), Cu (II) and
Zn (II) were selected as metal centers employing 4-methyl
nitrobenzene as representative substrate in the presence of
10% NHPI. As illustrated in Tables 3 and S9, for all the
porphyrin ligands investigated, the Co (II) complexes exhib-
ited the highest catalytic performance. In the oxidation of
4-methyl nitrobenzene catalyzed by the combination of
metalloporphyrins and NHPI, the conversion could reach
up to nearly 50% with the selectivity above 60% towards
1 3

Selective Solvent-Free and Additive-Free Oxidation of Primary Benzylic C–H Bonds with O2…
Table 3 Efect of central metal on oxidation of primary benzylic C–H bonds catalyzed by metalloporphyrins and NHPI
Entry
Catalysts
Conversion (%)
Selectivity (%)
R1-OOH
R1-OH
R2-CHO
R2-COOH
1
2
3
4
5
6
7
8
T(2-OCH3)PPCo
T(2-OCH3)PPMn
T(2-OCH3)PPNi
T(2-OCH3)PPFe
T(2-OCH3)PPCu
T(2-OCH3)PPZn
T(2-Cl)PPCo
T(2-Cl)PPMn
T(2-Cl)PPNi
T(2-Cl)PPFe
49.4
29.8
18.8
18.6
16.4
17.9
43.0
23.0
15.5
17.3
13.8
17.7
10.1
13.7
21.6
16.0
22.4
16.9
10.5
15.7
19.9
17.0
22.8
17.3
21.5
12.2
6.9
3.9
7.7
17.3
14.1
15.3
23.3
2.7
64.5
66.4
54.2
63.6
48.0
47.1
66.3
63.5
57.0
62.2
49.9
41.4
6.3
14.3
12.7
20.5
14.0
6.5
6.0
7.5
6.8
9
16.6
14.8
19.8
16.4
10
11
12
T(2-Cl)PPCu
T(2-Cl)PPZn
24.9
Reaction conditions: 4-methyl nitrobenzene (50 mmol, 6.86 g), metalloporphyrins (0.012%, mol/mol), NHPI (10%, mol/mol, 0.82 g), O2
(1.0 MPa), 8.0 h, 800 rpm
positive charges around cobalt (II), a more efcient cata-
T(2-OCH3)PPCo
T(2-OCH3)PPCu
T(2-OCH3)PPFe
T(2-OCH3)PPMn
T(2-OCH3)PPZn
T(2-OCH3)PPNi
6
4
lytic oxidation system for primary benzylic C–H bonds was
developed through combining T(2-OCH3)PPCo and NHPI as
binary catalysts under solvent-free and additive-free condi-
tions, in which the metalloporphyrin acted as initiator [72,
73].
2
0
3.5 Efect of Catalyst Loading and Pressure
-2
-4
In succession, the efect of catalyst loading and oxygen pres-
sure was explored further with the purpose to realize the
catalytic oxidation of primary benzylic C–H bonds more
efciently and under milder conditions, and the 4-methyl
nitrobenzene was employed as representative substrate, T(2-
OCH3)PPCo and T(2-Cl)PPCo as catalysts. As illustrated
in Tables 4, S11 and S12, with the increase in both of the
metalloporphyrins loading and NHPI loading, the conver-
sion of substrate was increased dramatically. For example,
the conversion of 4-methyl nitrobenzene was increased
from 15.4 to 49.4% as the loading of T(2-OCH3)PPCo was
increased from 0.003 to 0.012% (mol/mol), with the selec-
tivity above 60% towards 4-nitrobenzene acid. To make a
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Potential (V)
Fig. 4 Typical cyclic voltammograms of metalloporphyrins in DMF
in scan rate of 100 mV/s
phthalimide N-oxyl radical (PINO) and the initiation of the
catalytic oxidation cycle [72, 73]. Thus, due to the delicate
cooperation between charge transfer property and fewer
1 3

H.-M. Shen et al.
Table 4 Efect of metalloporphyrins loading on catalytic oxidation of 4-methyl nitrobenzene
Entry
Metalloporp. loading
(%)
Conversion (%)
Selectivity (%)
R1-OOH
R1-OH
R2-CHO
R2-COOH
1a
2a
3a
4a
5a
6b
7b
8b
9b
10b
0.003
0.006
0.008
0.010
0.012
0.003
0.006
0.008
0.010
0.012
15.4
24.1
30.8
35.9
49.4
16.1
28.9
32.1
36.0
43.0
13.3
13.2
12.4
10.3
10.1
12.8
12.2
11.9
11.6
10.5
17.1
17.9
17.9
20.9
21.5
19.8
20.1
20.4
20.3
20.5
8.9
6.9
6.1
4.6
3.9
4.0
3.8
3.7
3.4
2.7
60.7
62.0
63.6
64.2
64.5
63.4
63.9
64.0
64.7
66.3
Reaction conditions: 4-methyl nitrobenzene (50 mmol, 6.86 g), metalloporphyrins (mol/mol), NHPI (10%, mol/mol, 0.82 g), O2 (1.0 MPa),
120 °C, 8.0 h, 800 rpm
aT(2-OCH3)PPCo
bT(2-Cl)PPCo
balance between catalytic performance and catalyst con-
sumption, the metalloporphyrins loading was set as 0.012%
(mol/mol) and the NHPI loading was set as 10% (mol/mol).
As for reaction pressure, a similar upward trend like the cata-
lyst loading was observed (Table 5). When the combination
of T(2-OCH3)PPCo and NHPI was utilized as catalyst, the
conversion of 4-methyl nitrobenzene increased from to 22.3
to 51.6% with the increase of reaction pressure from 0.40 to
1.20 MPa, and the selectivity towards 4-nitrobenzene acid
reached up to 65.7%. Considering the mild conditions and
the slight increase in conversion and selectivity, the reaction
pressure was suggested not higher than 1.0 MPa. And when
air (1.0 MPa) was employed as oxidant, the conversion of
4-methyl nitrobenzene was just 8.2% (Entry 11, Table 5),
which could be ascribed to the low concentration of oxidant.
Thus, in this work, the oxidant was selected as molecular
oxygen (O2). Up to now, an efcient and mild protocol in
selective oxidation of primary benzylic C–H bonds by O2
was established employing 4-methyl nitrobenzene as sub-
strate, and the combination of T(2-OCH3)PPCo and NHPI
as catalysts, in which no solvent and no additive was used.
The conversion could reach up to nearly 50% with the selec-
tivity above 60% towards aromatic acid, which had a great
application potential in the industrial oxidation of primary
benzylic C–H bonds.
3.6 Substrate Scope and Comparison with Existing
Oxidation Systems
With the efcient protocol for oxidation of 4-methyl nitroben-
zene established, the substrate scope was extended from
4-methyl nitrobenzene to a series of typical compounds con-
taining primary benzylic C–H bonds, such as methylbenzene,
1-methylnaphthalene, 4-methyl methylbenzene, 4-chloro
methylbenzene and so on. As demonstrated in Table 6, for
all the typical substrates, the selective oxidation of primary
1 3

Selective Solvent-Free and Additive-Free Oxidation of Primary Benzylic C–H Bonds with O2…
Table 5 Efect of pressure on catalytic oxidation of 4-methyl nitrobenzene
Entry
Pressure (MPa)
Conversion (%)
Selectivity (%)
R1-OOH
R1-OH
R2-CHO
R2-COOH
1a
0.40
0.60
0.80
1.00
1.20
0.40
0.60
0.80
1.00
1.20
1.00
22.3
25.2
37.7
49.4
51.6
23.0
25.9
37.9
43.0
44.8
8.2
19.1
17.3
14.6
10.1
9.4
14.6
11.0
10.9
10.5
10.7
15.8
10.3
13.9
17.6
21.5
21.6
15.0
16.9
18.4
20.5
20.7
18.1
9.3
6.2
4.9
3.9
3.3
8.4
7.9
5.9
2.7
2.2
12.7
61.3
62.6
62.9
64.5
65.7
62.0
64.2
64.8
66.3
66.4
53.4
2a
3a
4a
5a
6b
7b
8b
9b
10b
11c
Reaction conditions: 4-methyl nitrobenzene (50 mmol, 6.86 g), metalloporphyrins (0.012%, mol/mol, 0.0049 g), NHPI (10%, mol/mol, 0.82 g),
O2, 120 °C, 8.0 h, 800 rpm
aT(2-OCH3)PPCo
bT(2-Cl)PPCo
cT(2-OCH3)PPCo (0.012%, mol/mol), NHPI (10%, mol/mol), air (1.0 MPa)
benzylic C–H bonds could be accomplished under mild con-
ditions with aromatic acids as the primary products, espe-
cially for methylbenzene, 4-phenyl methylbenzene, 4-chloro
methylbenzene, 4-fluoro methylbenzene and 2,4-dichloro
methylbenzene, for which the selectivity towards aromatic
acids reached up to 92.3%, 91.6%, 95.1%, 94.6% and 94.1%
respectively with the acceptable conversions (35.1%, 73.5%,
88.5%, 85.3% and 63.2%). For other typical substrates, the
selectivity towards aromatic acids also reached up to higher
than 60%, and the primary products were 1,4-dicarboxyben-
zene and 1,3-dicarboxybenzene for 4-methyl methylbenzene
and 3-methyl methylbenzene as substrates. Based on the sat-
isfying substrate tolerance, the method disclosed in this work
could be regarded as an efcient strategy in selective oxida-
tion of primary benzylic C–H bonds with O2 employing the
combination of T(2-OCH3)PPCo and NHPI as catalysts under
the solvent-free and additive-free conditions. Then the strategy
in this work was compared with some reported methods in
oxidation of primary benzylic C–H bonds with O2, which had
been demonstrated in Table 7 clearly that the major superiori-
ties of our route were no solvent, no additive, mild conditions,
and higher selectivity towards aromatic acids, which made the
route for primary benzylic C–H bonds oxidation more simple
and practical, and more suitable for industrial application. This
work also was a signifcant example in which the efect of the
metalloporphyrins structures on selective oxidation of C–H
bonds was explored systematically. In addition to the satisfying
substrate tolerance, an enormous function was endowed to the
current route as an important reference for other C–H bonds
1 3

H.-M. Shen et al.
Table 6 Oxidation of primary benzylic C–H bonds catalyzed by metalloporphyrins and NHPI
Entry
1
Substrates
Conversion (%)
35.1
Selectivity (%)
R1-OOH
R1-OH
N.D.a
R2-CHO
4.0
R2-COOH
92.3
3.7
2
30.4
14.8
N.D
6.3
78.9
3
35.4
73.5
13.4
1.8
N.D
N.D
5.3
6.6
81.3
91.6
4b
5c
27.3
9.6
N.D
N.D
81.3
6d
7
14.8
34.2
14.9
18.3
N.D
4.2
N.D
5.7
71.4
71.8
8e
8.4
20.1
N.D
23.1
56.8
9
11.3
88.5
14.4
1.0
N.D
1.6
20.2
2.3
65.4
95.1
10
11
85.3
1.2
2.4
1.8
94.6
1 3

Selective Solvent-Free and Additive-Free Oxidation of Primary Benzylic C–H Bonds with O2…
Table 6 (continued)
Entry
Substrates
Conversion (%)
63.2
Selectivity (%)
R1-OOH
R1-OH
N.D
R2-CHO
3.8
R2-COOH
94.1
12
2.1
13
49.4
10.1
21.5
3.9
64.5
Reaction conditions: substrate (50 mmol), T(2-OCH3)PPCo (0.012%, mol/mol, 0.0048 g), NHPI (10%, mol/mol, 0.82 g), O2 (1.0 MPa), 8.0 h,
120 °C, 800 rpm
aNo obvious product detected
b100 °C
c1,4-dicarboxybenzene as main product
d1,3-dicarboxybenzene as main product
eT(2-Cl)PPCo (0.012%, mol/mol, 0.0049 g)
oxidative functionalization using the clean O2 as oxidant under
mild conditions.
with the conversions of 35.1%, 73.5%, 88.5%, 85.3% and
63.2% respectively. The superior performance of T(2-OCH3)
PPCo among the metalloporphyrins involved was mainly
attributed to its high efciency in charge transfer and fewer
positive charges around central metal Co (II) which favored
the adduction of O2 to cobalt (II) forming the high-valence
metal-oxo complex, followed by the production of phthalim-
ide N-oxyl radical (PINO) and the initiation of the catalytic
oxidation cycle. Compared with the existing primary ben-
zylic C–H bonds oxidation systems, the main superiorities
of the route in this work were no solvent, no additive, mild
conditions, higher selectivity towards aromatic acids and sat-
isfying substrate tolerance. To the best of our knowledge,
this work is a very scarce and signifcant instance in the
oxidative functionalization of primary benzylic C–H bonds
without any solvent or additive used under milder condi-
tions, which not only provided an efcient and practical
protocol in utilization of widely available and inexpensive
hydrocarbons containing primary benzylic C–H bonds, but
also would present a signifcant reference in both academic
research and industrial application for the construction of
more efcient C–H bonds oxidation systems.
4 Conclusions
The efcient and selective oxidation of primary benzylic
C–H bonds by O2 under solvent-free and additive-free
conditions was achieved employing the combination of
metalloporphyrins and NHPI as binary catalysts. Based
on the systematical investigation on the efects of reaction
temperature, porphyrin structure, central metal, catalyst
loading and O2 pressure, the optimized catalytic system
was obtained as the combination of T(2-OCH3)PPCo and
NHPI, in which all the typical substrates possessing primary
benzylic C–H bonds could be catalytically transformed to
their oxygen-containing products at 120 °C and 1.0 MPa O2
with aromatic acids as the primary products. For the typi-
cal substrates such as methylbenzene, 4-phenyl methylben-
zene, 4-chloro methylbenzene, 4-fuoro methylbenzene and
2,4-dichloro methylbenzene, the selectivity towards aromatic
acids reached up to 92.3%, 91.6%, 95.1%, 94.6% and 94.1%
1 3

H.-M. Shen et al.
Table 7 Comparison with some existing primary benzylic C–H bonds oxidation systems employing O2 as oxidant
Entry Main products
1
Conditions
Conversion (%) Selectivity (%) Refs.
T(4-Cl)PPMnCl, NHPI, CTAB, O2, 100 °C
41.0
75.2
23.2
80.0
35.3
29.9
18.6
60.1
14.0
70.6
93.5
60.9
78.0
71.8
89.3
60.7
99.6
44.3
[52]
[14]
[75]
[41]
[53]
[32]
[33]
[37]
[44]
2
3
4
5
6
7
8
9
CoOx/SiO2, NHPI, AcOH, O2, room temperature
Cobalt in N-doped carbon nanotube, O2, 120 °C
Co(OAc)2, NHPI, ionic liquid, O2, 100 °C
Immobilized phthalocyanine iron, NHPI, O2, CH3CN, 80 °C
Co-SBA-15@ NDHPI, O2, CH3CN, 90 °C
Immobilized NDHPI, O2, CH3CN, 90 °C
Au nanoparticles, NHPI, O2, CH3CN, 60 °C
TPPMnCl, O2, Cyclohexene, CH3CN, 160 °C
10
11
Immobilized NHPI, Co(OAc)2, AIBN, AcOH, O2, 80 °C
Immobilized NHPI, Co(OAc)2, AIBN, AcOH, O2, 80 °C
17.7
17.7
24.0
44.0
[34]
[34]
12
13
Co(OAc)2, NHSI, Mn(OAc)2, O2, CH3CN, 150 °C
Co(OAc)2, NHSI, CH3CN, O2, 40 °C
99.5
99.6
96.0
[18]
[40]
>99.0%
14
CoOx/SiO2, NHPI, HFIP, O2, room temperature
92.2
69.7
[14]
1 3

Selective Solvent-Free and Additive-Free Oxidation of Primary Benzylic C–H Bonds with O2…
Table 7 (continued)
Entry Main products
15
Conditions
Conversion (%) Selectivity (%) Refs.
Copper particles in graphene, O2, acetone, 110 °C
9.0
62.0
5.1
98.0
99.0
48.0
56.4
66.7
[39]
[38]
[15]
[53]
[44]
16
17
18
19
Co(OAc)2, NHPI, HFIP, O2, room temperature
Anion modifed mesoporous oxides, O2, 120 °C
Immobilized phthalocyanine iron, NHPI, O2, CH3CN, 80 °C
TPPMnCl, O2, Cyclohexene, CH3CN, 160 oC
23.7
7.2
20
21
T(2-OCH3)PPCo, NHPI, O2, 120 °C
T(2-OCH3)PPCo, NHPI, O2, 120 °C
35.1
88.5
92.3
95.1
This
work
This
work
22
T(2-OCH3)PPCo, NHPI, O2, 120 °C
73.5
91.6
This
work
10. Zhao GX, Hul B, Busser GW, Peng BX, Muhler M (2019) Chem-
suschem 12:2795
Acknowledgements This work was supported by the National Natu-
ral Science Foundation of China (Grant Nos. 21776259, 21878275,
21476270, 21306176).
11. Jie SS, Lin X, Chen QZ, Zhu RL, Zhang LY, Zhang BS, Liu ZG
(2018) Appl Surf Sci 456:951
12. Bindu VH, Parvathaneni SP, Rao VJ (2017) Catal Lett 147:1434
13. Saha R, Sekar G (2019) Appl Catal B 250:325
14. Shi GJ, Xu SH, Bao Y, Xu JY, Liang YX (2019) Catal Commun
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