Copper-catalyzed aerobic oxidative dehydrogenative coupling of anilines leading to aromatic azo compounds using dioxygen as an oxidant

Article abstract of DOI:10.1002/anie.201001651

In the air tonight: A novel approach to symmetric and unsymmetric aromatic azo compounds from simple anilines catalyzed by inexpensive CuBr has been disclosed. Air (or dioxygen) was used as an oxidant under mild reaction conditions, with H2O as the byproduct, to make this transformation environmentally benign and very easy to handle.

Full text of DOI:10.1002/anie.201001651

Communications
DOI: 10.1002/anie.201001651
Azo Compounds
Copper-Catalyzed Aerobic Oxidative Dehydrogenative Coupling of
Anilines Leading to Aromatic Azo Compounds using Dioxygen as an
Oxidant**
Chun Zhang and Ning Jiao*
Aromatic azo compounds are ubiquitous motifs and are
widely used in industry as organic dyes, indicators, pigments,
food additives, radical reaction initiators, and therapeutic
agents.[1] Despite numerous efforts towards the synthesis of
azo derivatives, big challenges still remain because: 1) the
catalytic procedures that have been reported to afford high
yields have been less widely explored;[2] 2) stoichiometric and
environmentally unfriendly oxidants, such as manganese
salts,[3a] lead salts,[3b–c] mercury salts,[3d–e] or ferrates[3f–g] were
previously employed for their preparation form aromatic
amines; 3) unsymmetric aromatic azo compounds are not
easy to prepare. Usually, two step syntheses are used,
proceeding from anilines via diazonium salt[4] or nitrosoben-
zene intermediates,[5] using stoichiometric amounts of nitrite
salts or other oxidants, to produce unsymmetric aromatic azo
compounds, with inorganic salts as the by-products [Eq. (1)].
However, the employment of a noble gold catalyst and the
requirement of higher pressure and temperature may limit
their applications. Herein, we demonstrate a novel approach
to azo compounds from readily available anilines, under mild
conditions using an inexpensive and commercial available
copper catalyst and air or dioxygen as an oxidant [Eq. (2)]. To
the best of our knowledge, this is the first convenient catalytic
process for unsymmetric azo compounds from aromatic
amines using O2 (1 atm) as the oxidant.
The use of dioxygen as an ideal oxidant has attracted a
great deal of attention.[6,7] During our investigations toward
a-ketoamides synthesis using the copper-catalyzed oxidative-
amidation/diketonization of terminal alkynes,[8] we observed
that trans-1,2-diphenyldiazene (2aa) was easily prepared by
dehydrogenative coupling[9] using a copper salt as the catalyst
in air. (Table 1). Employing pyridine as a ligand was key for
high efficiency in this transformation (Table 1, entries 1 and
Table 1: Copper-catalyzed oxidative coupling of 1a under air.[a]
Recent breakthroughs were developed by Corma, Garcꢀa,
and co-workers.[2a] They reported an oxidation of aromatic
anilines to aromatic azo compounds catalyzed by gold
nanoparticles using O2 (3–5 bar) as the oxidant at 1008C.
Entry Catalyst
Additives (mol%)
Solvent Yield of 2aa [%][b]
1
2
3
4
5
6
7
8
CuBr
CuBr2
CuCl
CuBr
CuI
CuOAc
Pd(OAc)2
AuBr3
CuBr
CuBr
CuBr
none
pyridine (18)
pyridine (18)
pyridine (18)
pyridine (18)
pyridine (18)
pyridine (18)
pyridine (18)
–
toluene
toluene
toluene
toluene
12
60
80
96
[*] C. Zhang, Dr. N. Jiao
State Key Laboratory of Natural and Biomimetic Drugs
School of Pharmaceutical Sciences, Peking University
Xue Yuan Rd. 38, Beijing 100191 (China)
Fax: (+86)10-8280-5297
toluene trace
toluene
toluene
toluene
pyridine
DCE
8
n.r.
n.r.
11
62
29
E-mail: jiaoning@bjmu.edu.cn
Dr. N. Jiao
9
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes, Department of Chemistry, East China Normal University
Shanghai 200062 (China)
10
11
12
13
14
15
pyridine (18)
2,2’-bipyridine (9)
PPh3 (18)
toluene
toluene
CuBr
CuBr
CuBr
CuBr
n.r.
[**] Financial support from Peking University, the National Science
Foundation of China (Nos. 20702002, 20872003), and the National
Basic Research Program of China (973 Program; Grant No.
2009CB825300) are greatly appreciated. We thank Chong Qin in this
group for reproducing the results of 2ii, 2ah, and 2lb.
1,10-phenanthroline (9) toluene trace
pyridine (12)
pyridine (24)
toluene
toluene
65
94
[a] Reaction conditions: 1a (1 mmol), cat. (0.03 mmol), solvent (4 mL),
air (1 atm), 20 h. [b] Yield of isolated product. DCE=1,2-dichloroethane,
n.r.=no reaction.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001651.
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4). Attempts to use other transition-metal catalyst, such as
silver, gold, iron, palladium, cobalt, or manganese were
unsuccessful (see the Supporting Information). Of the copper
salts tested as catalysts in the reaction, CuBr performed the
best (Table 1, entries 2–6; also see the Supporting Informa-
tion). We then surveyed the effect of different solvents: the
reactions gave low yields in 1,2-dichloroethane, pyridine, and
tetrahydrofuran, respectively (Table 1, entries 4, 9, and 10;
also see the Supporting Information). Further studies indi-
cated that the efficiency of this transformation decreased
when other ligands, such as PPh3, 2,2’-bipyridine, or 1,10-
phenanthroline were used (Table 1, entries 11–13). We envi-
sioned that the copper intermediate should have a flexible
structure that could readily dissociate with the ligand and
combine with a reactant. Compared with the copper/pyridine
complex, the copper/bipyridine complex is harder and more
inflexible; as such, the reaction with 9 mol% bipyridine gave
poor yield (Table 1, entries 4 and 11). The yields decreased
with higher or lower ligand loading (Table 1, entries 14 and
15). After extensive screening with the other reaction
parameters (see the Supporting Information), it was discov-
ered that 6 mol% CuBr, 18 mol% pyridine, in toluene at
608C under air were the optimized reaction condition (96%;
Table 1, entry 4).
diazonium salt with electron-rich aromatic compounds, can
be constructed using this oxidative dehydrogenative method
with H2O as the by-product (Table 3). Notably, even when 4-
bromo- or 4-chloro-aniline were employed as substrates,
which reacted with 1l to afford unsymmetric azo compounds
Table 3: Copper-catalyzed oxidative coupling of anilines for unsymmetric
azo compounds.[a]
Entry
1
R1
R2
Yield of 2 [%][b]
2
3
1h
1h
Under the optimized conditions, various aromatic azo
compounds were produced from their corresponding simple
and readily available amines. The results in Table 2 demon-
Table 2: Copper-catalyzed oxidative coupling of aniline.[a]
4
5
6
7
8
9
10
11
1l
1l
1l
1l
1l
1l
1l
1l
R’=H (1a)
R’=p-Me (1b)
R’=m-Me (1c)
R’=o-Me (1d)
R’=p-OCF3 (1 f)
R’=p-F (1i)
2la: 69%
2lb: 43%
2lc: 54%
2ld: 60%
2lf: 64%
2li: 53%
2lj: 73%
2lk: 73%
Entry
R1
Yield of 2 [%][b]
1
2
3
Ph (1a)
96 (2aa)
97 (2bb)
96 (2cc)
65 (2dd)
91 (2ee)
91 (2 ff)
84 (2gg)
66 (2hh)
97 (2ii)
R’=p-Cl (1j)
R’=p-Br (1k)
4-Me-C6H4 (1b)
3-Me-C6H4 (1c)
2-Me-C6H4 (1d)
4-Cy-C6H4 (1e)
4-CF3O-C6H4 (1 f)
2,4-Me2-C6H4 (1g)
4-OMe-C6H4 (1h)
4-F-C6H4 (1i)
4[c]
5
6
7[c]
8[c]
9
12
13
1m
1m
R’=H (1a)
R’=p-OCF3 (1 f)
2ma: 42%
2mf: 50%
10
11
12[c]
4-Cl-C6H4 (1j)
4-Br-C6H4 (1k)
4-COOEt-C6H4 (1l)
93 (2jj)
67 (2kk)
61 (2ll)
1
2
À
À
[a] Standard reaction conditions:
(0.2 mmol), CuBr (0.02 mmol), pyridine (0.06 mmol), toluene (4 mL),
O2 (1 atm), 24 h. [b] Yield of isolated product.
R
NH2 (1 mmol),
R
NH2
[a] Standard reaction conditions:
1
(1 mmol), CuBr (0.03 mmol),
pyridine (0.09 mmol), toluene (4 mL), air (1 atm), 20 h. [b] Yield of
isolated product. [c] The reaction was carried out under O2 (1 atm).
2lj and 2lk, respectively (73% yield; Table 3, entries 9 and
10). The copper-catalyzed Ullmann-amination reaction,[10]
À
strate that this reaction has a high degree of functional-group
tolerance. Both electron-rich (para, meta, and ortho substi-
tuted) and electron-deficient substrates were well-tolerated,
giving moderate to excellent yields. It is noteworthy that halo-
substituted anilines survived well, leading to halo-substituted
aromatic azo compounds (Table 2, entries 9–11), which could
be used for further transformations.
which results in the formation of a new C N bond, was not
observed in these cases, nor in Table 2, entries 10 and 11. The
homocoupling of anilines with electron-donating aryl sub-
stituents react faster than those of anilines with electron-
withdrawing aryl substituents. The kinetic experiments of 1b
and 1l clearly illustrated the different reaction rates (see the
Supporting Information). Therefore, a larger excess of
electron-poor anilines was employed to enhance the yield of
the cross-coupled diazo compounds.
Importantly, unsymmetrically substituted azobenzenes,
which are typically synthesized through reaction of the
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Communications
To probe the mechanism of this transformation, the
On the basis of the above results, a plausible mechanism
for the oxidative dehydrogenative coupling is illustrated in
Scheme 1. The copper(I) salt was initially chelated by a
pyridine ligand[6h,8] and oxidized by dioxygen to form the
more-active (m-h2:h2-peroxo)dicopper(II) complex A. Then, a
single-electron oxidation of aniline, mediated by the cop-
per(II) complex, into corresponding radical cation 3, followed
by subsequent coupling of 3 with 1a forms a three-electron
sigma bond 4,[2a,12] which consecutively donates two protons
and one electron leading to hydrazine 5. Hydrazine 5 is
further oxidized by the (m-h2:h2-peroxo)dicopper(II) complex
A or dioxygen to generate the corresponding aromatic azo
product.[2a,12]
In conclusion, we have developed a novel copper-cata-
lyzed approach to aromatic azo compounds, which are highly
valued chemicals and widely used in industry. Both symmetric
and unsymmetric substituted azobenzenes can be conven-
iently prepared by this method. Notably, air (or dioxygen), the
most environment friendly oxidant, was employed under mild
reaction conditions. Studies are ongoing in our laboratory to
understand the reaction mechanism and investigate further
synthetic applications.
reaction of 1a in the presence of 2.0 equivalents of a copper(I)
or copper(II) catalyst under N2 was investigated respectively
[Eq. (3); see Eq. S1 in the Supporting Information]. How-
ever, no product, such as 1,2-diphenylhydrazine, nitrosoben-
zene, or the expected aromatic azo 2aa was detected in both
of these reactions. We therefore postulated that the dioxygen
acted not only as the oxidant, but also as an initiator to trigger
this catalytic process. Enlightened by the growing amount of
information about copper(I)–dioxygen reactivity,[11] we
2
2
[11]
À
À
hypothesized that a (m h :h peroxo)dicopper(II) complex
A (Scheme 1) might be the active catalytic species, produced
Experimental Section
(E)-1,2-Diphenyldiazene (2aa).[13] Typical procedure: CuBr (4.2 mg,
0.03 mmol), pyridine (8.7 mg, 0.09 mmol), and aniline 1a (93 mg,
1 mmol) were mixed in toluene (4 mL) under air (1 atm). The
reaction mixture was stirred vigorously at 608C for 20 h. After cooling
down to room temperature and concentrating under vacuum, the
residue was purified by flash chromatography on a short silica gel
(eluent: petroleum ether) to afford 87.6 mg (96%) of 2aa; yellow
solid; 1H NMR (CDCl3, 400 MHz): d = 7.93–7.91 (m, 4H), 7.52–
7.44 ppm (m, 6H); 13C NMR (CDCl3, 100 MHz): d = 152.7, 131.0,
129.1, 122.8 ppm; Ms (70 ev): m/z (%): 182.1 (32) [M+], 77.1 (100); IR
Scheme 1. The proposed mechanism for the direct transformation.
(neat): n = 3418, 1581, 1481, 1452, 775, 688 cmÀ1
.
(E)-1-(4-Methoxyphenyl)-2-phenyldiazene (2ah). Typical proce-
dure: CuBr (2.9 mg, 0.02 mmol), pyridine (4.8 mg, 0.06 mmol), aniline
1a (93 mg, 1 mmol), and 4-methoxybenzenamine 1h (25 mg,
0.2 mmol) were mixed in toluene (4 mL) under an O2 atmosphere
(1 atm). The reaction mixture was vigorously stirred at 608C for 24 h.
After cooling down to room temperature and concentrating under
vacuum, the residue was purified by flash chromatography on a short
silica gel (eluent: petroleum ether/ethyl acetate = 200:1) to afford
21.2 mg (50%) of 2ah; yellow solid; 1H NMR (CDCl3, 400 MHz): d =
7.93 (d, J = 8.8 Hz, 2H), 7.88 (d, J = 7.2 Hz, 2H), 7.52–7.41 (m, 3H),
7.02 (d, J = 8.8 Hz, 2H), 3.89 ppm (s, 3H); 13C NMR (CDCl3,
100 MHz): d = 162.0, 152.8, 147.1, 130.3, 129.0, 124.7, 122.6, 114.2,
55.6 ppm; Ms (70 ev): m/z (%): 212.1 (64) [M+], 107.0 (100); IR (neat)
n = 2923, 2852, 1601, 1251, 1029, 840 cmÀ1; HRMS m/z (ESI) calcd for
C13H13N2O [M+H]+ 213.1022 found 213.1020.
in situ through the reaction of the Ln/CuI complex with O2.
Furthermore, nitrosobenzene was employed as the substrate
in the coupling reaction with ethyl 4-aminobenzoate 1l (see
Eq. S3 in the Supporting Information); however, no hetero-
coupling product was detected, which indicated that nitro-
sobenzene is not an intermediate of this oxidative process.
Interestingly, 1,2-diphenylhydrazine can easily be converted
into aromatic azo product 2aa (98% yield) under the
standard conditions [Eq. (4)]. We also investigated the
effect of copper and pyridine in this progress (see Table S6
Received: March 19, 2010
Published online: July 22, 2010
Keywords: azo compounds · copper · cross-coupling ·
.
ligand effects · oxidation
in the Supporting Information). It is noteworthy that pyridine
play an important role in this oxidation step. The fast
oxidation of 1,2-diphenylhydrazine into aromatic azo under
the standard conditions [15 min; Eq. (4)] indicates that this
step is not the rate-determining step in this transformation.
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