Selective N-oxidation of aromatic amines to nitroso derivatives using a molybdenum acetylide oxo-peroxo complex as catalyst

Article abstract of DOI:10.1016/j.tetlet.2008.04.005

The molybdenum acetylide oxo-peroxo complex obtained in situ by the treatment of the corresponding molybdenum acetylide carbonyl complex, CpMo(CO)₃(C{triple bond, long}CPh); Cp = η⁵-C₅H₅ with H₂O₂, has been used as an efficient catalyst for selective N-oxidation of primary amines to nitroso derivatives. Excellent amine conversion (up to 100%) and very high selectivity for nitroso compounds (99%) have been obtained using 30% hydrogen peroxide as an oxidant. The oxo peroxo Mo(VI) complex has also been found to be very active for the oxidation of various substituted primary aromatic amines with electron donating as well as electron withdrawing substituents on the aromatic ring.

Full text of DOI:10.1016/j.tetlet.2008.04.005

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 3616–3619
Selective N-oxidation of aromatic amines to nitroso derivatives
using a molybdenum acetylide oxo-peroxo complex as catalyst
Ankush V. Biradar, Trupti V. Kotbagi, Mohan K. Dongare, Shubhangi B. Umbarkar *
Catalysis Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
Received 25 February 2008; revised 27 March 2008; accepted 1 April 2008
Available online 4 April 2008
Abstract
The molybdenum acetylide oxo-peroxo complex obtained in situ by the treatment of the corresponding molybdenum acetylide
carbonyl complex, CpMo(CO)3(C„CPh); Cp = g5-C5H5 with H2O2, has been used as an efficient catalyst for selective N-oxidation
of primary amines to nitroso derivatives. Excellent amine conversion (up to 100%) and very high selectivity for nitroso compounds
(99%) have been obtained using 30% hydrogen peroxide as an oxidant. The oxo peroxo Mo(VI) complex has also been found to be very
active for the oxidation of various substituted primary aromatic amines with electron donating as well as electron withdrawing
substituents on the aromatic ring.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Molybdenum complex; Oxo-peroxo; Aromatic amine; Oxidation; Homogenous catalyst; Nitroso; Hydrogen peroxide
Catalytic oxidation of amines is an important functional
transformation in organic synthesis.1,2 Amongst the possi-
ble amine oxidation products (hydroxyl, nitroso, nitro, azo
and azoxy), aromatic nitroso compounds are utilized
extensively as chemical feedstocks for a wide range of use-
ful materials such as dyes, pharmaceuticals, perfumes and
plastics.3 Oxidation of amines pose problems due to nonre-
gioselective or over-oxidation of amines and competitive
oxidation of substrates. General methods for the prepara-
tion of nitroso compounds have been addressed by Gowen-
lock et al.4 Several organo catalysts5,6 as well as metal
catalysts7 are reported for the oxidation of amines. How-
ever, mainly the nitro compounds are obtained in many
cases. FeCl2–Py4 complex is reported to give selectively
nitroso or nitro product depending on the oxidant used
and also the oxidant used is a limiting agent leading to a
very low TON for the product.8 TS-1 is reported to give
high selectivity (73%) for nitroso compound at very low
conversions (7.3%).9
Amongst the early transition-metal complexes, molyb-
denum(VI) complexes have functional as well as structural
similarity with molybdo-enzymes and have the ability to
catalyze a variety of oxidation reactions.10,11 Various
molybdenum organometallic complexes and oxides are
known to be very good homogeneous and heterogeneous
catalysts for the oxidation/epoxidation reactions.12–14
Preyssler’s catalysts (W and mixed W/Mo) have been
reported to oxidize anilines with maximum selectivity
(82%) for the azoxy compound.15 Several Mo catalysts
with acac16 or hmpa17 ligands as well as Mo- and W-based
coordination polymers18 have been used for aniline oxida-
tion. Recently, we have used molybdenum acetylide com-
plex, CpMo(CO)3(C„CPh); Cp = g5-C5H5 complex, (1)
as oxidation catalyst for cis-dihydroxylation of various ole-
fins.19 In continuation of our efforts to find out the catalytic
activity of this catalysts for the oxidation of other sub-
strates, selective N-oxidation of amines to nitroso deriva-
tive has been investigated. To our knowledge there are no
reports on the use of a molybdenum acetylide complex
for the oxidation of amines. This study is particularly inter-
esting since it offers the possibility of extending the usually
*
Corresponding author. Tel.: +91 20 25902044; fax: +91 20 25902633.
E-mail address: sb.umbarkar@ncl.res.in (S. B. Umbarkar).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.04.005

A. V. Biradar et al. / Tetrahedron Letters 49 (2008) 3616–3619
3617
Table 1
versatile catalytic potential of organometallic complexes to
aqueous media.
Aniline oxidation in various solventsa
Solvent
Conv. (%)
Selectivity (%)
TONb
The molybdenum acetylide complex, CpMo(CO)3-
(–C„CPh), was prepared according to the literature
method.20 The oxidant used was hydrogen peroxide and
hence to confirm the stability of the catalyst and to study
the nature of the catalytically active species under the reac-
tion conditions, catalyst 1 was treated with H2O2 and char-
acterized by FTIR. On treatment of complex 1 with 30%
aqueous hydrogen peroxide, the corresponding oxo peroxo
Mo(VI) acetylide complex was formed after loss of the car-
bonyl ligands (Scheme 1).
Nitroso Azo Azoxy
1
2
3
4
5
6
t-BuOH
MeOH
CH2Cl2
CCl4
Toluene
CH3CN
97
95
96
97
84
72
97
90
80
84
83
90
2
7
14
11
12
7
1
3
6
5
5
3
470
428
384
407
349
324
a
Reaction conditions: PhNH2 0.01 mol; 30% H2O2 0.02 mol, solvent
10 g, catalyst 1 0.02 mmol; time 12 h; temp. rt.
b
TON for nitroso derivative = moles of nitroso product formed per
The FTIR spectrum of 1 after the addition of H2O2 con-
firmed the formation of the oxo peroxo species. The IR
band at 953 cmÀ1 indicates the presence of a Mo@O termi-
nal bond. The band at 858 cmÀ1 corresponds to the O–O
stretching vibration of the peroxo species. The weak bands
at 664 and 577 cmÀ1 can be assigned to the Mo–O2 (per-
oxo) asymmetric and symmetric stretching vibrations,
respectively. The IR positions for the Mo oxo and peroxo
moieties are in good agreement with the literature reports
for various Mo oxo peroxo complexes.21 The acetylide
moiety remained intact even after the addition of H2O2,
as confirmed by broad low intensity band centred at
2100 cmÀ1 assigned to m(C„C). The bands due to C–H
stretching vibrations of the phenyl ring were observed in
the range 2854–2955 cmÀ1 and the C@C stretching vibra-
mole of catalyst.
in t-BuOH at rt (Table 1, entry 1) to form single phase sys-
tem with aqueous oxidant and organic substrates and
product. Very high aniline conversion (97%) and excellent
selectivity for nitroso benzene (97%) were obtained. When
other solvents were tested for the same reaction, alcohols
were found to be better with high conversion and selectivity
for the nitroso compound. In the case of the chlorinated
solvents, the conversion was high, however, the selectivity
for the nitroso product decreased to 80% and 84%, respec-
tively (entries 3 and 4).
Comparatively lower conversions were obtained in the
case of toluene and acetonitrile as a solvent (Table 1,
entries 5 and 6). In all the cases, the turnover number
(TON) for the nitroso compound was very high (up to
470) compared to the TON reported previously using W
and Mo based coordination polymers (TON < 10),18
[Mo(O2)(O)(H2O)(hmpa)] (TON ꢀ 9)17 or Mo(O)2(acac)2
(TON < 10).16 This shows the high efficiency of catalyst 1
for selective aniline oxidation.
tions of the ring were observed at 1464 and 1377 cmÀ1
.
The bands due to the carbonyl stretching vibrations
(1940, 2031 cmÀ1) disappeared after the addition of
H2O2. This clearly indicates the elimination of all the CO
ligands and the formation of a higher oxidation state
Mo(VI) complex with the retention of the acetylide moiety
attached to the Mo centre.
The influence of temperature on aniline conversion and
product selectivity was also examined (Table 2). When the
reaction temperature was increased gradually from rt to
100 °C, the rate of the reaction increased and the product
distribution changed significantly. The reaction was com-
plete in 12 h at rt (entry 1), whereas at 100 °C it was com-
plete in only 4 h (entry 5); however, the selectivity for the
nitroso compound was maximum (97%) at rt which
decreased to only 8% at 100 °C. At higher temperatures
azo and azoxy compounds were formed as major products
compared to nitroso derivative. Formation of the nitro
product was also observed at high temperatures.
The applicability of the oxo peroxo Mo(VI) catalyst for
amine oxidation was initially investigated for aniline oxida-
tion (Scheme 2).22 Based on our previous experience,19 the
liquid-phase catalytic oxidation of aniline was carried out
H2O2, RT
Ph
Ph
C
C
Mo
C
C
Mo
O
- 3CO
O
OC
O
CO
CO
1
Considering the good conversion and highest selectivity
for aniline oxidation in t-butanol at rt, catalyst 1 was tested
Scheme 1. In situ generation of the oxo-peroxo Mo(VI) acetylide
complex.
N
O
RT
R
Cat
-H2O
NH2
+ H2O2
O-
N
R
60 ºC
N
N
N
+
R
R
R
R
Scheme 2. Oxidation of primary aromatic amines.

3618
A. V. Biradar et al. / Tetrahedron Letters 49 (2008) 3616–3619
Table 2
Mo/W-based coordination polymers were used for p-nitro-
Effect of temperature on aniline oxidationa
aniline oxidation, the yield dropped drastically from 82%
for aniline to 20% for p-nitroaniline.18 Similar drastic
reduction in yield was reported for [Mo(O2)(O)(-
H2O)(hmpa)] (77% for aniline and 10% for p-nitroani-
line).17 When Mo(O)2(acac)2 was used as a catalyst no
catalytic activity was observed with anilines with electron
withdrawing substituents on the aromatic ring compared
to a 77% yield for aniline using the same catalyst.16 In
the present case, the conversion was marginally decreased
from 97% for aniline to 85% for p-nitroaniline with almost
similar selectivity for nitroso compound (97% for aniline
and 95% for p-nitroaniline).
These results show that molybdenum acetylide complex
1 is a very efficient catalyst for the oxidation of variety of
primary aromatic amines with excellent selectivity for the
nitroso compound. It tolerates electron rich as well as elec-
tron withdrawing substituents on aromatic ring demon-
strating the wider applicability of this catalyst for
selective oxidation of primary aromatic amines to nitroso
derivatives.
Molybdenum acetylide complex CpMo(CO)3(C„CPh)
was used for the first time for the selective oxidation of pri-
mary aromatic amines to nitroso derivatives with very high
conversion and selectivity for the nitroso compounds. In
situ generated oxo-peroxo molybdenum acetylide complex
proved to be the catalytically active species. A variety of
substituted anilines were successfully oxidized to the corre-
sponding nitroso compounds under very mild conditions
with very high TON.
Temperature
(°C)
Time
(h)
Conv.
(%)
Selectivity (%)
Nitroso Azo Azoxy Nitro
1
2
3
4
5
rt
12
10
6
4
4
97
99
99
99
99
97
80
56
24
8
2
14
18
25
30
1
6
21
43
47
0
0
5
8
15
40
60
80
100
a
Reaction conditions: aniline 0.01 mol; 30% H2O2 0.02 mol; t-BuOH
10 g; catalyst 1 0.02 mmol.
for the oxidation of a variety of substituted aromatic
amines under these conditions and the results are summa-
rized in Table 3. Amongst the substrates tested, the best
results were obtained in the case of p-methylaniline with
100% conversion and 99% selectivity for the nitroso com-
pound at rt. High conversions and selectivity for nitroso
compounds were obtained for activated and electron rich
anilines (entries 2 and 4); however, the conversion
decreased for sterically hindered 2,6-dimethylaniline
though with very high selectivity (99%) for the nitroso
product. Halogenated anilines (entries 9–12) were oxidized
successfully with a high conversion and a very high selectiv-
ity for the nitroso compound. In the case of hydroxyani-
lines the conversion decreased slightly but the selectivity
remained very high (98%) (entries 7 and 8). In the case of
all amines tested, very high selectivity for nitroso com-
pound was obtained. The TON for nitroso compound in
all the cases is high with the maximum TON of 495 for
p-methylaniline which is much higher than that reported
previously with different molybdenum/tungsten-based cat-
alysts (<10).16–18
Acknowledgements
It is interesting to note that p-nitroaniline was also oxi-
dized successfully using catalyst 1. The conversion as well
as selectivity for p-nitroaniline oxidation did not decrease
significantly compared to aniline. In earlier reports, when
S.B.U. acknowledges the financial support from the
Department of Science and Technology, and A.V.B.
acknowledges the Council of Scientific and Industrial
Research for a fellowship.
Table 3
Oxidation of various aminesa
Anilines, substituents=
Conv. (%)
Selectivity (%)
TONb
Nitroso
Azo
Azoxy
Nitro
1
2
3
4
5
6
7
8
9
10
11
12
13
14
a
—
p-Me
2,6-Dimethyl
p-OCH3
o-Phenylene diamine
p-Phenylene diamine
o-OH
p-OH
o-Cl
m-Cl
o-F
m-F
p-NO2
Benzyl amine
97
100
75
83
77
84
73
75
60
72
79
70
85
80
97
99
99
97
98
99
98
98
96
99
79
74
95
75
2
1
1
2
1
1
1
1
2
0
8
10
2
12
1
0
0
1
1
0
1
1
2
0
0
0
0
0
0
0
0
0
0
2
2
2
6
470
495
371
403
371
416
358
368
288
356
312
259
404
300
1
11
14
1
7
Reaction conditions: amine 0.01 mol; oxidant 30% H2O2 0.02 mol; solvent t-BuOH 10 gm; catalyst 1 0.02 mmol; temperature rt, time 12 h.
TON for nitroso derivative = moles of nitroso product formed per mole of catalyst.
b

A. V. Biradar et al. / Tetrahedron Letters 49 (2008) 3616–3619
3619
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hydrogen peroxide (0.02 mol), 10 g solvent and catalyst
1
(0.02 mmol). The reaction mixture was stirred at rt for 12 h until
the completion of the reaction. The reaction was monitored using GC
(Agilent 6890 Gas Chromatograph equipped with a HP-5 dimethyl
polysiloxane, 60 m length, 0.25 mm internal diameter, 0.25 lm film
thickness) column with flame ionization detector. Products were
confirmed using GC–MS (Model GC Agilent 6890N with HP5 MS
30 m capillary column, MS Agilent 5973 Network MSD) and GC–IR
(Perkin Elmer Spectrum 2001, column DB-1, 25 m length, 0.32 mm
internal diameter).
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