Bifunctional Cs?Au/Co3O4 (Basic and Redox)-Catalyzed Oxidative Synthesis of Aromatic Azo Compounds from Anilines

Article abstract of DOI:10.1002/ejoc.202100976

An eco-friendly alkali-promoted (Cs?Au/Co₃O₄) catalyst, with redox and basic properties for the oxidative dehydrogenative coupling of anilines to symmetrical and unsymmetrical aromatic azo compounds, was developed. We realized a base additive- and molecular O₂ oxidant-free process (using air), with reasonable reusability of the catalyst achieved under milder reaction conditions. Notably, the enhanced catalytic activity was also linked to the increased basic site concentration, low reduction temperatures, and the effect of lattice oxygen on the nanomaterials. The increased basic strength of the cation-promoted catalyst improved the electron density of the active Au species, resulting in higher yields of the desired aromatic azo compounds.

Full text of DOI:10.1002/ejoc.202100976

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Bifunctional Au/Co3O4-Cs (Basic and Redox)- Catalyzed
Oxidative Synthesis of Aromatic Azo Compounds from Anilines
Authors: Charles O. Oseghale, Dele Peter Fapojuwo, Oyekunle Azeez
Alimi, Christianah Aarinola Akinnawo, Batsile M. Mogudi,
Oluwatayo Racheal Onisuru, and Reinout Meijboom
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content of this Accepted Article.
To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202100976
Link to VoR: https://doi.org/10.1002/ejoc.202100976
01/2020

10.1002/ejoc.202100976
European Journal of Organic Chemistry
Bifunctional Cs-Au/Co3O4 (Basic and Redox)-Catalyzed Oxidative Synthesis of Aromatic
Azo Compounds from Anilines
Charles O. Oseghale, Dele Peter Fapojuwo, Oyekunle Azeez Alimi, Christianah Aarinola Akinnawo, Batsile M.
Mogudi, Oluwatayo Racheal Onisuru, and Reinout Meijboom*
Research Center for Synthesis and Catalysis, Department of Chemical Sciences, University of Johannesburg, PO Box
524, Auckland Park 2006, Johannesburg, South Africa Tel.: +27 (0)72 894 0293, Fax: +27 (0)11 559 2819
Corresponding Author* E-mail: rmeijboom@uj.ac.za (R.M.)
Graphical abstract
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10.1002/ejoc.202100976
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Abstract
An eco-friendly alkali-promoted (Cs-Au/Co3O4) catalyst, with redox and basic properties for the
oxidative dehydrogenative coupling of anilines to symmetrical and unsymmetrical aromatic azo
compounds, was developed. We realized a base additive and toxic molecular O2 oxidant free
process (using air), with reasonable reusability of the catalyst achieved under milder reaction
conditions. Notably, the enhanced catalytic activity was also linked to increased basic site
concentration, low reduction temperatures, and the effect of lattice oxygen on the nanomaterials.
The increased basic strength of the cation-promoted catalyst improved the electron density of the
active Au species, resulting in the higher yield of the desired aromatic azo compounds.
Keywords: Alkali-cations; Aniline; Basicity Gold; Oxidation.
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Introduction
Concerns about the high energy consumption associated with large-scale chemical synthesis and
environmental toxicity have fueled current attempts to create benign methods with a low negative
environmental impact. Catalysis has been critical from the green chemistry perspective, owing to
its ability to improve product selectivity and reduce reactor temperatures. Heterogeneous catalysis
is of particular interest due to the ease with which it can be separated and the fact that it does not
produce harmful byproducts throughout the reactions. Recently, gold catalysts for sustainable and
green organic synthesis have attracted considerable scientific attention due to their broad range of
applications and efficiency [1–5]. Initially thought inert, gold became apparent as a selective and
active catalyst to others, such as platinum, for a range of reactions after a significant breakthrough
in its usage at the nanoscale level [6]. Recent advancements in the gold catalyst's practical utility
and applications, especially in gold metal and support interactions, have paved the way for new
methods for their practical utility and applications. The higher catalytic activity of these gold
catalysts has been attributed to their choice of support and, at times, the size of the gold
nanoparticles. Catalyst design based on mutual cooperation between the gold and support has been
shown to be advantageous for catalyzing a wide variety of reactions. Metal-support interactions
have resulted in significant advancements in the development of commercially viable organic
compounds. On the other hand, modifying (doping) support materials and catalysts with alkali
metals has previously been shown to induce a phenomenon, increase their binding properties, and
significantly improve the overall catalytic performance of the catalyst[4,7]
.
Corma and coworkers demonstrated that this supported gold catalyst has the unique capacity to
activate both a two-step, one-pot process to synthesize azo compounds from nitrobenzene through
hydrogenation and the oxidative coupling of anilines to azobenzenes [8]. In the following years, Pd
or Pt nanowires in the presence of KOH [9], Au/ZrO2 through a photocatalytic route [10], and
CuBr/pyridine [11] systems have all been investigated for this reaction.
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Scheme 1: Synthetic routes for aromatic azo compounds. (a) [8], (b) [12,13], (c) [14,15]
.
Azo compounds with -N=N- functionalities are an essential class of chemicals used in food
additives, pharmaceuticals, and other modern science applications such as liquid crystal displays
[16], dyes [17], therapeutic agents [18,19], pigments [20], optical media storage [21], and reaction
indicators [22]. They are mostly suitably accessed by oxidative dehydrogenative coupling of
anilines. However, it has been reported that these azo compounds are synthesized via the
diazonium route [12,13,23], which employs flammable organic bases, toxic oxidants, and nitrite salts
in stoichiometric amounts, thereby generating inorganic waste in the process. These current
methods have been linked with poorer yields, limited reusability, and severe reaction conditions
(higher temperature and pressure) [12,13]. A more versatile chemical procedure, such as a
multifunctional and effective heterogeneous catalyst for the selective synthesis of azo compounds
under mild reaction conditions, is highly desired to supplement the traditional techniques.
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In continuation of our ongoing research on the use of alkali-promoted catalysts for sustainable
catalysis [4,5,24], herein, we present a more environmentally friendly method for the oxidative
coupling of anilines to symmetrical and unsymmetrical aromatic azo compounds using a Au-based
catalyst with redox and basic properties. In general, the synthesis of unsymmetric aromatic azo
compounds is very complex. Usually, it proceeds via the nitrosobenzene intermediates and
diazonium salts pathway [13], using organic bases and a stoichiometric quantity of harmful oxidants
such as nitrates, ferrates, mercury salts, and palladium salts, which produce inorganic wastes as a
byproduct. This study describes a base additive and toxic oxidant-free reaction that utilizes air as
the terminal oxidant aided by a convenient 3D-printed air-flow splitter. To our knowledge, this is
the first Au-based catalyst with both redox and basic properties for the selective oxidative coupling
of anilines to aromatic azo compounds (Scheme 1). Besides, ascribing the catalytic performance
of this type of reaction to the concentration of basic sites in the catalyst has not been reported.
Additionally, our catalyst's exceptional reusability makes our system more intriguing and
competitive. Similarly, the effect of alkali-cations on the catalyst was investigated in terms of
basicity, low reduction temperatures, and lattice oxygen defects.
Experimental
Materials
The palladium(II) acetate (98%), platinium(II) acetate (98%), gold(III) chloride trihydrate
(99.9%), cobalt(II) nitrate hexahydrate (99%), manganese(II) nitrate tetrahydrate (97%), DMF
(98%), acetonitrile (98%), ethanol (99%), 1, 4-dioxane (99%), toluene (99.9%), nitric acid (HNO3)
(70%), Pluronic (P-123), urea (≥99.9%), sodium borohydride (≥96%), 1-butanol (≥99.8%), aniline
(≥99.5%), 4-methoxyaniline (99%), p-toluidine (99.6%), 4-fluoroaniline (98%), 4-chloroaniline
(98%), nitrate salts of lithium (98%), sodium (98%), potassium (99%), calcium (98%) and cesium
(98%) were supplied from Sigma-Aldrich and used without the need for further purification.
Catalyst synthesis
The deposition-precipitation technique was used to synthesize the undoped Au/Co3O4 and alkali
metal salt x(NO3) (x = Cs, Na, Li, Ca, and K) doped x-Au/Co3O4 catalysts [25]. Typically, 1 g of
[5]
mesoporous Co3O4-350 °C
was introduced to an aqueous solution of the metal salt
(HAuCl4.3H2O) and agitated at room temperature (RT) for 2 h. Urea in a molar ratio of 2:1 to the
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gold salt was added to the reaction mixture while maintaining the temperature and stirring speed
at 80 °C and 450 rpm, respectively. After cooling to room temperature, freshly prepared NaBH4
in a 10:1 molar excess was added to the reaction mixture and stirred for 4 h at RT. The alkali metal
salt was then added to the mixture and agitated for 1 h at RT. The precipitate obtained was rinsed
with excess distilled water (80 mL), dried, and calcined for 3 h at 350 °C. Mesoporous MnO2 and
the Pt and Pd-based catalysts using Pt(OAc)2 and Pd(OAc)2 precursors were synthesized according
to the procedure earlier reported in [5]. The % composition of the catalysts ranges from 0.4-5%,
with the main samples denoted as Co3O4, MnO2, M/Co3O4, M/MnO2, x-M/Co3O4 or M/Co3O4-x,
and x-M/MnO2 or M/MnO2-x. Where M = Au, Pd, and Pt. The initial nanoparticle loading was set
at 5 wt.%.
Catalyst characterization
The temperature-programmed oxidation (O2-TPO) by O2 was performed using a Micromeritics
AutoChem II instrument at a temperature range of 50-700 °C (10 °C min-1). Additionally, X-ray
diffraction (XRD), N2-sorption, Fourier transform infrared spectroscopy (FT-IR), temperature-
programmed reduction by H2 (H2-TPR), transmission electron microscopy (TEM), inductively
coupled plasma optical emission spectrometry (ICP-OES), Brunauer-Emmett-Teller (BET),
scanning electron microscopy (SEM), temperature-programmed desorption by CO2 (CO2-TPD),
gas chromatography spectroscopy (GC–MS/FID) and nuclear magnetic resonance spectroscopy
(NMR), were performed; the instruments and methods employed for the characterization of the
nanomaterials were the same as in our previously published works [4,5]
.
Catalytic activity for the oxidative coupling of anilines
The oxidation reactions were carried out in a carousel reactor equipped with twelve 50 mL tubes
from the Radley Discovery Technologies. Typically, a 50 mL tube was charged with the catalyst
(50 mg), toluene (3 mL), and 1 mmol (200 μL) decane (internal standard) and aniline substrates (1
mmol). The mixture was heated to a reaction temperature in the range of 80-110 °C during a
specified period while maintaining a stirring speed of 550 rpm under an air-flow. The air-flow was
assisted by a 12-way 3D-printed air-flow splitter built on-site. After separating the catalyst, the
mixture was cooled to room temperature, and the filtrate was analyzed using a GC-FID instrument.
Prior to the NMR investigations, water (40 mL) was slowly added to separate the organic phase,
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10.1002/ejoc.202100976
European Journal of Organic Chemistry
and the aqueous layer was extracted with ethyl acetate (2 x 20 mL). The mixed layer was washed
with brine solution and dried over sodium sulphate. The remaining solvent was evaporated using
a rotary evaporator, and the purification procedure was carried out using flash column
chromatography on silica gel with ethyl acetate and hexane as eluents. The catalyst was separated
using a simple filtering technique for the reusability experiments and then washed, dried, and
activated in an oven before reuse.
Results and discussions
The soft template inverse-micelle and deposition-precipitation techniques were employed for the
supports and catalysts syntheses. Alkali metal salts (x.NO3) in trace amounts were used to dope
the catalyst materials. However, these cations significantly enhanced the catalytic performance for
the oxidative coupling of a diverse array of anilines to symmetric and unsymmetric aromatic azo
compounds. As previously demonstrated by our group, alkali metal ions on the surface of reducible
metal oxides and catalysts can improve the nanomaterials' basic strength concentration and
catalytic activity [4,5,24]. According to Helwani and coworkers, the presence of these highly
electropositive cations in catalysts, which create basic sites, can cause defects in the materials [26]
.
.
[27]
Increased basic site concentrations in materials also aided in the oxidation of alkylbenzenes
With this, increasing the basicity of these catalysts can provide reasonable reactivity and activity
for the oxidative coupling of anilines.
Structural characterization of the catalyst
The carbon monoxide temperature-programmed desorption (CO2-TPD) measurement was used to
determine the nanomaterial's relative basicity. As shown in Figure 1, the Cs promoted catalyst
exhibited the highest basic concentration (1.058 mmol CO2 g-1) to the others. Comparing the
basicity values of Cs-Au/Co3O4 to the unpromoted Au/Co3O4 showed the promotional effect of the
alkali cations on the nanomaterials (Figure 1b). A fascinating finding is the increasing basicity of
the catalysts, which corresponds with the size and charge of the cations; a similar pattern has been
reported, with the Cs+ promoted catalyst being more basic than the others and providing a much
higher activity for the reaction [4] (Figure 1a). In many oxidation reactions, the addition of organic
bases (as additives) can enhance catalytic activity. However, an efficient and environmentally
sustainable oxidation process should not involve the use of additives (bases or acids). In this study,
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the bifunctional nature (basic and redox) of our catalytic system can promote the oxidative
dehydrogenative synthesis of aromatic azo compounds without using base additives. Therefore,
we believe that our catalyst’s basic and redox nature plays an essential role in the selective
oxidative coupling of the anilines.
Figure 1 CO2-TPD of the catalysts. The gold catalyst loadings were set at 5 wt.%.
The physicochemical properties of the catalysts (pore diameters/volumes and BET surface areas)
were also considered a factor in their high catalytic activity (Table S1 and Figure 2a). The 1%
Au/Co3O4-Cs loading, along with the reduced particle size and intra-particle voids, provided more
defects on the catalyst surface, which act as active catalytic sites. As a result, we hypothesized that
the decreased surface area, pore-volume, and pore size of the promoted catalyst with a gold loading
of up to 5% was the major reason for its slightly decreased catalytic activity. The TEM pictures
displayed aggregated particle sizes as the gold loading on the support matrix increased (Figure 3).
Additionally, the XRD study shows that the alkali-cations and AuNPs did not alter the structure
of the mesoporous cobalt oxide since there were no peaks attributed to AuNPs in the spectra
(Figure 2b).
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Figure 2 (a) BET and (b) p-XRD of the nanomaterials.
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Figure 3 TEM images and particle size distribution of the catalysts (a) (b) 1% Au/Co3O4-Cs, (c) (d) 3%
Au/Co3O4-Cs and (e) (f) 5% Au/Co3O4-Cs.
Temperature-programmed reduction (H2-TPR) and oxidation (O2-TPO) measurements were used
to determine the reduction-oxidation profiles of the nanomaterials (Figure 4). The catalyst activity
was found to be related to the reducibility of materials; the low reduction temperatures observed
for Cs-Au/Co3O4 in comparison to Au/Co3O4 indicate the catalyst's easy reducibility due to the
presence of Cs+, which also reveals an increase in the accessibility and mobility of the lattice
oxygen (Figure 4a). The two major reduction peaks of 143 and 312 °C (Cs-Au/Co3O4) and 224
and 284 °C (Au/Co3O4) indicate a two-step reduction of Co3O4-CoO (reducible capping oxygen at
a lower temperature) and CoO to Co (bulk reduction at a higher temperature). On the other hand,
it is thought that the Cs cation in the catalyst increases lattice oxygen supply, resulting in increased
defects in the nanomaterials [28]. The defects contributing to the high adsorption capacity and
efficient adsorption of nanomaterial substrates have been extensively studied [29]. In contrast to the
temperature reduction profiles, the oxidation peaks shifted to higher temperatures for Cs-
Au/Co3O4 (351 °C) than for unpromoted Au/Co3O4 (328 °C), indicating that the O2 consumption
at higher temperatures may be necessary for the oxidation reaction to proceed smoothly (Figure
4b). It is worth noting that the alkali cation (Cs) in the catalyst was critical for the nanomaterial’s
O2 (at higher temperatures) and H2 consumptions (at lower temperatures).
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Figure 4 (a) H2-TPR and (b) O2-TPO profiles of Cs-Au/Co3O4 and Au/Co3O4. The measurements were
conducted from RT to 600°C (10°C/min) under a stream of 10 % O2/Ar for O2-TPO and 10 % H2/Ar for
H2-TPR.
Catalytic studies
Catalyst screening
Next, we screened the different catalysts at 100 °C for 8 h using aniline 1a as the model substrate
(Table 1). With the 5% Au/Co3O4 catalyst, a moderate yield (60%) of the aromatic azo compound
2a was obtained (Table 1, entry 1). Not satisfied with the effectiveness of this unpromoted catalyst,
we further modified it by doping with different alkali-metal ions (x = Cs, Na, Li, Ca, and K).
Notably, the Cs-promoted catalyst was found to be the most active, with excellent 2a yield
obtained when compared to the others; this was attributed to the catalyst's highly electropositive
nature, which increased the basic site concentration and improved its catalytic activity [26] (Table
1, entry 2). As previously stated, the charge and size of these alkali cations in the catalysts are well
correlated with their catalytic activity. The 1% and 3% Au loadings provided a much better 2a
yield than the 5%. Apart from the Au/MnO2 catalyst and its Cs-promoted counterpart, which gave
satisfactory reactions (Table 1, entries 9 and 10), the oxidative dehydrogenative coupling of 1a
with the Pd and Pt-based catalysts was unsuccessful, indicating the involvement of the active
nanoscale gold species in the reactions. Likewise, increasing the time up to 24 h did not
significantly improve the reaction, but instead, it gave a trace amount of the product (Table 1, entry
11-16 ). Besides, the choice of support was also crucial and has been linked to the high catalytic
activity of many Au-based catalysts [30–34]. However, in this study, the unsaturated Au sites were
the active species. At an 8 h reaction time, attempts to use the support for the oxidative coupling
of 1a to azobenzene resulted in a reduced 2a yield and selectivity (Table 1, entry 17); however,
when the reaction time was increased to 16 h, the conversion and yield improved somewhat (Table
1, entry 18). Additionally, the difference in catalytic activity between the gold and platinium or
palladium catalysts may be attributed to the high affinity of these metals (Pt and Pd) for the
adsorption and diffusion of the aniline substrates in the reaction, thereby altering its catalytic
performance.
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Table 1 Catalysts screening [a]
Entry Catalyst
Dopants Conversion
Selectivity
(%) [b]
94
Yield
(%) [b]
60
(%) [b]
1
5% Au/Co3O4
-
63
2
5% Au/Co3O4
5% Au/Co3O4
5% Au/Co3O4
5% Au/Co3O4
5% Au/Co3O4
*1% Au/Co3O4
3% Au/Co3O4
1.2% Au/MnO2
1% Au/MnO2
1% Pd/Co3O4
1% Pd/Co3O4
1% Pd/Co3O4
0.8% Pt/Co3O4
1.3% Pt/Co3O4
0.4% Pt/MnO2
Co3O4
Cs
Na
Li
Ca
K
Cs
Cs
-
88
98
87
3
94
83
78
4
91
90
81
5
86
82
70
6
94
79
74
7
96
98
95
8
93
96
90
9
98
66
65
10
11 [c]
12 [c]
13 [c]
14 [c]
15 [c]
16 [c]
17
Cs
-
97
93
91
n.r
n.r
n.r
n.r
n.r
n.r
31
n.r
n.r
n.r
n.r
n.r
n.r
n.r
11
Li
Cs
-
n.r
n.r
n.r
Cs
Cs
-
n.r
n.r
35
18 [d] Co3O4
-
60
28
17
[a] Reaction conditions: Aniline 1a (1 mmol), catalyst (50 mg), temperature (100 °C), toluene (3 mL) and
8 h reaction time. [b] Selectivity, conversion, and yield (%) determined by GC-FID. [c] 24 h. [d] 16 h. n.r
= No reaction, although with a trace amount of the product detected for some of the catalysts.
Optimization of reaction parameters and substrate scope
For the solvent screening, the toluene showed the highest conversion and yield of 1a and 2a at 96
and 95%, respectively (Table 2, entry 1). The low reactivity of acetonitrile and ethanol may be due
to the solvent's inability to bind to the catalyst's active Au sites, resulting in low conversions and
yields (Table 2, entries 4 and 5). When the solvent volume was reduced to 1 mL, the conversion
of 1a improved significantly (Table 2, entry 6). Interestingly, when the reaction period was
extended to 18 h, the Cs-Au/Co3O4 also showed solvent-free green oxidation of 1a to 2a;
conversion and yield were 54% and 50%, respectively (Table 2, entry 7 ). Following that, we
investigated the effect of time on the reactions. Extending the time from 8 to 24 h resulted in a
substantial increase in 1a conversion and 2a yield (Table 2, entry 8). The temperature may be
critical in this reaction. Increasing the temperature to 110 °C seems to have a detrimental effect on
the 2a yields, with the 1a completely converted at > 99% (Table 2, entry 9). However, the reaction
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was not favorable at temperatures below 50 °C. Notably, due to this catalytic system's basic and
redox features, no side products were detected for several reactions, suggesting that the
intermediate in this oxidation step was readily converted to the product. The effect of catalyst
loading was also examined, and it was shown that reducing the catalyst loading to 20-30 mg had
no discernible influence on the 2a yield (Table 2, entry 10). When the amount is increased to 70
mg, a significant increase in the activity and reactivity of 2a occurs (Table 2, entry 11). However,
given the expense of the gold catalyst and its activity in this reaction, a loading of 50 mg of 1%
Au/Co3O4-Cs or less is ideal. Based on the satisfactory reactions, we proceeded with 100 °C, 8 h,
50 mg, and 1a (1 mmol) as the optimized reaction parameters for subsequent reactions.
Table 2 Optimization of reaction parameters using 1% Au/Co3O4-Cs [a]
Entry Solvents
Time (h) Temp Catalyst Conversion
Selectivity Yield
(°C)
100
100
100
80
(mg)
50
50
50
50
50
50
50
50
50
30
70
(%) [b]
(%) [b]
(%) [b]
1
Toluene
DMF
8
96
98
95
2
8
94
83
78
3
Dioxane
Acetonitrile
Ethanol
Toluene
-
8
71
96
69
4
8
20
57
12
5
8
70
8
70
6.2
97
6 [c]
7 [d]
8
8
100
100
100
110
100
100
98
98
18
24
8
54
91
50
Toluene
Toluene
Toluene
Toluene
100
100
95
98
98
9
85
85
10
11
8
98
92
8
100
>99
>99
[a] Reaction conditions: Aniline 1a (1 mmol), catalyst (50 mg), temperature (100 °C), toluene (3 mL) and
8 h reaction time. [b] Selectivity, conversion, and yield (%) determined by GC-FID. [c] Toluene (1 mL).
[d] 1a (2 mmol), 18 h.
To further our understanding of using this base additive-free system, we investigated various
substituted anilines with electron-donating and electron-withdrawing groups under the optimal
reaction conditions (100 °C, 8 h). Notably, high selectivity and yields of up to 94% were achieved.
As shown in Figure 5A, the yields of the desired aromatic azo compounds were well tolerated
regardless of the substitution manner of the electron-rich and electron-deficient substituents.
Except for the nitro substituted aniline 1i, which did not provide the intended product 2i, the other
electron-withdrawing substrates (1f-1h) survived the reaction and were successfully converted to
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azobenzene. The poor yields, on the other hand, showed the importance of electronic effects in the
reactions. The no reaction (n.r.) observed for the nitro-substituted aniline 1i may be attributed to
the amine radical cation's instability and the presence of weak nucleophiles. The yield and
conversion of the 1-naphthylamine substrate 1j were much lower, presumably due to the bulky
substituents' steric effect on the molecules. Interestingly, compound 1k with an N-heteroatom was
tolerated slightly; the poor conversion and yield may result from the poisoning effect of the
pyridine N atom coordinated with the catalyst's metal center [35]
.
Given the remarkable activity of the catalyst (1% Au/Co3O4-Cs) in the synthesis of the
symmetrical azobenzene, we hypothesized that the electron-rich substrates might undergo
oxidative coupling to unsymmetric aromatic azo compounds under the optimized reaction
conditions (Figure 5B). Unsymmetrically substituted azo compounds have been reported to be
formed by coupling diazonium salts with both electron-deficient and electron-rich aromatic
compounds in the presence of base additives and toxic oxidants, yielding a significant amount of
aromatic azo compounds and water as a byproduct. To our delight, the oxidative dehydrogenative
coupling of these electron-rich and electron-deficient substrates resulted in the formation of some
active unsymmetrically substituted azobenzene and derivatives in the absence of base additives,
diazonium salts, and hazardous oxidants. It is worth mentioning that the homocoupling of these
aryl-substituted anilines with electron-withdrawing groups was slower than those with electron-
donating groups.
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Figure 5 Reaction conditions: Catalyst (50 mg), substrates (1 mmol), 100 °C, toluene (3 mL), and 8 h.
Conversion and isolated yields determined by GC-FID analysis. [a] Reaction conditions: Catalyst (70 mg),
substrate (1 mmol), 100 °C, and 18 h.
Control experiment and mechanism of the reactions
To gain mechanistic insight into the oxidative coupling of the anilines with our redox-base catalyst
(Cs-Au/Co3O4), a series of control experiments were performed under the N2 atmosphere and the
results of which are presented in Scheme 2 [a]. Fortunately, just a trace amount of the aromatic
azo compound was found, suggesting that air (O2) was primarily responsible for initiating and
catalyzing the transition of the anilines to symmetric and unsymmetric azo compounds.
Additionally, since no crossover product was detected, Scheme 2 [b] shows that the reaction did
not proceed through the nitrosobenzene pathway as a viable intermediate. Minor amounts of the
nitrosobenzene intermediates were detected in the oxidative coupling reported by Corma and
coworkers, resulting in a slightly reduced yield of the desired azobenzene [8]. However, our
catalytic system utilized the synergistic combination of alkali metal cations, AuNPs, and Co3O4
support, with air as the terminal oxidant, compared to their system, which used molecular O2 and
pressure up to 5 bar. The low or no activity of these control experiments involving nitrogen when
compared to the use of air could be ascribed to the labile lattice O2 and adsorbed O2 on the surface
of the catalyst, indicating the role of O2 supply from the air in these reactions since the loss of
lattice O2 is replenished by the O2 coming from air [36]. Additionally, we found that the oxidation
of 1, 2 diphenylhydrazine to azobenzene occurred more rapidly, which explains why the hydrazine
intermediate (41) was not detected during the reaction (Scheme 2 [c]).
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10.1002/ejoc.202100976
European Journal of Organic Chemistry
Scheme 2 [a] Control experiment with N2 and [c] Reaction of 1, 2 diphenylhydrazine with 1% Au/Co3O4-
Cs.
Based on the above, we proposed a probable reaction mechanism for the oxidative coupling of
aniline catalyzed by 1% Au/Co3O4-Cs (Scheme 3). Although, similar to that described by Corma
and coworkers [8], except for the azoxybenzene and nitrosobenzene intermediates, which were not
detected in our reactions. In step I, the 1% Au/Co3O4-Cs absorbs oxygen (O2) from the
surrounding air through interaction with the active Au species. The aniline (1a) nitrogen atom is
oxidized by adsorption on the oxygenated catalyst surface, forming an aniline radical cation (3),
step II. Following that, the aniline radical species (3) combined with another aniline molecule
(1a*) to produce a 3e- δ bond hydrazine radical intermediate (4), which was assumed to constitute
the rate-determining step, step III. At step IV, the hydrazobenzene radical species (4) donates one
electron and two protons by oxidation and removal of water molecules from the catalyst surface
to form a hydrazine intermediate (41), which further undergoes oxidation rapidly, by going through
step IV again to form the aromatic azo product 2a, step V. A critical observation is that the aniline
radical cation formed in step II was assumed to be involved in the reduction of the active Au+ to
Au, which hypothetically resulted in the superficial release of the labile lattice O2 from the air for
the entire cycle.
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10.1002/ejoc.202100976
European Journal of Organic Chemistry
Scheme 3 Plausible reaction mechanism for the oxidative dehydrogenative coupling reaction.
Reusability and heterogeneity studies
Notably, the 1% Au/Co3O4-Cs catalyst may be recycled and reused for up to six consecutive cycles
with no substantial loss of catalytic activity using the simple filtration method (Figure 6). To
prepare the catalyst for subsequent reuse, it was washed with 25 mL (x3) of toluene and reactivated
at 120 °C to remove physically absorbed species. Following that, we performed a series of
reactions using the filtrate after the catalyst separation (starting at 2 h reaction time). There was no
observable change in the 1a conversion or detectable amount of aromatic azo compounds (Figure
S1). Additionally, an ICP-OES analysis of the filtrate after the sixth cycle showed a negligible
amount of Au species. The results indicate that the reaction was catalyzed by the active Au species
rather than leached Au ions.
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10.1002/ejoc.202100976
European Journal of Organic Chemistry
Figure 6 Reusability studies of 1% Au/Co3O4-Cs with the optimized reaction conditions: 100 °C, 8 h,
toluene (3 mL), air (1 atm), and catalyst (50 mg).
Conclusion
In conclusion, a novel bifunctional catalyst (Cs-Au/Co3O4) with redox and basic properties was
designed for the oxidative coupling of anilines to symmetric and unsymmetric azo compounds.
The reaction was carried out without toxic oxidants and base additives, with air as the terminal
oxidant (aided by a 3D-printed air-flow splitter). The results indicate that the active Au species
were responsible for the high catalytic activity and essential for activating the O2 (air) and forming
the radical intermediates. Additionally, the higher catalytic activity of the catalyst was easily
correlated with the presence of the highly electropositive Cs+, which increased the basic site
concentration of the catalyst and improved its catalytic efficiency. The basic and redox features of
the catalyst, moderate reaction conditions, proper reusability, and the tested solvent-free green
system all contribute to our system being more viable and competitive than others.
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10.1002/ejoc.202100976
European Journal of Organic Chemistry
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