Efficient and selective oxidation of tertiary benzylic C[sbnd]H bonds with O2 catalyzed by metalloporphyrins under mild and solvent-free conditions

Article abstract of DOI:10.1016/j.apcata.2020.117599

The direct and efficient oxidation of tertiary benzylic C[sbnd]H bonds to alcohols with O₂ was accomplished in the presence of metalloporphyrins as catalysts under solvent-free and additive-free conditions. Based on effective inhibition on the unselective autoxidation and deep oxidation, systematical investigation on the effects of porphyrin ligands and metal centers, and apparent kinetics study, the oxidation system employing porphyrin manganese(II) (T(2,3,6-triCl)PPMn) with bulkier substituents as catalyst, was regarded as the most promising and efficient one. For the typical substrate, the conversion of cumene could reach up to 57.6% with the selectivity of 70.5% toward alcohol, both of them being higher than the current documents under similar conditions. The superiority of T(2,3,6-triCl)PPMn was mainly attributed to its bulkier substituent groups preventing metalloporphyrins from oxidative degradation, its planar structure favoring the interaction between central metal with reactants, and the high efficiency of Mn(II) in the catalytic transformation of hydroperoxides to alcohols.

Full text of DOI:10.1016/j.apcata.2020.117599

AppliedCatalysisA,General599(2020)117599
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Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Efficient and selective oxidation of tertiary benzylic CeH bonds with O2
catalyzed by metalloporphyrins under mild and solvent-free conditions
Hai-Min Shen, Meng-Yun Hu, Lei Liu, Bei Qi, Hong-Liang Ye, Yuan-Bin She*
College of Chemical Engineering, State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology, Hangzhou 310014,
China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Benzylic CeH bond
Oxidation
Molecular oxygen
Metalloporphyrin
Alcohol
The direct and efficient oxidation of tertiary benzylic CeH bonds to alcohols with O2 was accomplished in the
presence of metalloporphyrins as catalysts under solvent-free and additive-free conditions. Based on effective
inhibition on the unselective autoxidation and deep oxidation, systematical investigation on the effects of
porphyrin ligands and metal centers, and apparent kinetics study, the oxidation system employing porphyrin
manganese(II) (T(2,3,6-triCl)PPMn) with bulkier substituents as catalyst, was regarded as the most promising
and efficient one. For the typical substrate, the conversion of cumene could reach up to 57.6% with the se-
lectivity of 70.5% toward alcohol, both of them being higher than the current documents under similar con-
ditions. The superiority of T(2,3,6-triCl)PPMn was mainly attributed to its bulkier substituent groups preventing
metalloporphyrins from oxidative degradation, its planar structure favoring the interaction between central
metal with reactants, and the high efficiency of Mn(II) in the catalytic transformation of hydroperoxides to
alcohols.
1. Introduction
devoted to the direct transformation of cumene and its derivatives to
acetophenone, 2-benzyl-2-propanol and their derivatives. Thus, besides
The oxidative functionalization of benzylic CeH bonds in alkyl
aromatics with molecular oxygen directly under solvent-free conditions
are a series of extremely useful and important processes in chemical
industry, for the high value and extensive application of their oxyge-
nated products, the shortest conversion paths, the highest atom
economy and lower environmental impact [1–4]. Among these trans-
formations, one significant process is the oxidation of tertiary benzylic
CeH bonds represented by the typical oxidation of cumene, in which
valuable cumene hydroperoxide in chemical industry usually is ex-
pected as the target product, since cumene hydroperoxide not only is
the irreplaceable precursor in the production of phenol and acetone, but
also is the essential oxidant in the industrial oxidation of propylene to
propylene oxide [4–7]. Besides cumene hydroperoxide, another two
main oxygenated products in oxidation of cumene are acetophenone
and 2-benzyl-2-propanol, both of which are regarded as by-products
[5,8]. And the main objective in oxidation of cumene is focused on the
production of hydroperoxide. But in chemical industry, all of acet-
ophenone, 2-benzyl-2-propanol and their derivatives are very im-
portant fine chemical materials, and applied widely in the manufacture
of pharmaceuticals [9–11], pesticides [12,13], dyes [14,15], perfumes
[16–18], and so on [19], meanwhile scarce attempt and report were
hydroperoxides, the direct conversion of tertiary benzylic CeH bonds to
aromatic ketones or tertiary benzylic alcohols with O2 is also an at-
tractive route to synthesize these important fine chemical materials,
especially to tertiary benzylic alcohols for the high atom economy and
the high-value of the oxygenated products [9–11].
In these scarce documentations on the direct transformation of
tertiary benzylic CeH bonds to aromatic alcohols, Leung and co-
workers documented the oxidation of cumene with tert-butyl hydro-
peroxide catalyzed by water-soluble ruthenium(III) complexes at 25 °C
in water, and the yield of 2-benzyl-2-propanol reached up to 75% [20].
Similarly, Song and co-workers employed hollow carbon shell doped
with nitrogen, phosphorus and sulfur as catalyst to conduct the selec-
tive oxidation of tertiary benzylic CeH bonds with tert-butyl hydro-
peroxide at 80 °C in water, and for cumene as substrate, the selectivity
toward tertiary benzylic alcohol reached up to 77% in the conversion of
90% [21]. Besides tert-butyl hydroperoxide, Lachter and co-workers
reported the oxidative functionalization of aromatic compounds with
hydrogen peroxide employing mononuclear non-heme iron(III) com-
plexes as catalysts at 50 °C in acetonitrile, and the selectivity toward 2-
benzyl-2-propanol was 12% in the conversion of 31% for cumene as
substrate [22]. Bryliakov and co-workers reported the aminopyridine
⁎ Corresponding author.
E-mail addresses: haimshen@zjut.edu.cn (H.-M. Shen), haimshen@zjut.edu.cn (Y.-B. She).
https://doi.org/10.1016/j.apcata.2020.117599
Received 19 February 2020; Received in revised form 20 April 2020; Accepted 24 April 2020
Availableonline05May2020
0926-860X/©2020ElsevierB.V.Allrightsreserved.

H.-M. Shen, et al.
AppliedCatalysisA,General599(2020)117599
manganese(II) complexes catalyzed oxidation of cumene employing
hydrogen peroxide as oxidant and acetic acid as additive at 0 °C in
acetonitrile with the selectivity of 81% toward 2-benzyl-2-propanol in
the conversion of 76% [23]. As for the green oxidant O2, our group
reported the oxidative transformation of 4-nitrocumene in 4.5 M
ethanol solution of sodium hydroxide at 60 °C under 1.80 MPa O2, and
the selectivity of tertiary benzylic alcohol reached up to 87% in the
conversion of 97% [24]. Another typical example in conversion of
tertiary benzylic CeH to alcohol was reported by Peng employing
carbon nanotubes doped with nitrogen as catalysts and O2 as oxidant,
and the conversion of cumene reached up to 86% with the selectivity of
56% toward tertiary benzylic alcohol [8]. Based on the concise sum-
mary above and comprehensive review on related literatures
[5,6,23,25–27], it is obvious that the direct and selective conversion of
tertiary benzylic CeH bonds to alcohols has not been paid enough at-
tention and systematical study considering the high-value and extensive
application of tertiary benzylic alcohols, especially employing O2 as
oxidant under mild and solvent-free conditions. Therefore, it would be a
novel, meaningful, and promising work to carry out the direct and se-
lective oxidation of tertiary benzylic CeH bonds to alcohols with O2
under mild and solvent-free conditions. To realize this objective, sui-
table catalysis system would be the key.
Metalloporphyrins, as the chemical model compounds of
Cytochrome P450, are composed by a series of transition metal com-
plexes and have been applied widely in the oxidative functionalization
of CeH bonds with O2 for their high efficiency and selectivity in activity
of O2 and functionalization of CeH bonds [28–34]. Metalloporphyrins
in molar ratio of one in ten thousand to the substrates would be enough
to realize the oxidative transformation of CeH bonds in various al-
kanes. Hence, metalloporphyrins were regarded as a kind of green
catalysts in oxidation reactions [35–37]. In addition, the catalytic
performance of metalloporphyrins are very adjustable. Not only the
central metal in metalloporphyrins can be varied among almost all the
transition metal elements, but also the substituent groups around por-
phyrin ring can be modified with purpose. Both of them would regulate
the performance of metalloporphyrins in catalytic reactions, and pro-
vide a large number of catalyst candidates with various catalytic per-
formance, which is in strong favor of the efficient and selective oxi-
dation of CeH bonds in different structures, including the tertiary
benzylic CeH bonds.
2. Experimental
2.1. Materials
The aromatic aldehydes employed in the syntheses of porphyrins
were purchased from Energy Chemical Co. Ltd. China, Xilong Chemical
Reagent Co. Ltd. China, Adamas Reagent Co. Ltd. China and Macklin
Biochemical Co. Ltd. China respectively. Pyrrole was purchased from
Xilong Chemical Reagent Co. Ltd. China, which should be redistilled
under reduced pressure before used. The metal acetates in the syntheses
of metalloporphyrins were purchased from Energy Chemical Co. Ltd.
China and Adamas Reagent Co. Ltd. China. Cumene and its derivatives
as well as their oxidation products were purchased from Energy
Chemical Co. Ltd. China, Adamas Reagent Co. Ltd. China, Macklin
Biochemical Co. Ltd. China and Xilong Chemical Reagent Co. Ltd. China
respectively. All the chemical reagents involved in the experiments
were analytically pure and used directly as received. No additional
purification operations were performed for all the reagents unless
otherwise noted.
2.2. Characterizations
The 1H NMR and 13C NMR spectra were obtained employing a
Bruker AVANCE III 500 MHz NMR spectrometer to determine the
structure of porphyrins with CDCl3 as solvent and tetramethylsilane as
internal standard. The ESI-MS data of porphyrins and metallopor-
phyrins were recorded on a Agilent 6210 LC/TOF mass spectrometer
through direct injection technique. The UV–vis data were collected on a
HITACHI U-3900 spectrometer using DMF as solvent in a quartz cuvette
at room temperature. The thermal stability of metalloporphyrins was
studied through thermal gravimetric analysis (TGA) on a PerkinElmer
Diamond TG/DTA instrument in the atmosphere of air from room
temperature to 800 °C with the ramping speed of 10 °C/min. The redox
potentials of representative metalloporphyrins were measured on a
ZAHNER Zennium electrochemical workstation through three-electrode
system in which a glassy carbon was employed as working electrode, a
platinum wire was used as counter electrode and Ag/AgCl was em-
ployed as the reference electrode in the presence of tetra-
butylammonium hexafluorophosphate (TBAPF6) (0.025 mol/L in DMF)
as supporting electrolyte. All the electrochemical measurements were
conducted under the atmosphere of nitrogen at 25 °C.
In this research, the direct, efficient, and selective oxidation of
tertiary benzylic CeH bonds to alcohols was conducted with O2 as
oxidant catalyzed by metalloporphyrins under mild and solvent-free
conditions following the previous investigation on the selective oxida-
tion of CeH bonds in our group [38–42]. Owing to the diverse catalytic
performance of metalloporphyrins, almost all the tertiary benzylic CeH
bonds were converted to tertiary benzylic alcohols directly at
70∼120 °C in high conversion and selectivity with satisfying substrate
tolerance. The conversion of cumene reached up to 57.6% with the
selectivity of 70.5% toward 2-benzyl-2-propanol at 70 °C and atmo-
spheric pressure of O2, a very mild condition. For other typical sub-
strates possessing tertiary benzylic CeH bonds, satisfying conversion
and selectivity were achieved too. Compared with current reports on
tertiary benzylic CeH bonds oxidation, the main advantages of this
work were the direct conversion of tertiary benzylic CeH bonds to al-
cohols, high selectivity toward tertiary benzylic alcohols, high con-
version, and mild conditions. To the best of our knowledge, this work is
an extremely novel, meaningful, and practical example in direct and
selective oxidation of tertiary benzylic CeH bonds to alcohols with O2
under mild and solvent-free conditions, which not only is a useful re-
ference for the direct and efficient oxidative functionalization of CeH
bonds, but also is an enormous promotion for the extensive application
of metalloporphyrins in the field of catalysis.
2.3. Syntheses of porphyrins
The porphyrin ligands involved in this work were synthesized
mainly following the Adler–Longo method [43–45] and Lindsay
method [46–48] with some modifications. In the typical procedure of
Adler–Longo method, aromatic aldehyde (150 mmol) was dissolved in
propionic acid (450 mL) followed by heating and stirring to refluxing
(145 °C) under the nitrogen atmosphere, and then pyrrole (150 mmol)
which had been redistilled was injected into the refluxing solution
dropwise. The resultant mixture was stirred with refluxing for another
2.0 h and cooled to the ambient temperature naturally. All of above
operations were conducted in the shield from ambient light. After kept
standing at 20 °C for 24.0 h, the reaction mixture was transferred to
suction filtration. The collected precipitate was suspended in methanol
(200 mL) and kept stirring at ambient temperature for 6.0 h. The second
suction filtration was carried out to collect the purplish red precipitate,
which was washed with methanol two times (2 × 100 mL) successively
and then subjected to the silica column chromatography separation
with eluent of cyclohexane and dichloromethane in the volume ratio of
10:1∼4:1. All the porphyrins synthesized via Adler–Longo method
were dried at 80 °C for 8.0 h under reduced pressure before further
used, and characterized qualitatively through 1H NMR, 13C NMR and
ESI-MS. The details for the syntheses and characterizations of por-
phyrins have been illustrated in the Supplementary Information.
2

H.-M. Shen, et al.
AppliedCatalysisA,General599(2020)117599
In the typical procedure of Lindsay method, aromatic aldehyde
O2 for 3.0 min. Then the reaction tube was sealed with an oxygen
balloon, and heated with stirring to the set reaction temperature. After
stirring with the oxygen balloon for 8.0 h, the resultant mixture was
cooled to room temperature naturally followed by the addition of tri-
phenylphosphine (6.0 mmol) and another stirring of 30 min at room
temperature. The obtained mixture was transferred to a volumetric
flask with the capacity of 50 mL using acetone as solvent. In succession,
GC analyses and HPLC analyses were carried out to determine the
conversion and selectivity in the catalytic oxidation of tertiary benzylic
CeH bonds.
(6.0 mmol) and pyrrole (6.0 mmol) which had been redistilled, were
dissolved in dry dichloromethane (600 mL) under the nitrogen atmo-
sphere, and the obtained solution was stirred at ambient temperature
for 15 min in the shield from ambient light. BF3%OEt2 (2.0 mmol) was
injected into the reaction mixture in one portion, and the resultant
reaction solution was kept stirring for 3.0 h at ambient temperature
followed by the addition of p-chloranil (6.0 mmol) and stirring for an-
other 3.0 h. Then the reaction was quenched through addition of trie-
thylamine (2.0 mL). After evaporation of solvent, the crude products
were passed through neutral alumina column for purification with di-
chloromethane as eluent and all of the eluent from black to red was
collected. The crude products obtained from concentrating the collected
eluent were purified further through silica column chromatography
using the eluent of cyclohexane and dichloromethane in the volume
ratio of 4:1∼1:1. All the porphyrins synthesized via Lindsay method
were dried at 80 °C for 8.0 h under reduced pressure before further
used, and characterized qualitatively through 1H NMR, 13C NMR and
ESI-MS. The details for the syntheses and characterizations of por-
phyrins have been illustrated in the Supplementary Information.
2.7. Kinetic study
The kinetic studies of tertiary benzylic CeH bonds oxidation were
conducted employing cumene as representative at the temperatures of
75, 80 and 85 °C. In the typical operating procedure, cumene (1.2019 g,
10.0 mmol) was added to a reaction tube (25 mL) with metallopor-
phyrins (0.012%, mol/mol) as catalysts, and purged with O2 for
3.0 min. Then the reaction tube was sealed with an oxygen balloon, and
heated with stirring to the set reaction temperature. After stirring with
the oxygen balloon for set reaction time (1.00, 1.50, 2.00, 2.50, and
3.00 h), the reaction was stopped and cooled to room temperature
quickly in ice-water bath followed by the addition of triphenylpho-
sphine (6.0 mmol) and another stirring of 30 min at room temperature.
The resultant mixture of cumene and its oxidation products was
transferred to a volumetric flask with the capacity of 50 mL using
acetone as solvent. In succession, GC analyses and HPLC analyses were
carried out to determine the conversion and yield in the oxidation of
cumene.
2.4. Syntheses of metalloporphyrins
The metallization of porphyrins was achieved through refluxing the
ligand with corresponding metal acetate in DMF according to the re-
ported procedure [48–50]. In a typical procedure, porphyrin ligand
(0.20 mmol) and corresponding anhydrous metal acetate (2.0 mmol)
were dissolved in DMF (60 mL), and the obtained solution was heated
to refluxing (150 °C) with stirring under N2 atmosphere. After refluxing
for 16.0 h, the resultant mixture was cooled to room temperature
naturally. Then the obtained solid residue from rotary evaporation
under reduced pressure was dissolved in dichloromethane (60 mL), and
the purplish red solution was washed with water five times
(5 × 200 mL) until the upper water layer became clear and colorless.
The isolated lower layer liquid was dried over anhydrous Na2SO4. The
purple solid obtained from rotary evaporation of the solvent was sub-
jected to silica column chromatography separation with eluent of cy-
clohexane and dichloromethane in the volume ratio of 4:1∼1:1. The
metalloporphyrins synthesized in this work were dried at 80 °C for 8.0 h
under reduced pressure before used, and characterized qualitatively
through 1H NMR, 13C NMR and ESI-MS. The details for the syntheses
and characterizations of metalloporphyrins have been illustrated in the
Supplementary Information.
2.8. Products analyses
The qualitative determinations of the unreacted substrates and
oxidation products were conducted through comparison of the reten-
tion times in chromatographic analyses with the authentic samples. The
quantitative determinations of the unreacted substrates and oxidation
products such as alcohols, ketones and triphenylphosphine oxide, were
carried out through GC analyses with the internal standard of naph-
thalene. The yield of hydroperoxide was calculated via the quantity of
triphenylphosphine oxide from reduction of hydroperoxide with tri-
phenylphosphine. All the GC analyses were carried out employing a TG-
5MS capillary column (30 m × 0.32 mm × 0.25 μm) on Thermo
Scientific instrument of Trace 1300 with a Flame Ionization Detector.
The quantitative determinations of aromatic carboxylic acids were
conducted through HPLC analyses with the internal standard of 2-
naphthalenecarboxylic acid. All the HPLC analyses were conducted
2.5. Autoxidation of tertiary benzylic CeH bonds
The autoxidation of tertiary benzylic CeH bonds with O2 were
conducted employing cumene as representative. In the typical oper-
ating procedure, cumene (1.2019 g, 10.0 mmol) was added to a reaction
tube (25 mL) and purged with O2 for 3.0 min. Then the reaction tube
was sealed with an oxygen balloon, and heated with stirring to the set
reaction temperature. After stirring with the oxygen balloon for 8.0 h,
the reaction mixture was cooled to room temperature naturally fol-
lowed by the addition of triphenylphosphine (6.0 mmol) and another
stirring of 30 min at room temperature. The resultant mixture of cu-
mene and its oxidation products was transferred to a volumetric flask
with the capacity of 50 mL using acetone as solvent. In succession, GC
analyses and HPLC analyses were carried out to determine the con-
version and selectivity in the autoxidation of tertiary benzylic CeH
bonds.
with
a
Amethyst C18-H liquid chromatography column
(250 mm × 4.6 mm × 0.25 μm) on Thermo Scientific chromatography
of Ultimate 3000 equipped with a Photodiode Array Detector. All the
conversion and selectivity were the average values from three experi-
ments.
3. Results and discussion
3.1. Characterizations
Besides the qualitative characterizations of NMR and ESI-MS, the
UV–vis absorption spectra of porphyrins and metalloporphyrins in-
volved in this work were collected as illustrated in Figs. 1 and S1. The
appearance of characteristic absorption peaks of porphyrins in about
420 nm, porphyrin manganeses(II) in about 460 nm and porphyrin co-
balts(II) in about 420 nm was a strong evidence for the successful
syntheses of porphyrins and metalloporphyrins [51–56]. When the
porphyrin manganeses(II) were formed, obvious shifts in the char-
acteristic absorption peaks were observed from 420 to 460 nm. And the
differences in the absorption intensity were mainly attributed to the
2.6. Catalytic oxidation of tertiary benzylic CeH bonds
In the typical operating procedure, metalloporphyrin catalyst
(0.012%, mol/mol) was dissolved in cumene or its derivative
(10.0 mmol) with stirring in a reaction tube (25 mL), and purged with
3

H.-M. Shen, et al.
AppliedCatalysisA,General599(2020)117599
Fig. 1. The UV–vis absorption spectra of representative porphyrins (a) and metalloporphyrins (b) in DMF (4.0 × 10−6 mol/L).
Fig. 2. The thermal gravimetric analysis (TGA) curves of representative me-
Fig. 3. The cyclic voltammetry curves of representative metalloporphyrins in
talloporphyrins.
DMF at the scan speed of 100 mV/s with a reference electrode of Ag/AgCl.
diversities in molecular structure and solubility of porphyrins and
metalloporphyrins in DMF. To investigate the thermal stability of me-
talloporphyrins as catalysts in oxidation, the thermal gravimetric ana-
lyses (TGA) of representative metalloporphyrins were conducted in the
atmosphere of air from room temperature to 800 °C. As demonstrated in
Figs. 2 and S2, no obvious weight loss were observed before 300 °C for
most of the metalloporphyrins investigated except for the ones con-
taining cyano groups. And when the temperature was below 200 °C, the
metalloporphyrins possessing cyano groups were stable enough without
obvious weight loss. As for the tetrakis(4-carboxylphenyl)porphyrin
cobalt(II) (T(4−COOH)PPCo(II)), the weight loss before 100 °C was
mainly ascribed to the desorption of adsorbed water. Thus, based on the
TG analyses, it could be regarded to some extent, that the metallopor-
phyrins were relatively stable as catalysts in oxidation reaction when
the reaction temperature was not higher than 200 °C. It was also an
important direction for the determination of reaction temperature in
oxidation of tertiary benzylic CeH bonds catalyzed by metallopor-
phyrins. As catalysts in oxidation, the redox performances of the re-
presentative metalloporphyrins were measured through cyclic voltam-
metry employing tetrabutylammonium hexafluorophosphate (TBAPF6)
(0.025 mol/L in DMF) as supporting electrolyte. It was shown in Fig. 3,
when a suitable applied voltage was exerted, obvious oxidation po-
tentials and reduction potentials could be observed for all of the me-
talloporphyrins investigated, which indicated that they possessed fine
performance in electron transfer and potential to act as catalysts in
oxidation reactions.
point needed to be considered, because the unselective autoxidation of
tertiary benzylic CeH bonds and deep oxidation of the target products
would happen at higher temperature. Thus, the effect of temperature on
the oxidation of tertiary benzylic CeH bonds was investigated sys-
tematically employing cumene as representative substrate, tetrakis(4-
chlorophenyl) porphyrin cobalt(II) (T(4-Cl)PPCo) and tetrakis
(2,3,4,5,6-pentafluorophenyl) porphyrin cobalt(II) (TPFPPCo) as cata-
lysts. It was illustrated in Table 1 that, no obvious autoxidation of cu-
mene happened when the reaction temperature was below 90 °C (85, 80
and 75 °C), and as reaction temperature increased from 85 to 90 °C, a
slight autoxidation was observed with cumene hydroperoxide as the
main product. And when metalloporphyrin such as T(4-Cl)PPCo or
TPFPPCo was introduced to the oxidation of cumene as catalyst, the
oxidative transformation of cumene was obtained at the reaction tem-
perature of 85, 80 and 75 °C, at which no autoxidation of cumene
happened. Especially for TPFPPCo, the conversion of cumene reached
up to 56.6% with the selectivity of 51.3% toward 2-phenyl-2-propanol
(Entry 12). It was also demonstrated in Table 1 that, the catalytic per-
formance was mainly from the metalloporphyrin, and the porphyrin
ligand did not exhibit obvious catalytic activity (Entry14, 15). In ad-
dition to cumene as substrate, the target product of cumene oxidation in
this work, 2-phenyl-2-propanol was also employed as substrate to in-
vestigate whether the deep oxidation would happen at current reaction
conditions. As shown in Entry 16∼19, no oxidation transformation
occurred for 2-phenyl-2-propanol as substrate, not only in the absence
of catalyst, but also in the presence of metalloporphyrin as catalyst.
Therefore, it is a very suitable temperature range to carry out the cat-
alytic oxidation of tertiary benzylic CeH bonds to alcohols at tem-
peratures below 90 °C, such as 85, 80, and 75 °C, at which not only the
unselective autoxidation of tertiary benzylic CeH bonds, but also the
deep oxidation of target products alcohols would not occur. The
3.2. Determination of reaction temperature
To achieve the selective oxidation of tertiary benzylic CeH bonds to
alcohols under mild conditions, suitable reaction temperature is a key
4

H.-M. Shen, et al.
AppliedCatalysisA,General599(2020)117599
Table 1
Effect of temperature on the oxidation of cumene in the absence or presence of metalloporphyrins as catalysts.a
Entry
Catalysts
Temp.(ºC)
Conversion (%)
Selectivity (%)
R1–OOH
R1–OH
R2=O
R3–COOH
1
2
3
4
5
6
7
8
–
–
–
–
75
80
85
90
95
75
80
85
90
95
75
80
85
80
85
80
85
80
85
N. D.b
N. D.
N. D.
2.4
13.9
31.9
34.4
38.0
39.0
39.4
56.1
56.6
57.4
7.1
N. D.
N. D.
N. D.
100.0
100.0
68.7
66.5
73.6
73.8
74.1
21.4
26.7
31.6
100.0
100.0
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
24.2
26.3
19.0
17.2
15.4
56.6
51.3
46.4
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
7.1
7.2
7.4
9.0
10.5
22.0
22.0
22.0
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
N. D.
–
T(4-Cl)PPCo
T(4-Cl)PPCo
T(4-Cl)PPCo
T(4-Cl)PPCo
T(4-Cl)PPCo
TPFPPCo
TPFPPCo
TPFPPCo
TPFPP
TPFPP
–
–
T(4-Cl)PPCo
T(4-Cl)PPCo
9
10
11
12
13
14
15
16c
17c
18c
19c
9.4
N. D.
N. D.
N. D.
N. D.
a
Reaction conditions: cumene (10 mmol, 1.2019 g), catalyst (0.012%, mol/mol) or no catalyst, O2 (1.0 atm), 8.0 h.
No reaction or no product detected.
2-Phenyl-2-propanol as substrate, (10 mmol, 1.3619 g), catalyst (0.012%, mol/mol) or no catalyst, O2 (1.0 atm), 8.0 h.
b
c
Table 2
Effect of porphyrin structures on catalytic oxidation of tertiary benzylic CeH bonds.a
Entry
Catalysts
Conversion (%)
Selectivity (%)
R1–OOH
R1–OH
R2=O
R3–COOH
1
2
3
4
5
6
7
8
T(2-CH3)PPCo
T(3-CH3)PPCo
T(4-CH3)PPCo
TPPCo
T(2-Cl)PPCo
T(3-Cl)PPCo
40.8
25.4
28.2
37.2
46.6
31.1
34.4
30.3
34.4
46.3
30.6
32.2
44.3
61.7
62.9
50.1
66.3
64.7
57.9
41.0
60.6
66.5
67.1
72.5
48.7
78.6
62.3
48.6
27.0
22.5
42.6
29.1
28.4
33.2
46.9
34.5
26.3
25.7
22.6
41.9
15.3
29.8
40.2
53.2
57.8
7.3
4.5
6.9
8.9
12.1
4.9
7.2
7.2
4.9
9.4
6.1
7.9
11.2
19.8
19.7
N.D.b
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
T(4-Cl)PPCo
T(4-F)PPCo
9
T(4-COOH)PPCo
T(2-CN)PPCo
T(3-CN)PPCo
T(4-CN)PPCo
T(2,5-diCl)PPCo
T(2,6-diCl)PPCo
T(2,3,6-triCl)PPCo
10
11
12
13
14
15
a
Reaction conditions: cumene (10 mmol, 1.2019 g), catalyst (0.012%, mol/mol), 80 °C, O2 (1.0 atm), 8.0 h.
No product detected.
b
efficient inhibition of the autoxidation and deep oxidation would un-
doubtedly promote the accomplishment of selective oxidation of ter-
3.3. Effect of ligand structures on catalytic oxidation of tertiary benzylic
CeH bonds
tiary benzylic CeH bonds to alcohols.
The ligand structure is an important influencing factor on the cat-
alytic performance of metal complexes, which is the primary source of
5

H.-M. Shen, et al.
AppliedCatalysisA,General599(2020)117599
Fig. 4. Optimized conformations of representative metalloporphyrins T(2,3,6-triCl)PPCo (a, b), T(2,6-diCl)PPCo (c, d), T(4-CH3)PPCo (e, f) and T(2-Cl)PPCo (g, h).
the diversity in catalytic property for metalloporphyrins. So, a series of
porphyrin cobalts(II) with different substituent groups were applied to
the catalytic oxidation of tertiary benzylic CeH bonds employing cu-
mene as representative substrate in order to realize the direct, efficient,
and selective oxidation of tertiary benzylic CeH bonds to alcohols. It
was demonstrated in Tables 2 and S1 that, although all the porphyrin
cobalts(II) employed could catalyze the transformation of cumene to
corresponding oxygen containing compounds, the higher catalytic
performance was exhibited in the catalytic transformation using por-
phyrin cobalts(II) with bulky substituent groups as catalysts, in which
not only higher conversion of cumene was obtained, but also higher
selectivity toward 2-phenyl-2-propanol was achieved. For T(2,3,6-triCl)
PPCo, which possessed the largest substituent group in Table 2, both of
the highest conversion and selectivity toward 2-phenyl-2-propanol
6

H.-M. Shen, et al.
AppliedCatalysisA,General599(2020)117599
Table 3
Effect of central metal on catalytic oxidation of tertiary benzylic CeH bondsa.
Entry
Catalysts
Conversion (%)
Selectivity (%)
R1−OOH
R1−OH
R2=O
R3−COOH
1
2
3
4
5
6
7
8
T(3-Cl)PPCo
T(3-Cl)PPMn
T(3-Cl)PPCu
T(3-Cl)PPZn
T(3-Cl)PPFe
T(3-Cl)PPNi
T(2,3,6-triCl)PPCo
T(2,3,6-triCl)PPMn
T(2,3,6-triCl)PPCu
T(2,3,6-triClPP)Zn
T(2,3,6-triCl)PPNi
T(2,3,6-triCl)PPFe
31.1
65.3
13.5
13.6
15.0
8.4
62.9
64.0
20.7
13.3
15.8
6.6
60.6
25.1
95.9
96.1
64.6
92.6
22.5
14.6
93.5
95.5
95.5
100.0
34.5
63.2
0.0
0.0
28.3
0.0
57.8
66.3
2.7
0.0
0.0
4.9
11.7
4.1
3.9
7.1
7.4
19.7
19.1
3.8
4.5
4.5
–
N.D.b
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
9
10
11
12
–
a
Reaction conditions: cumene (10 mmol, 1.2019 g), catalyst (0.012%, mol/mol), 80 °C, O2 (1.0 atm), 8.0 h.
No product detected.
b
were achieved, and the conversion of cumene reached up to 62.9% with
the selective of 57.8% toward 2-phenyl-2-propanol. For other porphyrin
cobalts(II) with smaller substituent groups, not only the conversion was
at a low level (below 45%), but also the primary product was cumene
hydroperoxide, not 2-phenyl-2-propanol, and the selectivity toward
alcohols was less than 50%. The superior performance of porphyrin
cobalts(II) with bulky substituent groups, especially for T(2, 6-diCl)
PPCo and T(2,3,6-triCl)PPCo, was mainly attribute to their high stabi-
lities and plane structures, which had been verified through the struc-
tural optimization employing quantum chemical calculation at the level
of B3LYP/6-31G(d) [57]. The bulkier substituent groups not only pre-
vented the metalloporphyrins from being attacked by the reactive
species in reaction, but also resulted in a planar molecular structure as
illustrated in Figs. 4 and S3 which would favor the interaction between
metal catalytic active center and reactants, and the catalytic oxidation
of cumene, not the unselective autoxidation. Thus, owing to the higher
stability and planar molecular structure, the catalytic transformation of
tertiary benzylic CeH bonds could be realized smoothly in the presence
Fig. 5. The cyclic voltammetry curves of representative metalloporphyrins in
DMF of 0.025 mol/L TBAPF6 at the scan speed of 100 mV/s with a reference
electrode of Ag/AgCl.
Table 4
Catalytic decomposition of cumene hydroperoxide in the presence of metalloporphyrins.a
Entry
Catalysts
Decomposition Ratio (%)
Selectivity (%)
R1−OH
R2=O
R3−COOH
1
2
3
4
5
6
7
–
N.D.b
95.8
53.8
14.5
9.0
N.D.
71.8
97.5
78.8
66.5
50.7
31.9
N.D.
28.2
2.5
21.2
33.5
49.3
68.1
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
T(2,3,6-triCl)PPCo
T(2,3,6-triCl)PPMn
T(2,3,6-triCl)PPCu
T(2,3,6-triCl)PPNi
T(2,3,6-triCl)PPFe
T(2,3,6-triCl)PPZn
6.1
4.6
a
Reaction conditions: cumene hydroperoxide (10 mmol, 1.5219 g), catalyst (0.012%, mol/mol), 80 °C, 8.0 h.
No product detected.
b
7

H.-M. Shen, et al.
AppliedCatalysisA,General599(2020)117599
Table 5
Effect of catalyst loading and temperature on oxidation of tertiary benzylic CeH bonds.a
Entry
Catalysts
Temp.(ºC)
Conversion (%)
Selectivity (%)
R1−OOH
R1−OH
R2=O
1
2
3
4
5
6
7
8
T(3-Cl)PPMn
60
65
70
75
80
85
60
65
70
75
80
85
70
70
29.1
41.7
50.3
57.8
65.3
71.9
33.1
49.7
57.6
61.3
64.0
71.2
62.6
66.7
35.1
26.8
25.5
25.0
25.1
26.2
26.9
16.3
14.0
14.4
14.6
16.9
9.2
60.0
66.6
66.7
64.4
63.2
61.5
67.2
70.9
70.5
67.3
66.3
62.2
69.1
64.8
4.9
6.6
7.8
10.6
11.7
12.3
5.9
12.8
15.5
18.3
19.1
20.9
21.7
29.0
T(2,3,6-triCl)PPMn
9
10
11
12
13b
14c
6.2
a
Reaction conditions: cumene (10 mmol, 1.2019 g), catalyst (0.012%, mol/mol), O2 (1.0 atm), 8.0 h. No aromatic acid detected.
Catalyst (0.024%, mol/mol).
Catalyst (0.048%, mol/mol).
b
c
of metalloporphyrins with bulky substituent groups as catalysts. Similar
oxidation phenomena were also found at the temperature of 75 °C and
85 °C shown in Tables S2 and S3.
selectivity toward 2-phenyl-2-propanol (97.5%). Although the highest
decomposition ratio was obtained in the employment of T(2,3,6-triCl)
PPCo as catalyst, the selectivity to alcohol was just 71.8%. Therefore,
the most suitable catalyst for oxidation of tertiary benzylic CeH bonds
to alcohols was regarded as T(2,3,6-triCl)PPMn in this work, for which
both promising conversion and selectivity could be achieved.
3.4. Effect of central metal on catalytic oxidation of tertiary benzylic CeH
bonds
3.5. Effect of temperature and catalyst loading
Central metal is the determining factor in the catalytic performance
of metalloporphyrins, thus the effect of central metal on the catalytic
oxidation of tertiary benzylic CeH bonds was explored deeply, in which
representative porphyrins such as T(4-CH3)PP, TPP, T(2-Cl)PP, T(3-Cl)
PP, T(4-Cl)PP, T(4−COOH)PP and T(2,3,6-triCl)PP were employed as
ligands, and Co, Mn, Cu, Zn, Fe, and Ni were employed as central metals
with cumene as typical substrate. As demonstrated in Tables 3 and
S4–S8, for all the porphyrin ligands investigated, Mn(II) complexes
exhibited the most satisfying catalytic performance, and the conversion
of cumene reached up to 64.0% with the selectivity of 66.3% toward 2-
phenyl-2-propanol in the presence of T(2,3,6-triCl)PPMn as catalyst.
Based on the systematical investigation on the redox properties and the
electronic structures of the metalloporphyrins used as catalysts, the
high efficiency of porphyrin Mn(II) in the oxidation of cumene was
mainly attributed to the lower oxidation potential and higher positive
electricity around central metal Mn. It was illustrated in Fig. 5 that, for
all the metalloporphyrins investigated in the cyclic voltammetry,
T(2,3,6-triCl)PPMn possessed the lowest oxidation potential, which
would favor the electron transfer from T(2,3,6-triCl)PPMn to O2, re-
sulting in the catalytically active high-valence manganese-oxo com-
plexes. And the higher positive electricity around Mn shown in Table
S9, could increase the activity of high-valence manganese-oxo com-
plexes to abstract hydrogen atom from CeH bonds, thus high conver-
sion was achieved. For the high selectivity, it could be ascribed to the
high efficiency of central metal Mn in the catalytic decomposition of
oxidation intermediate cumene hydroperoxide to alcohol, which was
explained through experiments in Table 4. As shown in Table 4, when
the catalytic decomposition of oxidation intermediate cumene hydro-
peroxide was carried out in the presence of T(2,3,6-triCl)PPCo, Mn, Cu,
Ni, Fe, Zn as catalysts, the porphyrin Mn exhibited the highest
The obtained method in oxidation of tertiary benzylic CeH bonds
catalyzed by metalloporphyrins was further optimized from the aspect
of temperature and catalyst loading with cumene as typical substrate.
As demonstrated in Table 5, for both of the metalloporphyrins (T(3-Cl)
PPMn and T(2,3,6-triCl)PPMn), higher selectivity toward 2-phenyl-2-
propanol was achieved at 70 °C with the conversion of above 50%
(Entry 3 and Entry 9). The conversion of cumene was 57.6% with the
selectivity of 70.5% catalyzed by T(2,3,6-triCl)PPMn at 70 °C. Further
decrease or increase in reaction temperature did not result in any en-
hancement on the selectivity toward alcohol. Besides the reaction
temperature, the catalyst loading was increased from 0.012% to
0.048% with the purpose to increase the selectivity toward alcohol to a
higher level (Entry 13 and Entry 14). Although the conversion was in-
creased from 57.6% to 66.7%, the selectivity decreased from 70.5% to
64.8%. To keep the selectivity toward alcohol at a higher level, the
catalyst loading in this work was chosen as 0.012% (mol/mol), which
was a very low catalyst loading. In a brief summary, an efficient pro-
tocol for direct and selective oxidation of tertiary benzylic CeH bonds
to tertiary benzylic alcohols was developed with cumene as typical
substrate and T(2,3,6-triCl)PPMn as the most promising catalyst, in
which the conversion of cumene was 57.6% with the selectivity of
70.5% toward 2-phenyl-2-propanol.
3.6. Kinetic study
With the efficient protocol for selective oxidation of tertiary
benzylic CeH bonds in hands, the kinetic study was carried out to in-
vestigate the source of diverse catalytic performances exhibited by
8

H.-M. Shen, et al.
AppliedCatalysisA,General599(2020)117599
Fig. 6. Pseudo-first-order fits for the oxidation of cumene catalyzed by T(4-Cl)PPCu (a, b), T(4-Cl)PPCo (c, d), T(4-CH3)PPMn (e, f), T(4-Cl)PPMn (g, h) and T(2,3,6-
triCl)PPMn (i, j).
9

H.-M. Shen, et al.
AppliedCatalysisA,General599(2020)117599
performances of different metalloporphyrins were mainly from their
different ability in decreasing the apparent activation energies in oxi-
dation of cumene. The apparent activation energies (Ea) obtained is
another rational explanation for the different catalytic performances
exhibited by different catalysts.
Table 6
The pseudo-first-order kinetic parameters for catalytic oxidation of cumene.
Entry Catalysts
Temp. (°C) k (h−1
)
R2
Average
intercepts
Ea (kJ/
mol)
1
T(4-Cl)PPCu
75
80
85
75
80
85
75
80
85
0.03002 0.9891 0.00330
0.04681 0.9959
0.09548 0.9941
0.01972 0.9978 0.09745
0.03347 0.9934
0.04907 0.9768
0.06696 0.9915 0.01317
0.10802 0.9936
0.14751 0.9917
119.8
94.6
82.0
80.6
70.7
2
3
3.7. Substrate scope and comparison with existing oxidation systems
4
T(4-Cl)PPCo
5
6
With the optimized strategy for direct oxidation of tertiary benzylic
CeH bonds to alcohols obtained, the substrate scope was extended from
cumene to a series of cumene analogues containing tertiary benzylic
CeH bonds. It was demonstrated in Table 8 that, in the presence of
T(2,3,6-triCl)PPMn as catalyst and O2 as oxidant, all the tertiary
benzylic CeH bonds investigated could be converted to their corre-
sponding tertiary benzylic alcohols directly in the selectivity of
50%∼70% with acceptable conversion, except for 1-isopropyl-4-
methoxybenzene and 4-isopropylbenzoic acid. In the oxidation of 1-
isopropyl-4-methoxybenzene, the major product was hydroperoxide in
selectivity of 54.3%, and the selectivity toward tertiary benzylic alcohol
was 30.2%, which might be attributed to the high stability of 4-
methoxy cumene hydroperoxide. As for 4-isopropylbenzoic acid, the
poor conversion was ascribed to the quenching effect of carboxyl on the
free radicals in the oxidative transformation. For the satisfying sub-
strate scope, the protocol presented in this work was a very promising
and practical choice to convert tertiary benzylic CeH bonds to valuable
alcohols directly with clean O2 as oxidant catalyzed by T(2,3,6-triCl)
PPMn under solvent-free and additive-free conditions. In succession,
the protocol disclosed in this work was compared with some typical
tertiary benzylic CeH bonds oxidation systems reported, which were
listed in Table 9. It was shown in Table 9 clearly that, although the
catalytic transformation of tertiary benzylic CeH bonds had been in-
vestigated extensively, the substrate was just focused on cumene in
most cases, and the major product was cumene hydroperoxide. In this
work, the tertiary benzylic CeH bonds were converted to corresponding
alcohols directly employing clean O2 as oxidant and readily available
T(2,3,6-triCl)PPMn as catalyst under solvent-free and additive-free
conditions. For the typical substrate, the conversion of cumene could
reach up to 57.6% with the selectivity of 70.5% toward tertiary
benzylic alcohol, which was higher in both conversion and selectivity
than the existing solvent-free and additive-free oxidation systems using
O2 at present. For other oxidants such as hydrogen peroxide, m-chlor-
operoxybenzoic acid and tert-butyl hydroperoxide, in spite of the higher
selectivity and conversion, they were not so suitable for industrial ap-
plication like O2 because of the extensive utilization of the toxic, cor-
rosive solvent and additive, and the large amount of by-products from
7
8
T(4-CH3)
PPMn
9
10
11
12
13
14
15
T(4-Cl)PPMn 75
0.06420 0.9988 0.02335
0.09661 0.9990
0.13968 0.9951
80
85
T(2,3,6-triCl) 75
PPMn
0.07498 0.9845 0.07562
0.10125 0.9932
0.14837 0.9980
80
85
different catalysts, in which cumene was employed as substrate, T(4-Cl)
PPCu, T(4-Cl)PPCo, T(4-CH3)PPMn, T(4-Cl)PPMn and T(2,3,6-triCl)
PPMn were employed as representative metalloporphyrins, and pseudo-
zero-order kinetic, pseudo-first-order kinetic and pseudo-second-order
kinetic were employed as models. As illustrated in Figs. 6, S4 and S5,
pseudo-first-order kinetic model (Fig. 6) fitted better the oxidation of
cumene with O2 catalyzed by metalloporphyrins under current condi-
tions than others, and very high correlation coefficients were obtained
for all the metalloporphyrins investigated (Table 6). And from the ki-
netic fittings in Fig. 6 and Table 6, larger average intercepts were ob-
served for T(4-Cl)PPCo and T(2,3,6-triCl)PPMn as catalysts, which in-
dicated their higher performances in initiation of the catalytic oxidation
and the shorter induction periods. For T(4-Cl)PPCu with lower catalytic
performance as illustrated in Table 7, and the autoxidation of tertiary
benzylic CeH bonds, longer induction periods would be needed with
smaller average intercepts. Based on the systematical kinetic studies,
the apparent activation energies (Ea) in the oxidation of cumene cata-
lyzed by different metalloporphyrins were calculated following the
Arrhenius equation: lnk = − (Ea / R) × (1 / T) + lnk0 and summarized
in Table 6, which decreased in the following order, Ea(T(4-Cl)PPCu,
119.8 kJ/mol) > Ea(T(4-Cl)PPCo, 94.6 kJ/mol) > Ea(T(4-CH3)PPMn,
82.0 kJ/mol) > Ea(T(4-Cl)PPMn, 80.6 kJ/mol) > Ea(T(2, 3, 6-triCl)
PPMn, 70.7 kJ/mol) in accordance with the sequence in the catalytic
activity and the selectivity toward 2-phenyl-2-propanol shown in
Table 7. The lower the apparent activation energy is, the higher the
conversion of cumene and selectivity toward 2-phenyl-2-propanol are.
Thus, from the aspect of kinetic study, the discrepancies in the catalytic
Table 7
Oxidation of tertiary benzylic CeH bonds catalyzed by different metalloporphyrins.a
Entry
Catalysts
Conversion (%)
Selectivity (%)
R1−OOH
R1−OH
R2=O
R3−COOH
1
2
3
4
5
T(4-Cl)PPCu
T(4-Cl)PPCo
T(4-CH3)PPMn
T(4-Cl)PPMn
T(2,3,6-triCl)PPMn
30.4
34.4
47.0
59.5
64.0
80.5
66.6
61.1
45.2
14.6
15.5
26.3
33.8
46.8
66.3
4.0
7.1
5.1
8.0
19.1
N.D.b
N.D.
N.D.
N.D.
N.D.
a
Reaction conditions: cumene (10 mmol, 1.2019 g), catalyst (0.012%, mol/mol), 80 °C, O2 (1.0 atm), 8.0 h.
No product detected.
b
10

H.-M. Shen, et al.
AppliedCatalysisA,General599(2020)117599
Table 8
Oxidation of tertiary benzylic CeH bonds with O2 catalyzed by T(2,3,6-triCl)PPMna.
Entry
1
Substrates
Temp.(ºC)
70
Conv.(%)
57.6
Selectivity (%)
R1−OOH
14.0
R1−OH
R2=O
15.5
R3−COOH
70.5
N.D.c
2b
95
58.6
21.8
65.1
13.1
N.D.
3
80
70
51.8
41.2
21.7
29.3
64.0
62.2
14.3
8.5
N.D.
N.D.
4b
5
6
80
58.4
31.9
30.5
39.5
60.4
56.3
9.1
4.2
N.D.
N.D.
120
7
8
9
70
15.3
29.7
N.D.
49.3
54.3
N.D.
50.7
30.2
N.D.
0.0
N.D.
N.D.
N.D.
85
15.5
N.D.
100
a
Reaction conditions: substrate (10 mmol), T(2,3,6-triCl)PPMn (0.012%, 0.0013 g, mol/mol), O2 (1.0 atm), 8.0 h.
T(2,3,6-triCl)PPMn (0.024%, 0.0026 g, mol/mol).
No product detected.
b
c
oxidant reduction. Thus, considering the satisfying substrate tolerance,
important reference for the efficient utilization of widely available and
inexpensive hydrocarbons and the convenient acquirement of the high-
value-added benzylic alcohols through the shortest catalytic oxidation
path employing O2 as oxidant and metalloporphyrins as catalysts.
operational simplicity, higher atom economy, and higher selectivity
toward valuable alcohols, our route presented in this work was a pro-
mising and practical strategy to transform tertiary benzylic CeH bonds
to important tertiary benzylic alcohols directly employing clean O2 as
oxidant and readily available T(2,3,6-triCl)PPMn as catalyst under
solvent-free and additive-free conditions. Our work was also an
11

H.-M. Shen, et al.
AppliedCatalysisA,General599(2020)117599
Table 9
Table 9 (continued)
Comparison with some existing tertiary benzylic CeH bonds oxidation systems.
Entry Main
products
Conditions
Conversion (%) Selectivity (%) Ref.
Entry Main
products
Conditions
Conversion (%) Selectivity (%) Ref.
14
15
16
H2O2, Fe(III)
complex, CH3CN,
50 °C, 24.0 h
31.7
76.0
44.0
12.0
81.6
93.2
[22]
[23]
[23]
1
2
3
4
5
6
O2, Co-Ni MOF,
0.40 MPa, 90 °C,
7.0 h
30.0
42.6
24.1
84.8
59.8
59.3
91.0
95.1
88.4
46.8
57.1
62.4
[5]
H2O2, Mn(II)
complex, CH3CN,
AcOH, 0 °C, 4.0 h
O2, NHPI, AIBN,
ionic liquid,
0.10 MPa, 60 °C,
6.0 h
[4]
H2O2, Mn(II)
complex, CH3CN,
AcOH, 0 °C, 4.0 h
O2, carbon
nanotube, 80 °C,
8.0 h
[58]
[59]
[60]
[61]
TBHP, cobalt-
17
18
H2O2, Mn(II)
complex, CH3CN,
AcOH, 0 °C, 4.0 h
64.0
39.0
73.4
82.1
[23]
[23]
copper- aluminum
layered hydroxide,
120 °C, 12.0 h
TBHP, Co–Cu
mixed metal oxide,
105 °C, 7.0 h
H2O2, Mn(II)
complex, CH3CN,
AcOH, 0 °C, 4.0 h
O2, cobalt in N-
doped carbon
nanotube,
0.80 MPa, 120 °C,
5.0 h
O2, immobilized
NHPI, 0.10 MPa,
140 °C, 24.0 h
19
20
Air, cumene
hydroperoxide,
ionic liquid, 90 °C,
11.0 h
63.5
97.0
61.0
89.7
[65]
[24]
7
8
9
97.0
91.0
69.2
95.0
74.0
63.0
[25]
[62]
[26]
O2, NaOH,
CH3CH2OH,
1.80 MPa, 60 °C,
24.0 h
O2, Cu−Lu MOF,
0.50 MPa, 120 °C,
4.0 h
O2, cobalt
nanoparticles in
nitrogen-doped
carbon nanotube,
0.80 MPa, 120 °C,
5.0 h
21
22
23
O2, nitrogen-doped 53.9
carbon nanotube,
80 °C, 8.0 h
57.2
70.5
65.1
[8]
10
11
KHSO5,
98.0
81.0
77.6
92.6
[63]
[20]
heterogeneous
cobalt, d6-acetone,
D2O, R.T., 15.0 h
O2, T(2,3,6-triCl)
PPMn, 70 °C, 8.0 h
57.6
58.6
This
work
TBHP, Ru(III)
complex, H2O,
25 °C, 16.0 h
O2, T(2,3,6-triCl)
PPMn, 95 °C, 8.0 h
This
work
12
13
TBHP, Co-doped
carbon, H2O, 80 °C,
12.0 h
90.0
26.0
76.7
88.5
[21]
[64]
24
O2, T(2,3,6-triCl)
PPMn, 80 °C, 8.0 h
51.8
64.0
This
work
m-CPBA, Ni(II)
complex, CH3CN,
25 °C, 1.5 h
12

H.-M. Shen, et al.
AppliedCatalysisA,General599(2020)117599
4. Conclusions
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The tertiary benzylic CeH bonds were converted to their corre-
sponding tertiary benzylic alcohols directly and efficiently employing
O2 as oxidant and metalloporphyrins as catalysts under solvent-free and
additive-free conditions. The higher catalytic performance was
achieved in the employment of porphyrin manganeses(II) possessing
bulkier substituent groups as catalysts, such as T(2,3,6-triCl)PPMn. For
the typical substrate, the conversion of cumene could reach up to 57.6%
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bulkier substituent groups not only prevented the metalloporphyrins
from being attacked by the reactive species in reaction, but also resulted
in a planar molecular structure favoring the interaction between metal
catalytic active center and reactants. For the higher selectivity toward
alcohols exhibited by porphyrin Mn(II), it was ascribed to the high ef-
ficiency of central metal Mn(II) in the catalytic transformation of oxi-
dation intermediates hydroperoxides to alcohols, which had been ver-
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benzylic CeH bonds to alcohols under mild conditions, clean oxidant,
shortest reaction route, high conversion and selectivity to valuable al-
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the best of our knowledge, our work is an extremely novel, significant,
and practical example in direct and selective oxidation of tertiary
benzylic CeH bonds to alcohols, which would be a useful reference for
the direct and efficient oxidative functionalization of CeH bonds, and
an enormous promotion for the extensive application of metallopor-
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Declaration of Competing Interest
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The authors declare that they have no known competing financial
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