Mechanistic Understanding towards the Role of Cyclohexene in Enhancing the Efficiency of Manganese Porphyrin-Catalyzed Aerobic Oxidation of Diphenylmethane

Article abstract of DOI:10.1002/ejic.201800317

The selective aerobic oxidation of nonactivated C–H bonds is a key challenge in both industrial and academic research. The catalytic activity of manganese porphyrins (MnTPPCl) towards the aerobic oxidation of diphenylmethane (DPM) is investigated. Compared with the oxidation-catalyzed MnTPPCl alone, the conversion of DPM is enhanced ca. 6 times (from 7.0 % to 42.5 %) in the presence of cyclohexene. Some in situ characterizations, such as in situ EPR (electron paramagnetic resonance) spectroscopy and in situ UV/Vis spectroscopy are used to investigate the role of cyclohexene in catalytic oxidation. Mechanistic studies suggest that both high-valent manganese species and cyclohexenyl alkoxyl radicals are favorable for the formation of the carbon radicals. The enhanced activity is related to the easier generation of secondary carbon radicals, triggered by high-valent manganese species and cyclohexenyl alkoxyl radicals.

Full text of DOI:10.1002/ejic.201800317

A Journal of
Accepted Article
Title: Mechanistic understanding towards the role of cyclohexene
in enhancing efficiency for manganese porphyrins-catalyzed
aerobic oxidation of diphenylmethane
Authors: Jun Jiang, Jiexiang Wang, Xiantai Zhou, Hongyu Chen, and
Hongbing Ji
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800317
Link to VoR: http://dx.doi.org/10.1002/ejic.201800317

10.1002/ejic.201800317
European Journal of Inorganic Chemistry
Mechanistic understanding towards the role of cyclohexene
in enhancing efficiency for manganese porphyrins-catalyzed
aerobic oxidation of diphenylmethane
*
Jun Jianga, Jie-Xiang Wangc, Xian-Tai Zhoub*, Hong-Yu Chena, Hong-Bing Jia
a Fine Chemical Industry Research Institute, School of Chemistry, Sun Yat-sen
University, Guangzhou 510275, PR China
b Fine Chemical Industry Research Institute, School of Chemical Engineering and
Technology, Sun Yat-sen University, Zhuhai, 519082, PR China
c Huizhou Research Insititute of Sun Yat-sen University, Huizhou, 516081, PR China
*Corresponding author. Tel.: +86-20-84113653; fax: +86-20-84113654.
E-mail: zhouxtai@mail.sysu.edu.cn (X.T. Zhou), jihb@mail.sysu.edu.cn (H. B. Ji)
ABSTRACT: The selective aerobic oxidation of non-activated C–H bonds is a key
challenge in both industrial and academic research. The catalytic activity of manganese
porphyrins (MnTPPCl) was investigated towards the aerobic oxidation of
diphenylmethane (DPM). Compared to the oxidation catalyzed MnTPPCl alone, the
conversion of DPM was enhanced to ~6 times (from 7.0% to 42.5%) in the presence of
cyclohexene. Some in situ characterizations, such as in situ EPR (Electron
Paramagnetic Resonance) and in situ UV–vis spectroscopy were used to investigate the
role of cyclohexene in the catalytic oxidation. Mechanistic studies suggested that both
high-valent manganese species and cyclohexenyl alkoxyl radical were favourable for
the formation of carbon radical. The enchanced activity was related to the easier
generation of secondary carbon radical triggered by high-valent manganese species and
cyclohexenyl alkoxyl radical.
Introduction
The selective oxidation of non-activated C–H bonds to the corresponding oxygen-
containing compounds has become one of the most rapidly developing areas [1]. As a
model of benzylic C-H oxyfunctionalization, the oxidation of diphenylmethane to
1
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
benzophenone is gathering much attention, as benzophenone is a key intermediate for
the synthesis of pharmaceuticals, insecticides, photo initiators, perfumes and also
ultraviolet curing agents [2]. The oxidative process using stoichiometric quantities of
oxidizing agents like KMnO4, SeO2 and CrO3 has gradually been eliminated due to the
generation of large amount of waste [3]. Consequently, it is of great importance to
develop more economic and environmentally benign processes. Hence, hydroperoxides
such as hydrogen peroxide (H2O2) or tert-butylhydroperoxide (TBHP) as oxidant is
gathering much attention, in which water and tert-alcohol are formed as by products [4].
As compared to the above oxidants, the oxidation of C–H bonds to corresponding
oxygen-containing compounds using molecular oxygen could be a cost-effective
method [5]. However, because of the cleavage of high-energy C–H bonds, oxidation
typically requires harsh reaction conditions [6]. High temperature is usually required for
the oxidation of diphenylmethane with molecular oxygen as sole oxidant. Medium
conversions of diphenylmethane were obtained when the oxidation catalyzed by
-1
heterogeneous catalysts such as MnO4 exchanged Mg-Al hydrotalciate [7], Ni-Al
[9]
hydrotalcites [8], Mn-MgAl hydrotalcite
at a reaction temperature over 130oC.
Excellent conversion (87%) could be achieved when the oxidation was conducted at
higher temperature (200 oC), in which hybrid Co/Mn nano supported on calcined bone
was used as catalyst [10]
.
In the catalytic oxidation of C-H bond, the metalloporphyrins-mediated
biomimetic catalytic system is the promising approach, attributed to the similar
reactivity as that observed for enzyme-catalyzed oxidation [11]. Our group have reported
that simple iron porphyrins exhibited excellent activity for the one-step aerobic
oxidation of cyclohexane to adipic acid [12]. Other biomimetic approaches were
effective for achieving the selective oxidation of C-H bonds with metalloporphyrins as
heterogeneous catalysts [13]. Cobalt porphyrins immobilized on CeO2@SiO2 core-shell
nanoparticles showed satisfied activity with 41.6% conversion in the oxidation of
o
diphenylmethane at 130 C [14]. Although metalloporphyrins-based heterogenesous
2
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
catalytic systems for aerobic oxidation of C-H bond have been reported, the
homogeneous oxidation of diphenylmethane catalyzed by metalloporphyrins with
dioxygen is still limited.
The major challenges faced in metalloporphyrins-catalyzed homogeneous
oxidation of C-H bonds are concluded in two different aspects: the initiation of free
radical, as well as the activation of dioxygen. In bio-oxidation mediated by cytochrome
P450 enzymes, hydroxylation can be achieved by utilizing NAD(P)H as the hydrogen
donor for activating molecular oxygen [15]. Inspired by the systems found in nature, one
compound plays the role of NAD(P)H-like could promote the initiation of radical and
the activation of dioxygen. Recently, our group reported the biomimetic aerobic
epoxidation of olefins with isopropylbenzene as the NAD(P)H-like, with excellent
conversion and selectivity up to 94% [16]
.
We initiated continously our study by examining the compound could act as the
role of NAD(P)H-like compound in the activation of C-H bonds. After a series of
screening experiments, cyclohexene containing active -hydrogen is the desirable
compound. In this work, the metalloporphyrins-catalyzed oxidation of C-H bond in the
presence of cyclohexene by using diphenylmethane as model substrate has been
developed. The process also provides a biological-oxidation-like protocol for the
aerobic oxidation of non-activated C–H bonds under milder conditions. Furthermore,
based on the results obtained from experiments as well as in situ electron paramagnetic
resonance (EPR) and UV–vis spectroscopy, the mechanistic understanding the role of
cyclohexene towards in enhancing the catalytic efficiency has been presented.
Results and discussion
Metalloporphyrins-catalyzed oxidation of DPM with cyclohexene
As shown in Scheme 1, the oxidation of DPM by dioxygen in the presence of
MnTPPCl and cyclohexene was explored, resulting in the formation of benzhydrol and
benzophenone as the oxidation products. Figure 1 shows the results obtained.
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
OH
O
MnTPPCl
+
Cyclohexene, 110oC, 1.0MPa
DPM
1
2
N
N
N
Mn
N
Cl
MnTPPCl
Scheme 1. Oxidation of DPM catalyzed by MnTPPCl with cyclohexene
No reaction occurred in the absence of either MnTPPCl or cyclohexene by stirring
the solution after 8 h (Figure 1, ▼; Table 1, entry 1). On the other hand, when the
reaction was conducted with either MnTPPCl or cyclohexene as NAD(P)H-like model,
almost the same efficiency was obtained (Figure 1, ■ and ●; Table 1, entries 2 or 3,
respectively). The conversion of DPM in the system with only MnTPPCl was slightly
greater than that observed with the use of only cyclohexene (7.0% vs 6.1%). In addition,
the reaction in the absence of cyclohexene was conducted under more harsh reaction
conditions (150°C and 2.0 MPa), and conversion increased only slightly (7.6%) even
with prolonging the reaction time to 16 h (Table 1, entry 4).
In contrast, when the reaction was conducted in the presence of MnTPPCl and
cyclohexene, conversion was appreciably enhanced to 42.5% after 8 h (Figure 1, ▲;
Table 1, entry 5). TON was improved to 26562, which is approximately 6–7 times
greater than obtained with the use of either MnTPPCl or cyclohexene. Notably,
remarkable acceleration was observed with the multiplication of MnTPPCl and
cyclohexene alone. This result also indicated that cyclohexene plays a significant role
in the biomimetic catalytic system, which could trigger the activation of aliphatic C–H
bond of DPM.
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
From Figure 1, the conversion of DPM, as well as the yield of oxidized products,
slowly increased within the first 2 h. Then, the reaction rate rapidly increased. An
induction period was apparently observed for the aerobic oxidation of DPM, which was
consistent with reactions involving radicals. For verifying the free-radical mechanism,
2,6-di-tert-butylphenol (1 mmol), serving as the free-radical inhibitor, was used. After
the addition of this inhibitor, the oxidation was subsequently quenched (entry 6 of Table
1).
MnTPPCl
Cyclohexene
MnTPPCl/Cyclohexene
None
40
20
0
40
20
0
2
4
6
8
Time (h)
Figure 1. Time course for the oxidation of DPM with cyclohexene. DPM (6 mmol),
cyclohexene (1.5 mmol), dichloromethane (25 mL), MnTPPCl (1.6  10−3 mol%),
pressure (1.0 MPa), temperature (110 °C), 8 h.
Furthermore, the reaction was conducted for 3 h according to typical experimental
procedures, that is, DPM (6 mmol), dichloromethane (25 mL), MnTPPCl (1.610−3
o
mol%), cyclohexene (1.5 mmol), 110 C and O2 (1.0 MPa), which afforded a DPM
conversion of 16.1% (entry 7 of Table 1). Then, the 2,6-di-tert-butylphenol (1 mmol)
as a free-radical inhibitor was added to the reaction mixture, and the reaction was
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
subsequently quenched (entry 8 of Table 1), indicating that products are generated via
the transfer of a series of free-radical reactions.
Also, effects of the amount of catalyst, temperature, reaction pressure and the
amount of cyclohexene on the aerobic oxidation of diphenylmethane were investigated,
results were shown in Figure S1-S4.
Table 1. Aerobic oxidation of DPM catalyzed by MnTPPCl in the presence of cyclohexenea
Select. (%)
2
Cyclohexene
(mmol)
Entry
Catalyst
Conv. (%)
TONb
1
1
2
None
-
-
-
-
-
MnTPPCl
-
-
7.0
6.1
7.6
42.5
-
4375
-
25
26
24
22
-
75
3
1.5
-
74
76
78
-
4c
5
MnTPPCl
MnTPPCl
MnTPPCl
MnTPPCl
MnTPPCl
4750
26562
-
1.5
1.5
1.5
1.5
6d
7e
8f
16.1
16.1
10062
10062
24
24
76
76
a DPM (6 mmol), solvent (dichloromethane, 25 mL), MnTPPCl (1.6  10−3 mol%), 110 °C, O2 (1.0 MPa), 8 h.
b TON (Turnover number), [oxidation products]/[MnTPPCl].
c Reaction conditions: 150 °C, O2 (2.0 MPa), 16 h.
d Added free-radical inhibitor (1 mmol).
e Reaction was initially performed for 3 h.
f After the reaction was performed for 3 h, the free-radical inhibitor was added.
Kinetic studies
To explore the mechanisms of MnTPPCl-catalyzed oxidation, DPM was
considered for kinetics in terms of the initial concentration. As for various reactant
concentrations, the classic Michaelis–Menten (MM) equation (equation 1 was
employed to analyze kinetic data by the Lineweaver–Burk plot equation 2) was utilized
for enzyme-like kinetics analysis in the MnTPPCl/cyclohexene catalytic sysytem.
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
Vmax
S
 
V 
(1)
(2)
K  S
 
M
K  S
1
  KM
S
 
1
1
M



V
Vmax
Vmax
S
 
Vmax
Here, [S] is the substrate concentration, V is the reaction rate, Vmax is the maximum
reaction rate and KM is the Michaelis–Menten constant, which represents the substrate
concentration at which the reaction rate is half of Vmax.
10
Vmax=5.2110-3mol  L-1 s-1
8
KM=7.82 mol  L-1
Vmax/KM=6.6610-4 s-1
6
4
2
0
1000
2000
3000
4000
5000
6000
1/[DPM] (mol-1 L)
Figure 2. Lineweaver–Burk plots for the catalytic oxidation of DPM catalyzed by MnTPPCl (0.2
mM). cyclohexene (0.6 M), dichloromethane (25 mL), pressure (1.0 MPa), temperature (110 °C).
As shown in Figure 2, the oxidation of DPM well fitted the MM equation with a
high correlation coefficient R2 = 0.9999, indicating that the oxidation of DPM follow
the enzyme-like kinetics. According to enzyme kinetics, the Michaelis constant KM is a
measure of the affinity of the enzyme for substrate molecules, where a small KM value
implies strong affinity of the enzyme,[17] and Vmax is implied to the calculation of the
catalytic constant. From Figures 2, the corresponding KM and Vmax values were equal to
7.82 M and 5.21 mM s-1, respectively. Furthermore, according to the equation, i.e. Kcat
= Vmax/E0, where Kcat is the catalytic constant, which represents the speed of enzymatic
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
catalysis reaction, and E0 is the concentration of the catalyst, the corresponding Kcat
values was equal to 26.0 s-1. Moreover, the KM value for the oxidation of DPM by
MnTPPCl with cyclohexene exhibited the same order as KM for the H2O2 of catalase
[18]. In general, P450BM3 which belongs P450 cytochrome (CYP) monooxygenases
exhibits KM in the range of 300 M, Vmax in the range of 100 M s-1 and Kcat in the
range of 300 s-1 [19]. Compared to the reported KM and Vmax of cytochrome P450, in this
paper, the KM and Vmax were much large, and Kcat was much less, which could be
determined by lower catalytic activity of MnTPPCl compared to cytochrome P450.
Hence, kinetic studies of DPM follow the enzyme-like kinetics based on high R2 value,
but the catalytic activity of MnTPPCl is much less than cytochrome P450.
in situ UV–vis absorption of the aerobic oxidation of DPM
As has been previously reported, the mechanism of metalloporphyrin-catalyzed
oxidation involves the generation of metal–oxo species. For confirming the formation
of a high-valent Mn porphyrin intermediate, in situ UV–vis spectra were recorded on a
spectrophotometer connected to an AvaSpec-2048 spectrometer under the given
reaction conditions. Figure 3 shows the in situ UV–vis spectra of Mn porphyrins during
the reaction.
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
0.8
0.6
0.4
0.2
0.0
3 nm
400
450
500
550
600
650
Wavelength (nm)
Figure 3. in situ UV–vis spectra of the oxidation of DPM catalyzed by MnTPPCl in the presence of
cyclohexene (time scan: 180 min, interval 60 min). DPM (6 mmol), cyclohexene (1.5 mmol),
dichloromethane (25 mL), MnTPPCl (1.6  10−3 mol%), pressure (1.0 MPa), temperature (110 °C).
The spectrophotometer was programmed for acquiring UV–vis spectrum every 60
min. As shown in Figure 3, as the reaction proceeded, the Soret band gradually
decreased at 477 nm, accompanied with the decrease of the peak at 518 nm and 617
nm. In addition, a concomitant slight red-shift in the Soret band from 477 nm to 480
nm was observed. Moreover, the peak at 401 nm gradually increased with a blue-shift.
Such changes in the in situ UV–vis spectra is likely to be attributed to the generation of
high-valent manganese (IV or V)–oxo porphyrins, that is, MnV = O or MnIV = O
(Scheme 2). After reaction, the color of the reaction mixture changed from green to
yellow, also indicative of the presence of high-valent species. GC-MS analysis revealed
the consumption of DPM and the formation of diphenylmethanol and benzophenone,
indicative of the presence of an active oxidation species. Unfortunately, because UV-
vis spectra of MnV-oxo porphyrin and MnIV-oxo porphyrin are approximate [20], both of
them were hardly distinguished according to the UV-vis spectra in Figure 3. In general,
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
MnV-oxo porphyrin complex and MnIV-oxo porphyrin complex, which are generated
though O-O bond heterolysis and hemolysis, respectively (Scheme 2) [20], play key roles
with high reactivities in the oxidation of organic substrates by manganese porphyrin
catalysts [21]
.
However, both reactive activities have huge difference in the hydrogen atom
abstraction (HAA) reactions, and MnV-oxo porphyrin complex are often considered to
be reactive species [22]. For example, the reactivity of MnV-oxo porphyrin complex is
150 times greater than that of a MnIV-oxo porphyrin complex in the oxidation of
xanthene [22d]. Furthermore, computational study shows that the MnV-oxo porphyrin
complex and MnIV-oxo porphyrin complex are capable of hydroxylating 10-methyl-
9,10 –dihydroacridine, and MnV-oxo porphyrin complex is the more active oxidant
based on the result of density functional theory (DFT) [23]. Besides, MnV-oxo porphyrin
complex are commonly regarded as active species in the oxygen atom transfer reactions
[24] and chlorite oxidation reaction [25]. In a word, based on the above reported literature,
we estimate that MnV = O is likely to be active species for DPM oxidation.
Kinetic profiles of the content of DPM and the oxidation products
Moreover, the kinetics profile of DPM oxidation catalyzed by
MnTPPCl/cyclohexene was investigated to understand the distributions of substrate
and products along with the reaction time (Figure 4).
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
6
5
4
3
2
1
DPM
Benzophenone
Benzhydrol
0
0
1
2
3
4
5
6
7
8
Time ( h )
Figure 4. Kinetic profiles of DPM consumption, the generation of corresponding
benzophenone and benzhydrol in the presence of MnTPPCl and cyclohexene. DPM (6
mmol), cyclohexene (1.5 mmol), dichloromethane (25 mL), MnTPPCl (1.6  10−3
mol%), temperature (110 °C), pressure (1.0 MPa).
During the initial stage of reaction, the content of DPM was slightly decreasing
accompanied by slightly increasing content of benzophenone and benzhydrol,
suggesting that DPM oxidation catalyzed by MnTPPCl/cyclohexene has a relatively
long induction period. Subsequently, in the period of 2-6 h, the content of DPM was
quickly decreasing accompanied by rapidly increasing content of benzophenone and
slowly increasing content of benzhydrol. Meanwhile, the content of benzophenone was
always much large than that of benzhydrol, and this probably suggests that
benzophenone is not originated from the overoxidation of benzhydrol, and oxidation of
DPM goes through a process of competing reaction rather than tandem reaction [26]. In
iron porphyrin catalyzed C-H functionalization, traditional oxygen rebound mechanism
is responsible for hydroxylation of the C–H bond [27]. Recently, some studies show that
because rebound barriers are higher than dissociation energies of the substrate radical
based on the result of DFT, C–H bond activation reactions by MnV-oxo porphyrin
complex possibly does not occur via the well-known oxygen rebound mechanism [28]
.
Accordingly, substrate radical derived from hydrogen abstraction by metal-oxo
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
complex possibly escape from the solvent cage to react with other species in solution
[28b, 29], such as residual metal-oxo species or radical traps such as O2. Therefore, in this
paper, diphenylmethanyl radical (C13H11•) generated from hydrogen abstraction by
MnV = O possibly escape from the solvent cage to react with residual MnV = O to afford
benzophenone or react with O2 to afford diphenylmethanyl peroxyl radical (C13H11OO•),
as shown in Scheme 4.
in situ EPR spectral investigation of the aerobic oxidation of DPM
For obtaining further insights into the role of cyclohexene in the observed
acceleration, in situ EPR spectra for the oxidation of cyclohexene were first recorded
during the reaction. Aerobic catalytic oxidation in CH3CN was performed using a
Wilmad WG-810-A quartz-flat cell using a mixture of the CH3CN solution of
MnTPPCl, cyclohexene, and PBN (free-radical spin-trapping agent). The cell was
sealed after filling dioxygen. EPR analyses were performed at a microwave frequency
of 9.05 GHz at 383 K. As we know, in the reported studies about PBN as a trapping
agent, cyclohexene derivative radicals and diphenylmethane derivative radicals had
been hardly reported. Although the studies about the carbon-centered radicals and
oxygen-centered radicals with PBN as a trapping agent had been widely reported, the
magnitude of hyperfine couplings constants was markedly dependent on the nature and
structure of the added radical [30]. Therefore, it was relatively difficult to confirm the
radicals mentioned in this article according to the reported hyperfine couplings
constants. Hence, stable PBN radical adducts were confirmed via the comparison of
hyperfine splitting constants, the computer simulation and the corresponding reaction
mechanism.
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
2,3
2,3
3
3
3
2
2
3
3
3
3
2
2
c
2
2
2,3
2,3
3
b
a
2
2
1
1
1
1
1
1
3200
3210
3220
3230
3240
3250
Magnetic Field (Gauss)
Figure 5. in situ EPR spectra oxidation of cyclohexene at 383K. (a) Under argon. (b) Under
molecular oxygen. (c) Under molecular oxygen with MnTPPCl.
Figure 5 shows the results obtained from the in situ EPR of the oxidation of
cyclohexene under different reaction conditions. As has been reported previously, the
α-hydrogen atom of cyclohexene was easily abstracted under high temperature,
affording an allylic radical. As shown in Figure 5, the only stable PBN radical adduct
was obtained when cyclohexene was immersed in argon (Figure 5a). Hence, Adduct 1
was characterized by hyperfine splitting constant (aN1 = 15.3 G, aH1 = 3.5 G, g = 2.0033),
which was in accordance with that of the computer simulation (Figure S5). On account
of the absence of oxygen, adduct 1 should be assigned to carbon-centred cyclohexenyl
radical (C6H9) [31]. When argon was adjusted by molecular oxygen, two stable PBN
radical adducts (2 and 3) were observed (Figure 5b), which were in accordance with
that of the computer simulation (Figure S6). Adduct 2 and 3 was characterized by
hyperfine splitting constants, that is, aN2 = 14.0 G, aH2 = 2.3 G, g = 2.0033 and aN3 =
15.5 G, aH3 = 3.4 G, g = 2.0031), respectively. In general, carbon-centered alkyl radicals,
such as C6H9, easily and quickly combine with dioxygen, generating oxygen-centred
radical [31, 32], so adduct 2 and 3 are likely to be oxygen-centred radical, that is,
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
cyclohexenyl peroxyl radical (C6H9OO) and cyclohexenyl oxyl radical (C6H9O).
When MnTPPCl catalyst was added in the solution, the same two adducts (2 and 3)
were obtained (Figure 5c). However, the intensity of adduct 2 significantly decreased,
coupled with the increase of adduct 3 (comparison of the two curves shown in Figure
5b and 5c). The manganese porphyrin catalyst has been suggested to be favorable for
the transfer of radical 2 to radical 3. In the aerobic oxidation of cyclohexene,
cyclohexenyl hydroperoxide derived from H-abstraction of C6H9OO was an important
intermediate [33], which was possibly decomposed by MnTPPCl to afford C6H9O and
MnV=O though the O-O bond homolysis and heterolysis, respectively (Scheme 2) [20]
.
Therefore, adducts 2 and 3 are considered to be C6H9OO and C6H9O, respectively.
For confirming the above mentioned results, further in situ EPR measurement
related to the MnTPPCl-catalyzed aerobic oxidation of cyclohexene with reaction time
was conducted.
3
3
3
3
3
3
c
b
3
3
3
3
3
3
3
2
3
2,3
2,3
3
2
3
2
a
2
2
3210
3220
3230
3240
3250
Magnetic Field (Gauss)
Figure 6. in situ EPR spectra for the oxidation of cyclohexene catalyzed by MnTPPCl at 383 K. (a)
after 5 min; (b) after 10 min; and (c) after 30 min.
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10.1002/ejic.201800317
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Figure 6 shows the time courses of in situ EPR spectra of cyclohexene oxidation
in the presence of MnTPPCl. With the progress of the reaction, the intensity of spectra
significantly changed. The intensity of adduct 2 gradually weakened until it completely
disappeared at 30 min, while that of adduct 3 constantly increased, indicating that
radical 2 is transformed to radical 3 as reaction progresses, reflecting the same trend
exhibited by radical 2 and 3 in Figure 5. Hence, the results further confirmed the
presence of free radicals, as mentioned before.
Scheme 2 shows the initiation mechanism of a free-radical chain with cyclohexene.
Due to the control experiment, 7% conversion of DPM was observed with only
MnTPPCl (entry 2 in Table 1). It indicates that high-valent Mn porphyrin intermediate
could be generated by MnTPPCl interaction with oxidant O2.
Scheme 2. Initiation mechanism of the free-radical chain with cyclohexene.
In addition, in situ EPR spectra for DPM oxidation were recorded to explore the
generation of the DPM derivative radicals. Aerobic catalytic oxidation in CH3CN was
conducted on a Wilmad WG-810-A quartz-flat cell with a mixture of the CH3CN
solution of MnTPPCl, cyclohexene, DPM, and PBN. Figure 7 shows the in situ EPR
spectra of DPM oxidation under different conditions. The cell was sealed after filling
dioxygen.
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
4,5
(B)
(A)
4,5
5
5
5
4
aN5
aN4
d
c
4
aH5
3
3
aH4
4,5
4,5
4
5
4
3
3
b
a
3210
3220
3230
3240
3250
3210
3220
3230
3240
3250
Magnetic Field (Gauss)
Magnetic Field (Gauss)
Figure 7. (A) in situ EPR spectra for the aerobic oxidation of DPM at 383K. (a) DPM under
molecular oxygen after 10 min. (b) DPM/MnTPPCl after 10 min. (c) DPM/MnTPPCl/cyclohexene
after 2 min. (d) DPM/MnTPPCl/cyclohexene after 20 min. (B) Hyperfine splitting constants of
adduct 4 and 5 for DPM/MnTPPCl/cyclohexene after 2 min.
During the aerobic oxidation of DPM at 383 K, radical signals were not observed
after 10 mins, irrespective of the presence or absence of catalyst (Figure 7A-a and -b)).
The abstraction of hydrogen from the secondary carbon of DPM has been suggested to
be difficult, attributed to the cleavage of high-energy C–H bonds. As shown in Figure
1, with the introduction of cyclohexene in the reaction, remarkable enhancement in
efficiency was observed. Figure 7A-c shows sensitive changes in the corresponding
EPR signals. According to the analytical method [34], two radicals were obtained by the
distinction of hyperfine splitting constants, referred to as adduct 4 (aN4 = 14.2 G, aH4 =
2.2 G, g = 2.0042) and adduct 5 (aN5 = 14.9 G, aH5 = 2.5 G, g = 2.0036) (Figure 7B),
respectively. Due to the nearly disappearance of adduct 2 after 10 min in Figure 6,
therefore, adduct 2 cannot contribute to the signal in Figure 7A. In addition, hyperfine
splitting constants of adduct 4 and 5 were different from those of adducts 3, generated
from cyclohexene. Hence, two free radicals are generated from DPM. Because of the
presence of oxygen, carbon-centred diphenylmethanyl radical (C13H11•) cannot
contribute to the signal in Figure 7A [31], therefore, radical 4 and 5 are oxygen-centred
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
radical including diphenylmethyl oxyl radical (C13H11O) and diphenylmethyl peroxyl
radical (C13H11OO).
From Figure 7A-c and -d, with the progress of the reaction, the intensity of spectra
significantly changed. The intensity of adduct 4 gradually weakened, while that of
adduct 5 constantly increased, indicating that radical 4 is transformed to radical 5 as the
reaction progressed. Due to the convenient conversion of alkylperoxyl radicals to
alkyloxyl radicals in the free-radical chain reaction (Scheme 2), hence, adduct 4 and 5
are attributed to C13H11OO and C13H11O, respectively.
Except adduct 4 and 5, because of the overlap of peaks, the single peak at the
rightmost of Figure 7A-c was hard characterized by hyperfine splitting constant.
According to the above results, we speculated that the single peak was likely to be
adduct 3 (C6H9O) though excluding the possibility of C6H9OO, C13H11OO and
C13H11O. Because alkyloxyl radicals possess strong ability to abstract active hydrogen
[33], C6H9O is able to abstract not only the secondary hydrogen of DPM yielding
C13H11• (Scheme 3, pathway B) but also -H of cyclohexene yielding C6H9 (Scheme 3,
pathway A).
Scheme 3. Possible pathway for hydrogen abstraction by cyclohexenyl alkoxyl radical (C6H9O)
Control experiments for the aerobic oxidation of cyclohexene were performed to
determine the possible pathway of hydrogen abstraction by cyclohexenyl alkoxyl
radical. Table 2 summarizes the results obtained. The oxidation products mainly
consisted of cyclohexene oxide (1), 2-cyclohex-2-enol (2), and 2-cyclohexen-1-one (3).
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European Journal of Inorganic Chemistry
Table 2. Aerobic oxidation of cyclohexene in the presence or absence of DPMa
OH
O
O2/MnTPPCl
O
+
+
+
Others
110oC, 1.0MPa, 8h
1
3
2
Selectivity (%)
Entry
Catalyst
DPM (mmol)
Conversion (%)
1
2
7
3
others
20
1
2
MnTPPCl
MnTPPCl
-
92
65
35
17
38
48
6
18
17
a cyclohexene (1.5 mmol), solvent (dichloromethane, 5 mL), MnTPPCl (1.6  10−3 mol%), 110 °C, O2 (1.0 MPa), 8
h.
With the use of MnTPPCl for directly catalyzing the oxidation of cyclohexene, 92%
conversion was obtained (entry 1 in Table 2). However, by the addition of DPM into
the reaction solution, cyclohexene conversion sharply decreased to 65% (entry 2 in
Table 2). This result indicated that cyclohexenyl alkoxyl radical (C6H9O) cannot
possibly abstract the -H of cyclohexene. Else, cyclohexene conversion would be
increased. In addition, the total selectivity for 2 and 3 (representing the oxidized
products generated by the removal of -H) significantly enhanced. In a direct
MnTPPCl-catalyzed system, epoxide was generated through the oxygen atom transfer
of high-valent species, resulting in the high selectivity of epoxide [35]. The enhanced
selectivity for 2 and 3 indicated that the mechanism is different from that for the direct
MnTPPCl-catalyzed system. Cyclohexenyl alkoxyl radical (C6H9O) as the active
species, could trigger the generation of the secondary carbon radical. Therefore, it is
reasonable to speculate that cyclohexenyl alkoxyl radical tends to abstract the
secondary hydrogen of DPM (pathway B in Scheme 3). In addition, in the MnTPPCl-
catalyzed oxidation of DPM without cyclohexene, the conversion was 7.0%. The
initiation of the secondary carbon radical was attributed to the high-valent species.
Plausible catalytic mechanism of diphenylmethane oxidation
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
Based on the above mentioned discussions, a plausible mechanism was proposed
for the aerobic oxidation of DPM in the presence of MnTPPCl and cyclohexene, as
shown in Scheme 4.
O O
O OH
H-abstraction
O2
2
1
PorMn(III)
heterolysis
homolysis
PorMn(III)
O
3
O2
O
+
PorMn(V)
O
PorMn(V)
O2
path A
O
O
4
O
4
H
H-abstraction
OOH
OH
O
5
H-abstraction
PorMn(III)
homolysis
Scheme 4. Possible biomimetic catalytic mechanism for the oxidation of DPM
First, cyclohexenyl radical (1, C6H9) is derived from the abstraction of hydrogen
at the allylic position under high-temperature conditions. Cyclohexenyl peroxyl radical
(2, C6H9OO) is rapidly formed by the insertion of oxygen. Subsequently, cyclohexenyl
hydroperoxide, derived from H-abstraction of C6H9OO, is decomposed by PorMn(III)
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
though the homolysis of O-O bond yielding cyclohexenyl alkoxyl radical (3, C6H9O)
or the heterolysis of O-O bond yielding PorMn(V)=O. The parahydrogens of DPM are
able to be despoiled by C6H9O or PorMn(V)=O, yielding a diphenylmethanyl alkyl
radical. On the one hand, diphenylmethanyl alkyl radical (C13H11•) possibly escape
from the solvent cage, and then react with residual PorMn(V)=O to afford
benzophenone (path A). On the other hand, C13H11• is likely to be inserted by O2,
yielding a diphenylmethanyl peroxyl radical (4, C13H11OO). Next, the termination of
two C13H11OO allows for the formation of benzophenone (path B) [36]. Besides,
C13H11OO is converted to diphenylmethanyl hydroperoxide by H-abstraction reaction,
which is decomposed though the homolysis of O-O bond to afford diphenylmethanyl
alkoxyl radical (5, C13H11O). This alkoxyl radical may be converted to benzophenone
through the abstraction of the weakly bonded α-H atom by O2 (path C) [37]. In addition,
C13H11O is also likely to be converted to benzhydrol though H-abstraction.
4. Conclusions
In this study, a system mimicking that of enzyme-catalyzed oxidation, in which
cyclohexene was used as the NAD(P)H-like model. This MnTPPCl-mediated
biomimetic catalytic protocol was efficient for the aerobic oxidation of non-active C–
H bonds under benign conditions. With diphenylmethane as the model substrate, the
conversion and TON of the catalyst reached to 42.5% and 26562, respectively.
Efficiency was approximately 6–7 times greater than that observed for the system
catalyzed by only MnTPPCl. The effects of reaction temperature, pressure, and catalyst
and cyclohexene amounts on the aerobic oxidation of diphenylmethane were optimized.
From the results obtained from experiments as well as in situ EPR and UV–vis
spectroscopy, the mechanism of this biomimetic catalytic system for the activation of
C–H bonds was investigated. Both high-valent manganese species and cyclohexenyl
alkoxyl radical were triggers for the secondary carbon radical, which could be easily
converted to corresponding alcohol or ketone.
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
Experimental section
Materials and methods
DPM, diphenylmethanol, and benzophenone, cyclohexene, cyclohexene oxide, 2-
cyclohexen-1-one, and 2-cyclohexen-ol, N-tert-butyl-alpha-phenylnitrone (PBN) were
obtained from Alfa Aesar. All other reagents were of analytical grade and used without
further purification unless indicated. Pyrrole was redistilled before use.
Metalloporphyrin catalysts were prepared according to previous procedures [38]
.
UV–vis spectra were recorded on a Shimadzu UV-2450 spectrophotometer. An
AvaSpec-2048 spectrometer (Avantes, Netherlands) was used for recording in situ UV.
Elemental analysis data were obtained on a Vario EL III system. FTIR spectra were
obtained on a Bruker 550 spectrometer. in situ EPR spectroscopy measurements were
performed on a JESFA-200 (JEOL, Japan) spectrometer using N-tert-butyl-alpha-
phenylnitrone (PBN) as a free-radical spin-trapping agent. All reactions were
conducted in triplicate.
General procedures for the oxidation of DPM
In a typical experiment, DPM (6 mmol), cyclohexene (1.5 mmol), dichloromethane
(25 mL), MnTPPCl, and diphenyl (as internal standard) were first added into a 100 mL
stainless-steel autoclave equipped with a magnetic stirrer and a temperature-controlling
device. Second, after the reaction mixture was rapidly heated to 110 °C, the autoclave
was pressurized with dioxygen from a reservoir tank for maintaining a constant pressure
of 1.0 MPa. Third, after the reaction mixture was stirred for the designated reaction
time, the autoclave was rapidly cooled to −5 °C, and the remaining O2 was slowly
released and absorbed in a small amount of ethyl acetate. Each catalytic reaction was
repeated three times for achieving reproducibility. The consumption of DPM and
formation of the oxidized products were monitored by GC (Shimadzu GC-2010 plus)
and GC-MS (Shimadzu GCMS-QP2010).
Oxidation monitored by in situ UV–vis spectroscopy
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
In a typical experiment, the stainless-steel reactor (Parr 5510) was sealed after the
addition of liquid reagents and the MnTPPCl solution. After the reaction mixture was
rapidly heated to 110 °C, the autoclave was pressurized with dioxygen for maintaining
a constant pressure of 1.0 MPa. The in situ UV–vis spectra of MnTPPCl were recorded
on the AvaSpec-2048 spectrometer, which was equipped with a high-pressure, high-
temperature probe and connected to the stainless-steel reactor. The spectrophotometer
was programmed at 5 min intervals.
Oxidation monitored by in situ EPR spectroscopy
Aerobic catalytic oxidation in CH3CN was performed in a Wilmad WG-810-A
quartz-flat cell with a mixture of the CH3CN solution of MnTPPCl (1 × 10−4 mol L−1)
as well as cyclohexene and DPM in a molar ratio of 1:4. A JESFA-200 (JEOL, Japan)
spectrometer was utilized to record in situ EPR spectra. The amount of PBN, serving
as the radical scavenger, was 60 mM. The cell was sealed after filling dioxygen. EPR
analyses were performed at a microwave frequency of 9.05 GHz at 383K. Instrument
conditions for all analyses were as follows: microwave frequency, 9.05 GHz;
modulation amplitude, 2 mT; modulation frequency, 100 kHz; power, 0.998 mW; and
time constant, 0.03 s. All spectra represent the signal-averaged sum of six scans unless
otherwise noted.
General procedures for kinetic experiments
Each experiment in this study was performed in triplicate. Kinetic experiments
were performed by the initial concentration method under [substrate] >> [MnTPPCl]
conditions. In a typical experiment, the stainless-steel reactor (Parr 5510) was sealed
after the addition of liquid reagents and the MnTPPCl solution. After the reaction
mixture was rapidly heated to 110 °C, the autoclave was pressurized with dioxygen for
maintaining a constant pressure of 1.0 MPa. The initial rate for the oxidized product
was measured as a function of the concentration of the product produced. The
reproducibility for the rate constants measured was within 5%.
22
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10.1002/ejic.201800317
European Journal of Inorganic Chemistry
Acknowledgements
This study was supported by the National Key Research and Development Program
of China (2016YFA0602900), the National Science for Distinguished Young Scholars
of China (No. 21425627), the National Natural Science Foundation of China (Nos.
21576302), the Natural Science Foundation of Guangdong Province
(2015A030313104).
Keywords: manganese porphyrins; diphenylmethane; oxidation; cyclohexene
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Graphical Abstract
Mechanistic understanding towards the role of cyclohexene
in enhancing efficiency for manganese porphyrins-catalyzed
aerobic oxidation of diphenylmethane
*
Jun Jianga, Jie-Xiang Wangc, Xian-Tai Zhoub*, Hong-Yu Chena, Hong-Bing Jia
a Fine Chemical Industry Research Institute, School of Chemistry, Sun Yat-sen
University, Guangzhou 510275, PR China
b Fine Chemical Industry Research Institute, School of Chemical Engineering and
Technology, Sun Yat-sen University, Zhuhai, 519082, PR China
c Huizhou Research Insititute of Sun Yat-sen University, Huizhou, 516081, PR China
*Corresponding author. Tel.: +86 20 84113653; fax: +86 20 84113654.
E-mail: zhouxtai@mail.sysu.edu.cn (X.T. Zhou), jihb@mail.sysu.edu.cn (H. B. Ji)
Efficiency enhanced by cyclohexene: The efficiency towards diphenylmethane
oxidation catalyzed by metalloporphyrins was improved remarkably in presence of
cyclohexene. As NAD(P)H-like model, the role of cyclohexene that promotes the
initiation of radical and the activation of dioxygen are demonstrated by a mechanistic
study.
26
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