Simple preparation of nitroso benzenes and nitro benzenes by oxidation of anilines with H2O2 catalysed with molybdenum salts

Article abstract of DOI:10.1055/s-2004-815964

Nitroso arenes 2a-k are prepared in 53-80% yield from anilines 1a-k by oxidation with H₂O₂ catalysed with MoO₃/KOH, ammonium molybdate or other molybdenum salts. Further oxidation to nitro arenes 3a,d j in 66-90% is also described.

Full text of DOI:10.1055/s-2004-815964

706
PAPER
Simple Preparation of Nitroso Benzenes and Nitro Benzenes by Oxidation of
Anilines with H2O2 Catalysed with Molybdenum Salts
Nitroso Benzenes
l
a
nd Nit
b
ro
B
enzene
s
e
by
O
xidati
r
on of
A
n
t
ilines Defoin*
Laboratoire de Chimie Organique et Bioorganique, UMR 7015, Ecole Nationale Supérieure de Chimie de Mulhouse, Université de Haute-
Alsace, 3, rue Alfred Werner, 68093 Mulhouse Cédex, France
Fax +33(3)89336875; E-mail: A.Defoin@uha.fr
Received 9 January 2004
the previous two-phase preparation.4,8,9,11 A complete ox-
Abstract: Nitroso arenes 2a–k are prepared in 53–80% yield from
idation and a sufficient reaction rate required an excess of
H2O2 (4 equiv) and 0.1 mol of molybdic derivative per 1
anilines 1a–k by oxidation with H2O2 catalysed with MoO3/KOH,
ammonium molybdate or other molybdenum salts. Further oxida-
mol of aniline, smaller proportions of oxidant or catalyst
led to slower or incomplete reactions and favoured the
formation of azo- azoxy- or coloured-compounds.
tion to nitro arenes 3a,d,j in 66–90% is also described.
Keys words: nitroso arenes, nitro arenes, oxidation, molybdenum
salts
NH2
NH2
NH2
H2O2/EtOH
[MoO3]
H2O2/EtOH
[MoO3]
Classic methods for the preparation of aromatic nitroso
compounds are oxidation of anilines with peracids,1 par-
ticularly Caro acid,2 peracetic acid,1 perbenzoic acid,3,4
trifluoroperacetic acid5 or more recently molybdic per-
oxo-complexes.6 Catalytic methods using H2O2 or tert-
butyl peroxide as oxidant and sodium tungstate,7 oxo-
molybdate complexes,8 phosphotungstate,9–11 phospho-
molybdate10,11 or zirconate salts12 were published later.
Dimeric products such as azo- or azoxy-derivatives9,13a as
well as over-oxidation to the nitro compounds7,10,11 were
often observed as by-products or sometimes as main prod-
ucts.6,14 Some methods for the direct oxidation of aromatic
amines to nitro derivatives were described with perac-
ids,15 and more recently with dioxirane,16 tert-butyl perox-
ide or H2O2 as oxidants and metal peroxocomplexes
0 °C or r.t.
60 °C
R
R
R
1
2
3
Scheme 1
a) Dependence on the pH–Value
p-Chloroaniline 1a was used as the model compound and
the standard catalyst was 10 mol% MoO3 in the presence
of KOH as the base, which was added in various quanti-
ties. Oxidation of p-chloroaniline is very dependent on
MoO3/KOH molar ratio. The results are listed in Table 1,
and summarised here.
(Mo,17 W,6,10,11 V,17 Zr,12 Re18) or with chromium – With MoO3 only without added base, the oxidation oc-
silicalit13 as catalyst.
curred cleanly, but the progress of the reaction was slow
(15% reaction after 1 d).
We describe herein a simple oxidation method of anilines,
resulting in the corresponding nitroso compounds using – With MoO3 and 1–1.8 equiv KOH, the oxidation rate in-
30% H2O2 and molybdenum trioxide or other molybde- creased with the concentration of base, but coloured im-
num salts as catalyst without the formation of azo- or purities were formed when 1.5–1.8 equiv of base (pH
azoxy-derivatives. Further oxidation to nitro compounds value 5–7) were added. The best condition for the oxida-
was examined in some cases.
tion to the nitroso compound was with 1–1.2 equiv KOH,
then the oxidation occurred cleanly and gave good yields
(86–94%) of the nitroso compounds. Observing the reac-
tion, the solution turned from red at the beginning to yel-
low after 1 hour and the pH value varied with the
Oxidation of Anilines to Nitroso Arenes
Oxidation of anilines 1a–k was carried out in a water– consumption of aniline; from pH 6 to pH 5 after 1 hour,
methanol solution in which the anilines were only slightly pH 4–4.5 after 3 hours and pH 3–4 at the end of the oxi-
soluble but the resulting nitroso compounds 2a–g,j,k were
insoluble and could be isolated by filtration (Scheme 1).
Side-reactions such as formation of nitro derivatives and
dimeric compounds such as azo- or azoxy-derivatives was
also minimised. This method appeared to be better than
dation.
– With Na2MoO4 as catalyst, which corresponds to 2
equivalents of base, the pH-value was 6–7, the solution re-
tained its red colour with emission of oxygen and the con-
version was incomplete.
It is possible to specify the major molybdenic species in
solution from Aubry’s and Griffith’s work.19–21 The rela-
tionship between the structure of monomeric or dimeric
peroxomolybdic acids and pH values in aqueous solution
SYNTHESIS 2004, No. 5, pp 0706–0710
Advanced online publication: 10.02.2004
DOI: 10.1055/s-2004-815964; Art ID: Z00504SS
© Georg Thieme Verlag Stuttgart · New York
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Nitroso Benzenes and Nitro Benzenes by Oxidation of Anilines
707
was already studied: under the optimal conditions for ox- In practice, ammonium molybdate proved to be the most
idation to nitroso compounds (H2O2 concentration ca. 3 suitable, the reaction rate is faster than with MoO3/KOH.
M, MoO3 concentration ca 7 × 10–2 M), the major species
(2–)
was the yellow anion Mo2O3(O2)4 (pH< 5); while at
c) Generalisation of the Reaction
pH> 5 the unstable red anion Mo(O2)4(2–), which easily
lost molecular oxygen was present.19,20 It seems then that The oxidation of several p-substituted anilines was per-
Mo2O3(O2)4(2–) is the main reactive species for this oxida- formed using mainly standard conditions (10% molar
tion at pH 3–5. This Mo-peroxide had already been used MoO3 and KOH). The results are reported in Table 1.
at this pH range to oxidise alcohols to ketones20 or to Yields were generally good (80–90%). The reaction time
epoxidise20,21 unsaturated compounds.22
varied from 6 hours to some days according to the aryl-
substitution and solubility of the aniline. In most cases, a
simple filtration provided pure nitroso compounds. Some-
times partial over-oxidation to nitro derivative was ob-
b) With other Molybdenum Salts as Catalyst
It was possible to use other molybdenum salts as catalysts served and the standard conditions had to be modified:
and the resulting yields of nitroso compounds were the
– aniline 1d (R = H) and toluidine 1e (R = Me), the reac-
same so long as the ratio molybdenum/base was near 1
tion was carried out at 0 °C.
(complex salts were related as their MoO3 equivalents).
– p-methoxy derivatives 1h (R = OMe), the nitroso com-
pound is a green monomeric species which is soluble in
the reaction medium. The reaction was performed at 0 °C
The results given in Table 1 and summarized below.
– MoO3 and 1.0 equivalents of NaOH or KOH (pH 6 to 4–3)
– Na2MoO4 and 0.5 equivalents of H2SO4 (pH 6 to 4–3)
– 0.143 equivalents of ammonium
(NH4)6Mo7O24·4H2O (pH 2)
in two phases with hexane.
molybdate – p-nitro derivatives 1j,k, very good results were obtained
only with MoO3 as catalyst without base (pH value 1–
1.5). The excellent result validates the long reaction time
– 0.083 equivalents of phosphomolybdic acid Mo12O40PH3
and 1.0 equivalents of NaOH (pH 5 to 4–3)
(almost 1 week).
Table 1 Oxidation of Anilines 1a–k to Nitroso Compounds 2a–k
R
Compound
Catalyst (10 mol%)
Conditions
25 °C, 10 h
25 °C, 11 h
25 °C, 6 h
25 °C, 3 h
25 °C, 3 h
Crude Yield
87–94%
94%
Pure Yielda
p-Cl
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1f
MoO3, KOH (1 mmol)
78–84% (EtOH)
MoO3, NaOH (1 mmol)
MoO3, KOH (1.2 mmol)
(NH4)6Mo7O24·4H2O (0.143 mmol)
MoO4Na2, H2SO4 (0.5 mmol)
89%
92%
92%
Mo12O40PH3 (0.083 mmol), KOH (1 mmol)
MoO3, KOH (1 mmol)
MoO3, KOH (1 mmol)
MoO3, KOH (1 mmol)
MoO3, KOH (1 mmol)
MoO3, KOH (1 mmol)
MoO3, KOH (1 mmol)
MoO3, KOH (1 mmol)
MoO3
25 °C, 8 h
15–20 °C,
25 °C, 43 h
0 °C, 1 d
92%b
81%
4-Cl, 3-Me
p-Br
77% (MeOH)
87% (EtOH)
65% (MeOH)
97%
H
77–85%
85%c
p-Me
0 °C, 16 h
25 °C, 4 d
25 °C, 4 d
0 °C, 16 h
25 °C, 10 d
25 °C, 7 d
p-PhCO
p-CO2Me
p-OMe
p-NO2
98%d
91–94%
63%
78% (EtOH)
86% (EtOAc)
53% (hexane)
80% (EtOH)
72% (MeOH)
1g
1h
1j
96%
4-NO2, 3-Me
1k
MoO3
89%
a After recrystallisation.
b Darker product, containing ca. 4% impurities.
c Containing ca. 5% nitro derivative.
d Containing 15% nitro derivative.
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708
A. Defoin
PAPER
Table 3 Direct Oxidation of Anilines 1a,d,j to Nitro Derivatives
3a,d,j
Preparation of Nitro Compounds
At a higher temperature (60 °C), nitroso arenes became
partially soluble in methanol–H2O2 solution and were ox-
idised to the corresponding nitro derivatives, without de-
composition of the oxidant. It is therefore possible to
oxidise directly anilines to nitro compounds in two steps,
solely by raising the temperature and without isolation of
the intermediary nitroso compounds. These oxidations
were only studied in the cases R = H and its p-chloro and
p-nitro derivatives.
R
Catalyst (10 mol%)
Conditions
Yield
p-Cl
1a
1d
1j
(NH4)6Mo7O24·4H2O
(0.143)
4.5 h, 20 °C 92%a
1 d, 60 °C
H
(NH4)6Mo7O24·4H2O
(0.143)
24 h, 0 °C
6 h, 60 °C
66%b
p-NO2
(NH4)6Mo7O24·4H2O
(0.143)
7 d, 25 °C
1 d, 60 °C
68%c
a Containing 3% nitroso derivative.
b Purified by distillation (Eb6 = 69 °C)
c Purification by sublimation (5–10 Torr at 125 °C) containing 5% ni-
troso derivative.
From Nitroso Compounds
Oxidation of nitroso compounds 2a,d,j was performed
with ammonium molybdate (10 mol% MoO3) as catalyst
in methanol and 30% H2O2 (4 equiv) at 60 °C. In all cases,
the corresponding nitro derivatives are obtained in good
yields (75–90%; Table 2) after purification by distillation
for nitrobenzene 3d or by sublimation for the p-dini-
trobenzene 3j.
Conclusion
We have described the oxidation of anilines to nitroso de-
rivatives and further oxidation to nitro derivatives using
30% H2O2 and molybdic salts as the catalyst. The result-
ing yields of pure compounds are good to excellent (53–
90%) after purification. Due to their simple work-up pro-
cedure, these methods can be used for large-scale synthe-
sis.
Table 2 Oxidation of Nitroso Compounds 2a,d,j to Nitro Deriva-
tive 3a,d,j
R
Catalyst (10 mol%)
Conditions
Yield
p-Cl
2a
2d
2j
(NH4)6Mo7O24·4H2O
(0.143)
60 °C, 16 h 93%b
Flash chromatography (FC): silica gel (Merck 60, 230–400 mesh).
TLC: Al-roll silica gel (Merck 60, F254). Mp: Kofler hot bench, cor-
rected. IR spectra : Perkin-Elmer 157 G. 1H- and 13C NMR spectra:
Bruker 400 MHz, TMS and CDCl3 [13C NMR; d (CDCl3) = 77.0
ppm with respect to TMS] as internal references. Microanalyses
were carried out by the Service Central de Microanalyses du CNRS,
Vernaison.
H
(NH4)6Mo7O24·4H2O
(0.143)
60 °C, 6 h
60 °C, 4 d
77%a
76%c
p-NO2
(NH4)6Mo7O24·4H2O
(0.143)
a After distillation.
H2O2 was purchased from Prolabo and was titrated by iodometry as
7.5 M. MoO3, phosphomolybdic acid were obtained from Fluka pu-
rum, ammonium molybdate from Riedel de Haen, anilines 1a, 1c,
1h, 1j, 1g from Fluka purum, 1f from Fluka technicum; 1d was dis-
tilled; 1b mp 81–84 °C, 1j mp 146–147 °C, 1e mp 45 °C were re-
crystallised from cyclohexane.
b The crude product is pure.
c After sublimation (5–10 Torr at 125 °C).
From Anilines
Unless otherwise stated IR, 13C, 1H NMR data agreed with the liter-
ature values.
Oxidation of aniline and derivatives 1a,d,j to their insolu-
ble nitroso compounds was performed in methanol with 6
equivalents of 30% H2O2 at room temperature for 1a,j or
at 0 °C for 1d until quasi-consumption of anilines had oc-
cured; under these conditions, variable amounts of nitro
compounds were already formed (5% of 3a, p-Cl, 7% of
3d, aniline, 77% of 3j and 10% azoxy-derivative, p-NO2).
The oxidation to nitro compounds 3a,d,j was completed at
60 °C and the latter were then isolated in acceptable yields
(66–92%; Table 3) after purification as above.
Oxidation of Anilines 1a–h to Nitroso Arenes 2a–h; General
Procedure
To a stirred solution of aniline 1 (10 mmol) in MeOH (3 mL) were
added H2O2 (5.5 mL, 40 mmol, 4 equiv) and H2O (4.5 mL). Aniline
1 precipitated as fine crystals and then MoO3 (0.144 g, 1 mmol) and
aqueous KOH solution (1 mL, 1 mmol) were added and the solution
stirred at 25 °C. The solution became brown and then yellow with
formation of a precipitate, pH value 3–3.5. The reaction was moni-
1
tored by H NMR in CDCl3. After a given time at 25 °C (see
Direct oxidation of anilines 1a,d,j at 60 °C provided sub-
stantial amounts of azoxy-derivatives which are formed
by condensation of the intermediary N-hydroxyaniline in
the presence of a nitroso derivative.
Table 1), H2O (15 mL) was added and 2 formed as an ochre precip-
itate, which was filtered, washed with H2O (2 × 10 mL), cold
MeOH (5 mL) and dried. Crude 2 is mainly pure and can be recrys-
tallised to give pure 2. Crude and recrystallised yields are given in
Table 1.
1-Chloro-4-nitrosobenzene (2a)10,23
Yellowish crystals; mp 115–118, 130–140 °C (dec.) (EtOH) (lit.23
130 °C, lit.25 92–3 °C).
Synthesis 2004, No. 5, 706–710 © Thieme Stuttgart · New York

PAPER
Nitroso Benzenes and Nitro Benzenes by Oxidation of Anilines
709
1H NMR (CDCl3): d = 7.86 (dm, H-3, H-5), 7.60 (dm, H-2, H-6,
J = 8.6 Hz).
carried out as above; crude and recrystallised yields are given in
Table 1.
2-Chloro-5-nitrosotoluene (2b)
1-Nitro-4-nitrosobenzene (2j)10,23
Yellowish crystals; mp 84–87 °C (MeOH) {lit.23 73–74 °C
(EtOH)}.
Mp 128–130 °C (EtOH) (lit.23 128–130 °C; lit.24 118.5–119 °C).
2-Nitro-5-nitrosotoluene (2k)
1H NMR (CDCl3): d = 2.53 (s, Me-1), 7.59 (d, H-3, J3,4 = 8.3 Hz),
7.68 (dd, H-4, J4,6 = 2.0 Hz), 7.79 (d, H-6).
13C NMR (CDCl3): d = 20.1 (Me-1), 119.3 (C-4), 123.4 (C-6), 130.0
(C-3), 137.7 (C-1), 141.6 (C-2), 164.1 (C-5).
The same procedure as above 1k (1.52g, 10 mmol), MeOH (3 mL),
H2O2 (15 mL, 0.11 mol, 11 equiv) and MoO3 (0.144 g, 1 mmol), 7
d; mp 112–114 °C (EtOH) (lit.24 113 °C).
IR (KBr): 3100, 1610, 1575, 1515, 1345, 1308, 1255, 840, 825
cm–1.
1H NMR (CDCl3): d = 2.74 (s, Me-1), 7.74 (dd, H-4, J3,4 = 8.5 Hz),
7.98 (d, H-6, J4,6 = 2.0 Hz.), 8.13 (d, H-3).
13C NMR (CDCl3): d = 20.0 (Me-1), 117.7 (C-4), 125.8, 126.0 (C-
3,C-6), 135.4 (C-1), 152.3 (C-2), 162.1 (C-5).
1-Bromo-4-nitrosobenzene (2c)
Yellowish crystals; mp 98–100 °C (EtOH) (lit.24 95 °C).
IR (KBr): 3085, 1575, 1475, 1400, 1283, 1253, 1100, 1063, 1010,
853, 805, 705 cm–1.
1H NMR (CDCl3): d = 7.78 (s, 4 H).
13C NMR (CDCl3): d = 122.1 (C-3, C-5), 132.7 (C-2, C-6), 131.7
(C-1); 163.8 (C-4).
Oxidation to Nitro Derivatives 3a,d,j; General Procedure
Oxidation of Nitroso Compounds 2a,d,j
To a stirred suspension at 60 °C of 2a,d,j (10 mmol) in MeOH (7.5
mL) were added H2O2 (5.5 mL, 41 mmol, 4 equiv) and
(NH4)6Mo7O24·4H2O (0.178 g, 1 mmol MoO3). The reaction was
monitored by 1H NMR (CDCl3) and after the given time (see
Table 2) the suspension was diluted with H2O (10 mL), chilled and
(except for 3d, see below) filtered, the solid 3 was washed with H2O
and dried over P2O5 (for yields see Table 2).
Nitrosobenzene (2d)10,26
Reaction at 0 °C; yellowish crystals; mp 76–80 °C (MeOH) (lit.24
67.5–68 °C).
IR,1H NMR and 13C NMR (CDCl3) agreed with the literature.
4-Nitrosotoluene (2e)10
Reaction at 0 °C; yellowish crystals; mp 58–60 °C (lit.24 48.5 °C,
lit.10 47–49 °C).
Oxidation of Anilines 1a,d,j
To a stirred solution of anilines 1a,d,j (10 mmol) in MeOH (10.5
IR (KBr): 3030, 1595, 1500, 1295, 1250, 1160, 1115, 1018, 860,
820, 815, 758 cm–1.
mL) were added H2O2 (8 mL, 60 mmol,
6 equiv) and
(NH4)6Mo7O24·4H2O (0.178 g, 1 mmol MoO3). The solution was
stirred at 20 °C for 1a, 0 °C for 1d, 25 °C for 1f for the given time,
then heated at 60 °C for 6–24 h (see Table 3). Same work-up as
above (for yields see Table 3).
4-Benzoylnitrosobenzene (2f)
Yellowish crystals; mp 120–122 °C (EtOH).
IR (KBr): 3100, 3050, 1645, 1592, 1524, 1403, 1314, 1302, 1276,
1260, 1105, 940, 923, 876, 870, 822, 788, 740, 695, 685 cm–1.
1H NMR (CDCl3): d = 7.53 (tm, 2 H, Arm, Jo,m = 7.4 Hz ), 7.66 (tm,
H, Arp, Jm,p = 7.6 Hz), 7.83 (dm, 2 H, Aro), 8.00, 8.02 (2 d, H-2, H-
6, H-3, H-5, J2,3 = 8.8 Hz).
13C NMR (CDCl3): d = 120.4 (C-3, C-5), 128.6 (2 Cm), 130.1, 131,0
(2 Co, C-2,C-6), 133.3 (Cp), 136.5 (C), 142.7 (C-1), 164.0 (C-4),
195.5 (CO).
4-Chloronitrobenzene (3a)26
Pale yellow crystals; mp 81–82 °C (lit.24 83–84 °C).
Nitrobenzene (3d)28
The dark organic phase was separated, the aqueous phase was dilut-
ed with H2O (10 mL) and extracted with CH2Cl2 (3 × 1 mL), organ-
ic phases were combined and distilled to give 3d.
Yellow oil; Eb6 68 °C (lit.24 Eb10 85 °C); nD20 1.5518 (lit.24 1.5530).
Anal Calcd for C13H9NO2 (211.22): C, 73.92; H, 4.30; N, 6.63.
Found: C, 73.6; H, 4.1; N, 6.8.
4-Dinitrobenzene (3j)13a,26,28
Crude product isolated as yellow crystals (83–85%) containing 5%
of tetranitroazoxybenzene (Method 1) and 11% (Method 2), which
was purified by sublimation (130 °C, 4–10 Torr, 2 h, containing 2%
azoxyderivative).
Methyl 4-nitrosobenzoate (2g)23,27
After 24 h, MeOH (6 mL) and H2O (4 mL) were added; yellowish
crystals; mp 145–150 °C (EtOAc) (lit.23 130 °C, lit.24 128.5–
129.5 °C)
Pale yellow crystals; mp 172–174 °C (lit.24 173–174 °C).
4-Nitrosoanisole (2h)23
Reaction carried out at 0 °C in two phases with addition of hexane
(20 mL) for 16 h. The hexane phase was decanted and the aqueous
phase extracted with hexane (4 × 20 mL), the combined hexane
phases were evaporated at 30 °C and the resulting green resin (0.97
g, 63%) recrystallised from hexane at –30 °C; green crystals;
mp 20–21 °C (hexane) (lit.23 22 °C, lit.24 23 °C).
Acknowledgement
The support of the Centre National de la Recherche Scientifique
(UMR-7015) is gratefully acknowledged. We thank also Prof. J.
Streith for his interest in this work and the diploma student Isabelle
Guy for her cooperation.
13C NMR (CDCl3): d = 55.9 (OMe-1), 113.8 (C-2, C-6), 124.2 (C-
3, C-5), 164.0, 165.6 (C-4), C-1).
References
Nitronitrosobenzenes 2j,k; Typical Procedure
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with MeOH (3 mL) and H2O (2 mL); after 10 d the work-up was
Synthesis 2004, No. 5, 706–710 © Thieme Stuttgart · New York

710
A. Defoin
PAPER
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2–
(22) At pH 11–12, it was shown that another species MoO(O2)3
decomposed with formation of singlet oxygen (1O2).21 See a
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