A free radical process for oxidation of hydrocarbons promoted by nonmetal xanthone and tetramethylammonium chloride under mild conditions

Article abstract of DOI:10.1016/j.tetlet.2009.01.077

A nonmetal catalytic system consisting of N-hydroxyphthalimide, xanthone, and tetramethylammonium chloride was developed. A wide range of hydrocarbons could be oxidized efficiently with dioxygen under mild conditions. In the presence of xanthone and tetramethylammonium chloride, catalytic activity of N-hydroxyphthalimide was greatly improved, and selectivity for alkyl hydroperoxide was remarkably decreased.

Full text of DOI:10.1016/j.tetlet.2009.01.077

Tetrahedron Letters 50 (2009) 1677–1680
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Tetrahedron Letters
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A free radical process for oxidation of hydrocarbons promoted by
nonmetal xanthone and tetramethylammonium chloride under mild conditions
a,b
a
a
a
a
a,*
Zhongtian Du , Zhiqiang Sun , Wei Zhang , Hong Miao , Hong Ma , Jie Xu
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A nonmetal catalytic system consisting of N-hydroxyphthalimide, xanthone, and tetramethylammonium
chloride was developed. A wide range of hydrocarbons could be oxidized efficiently with dioxygen under
mild conditions. In the presence of xanthone and tetramethylammonium chloride, catalytic activity of N-
hydroxyphthalimide was greatly improved, and selectivity for alkyl hydroperoxide was remarkably
decreased.
Received 10 November 2008
Revised 6 January 2009
Accepted 7 January 2009
Available online 19 January 2009
Ó 2009 Published by Elsevier Ltd.
Keywords:
Aerobic oxidation
Hydrocarbons
Xanthone
Nitroxyl radical
Quaternary ammonium salts
Liquid-phase oxidation of hydrocarbons is an important indus-
it was possible for oxygenated compounds to perform as a catalyst
if they could be recycled during the oxidation process. We noticed
that xanthene could be facilely oxidized to xanthone nearly quan-
1
,2
trial process. However, the reaction conditions are usually harsh
industrially. For example, the Snia-viscosa process for oxidation of
2
5
toluene operates at 165 °C. The oxidation catalysts usually contain
titatively through NHPI-catalyzed autoxidation process. It was as-
2
+
2+ 3
transition metals such as Co and Mn
.
Though great advances
sumed that it could be used with NHPI to promote oxidation of
other hydrocarbons. Primary investigations were carried out using
ethylbenzene as a probe molecule (Table 1). To our delight, the
combination of xanthone and NHPI could significantly promote
the oxidation of ethylbenzene. The conversion of ethylbenzene in-
creased to 40.8% compared with only about 10% conversion using
NHPI alone. (Table 1, entry 4). The property of aromatic heterocy-
clic compounds is greatly affected by large conjugation system
containing heteroatom in backbone. Then, other heterocyclic com-
pounds were also studied besides xanthone (Table 1). However, 1-
azaxanthone had no significant effect on both conversion and
selectivity (Table 1, entry 3). Thioxanthone, acridone which con-
tains S or N atom, decreased the catalytic activity (Table 1, entries
1 and 2). It was notable that the selectivity of 1-phenylethyl hydro-
peroxide (PEHP) was rather high (56–97%). Promoting the further
conversion of PEHP was beneficial for favorable ketone/alcohol
yield.
3
have been made, it is still a challenge to develop efficient oxida-
tion catalytic systems from both an economic and environmental
point of view. In the past several years, N-hydroxyphthalimide
(
NHPI) has attracted much attention for aerobic oxidation of
4
–8
hydrocarbons.
2
However, cocatalyst such as Co(OAc) was re-
quired for the generation of the corresponding phthalimide N-oxyl
radical (PINO) under mild conditions. Transition metals combined
with NHPI have been intensely studied;4 while the inherent
advantages of a nonmetal catalytic system were often ignored.
Some nonmetallic compounds such as aldehyde, AIBN, have been
used as mediators combined with NHPI.9 However, these com-
pounds were often not recoverable. Our group has developed sev-
–6
1
0–14
eral organocatalytic systems such as NHPI/anthraquinones.
We are particularly interested in organocatalysis, and here we dis-
closed a novel nonmetal catalytic system for oxidation of hydrocar-
bons under mild conditions.
Free radical processes are usually involved in the liquid-phase
aerobic oxidation of hydrocarbons. Oxygenated intermediates are
usually more active than substrate itself. Some oxygenated com-
pounds can play important roles during the oxidation process
Recently, using quantum chemical calculations, Ive Hermans
and co-workers believed that there was an established equilibrium
Å
ROO + NHPI ꢀ ROOH + PINO in NHPI-based oxidation process;
1
6
the reverse reaction was very fast. PINO is highly electrophilic
1
5
though their effects on the oxidation were often ignored. Thus,
and readily abstracts H-atoms from hydrocarbons much faster
Å 17,18
than ROO .
In order to shift the equilibrium right to form more
PINO, a third component could be introduced to reduce the con-
centration of ROOH.
*
Corresponding author. Tel./fax: +86 411 8437 9245.
E-mail address: xujie@dicp.ac.cn (J. Xu).
0
040-4039/$ - see front matter Ó 2009 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2009.01.077

1
678
Z. Du et al. / Tetrahedron Letters 50 (2009) 1677–1680
Table 1
1
00
Oxidation of ethylbenzene using NHPI and different aromatic heterocyclic
compounds
Conversion of ethylbenzene (%)
Selectivity for PEHP (%)
a,b
Entry
Aromatic heterocyclic compounds
Conv. (%)
Selectivity (%)
8
0
AcPO
PEA
PEHP
1
2
3
4
Thioxanthone
Acridone
1-Azaxanthone
Xanthone
5.5
7.2
10.3
40.8
Trace
1.2
Trace
34.0
4.3
1.6
7.7
9.7
95.7
97.2
92.2
56.3
60
a
Reaction conditions: 10 mmol ethylbenzene, 1 mmol NHPI, 1 mmol aromatic
CN, 80 °C, 0.5 MPa O ,4 h; AcPO: acetophenone,
heterocyclic compound, 10 mL CH
3
2
4
2
0
0
PEA: 1-phenylethanol, PEHP: 1-Phenylethyl hydroperoxide. Conversion and Selec-
tivity of the products were determined by GC using an internal standard. Trace: it
was detected, but it was too little to be accurately determined.
b
The corresponding chemical structures of some compounds:
0
a
b
c
c
blank blank NH Cl
CTAB TMBAC TBAB TMAC
4
TEAC
Figure 1. Oxidation of ethylbenzene using NHPI, xanthone and different quaternary
ammonium salts. Reaction conditions: 10 mmol ethylbenzene, 1 mmol NHPI,
1
mmol xanthone, 0.1 mmol quaternary ammonium salts,10 mL CH
3
CN, 60 °C,
0
.5 MPa O , 6 h, TEAC: triethylamine hydrochloride; CTAB: Hexadecyltrimethyl
2
ammonium bromide; TMBAC: benzyltrimethylammonium chloride; TBAB: tetra-
butyl ammonium bromide; NHPI alone; b NHPI and xanthone; 0.2 mmol feed.
a
c
Quaternary ammonium salts have been reported for decompo-
sition of alkyl hydroperoxide.19 Then they were introduced to fur-
ther optimize our nonmetal system (Table 2). Even when reaction
temperature was decreased to 60 °C, NHPI/xanthone was still
effective (Table 2, entry 5), but it was not efficient enough. With
addition of 1 mol % tetramethylammonium chloride (TMAC) to
NHPI/xanthone system, the conversion increased strikingly to
trimethyl ammonium chloride (TMBAC) and tetrabutyl ammonium
bromide (TBAB), were also effective. The conversions were more
than 75% with more than 98% selectivities for acetophenone and less
than 1% selectivities for 1-phenylethyl hydroperoxide. But the TMAC
seemed to be the best. Anion, either bromine ion or chloride ion, was
nonessential in this system. Alkyl hydroperoxide is one of key inter-
mediates during autoxidation of hydrocarbons. It cannot be deter-
mined directly by GC due to its thermolability. In some literatures,
hydroperoxides were often ignored or directly measured as their
decomposed products. However, selectivities for 1-phenylethyl
hydroperoxide in the reaction mixture seemed to be adverse to the
conversions of ethylbenzene (Fig. 1). With low conversion of ethyl-
benzene, the selectivity for 1-phenylethyl hydroperoxide was high.
When the conversion increased, the selectivity for 1-phenylethyl
hydroperoxide decreased. This observation was also in accordance
8
4.4% with 98.5% selectivity for acetophenone (AcPO), while the
selectivity for PEHP remarkably decreased to less than 1% (Table
, entry 7). It contrasted with the results obtained using NHPI/xan-
2
thone or NHPI/TMAC (Table 2, entries 4–7). NHPI alone was not
efficient with high selectivity for PEHP (>98%) under 60 °C. TMAC
and/or xanthone were inactive (Table 2, entries 2–4). Though
NHPI/TMAC also showed some activities which were mentioned
in the previous literatures,20 the combination of xanthone and
TMAC/NHPI resulted in much higher conversion and in quite differ-
ent products distribution. Three components were indispensable to
be efficient under mild conditions. Reaction in the beginning was
severe in NHPI /xanthone/TMAC-mediated oxidation. The phenom-
1
6
with Ive Hermans’s theoretical study.
To further probe the potential of this nonmetal system, various
hydrocarbons were successfully oxygenated by NHPI/xanthone/
TMAC system under mild conditions (Table 3). Aromatic hydrocar-
bons with methylene such as diphenyl methane, tetralin, and fluo-
rene could be oxidized smoothly with high conversions (65–94%)
2
enon was similar to oxidation using NHPI/Co(OAc) .
Some other quaternary ammonium salts were also studied
Fig. 1). Hexadecyltrimethyl ammonium bromide (CTAB), benzyl-
(
(Table 3, entries 1–3). The selectivity for corresponding ketone
Table 2
was also high and little hydroperoxide was detected (Table 3, en-
tries 1 and 2). In contrast, the selectivity for 9-fluorenyl hydroper-
oxide was over 23% using NHPI/anthraquinone at 80 °C. Toluene,
p-xylene, and durene with methyl group could also be efficiently
oxygenated with aromatic acid as main products (Table 3, entries
a
Oxidation of ethylbenzene using different catalysts
1
1
Entry
Catalysts
Conv. (%)
Selectivity (%)
PEA
AcPO
PEHP
1
2
3
4
5
6
None
Xanthone
TMAC
Xanthone/ TMAC
NHPI/xanthone
NHPI/ TMAC
NHPI/xanthone/TMAC
NHPI/xanthone/TMAC
0
0
—
—
—
—
20.4
26.7
98.5
91.3
—
—
—
—
15.8
12.3
0.3
0.7
—
—
—
—
63.8
61.0
0.2
0.7
4
2
–6). Cyclohexene was also oxygenated smoothly with cyclohex-
-enone as the main product (Table 3, entry 7). Cyclohexane was
Trace
Trace
24.7
33.4
84.4
86.4
difficult to be oxidized under 60 °C, and few products were de-
tected within 10 h, but when the reaction temperature was raised
to 80 °C, good conversion of cyclohexane (33%) could also be ob-
tained (Table 3, entry 9). Long induction period was observed dur-
ing the oxidation of adamantane and toluene under 60 °C. For
example, induction period was about 2 h during the oxidation of
toluene, and it decreased to 20 min under 80 °C. This resulted in
long reaction time under 60 °C (Table 3, entries 6 and 8).
b
7
c
8
a
Reaction conditions: 10 mmol ethylbenzene, 1 mmol NHPI, 1 mmol xan-
CN, 60 °C, 0.5 MPa 6 h, Conversion and
thone,0.1 mmol TMAC,10 mL CH
3
2
O ,
Selectivity of the products were determined by GC using an internal standard.
TMAC: tetramethylammonium chloride.
b
To explain these findings, several experiments were carried out.
Firstly, addition of 5 mol % hydroquinone to the system completely
1
8
% benzoic acid.
0 °C, 0.05 mmol TMAC, 3 h, 7.2% benzoic acid was detected.
c

Z. Du et al. / Tetrahedron Letters 50 (2009) 1677–1680
1679
Table 3
Oxidation of various substrates by NHPI/xanthone/TMAC system
a
Entry
Substrate
Time (h)
Conv. (%)
Main products select (%)
Benzhydrol (2)
1
2
3
4
5
6
7
8
Diphenyl methane
Fluorene
Tetralin
Durene
p-Xylene
4
4
4
3
3
8
4
8
4
65
91
94
94
69
35
77
92
33
Benzophenone (98)
9-Fluorenone (96)
1-Tetralone (92)
9-Fluorenol (4)
1-Tetralol (4)
2,4,5-Trimethyl benzaldehyde (23)
p-Tolualdehyde (23)
Benzaldehyde (16)
2,4,5-Trimethyl benzoic acid (59)
4-Toluic acid (76)
Benzoic acid (82)
Toluene
Cyclohexene
Adamantane
Cyclohexane
Cyclohex-2-enone (61)
2-Adamantanol (52)
Cyclohexanone (50)
Cyclohex-2-enol (7)
1,3-Damantanediol (34)
Acid (34)
b
9
a
Reaction conditions: 10 mmol liquid substrates or 5 mmol solid substrates, 10 mol % NHPI, 10 mol % xanthone, 0.5 mol % TMAC,10 mL CH
3 2
CN, 60 °C, 0.5 MPa O . Con-
version and selectivity were determined by GC.
b
Reaction was carried out at 80 °C. Acid was determined by titration.
carbons. A wide range of hydrocarbons could be efficiently oxygen-
ated with molecular oxygen under mild conditions. Impressive
performance for aerobic oxidation of hydrocarbons catalyzed by
nonmetal molecules was demonstrated. This report may be helpful
for further development of nonmetal catalysis for oxidation.
xanthone
ROOH
N-OH
RH
N-O
CH ) NCl
(
3
4
Products
Scheme 1. Proposed catalytic mechanism.
Acknowledgments
stopped the reaction. This indicated that a radical pathway was in-
volved in the main reaction course. When oxidation of 1-phenyl-
ethanol was carried out using NHPI/xanthone/TMAC system
under 60 °C, only 36% conversion of 1-phenylethanol was obtained.
Without additive, alkyl peroxide was believed to decompose to ke-
tone and alcohol through Russell pathway.6 Quaternary ammo-
nium salts were reported for homolytic cleavage of alkyl
We gratefully acknowledge the financial support from the Nat-
ural Science Foundation of China (20672111 and 20736010) and
the Knowledge Innovation Program of the Chinese Academy of Sci-
ences (DICP K2007D6).
Supplementary data
1
9
hydroperoxide. TMAC was proposed to be responsible for high
selectivity of AcPO. In the presence of TMAC, AcPO was possibly di-
rectly produced from decomposition of 1-phenylethyl hydroperox-
ide rather than from further oxidation of 1-phenylethanol. Such
catalytic decomposition might also account for excessive oxidation
of ethylbenzene to benzoic acid (Table 2, entries 7 and 8). More
than 7% benzoic acid was detected within 3 h if the reaction was
performed at 80 °C (Table 2, entry 8). This observation was quite
different from our previous reports of heterogeneous catalytic
decomposition using HY.10 In that case, nonradical cleavage route
was proposed, and few excessive oxidation products were
detected.
General procedure for the oxidation, hydroperoxide analysis
and detailed GC measurement methods are available. Qualitative
analysis of products by GC–MS. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2009.01.077.
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10,11
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