Phenyliodine(III) diacetate (PIDA) mediated synthesis of aromatic azo compounds through oxidative dehydrogenative coupling of anilines: Scope and mechanism

Article abstract of DOI:10.1002/ejoc.201301209

An efficient and environmentally benign method has been developed for the synthesis of symmetrical and unsymmetrical aromatic azo compounds through phenyliodine(III) diacetate (PIDA) mediated oxidative dehydrogenative coupling of anilines in high yields.

Full text of DOI:10.1002/ejoc.201301209

FULL PAPER
DOI: 10.1002/ejoc.201301209
Phenyliodine(III) Diacetate (PIDA) Mediated Synthesis of Aromatic Azo
Compounds through Oxidative Dehydrogenative Coupling of Anilines: Scope
and Mechanism
Kamarul Monir,[a] Monoranjan Ghosh,[a] Subhajit Mishra,[a] Adinath Majee,*[a] and
Alakananda Hajra*[a]
Keywords: Synthetic methods / Azo compounds / Oxidation / Dyes/pigments / Hypervalent compounds / Dimerization
An efficient and environmentally benign method has been
developed for the synthesis of symmetrical and unsymmetri-
cal aromatic azo compounds through phenyliodine(III) di-
acetate (PIDA) mediated oxidative dehydrogenative cou-
pling of anilines in high yields. The scope of the reaction is
broad for both homo- and cross-dimerization. A plausible re-
action mechanism has been proposed based on a structurally
characterized key intermediate, suggesting that ethanol is
involved in the reaction pathway. Fast reaction, metal-free
conditions, and functional-group tolerance are advantages of
this method.
pling for unsymmetrical azo compounds from aromatic
amines by using O2 (1 atm).[12] Recently He and co-workers
found that metal nanoparticles (Ag) are suitable for aerobic
oxidation of anilines to form azobenzenes.[13] Although
these methods have their own advantages, considering the
importance of modern aspects of organic chemistry, metal-
free synthesis of azobenzenes remains in clear demand.
Very recently, Minakata et al. reported efficient oxidative
homo- and cross-dimerization of anilines by tert-butyl hy-
poiodite (tBuOI).[14] The use of hypervalent iodine reagents
as oxidizing agents in organic synthesis has drawn consider-
able interest[15] because they are considered to be mild, se-
lective, and nontoxic oxidizing reagents that constitute
good alternatives to common metal oxidants such as CrVI,
HgII, TIIII, and PbIV, which are highly toxic. Among the
various iodine derivatives, phenyliodine(III) diacetate
(PIDA) has been used extensively to oxidize the nitrogen
atom in C–N[16] or N–N[17] bonds. However, the use of
PIDA-mediated intermolecular N–N bond forming reac-
tions to give azo compounds is very limited.[18] Very re-
cently, Ma and Lei reported the synthesis of azobenzenes
by using PIDA in dichloromethane (CH2Cl2) and proposed
a reaction mechanism involving radical intermediates.[19]
However, this method has only limited application for the
preparation of unsymmetrical azo compounds, and only 3-
ethynylbenzenamine has been applied as a cross-dimeriza-
tion partner.
Introduction
Aromatic azo compounds are widely used as organic
dyes, indicators, pigments, and food additives.[1] In addition,
they have potential applications in material and pharma-
ceutical chemistry.[2] The crucial importance of such com-
pounds has motivated chemists to develop syntheses in
more facile and smarter ways, and various synthetic meth-
ods have been developed over the past few decades.[3] Con-
ventionally, symmetrical azobenzenes are prepared by re-
duction of nitrobenzenes[4] and by oxidation of anilines.[5]
Usually, these oxidations are carried out by various oxi-
dants such as manganese salt,[6a] lead salt,[6b,6c] mercury
salt,[6d,6e] or ferrate,[6f,6g] which are not environmentally be-
nign. Recently, Li and Gu reported the synthesis of azo-
benzenes from nitrobenzene by using nano-Pd and nano-Pt
catalysts.[7]
The Mills[8] and Wallach reactions[9] are two protocols
for the synthesis of unsymmetrical azobenzenes. Another
method is the coupling of aryl diazonium salts with nucleo-
philes.[10] However, these are not facile reactions from the
synthetic point of view. In this regard, two elegant ap-
proaches have been reported that use molecular oxygen as
a green oxidant. Corma and co-workers reported the first
oxidation of anilines to azo compounds catalyzed by gold
nanoparticles using O2 (3–5 bar) at 100 °C.[11] Jiao and co-
workers developed copper-catalyzed dehydrogenative cou-
A review of the reported methods reveals that the full
potential of PIDA has not been explored for intermolecular
N–N bond formation through dehydrogenative coupling of
anilines to azo compounds. Considering the current intense
interest in hypervalent iodine in organic synthesis, develop-
ment of an efficient method that is applicable for both
[a] Department of Chemistry, Visva-Bharati (Central University),
Santiniketan, 731235, India
E-mail: alakananda.hajra@visva-bharati.ac.in
adinath.majee@visva-bharati.ac.in
www.visva-bharati.ac.in
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301209.
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PIDA-Mediated Synthesis of Aromatic Azo Compounds
homo- and cross-couplings, and studies on the associated tetrahydrofuran (THF), however, ethanol was the best
mechanistic path, are in demand. In a continuation of our among the solvents tested. Thus the use of 2 equiv. PIDA
studies on the use of hypervalent iodine derivatives as oxid- in ethanol (60 °C starting from –20 °C) was found to be the
izing reagents,[20] herein, we report a PIDA-mediated homo- optimized reaction conditions, the use of which afforded
and cross-dimerization of anilines to afford azobenzenes in 67% yield (Table 1, entry 1).
ethanol (Scheme 1).
After optimization of the reaction conditions, the sub-
strate scope of the reaction was explored by applying the
conditions to a library of substituted anilines; the results
are summarized in Table 2. It was found that anilines are
efficiently converted into azobenzenes within 30 minutes.
The present reaction conditions have a high degree of func-
tional-group tolerance, and substituted anilines with halo
(-F, -Cl, -Br, -I), nitro, and methoxy substituents produced
the desired products smoothly. Indeed, even a heteroaryl-
substituted aromatic amine with considerable steric bulk
also afforded the corresponding azo compounds in moder-
Scheme 1. PIDA-mediated synthesis of azobenzenes.
Results and Discussion
To optimize the reaction conditions, aniline was taken as ate yield (Table 2, entry 2). The electron-donating or elec-
a model substrate using PIDA as reagent and EtOH as sol- tron-withdrawing nature of the substituent played no sig-
vent (Table 1). Because the reaction is very exothermic, nificant role in this reaction, but the position of the substit-
PIDA was added portionwise to the reaction mixture keep- uent was important (Table 2, entries 6 vs. 9; 7 vs. 10). Gen-
ing the reaction vessel in an ice-salt bath (–20 °C). After erally, para-substituted anilines produced higher yields than
complete addition of PIDA, the reaction temperature was ortho-substituted anilines.
allowed to gradually increase and the progress of the reac-
Oxidative cross-dimerization between two different anil-
tion was monitored by TLC. It was found that the reaction ines is very challenging because of competing homodimer-
was complete within 30 minutes at 60 °C. The quantity of ization of the more reactive aniline (electron-rich), and usu-
PIDA was important in this transformation and it was ob- ally a large excess of the less reactive aniline (electron-poor)
served that the use of 2 equiv. PIDA was necessary to is used to obtain cross-coupling products. To explore the
achieve optimal yield. The effect of temperature was also general applicability of the present protocol, we have ap-
monitored. At higher temperature (80 °C) the yield of the plied it for the synthesis of unsymmetrical azobenzenes.
reaction decreased (Table 1, entry 3) and lower yield was
The nature of the solvent plays a major role in the pres-
obtained at 40 °C even after longer reaction time (Table 1, ent cross-dehydrogenative coupling reaction (Table 3).
entry 4). Other common solvents were screened, however, Highly selective cross-coupling product was obtained in
only a trace amount of the desired product was formed in ethanol by using 2.2 equiv. PIDA. Changing the solvent af-
N,N-dimethylformamide (DMF) and dimethyl sulfoxide fected the selectivity significantly. The formation of only a
(DMSO). The use of 1,2-dichlorobenzene (DCB), 1,2- trace amount of cross-coupling product was detected when
dichloroethane (DCE) and toluene were also not effective CH2Cl2 or THF were used as solvent (Table 3, entries 1 and
and afforded lower yields. Moderate yield was obtained in 2). Homodimerization occurred predominantly over cross-
dimerization for these cases. However, when a mixture of
THF and ethanol (1:1) was used, the formation of the
cross-dimerized product increased, and a similar result was
obtained when the reaction was carried out in a mixture of
CH2Cl2 and ethanol (1:1). These observations suggest that
ethanol plays a crucial role in determining the selectivity
for cross-dimerization. A larger amount of cross-dimerized
product was obtained in ethanol and, finally, the use of
1.2 equiv. of one aniline (1c) between two reacted anilines
was used to achieve the optimal yield under the present
reaction conditions (Table 3, entry 6). Ma and Lei also re-
ported that PIDA-mediated oxidative couplings between 3-
ethynylbenzenamine and anilines produced a mixture with
only detectable amounts of unsymmetrical azobenzenes in
CH2Cl2.[19] They proposed a mechanism involving radical
intermediates, however, tBuOI-mediated azo formation
from anilines does not proceed through the radical interme-
diate.[14]
Table 1. Optimization of the formation of azobenzene from aniline
by using PIDA.[a]
Entry
Solvent
Temp. (°C)
Time (min)
Yield (%)[b]
1
2
3
4
5
6
7
8
9
10
EtOH
EtOH
EtOH
EtOH
DCE
toluene
THF
DCB
DMSO
DMF
60
60
80
40
60
60
60
60
60
60
30
60
30
180
60
60
30
60
60
55
67
66
55
40
20
10
50
25
trace
trace
Gratifyingly, we have successfully synthesized various un-
symmetrical azobenzenes with moderate to high yields in
ethanol (Table 4). One notable advantage of this method is
[a] Reaction conditions: aniline (1.0 mmol), PIDA (1 mmol), sol-
vent (2 mL), starting from –20 °C to higher temperature. [b] Iso-
lated yield.
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K. Monir, M. Ghosh, S. Mishra, A. Majee, A. Hajra
Table 3. Effect of solvents on cross-dimerization.[a]
FULL PAPER
Table 2. Synthesis of symmetric azobenzenes under the optimized
reaction conditions.[a]
Entry Solvent
Amine
1a
(equiv.) (equiv.)
Amine
1c
Cross-coupling
product 4ac yield
(%)[b]
1
2
3
4
5
6
7
CH2Cl2
THF
THF/EtOH (1:1)
CH2Cl2/EtOH (1:1)
EtOH
1
1
1
1
1
1
1
1
1
1
1
1
trace
trace
25
28
51
EtOH
EtOH
1.2
1.5
56
57
[a] Reaction conditions: aromatic amine 1a (1.0 mmol), amine 1c
(1.0 mmol), PIDA (2.2 mmol), solvent (4 mL), –20 to 60 °C. [b] Iso-
lated yield.
that the reaction is highly efficient for cross-dimerization of
electron-rich as well as electron-deficient anilines, whereas
tBuOI is not effective for cross-dimerization of two elec-
tron-rich anilines.[14] Only a small excess (1.2 equiv.) of one
arylamine was required over the second aniline to achieve
the unsymmetrical azobenzene as a major product over
homodimerization. The formation of only a small amount
of homodimerized product was observed. Different combi-
nations of substituted anilines were subjected to the reac-
tion conditions and, in most cases, efficient cross-dimeriza-
tion was observed under the present reaction conditions. It
is interesting to note that combinations of electron-donat-
ing and electron-withdrawing substituents on the phenyl
ring did not change the selectivity of cross-dimerization.
Aniline reacted with p-toluidine smoothly under the present
reaction conditions, affording the corresponding cross-cou-
pled product with 56% yield. Similarly, this method was
efficient for cross-dimerization between p-anisidine and 3-
nitroaniline (54% yield), and ethyl 4-aminobenzoate suc-
cessfully coupled with p-toluidine (60% yield). Unsymmet-
rical azobenzenes with two different halo substituents were
also successfully synthesized. The observed cross-coupling
product was predominant over homo-coupling due to the
selective formation of nitrenium ions from two different an-
ilines. Hypervalent iodine reagents (PIDA) tend to preferen-
tially react with the more electron-rich aniline rather than
the electron-deficient, less-reactive aniline. Thus, the cross-
coupling seems to successfully occur in combinations of
electronically-differentiated anilines. Accordingly, a variety
of unsymmetrical azobenzenes were obtained by employing
this protocol.
A proposed mechanism for the oxidative dehydrogenative
coupling reaction is outlined in Scheme 2. Probably, the re-
action proceeds through the rapid formation of hydrazo-
benzene A via a nitrenium ion intermediate.[21] Intermediate
A reacts further with PIDA in the presence of EtOH, which
acts as a nucleophile, to form the corresponding phenyl-
[a] Reaction conditions: aromatic amine 1 (1.0 mmol), PIDA
(1 mmol), EtOH (2 mL), –20 to 60 °C. [b] Isolated yield.
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PIDA-Mediated Synthesis of Aromatic Azo Compounds
Table 4. Synthesis of unsymmetrical azobenzenes under the opti- hydrazine intermediate[21c] B at room temperature. The lat-
mized reaction conditions.[a]
ter intermediate has been successfully isolated and charac-
terized. Intermediate
B was subsequently aromatized
through elimination of EtOH by heating the reaction mix-
ture at 60 °C to form the corresponding azobenzene. In a
separate experiment, the isolated intermediate B was heated
at 60 °C in ethanol without PIDA and the desired azobenz-
ene 2cc was isolated in excellent yield (95%) [Scheme 2,
Equation (1)].
Scheme 2. Plausible reaction mechanism for the synthesis of azo-
benzene.
Further experiments were conducted to verify the pro-
posed mechanism (Scheme 3). When N-methylaniline was
treated with 1.1 equiv. PIDA, N,N-dimethylhydrazobenzene
was obtained in 75% yield in even shorter reaction time
[Equation (2)]. Furthermore, azobenzene was obtained di-
rectly from hydrazobenzene by the reaction of PIDA
[a] Reaction conditions: anilines
1
(1.0 mmol), anilines
3
(1.2 mmol), PIDA (2.2 mmol), EtOH (4 mL), –20 to 60 °C. [b] Iso-
lated yield.
Scheme 3. PIDA-mediated inter- and intramolecular oxidative re-
actions.
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K. Monir, M. Ghosh, S. Mishra, A. Majee, A. Hajra
FULL PAPER
Scheme 4. PIDA-mediated oxidation in the presence of radial scavenger.
(1.1 equiv.) [Equation (3)]. These observations indicate that both oxidation steps, 2 equiv. were required to achieve full
the reaction proceeds through the formation of intermedi- conversion.
ate A.
To gain more insight into the reaction mechanism, the
possibility of a radical reaction pathway as suggested by
Ma and Lei et al. was investigated. To this end, several radi-
Conclusions
cal-trapping experiments were carried out (Scheme 4).[19]
Two different types of radical scavengers, 1,1-diphenylethyl-
We have developed a straightforward, fast, and efficient
ene and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) procedure for the synthesis of aromatic azo compounds by
were used for this purpose but no effect was observed in using the commercially available, environmentally benign
presence of 2 equiv. of either for the formation of the corre- organic oxidant, PIDA under an open atmosphere. This
sponding azobenzene (2aa) from aniline (1a). Moreover, ad- method tolerates a wide range of functionalities, especially
ditional experiments were conducted on PIDA-mediated re- iodo- and bromo-derivatives, which have high potential for
actions that were carried out by reacting N-methylaniline coupling reactions. Moreover, we could conveniently syn-
(5a) in the presence of radical scavengers in ethanol, how- thesize unsymmetrical azobenzenes, which has been an im-
ever, neither scavenger retarded the reactions. Similarly, mense synthetic challenge for a long time. A plausible reac-
when the reaction was carried out with 1,2-diphenylhydra- tion mechanism has been proposed on the basis of an iso-
zobenzene (A) in presence of scavengers, the yield of azo- lated reaction intermediate and on the results of other ex-
benzene remained almost unchanged. In addition, these re- periments, suggesting ethanol is involved in this coupling
actions proceeded efficiently under an O2 atmosphere. reaction and plays a vital role in making this protocol
These observations do not support the formation of a radi- highly efficient for cross-dimerization. We envision that our
cal intermediate in EtOH. Isolation of intermediate B findings will open a new arena of PIDA-mediated oxidative
strongly supports the mechanism as proposed in Scheme 2. coupling and render this approach attractive for academic
Finally, it should be noted that because PIDA took part in and industrial use.
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PIDA-Mediated Synthesis of Aromatic Azo Compounds
(grant number SR/S5/GC-05/2010). K. M. thanks the Council of
Scientific Research (CSIR), New Delhi, and M. G. and S. M. thank
the University Grants Commission (UGC), New Delhi for their
fellowships. The authors are also thankful to the Department of
Science and Technology – Fund for Improvement of Science and
Technology and University Grants Commission – Special Assist-
ance Programme (DST-FIST and UGC-SAP) for their support in
improving the infrastructural facility. The authors thank the re-
viewers for their constructive suggestions.
Experimental Section
Typical Procedure for the Synthesis of (E)-1,2-Di-p-tolyl-diazene
(2cc): In a round-bottomed flask fitted with a cap, p-toluidine (1b,
107 mg, 1.0 mmol) was dissolved in ethanol (2 mL) and cooled in
an ice-salt mixture at –20 °C under an open atmosphere. PIDA
(1 mmol) was added portionwise during 5 min and the reaction
mixture was then allowed to reach room temperature and then
heated to 60 °C for 25 min. The progress of the reaction was moni-
tored by TLC. Upon completion of the reaction, ethanol was evap-
orated and the residue was purified by column chromatography
over silica gel (petroleum ether) to furnish the corresponding aro-
matic azo compound (75 mg, 71%)[12] as a yellow solid; m.p. 137–
140 °C. 1H NMR (400 MHz, CDCl3): δ = 7.73 (d, J = 8.0 Hz, 2
H), 7.22 (d, J = 8.4 Hz, 2 H), 2.35 (s, 3 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 150.8, 141.1, 129.6, 122.7, 21.4 ppm.
[1]
a) H. Zollinger, Color Chemistry: Synthesis Properties and Ap-
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1987, p. 85; b) K. Hunger, Industrial Dyes: Chemistry, Proper-
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P. F. Gordon, P. Gregory, Organic Chemistry in Colour,
Springer, NY, 1983, p. 95.
(E)-1-(4-Chlorophenyl)-2-(4-methoxyphenyl)diazene (4dh): A mix-
ture of p-toluidine 1b (107 mg, 1.0 mmol) and p-chloroaniline 1h
(152.4 mg, 1.2 mmol), in a round-bottomed flask fitted with a cap,
was dissolved in ethanol (4 mL) and cooled in a ice-salt mixture at
–20 °C under an open atmosphere. PIDA (2.2 mmol) was added
slowly during 5 min, then the reaction mixture was allowed to reach
room temperature and heated at 60 °C for another 25 min. The
progress of the reaction was monitored by TLC. Upon completion
of the reaction, ethanol was evaporated and the residue was puri-
fied by column chromatography on silica gel (ethyl acetate/petro-
leum ether, 1:9) to furnish the corresponding azo compound
(133 mg, 53%). 1H NMR (400 MHz, CDCl3): δ = 7.93 (d, J =
8.8 Hz, 2 H), 7.85 (d, J = 8.8 Hz, 2 H), 7.48 (d, J = 8.8 Hz, 2 H),
7.03 (d, J = 9.2 Hz, 2 H), 3.91 (s, 3 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 162.2, 151.0, 146.7, 136.1, 129.2, 124.8, 123.7, 114.2,
55.5 ppm. C13H11ClN2O (246.70): calcd. C 63.29, H 4.49, N 11.36;
found C 63.10, H 4.35, N 11.15.
[2]
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[4]
[5]
[6]
Isolation of Intermediate B [N-(4-Ethoxy-4-methylcyclohexa-2,5-di-
enylidene)-NЈ-p-tolylhydrazine] (B): p-Toluidine (1c, 107 mg,
1.0 mmol) was taken in a sealed tube, dissolved in ethanol (2 mL)
and kept in an ice-salt mixture at –20 °C under an open atmo-
sphere. PIDA (1.0 mmol) was added slowly during 5 min, then the
reaction mixture was allowed to reach room temperature and kept
for 1 h. The progress of the reaction was monitored by TLC. Upon
completion of the reaction, ethanol was evaporated by rotary evap-
orator. The residue was purified by column chromatography over
silica gel (petroleum ether/ethyl acetate, 10:1) to furnish the corre-
sponding intermediate B (68 mg, 53%). 1H NMR (400 MHz,
CDCl3): δ = 7.13 (d, J = 7.6 Hz, 2 H), 6.73 (d, J = 8.0 Hz, 2 H),
6.54 (dd, J = 2.0, 10 Hz, 1 H), 6.42 (dd, J = 2.0, 10.4 Hz, 1 H),
6.36 (dd, J = 2.4, 10 Hz, 1 H), 6.25 (dd, J = 2.4, 10 Hz, 1 H), 3.39–
3.27 (m, 2 H), 2.34 (s, 3 H), 1.40 (s, 3 H), 1.16 (t, J = 7.2 Hz, 3
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 156.4, 147.3, 145.0,
143.0, 133.6, 131.0, 129.3, 120.9, 120.4, 72.5, 60.4, 27.4, 20.8,
15.9 ppm. C16H20N2O (256.35): calcd. C 74.97, H 7.86, N 10.93;
found C 74.86, H 7.98, N 10.9.
[7]
[8]
[9]
All products were characterized on the basis of their spectral (1H
and 13C NMR) and analytical data.
[10]
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details, product characterization, and copies of
1H and 13C NMR spectra of all synthesized compounds.
[11]
[12]
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Received: August 10, 2013
Published Online: December 3, 2013
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