Metalloporphyrin and heteropoly acid catalyzed oxidation of C=NOH bonds in an ionic liquid: Biomimetic models of nitric oxide synthase

Article abstract of DOI:10.1016/j.tetlet.2005.02.088

Water soluble iron(III) porphyrins and phosphotungstic acid in an ionic liquid are effective catalysts for the H₂O₂ mediated oxidation of the CNOH bond in N-hydroxyarginine and other oximes. The carbonyl compounds generated as the oxidation products can be easily isolated from the reaction media. These systems serve as biomimetic models of nitric oxide synthase (NOS) and the catalyst immobilized in an ionic liquid can be easily recycled and reused.

Full text of DOI:10.1016/j.tetlet.2005.02.088

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 2599–2602
Metalloporphyrin and heteropoly acid catalyzed oxidation
of C@NOH bonds in an ionic liquid: biomimetic models of
nitric oxide synthase
Nidhi Jain, Anil Kumar and Shive M. S. Chauhan*
Department of Chemistry, University of Delhi, Delhi 110 007, India
Received 21 December 2004; revised 5 February 2005; accepted 15 February 2005
Abstract—Water soluble iron(III) porphyrins and phosphotungstic acid in an ionic liquid are effective catalysts for the H2O2 medi-
ated oxidation of the C@NOH bond in N-hydroxyarginine and other oximes. The carbonyl compounds generated as the oxidation
products can be easily isolated from the reaction media. These systems serve as biomimetic models of nitric oxide synthase (NOS)
and the catalyst immobilized in an ionic liquid can be easily recycled and reused.
Ó 2005 Elsevier Ltd. All rights reserved.
Nitric oxide synthase (NOS) catalyzes the five-electron
oxidation of L-arginine to L-citrulline and nitric oxide
(NO) by O2 in two steps.1 In the first step, two NADPH
derived reducing equivalents and O2 lead to the forma-
tion of N-hydroxy-L-arginine (NHA) 3.2 The redox stoi-
chiometry for this step is that of a typical cytochrome
P450 hydroxylation. The second step of the NOS reac-
tion involves a three electron oxidation of N-hydroxy-
L-arginine to NO and citrulline 4 by O2.3 Several mech-
anisms have been proposed to account for the unusual
redox chemistry of the NOS catalyzed oxidation of
NHA.4 These include generation of either a high valent
iron-oxo intermediate5 or a nucleophilic peroxo Fe(III)P
species during the reaction.6
stability of anionic water soluble iron(III) porphyrins
and cobalt(II) phthalocyanines in imidazolium ionic liq-
uids.16,17 In continuation of our ongoing research into
the application of ionic liquids as green solvents in or-
ganic synthesis,18 herein we report the oxidative cleav-
age of the C@NOH bond in N-hydroxyarginine, the
typical biological precursor of nitric oxide in the NOS
reaction, as well as other oximes with hydrogen peroxide
catalyzed by water soluble iron(III) porphyrins as chemi-
cal models of NO synthase and molecular oxygen
(Schemes 1 and 2).19,20 We have also examined the same
reaction using another catalyst, phosphotungstic acid
immobilized in an ionic liquid. Phosphotungstic acid
belongs to the class of heteropoly acids, sometimes
referred to as inorganic metalloporphyrins,21 which are
useful and versatile catalysts in a number of homoge-
neous, biphasic and heterogeneous liquid phase reac-
tions.22,23 Aqueous hydrogen peroxide coupled with a
tungstate complex and a quaternary ammonium hydro-
gen sulfate is a clean, safe and ideal oxidant for oxida-
tive dehydrogenation and epoxidation of alkenes and
Metalloporphyrins as chemical models of cytochrome
P450 and related monooxygenases have been used to
carry out oxygenation and oxidation reactions of vari-
ous drugs and biologically active compounds.7–10 Air-
and moisture-stable room temperature ionic liquids
(ILs) based on imidazolium cations are environmentally
benign solvents for carrying out various chemical and
biochemical transformations11–13 and for immobiliza-
tion of transition metal catalysts in biphasic and mono-
phasic processes.14,15 We have reported the enhanced
R
R'
R
H2O2/Cl8TPPS4 Fe(III) or H2O2/H3PW O40
12
OH
N
O
[bmim][BF4]
'
R
2
1
Keywords: Nitric oxide synthase (NOS); Heteropoly acid; Porphyrins;
Ionic liquid.
Scheme 1. Oxidation of the C@NOH bond by hydrogen peroxide
catalyzed by Cl8TPPFe(III) 5 or phosphotungstic acid in an ionic
liquid.
*
Corresponding author. Tel./fax: +91 1127666845; e-mail: smschauhan@
chemistry.du.ac.in
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.02.088

2600
N. Jain et al. / Tetrahedron Letters 46 (2005) 2599–2602
OH
N
product analysis and yield determination were based
on HPLC analysis. The oxidation was more facile with
ketoximes which gave higher yields of the carbonyl com-
pounds as compared to aldoximes. A range of aldoximes
were examined for this reaction and it was found that
aldoximes 1b,c possessing electron donating substituents
gave higher yields of the products than aldoximes 1f–h
having electron withdrawing substituents. The mecha-
nism of deoximation may involve formation of a hydro-
peroxide intermediate which adds in a nucleophilic sense
to the C@N bond to give a peroxy anion adduct 6 which
on subsequent decomposition yields 4 (Scheme 3).
Nucleophilic attack of a ferric peroxy anion (FeIII–O–
OÀ) to C@O and C@N groups in substrates has been re-
ported in the literature.27 The same mechanism may be
operative in the oxidation of oximes into carbonyl com-
pounds by hydrogen peroxide in the presence of iron(III)
porphyrins in ionic liquids, although a detailed mecha-
nistic study would be required to prove this.
NH2
O
NH2
NH
NH
Cl8TPPS4Fe(III)/H2O2
[bmim][BF
4]
+
+
-
-
H3N
CO2
H3N
CO2
N-hydroxy–L-arginine
L-citrulline
3
4
Ar
Cl
Cl
SO3Na
Cl8TPPS4Fe(III) 5
N
N
N
Ar =
=
Ar
Ar
Fe(III)
N
Ar
The oxidation of oximes with H2O2 catalyzed by the sec-
ond catalyst, phosphotungstic acid in ionic liquid gave
higher yields of the carbonyl compounds in comparison
to 5. It is believed that the interaction of phosphotung-
stic acid with hydrogen peroxide in an ionic liquid leads
to initial formation of an active peroxo polyoxometalate
species 7, which reacts with respective aldoxime or ket-
oxime in a nucleophilic manner followed by subsequent
transformations leading to the formation of the corre-
sponding aldehyde or ketone (Scheme 4). This peroxo
species 7 has been identified as a tungsten dioxygen
intermediate by its characteristic peak at 866 cmÀ1 in
its IR spectrum indicating a tungsten dioxygen ring.28
Scheme 2. Oxidation of N-hydroxyarginine with hydrogen peroxide
catalyzed by 5 in an ionic liquid.
for oxidation of alcohols, glycols, phenols and sul-
fides.24–26 To the best of our knowledge, this is the first
report for the use of water soluble iron(III) porphyrins
and heteropoly acids in an ionic liquid as model cata-
lysts of NO synthase (Table 1).
The reaction of 3 with hydrogen peroxide catalyzed by
5 in [bmim][BF4] gave 4 in 37% yield after 4 h. The
Table 1. Oxidation of oximes (1a–p) with hydrogen peroxide catalyzed by phosphotungstic acid or 5 in [bmim][BF4]a
Entry
Substrate 1
Product 2b
Time (h)
Yieldc(%)
H3PW12O40
5
a
b
c
Benzaldehyde oxime
Benzaldehyde
3
4
4
4
6
5
5
5
3
4
5
4
3
3
3
6
80
86
90d
88
83
78
74
70
80
84
76
74
92
86
97f
71
57
68
71
64
62e
59
45
49
63
57
45
70
81
73
85
65
4-Methylbenzaldehyde oxime
4-Methoxybenzaldehyde oxime
Piperonal oxime
4-Methylbenzaldehyde
4-Methoxybenzaldehyde
Piperonal
d
e
4-Methylamino-3-nitrobenzaldehyde oxime
4-Chlorobenzaldehyde oxime
4-Fluorobenzaldehyde oxime
4-Nitrobenzaldehyde oxime
Cinnamaldehyde oxime
4-Methylamino-3-nitrobenzaldehyde
4-Chlorobenzaldehyde
4-Fluorobenzaldehyde
4-Nitrobenzaldehyde
Cinnamaldehyde
f
g
h
i
j
2-Furancarboxaldehyde oxime
3-Pyridine carboxaldehyde oxime
2-Butanone oxime
2-Furancarboxaldehyde
3-Pyridine carboxaldehyde
2-Butanone
k
l
m
n
o
p
Acetophenone oxime
Benzophenone oxime
Acetophenone
Benzophenone
Cyclohexanone oxime
9-Fluorenone oxime
Cyclohexanone
9-Fluorenone
a Reaction conditions: All reactions were performed under nitrogen. Oxime (1.0 mmol), H2O2 (1.0 mmol), H3PW12O40 or Cl8TPPS4Fe(III)Cl
(0.01 mmol), [bmim][BF4] (3.0 mL).
b All products were characterized by 1H NMR, IR and EI-MS spectroscopic methods.
c Yields refer to isolated products.
d Yield of 4-methoxybenzaldehyde was only 10% when deoximation was carried out under phase transfer conditions in dichloromethane/water using
tetrabutylammonium bromide as the phase transfer reagent and the starting oxime was recovered in 80% yield. 10% of 4-methoxybenzoic acid was
obtained as an over-oxidation product.
e N-Demethylated aldehyde was also obtained in 7% yield after 6 h.
f Cyclohexanone was not regenerated and starting material was recovered unchanged on reaction of cyclohexanone oxime (1.0 mmol) with H2O2
(1.0 mmol), H3PW12O40 (0.01 mmol) in dichloromethane and methanol. Instead, the over-oxidized product adipic acid was obtained in 20% yield
after 1 h.

N. Jain et al. / Tetrahedron Letters 46 (2005) 2599–2602
2601
III
Fe-O-O
III
Fe
O
O
R'
HO
O
N
O
R'
+
N
R'
O2
-
-
+
NO3
NO
NO2
R
R
R
H+
IV
6
Fe=O
Scheme 3.
O
O
H
O
W
O
O
O
O
O
O
H2O
O
OH
H2O2
rapid
W
O
O
O
W
O
O
O
H2O2
O
7
HO
N
R'
R
O
O
R'
O
O
O2
-
NO
NO2- NO3
+
W
+
O
HO
R
O
N
O
R'
O
R
Scheme 4.
Further, on addition of hydrogen peroxide to phospho-
tungstic acid immobilized in an ionic liquid, the colour
of the solution changed to yellow, which is also charac-
teristic of a three-membered tungsten dioxygen ring sys-
tem. The ionic liquids are believed to stabilize this
reactive intermediate as they have been reported earlier
to stabilize manganese-oxo intermediate in manga-
nese(III) porphyrins catalyzed oxygenation reactions.29
Furthermore, on addition of H2O2 to the solution of
phosphotungstic acid in the ionic liquid, a new peak at
275 nm was observed in the UV–visible spectra which
in the presence of oximes decreased and finally disap-
peared depending on the amount of oxime (see Supple-
mentary data). However, the peak reappeared on
addition of fresh H2O2 and subsequently decreased in
the presence of substrate. A similar peak at 280 nm
was reported in the oxygen atom transfer reactions cata-
lyzed by peroxotungstate complexes.30
reusable biomimetic model systems to study oxidation
reactions of nitric oxide synthase in ionic liquids.
Acknowledgements
The author (N.J.) thanks the Council of Scientific and
Industrial Research (CSIR), New Delhi for providing
a senior research fellowship (SRF).
Supplementary data
A detailed experimental section and spectroscopic data
of the oximes and carbonyl compounds prepared is
available. Supplementary data associated with this arti-
cle can be found, in the online version, at doi:10.1016/
j.tetlet.2005.02.088.
In conclusion, a simple chemical model for biomimetic
oxidation of C@NOH bonds has been developed with
H2O2 catalyzed by water soluble iron(III) porphyrins
as well as phosphotungstic acid. Phosphotungstic acid
was found to be more efficient in bringing about this
transformation as compared to water soluble iron(III)
porphyrins. This study provides eco-friendly and
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