Study of special catalytic behaviors of the metal porphyrins with different central metal ions in the aerobic oxidation of 4-nitroethylbenzene to 4-nitroacetophenone

Article abstract of DOI:10.1134/S1070427215050250

The special catalytic behaviors of metalloporphyrins with different central metal ions in the oxidation of 4-nitroethylbenzene to 4-nitroacetophenone with molecular oxygen as an oxidant were investigated, including the initiated temperature and catalytic

Full text of DOI:10.1134/S1070427215050250

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2015, Vol. 88, No. 5, pp. 885−890. © Pleiades Publishing, Ltd., 2015.
VARIOUS
TECHNOLOGIES
Study of Special Catalytic Behaviors of the Metal Porphyrins
with Different Central Metal Ions
in the Aerobic Oxidation
of 4-Nitroethylbenzene to 4-Nitroacetophenone1
Pan Wanga, Yuanbin Sheb,a*, Haiyan Fuc**, Wenbo Zhaoa, and Meng Wanga
aInstitute of Green Chemistry and Fine Chemicals, college of Environmental & Energy Engineering,
Beijing University of Technology, Beijing, 100124 China
b College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014 China.
cCollege of Pharmacy, South-Central University for Nationalities, Wuhan, 430074 China
e-mail: *sheyb@zjut.edu.cn, **fuhaiyan@mail.scuec.edu.cn
Received May 23, 2015
Abstract—The special catalytic behaviors of metalloporphyrins with different central metal ions in the oxidation
of 4-nitroethylbenzene to 4-nitroacetophenone with molecular oxygen as an oxidant were investigated, including
the initiated temperature and catalytic activities of mettalloporphyrins, as well as the selectivity of 4-nitroace-
tophenone. The 55.3% conversion of 4-nitroethylbenzene and 87.8% selectivity to 4-nitroacetophenone were
obtained in the above process with cobalt(II) meso-tetrakis (4-methoxyphenyl) porphyrin as a catalyst at 140°C
reaction temperature for 12 h. In particular, when the above process was carried out with zinc(II) meso-tetrakis-
(4-methoxyphenyl) porphyrin as a catalyst at the same reaction condition, a predicted intermediate bis(α-methyl-
4-nitrobenzyl) peroxide was discovered and selectivity to the compound was realized as 44.0%. In addition, a
possible oxidation mechanism of 4-nitroethylbenzene to 4-nitroacetophenone catalyzed by metalloporphyrins
was proposed.
DOI: 10.1134/S1070427215050250
Metalloporphyrins, as the mimics of cytochrome
P-450, can activate molecular oxygen to oxidize
various organic compounds. Therefore, they have been
widely used as the catalysts in the selective oxidation
of hydrocarbons [1–7]. 4-Nitroacetophenone (PNAP)
is an important organic intermediate with prochirality
for the synthesis of pharmaceuticals [8, 9], pesticides
[10], and chiral compounds [11, 12]. The aerobic
oxidation of 4-nitroethylbenzene (PNEB) is one of
the most promising methods for preparing PNAP [13]
because the nitro and the acetyl are the second locating
groups. Metalloporphyrins would be the effective
catalysts in the aerobic oxidation of PNEB. However,
the catalytic activities of metalloporphyrins and
selectivities of aromatic ketones may be interrelated with
the redox electric potential [14–17]. Thus, the catalytic
performances of different metalloporphyrins may vary
in the oxidation of PNEB.
The free radical autoxidation mechanism of the
ethylbenzene catalyzed by iron porphyrins was confirmed
by Evans and Smith [18]. In addition, they predicted a
minor pathway [Eq. (1)], but was not able to detect it.
According to the works [19, 20], this pathway forms
peroxide. Moreover, the peroxide was only detected from
the decomposition and coupling [Eq. (2)] of the peroxide
radicals, instead of the hydrocarbon oxidation. However,
nitro substituent as a strong electron-withdrawing group
can enhance the stability of the phenylethyl radical
1 The text was submitted by the authors in English.
885

886
PAN WANG et al.
Scheme 1. Aerobic oxidation of PNEB catalyzed by metalloporphyrins.
OCH3
A: M = Fe
B: M = Co
C: M = Mn
D: M = Zn
OCH3
H3CO
OCH3
through the p-π conjugation. Thus, the Eq. (1) seems to
occur when the substrate is PNEB.
was separated by silica gel column chromatography
using ethyl acetate/petroleum ether (80/20) as the eluting
solvent and identified with melting point, IR, Raman,
1H NMR, 13C NMR, and FAB-MS.
ROO· + ROO· → ROOR + O2
ROO· + R· → ROOR + O2
(1)
(2)
BMNP: pale yellow solid; mp, °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): δ 8.23 (d, 3JHH = 8.4Hz, 4H),
Accordingly, PNEB was employed as the substrate
and four metalloporphyrins were utilized as the catalysts
in this study. The specificities of metalloporphyrins were
investigated and the peroxide [21] was expected to be
found as the oxidation product of PNEB.
3
3
7.55 (d, JHH = 8.4Hz, 4H), 5.18 (q, JHH = 6.4Hz, 2H)
and 1.48 (d, JHH = 6.4Hz, 6H); 13C NMR (100 MHz,
3
CDCl3, ppm): δ 20.2, 82.7, 123.8, 127.2, 147.5, 149.5.
FAB-MS m/z: 333([M+H]+).
All solvents and reagents were AR reagent or
chromatography pure reagent unless otherwise stated.
PNEB was supplied by Anqin chemical Co., Ltd. of
Funing County (purity: 99%). Deionized water was made
by PALL PL5241. Catalysts were prepared by ourselves
according to the process previously reported [22, 23].
EXPERIMENTAL
Aerobic oxidation of PNEB to PNAPwas catalyzed by
different metalloporphyrins, as represented in Scheme 1.
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, there appeared a novel substance,
bis(α-methyl-4-nitrobenzyl) peroxide (BMNP), which
Ultraviolet-visible spectra were obtained on HITACHI
U-3010; FT–IR, FT-Raman spectra on Bruker VERTEX
1
70, Bruker RFS 100/S; H and 13C NMR spectra on
Bruker Avance 400; and FAB-MS on Thermo Finnigan
MAT 90. HPLC analyses were performed byAgilent 1200
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STUDY OF SPECIAL CATALYTIC BEHAVIORS OF THE METAL PORPHYRINS
887
Table 1. Initiation temperatures and orbital energy levels of different metalloporphyrins in the oxidation of PNEBa
Entry
Catalystsb
Initiation temperaturec, ºC
EHOMO of catalyst, eV
ELUMO of catalyst, eV
1
2
3
4
5
Blank
190
190
170
165
155
–
–
T(p-OCH3)PPCo
T(p-OCH3)PPZn
T(p-OCH3)PPFe
T(p-OCH3)PPMn
–3.861
–4.435
–4.575
–4.664
–2.734
–2.791
–3.060
–3.469
a Reaction conditions: PNEB (14.1 g, 100 mmol), metalloporphyrins (0.1×10−2 mmol), and O2 bubbling (1 atm). The reactions were initiated
at the initiation temperature for 1 min and performed at 140ºC as a reaction temperature for 12 h.
b T(p-OCH3)PPFe: iron(II) meso-tetrakis (4-methoxyphenyl) porphyrin; T(p-OCH3)PPCo: cobalt(II) meso-tetrakis (4-methoxyphenyl) porphyrin;
T(p-OCH3)PPMn: manganese(II) meso-tetrakis (4-methoxyphenyl) porphyrin; T(p-OCH3)PPZn: zinc(II) meso-tetrakis (4-methoxyphenyl)-
porphyrin.
c
Initiation temperature: the PNAP could be detected by HPLC when the oxidation reaction is maintained at this temperature for 1 min.
LC, LC-MS (ESI) were made on Bruker esquire 6000,
and GC-MS (EI) were completed on Bruker Trace DSQ.
Notably, a positive correlation existed between the
initiation temperature and EHOMO or ELUMO of T(p-
OCH3)PPFe, T(p-OCH3)PPCo, T(p-OCH3)PPMn and
T(p-OCH3)PPZn. Therefore, the lower the EHOMO or
ELUMO was, the lower the initiation temperature became
(entries 2–5). According to the frontier molecular orbital
theory [26], if the EHOMO and ELUMO of the central metal
ions in metalloporphyrins are closer to the ELUMO and
EHOMO of molecular oxygen (under the same calculated
levels, the EHOMO and ELUMO of O2 are –8.698 and
–6.631 eV), the energy needed for the initiation of the reaction
decreases. Therefore, the initiation temperature would be
lowered.
A typical reaction procedure was as follows: 14.1 g
(100 mmol) of PNEB and 0.7 mg (1 × 10–3 mmol) of
T(p-OCH3)PPCo were loaded in the reactor and oxygen
at a rate 40 mL min–1 was bubbled through the mixture
obtained. The reaction mixture was heated to 190°C and
kept at this temperature for 1 min, then it was quickly
cooled to 140°C and kept at this temperature for 12 h
under stirring with magnetic stirrer. The composition of
reaction mixture was analyzed by means of HPLC.
RESULTS AND DISCUSSION
In addition, the by-products sharply increased when
the reaction was conducted above 140°C. Thus, the
initiation and reaction temperatures must be controlled
precisely to obtain the high yields for products.
Various manifestations of the initiation temperature
in the oxidation of PNEB. The initiation of the reaction
is a key step for the PNEB oxidation [24], which is a free
radical reaction. Therefore, the initiation temperature was
investigated. In addition, the orbital energy levels of the
central metal ions in diverse metalloporphyrins were
calculated by Dmol3 program [25]. The results were
summarized in Table 1.
Catalytic activities and selectivities of metallo-
porphyrins in the aerobic oxidation of PNEB to PNAP.
The catalytic performances of metalloporphyrins with the
different central metal ions were discussed. The results
were summarized in Table 2.
As shown in Table 2, the metalloporphyrins work
effectively in the oxidation of PNEB compared with the
blank experiment (entry 1). Notably, the selectivity for
PNAP was substantially increase when the metallopor-
phyrins used as catalysts. However, the catalytic per-
formances of the metalloporphyrins distinctly exhibited
diversities (entries 2–5). This finding suggested that the
catalytic activities of metalloporphyrins and selectivities
of PNAP depend on the nature of central ions. Based on
the results of this study, T(p-OCH3)PPCo has presented
As shown in Table 1, the initiation temperatures varied
with various metalloporphyrins. In comparison with the
blank reaction without catalyst (Entry 1), the metallo-
porphyrins served as the effective catalysts to lower the
initiation temperatures, except for cobalt (II) meso-tetra-
kis(4-methoxyphenyl)porphyrin (T(p-OCH3)PPCo). In
particular, manganese(II) meso-tetrakis(4-methoxy-
phenyl)porphyrin (T(p-OCH3)PPMn) lowered the
initiation temperature from 190°C to 155ºC (Entry 5).
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STUDY OF SPECIAL CATALYTIC BEHAVIORS OF THE METAL PORPHYRINS
889
Table 2. Oxidation of PNEB catalyzed by different metalloporphyrins in the presence of molecular oxygena
Conversion, Selectivity for Selectivity for Selectivity for Selectivity for Selectivity for
Entry
Catalysts
Blank
%
PNAP, %
PNPEb, %
PNBA, %
BMNP, %
PNBD, %
1
2
3
4
5
11.3
55.3
16.2
48.1
53.2
52.5
87.8
38.9
80.6
77.8
2.9
5.9
44.4
6.3
6.9
7.5
8.5
0
0
0.2
0
T(p-OCH3)PPCo
T(p-OCH3)PPZn
T(p-OCH3)PPFe
T(p-OCH3)PPMn
10.0
7.8
44.0
2.1
5.7
0.2
2.0
1.2
6.8
a Reaction conditions: PNEB (14.1 g, 100 mmol), metalloporphyrins (0.1 × 10-2 mmol), and O2 bubbling (1 atm). The reactions were initiated
at the initiation temperature for 1 min and performed at 140°C as the reaction temperature for 12 h.
b PNPE: 1-(4-nitrophenyl) ethanol, PNBA: 4-nitrobenzoic acid, BMNP: bis(α-methyl-4-nitrobenzyl) peroxide, PNBD: 4-nitrobenzaldehyde.
high conversion of PNEB and good selectivity for PNAP,
and BMNP cannot be detected (entry 2). As for the T(p-
OCH3)PPZn (entry 3), the great volume of BMNPproduct
indicated that BMNPcannot be effectively converted into
PNAP with the T(p-OCH3)PPZn as the catalyst. This
result suggested that the conversion of BMNP required
the variable valence metalloporphyrins[27,28]. According
to the above discussion, the specificities of metallopor-
phyrins functioning as metal ions were remarkable.
in presence of T(p-OCH3)PPZn continued due to the
presence of BMNP as the free radical initiator. However,
no change was observed in other cases. It means that the
oxidation of PNEB at the addition of BNMP can proceed
without heating to the initiation temperature.
CONCLUSIONS
In conclusion, the remarkable catalytic specificities of
metalloporphyrins, such as catalytic activities and selec-
tivities, in the aerobic oxidation of PNEB were observed,
including the effects on the initiation temperatures and
diverse selectivities for different products. The initiation
temperature was low when T(p-OCH3)PPMn was used as
the catalyst, whereas it was the highest when the oxidation
of PNEB was catalyzed by T(p-OCH3)PPCo. However,
T(p-OCH3)PPCo was demonstrated to have a reasonable
catalytic activity for the oxidation of PNEB (55.3%) and
a high selectivity for PNAP (87.8%). In particular, bis(α-
methyl-4-nitrobenzyl)peroxide (BMNP) as a predicted
intermediate was produced at 44.0% selectivity in the
oxidation of PNEB with T(p-OCH3)PPZn as the catalyst.
Finally, the radical mechanism of the above reaction was
proposed and confirmed for this oxidation process.
Possible oxidation mechanism of PNEB to PNAP
catalyzed by the metalloporphyrins with variable
valences. Combining the researches of Smith [18],
Ba1ci [26], Taylor [28] and their co-workers, a possible
oxidation mechanism of PNEB to PNAPcatalyzed by the
metalloporphyrins with variable valence metal ions has
been proposed according to the above results (Scheme 2).
The nitro group as the para-substituent of the
ethylbenzene enhanced the structural stability of the
phenylethyl radical because of conjugative effect.
Furthermore, the path (a) was a fast reaction because the
hydroperoxide was not observed when PPh3 was applied
to the PNEB oxidation mixture. BMNPwas decomposed
by metalloporphyrins with variable valence metal ions
(PMII) according to Haber–Weiss recycle mechanism
[29] [path (b)] to PNAP and PNPE [path (c) and (d)],
and then PNPE was oxidized to PNAP and PNBD [path
(e) and (f)]. These pathways were further confirmed
with the oxidative reaction of BMNP and PNPE as the
reactants respectively. In addition, two oxidation reactions
of PNEB, which were catalyzed by T(p-OCH3)PPCo or
T(p-OCH3)PPZn, were desired and investigated. It was
shown that, when oxidation lasted for 3 h, the temperature
was reduced from 140 to 25°C, maintained for 1 h, and
the again rose to 140°C and maintained for 6 h, reaction
ACKNOWLEDGMENTS
This work was supported by the Major, General
and Youth Projects of National Natural Science
Foundation of China (Grant No. 21476270; 21276006,
21205145), the Funding Project for Academic Human
Resources Development in Institutions of Higher
Learning under the Jurisdiction of Beijing Municipality
(grant no. PHR201107104), and the Open-Funding
Project of State Key Laboratory Breeding Base of
Green Chemistry-Synthesis Technology in Zhejiang
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