Mesoporous manganese oxide catalyzed aerobic oxidative coupling of anilines to aromatic azo compounds

Article abstract of DOI:10.1002/anie.201508223

Herein we introduce an environmentally friendly approach to the synthesis of symmetrical and asymmetrical aromatic azo compounds by using air as the sole oxidant under mild reaction conditions in the presence of cost-effective and reusable mesoporous manganese oxide materials.

Full text of DOI:10.1002/anie.201508223

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201508223
German Edition: DOI: 10.1002/ange.201508223
Heterogeneous Catalysis
Mesoporous Manganese Oxide Catalyzed Aerobic Oxidative Coupling
of Anilines To Aromatic Azo Compounds
Biswanath Dutta+, Sourav Biswas+, Vinit Sharma, Nancy Ortins Savage, S. Pamir Alpay, and
Steven L. Suib*
Abstract: Herein we introduce an environmentally friendly
approach to the synthesis of symmetrical and asymmetrical
aromatic azo compounds by using air as the sole oxidant under
mild reaction conditions in the presence of cost-effective and
reusable mesoporous manganese oxide materials.
A
romatic azo compounds are an important class of mole-
cules used as dyes,[1] food additives,[2] pigments,[3a] indica-
tors,[3c] and therapeutic agents,[3b,d] and as precursors in the
synthesis of natural products.[4] Conventional methods for the
formation of aromatic azo compounds involve the use of
stoichiometric amounts of nitrite salts (NaNO2) or toxic
oxidants and proceed via diazonium[5] or nitrosobenzene
intermediates[6] (Scheme 1a). Catalytic pathways can make
these conventional processes facile and more selective. A
number of catalytic protocols have been developed for the
synthesis of azo derivatives by the oxidation of anilines.[1]
However, the existing systems[7] have several drawbacks, such
as lower yields, undesirable overoxidation products, limited
functional-group compatibility, the presence of additives, the
use of harsh reaction conditions (high pressure, high temper-
ature), and poor reusability of the catalyst. In 2008, Corma
and co-workers made a breakthrough when they developed
a Au/TiO2-mediated synthesis of aromatic azo compounds
from the corresponding aniline derivatives[8a,9] (Scheme 1b).
In the following years, many important attempts were
made to synthesis azo derivatives in good yields, for example,
by the use of a CuBr/pyridine/O2 reaction system,[10] a photo-
catalytic pathway in the presence of Au/ZrO2,[11] and Pt or Pd
nanowires in the presence of KOH.[12] Recently, Corma and
co-workers reported another way of preparing azo com-
pounds from aromatic nitro compounds: with Au/CeO2 under
high pressure of hydrogen.[8b, 13] However, the use of precious
metals (Au, Pt, or Pd), high pressure, and base additives make
Scheme 1. Catalytic routes for synthesizing aromatic azo compounds.
these processes industrially and environmentally unfriendly.
Therefore, the development of cost-effective, efficient,
heterogeneous catalysis for the synthesis of aromatic azo
compounds under mild, atmospheric reaction conditions in
the absence of additives is highly desirable.
The use of catalytic systems based on transition-metal
oxides has drawn significant attention for the synthesis of
small organic molecules.[14] Recently, our research group
introduced a new class of inverse-micelle-templated meso-
porous manganese oxide materials [University of Connecticut
(UCT) materials],[15,16] which were found to be effective in
different organic transformations, such as the oxidation of
alcohols[17] and amines,[18] the oxidation and esterification of
long-chain alcohols, and methylbenzoic acid production by
the oxidation of alkyl benzenes.[15] These mesoporous man-
ganese oxide (meso-MnOx) materials have versatile structural
forms and high thermal stability, which influence their
catalytic activity.[19] The redox cycle of manganese (Mn),
along with its labile lattice oxygen molecules, have been found
to be the dominant factors in the catalysis of different
oxidation reactions.[18] Herein, we demonstrate a facile, cost-
effective approach for the aerobic oxidative coupling of
anilines in the presence of meso-MnOx materials to synthesize
diverse symmetrical and unsymmetrical aromatic azo com-
pounds (Scheme 1c). The absence of precious metals and
additives, the use of air as the sole oxidant under atmospheric
[*] B. Dutta,[+] S. Biswas,[+] Prof. S. L. Suib
Department of Chemistry, University of Connecticut
Storrs, CT 06269 (USA)
E-mail: steven.suib@uconn.edu
Dr. V. Sharma, Prof. S. P. Alpay, Prof. S. L. Suib
Institute of Materials Science, University of Connecticut
Storrs, CT 06269 (USA)
Prof. N. O. Savage
Department of Chemistry and Chemical Engineering
University of New Haven
West Haven, CT 06516 (USA)
[+] These authors contributed equally.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508223.
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Table 1: Optimization of the oxidative coupling of aniline by meso-
conditions, and the proper reusability of the catalyst make our
catalytic protocol competitive with other existing systems.
According to a previous report, the metal-oxide-catalyzed
oxidative coupling of aniline is initiated by the formation of
radical intermediates through the transfer of electrons from
aniline to a metal center.[8a] The target molecule (azobenzene)
is formed after the successive loss of protons and electrons
from the corresponding intermediates. Initially, we investi-
gated the oxidative coupling of aniline as the model reaction
for developing optimal reaction conditions. In a comparative
study, aniline (1a) was converted into trans-1,2-diphenyldia-
zene (Iaa) by the use of different phases of manganese oxide
materials synthesized by the UCT method (see Table S1 in the
Supporting Information).[16] The Mn3+-rich (as revealed from
X-ray photoelectron spectra; see Figure S1 and Table S2 in
the Supporting Information) meso-MnOx material (meso-
MnOx-450) was found to be the best material in terms of
conversion and selectivity (see Table S1). The meso-MnOx-
450 material had the crystalline Mn2O3 phase (see Figure S2)
with a mesoporous size distribution (see Figure S3). The
surface area was calculated to be 100 m2 gÀ1, and the pore size
was 5.3 nm (see Table S3). A reaction with commercially
available nonporous Mn2O3 produced the target compound in
only 3% yield, which is similar to that observed under
catalyst-free conditions (see Table S1). This result can be
attributed to the presence of the mesoporous network of
meso-Mn2O3, which not only provides a higher surface area
(100 m2 gÀ1) than commercial Mn2O3 (11 m2 gÀ1), but also
facilitates the adsorption and diffusion of aniline. A continual
increase in conversion was observed as the catalyst loading
was increased (see Figure S4), thus indicating that the system
does not have limitations due to adsorption or mass transfer.
During the investigation of the effects of oxidants, higher
conversion was observed for reactions conducted under air or
oxygen than under nitrogen (see Table S4). This result
indicates the importance of aerial oxygen in the reaction.
The effects of different solvents with variable polarities were
also surveyed (see Table S5), and toluene emerged as the best
solvent, with 97% conversion. While optimizing the amount
of solvent used, we found that 0.5 mL was optimal (> 99%
conversion; Table 1, entry 5). The formation of a trace
amount of imine 2 as a side product can be attributed to the
condensation of benzaldehyde (from the oxidation of tolu-
ene) with aniline. This undesirable formation of benzalde-
hyde increased proportionally with the volume of toluene
(Table 1, entry 1–5), thus reflecting higher selectivity towards
the undesired formation of imine 2. The use of oxidative
additives, such as tert-butyl hydroperoxide (TBHP; Table 1,
entry 6), enhanced the formation of benzaldehyde, thus
leading to an increase in selectivity for the imine product to
79%. Hence, after extensive screening of all reaction
parameters, we concluded that the optimal conditions were
the use of 50 mg of meso-Mn2O3 in 0.5 mL of toluene at 1108C
under an air balloon (Table 1, entry 5). We used these
reaction conditions for the rest of our study.
Mn2O3.[a]
Entry
Solvent
volume [mL]
t [h]
Conv.
[%][b]
Selectivity [%][b]
Iaa
TON[c]
2
1
2
3
4
10
25
5
24
24
24
24
8
71
39
75
97
>99
58
70
30
38
<2
<1
<1
79
2.2
1.2
2.3
3.0
3.1
1.8
1.6
62
>98
>99
>99
21
1
5
0.5
10
no
6[d]
7
36
36
51
100
0
[a] Reaction conditions: 1a (93 mg, 1.0 mmol), meso-Mn2O3 (50 mg,
0.32 mmol), toluene as the solvent, 1108C, air balloon. [b] Conversion
and selectivity were determined by GC–MS. [c] Turnover number: moles
of aniline converted per mole of catalyst. [d] The reaction was carried out
in the presence of TBHP.
64% conversion of aniline). The filtrate was then kept under
the same reaction conditions for the next 7 h. Successive
aliquots were taken after each hour and analyzed by GC–MS,
but no further production of azobenzene was observed (see
Figure S5). Moreover, inductively coupled plasma (ICP)
analysis revealed a very low amount (79.6 ppb) of Mn in the
filtrate. To check the reusability of the meso-Mn2O3 catalyst,
we retrieved it from the reaction mixture by a simple filtration
method with > 90% recovery. The reused catalyst was washed
with excess toluene and acetonitrile, and finally reactivated at
2508C (to remove the adsorbed species from the catalyst
surface) for 50 min prior to reuse. The retrieved catalyst could
be used at least five times, although a slight gradual decrease
in performance was observed (see Figure S6).
We then focused our attention on evaluating the scope
and general applicability of our catalytic protocol. Under the
operationally simple and optimized reaction conditions,
diverse aniline derivatives with electron-rich (Table 2,
entries 2–5) and electron-deficient groups (entry 6) were
converted into the corresponding aromatic azo compounds
in excellent yield (as high as 98%) and with excellent
selectivity (> 99%).
The reaction of a heterocyclic aniline analogue, 1g
proceeded to comparatively lower conversion (45%;
Table 2, entry 7) with a greater amount of the catalyst. This
result may be due to poisoning of the metal center of the
catalyst by coordination with the pyridine N atom.[20] The
excellent yield (82%) of the azo compound Ihh (Table 2,
entry 8) derived from bulky 1-naphthylamine (1h) indicated
that steric hindrance did not have a significant impact on this
reaction. Moreover, the survival of halo-substituted com-
pounds, such as 4-chloroaniline (1 f), which was converted
into the corresponding azo compound Iff (84%; Table 2,
entry 6), is noteworthy, as the prevention of dehalogenation is
a challenge in catalytic reactions involving halogen deriva-
tives.[21] We found that this N,N coupling is only possible when
To verify whether the observed catalysis was the result of
solid meso-MnOx or leached active metal species, we carried
out the oxidative coupling of aniline and separated the
catalyst from the system after a reaction time of 3 h (at about
À
the NH2 group is attached to an aromatic ring; with
benzylamine (1j), the imine was formed by self-coupling
(Table 2, entry 10).
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Table 2: Aerobic oxidation of anilines to aromatic azo compounds by
meso-Mn2O3.[a]
thylaniline, p-N,N’-dimethylaniline, p-chloroaniline, and nap-
thylamine, were subjected to the reaction with aniline in this
way. The unsymmetrical aromatic azo compound IIba was
synthesized from p-toluidine (1b) and aniline (1a) with 56%
selectivity (Table 3, entry 4).
Entry R1
t [h] Conv. Selectivity
TON[e]
Anilines with electron-donating groups undergo homo-
coupling much faster (Table 2, entries 2–5) than those with
electron-withdrawing groups (Table 2, entry 6). Therefore, for
cross-coupling, an excess of the more electron deficient
aniline was used to enhance the yield of the cross-coupled
product (Table 3, entry 1–4). The molar ratio was flipped to
3:1 (Table 3, entry 5–8) for the reactions with aniline of
aniline derivatives 1d, 1e, 1 f, and 1h, which required equal or
more time for homocoupling as compared to aniline In these
reactions, the cross-coupled products were formed in moder-
ate to high yields (Table 3, entries 5–8).
[%][c]
for I [%][c,d]
1
2
3
4
5
6
C6H5 (1a)
8
7
4
4
8
>99
>99
>99
88
>99
84
45
82
97
>99
>99 (93; Iaa) 3.13
>99 (95; Ibb) 3.13
4-MeC6H4 (1b)
4-OMeC6H4 (1c)
3,5-Me2C6H3 (1d)
4-N(Me)2C6H4 (1e)
4-Cl-C6H4 (1 f)
2-NH2C5H5N (1g)
1-naphthyl (1h)
2-Cl-4-MeC6H3 (1i)
C6H5CH2 (1j)
>99 (89; Icc)
>99 (Idd)
>99 (Iee)
>99 (Iff)
>99 (Igg)
>99 (Ihh)
>99 (Iii)
0
3.13
2.78
3.13
2.65
0.71
2.59
3.06
0
6
7[b]
8
9
10
12
12
16
9
[a] Reaction conditions: R1NH2 (1.0 mmol) and meso-Mn2O3 (50 mg,
0.32 mmol) were heated in toluene (0.5 mL) at 1108C under air balloon.
[b] The reaction was carried out with 100 mg of the catalyst. [c] Con-
version and selectivity were determined by GC–MS on the basis of the
concentration of the aniline starting materials. [d] The yield of the
isolated product is shown in parenthesis in entries 1–3. [e] The TON
value is the number of moles of the aniline derivative converted per mole
of catalyst.
To investigate the kinetic aspects of the reaction, we
conducted a time-dependent study (see Figure S7). The
formation of a minute amount of the imine resulting from
the coupling of aniline with benzaldehyde (formed by the
oxidation of toluene) was tracked during the reaction. Kinetic
experiments revealed a first-order rate equation with respect
to aniline (1a; see Figure S8), with a rate constant of 0.32 hÀ1.
The apparent activation energy (Ea), as determined from an
Arrhenius plot in the temperature range of 20–1108C (see
Figure S9), was 29.8 kJmolÀ1.
We conducted several experiments to gain an under-
standing of the mechanistic details of the catalytic process. In
the presence of 2,6-di-tert-butyl-4-methylphenol (a radical
inhibitor), no trace of azobenzene was observed. Rather, the
reaction gave the product derived from the trapping of 3a
(Scheme 2) by 2,6-di-tert-butyl-4-methylphenol (see Fig-
ure S10). The formation of this intermediate suggests that
our mechanism is similar to that proposed by Corma and co-
workers.[8a] They assumed that the radical species 3a formed
from aniline (1a) was coupling with another molecule of
aniline 1a to form 4 (a compound with a 3eÀ s bond), which
through the successive loss of a proton, an electron, and
another proton formed intermediate 6. Intermediate 6 goes
through all the aforementioned steps once again to provide
the final product Iaa.
The excellent activity of the meso-Mn2O3-mediated
homocoupling of anilines inspired us to study the feasibility
of the protocol for the synthesis of unsymmetrical aromatic
azo compounds. Unsymmetrical aromatic azo compounds are
usually synthesized by treating diazonium salts with electron-
rich aromatic compounds.[5,6] Initially, we examined reactions
between p-anisidine (1c) and aniline (1a) in different molar
ratios (Table 3, entries 1–3). A 1:3 molar ratio of p-anisidine
to aniline was selected as the optimum ratio for the synthesis
of the unsymmetrical aromatic azo compound IIca (35%
selectivity; Table 3, entry 2). Various structurally different
aromatic aniline derivatives, such as p-toluidine, 3,5-dime-
Table 3: Aerobic oxidative cross-coupling of aniline with different aniline
derivatives.[a]
The generation of the radial intermediate 3a caused the
reduction of surface-active Mn3+ to Mn2+, which led to the
facile release of labile lattice oxygen (Scheme 2, cycle 2).[22]
That lattice oxygen combines with the protons (4H+) and
electrons (2eÀ) released in cycle 1 to form water. The supply
Entry
R1
Molar
ratio[b]
Conv.
[%][c]
Selectivity
Yield
[%][c]
for II [%][c,d]
1
2
3
4
5
6
7
8
4-OMeC6H4 (1c)
4-OMeC6H4 (1c)
4-OMeC6H4 (1c)
4-MeC6H4 (1b)
3,5-Me2C6H3 (1d)
4-N(Me)2C6H4 (1e)
4-ClC6H4 (1 f)
1:2
1:3
1:4
1:3
3:1
3:1
3:1
3:1
>99
>99
>99
>99
>99
>99
>99
80
28 (IIca)
35 (IIca)
37 (IIca)
56 (IIba)
92 (IIda)
43 (IIea)
88 (IIfa)
72 (IIh)
28
35
37
55
91
43
87
58
1-naphthyl (1h)
[a] Reaction conditions: R1NH2 and PhNH2 (total amount, according to
the molar ratio indicated: 1 mmol), meso-Mn2O3 (50 mg, 0.32 mmol),
toluene (0.5 mL), 1108C, air balloon, 12 h. [b] Aniline derivative/aniline.
[c] Conversion and selectivity were determined by GC–MS on the basis of
the concentration of the aniline starting materials. [d] Selectivity was
calculated on the basis of the self-coupled (I) and cross-coupled
products (II) derived from the limiting reagent.
Scheme 2. Proposed mechanism of the reaction.
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of oxygen from the air was crucial for
this catalytic activity, as the lattice
oxygen lost should be replenished by
the oxygen from air.[18] This catalytic
cycle is consistent with our observation
of a diminished reaction rate when the
reaction was performed under nitrogen
only, instead of under air (see Table S2).
When 1,2-diphenylhydrazine (6)
was heated with meso-Mn2O3 under
the optimized conditions, azobenzene
(Iaa) was formed in just 20 min. This
reaction did not occur in the absence of
the catalyst (Table 4). The very fast
reaction rate could be the reason that
intermediate 7 was not trapped.
To further validate the reaction
mechanism, we also performed quan-
tum-mechanical computations on the
system in the gas phase by using density
Figure 1. Energy profile of the reaction in the gas phase, as determined by DFT calculations.
Table 4: Reactivity of 1,2-diphenylhydrazine (6) in the presence and
coupling in moderate to excellent yields (35–99%). Air as the
terminal oxidant, the absence of precious metals and addi-
tives, and the atmospheric conditions with proper reusability
make our catalytic protocol superior to other reported
systems. Through mechanistic investigations, we were able
to reveal the involvement of surface-active Mn3+ species,
labile lattice oxygen, and radical intermediates in the catalytic
cycle. A manganese-mediated electron-transfer mechanism
from aniline was proposed and was supported by DFT
calculations. Studies aimed at modifying the reaction mech-
anism to make the reaction more facile are under way.
absence of meso-Mn2O3.[a]
Entry
Catalyst
Conversion
[%][b]
Selectivity
for Iaa [%][b]
1
2
meso-Mn2O3
none
>99
>99
0
0
[a] Reaction conditions: 1,2-diphenylhydrazine (6; 1 mmol) and meso-
Mn2O3 (50 mg, 0.32 mmol) were heated in toluene (0.5 mL) at reflux
(1108C) under an air balloon for 20 min. [b] Conversion and selectivity
were determined by GC–MS.
Acknowledgements
functional theory (DFT) as implemented in the Vienna
ab initio package. Furthermore, to estimate the Mn oxidation
states, DFT calculations were carried out on mesoporous
manganese oxide (meso-MnOx). A DFTenergy profile for the
reaction in the gas phase is shown in Figure 1. The relative
energies [kcalmolÀ1] are based on the summation of the
representative total energies Etot of all species. Our Bader
charge analysis based on DFT calculations supports the
hypothesis that in step 1, the active Mn3+ species was reduced
to Mn2+. The calculated charge on Mn was 2.18 eÀ. In step 2,
the generated radical is further stabilized by coupling with
another molecule of aniline, and the resulting intermediate
readily loses two protons and an electron in steps 3 and 4 to
give the more stable intermediate 1,2-diphenylhydrazine (6).
In the subsequent steps (transformation of 6 into Iaa), our
DFT computations support the suggested mechanism, accord-
ing to which a similar pathway is followed to that for the
conversion of 1a into 4.
S.L.S. is grateful for support from the US Department of
Energy, Office of Basic Energy Sciences, Division of Chem-
ical, Biological and Geological Sciences under Grant DE-
FG02-86ER13622.A000. N.O.S. is grateful for a sabbatical
leave award from the University of New Haven. We thank
Sniega Stapcinskaite of the Center of Environmental Science
and Engineering, University of Connecticut, Storrs for
assistance with ICP-MS.
Keywords: aniline oxidation · azo compounds ·
heterogeneous catalysis · manganese oxide ·
mesoporous materials
How to cite: Angew. Chem. Int. Ed. 2016, 55, 2171–2175
Angew. Chem. 2016, 128, 2211–2215
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Received: September 2, 2015
Published online: January 8, 2016
Angew. Chem. Int. Ed. 2016, 55, 2171 –2175
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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