Highly efficient and metal-free aerobic hydrocarbons oxidation process by an o-phenanthroline-mediated organocatalytic system

Article abstract of DOI:10.1002/adsc.200505183

A highly efficient o-phenanthroline-mediated, metal-free catalytic system has been developed for oxidation of hydrocarbons with dioxygen in the presence of N-hydroxyphthalimide; various hydrocarbons were efficiently and high selectively oxidized, e.g., ethylbenzene to acetophenone in 97% selectivity and 76% conversion, under mild conditions.

Full text of DOI:10.1002/adsc.200505183

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DOI: 10.1002/adsc.200505183
Highly Efficient and Metal-Free Aerobic Hydrocarbons
Oxidation Process by an o-Phenanthroline-Mediated
Organocatalytic System
Xinli Tong, Jie Xu,* Hong Miao
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Graduate School of the Chinese Academy of
Sciences, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, Liaoning 116023, Peopleꢀs Republic of China,
Fax: (þ86)-(0)411-8437-9245, e-mail: xujie@dicp.ac.cn
Received: May 1, 2005; Accepted: August 30, 2005
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A highly efficient o-phenanthroline-medi-
ated, metal-free catalytic system has been developed
for oxidation of hydrocarbons with dioxygen in the
presence of N-hydroxyphthalimide; various hydro-
carbons were efficiently and high selectively oxi-
dized, e.g., ethylbenzene to acetophenone in 97% se-
lectivity and 76% conversion, under mild conditions.
(or analogues) in combination with a metal has been em-
ployed to the electron-transfer process or to some oxida-
tion processes,[7] but has rarely employed for hydrocar-
bon oxidation in the absence of metal complexes. Herein,
focusingonmoreefficient organic mediators, wehavede-
veloped a new o-phenanthroline (or analogues)-mediat-
ed, metal-free catalytic system for the oxidationof hydro-
carbons in the presence of NHPI and a cocatalyst. In this
communication, molecular bromine (Br2), which has
shown a particular promotion effect in some oxidation
process,[8] was chosen as a cocatalyst.
À
Keywords: C H activation; hydrocarbons; molecular
oxygen; organic catalysis; oxidation; o-phenanthro-
line
To test the efficiency of our catalytic system, ethylben-
zene was used as a model substrate (Scheme 1). The re-
action was carried out under 0.3 MPa of O2 at 353 K for
2 h. Table 1 shows the results of oxidation using Phen or
The aerobic selective oxidation of hydrocarbons is a ma- different analogues as catalyst mediators in the presence
jor goal of todayꢀs research in catalysis as selectively oxi- of NHPI and Br2. When Phen was employed, a 76% con-
dizedhydrocarbonscanbeusedasfeedstockfortheprep- version and 97% selectivity for acetophenone (1) was
aration of fine chemicals.[1] In catalysis research field, cat- obtained (entry 1). The conversion decreased (67 or
alytic efficiency and product selectivity remain a chal- 65%) and the selectivity for 1 was almost unchanged
lengewhenmolecularoxygenisemployedasterminalox- when 2, 2’-bipyridine or bathophenanthroline replaced
idant.[2] Traditional approaches for oxidation are based Phen as catalyst mediators (entries 2 and 3). However,
on metal-catalyzed activation of hydrocarbons.[3] Aiming when neocuproine, bathocuproin, or 2,2’-bichinolin
atimprovingoxidationefficiency, Ishiiꢀsgroup and others was employed, the conversion (35%–49%) and selectiv-
developed an efficient catalytic system of N-hydroxyph- ity (75%–88%) for 1 were evidently reduced (entries 4–
thalimide (NHPI) combined with metal mediators (co- 6). Even as expected, only 28% conversion and 67% se-
balt, manganese, etc.), which exhibited considerable ac- lectivity for 1 were obtained when 4, 4’-bipyridine was
tivity in oxidation processes.[4] Recently, due to concerns used instead of Phen (entry 7). It can be seen that
of metal toxicity and metal-sensitive products, extensive Phen is a most efficient catalyst mediator, and differen-
À
research has been carried out on metal-free C H activa- ces in the oxidations mediated by different analogues
tions,[5] thus combining organic mediators with NHPI be- are probably ascribed to effects of conjugated structures
comes an ideal catalytic system. Our group has reported a or substituents.
biomimetic non-metal system composed of anthraqui-
nones, HY zeolite and NHPI.[6] Excellent selectivity for
hydrocarbon oxidationwas obtained. In the above autox-
idation processes, the active catalytic species of phthali-
mide N-oxyl radical (PINO) originated from NHPI via
proton and electron transfer with the assistance of cata-
lyst mediators. O-Phenanthroline (Phen) or bipyridine Scheme 1. Oxidation of ethylbenzene with molecular oxygen.
Adv. Synth. Catal. 2005, 347, 1953 – 1957
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Table 1. Oxidation of ethylbenzene by Phen (or analogues)-mediated catalysis.[a]
Entry
Mediator
Conversion[b] [%]
Product selectivity [%]
1
2
3
3
<1
1
2
3
76
67
65
97
96
1
1
5
3
2
97
75
4
5
6
35
45
20
11
87
2
49
28
88
67
11
17
1
7
17
[a]
Reaction conditions: the reaction of ethylbenzene was performed on a 1-mL scale, in the presence of NHPI (7.5 mol %),
Phen hydrate or analogues (2.5 mol %), Br2 (3.0 mol %), in 10 mL CH3CN, pressure¼0.3 MPa, time¼2 h, temperature¼
353 K.
[b]
The data were obtained by GC and GC-MS analysis using 1, 3-dichlorobenzene as an internal standard.
As indicated in Table 2, various hydrocarbons have uene, the selectivity for benzaldehyde (47 to 20%) was
been successfully oxidized using Phen as a catalyst medi- obviously reduced along with the increase of the conver-
ator. Tetralin was oxidized to 1-tetralone in 98% selec- sion, which suggests that the aldehyde was not stable in
tivity at 93% conversion only for 1 h (entry 1). When in- this radical reaction. From these results, it could be seen
dan was oxidized, an 88% conversion and 89% selectiv- that the remarkable conversion and selectivity of oxy-
ity for 1-indanone was obtained (entry 2). Moreover, no genated hydrocarbons have been demonstrated under
induction periods were observed during their oxidation, mild conditions when our metal-free organocatalytic
À
possibly due to the C H bonds being affected by ring system was employed.
strain. In the case of diphenylmethane, 100% selectivity
In order to clearly reveal the function and character of
for benzophenone, as no by-products were detected by catalyst components, compared data are summarized in
GC, was obtained at 34% conversion (entry 3). In addi- Table 3. It was observed that only 3% of ethylbenzene
tion, oxidations of cyclohexane and toluene were also was oxidized when 7.5 mol % NHPI was used individu-
performed to extend the present method with our cata- ally (entry 1), and oxidation hardly occurred when
lytic system. It was found that 14% or 25% of toluene 2.5 mol % Phen was solely employed (entry 2). But
was oxidized after 3 h or 5 h (entries 4 and 5), while then, when Phen and NHPI were coupled as a catalytic
21% or 48% conversion was obtained in the oxidation system, the conversion was improved to 9% but 71% se-
of cyclohexane after 2 h or 5 h (entries 6 and 7). The ma- lectivity for 1-phenylethyl hydroperoxide (3) was ob-
jor oxygenated products were ketone/aldehyde and tained (entry 3). In succession, we investigated the oxi-
acid/esters. In the mean time, GC-MS measurements in- dation of ethylbenzene by Br2, NHPI-Br2 or Phen-Br2
dicated that a few brominated products existed in the re- under the same reaction conditions; nevertheless, only
action, which was probably affected by the randomness 1%, 3% or 2% of ethylbenzene was respectively oxi-
of radical reactions. For the oxidation of cyclohexane, dized (entries 4–6). Furthermore, when bromine ion
the selectivity of cyclohexanone (25 to 22%) changed a was used as a cocatalyst in the presence of Phen and
little when the reaction time was prolonged and the con- NHPI, a promising result (43% conversion and 75% se-
version was enhanced. However, for the oxidationof tol- lectivity for 1) was obtained (entry 7). Owing to PINO
1954
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Table 2. Oxidation of various substrates by the Phen-mediated catalytic system.[a]
Entry
Substrate
t [h]
Conversion[b] [%]
Products selectivity [%]
1
Tetralin
Indan
Diphenylmethane
Toluene
Toluene
1
1
2
3
5
2
5
93
88
34
1-Tetralone
1-Indanone
98
89
100
47
20
25
1-Tetralol
1-Indanol
–
2
11
–
50
78
71
75
2
3[c]
4
Benzophenone
Benzaldehyde
Benzaldehyde
Cyclohexanone
Cyclohexanone
[d]
14
Benzoic acid
Benzoic acid
AE[f]
5
25[d]
21[e]
48[e]
6[c]
7[c]
Cyclohexane
Cyclohexane
22
AE[f]
[a]
General reactions condition: reactants were performed on a 1 mL scale, in the presence of NHPI (7.5 mol %), Phen hydrate
(2.5 mol %), Br2 (3.0 mol %), in 10 mL CH3CN, pressure¼0.3 MPa, temperature¼353 K.
The data were obtained by GC and GC-MS analysis using 1,3-dichlorobenzene as an internal standard.
Reaction was performed at 373 K in 5 mL CH3CN and 5 mL CCl4.
1,2,4,5-Tetramethylbenzene as an internal standard.
[b]
[c]
[d]
[e]
[f]
Toluene as an internal standard.
AE represents adipic acid or ester.
Table 3. Oxidation of ethylbenzene in different catalyst combinations.[a]
Entry
Catalyst
Conversion [%]
Products selectivity [%]
1
2
3
1
2
NHPI
Phen
3
0
9
–
12
–
79
–
3
4
5
6
PhenþNHPI
9
1
3
2
43
4
6
11
28
18
86
75
72
71
18
47
80
13
12
25
14
71
25
2
1
13
3
Br2
NHPIþBr2
PhenþBr2
PhenþNHPIþNaBr
PhenþBr2
PhenþBr2 þNaBr
7[b]
8[c]
9[d]
15
[a]
Reaction conditions and analytical methods were similar with the situations in Table 1.
5.0 mol % NaBr was employed.
12.5 mol % Phen and 15.0 mol % Br2 were employed.
[b]
[c]
[d]
5.0 mol % Phen, 3.0 mol % Br2 and 5.0 mol % NaBr were employed.
and bromine radical being provided with similar ther- PINO radicals. Moreover, PINO signals could be found
À
modynamic and kinetic properties in C H abstraction by means of in situ FT-IR spectroscopy. Now that the re-
reactions,[4] the control experiments were carried out action can proceed efficiently, a catalytic oxidation cycle
with larger contents of Phen-Br2 and the combination must exist. A proposed mechanism is shown in
of Phen-Br2-BrÀ (entries 8 and entry 9), and only 4% Scheme 2. From our viewpoint, p-phenylenediamines
and 6% conversion were obtained, which showed that containing tertiary nitrogens are facilely oxidized to
the bromine radical could rarely exist and PINO radical the cation radicals by Br2 or electrode through a single
could mainly perform activation of C H bond in this or- electron transfer process,[10] so Phen can potentially be
À
ganocatalytic oxidation. All these results directly exhib- converted to cation radicals through single-electron ox-
ited the importance of Phen-mediationand the necessity idation of the nitrogen atom in the beginning of the re-
for cooperating functions of the catalyst components in action, then the cation radicals promote the generation
the oxidation process.
of PINO radical under the metal-free conditions via
Sequentially, several parameters have been investi- electron and proton transfer between cation radicals
gated about the reaction mechanism. No oxidation oc- and NHPI (E1 of Scheme 2). The next step involves
curred in the presence of hydroquinone, which con- the hydrogen atom abstraction from the hydrocarbon
firmed the free radical pathway of the reaction. In addi- by PINO, and the resulting hydrocarbon radical being
tion, after Phen, NHPI and Br2 were mixed without any trapped by dioxygen provides the peroxy radicals which
substrate in acetonitrile, the solutionꢀs color gradually are eventually converted into products through hydro-
changed from brown-red (color of Br2) to yellow (color peroxide (E2). Otherwise, the one electron-deficient
of PINO),[9] which proved visually the production of Phen cation radical can be probably produced and acti-
Adv. Synth. Catal. 2005, 347, 1953 – 1957
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1955

Xinli Tong et al.
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to quantify the selectivity of the oxidation products of various
organic substrates, the reaction mixtures were all treated with
an excess of Ph3P for 1 h after the first GC measurement and
were analyzed again. For example, in the oxidation of ethyl-
benzene (Scheme 1), due to the fact that the compound 3 could
be reduced quantitatively to 2 by Ph3P at room temperature,
the selectivity of 1, 2 and 3 could be accurately attained after
a second GC measurement.
Typical Separation Procedure for Acetophenone (1)
After ethylbenzene was oxidized, the reaction mixture was
transferred into a flask and an aqueous solution of semicarba-
zide hydrochloride and sodium acetate was added. The ob-
tained liquid mixtures were stirred for 4 h under reduced pres-
sure and left in a refrigerator for 5 h, and then the resulting sol-
id was collected by filtration. The solid mixtures, which were
composed of catalysts and formed semicarbazone of 1, were
washed with diethyl ether to remove ethylbenzene and by-
products. In the following, the washed solid was transferred
into a two-neck flask and dissolved in oxalic acid and water,
and then was refluxed for 2 h. The regenerated 1 was distilled
out by steam distillation. The distillate was saturated with so-
dium chloride and extracted with diethyl ether. After being
dried, the diethyl ether layer was distilled to obtain pure 1 as
a liquid. The purity was more than 99% from GC analysis.
The yield was 71% (Entry 1 in Table 1).
Scheme 2. Proposed mechanisms for hydrocarbon oxidation
using the Phen-mediated catalytic system.
vates molecular oxygen. Further investigations about
the reaction mechanism, especially including more di-
rect information about the PINO radical and Phen cati-
on radicals in the oxidation, are underway.
In summary, a new Phen (or analogues)-mediated,
metal-free organocatalytic system has been developed
which can efficiently and with high selectivity promote
aerobic oxidation of various hydrocarbons under mild
conditions, and will therefore constitute a promising
strategy in the petrochemical industry.
Supporting Information
Experimental Section
Detailed GC measurement conditions and correlative IR spec-
tra are contained in the Supporting Information.
Reagents
Ethylbenzene and other substrates were purified by distilla-
tion. Spectro-grade acetonitrile was purchased from Tedia
Company, Inc. All other reagents were of analytical grade.
Oxygen supplied in a high-pressure cylinder was used through
a reducing valve without further treatment.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (Project 20473088) and the Na-
tional High Technology Research and Development Program
of China (Project 2004AA32G020).
General Procedure for Oxidation of Various
Hydrocarbons
References and Notes
All oxidation experiments were performed in a 100-mL auto-
clave equipped with magnetic stirring and automatic tempera-
ture control. A typical procedure for the oxidation of ethylben-
zene was as follows: an acetonitrile (10 mL) solution of ethyl-
benzene (1 mL, 8.3 mmol), NHPI (7.5 mol %), Phen
(2.5 mol %), and bromine (3.0 mol %) was charged into the re-
actor; The atmosphere inside were replaced with oxygen be-
fore the reactor was sealed. Under stirring, the autoclave was
preheated to 353 K, and then oxygen was charged to 0.3 MPa
and kept for 2 h. After the reaction, the autoclave was cooled,
and the excess gas was purged.
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asc.wiley-vch.de
1957