Metal free: A novel and efficient aerobic oxidation of toluene derivatives catalyzed by N′, N″, Nprime;, -trihydroxyisocyanuric acid and dimethylglyoxime in PEG-1000-based dicationic acidic ionic liquid

Article abstract of DOI:10.1016/j.catcom.2012.07.019

A non-metal catalytic system containing of N′, N″, Nprime;, -trihydroxyisocyanuric acid (THICA) and dimethylglyoxime (DMG) is described for the selective oxidation of toluene derivatives with dioxygen in PEG-1000-based dicationic acidic ionic liquid (PEG₁₀₀₀-DAIL). Several toluene derivatives were efficiently oxidized to corresponding acids under mild conditions. The oxidation followed a radical pathway and a possible mechanism was proposed. Both the catalyst and PEG₁₀₀₀-DAIL could be reused at least eight times without significantly decreasing the catalytic activity.

Full text of DOI:10.1016/j.catcom.2012.07.019

Catalysis Communications 27 (2012) 124–128
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Metal free: A novel and efficient aerobic oxidation of toluene derivatives catalyzed by
N′, N″, N‴, -trihydroxyisocyanuric acid and dimethylglyoxime in PEG-1000-based
dicationic acidic ionic liquid
Tingting Lu, Yang Mao, Kai Yao, Jian Xu, Ming Lu ⁎
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 May 2012
Received in revised form 19 July 2012
Accepted 19 July 2012
Available online 28 July 2012
A non-metal catalytic system containing of N′, N″, N‴, -trihydroxyisocyanuric acid (THICA) and
dimethylglyoxime (DMG) is described for the selective oxidation of toluene derivatives with dioxygen in
PEG-1000-based dicationic acidic ionic liquid (PEG1000–DAIL). Several toluene derivatives were efficiently
oxidized to corresponding acids under mild conditions. The oxidation followed a radical pathway and a pos-
sible mechanism was proposed. Both the catalyst and PEG1000–DAIL could be reused at least eight times with-
out significantly decreasing the catalytic activity.
Keywords:
©
2012 Elsevier B.V. All rights reserved.
Aerobic oxidation
Alkylaromatics
THICA-DMG
PEG1000–DAIL
1
. Introduction
[11,12]. Furthermore, ILs have excellent advantages like negligible vol-
atility, thermal stability, remarkable solubility, and a variety of available
structures [13]. Recently, the novel PEG-1000-based dicationic acidic
ionic liquid (PEG1000 -DAIL), which exhibits a temperature-dependent
phase behavior with toluene, is widely used in the organic reaction
(Scheme 1) [14].
Herein we introduce an efficient, metal-free protocol for the aerobic
oxidation of toluene derivatives to the corresponding acids catalyzed by
THICA/dimethylglyoxime (DMG) with PEG1000–DAIL as solvent, where-
in both the catalyst and PEG1000–DAIL can be successfully recovered and
reused.
The development of new, efficient and sustainable processes for
the mild oxidative functionalization of toluene derivatives continues
to be a challenging problem of modern chemistry, as evidenced by
the increasing number of research publications in this field [1]. Tolu-
ene derivatives are known as the abundant carbon raw materials but
are inert for selective transformations under mild conditions [2].
Traditional approaches for oxidation were based on metal-catalyzed
activation, but the disadvantages of metallic toxicity could not be
avoided [3]. N-hydroxyphthalimide (NHPI), which is a valuable
carbon radical producing catalyst (CRPC) for the aerobic oxidation,
are unstable at >80 °C and has bad performance on the oxidation of
electron-withdrawing substituted toluene [4–6]. In recent years, the
more thermally stable CRPC N′, N″, N‴, -trihydroxyisocyanuric acid
2. Experimental section
2.1. Materials and methods
(
THICA) is attracting attention due to its highly catalytic efficiencies
for the aerobic oxidation of both electron-donating and electron-
withdrawing substituted toluene [7–9]. Nonetheless, THICA suffers
from two major disadvantages: (1) it usually combines with
Co(OAc) to complete the catalytic oxidation and employs corrosive
2
acetic acid as solvent. (2) THICA is expensive but no attempt is
made on recycling of it [10].
Ionic liquids (ILs), as environmental friendly reaction media or cata-
lysts, have attracted increasing attention over the last decade, and one
reason is the expected dissolution or immobilization of the catalyst in
the IL that would allow the recycling of the tandem catalyst/solvent
All starting materials were purchased from commercial sources
and used without further treatment. 1H NMR (500 MHz) and
13
C
NMR (125 MHz) were recorded on a Bruker 500 spectrometer with
tetramethylsilane (TMS) as an internal standard. IR spectra were
recorded on a Bruker Vector 22 infrared spectrometer, as KBr pellets
−
1
with absorption in cm . EPR spectra were obtained with a Bruker
EMX-10/12 spectrometer. High performance liquid chromatography
(HPLC) experiments were performed on a liquid chromatograph
(Dionex Softron GmbH, America). The conversions of the substrates
and the selectivities of products were estimated from the peak areas
based on the internal standard technique. The products were deter-
mined in some cases by comparison of their HPLC with those of au-
thentic samples.
⁎
Corresponding author. Tel.: +86 25 84315514; fax: +86 25 84315030.
E-mail addresses: tt252525@gmail.com (T. Lu), luming302@126.com (M. Lu).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2012.07.019

T. Lu et al. / Catalysis Communications 27 (2012) 124–128
125
Scheme 1. The preparation of the PEG-DAIL.
2
.2. General procedure for the oxidation
N-oxyl were also examined (Runs 5–7). Unsatisfactorily, when an-
thraquinone [17] or N′, N″-azobisisobutyronitrile [18] (ABIN) was
employed, the reaction proceeded in low conversion to result in lots
of BAl. And the conversion reached only 9.4% and no benzoic acid
THICA was prepared by the literature procedure [8,15]. 1H NMR
1
3
(
6 6
DMSO-d ): δ (ppm) 11.03 (s, 3H); C NMR (DMSO-d ): δ (ppm)
−
1
1
46.6; IR (cm ): 3539, 3166, 2827,1720, 1433, 1211, 1010, 701.
appeared in the presence of the HNO
Subsequently, various solvents were screened (Runs 8–14). In
contrast to the traditional NHPI/Co(OAc) catalyzed oxidation of
3
/THICA [19].
The PEG1000–DAIL was prepared by the procedure given in the lit-
1
erature (Scheme 1) [14]. H NMR (D
2
O): δ (ppm) 2.15 (t, 4H, J=7 Hz,
), 3.45–3.66 (m, 90.3H, (OCH CH ),
), 4.21–4.30 (8H, 4×NCH ), 7.41 (s, 4H, 4×CH),
O): δ (ppm) 25.2, 47.7, 47.9, 49.1,
2
2
3
8
6
2
5
×CH
.74 (4H, 2×CH
.71 (s, 2H, 2×CH); C NMR (D
2
), 2.77 (m, 4H, 2×CH
2
2
2
)
n
alkylaromatics where acetic acid or acetonitrile was the appropriate
solvent [7], the employment of them in the present oxidation led to
a decrease of the conversion and selectivity of BAC. Meanwhile,
2
2
1
3
2
−
1
8.6, 68.7, 69.6, 122.3, 123.1, 136.0; IR (cm ): 3849, 3400, 3020,
4
1-butyl-3-methylimidazolium tetrafluoroborates ([Bmim]BF ), which
883, 1732, 1591, 1456, 1431, 1377, 1122, 1109, 972, 883, 798, 677,
77.
had been successfully applied in the nitration of methylbenzenes cata-
lyzed by NHPI [20], did not adapt to oxidation system. The reaction in
PEG 1000-based dicationic ionic liquid (PEG1000-DIL) resulted in lower
conversion due to its lack of acidity. It also shown PEG1000–DAIL was
−
3
−5
The substrate (5.0*10
mol), PEG1000–DAIL (2.8*10
mol),
mol, 5 mol%)
−
4
−4
THICA (2.5*10
mol, 5 mol %), and DMG (2.5*10
were placed in a three-necked flasks. O
2
was bubbled into the flask
−
1
at a flow rate of 20 mL min . The reaction mixture was stirred at a
specific temperature for a specific time, and the reaction progress
was monitored by HPLC. After completion of the reaction, the mixture
was cooled to room temperature and extracted with ether (10 mL)
three times. After concentration of the ether solution, the products
were taken for HPLC measurement and the PEG1000–DAIL was reused
without any treatment.
Table 1
Oxidation of toluene with O
under various conditions.a
Additive (mol %) Solventb
2
Run THICA
(mol %)
Conversion Products
[%] selectivity [%]
BOL BAl BAC
0.7 n.dc 99.0
1
2
3
4
5
5
5
5
DMG (5)
Acetoxime (5)
acetaldoxime (5) PEG1000-DAIL 73.9
Cyclohexanone
oxime (5)
PEG1000-DAIL 99.5
PEG1000-DAIL 83.0
n.d 2.6
5.2 5.8
94.6
84.2
3
. Results and discussions
PEG1000-DAIL 75.7
PEG1000-DAIL 65.8
PEG1000-DAIL 27.3
5.7 32.7 60.7
3
.1. Influence of reaction conditions on the oxidation of toluene with
5
5
Anthraquinone
n.d. 27.4 69.2
THICA
(10)
6
7
8
9
1
1
1
5
5
5
5
5
5
5
5
5
5
5
2.5
7.5
7.5
5
0
0
ABIN (3)
0.6 40.4 55.3
n.d. n.d. n.d.
As a representative example, the aerobic oxidation of toluene
Equation 1) was selected to optimize the reaction conditions
Table 1). It is pleasing that the oxidation of toluene with O
1 atm) catalyzed by THICA (5 mol%) combined with DMG (5 mol%)
nitric acidd (10) PEG1000-DAIL
9.4
99.0
89.8
10.7
90.7
(
(
(
DMG (5)
DMG (5)
DMG (5)
DMG (5)
DMG (5)
DMG (5)
DMG (5)
DMG (2.5)
DMG (7.5)
DMG (5)
DMG (5)
DMG (7.5)
HOAc
CH CN
[Bmim]BF
PEG1000-DIL
n.d 0.5
n.d 2.7
n.d 0.8
n.d 7.4
n.d 1.5
n.d 0.8
n.d 5.6
n.d 5.4
n.d 0.6
n.d 7.1
n.d 0.9
n.d 0.3
90.3
89.9
30.7
85.0
98.2
98.9
93.1
92.3
99.1
90.7
99.0
99.6
3
2
0
1
2
4
gave benzaldehyde (BAl), benzoic acid (BAC) in 0.7% and 99.0% selec-
tivity, respectively, at 99.5% conversion (Run 1). Several DMG ana-
logues were compared (Runs 2–4) [16]. Substitution by acetoxime,
acetaldoxime or cyclohexanoneoxime led to lower conversion with
comparable selectivity of BAC. Other non-metallic compounds (like
anthraquinone) which also can abstract the hydrogen atom from
the hydroxyimide moiety of the NHPI to generate phthalimide
PEG400-DAIL 97.9
PEG600-DAIL 98.3
PEG2000-DAIL 85.5
PEG1000-DAIL 64.4
PEG1000-DAIL 98.9
PEG1000-DAIL 60.5
PEG1000-DAIL 99.6
PEG1000-DAIL 99.5
PEG1000-DAIL 47.0
PEG1000-DAIL 31.1
PEG1000-DAIL 30.2
13
14
1
1
1
1
5
6
7
8
19
2
2
2
0
1
2
n.d 20.0 58.1
n.d 28.7 53.1
n.d 27.9 52.4
DMG (5)
a
Toluene (0.05 mol) was allowed to react with O
2
(1 atm) with THICA and additive
in solvent at 80 °C for 10 h.
b
−5
PEG1000 -DAIL was 2.8*10
Not detected.
mol and other solvent was 10 ml.
c
d
Equation 1. The oxidation of toluene.
The concentration of nitric acid was 67%.

126
T. Lu et al. / Catalysis Communications 27 (2012) 124–128
more suitable than other PEG-based dicationic acidic ionic liquids such
as PEG400–DAIL.
Then the catalytic performances of THICA /DMG in PEG1000–DAIL
were examined in detail (Runs 14–22). In comparison with the oxida-
tion using 5 mol% THICA and 5 mol% DMG, the conversions markedly
decreased when the amount of one component was contracted to
2
.5 mol% no matter whether the other component was changed. On
the other hand, when the amounts of either one component or both
the components were increased to 7.5 mol%, the conversions stayed
at 99.5%. These mean that the catalytic ability was not improved obvi-
ously by increasing the amounts of the catalysts beyond 5 mol%. Con-
trol experiments showed that lower conversion was detected with
only THICA added, and the conversion was less than 32% without a
catalyst or in the presence of DMG alone.
The influences of time and temperature on the oxidation were also
investigated. As illustrated in Fig. 1, the conversion of toluene en-
hanced with prolonging the reaction time. Simultaneously, the selec-
tivity of BAC continuously increased, and the selectivity of benzyl
alcohol (BOL) decreased owing to the continuous oxidation of BOL
to BAC. However, after 8 h the conversion changed slowly and be-
came stagnated. As a result, in an 8 h reaction course, 99.5% toluene
was oxidized with 99.0% selectivity of BAC. The change of the selectiv-
ity of BAl was similar to BOL except the concentrate was always low.
It was also shown that when the temperature was lower than
Fig. 2. Temperature-dependence curves for the aerobic oxidation of toluene . aReaction
a
conditions: 5*10− mol toluene, 5 mol% DMG, 5 mol% THICA, 2.8*10
3
−5
mol PEG1000
-
DAIL, O , 8 h.
2
6
0 °C, the low conversion was obtained due to incomplete dissolution
of toluene in PEG1000–DAIL, and it reached a maximum at 80 °C
Fig. 2). However, the conversion gradually decreased as a result of
be oxidized in 54.7% conversions, but the selectivity of o-nitrobenzoic acid
was only 29.9%.
(
3.3. The mechanism of the oxidation
continuing to elevate the temperature because lots of toluene was
exhausted by evaporation at higher temperature. On the contrary,
the selectivity of BAC was almost unchanged and almost no side prod-
ucts were found.
Ishii suggested THICA can generate an N-oxyl radical in higher
temperature [7]. The research on the catalytic mechanism of THICA/
HNO
3
system [19] was done and it was suggested the catalytic
3
.2. Oxidation of different substrates
With optimized conditions in hand, we then turned our attention to
Table 2
a
the scope of the reaction (Table 2). First, a variety of toluene with
electron-rich substituted group was examined (Runs 1–6). In most
cases, they were oxidized to the corresponding products in moderate to
excellent conversions. For example, using xylenes as the substrates was
obtained the diacids. The sterically hindered p-tert-butyltoluene was
also reacted to furnish p-tert-butylbenzoic acid in 80.7% conversion
with 95.7% selectivity. Then a series of the electron-withdrawing
substituted toluene were tested (Runs 7–14). Toluenes with cyano-,
carboxyl- and halo-substituted groups were converted into the
corresponding acids with good conversions. P-acetoxytoluene underwent
the oxidation with lower conversion. P-Nitrotoluene which had a strong
electron-withdrawing substituent gave p-nitrobenzoic acid in fair conver-
sion. Since THICA was stable at high temperature [7], o-nitrotoluene could
Aerobic oxidation of various substrates.
Run Substrate Times Conversion Main products and selectivity [%]
[
%]
1b
2b
13
13
94.3
94.2
89.7
85.0
10.2
13.9
3b
4
13
12
90.0
84.7
95.7
14.9
3.2
80. 7
5
6
7
8
9
8
9
95.2
92.0
94.4
91.1
90.9
96.0
86.9
96.7
93.4
92.0
2.1
12.1
3.0
10
11
11
5.5
7.6
1
1
1
0
1
2
9
9
93.8
94.6
86.8
94.5
95.0
77.0
4.8
4.6
10
22.1
1
1
a
3c
16
16
78.7
54.7
86.0
29.9
12.5
43.7
c,
4
d
Reaction conditions: 5*10−3 mol substrates, 5 mol% DMG, 5 mol% THICA, 2.8*10−5 mol
PEG1000–DAIL, O
2
, 80 °C.
b
c
Both THICA and DMG were 10 mol%.
Both THICA and DMG were 15 mol%.
The temperature was 140 °C.
a
a
Fig. 1. Time-dependence curves for the aerobic oxidation of toluene . Reaction conditions:
−
3
−5
d
5
2
*10 mol toluene, 5 mol% DMG, 5 mol% THICA, 2.8*10 mol PEG1000 -DAIL, O , 80 °C.

T. Lu et al. / Catalysis Communications 27 (2012) 124–128
127
Scheme 2. A possible reaction mechanism for the catalytic cycle of the THICA/DMG system.
process was similar with that of NHPI. In order to clarify the mecha-
nism of the THICA/DMG system, following experiments were carried
out.
A(N) =4.86 G). It was shown THICA was directly abstracted hydrogen
atom by N-oxyl radical derived from DMG with oxygen. (The
PEG1000–DAIL with only DMG was also examined by EPR and the
spectrum was much different from that of THICA.)
First, p-cresol (5 mol%), a free radical scavenger, was added into
the oxidation of toluene catalyzed by THICA/DMG. The result that
no reaction occurred indicated that the oxidation followed a radical
pathway. Secondly, dimethylglyoxime ester was used to replace
DMG in the oxidation of toluene and the conversion was only 47.0%,
which was consistent with the result of using THICA alone. This im-
plied that dimethylglyoxime ester had no catalytic effect, and that
the N-hydroxy groups of DMG played a key role in the catalytic oxida-
tion. However, it was indicated that the corresponding N-oxyl radical
of DMG could not promote the radical aerobic oxidation of hydrocar-
bons [16], although it was formed in the reaction. We did the similar
experiments and found the results were consistent.
3.4. The cycle of catalyst and PEG1000–DAIL
When the final reaction mixture was cooled to room temperature
and extracted with ether, the upper layer of ether, containing prod-
uct, was removed by decantation. Then only fresh substrate was
recharged to the residual PEG1000 -DAIL and the mixture was heated
to react once again. The results from Fig. 4 were shown that the pro-
cedure was repeated eight times with no appreciable decrease in con-
versions. The selectivity of toluene decreased obviously after 7 times.
A possible catalytic mechanism of THICA/DMG in the aerobic oxi-
dation was proposed in Scheme 2. The first step of the reaction was to
involve the corresponding N-oxyl radical of DMG, which was formed
by the reaction of DMG and dioxygen. And then this N-oxyl radical
abstracted hydrogen from THICA to recover to DMG. Simultaneously,
THICA was converted to its radical, which abstracted a hydrogen atom
from the toluene derivatives to form benzyl radicals. This benzyl rad-
ical was then readily captured by O to give a benzylperoxy radical,
2
followed by a complicated redox process to finally produce the car-
boxyl acid.
4. Conclusion
Summarising, we have developed a novel, metal-free catalytic sys-
tem consisting of THICA and DMG for the aerobic oxidation of
alkylaromatics in PEG1000–DAIL, avoiding the use of heavy metals
and toxic solvent. By using this catalytic approach, toluene with
electron-donating or electron-withdrawing groups can be oxidized
to the corresponding acid with high to moderate conversions. This
metal-free catalytic oxidation method has an environmentally friend-
ly feature and provides an attractive method which has long been de-
sired in the chemical industry.
The production of the THICA radical with DMG in PEG1000 -DAIL
was observed by EPR measurements (Fig. 3). Due to the limits of
the analytical condition, the PEG1000 –DAIL solution was examined
only in room temperature and the signal of THICA was much weaker
(
The sample was scanned for ten times) than that in nitric acid [19],
but the g and A(N) value (g=2.00060, A(N) =4.88 G) of the spectrum
in PEG1000 –DAIL were similar to them in nitric acid (g=2.00075,
Fig. 4. Repeating reaction using recovered PEG1000–DAILa. aThe PEG1000–DAIL was used
Fig. 3. EPR spectrum of the THICA radical.
in the further reaction without any disposal.

128
T. Lu et al. / Catalysis Communications 27 (2012) 124–128
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