Oxidation of alkylaromatics to aromatic ketones catalyzed by metalloporphyrins under the special temperature control method

Article abstract of DOI:10.1139/cjc-2014-0167

The aerobic oxidation of alkylaromatics to aromatic ketones catalyzed by metalloporphyrins under the special temperature control method was systematically investigated. Three novel and key points were found to have significant functions in this process, that is, the special temperature control method (initiation at higher temperature and reaction at lower temperature), the synergistic effect of the composite catalysts comprising cobalt and manganese porphyrins, and the amount of catalysts in the reaction. Subsequently, the effects of substitutes on alkylaromatics were also explored under the same conditions. Results showed that alkylaromatic conversions gradually increased from 8.5% to 54.3% with the para-substituents shifting from the electron-donating group to the electron-withdrawing group (i.e., methoxy < hydro < bromo < acyl < nitro). A possible mechanism for this reaction was also proposed.

Full text of DOI:10.1139/cjc-2014-0167

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ARTICLE
Oxidation of alkylaromatics to aromatic ketones catalyzed by
metalloporphyrins under the special temperature control method
Pan Wang, Yuanbin She, Haiyan Fu, Wenbo Zhao, and Meng Wang
Abstract: The aerobic oxidation of alkylaromatics to aromatic ketones catalyzed by metalloporphyrins under the special
temperature control method was systematically investigated. Three novel and key points were found to have significant
functions in this process, that is, the special temperature control method (initiation at higher temperature and reaction at lower
temperature), the synergistic effect of the composite catalysts comprising cobalt and manganese porphyrins, and the amount of
catalysts in the reaction. Subsequently, the effects of substitutes on alkylaromatics were also explored under the same condi-
tions. Results showed that alkylaromatic conversions gradually increased from 8.5% to 54.3% with the para-substituents shifting
from the electron-donating group to the electron-withdrawing group (i.e., methoxy < hydro < bromo < acyl < nitro). A possible
mechanism for this reaction was also proposed.
Key words: special temperature control method, metalloporphyrins, biomimetic catalysis, alkylaromatics, oxidation, aromatic ketones.
Résumé : L’oxydation aérobie des composés alkylaromatiques en cétones aromatiques catalysée par des métalloporphyrines a
été étudiée systématiquement suivant un schéma précis de réglage de la température. On a identifié trois éléments clés jouant
un rôle important dans ce processus : le schéma précis de réglage de la température (enclenchement a` température élevée et
réaction a` température plus basse), l’effet synergique des catalyseurs composites comprenant des porphyrines de cobalt et de
manganèse et la quantité de catalyseurs en jeu dans la réaction. On a ensuite étudié les effets des substituants sur les al-
kylaromatiques dans les mêmes conditions. Les résultats ont montré que la conversion des composés alkylaromatiques aug-
mente graduellement de 8,5 a` 54,3 % a` mesure que les substituants para passent du groupement le plus électrodonneur au
groupement le plus électroattracteur (méthoxy < hydro < bromo < acyl < nitro). On a également proposé un mécanisme pouvant
être a` l’origine de cette réaction. [Traduit par la Rédaction]
Mots-clés : schéma précis de réglage de la température, métalloporphyrines, catalyse biomimétique, composés alkylaromatiques,
oxydation, cétones aromatiques.
The study of alkylaromatic oxidation catalyzed by metalloporphy-
rins by using ethylbenzene as the major substitute is performed
Introduction
Aromatic ketones are important chemical intermediates for the
because the ethylbenzene is the basic structure of alkylaromatics.
synthesis of various pharmaceutical drugs,1 pesticides,2 dyes,3 and
Evans and Smith have reported an ethylbenzene oxidation mech-
perfumes.4 Researchers are focusing on the direct oxidation of
anism catalyzed by metalloporphyrins.11a Previous studies in
alkylaromatics for economic and environmental reasons, and the
our laboratory have shown that the oxidation mechanism of
Friedel–Crafts acylation of aromatics is widely used to prepare
p-nitrotoluene is different from that of toluene, because of nitro
the corresponding aromatic ketones.5 Mirkhani et al.,6 Shaabani
substituent effects.12 Therefore, p-nitroethylbenzene (PNEB) is se-
et al.,7 and Lee et al.8 have investigated the catalytic oxidation of
lected as the model substrate in this study.
alkylaromatics to targeted aromatic ketones using different oxi-
dants including KMnO4, NaIO4, NaBrO3, KBrO3, PhIO, t-BuOOH,
air, and O2. Although molecular oxygen is a more economical,
Constant temperature reactions are usually maintained during
the reaction processes. However, in the pre-analysis conducted by
our group for the aerobic oxidation of PNEB to p-nitroacetophenone
environment-friendly, and atom-efficient9 oxidant compared with
other oxidants, oxidation of hydrocarbons with molecular oxygen
is not feasible, because dioxygen and hydrocarbons, respectively,
exist in triplet and singlet states. Fortunately, metalloporphyrins
can catalytically activate molecular oxygen even at room tempera-
ture, thereby solving the spin-restrained problem of hydrocarbon
oxidation.10 The oxidation of alkylaromatics to aromatic ketones
catalyzed by metalloporphyrins with molecular oxygen as an ox-
idant is preferred.11
(PNAP) catalyzed by metalloporphyrins,13 a special temperature
control method (initiation at higher temperature and reaction at
lower temperature) was required. The reaction will slow down
upon maintaining the reaction temperature at the initiation tem-
perature for more than three minutes and will even stop once the
initiation temperature was maintained for more than five min-
utes, so one minute was adopted as the optimum initiation time
for the oxidation of PNEB. In this study, the effects (e.g., temper-
ature effects) on the aerobic oxidation of PNEB were analyzed as
Received 5 April 2014. Accepted 20 June 2014.
P. Wang, W. Zhao, and M. Wang. Institute of Green Chemistry and Fine Chemicals, college of Environmental & Energy Engineering, Beijing
University of Technology, Beijing 100124, China.
Y. She. Institute of Green Chemistry and Fine Chemicals, college of Environmental & Energy Engineering, Beijing University of Technology, Beijing
100124, China; State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering, Zhejiang University of
Technology, Hangzhou 310014, China.
H. Fu. College of Pharmacy, South-Central University for Nationalities, Wuhan, 430074, China.
Corresponding authors: Yuanbin She (e-mail: sheyb@bjut.edu.cn) and Haiyan Fu (e-mail: fuhaiyan@mail.scuec.edu.cn).
Can. J. Chem. 92: 1059–1065 (2014) dx.doi.org/10.1139/cjc-2014-0167
Published at www.nrcresearchpress.com/cjc on 30 June 2014.

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Can. J. Chem. Vol. 92, 2014
Scheme 1. Aerobic oxidation of PNEB to PNAP catalyzed by metalloporphyrins. PNEB: 4-nitroethylbenzene; PNAP: 4-nitroacetophenone;
PNPE: 1-(4-nitrophenyl) ethanol; PNBD: 4-nitrobenzaldehyde; PNBA: 4-nitrobenzoic acid; and BMNP: bis(␣-methyl-4-nitrobenzyl) peroxide.
Fig. 1. Effect of reaction temperature on the oxidation of PNEB to
PNAP with dioxygen as an oxidant. Reaction conditions: PNEB
(14.1 g, 0.1 mol), solvent-free, CoTPP (0.7 mg, 1.0 × 10−3 mol%), and O2
bubbling (1 atm, 1 atm = 101.325 kPa). The reactions were initiated at
190 °C for 1 min and performed at different temperatures for 12 h.
Scheme 2. Oxidation of PNEB to PNAP catalyzed by CoTPP with
BMNP as an oxidant at 190 °C.
Scheme 3. Decomposition of BMNP catalyzed by CoTPP at 140 °C.
flow, and then the reaction mixture was heated to the initial
temperature (190 °C). The initial reaction temperature was main-
tained for one minute, after which it was lowered to 140 °C, and
the mixture was stirred at this temperature for 12 h. The reaction
mixtures were analyzed by HPLC.
shown in Scheme 1. In addition, the special temperature control
was also employed for the aerobic oxidation of other alkylaromatics
to corresponding aromatic ketones.
The above five products were identified by LC-MS (ESI), GC-MS
(EI), or MS (FAB), in which, as the major products, PNAP ([M+H]+:
m/z 166), PNPE ([M+H]+: m/z 168), and PNBA ([M−H]−: m/z 166) were
detected by LC-MS, while 4-nitrobenzaldehyde (PNBD) (M+: m/z 151)
by GC-MS. Further, a new product, bis(␣-methyl-4-nitrobenzyl) per-
oxide (BMNP), which was separated by silica gel column chroma-
tography using ethyl acetate/petroleum ether (80:20) as the eluting
Experimental
All solvents and reagents were analytical grade or chromatog-
raphy pure reagents. Catalysts were freshly prepared according
to the previous reported procedures.14 HPLC analyses were per-
formed using an Agilent 1200 LC.
Oxidation of alkylaromatics with molecular oxygen catalyzed
by metalloporphyrins was performed in a 100 mL three-necked
flask immersed in a oil bath thermostat equipped with a magnetic
stirrer. A typical reaction was conducted in solvent-free condi-
tions and carried out as follows: the mixture of PNEB (14.1 g,
0.1 mol), Co (II) meso-tetraphenylporphyrin (CoTPP) (0.7 mg, 1.0 ×
10−3 mol%) were added to the reactor, pure oxygen was bubbled
into the reaction mixture for 12 h at a 40 mL/min rate of oxygen
solvent and identified by melting point, IR, Raman, 1H NMR, 13
C
NMR, and FAB-MS. BMNP: pale yellow solid; m.p. (°C): 55–56. IR
(KBr, cm−1): 2930, 2851, 1522, 1344, 1207, 908, and 856. Raman (KBr,
cm−1): 1598, 1334, 1106, 861, 627. 1H NMR (400 MHz, CDCl3, ppm) ␦:
8.23 (d, 3JHH = 8.4 Hz, 4H), 7.55 (d, 3JHH = 8.4 Hz, 4H), 5.18 (q, 3JHH
=
3
6.4 Hz, 2H) and 1.48 (d, JHH = 6.4 Hz, 6H). 13C NMR (100 MHz,
CDCl3, ppm) ␦: 20.2, 82.7, 123.8, 127.2, 147.5, 149.5. FAB-MS m/z:
333([M+H]+).
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Wang et al.
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Table 1. Oxidation of PNEB to PNAP catalyzed by different catalysts in the presence of oxygen.
Initiation
temperature (°C)
Conversion
(%)
Selectivity
for PNAP (%)
Selectivity
for PNPE (%)
Selectivity
for PNBA (%)
Selectivity
for BMNP (%)
Entry
Catalystsa
1
2
3
4
Blank
CoTPPCl
FeTPPCl
MnTPPCl
␮-oxodimer iron porphyrin
FeTPPCl plus CoTPPCl
CoTPPCl plus MnTPPCl
190
195
165
155
165
165
155
11.3
56.2
44.6
52.9
29.4
58.3
62.4
52.5
86.1
81.5
77.1
80.4
89.9
91.3
2.9
6.3
8.7
6.8
12.6
4.2
4.0
44.4
6.6
4.8
7.8
5.6
5.1
0
0
3.7
6.2
0
5
6b
7b
0
0
4.1
Note: Reaction conditions: PNEB (14.1 g, 0.1 mol), solvent-free, metalloporphyrins (1.0 × 10–3 mol%), and O2 bubbling (1 atm, 101.325 kPa). The reactions were initiated
at the specific temperature for 1 min and performed the reaction temperature of 140 °C for 12 h.
aThe catalysts are listed in Scheme 1. CoTPPCl: chloro-cobalt meso-tetraphenylporphyrin; FeTPPCl: chloro-iron meso-tetraphenylporphyrin; MnTPPCl: chloro-
manganese meso-tetraphenylporphyrin.
bConcentrations of FeTPPCl, CoTPPCl, and MnTPPCl: 5.0 × 10–4 mol%.
Fig. 2. Effect of the amount of CoTPP on the oxidation of PNEB to
PNAP. Reaction conditions: PNEB (14.1 g, 0.1 mol) and O2 bubbling
(1 atm), solvent-free. The reactions were initiated at 190 °C for 1 min
and performed at the reaction temperature of 140 °C for 12 h.
Results and discussion
Effect of reaction temperature on the aerobic oxidation of
PNEB to PNAP
Temperature was found to have a significant effect, based on
the specific temperature control procedures that were used. The
effect of the reaction temperature on the aerobic oxidation of
PNEB to PNAP catalyzed by Co (II) meso-tetraphenylporphyrin
(CoTPP) was further analyzed, as shown in Fig. 1.
Figure 1 shows that the conversion of PNEB and the selectivity
for PNAP were closely related to the temperature. In general, con-
version increases with the increase of reaction temperature. How-
ever, as seen in Fig. 1, an interesting phenomenon on the relation
between conversion and reaction temperature occurred in the
aerobic oxidation of PNEB catalyzed by CoTPP. The conversion
initially increased with temperature from 90 to 140 °C but then
unexpectedly decreased with temperature from 140 to 190 °C in a
nearly linear trend; even if the reaction is initiated, the reaction
will slow down if the temperature is maintained at 190 °C for more
than three minutes and even stop if it is maintained at 190 °C for
more than five minutes. The selectivity for PNAP indicates a trend
similar to that of conversion, but a plateau existed from 90 to
140 °C. The rapid decrease of this selectivity is due to the domi-
nance of by-product formation compared with that of PNAP when
the temperature was above 140 °C. The special temperature con-
ligands of metalloporphyrins were considered important factors
trol method, namely, initiation at 190 °C and reaction at 140 °C,
affecting the catalytic activities and selectivities for targeted prod-
was adopted in the aerobic oxidation of PNEB catalyzed by CoTPP.
ucts.18 Further, the catalytic performance of ␮-oxodimer iron por-
The decrease in conversion with rising reaction temperature
phyrin significantly deviated compared with that of the other iron
porphyrins (Table 1, entry 5). Specifically, PNEB had lower conver-
sion, but 1-(4-nitrophenyl) ethanol (PNPE) had higher selectivity.
Based on the hypothetical catalytic mechanism of iron porphyrin,
␮-oxodimer iron porphyrin as a by-product had low catalytic
activity.11e,11g In addition, the composite catalysts of the two different
metalloporphyrins were also probed (Table 1, entries 6 and 7). The
synergistic effects of cobalt and manganese porphyrins as com-
posite catalysts resulted in a low initiation temperature and high
catalytic performance, which benefited from the positive effects
of both catalysts.
was unusual. The previous research results15 have shown that
phenol or quinone was generated in the oxidation of ethylben-
zene with H2O2 as an oxidant. 2-Ethyl-5-nitro-1,4-benzoquinone, a
new by-product, was generated from the oxidation of PNEB at a
reaction temperature of 190 °C, detected using GC-MS (m/z: 181,
152, 125, 106, 96, 78, 50, and 28). Therefore, the reaction was ter-
minated with the formation of 2-ethyl-5-nitro-1,4-benzoquinone,
because the by-product is a strong free-radical inhibitor.16 Gener-
ation of the quinone might be attributed to the oxidation ability
of bis(␣-methyl-4-nitrobenzyl) peroxide (BMNP) occurring at a
high temperature (Scheme 2) and the faster rate of consumption
of BMNP as an oxidant compared with the decomposition rate
(Scheme 3).17
The catalytic activities and selectivities for different metallo-
porphyrins were investigated using the aerobic oxidation of PNEB
to PNAP. Table 1 shows that metalloporphyrins served as effective
catalysts in the oxidative reaction of PNEB. Moreover, the catalytic
activation of different metalloporphyrins followed the order based
on PNEB conversion: cobalt porphyrin > manganese porphyrin >
iron porphyrin > ␮-oxodimer iron porphyrin. The central ion
nature has a significant function in the catalytic performances.
Significant differences were not observed in either the conver-
sion of PNEB or the selectivity for PNAP at different metallopor-
phyrins with or without axial ligands. Nevertheless, the axial
Effect of the catalyst amount on the oxidation of PNEB to
PNAP
As the biomimetic catalysts, metalloporphyrins show distinct
catalytic performances even in reaction systems expressed in
parts-per-million levels. The effect of the CoTPP amount on the
PNAP yield was investigated, as shown in Fig. 2.
Figure 2 shows that the amount of CoTPP had a significant
influence in the oxidation of PNEB. The increase in the CoTPP
amount gradually decreases the PNAP yield after an initial rise of
up to 5.0 × 10−3 mol%. This behavior could be attributed to either
the susceptibility of metalloporphyrins to aggregation through
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Can. J. Chem. Vol. 92, 2014
Table 2. Oxidation of alkylaromatics to aromatic ketones catalyzed by CoTPP in the presence of dioxygen.
Initiation Selectivity
temperature (°C) Conversion (%) for ketones (%)
Entry Substrates
1
Products
150
8.5
90.6
2
3
136
160
25.2
53.3
36.8
99.1
4
160
53.5
89.6
5
190
54.3
87.8
6
7
160
200
30.4
31.0
91.2
89.2
Note: Reaction conditions: alkylaromatics (0.1 mol), solvent-free, CoTPP (0.7 mg, 1.0 × 10–3 mol%), and O2 bubbling (1 atm, 101.325 kPa). The
reactions were initiated at the specific temperature for 1 min and performed at the reaction temperature of 130 °C for 12 h.
␲–␲ interaction19 or on the interaction with free radicals.11a The
product yield was good when the amount of catalyst had a low
concentration of 1.0 × 10−3 mol%.
gradually increased from 8.5% to 54.3 % using para-substituents,
changing from a strong electron-donating group to a strong electron-
withdrawing group (i.e., methoxy < hydro < bromo < acyl < nitro;
Table 2, entries 1–5). In addition, the conversions of substrates
with para-substituents were much higher than those of the
substrates with ortho-substituents (Table 2, entries 3 and 6 or 5
and 7).
Oxidation of various substrates to ketones catalyzed by
CoTPP
Various alkylaromatics were subjected to the reaction system
under similar reaction conditions to investigate the effects of the
substitutes on the alkylaromatics (Table 2). The substrate conversion
The oxidation mechanism of ethylbenzene in a solvent-free sys-
tem is generally considered to be metalloporphyrin-catalysed free
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Wang et al.
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Table 3. Binding energies(BE) for C–H bonds of alkylaromatics catalyzed by CoTPP.
BE for C-H
(kj/mol)a
Initiation
temperature (°C)
Conversion
(%)
Entry
1
Substrates
Products
161.13
150
8.5
2
3
94.86
88.13
136
160
25.2
53.3
4
64.97
160
53.5
5
13.84
190
54.3
6
7
74.73
160
200
30.4
31.0
112.58
Note: Reaction conditions: alkylaromatics (0.1 mol), solvent-free, CoTPP (0.7 mg, 1.0 × 10–3 mol%), and O2 bubbling (1 atm,
101.325 kPa). The reactions were initiated at the specific temperature for 1 min and performed at the reaction temperature of 130 °C for
12 h.
aAll calculations were performed using ADF2010, calculated by unrestricted GGA BLYP method, with basis set TZP.
radical oxidation.11a In the autoxidation of ethylbenzene, the key
chain carrier is 1-phenylethyl free radical, and the generation of
1-phenylethyl free radical depends on the stability of the alpha
C–H bond of the side-chain in ethylbenzene. Meanwhile, binding
energy (BE) is the mechanical energy required to break a com-
pound into its components. To obtain the energy of each species
from the optimized geometries of reactants and free radicals, the
BE can be evaluated through equation below:
BE ϭ EH ϩ EEB(FR) Ϫ EEB
where BE is the binding energy of the C–H bond of the al-
kylaromatic, EH is the bond energy of the hydrogen atom, EEB (FR)
is the energy of 1-phenylethyl free radical, and EEB is the energy
of alkylaromatic molecule. To predict the relative stability of al-
kylaromatics, the variation of calculated binding energies for
the C–H bond of alkylaromatics as a function of stability was
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Can. J. Chem. Vol. 92, 2014
Fig. 3. Two possible pathways for the aerobic oxidation of alkylaromatics to aromatic ketones catalyzed by metalloporphyrins.
investigated (Table 3). Upon comparing the para-substituents
of alkylaromatics, the binding energies of alkylaromatics were
negatively correlated with the substrate conversions. By contrast,
the binding energies increased from 13.84 to 161.13 kJ/mol, and
the substrate conversions decreased from 54.3% to 8.5%. A low
binding energy of the substrate implies a low stability, and the
generation of free radicals for alkylaromatics with lower binding
energies is easier than that of alkylaromatics with higher bind-
ing energies. Hence, aerobic oxidation of alkylaromatics with low
binding energies catalyzed by metalloporphyrins appears to be
more efficient. The rules governing the para-substituents of al-
kylaromatics also hold for substrates with ortho-substituents.
The special temperature control affects the substrates with
electron-withdrawing substituents. Generation of free radicals
was much easier, and the structures of the free radicals were more
unstable compared with other alkylaromatics having electron-
donating substituents. The decomposition of the metallopor-
phyrins by free radicals occurred more rapidly, whereas more
hydroperoxides were produced as main by-products. The nitro
group (the strong electron-withdrawing substituent) on the para
position of ethylbenzene could enhance the structural stability of
the phenylethyl radical via a conjugative effect. Thus, the struc-
tures of the peroxide intermediates obtained from the aerobic
oxidation were different between PNEB (BMNP) and other al-
kylaromatics (hydroperoxide). The two peroxide intermediates
decomposed into alcohols and ketones [paths (a) and (b)], followed
by the further oxidation of alcohols to ketones [path (c)]. The
path (I) reacts faster, because hydroperoxide is not detected in
the oxidation mixture of PNEB upon the addition of PPh3. There-
fore, two possible pathways are available in the oxidation of
alkylaromatics to aromatic ketones with molecular oxygen cat-
alyzed by metalloporphyrins (Fig. 3).
Conclusions
The special temperature control method (initiation at higher
temperature and reaction at lower temperature) was proposed
and applied in the aerobic oxidation of alkylaromatics to aromatic
ketones catalyzed by metalloporphyrins. The temperature and
the amount of metalloporphyrins have significant influence on
the oxidation of substrates. Among all composite porphyrin cata-
lysts, the composite catalysts comprising cobalt and manganese
porphyrins succeeded in highlighting the excellent characteris-
tics of individual metalloporphyrins, such as the MnTPP has the
Published by NRC Research Press

Wang et al.
1065
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Acknowledgements
This work was supported by the Key Program, General and
Youth Projects of National Natural Science of China (Grant Nos.
21476270, 21036009, 21276006, 21076004, 21205145, 20903007), the
Funding Project for Academic Human Resources Development in
Institutions of Higher Learning under the Jurisdiction of Beijing
Municipality (Grant No. PHR201107104), and the Talents Fund of
South-Central University for Nationalities (YZZ12002, XTZ13002). The
authors declare no competing financial interests.
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