A mild CuBr-NMO oxidative system for the coupling of anilines leading to aromatic azo compounds

Article abstract of DOI:10.1039/c5ra12535a

An efficient, mild and cost-effective method has been developed utilizing CuBr with N-methylmorpholine N-oxide (NMO/NMMO) for the oxidative coupling of anilines to access symmetrical and unsymmetrical azo compounds in high yield. The reactivity was found to be governed by electronic and steric factors of anilines.

Full text of DOI:10.1039/c5ra12535a

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A mild CuBr-NMO oxidative system for the coupling of anilines
leading to aromatic azo compounds
Shikha Singh,a Parul Chauhan,a Makthala Ravi,a Isha Taneja,b Wahajuddin,b Prem. P. Yadav*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient, mild and cost-effective method has been developed utilizing hazardous diazonium salts and toxic nitrosobenzenes. More
CuBr with N-methylmorpholine N-oxide (NMO/NMMO) for the oxidative specifically the problem lies in the substrate scope as it is
coupling of anilines to access symmetrical and unsymmetrical azo limited to the electron donating amines. In this regard Corma
compounds in high yield. The reactivity was found to be governed by et al. reported an efficient path to synthesize aromatic azo
electronic and steric factors of anilines.
compounds by gold nanoparticles catalyzed process under O2
o
(3-5 bar) as an oxidant at 100 C, however, high temperature
and pressure limits this methodology.14 Again silver
nanoparticles catalyzed oxidative dimerization of amines
suffers from the use of stoichiometric amount of strong base
KOH.15 Jiao’s group proposed Cu/Pyridine catalyzed aerobic
Aromatic azo compounds have attracted considerable
attention for a long time due to their wide range of
applications such as organic dyes, indicators, food additives,
pigments and therapeutic agents.1 These motifs are
extensively used as smart polymers,2 liquid crystals3 and
photoswitches4 in biological system because of their exclusive
cis/trans isomerisation.5 Recently the azo group has been used
o
oxidation of aniline derivatives at 60 C, where an excess use
of electron deficient anilines was required for the synthesis of
unsymmetrical azobenzene derivatives.16 Ma et al. developed
an organomettallic approach using hypervalent iodine (III).17
Furthermore, Minakata group used t-BuOI in different reaction
conditions (temperature and solvents) for different substrates
under nitrogen atmosphere for oxidative dimerization of
aromatic amines leading to azo derivatives (Scheme 1).18
successfully as
a
directing group for ortho C-H
activation/functionalization,6 for the synthesis of valuable o-
acylazobenzenes,6a-d o-alkoxyazobenzenes6e and indazole
derivatives.6f
Innumerable methods7 are available for the synthesis of these
azo derivatives, however, development of new methods with
mild reaction conditions and the improvement of the existing
methodologies is in high demand. Traditional methods for the
preparation of symmetrical azo derivatives include oxidation of
primary amines8 and reductive homodimerization of
nitroarenes.9 These methods have some limitations such as in
the oxidative preparation method use of environmentally
unfriendly heavy metals10 (Hg,10d Mn10e and Pb10f salts) in
stoichiometric amount and poor control of product
distribution (the azo and azoxybenzene ratio) in reductive
homodimerization of nitroarenes. Diazonium coupling11, Mills
reaction12 and Wallach reaction13 are the most common
methodologies used to synthesize unsymmetrical azo
compounds, but these methods require explosive and
Scheme 1 Strategies for the synthesis of azobenzenes
Here we developed an efficient and
a mild synthetic
a.Division of Medicinal and Process Chemistry, CSIR-Central Drug Research Institute,
Lucknow-226031, India. E-mail: pp_yadav@cdri.res.in; ppy_cdri@yahoo.co.in;
Fax: +91-522-2771942/2771970; Tel: +91-522-2772450 ext. 4761
b.Division of Pharmacokinetics and Metabolism, CSIR-Central Drug Research
Institute, Lucknow-226031, India
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
methodology for the preparation of symmetrical and
unsymmetrical azo compounds from aromatic anilines in a
moderate to good yield. N-Methylmorpholine N-oxide (NMO)
is mostly utilized as a co-oxidant or a sacrificial oxidant with
TPAP,19 Osmium tetraoxide20 in various oxidation reactions and
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also is used as a solvent for dissolution of cellulose in Lyocell most of the copper-catalyzed reactions are conducted in
process.21 Herein, we have utilized CuBr catalyst in the acetonitrile,22 we started observing the reaction in neat
presence of NMO as an oxidant (Scheme 1) and it was found to acetonitrile (6 mL), which gave 48% and 62% yield with 10
be a better and mild oxidative system for the synthesis of mol% and 20 mol% CuBr respectively (entry 3-4, Table 1).
various azo derivatives as compared to some other previously Interestingly, when the reaction was carried out in 2:1 (v/v)
reported methods, where high temperature, pressure, excess acetonitrile and water solvent mixture, reaction yield was
use of strong base and explosive or toxic intermediate were abruptly increased to 86% (entry 5, Table 1). When the
the main limitations.
amount of water was increased in the solvent mixture the
reaction yield was decreased (entry 7, Table 1). It may be due
to enhanced disproportionation of Cu(I) salt in the aqueous
solution into Cu and Cu(II).21 When toluene was used instead
of acetonitrile in a solvent mixture to form biphasic solvent no
product was observed (entry 16, Table 1). Other solvents such
as DMSO, DMF, THF and ethanol were not effective in the
conversion. NMO was essential for the reaction since its
absence resulted in a profound decrease in the reaction yield
(entry 1, 6 and 8, Table 1).
Table 1 Optimization of catalyst for coupling reactiona
Entry
1
Cu source
CuBr
X
Y
-
Solvent
CH3CN
Time
5 h
Yield (%)b
25
10
Table 2 Homocoupling of substituted anilinesa
2
3
4
5
-
-
5
1
1
1
CH3CN
CH3CN
CH3CN
3 h
30 min
1 h
-
CuBr
CuBr
CuBr
10
20
10
48
62
86
CH3CN/ 30 min
H2O(2:1)
6
7
CuBr
CuBr
10
10
-
CH3CN/
H2O(2:1)
CH3CN/
H2O(1:2)
CH3CN
3 h
30
35
Entry
1
R
Product (2)
Time
30 min
Yield (%)b
86
1
3 h
4-OMe
MeO
N
N
OMe
2a
2
3
H
45 min
45 min
78
83
8
9
CuBr2
CuBr2
CuBr2
10
20
20
-
1 h
1 h
1 h
32
56
59
1
1
CH3CN
3-OMe
10
CH3CN/
H2O(2:1)
CH3CN/
H2O(1:2)
CH3CN
4
5
2-OMe
40 min
1 h
84
72
11
CuBr2
20
1
1.5 h
30
2-Me-4-OMe
12
13
Cu(OAc)2
Cu(OAc)2
20
20
1
1
1.5 h
2 h
42
48
CH3CN/
H2O(2:1)
CH3CN
6
7
8
9
4-F
4-Cl
30 min
45 min
1 h
79
83
73
74
14
15
CuCl
CuCl
20
20
1
1
1 h
1 h
40
55
CH3CN/
H2O(2:1)
Toluene/
H2O(2:1)
4-Br
16
CuBr
20
1
1 h
-
3,4-Di-Me
1 h
aReaction condition: 1a (0.81 mmol), catalyst (X mol%), NMO (Y equivalents),
solvent (6 mL) at rt. bIsolated yields.
10
11
4-Et
2-Ph
30 min
2 h
90
69
To optimize the reaction condition oxidation of 4-methoxy
aniline was chosen as a model reaction (Table 1). Various
copper salts were investigated among them copper bromide
gave the best conversion of 4-methoxy aniline to bis(4-
methoxyphenyl)diazene in 30 minutes (entry 5, Table 1). NMO
was essential for the reaction along with CuBr since in the
absence of any one of these showed lack of reactivity. Other
copper salts such as CuCl and copper acetate showed less
efficiency as compared to CuBr (entry 12-15, Table 1). CuBr2
gave a moderate yield (entry 8-11, Table 1). The decrease
reaction yields in case of Cu(II) salts may be linked with the
tendency of Cu(II) salts to directly react with NMO leading to
its decomposition into morpholine and formaldehyde.20
Selection of solvent system was crucial for the reaction, as
aReaction condition: 1a (0.81 mmol), CuBr (10 mol%), NMO (1 equivalent),
CH3CN/H2O (2:1) (6 mL) at rt. bIsolated yields mixture of cis:trans isomers.
Under the optimized reaction condition, the scope of
different anilines was investigated. The electronic and steric
factors were found to be crucial for the reactivity. The
experiment showed that anilines with electron donating
substituents gave corresponding azo compounds in excellent
yield (Table 2). Halogenated anilines (2f-2h) easily reacted
under the optimized reaction condition to afford respective
halogenated azo derivatives in good to high yield which can be
further utilized for functionalization (entry 6-8, Table 2).
2 | J. Name., 2012, 00, 1-3
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Successful results for homodimerization motivated us to Azo-pyridines have been widely utilized as photodissociable
find out the scope of cross-dimerization reaction, since it is axial ligands (PDL),23 photoactive liquid crystals24 and an
quite challenging to couple two different anilines to afford efficient reagent for esterification reaction.25 Thus, we tried to
unsymmetrical azo benzenes as a major product due to existed explore the scope of our methodology to synthesize 3-
Competitive homodimerization over cross dimerization azopyridine (2l) from 3-aminopyridine (1l). It was our delight
reaction. Thus to check the applicability of our protocol we that the desired product was obtained in good yield within 30
applied this methodology for the synthesis of the minutes. Similarly, 3-azoquinoline was also prepared within 2 h
unsymmetrical azobenzenes and gratifyingly, successfully we using the present protocol (Scheme 2).
could synthesize various unsymmetrical azo derivatives (Table
3).
Table 3 Cross-coupling of substituted anilinesa
Entry
R
R1
Cross coupling product Time
Yield (%)b
Scheme 2 Synthesis of 3-azopyridine and 3-azoquinoline
(3)
(3, 2, 2′)
1
2
3
4
5
6
7
4-OMe
4-OMe
4-OMe
H
Cl
1 h
45c
MeO
N
N
N
In order to understand the reaction mechanism and to
exclude the possibility of dioxygen as an oxidant, we used our
optimized reaction condition and allowed 1b to react under
nitrogen atmosphere (Scheme 3). The corresponding
azobenzene 2b was observed within 10 min of the reaction
and full conversion was obtained after 40 min. However, in the
absence of NMO, the reaction yielded only 14% of the product.
From these observations, it was clear that dioxygen was not
acting as an oxidant and NMO was responsible for the
oxidative coupling of the aniline along with CuBr.
3a
1.5 h
1 h
54, 20, 26
50c
MeO
MeO
N
Cl
F
3b
N
N
4-F
3c
MeO
N
N
4-OMe 3,4-DiMe
2.5 h
2.5 h
1.5 h
1 h
52, 33, 15
48c
3d
3-OMe
3-OMe
3-OMe
H
4-Cl
4-Et
53, 27, 20
50, 41, 9
CuBr (10 mol%)
NMO.H2O (1equivalent)
NH2
N
N
CH3CN/H2O (2:1)
6 mL, rt, N2, 40 min
2b (77%)
1b
8
9
2-OMe
2-OMe
H
2 h
2.5 h
2 h
45, 40, 15
51, 49, -d
49, 42, 9
50, 45, 5
Sheme 3 Homodimerization of aniline under N2 atmosphere
4-Cl
4-Et
4-Cl
1,2-diphenylhydrazine
(
4
)
was oxidized under the
was easily
optimized reaction condition and it was found that
4
10 2-OMe
transformed to corresponding azobenzene (2b) within 5 min
(Scheme 4). Although, it could be converted by CuBr alone, but
the full conversion of azobenzene was obtained after 6 h.
Thus, it showed that NMO played an important role in the
oxidative coupling of anilines.
11
H
1.5 h
aReaction condition: 1 (0.81 mmol), 1' (0.90 mmol), CuBr (10 mol%), NMO (1
equivalent), CH3CN/H2O (2:1) (6 mL) at rt. bHPLC yields, cIsolated yield, dNot
resolved in HPLC.
Anilines substituted at ortho, meta and para position smoothly
afforded unsymmetrical azobenzenes in moderate to good
yield. Halogen substituted anilines smoothly reacted to give
cross-dimerized halo substituted azo derivatives in high yield
over homodimerized products (Table 3). Unfortunately, nitro,
cyano, or ester substituted anilines did not afford azo
derivative and remained in a dormant state. Except these, In
all other cases cross-coupled azobenzene were the major
product over the homodimerized products and were easily
purified by column chromatography.
Scheme 4 Oxidation of 1, 2-phenylhydrazine
Through the studies reported above, we postulated a
plausible mechanism (Figure 1). Firstly, in the primary step a
single electron transfer from Cu(I) to NMO resulted in the
homolytic cleavage of N-O bond which subsequently produced
aminyl radical (
5
) and Cu(II) salt.21 This aminyl radical was
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methylmorpholine (NMM,
6
)
and Cu(II) salt. Cu(II) salt
which
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A
after one electron transfer resulted in radical cation
Dimerizaton of radical cations afforded respective hydrazine
B
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C
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the decrease in the reaction yield when Cu(II) salts were used
instead of Cu(I) salts in the reaction.21 In situ generated Cu(II)
salt oxidizes NMM to generate Cu(I) and aminyl radical which
further starts the catalytic cycle.
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O
N
O
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Cu(II)
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NMM
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NMO
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Cu(II)
Ar/Ar' NH2
Ar/Ar' NH2
B
(1)
Ar/Ar'
N
Ar/Ar' NH2
H
Ar/Ar'
N
N
Ar/Ar'
N
A
Ar/Ar'
H
Cu(II)
(2/2'/3)
C
---------------------------------------------------------------------------------------------------------------------------------
7
8
9
O
N
O
O
Cu(II)
Cu(I)
Cu(II)
HCHO
N
H
N
(5)
NMM
morpholine
Figure 1 Proposed mechanism
Conclusions
In summary, an efficient and a low cost CuBr-NMO oxidative
system was developed for the synthesis of symmetrical and
unsymmetrical azobenzenes under mild reaction condition.
Anilines were converted to corresponding homo and cross-
coupled azo products in good to excellent yield. Furthermore,
3-Azopyridine a highly valued hetero azo derivative was
successfully synthesized by our methodology. Studies are
ongoing in our lab for the further synthetic application of this
protocol.
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Acknowledgements
S.S., P.C. and M.R. are thankful to CSIR, New Delhi, India, for
Fellowship, Mr. A. K. Srivastava for providing technical support,
SAIF-CDRI, Lucknow for providing spectral and analytical data.
This work was supported by CSIR network project “HOPE”
(BSC0114). This is CDRI communication No. xxxx (will be
inserted after acceptance).
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