Ionic liquid effect: Selective aniline oxidative coupling to azoxybenzene by TiO2

Article abstract of DOI:10.1039/c4cy01257g

Simple commercially available TiO₂ in ionic liquids (ILs) promotes the oxidative catalytic coupling of anilines to diazo compounds. For example, the reaction of aniline with molecular oxygen (5 bars) in the presence of TiO₂ and functionalised imidazolium-based ILs affords azoxybenzene in up to 85% selectivity. The catalytic selectivity and activity are mainly related to the viscosity, oxygen solubility, and stability of the superoxide ion (O₂^-) and ionic species provided by the IL (IL effect). This effect is evident since most of the reactions performed in the presence of ILs gave as major products the azoxybenzene zwitterions. This is in opposition to that performed in toluene, which gave almost exclusively azobenzene.

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Ionic liquid effect: Selective aniline oxidative coupling to azoxybenzene
by TiO2
Muhammad I. Qadir,a Jackson D. Scholtena and Jairton Dupont*a,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Simple commercially available TiO2 in ionic liquids (ILs) promote the oxidative catalytic coupling of
anilines to diazo compounds. For example, the reaction of aniline with molecular oxygen (5 bars) in the
presence of TiO2 and functionalised imidazoliumꢀbased ILs affords azoxybenzene up to an 85% yield.
The catalytic selectivity and activity are mainly related to the viscosity, oxygen solubility, and stability of
10 the superoxide ion (O2•ꢀ) and ionic species provided by the IL (IL effect). This effect is evident since most
of the reactions performed in the presence of ILs gave as a major product the azoxybenzene zwitterions.
This is in opposition to that performed in toluene, which gave almost exclusively azobenzene.
nobleꢀmetalꢀfree catalyst system for the oxidation of anilines.
Moreover, we demonstrate that selectivity is strongly dependent
on the nature of both the cation and anion of the IL.
Introduction
There is no doubt that the aerobic oxidation of aniline is an
15 important reaction for the preparation of its oxygenated
derivatives such as nitroso, azo, hydroxylamine, nitro, and azoxy
aromatic compounds. Aromatic azoxy compounds are of great
interest in view of their large applications such as dyes, reducing
agents, chemical stabilisers, liquid crystal in electronic display,
20 and therapeutic medicines.1 In the same vein, aromatic azo
50
Results and discussion
The screening of the ILs was performed using the same reaction
conditions employed in the oxidation of aniline by P25 in toluene
(i.e., at 100 °C for 24 h under 5 bars of molecular oxygen).10 The
compounds are widely used in the industry as pigments, 55 reaction was performed by simple mixing of P25 (3.1 mmol) and
indicators, food additives, and radical reaction initiators.2ꢀ4
Although several methods for the synthesis of azo and azoxy
derivatives have been reported, challenges remain to afford high
25 yields and selectivities. Moreover, their preparation from
aromatic amines usually requires the use of large amounts of lead
salts,5 mercury salts,6 manganese salts,7 ferrates,8 or diazonium
salts.9
aniline (10.7 mmol) in 1 g of various ILs (Figure 1). The results
are summarised in Table 1.
However, the catalytic oxidation of anilines could be achieved
30 using supported gold nanoparticles on TiO2 under oxygen at 100
°C.10 More recently, supported transitionꢀmetal nanoparticles
have also been successfully used for the aerobic oxidation of
anilines.11 The employment of noble and expensive metals may
limit their application. Although these catalytic methods represent
35 a significant improvement as compared with classical ones, they
use noble metals that are still a major drawback. Nonetheless,
simple titanium dioxide is also active for the oxidation of aniline
in toluene at 100 °C, giving diazobenzene a 52.5% yield.10 It is
accepted that the aerobic oxidation of the anilines involves
40 radical and/or ionic species. In this respect, the use of compounds
that can stabilise such species may generate more active and
selective catalytic systems. It is well known that some
imidazoliumꢀbased ionic liquids (ILs) are able to have a strong
influence on the stability of radical and ionic species.12, 13
60 Figure 1. Structure of the ILs employed in the oxidative coupling of
anilines.
It is clear from the data on Table 1 that regardless of the nature of
the IL, aniline is oxidised (conversion from 64% to 95%) into
65 only three products: azoxybenzene (2a), azobenzene (3a), and
nitrobenzene (4a). In opposition to the reaction performed in
toluene (entry 11, Table 1), nitrosobenzene was not detected,
indicating that the IL induces the formation of nitrobenzene.
45 We report herein that simple ILs associated with commercially
available titanium dioxide (P25) indeed make up a highly active
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Table 1. Results of the aerobic oxidation of aniline (1a) in ILs catalysed
by TiO2
the presence of oxygen has been found to be essential for the
oxidation of aniline. At 1 bar of oxygen pressure, 40% conversion
was obtained, with 42% azoxybenzene, 9% azobenzene, and 49%
45 nitrobenzene. As expected, increasing the oxygen pressure
increases the conversion, with the selectivity of azoxybenzene
reaching up to 85% at 5 bars (Figure 2). The higher selectivity of
azoxybenzene compared with azobenzene even at 1 bar of oxygen
suggests that it may be difficult to control the overoxidation of
50 the preformed azobenzene to azoxybenzene using IL as a solvent.
To investigate this hypothesis, the oxidation of azobenzene was
carried out under the standard reaction conditions in
MOEMIm.NTf2 achieving 30% of conversion and total
selectivity of azoxybenzene. This low conversion indicates that
55 the overoxidation of azobenzene is not easy and that it is unlikely
that the obtained azoxybenzene might come in its major amount
from the oxidation of the preformed azobenzene (see the
discussion below).
a
Entry
Medium
Conv.(%)b
Selectivity (%)b
2a
3a
9
6
12
18
8
22
3
8
6
7
52.5
100
100
36
4a
6
7
42
6
22
55
95
72
19
57
0
1
2
3
4
5
6
7
8
9
10
11c
12
13
14
MOEMIm.NTf2
MOEMIm.BF4
BMIm.BF4
95
86
78
73
86
75
89
94
67
64
53
9
85
77
46
76
70
23
2
20
75
36
0
BMIm.NTf2
BMIm.B(CN)4
PCNMIm.NTf2
PCNMIm.BF4
MOEPy.NTf2
MOEBPyrr.NTf2
BMPyrr.B(CN)4
Toluene
CH3CN
DMSO
THF
0
0
43
0
0
21
3
37
a Reaction conditions: aniline (10.7 mmol) dissolved in IL (1.0 g), TiO2ꢀ
P25 (3.1 mmol), 5 bars of O2, 100 °C for 24 h. b Determined by gas
chromatography using nꢀUndecane as an internal standard. c Data from
reference 10.
5
The IL effect14 is more pronounced in the selectivity since in
most of the reactions performed in ILs, the major product is
10 azoxybenzene (2a) zwitterions (see entries 1–5 and 9). This fact
is in opposition to that performed in toluene, acetonitrile and
DMSO, which yields almost exclusively azobenzene (3a) (entries
11ꢀ13, Table 1), and no azoxybenzene (2a) was observed. This is
probably related to the stabilisation of ionic species promoted by
15 the ILs.15 Moreover, even in the cases where the selectivity is
towards nitrobenzene (4a), the azobenzene (3a) is mostly the
minor product (see entries 6, 8, and 10, Table 1) in the reactions
performed in ILs. It is important to note that using THF the
conversion was low and the selectivity was slightly superior for
20 product 2a (entry 14, Table 1).
60 Figure 2. Graph showing the conversion/selectivity versus the O2
pressure for aniline oxidation: (black) conversion, (red) 2a, (blue) 3a, and
(green) 4a. Aniline (10.7 mmol) dissolved in MOEMIm.NTf2 (1.0 g),
TiO2ꢀP25 (3.1 mmol), desired O2 pressure at 100 °C for 24 h.
The highest conversions are observed with methoxyꢀ
functionalisedꢀbased
imidazolium
(MOEIm.NTf2)
and
In addition, the selectivity of the products was followed during
65 the oxidation of aniline in MOEMIm.NTf2 at 5 bars of O2 and at
100 °C (Figure 3).
pyridinium (MOEPy.NTf2) ILs (see entries 1 and 8, respectively).
This relatively higher catalytic activity is likely to be related to
25 the lower viscosity and higher oxygen solubility displayed by
these ILs as compared with the nonfunctionalised ones.16
Moreover, it is known that the imidazoliumꢀbased ILs are able to
stabilise superoxide ion (O2•ꢀ),12 and this is probably not the case
for the nonconjugated pyrrolidinium cation (see entries 9 and 10,
30 Table 1). In fact, the highest aniline conversion (95%) and
selectivity (85%) on azoxybenzene (2a) was attained in the
reaction performed in MOEIm.NTf2 (entry 1), which has the
relative lowest viscosity, the higher oxygen solubility, and the
imidazolium ring that can stabilise the superoxide ion and
35 catalyse the aniline oxidative coupling.
For the evaluation of the scope of reaction, it was decided that the
IL MOEMIm.NTf2 be used as a standard solvent because of its
superior performance in terms of conversion and selectivity.
Control experiments showed that the thermal oxidation of aniline
40 does not occur. The effect of oxygen was also elucidated in an
experiment performed under N2, which showed no reaction. Then
Figure 3. Conversion/selectivity versus time for aniline oxidation: (black)
70 conversion, (red) 2a, (blue) 3a, and (green) 4a. Aniline (10.7 mmol)
dissolved in MOEMIm.NTf2 (1.0 g), TiO2ꢀP25 (3.1 mmol), 5 bars of O2
pressure at 100 °C.
2
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The reaction started only after 3 h, and the selectivity of
azoxybenzene was always much higher than that of azobenzene.
This fact indicates that the IL is stabilising the ionic transition
azoxy compounds by using ILs as a surface modifier. The
preferred selectivity of azoxy instead of azo compounds in ILs
may be related to the intrinsic properties of ILs in stabilising
species favouring the formation of the azoxy compound. 50 ionic species (IL effect). Indeed, it was found that the cage
5
Interestingly, nitrobenzene was not observed even after 9 h of
reaction, suggesting that it is not a preferred product in this case.
To demonstrate the general applicability of aerobic catalytic
oxidation, a screening of some primary aromatic substituted
lifetime for radical ion pairs in ionic liquid was estimated to be
much longer than the neutral radical pairs in the same fluid.25 The
nature of IL has a significant influence on the conversion and
selectivity of products. In particular, the IL cation seems to play a
anilines was performed (Table 2). In most of the cases, the 55 major role during the oxidation reaction. A higher catalytic
10 reaction was found to be selective for the azobenzene product.
activity and selectivity was attained in ILs displaying lower
The reaction rate depended qualitatively on the Hammett constant
of the substituent since the oxidation reaction is favoured in the
presence of electronꢀdonor substituents. The homocoupling of
anilines with electronꢀdonating substituents is faster than that
15 with electronꢀwithdrawing substituents. Under the optimised
conditions, pꢀmethylaniline and pꢀmethoxyaniline gave 99% and
90% of conversion, respectively. Interestingly, pꢀbromoaniline
showed 50% of conversion with total selectivity to the azo
derivative 3b after a long reaction time (66 h). Moreover, no
20 coupling was seen using pꢀnitroaniline, mꢀnitroaniline, and 2ꢀ
viscosity and higher oxygen solubility, such as in the case of
•ꢀ
MOEMIm.NTf2, which can also stabilise superoxide ion (O2
formed after the adsorption of oxygen on reduced titania.
)
60
Experimental section
General
The ILs were prepared following the procedures previously
reported: MOEMIm.NTf2,26 MOEMIm.BF4,26 PCNMIm.BF4,27
methoxyaniline. Unfortunately, attempts to produce only 65 PCNMIm.NTf2,27
BMIm.BF4,28
BMIm.NTf2,28
30
heterocoupling products from the reaction using different couples
of anilines failed because a mixture of homoꢀ and heterocoupled
products was observed in the system.
MOEBPyrr.NTf2,29,
and MOEPy.NTf2.31 In particular,
BMIm.B(CN)4 and BMPyrr.B(CN)4 were purchased from EMD
Chemicals Inc. All the other chemicals were purchased from
commercial sources. The substrates were distilled, degassed, and
70 stored under argon prior to use. Mass spectra were obtained using
25
Table 2. Results of the aerobic oxidation of substituted anilines (1b) in
a
a
GCꢀMS Shimadzu QPꢀ5050 (EI, 70 eV), and gas
the presence of TiO2
chromatography (GC) analyses were performed with an Agilent
6820 gas chromatograph with a flame ionisation detector and 30
m capillary column with a dimethylpolysiloxane stationary phase.
75 For GC analysis, nꢀUndecane was used as an internal standard.
The catalytic oxidation reactions were carried out in a modified
FischerꢀPorter steel bottle immersed in a silicon oil bath. Caution:
oxygen can form explosive mixtures with organic vapours in
particular working under pressure in the presence of a catalyst,
80 caution must be taken to avoid explosion.
Entry
R
Time (h)
Conv.(%)b
Selectivity (%)b
2b
3b
45
31
100
0
4b
1
2
3
4
CH3
OCH3
Br
24
24
66
66
99
90
50
0
34
56
0
21
13
0
NO2
0
0
Oxidation reactions
a Reaction conditions: 1b (5.5 mmol) dissolved in MOEMIm.NTf2 (1.0 g),
In a typical oxidation reaction, the IL (1.0 g) was mixed with
TiO2ꢀP25 (3.1 mmol) in a FischerꢀPorter reactor. After stirring
for 10 minutes, distilled aniline (10.7 mmol) was added, and the
85 reactor was loaded with oxygen (5 bars). The mixture was heated
at 100 °C under magnetic stirring (650 rpm). After the desired
time, the reactor was cooled at room temperature and
depressurised. Aliquots were removed and analysed by GC and
GCꢀMS. The validity of the identification and quantification of
90 the GC peak were verified through a comparison of the mass
spectra and gas chromatographic retention time with those of
authentic compounds using nꢀUndecane as a standard.
TiO2ꢀP25 (3.1 mmol), 5 bars of O2 at 100 °C. b Determined by gas
30 chromatography using nꢀUndecane as an internal standard.
The proposed mechanism for the oxidation of aniline probably
involves the defect sites or vacancies on TiO2 as active catalytic
sites that are able to abstract an electron from aniline, activate the
–
35 oxygen molecule, and also produce O2 radicals after the
adsorption of oxygen on reduced titania.17 After single electron
abstraction, the coupling of a cationic aniline radical with a
neutral aniline leads to the formation of a hydrazineꢀderived
compound.18ꢀ22 This hydrazineꢀderived compound undergoes
40 overoxidation by the ILꢀstabilised superoxide ion (O2 )
•ꢀ 12 to yield
azoxybenzene and/or nitrobenzene.23, 24 The presence of a radical
species was confirmed by using CCl4 and hydroquinone as
scavenger agents.
Acknowledgements
95 Special thanks to CNPq, TWAS, CAPES, and FAPERGS for the
financial support.
Conclusions
45 In conclusion, we have developed an efficient new heterogeneous
catalytic method for the aerobic oxidation of anilines to aromatic
Notes and references
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a Laboratório de Catálise Molecular, Instituto de Química-UFRGS, Av.
Bento Gonçalves, 9500, CEP 91501-970, Porto Alegre-RS, Brasil. Fax:
+55 51 3308 7304; Tel: +55 51 3308 9632; E-mail:
jairton.dupont@ufrgs.br
5
b School of Chemistry, University of Nottingham, University Park,
Nottingham, UK, NG7 2RD.
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Graphical Abstract
5
TiO2 promote the oxidative catalytic coupling of anilines to diazo compounds. In ionic liquids azoxybenzene is formed in high
selectivity whereas in toluene azobenzene is formed almost exclusively.
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