Oxidative coupling of anilines to azobenzenes using heterogeneous manganese oxide catalysts

Article abstract of DOI:10.1039/c5cy01015b

We herein report the transition metal oxide-catalyzed synthesis of azobenzenes through the oxidative coupling of anilines. An octahedral molecular sieve of manganese oxide, OMS-2, exhibited the best activity and selectivity. Nine examples of symmetric azobenzenes and twenty unsymmetric ones were synthesized with 62-99% conversion and 64-99% selectivity. In the aniline cross-coupling reactions, the difference of the Hammett constants of two substituted groups (Δσ) determines the selectivity to unsymmetric azobenzenes, which are the major products at Δσ < 0.32. In-depth studies reveal that the surface defect sites of the mixed-valence manganese oxide play a key role in facilitating electron transfer and activating molecular oxygen. The single-electron transfer (SET) reaction mechanism is proposed based on electron paramagnetic resonance and X-ray powder diffraction characterization.

Full text of DOI:10.1039/c5cy01015b

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DOI: 10.1039/C5CY01015B
ARTICLE
Oxidative coupling of anilines to azobenzenes using
heterogeneous manganese oxide catalysts
Cite this: DOI: 10.1039/x0xx00000x
Min Wang, Jiping Ma, Miao Yu, Zhe Zhang, and Feng Wang*
We herein report the transition metal oxide-catalyzed synthesis of azobenzenes through the
oxidative coupling of anilines. An octahedral molecular sieve of manganese oxide, OMS-2,
exhibited the best activity and selectivity. Nine examples of symmetric azobenzenes and twenty
unsymmetric ones were synthesized with 62-99% conversion and 64-99% selectivity. In the
aniline cross-coupling reactions, the difference of the Hammett constant of two substituted
groups (Δσ) determines the selectivity of unsymmetric azobenzenes, which is major product at
Δσ < 0.32. In-depth studies reveal the surface defect sites of the mixed-valance manganese oxide
play a key role in facilitating electron transfer and activating molecular oxygen. The single
electron transfer (SET) reaction mechanism is proposed based on electron paramagnetic
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
resonance and X-ray powder diffraction characterizations
.
C=N bonds,12 no example of catalyzing the formation of N=N bond
has been reported.
Introduction
Azobenzene and its derivatives are greatly important for our daily life.
Most colored textiles and leathers are treated with azo dyes and
pigments. Besides, azobenzenes are used as food additives, drugs,
reaction initiators, and therapeutic agents indicators.1 They are usually
obtained by the oxidative coupling of anilines with stoichiometric and
environmentally unfriendly oxidants (for example, lead tetraacetate,
mercury oxide, ferrates, sodium periodate).2 A more preferable way is
to employ heterogeneous catalyst and oxygen gas as oxidant. Up to
the present, only a few examples are known for catalytic oxidative
coupling of anilines to azobenzenes, such as Au/TiO2, Ag/C-NaOH,
CuBr-pyridine CuCl and Cu/NH4Br-pyridine reported by Corma, Li,
Jiao, Xi and Gu, respectively (Scheme 1).3 Although these works have
achieved high yields of azobenzenes, they have shortcomings of (i)
complex catalyst preparation procedures with difficulty in precise
particle size and morphology control; (ii) inevitable stoichiometric
reagents, such as NaOH or pyridine; and (iii) the use of precious
metals or halogen ions in reactions. Therefore, the development of
efficient catalytic procedures without the above-mentioned
disadvantages or with the use of transtion-metal catalysts without
additive will be greatly desirable. Especially if the catalyst is
abundantly available or its preparation is easily controlled at scale, it
will render the synthesis process more attractive and great
fundamental from the view of both academia and industry.4
Scheme 1 The known catalyst or catalytic system for the oxidative coupling of
aniline to azobenzene with oxygen gas as oxidant.
Experimental
Chemicals and Reagents. All the chemicals and metal oxides
are commercial available and used without further purification.
The metal oxides are purchased from Aladdin Industrial
Corporation.
Preparation of OMS-2. OMS-2 was prepared by refluxing a
mixture of potassium permanganate and manganese sulfate in
acidic medium according to the literature.13 Potassium
permanganate solution in deionized water (0.4 M, 100 mL) was
added to a mixture of manganese sulfate hydrate solution (1.75
M, 30 mL) and concentrated nitric acid (3 mL) to a 500 mL flask
fitted with a reflux condenser. The dark brown slurry was
refluxed for 24 h, then centrifuged and washed with deionized
water several times. The catalyst was dried at 100 oC overnight
before use.
An octahedral molecular sieve of manganese oxide (OMS-2) has
reliable and scale-up synthesis methods.5 This cryptomelane-type
pristine and crystalline manganese oxide contains the mixed valence
framework,6 and has wide applications in gas sensing, energy storage,
separation and catalysis.7 We here demonstrate OMS-2 is effective for
the oxidative coupling of anilines with molecular oxygen in the
absence of any additive. Although manganese oxides have been used
as catalysts in reactions of forming C–C,8 C-O,9 C=O,10 C–N,11 and
Catalytic tests. Catalytic reactions were performed in a 10 mL
autoclave reactor with Teflon-liner. Typically, 1 mmol aniline,
20 mg OMS-2 and 2 mL chlorobenzene were added into the
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reactor Then, the reactor was heated to the desired temperature
under magnetic stirring and charged with 0.8 MPa O2. When the
reaction finished and was cooled down to room temperature, the
reaction mixture was diluted with 5 mL ethanol. The catalyst was
separated by centrifugation. The products were identified by gas
chromatography−mass spectrometry (GC−MS) using an Agilent
7890A/5975C instrument equipped with an HP-5 MS column
(30 m in length, 0.25 mm in diameter) and quantitatively
analyzed by Agilent Technologies 7890D gas chromatograph
(GC) equipped with HP-5 MS column (30 m in length, 0.25 mm
in diameter) and flame ionization detector. The conversion is
Table 1 Oxidative coupling of substituted anilines to symmetric
azobenzenes using OMS-2 catalyst.a
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DOI: 10.1039/C5CY01015B
determined using
n-Dodecane as internal standard The
selectivity is determined by the area normalization. For the cross
coupling reaction, 0.25 mmol R1-C6H4NH2 and 0.75 mmol R2-
C6H4NH2 were used. The conversion and cross coupling
selectivity is calculated based on R1-C6H4NH2.
Characterizations. The X-ray powder diffraction (XRD)
patterns were obtained using a Rigaku D/Max 2500/PC powder
diffractometer with Cu Kα radiation (λ = 0.15418 nm).
Morphology of the samples was observed by field-emission
scanning electron microscopy (SEM, FEI Quanta 200F). The
electron paramagnetic resonance (EPR) were performed on
Bruker spectrometer at X-band under room temperature, with a
field modulation of 100 kHz. The microwave frequency was kept
at 9.401 GHz.
a Reaction conditions: 1 mmol aniline, 20 mg OMS-2, 2 mL PhCl, 0.8 MPa O2,
160 oC, 24 h. The conversion and selectivity were determined by GC-MS. The
data in parenthesis is the isolated yields.
The general applicability of the method was tested with a series of
substituted anilines (Table 1). Symmetric azobenzenes were
synthesized with moderate to excellent conversion and selectivity
regardless of the presence of electron-donating groups (para, meta,
and ortho substituted) or electron-withdrawing groups (Table 1, 1-9).
While, no reaction occurred for the extreme electron-withdrawing
groups, such as p-nitroaniline (not shown here). Note that the halo-
substituted anilines survived well, leading to the halo-azobenzenes
(Table 1, 6-9), which could be used for further transformations, such
as C–N and C–C coupling reactions.14 This approach is also effective
for the construction of heterocyclic compounds. Three N atom
containing heterocyclic compound (10) with 70% yield were
synthesized by the oxidative coupling of benzyl amine group and
aromatic aniline group in 2-aminobenzenemethanamine [Eq. (1)].12, 15
Furthermore, OMS-2 could be easily filtered from reaction mixture,
washed, and reused for three cycles with comparable activities (Fig.
S3).
Results and discussion
N
OMS-2
NH2
NH2
>99%
(1)
PhCl, 0.6 MPa O2
120 oC, 24 h
N
N
Fig. 1 Oxidative coupling of aniline to azobenzene over various catalysts. Reaction
conditions: 1 mmol aniline, 20 mg catalyst, 2 mL chlorobenzene (PhCl), 0.8 MPa
O2, 160 oC, 24 h.
10 (70%)
Not limited to symmetric azobenzenes, unsymmetric azobenzenes
were also synthesized with satisfied results (Table 2). These azo dyes
are usually synthesized through reaction of the diazonium salt with
electron-rich aromatic compounds.1a Electron-donating groups- (p-
OMe, o-Me, m-Me, p-Me) or electron-withdrawing groups- (p-F, p-
Cl, and p-Br) substituted anilines were successfully cross-coupled to
synthesize a wide range of unsymmetric azobenzenes with 62-99%
conversion and 65-99 % selectivity.
The oxidative coupling of aniline to azobenzene reactions were
conducted in an autoclave reactor with oxygen pressure up to 0.8 MPa
(Fig. 1). Initial explorations showed the reactions over Y2O3, ZrO2,
Fe2O3, Co3O4, Nb2O5 and MoO3 as well as a blank reaction without
adding catalyst gave no conversion of aniline. Oxides, such as Cr2O3,
Yb2O3, WO3, TiO2, Mn2O3, V2O5, Mn3O4, CeO2 and β-MnO2 showed
activity less than 60%. OMS-2 with rod morphology (Fig. S1) offered
the best result of 95% conversion of aniline and >99% selectivity for
azobenzene. The conversion increases with the increase of the
temperarue (Fig. S2). When decreasing the oxygen pressure to 0.1
MPa, the conversion decreases to 55%.
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The difference value (Δσ) of substituents’ Hammett constant [Δσ
= σ(R1)-σ(R2)] determines whether the cross-coupled product or
Table 2 Oxidative cross-coupling of anilines over OMS-2 for
unsymmetric azobenzenes.a
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homo-coupled product dominates in the reaction of two substituted
anilines (Fig. 2). When the Δσ is less than 0.D32O,I:th10e.1m03a9jo/Cr5pCroY0d1u0c1t5iBs
from cross-coupling reaction, and larger than 0.32, homo-coupling
reaction. This general rule of thumb is useful to control the product
selectivity.
R2
NH2
NH2
OMS-2
N
R1
+
N
R2
R1
PhCl, 0.8 MPa O2
160 oC, 24 h
13
16
11
14
12
N
N
N
N
N
N
95 / 65 (1.89)
93 / 82 (4.54)
97 / 77 (3.32)
15
F
Cl
O
N
N
N
N
N
N
91 / 72 (2.57)
56/ 85 (5.64)
>99 / 84 (5.07)
Br
19
17
18
O
N
N
N
N
N
N
99 / 78 (3.52)
>99 / 69 (2.2)
>99 / 88 (7.13)
20
21
22
F
Cl
Br
N
N
N
N
N
N
Fig. 3 (a) Time‐on‐stream course of conversion. C = conversion of aniline. (b)
Arrhenius plot for the oxidation of aniline.
84 / 72 (2.63)
99 / 60 (1.53)
>99 / 64 (1.8)
We then conducted kinetic study. Correlating -ln(1-C) (C stands
for conversion) against reaction time (ks) indicates a linear
relationship and a pseudo-first order reaction with respect to anilines
(Fig. 3a). The apparent activation energy, Ea, is determined from the
Arrhenius plot (Fig. 3b). The least-square fit analysis yields an Ea
value of 70.4 kJ mol–1.
23
24
25
28
O
F
Cl
N
N
N
N
N
N
>99 / 99 (99)
62 / 87 (6.45)
>99 / 78 (3.45)
27
26
Cl
Br
N
N
N
N
N
N
F
99 / 76 (3.08)
>99 / 76 (3.21)
96 / 78 (3.48)
29
30
Br
Br
N
N
N
N
F
Cl
99 / 70 (2.35)
96 / 82 (4.5)
a
Reaction conditions: 0.25 mmol R1-C6H4NH2, 0.75 mmol R2-C6H4NH2, 20
mg catalyst, 2 mL PhCl, 0.8 MPa O2, 160 oC, 24 h. The conversion and cross
coupling selectivity is calculated based on R1-C6H4NH2. The data in
parenthesis is the ratio of cross-coupled products and homo-coupled products.
Fig. 4 Hammett plot for the OMS‐2 catalyzed oxidation of substituted anlines.
A linear relationship between log(kX/kH) and Brown–Okamoto
constant σ+ is established for substituted anilines (X = p-OMe, p-Me
p-H, p-F) (Fig. 4). The resulting Hammett parameter ρ is -1.57,
suggesting the reaction is substituent-sensitive and aniline radical
cation intermediates are involved. Electron-donating groups can
stabilize the radical cation and thus are more favorable to be oxidized.
Two exceptions were observed for p-Cl and p-Br-aniline. This
phenomenon is further embodied by the dependence of the reaction
rate on the Brown–Okamoto constant σ+ of the substituent (Fig. S4).
Different from the previous results,3a, 3c we find the reaction is not
simply favored by the presence of electron-donating substituents.
With the increase of σ+ from -0.778 to 0.15, the reaction rate first
decreases and then increases after 0.062 (p-F), which agrees well with
the Hammett plot results. The conjugation effect may be more
favorable than the inductive effect for p-Cl and p-Br substituents in
Fig. 2 The relationship between the unsymmetric excess (ue) and Δσ for the
synthesis of unsymmetric azobenzenes. The “ue” is defined as (the selectivity of
unsymmetric azo – the selectivity of symmetric azo). The value “Δσ” is the
difference of Hammett constant of R1 and R. Reaction conditions are the same as
Table 2. The Hammett constant is from the literature.16
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this case. The electrons in p or d orbit can be delocalized to stabilize and resolved narrow line with g value of 1.9984 and ΔH value of 6.6
the nitrogen radical cations, which may account for this phenomenon. mT. These salient changes suggest that the Mn4+ was reduced to the
lower charged species (Mn2+ or Mn3+).22 This reducVeidewmAratinclgeaOnnelinsee
oxide returned to the original state upon oxidDatOioI:n10u.n10d3e9r/tCh5eCrYe0a1c0t1io5Bn
conditions deduced from that the signals in (a) and (c) show similar g
and ΔH value. Compared to the fresh OMS-2, the increased value of
ΔH accompanied by appearing of another superimposed line in (c) is
ascribed to the relative larger content of Mn3+ and Mn2+.19-20
To probe the mechanism of this transformation, the reaction of
aniline in the presence 2.0 equivalents of OMS-2 under N2 atmosphere
was investigated [Eq. (2)]. The aniline conversion of 70% with >99%
selectivity for azobenzene was obtained. This result indicated OMS-2
functions as oxidant under the reaction condition, resulting in the
oxygen transfer from manganese oxide lattice to aniline. It is
postulated that the oxygen transfer can be viewed as the half reaction.
The next half reaction is the restoration of the lattice oxygen by
molecular oxygen, forming a catalytic redox cycle. This postulation is
supported by the stepwise crystal phase evolution of OMS-2 tracked
by XRD (Fig. 5). Typical XRD pattern of OMS-2 is given in Fig. 4a.17
o
After treating OMS-2 with aniline under N2 at 160 C, the XRD
diffraction showed it changed to Mn3O4 phase (JCPDS Card no. 24-
0734, space group I41/amd[141]) as shown in Fig. S5. Azobenzene
was detected as major product, suggesting OMS-2 was reduced by
aniline. Upon treating the as-obtained Mn3O4 with oxygen gas under
the reaction condition without adding aniline, the OMS-2 phase was
restored (Fig. 4c), indicating Mn3O4 could be re-oxidized to OMS-2
with molecular oxygen. This is understandable because of the relative
larger redox potential of O2 than that of MnO2 [EΘ(O2/H2O) = 1.23 V;
Fig. 6 EPR spectra of (a) fresh OMS‐2, (b) OMS‐2 was treated with aniline at 160
under acid condition: EΘ(MnO2/Mn3+) = 0.95 V, EΘ(MnO2/Mn2+) =
1.23 V; under basic condition: EΘ(MnO2/Mn(OH)3) = -0.20 V,
EΘ(MnO2/Mn(OH)2) = -0.05 V].18
oC under N2 atmosphere for 24 h. (c) OMS‐2 in (b) was treated at 160 oC under O2
atmosphere for 24 h.
Furthermore, among the investigated manganese oxide, the β-
MnO2 gives only 43% yields of azobenzenes in the presence 2.0
o
equivalents of manganese oxide at 160 C under N2 atmosphere and
Mn2O3 and Mn2O3 were inactive. Temperature programmed
desorption- mass spectrometry curves (TPD-MS) shows the OMS-2
o
o
and β-MnO2 started to desorb oxygen at around 250 C and 350 C,
respectively (Fig. 7).21a, 23 And no oxygen was desorbed in the case of
Mn2O3 and Mn2O3 below 400 C. This indicates the Mn-O bond is
o
more active than that β-MnO2, Mn2O3 and Mn2O3, and accounts for
the higher reactivity in the catalytic oxidation of anilines.
Fig. 5 XRD patterns of (a) fresh OMS‐2, (b) OMS‐2 was treated with aniline at 160
oC under N2 atmosphere for 24 h. (c) OMS‐2 in (b) was treated at 160 oC under O2
atmospheres for 24 h.
This dynamic mixed-valence nature of OMS-2 during reaction
was investigated by electron paramagnetic resonance (EPR)
characterization (Fig. 6). The OMS-2 is featured with a wide line (a)
with g value of 2.2120, which is larger than 1.94 or 2.13 of the fully
oxidized MnO2 in literature.19 Its line width ΔH = 460 mT is larger
than the theoretical value of 430 mT for Mn4+ in the fully oxidized
MnO2.20 The two results indicate that the presence of defect sites (the
presence of lower charged Mnn+, n = 2, 3) in accordance with the
reported Mn charge of +3.8 in OMS-2.13, 17, 21 The broad EPR line
originates from the electrostatic and dipolar interactions between the
manganese ions with mixed valence states. When OMS-2 was treated
with aniline under N2 atmosphere, the EPR signal changed to a single
Fig. 7 TPD‐MS curves of manganese oxide.
Although in-depth investigations are required for revealing the
details of catalytic process, based on the above results, we may
propose a tentative reaction mechanism illustrated in Scheme 2.
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Aniline is adsorbed on the oxygen defect sites on MnO2. Then, single
electron transfer (SET) from aniline to the oxygen defect sites follows,
creating the aniline radical cation and reducing the Mn4+ to
Mn3+/Mn2+ with the activation of Mn–O bond. This aniline radical
cation is coupled with the surrounding neutral aniline to form a three-
electron sigma bond as reported.24 Further electron transfer and the
abstraction of two hydrogens by the activated oxygen on MnO2
surface forms hydrazine. Hydrazine is a very active intermediate. We
and other researchers failed to detect it.3a, 3c Fast oxydehydrogenation
of hydrazine generates azobenzene. The reduced OMS-2 is re-
oxidized by oxygen, ready for next catalytic cycle. Azobenzene is also
possible formed from the reaction of nitrosobenzene and aniline.1e But
we did not detected nitrosobenzene intermediate.
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Conclusion
In summary, we have reported a highly active, selective and
recyclable manganese oxide catalyst in the oxidative coupling of
anilines for the synthesis of symmetric and unsymmetric azobenzenes.
This study will help understand heterogeneous catalysis of oxides and
may provide the useful reference for producing azobenzenes in a
green and economical way.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21422308, 21303183, 21233008), and
Dalian Excellent Youth Foundation (2014J11JH126).
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Cryptomelaneꢀtype manganese oxide shows high efficienty for the N=N formation reaction in the
oxidative coupling of anilines to azobenzenes. The difference of the Hammett constant (ꢁσ) of
two substituted groups determines the selectivity of unsymmetric azobenzenes in crossꢀcoupling
reactions, which is favored at ꢁσ < 0.32.
.