Reactions of nitrosoarenes with nitrogen monoxide (nitric oxide) and nitrogen dioxide: Formation of diarylnitroxides

Article abstract of DOI:10.1002/ejoc.200800213

Nitrosoarenes react with nitrogen monoxide (nitric oxide) at room temperature and in aprotic media to afford the corresponding diarylnitroxides by the intermediate formation of N-nitrosoarylnitroxides. However, these latter spin adducts, in contrast with literature reports, have never been detected by us. N-nitrosophenylnitroxide is obtained only in the oxidation of the ammonium salt of N-nitrosophenylhydroxylamine (cupferron) with trace amounts of lead tetraacetate. However, it evolves with time to diphenylnitroxide, as demonstrated by following the reaction course in the ESR cavity. On a macroscale level, the reaction between nitrosobenzene and nitric oxide leads to the formation of N-nitrosodiphenylamine, 4-nitro-N-nitrosodiphenylamine, 4-nitrodiphenylamine and 4,4′-dinitrodiphenylamine, in addition to diphenylnitroxide. Diarylnitroxides are also obtained when nitrosoarenes react with small amounts of nitrogen dioxide; the mechanism of this reaction is proposed and discussed. The structure of 4-nitro-N-nitrosodiphenylamine was determined by X-ray analysis. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800213

FULL PAPER
DOI: 10.1002/ejoc.200800213
Reactions of Nitrosoarenes with Nitrogen Monoxide (Nitric Oxide) and
Nitrogen Dioxide: Formation of Diarylnitroxides
Paola Astolfi,[a] Patricia Carloni,[a] Elisabetta Damiani,[b] Lucedio Greci,*[a]
Milvia Marini,[a] Corrado Rizzoli,[c] and Pierluigi Stipa[a]
Dedicated to Professor Paolo Bruni on the occasion of his 70th birthday
Keywords: Nitrosoarenes / Nitrogen oxides / Diazonium salts / Diarylnitroxides
Nitrosoarenes react with nitrogen monoxide (nitric oxide) at
room temperature and in aprotic media to afford the corre-
sponding diarylnitroxides by the intermediate formation of
N-nitrosoarylnitroxides. However, these latter spin adducts,
in contrast with literature reports, have never been detected
and nitric oxide leads to the formation of N-nitrosodiphenyl-
amine, 4-nitro-N-nitrosodiphenylamine, 4-nitrodiphenyl-
amine and 4,4Ј-dinitrodiphenylamine, in addition to di-
phenylnitroxide. Diarylnitroxides are also obtained when ni-
trosoarenes react with small amounts of nitrogen dioxide; the
by us. N-nitrosophenylnitroxide is obtained only in the oxi- mechanism of this reaction is proposed and discussed. The
dation of the ammonium salt of N-nitrosophenylhydroxyl-
amine (cupferron) with trace amounts of lead tetraacetate.
However, it evolves with time to diphenylnitroxide, as dem-
onstrated by following the reaction course in the ESR cavity.
On a macroscale level, the reaction between nitrosobenzene
structure of 4-nitro-N-nitrosodiphenylamine was determined
by X-ray analysis.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Introduction
The oxidation of aromatic nitroso compounds to the cor-
responding nitro derivatives by nitrogen dioxide (·NO2) has
been considered in several studies,[1,2] most of which have
focused on the kinetic aspects of the reaction. The oxi-
dation of sulfur dioxide (SO2) to sulfur trioxide (SO3) by
nitric oxide (·NO) in the presence of oxygen (and hence
·NO2) is also a well-known process.[3] In this latter case, it
Scheme 1.
·
Unfortunately, the catalytic action of NO2 was not ob-
served and the complete consumption of nitrosobenzene
·
is proved that NO2 works in a catalytic way to transfer an
·
·
was obtained only when NO/O2 (or NO2) was added stoi-
chiometrically or in excess. A radical addition mechanism
for the oxidation of nitrosoarenes by ·NO2 has already been
reported and the kinetics of the reaction studied;[2a] how-
ever, some points on the reactivity of nitrosoarenes towards
·
oxygen atom to SO2 and to release a molecule of NO that
then reacts with oxygen thus fuelling another catalytic cycle.
On the basis of this application, in this study we tried to
·
use NO, in the presence of oxygen, as a catalyst for the
oxidation of nitrosoarenes to nitroarenes in solution as
shown in the cyclic reaction of Scheme 1.
·NO2 or NO still need clarification. For example, on the
·
one hand, the ability of nitroso compounds to act as radical
trapping agents is well established,[4] and on the other hand,
·
[a] Dipartimento di Scienze e Tecnologie Chimiche, Università Po- the use of these species for trapping NO and related radi-
litecnica delle Marche,
cals has not been of particular interest likely due to the
occurrence of more complex reactions other than simple
radical addition. For this reason, the reactions between ni-
trosoarenes and NO2 or NO were carried out in the elec-
tron spin resonance (ESR) cavity in order to show evidence
of the formation of radical intermediates during the oxi-
dation process or the formation of stable spin adducts. The
same reactions were also performed on a macroscale level
Via Brecce Bianche, 60131 Ancona, Italy
Fax: +39-071-2204714
E-mail: l.greci@univpm.it
[b] Istituto di Biochimica, Università Politecnica delle Marche,
Via Ranieri, 60131 Ancona, Italy
·
·
[c] Dipartimento di Chimica Generale ed Inorganica, Chimica An-
alitica, Chimica Fisica, Università,
Viale G. P. Usberti 17/A, 43100 Parma, Italy
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2008, 3279–3285
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P. Astolfi, P. Carloni, E. Damiani, L. Greci, M. Marini, C. Rizzoli, P. Stipa
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to identify other products possibly formed, together with
the nitroarenes, and to gain further information on the re-
action mechanism. In particular, attention was focused on
the reaction between nitrosobenzene and ·NO, as this latter
species is formed during the oxidative process of nitroso
compounds by ·NO2[2a] and likely responsible for the failure
of the catalytic process.
Results
A benzene solution of nitrosobenzene 1a was treated
·
·
with a catalytic amount (ca. 10%) of gaseous NO2 or NO
obtained from the decomposition of nitrous acid[5] (see Ex-
perimental Section) in the presence of oxygen; in both cases,
only trace amounts of nitrobenzene 2a were formed with
Figure 1. (a) Experimental and (b) computer-simulated spectra of
diphenylnitroxide 4a.
·
no catalytic action by NO2. If the reactions were carried
·
·
out by using an excess amount of NO2 or NO/O2, com-
pound 1a was oxidized to nitrobenzene 2a, which was iden-
tified by comparison with an authentic sample (Scheme 2).
When ·NO was the reacting species in the absence of oxy-
gen (decomposition of nitrous acid in well-deaerated solu-
tion, see Experimental Section), the signals of diarylni-
troxides 4a–g were recorded soon after mixing the reaction
solutions.
In all the experiments carried out in the ESR cavity, di-
phenylnitroxides 4a–g were the sole persistent radicals de-
tected: the spectra of nitrosonitroxides 3, which according
to the literature,[7,8] can be obtained from the trapping of
·NO by nitrosoarenes, were never observed. Nitrosoni-
troxide 3a was obtained only by oxidizing a dioxane solu-
tion of the ammonium salt of N-nitroso-N-phenylhydroxyl-
amine (cupferron, 5) with trace amounts of lead tetraace-
tate (Figure 2, Table 1). However, even in this case, the re-
corded signal evolved with time into that of diphenylni-
troxide 4a.
Scheme 2.
When the same reaction was carried out with benzene as
If nitrosoarenes 1a–g were treated in the ESR cavity with the solvent, diphenylnitroxide 4a was immediately formed.
a small amount of nitrogen dioxide, complex signals were Similarly, 4a was the sole radical obtained when 5 was oxid-
recorded due to the overlapping of two different spectra: a ized with larger amounts of Pb(OAc)4, but by following ex-
larger and unresolved signal disappearing in 15–30 min and actly the same experimental conditions described in the lit-
a narrower, persistent, and well-resolved signal correspond- erature,[8a] the expected N-nitrosonitroxide was never ob-
ing to diarylnitroxides 4a–g. All spectra were computer sim- tained.
ulated, and the obtained spectral parameters (hyperfine
From the macroscale reaction of nitrosobenzene 1a with
coupling constants and g factors) collected in Table 1 are in sodium nitrite and monochloroacetic acid (nitric oxide was
agreement with those reported in the literature.[6] At pres- always generated in situ by decomposition of nitrous
ent, the first radical detected during the reaction remains acid)[5] in a thoroughly degassed benzene solution, di-
unidentified. In Figure 1, the experimental and simulated phenylnitroxide 4a was obtained (but not isolated) together
ESR spectra of diphenylnitroxide 4a are shown.
with N-nitrosodiphenylamine 6 (traces, yields Ͻ3%), 4-ni-
Table 1. Hyperfine coupling constants (H.f.c.cs in Gauss) and g-factors of nitroxide 3a in dioxane and diarylnitroxides 4a–g in benzene.
Nitroxide aN
aH(o)
aH(p)
aH(m)
aH
aH
aN(NO)
aN(CN)
aCl
aBr
g factor
(CH3)
(OCH3)
3a
4a
4b
4c
4d
4e
4f
10.06
2.51
1.81
1.77
2.00
1.91
1.87
1.75
1.79
2.66
1.88
1.30
–
–
–
0.85
0.80
1.03
0.81
0.76
0.82
0.98
0.80
–
–
0.33
1.91
–
–
–
–
–
–
–
3.66
–
–
–
–
–
–
–
–
–
–
–
–
–
–
0.24
–
–
–
–
–
0.16
–
–
–
–
–
–
–
0.18
–
2.0059(6)
2.0058(1)
2.0060(7)
2.0057(3)
2.0057(7)
2.0061(5)
2.0064(2)
2.0059(8)
9.70
10.54
9.97
10.19
9.51
9.48
9.44
0.22
–
–
–
–
–
4g
–
–
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Reactions of Nitrosoarenes for the Formation of Diarylnitroxides
pound 9 was oxidised with m-chloroperbenzoic acid directly
in the ESR cavity to afford 4,4Ј-dinitrodiphenylnitroxide
(see Experimental Section).
Figure 2. (a) Experimental and (b) computer-simulated spectra of
phenyl-N-nitrosonitroxide 3a.
Discussion
tro-N-nitrosodiphenylamine 7 (40%), 4-nitrodiphenylamine
8 (21%) and 4,4Ј-dinitrodiphenylamine 9 (18%). Com-
pound 6 was identified by comparison with an authentic
sample; compounds 7, 8 and 9 were identified by compar-
ing their physical and spectroscopic data with those re-
ported in the literature. Moreover, the structure of diphenyl-
amine 7 was determined by single-crystal X-ray analysis
(Figure 3; selected bond parameters are listed in the cap-
tion). Bond lengths and angles within the molecule are un-
exceptional. The dihedral angle formed by the least-squares
planes of the aromatic ring is 70.99(9)°. The nitro group is
coplanar with the benzene ring [dihedral angle between the
least-squares planes is 1.0(2)°], and the N3–C4 bond length
of 1.449(4) Å is consistent with a π-electron delocalization
between the two groups. In the crystal packing, molecules
are linked by intermolecular C–H···N and C–H···O hydro-
gen-bonding interactions into chains running parallel to the
[1 2 0] and [1 –2 0] directions [C2–H2 0.93 Å, H2···N2i
142.2°; C5···H5 0.93 Å, H5···O2ii 2.42 Å, C5···O2ii
3.29(5) Å, C5–H5···O2ii 156.7°. Symmetry codes i: –x, –y,
–z; ii: 1 – x, –2 – y, –z].
In our attempts to use nitrogen dioxide as a catalyst in
the oxidation of nitrosobenzene to nitrobenzene we found
that the above oxidant cannot be used in a catalytic way
and that only when NO2 or NO/O2 was added in a stoi-
chiometric amount or in excess was nitrosobenzene oxidised
to nitrobenzene. This is in agreement with the stoichiometry
for reaction (1) established by Bonner and Hancock[1b] and
confirmed by Govenlock et al.[2a] according to which the
ratio ArNO/·NO2/ArNO2 was 1:1:1 [Equation (1)].
·
·
ArNO + NO2 ǞArNO2+ ·NO
(1)
·
Starting from this point, we decided to take a closer look
at the reaction of nitrosobenzene 1a and other substituted
·
·
nitrosoarenes 1b–g with NO2 and, above all, with NO and
to follow these reactions by ESR spectroscopy. In fact, very
few and also unclear data are reported in the literature con-
cerning the reactivity and the spin-trapping ability[7] of ni-
trosoarenes towards nitric oxide and related radicals (ni-
trosoarenes like other nitroso compounds should behave as
good spin-trapping agents). For example, Balaban[8] de-
scribed that nitrosobenzene was able to trap ·NO to give N-
nitrosophenylnitroxide when ·NO was bubbled into a dilute
dioxane solution of nitrosobenzene. We repeated this ex-
periment several times under the same conditions, by
changing the solvent (benzene instead of dioxane) and/or
nitrosobenzene concentration, but the signal of nitroxide 3a
was never detected. In all of our experiments with 1a and
all other nitrosoarenes 1b–g, the radicals detected were ni-
troxides 4a–g, as already observed by others upon the trap-
ping of ·NO by aromatic nitroso compounds.[4a,4c,4d] On the
basis of our knowledge concerning nitric oxide reactivity
towards indolinonic[9] and quinolinic[10] nitroxides, one can
Figure 3. ORTEP drawing of compound 7; displacement ellipsoids
are drawn at the 50% probability level. Selected bond lengths and
angles: N1–C1 1.417(4), N1–C7 1.446(4), N1–N2 1.352(3), N2–O1
1.230(3) Å; O1–N2–N1 114.9(3), N2–N1–C1 115.0(3), N2–N1–C7
120.0(3)°.
·
explain why NO cannot be detected by nitroso spin traps
and why the reactions of both NO and NO2 give diarylni-
·
·
·
Diphenylnitroxide 4a (prepared from nitrosobenzene and troxides. When nitrosoarenes react with NO, nitrosoni-
·
phenylmagnesium bromide) was also treated with NO and troxides 3 are indeed formed, but they react further with
products 6–9 were again obtained (data not shown). Com- ·NO to give nitrosated adduct 10, as shown in Scheme 3.
Eur. J. Org. Chem. 2008, 3279–3285
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FULL PAPER
(in the ESR cavity) of 5 with lead tetraacetate was at-
tempted as described in the literature.[19] In actual fact, ni-
troxide 3a was obtained but not under the reaction condi-
tions reported in that paper: if the concentrations indicated
by Balaban[19] were used, diphenylnitroxide 4a was immedi-
ately formed (in agreement with the results obtained also
by Rassat),[20] whereas if only trace amounts of Pb(OAc)4
were added, nitroxide 3a was obtained. The latter radical
was not stable and it rapidly evolved into diphenylnitroxide
4a, which thus indicates that it represents an intermediate
in the formation of 4a, as shown in Schemes 3 and 4 and
that it is in equilibrium with nitrosobenzene and ·NO. When
the same reaction was carried out in benzene instead of
dioxane, diphenylnitroxide was obtained right from the very
beginning. This could be due to a solvent effect on the equi-
librium of Scheme 3; this aspect has not been studied in
detail but it is likely that the dissociation of adduct 3 could
be favoured in benzene and hence also the formation of
·NO, which drives the reaction towards compound 10 and
then to diarylnitroxide.
Scheme 3.
Even if the values of the rate constants k1 and k2 are not
available, it can be assumed that the value of k2, which is
related to a radical–radical coupling, would be much higher
than that of k1 and, hence, that nitroxides 3 immediately
·
react with NO, which explains why signals due to radicals
3 (Scheme 3) are never observed. The formation of adduct
10 has already been reported in the literature,[2b] but with-
out consideration of the fate of this compound. As shown
in Scheme 4, adduct 10 may rearrange after cleavage of a
nitrogen–oxygen bond in the ϾN–ONO group (computed
BDEN–ON ≈ –1.512 kcal/mol),[11] a deoxygenation also ob-
served in the reaction between ·NO and aromatic ni-
troxides.[9,10] The NO2 group thus released may undergo
1,3-migration to give an arenediazonium nitrate, which can
be described by the three canonical structures 11, 12 and
13; a similar mechanism was proposed by us to justify the
formation of azoindoles in the reaction between nitroso-
indoles and nitric oxide in the presence of indole-free base
in aprotic solvent.[12]
Nitroxide 4a was also obtained when nitrosobenzene was
·
treated with trace amounts of NO2, although in this case
too it derives from the reaction between unreacted ni-
·
trosobenzene and NO formed during the oxidation of ni-
·
trosobenzene by NO2 [Equation (1)][2] through the same
pathways reported in Schemes 3 and 4. Actually, the first
spectrum recorded during the reaction was unresolved, be-
cause of 4a and to another nitroxide, which at present re-
mains unidentified, that rapidly disappears. However, it is
likely that this signal could be attributed to the Ar–N(O)–
·
O–N=O intermediate derived from the trapping of NO2,
·
which then evolves into nitrobenzene and NO.
The macroscale reaction of nitrosobenzene 1a with in
·
situ generated NO showed not only the formation of 4a,
but also that of C-nitro- and N-nitroso compounds 6–9.
Products 6–9 all derive from diphenylnitroxide through the
Scheme 4.
Similarly, it was reported that the reaction between tri- mechanisms already described in the literature for indoli-
·
fluoronitrosomethane and NO[13] affords a diazonium ni- nonic[9] and quinolinic[10] nitroxides. In particular, diphenyl-
trate through an intermediate (similar to compound 10) nitroxide 4a couples with another ·NO molecule to give ad-
that in some cases has also been isolated.[14,15] The forma- duct 15 (Scheme 5), which is the key product for the forma-
tion of a phenyldiazonium nitrate from nitrosobenzene and tion of 6 and 8 (Scheme 3) through aminyl radical 16, ·NO2
·
·NO is a well-known fact, as it was already reported by and NO. As for intermediate 10, a low value for the com-
Bamberger[16] at the end of the 19th century, but it was puted BDE of the ϾN–ONO bond in 15 (1.099 kcal/mol)
explained through a diazoniumdiolate intermediate.[17] [11] may justify its cleavage and the formation of aminyl rad-
However, on the basis of the spin-trapping ability of nitroso ical 16. Actually, N-nitrodiphenylamine (with or without
derivatives[4] and nitroxides towards ·NO,[9,10] intermediates other nitro groups also on the aromatic rings) could have
3 and 10 seem to be more likely than diazoniumdiolates, been obtained together with nitroso analogues 6 and 7, as
even if the formation of these latter intermediates cannot obtained, for example, in the reaction of indolinonic ni-
·
be excluded due to the results obtained in the present study. troxides with NO;[9] however, we were unable to observe
Once formed, aryldiazonium nitrates 11–13 (Scheme 4) de- the formation of such compounds.
compose at room temperature and in aprotic solvent to give
To explain the formation of nitroso derivative 7, ni-
aryldiazenyl and nitrate radicals; the former radical col- troxide 17 (Scheme 5) must be considered as an intermedi-
lapses[18] to dinitrogen and aryl radical 14, which is trapped ate, and it is likely derived from the trapping of ·NO2 on C-
by unreacted nitrosoarene to afford diarylnitroxide 4.
4 of the aromatic ring. Nitroxide 17 can further react with
·
As already mentioned above, the trapping of NO (or ·NO to give adduct 18, which then evolves into product 7.
·NO2) in the ESR cavity leading to the formation of N- Compound 18 is also involved in the generation of diphe-
nitrosonitroxide 3 was never observed. However, to gain in- nylamine 9, as cleavage of the ϾN–ONO bond in 18 fol-
·
formation on this radical and on its evolution, the oxidation lowed by addition of NO2 leads to its formation.
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Reactions of Nitrosoarenes for the Formation of Diarylnitroxides
(www.epr.niehs.nih.gov/).[25] Gaseous nitrogen dioxide (98.5% pu-
rity) was purchased from Fluka. It was used without purification
in the volumetric preparation of the solutions: a 20-mL volumetric
flask was filled with the indicated volume of the chosen solvent
and weighed. ·NO2 was bubbled through for a few seconds and the
flask was reweighed: the difference between the two weights gave
·
the amount, and hence the number of mols of NO2 dissolved in
20 mL. Solutions of ·NO2 generally contained between 5ϫ10–4 and
·
2ϫ10–3 mol in 20 mL. Nitrogen monoxide (nitric oxide, NO) was
obtained from the spontaneous decomposition of nitrous acid gen-
erated when monochloroacetic acid is added to sodium nitrite and
passed through a deaerated 20% KOH aqueous solution under an
atmosphere of argon. Two millimoles of sodium nitrite were used
·
to obtain 1 mmol of NO. Benzene and dioxane were Carlo Erba
Scheme 5.
or Aldrich RP-ACS grade solvents and were distilled prior to use.
N-nitrosophenylhydroxylamine ammonium salt (cupferron) and
lead tetraacetate were purchased from Sigma–Aldrich and were
used as received. Nitrosobenzene and 2-methylnitrosobenzene were
purchased from Aldrich, whereas 4-methyl-,[26] 4-methoxy-,[27] 4-
chloro-,[28] 4-bromo-[28] and 4-cyano-[29] nitrosobenzenes were pre-
pared according to literature reports. BDE calculations were per-
formed by means of the Gaussian 03 package[30] on an IBM SP5
supercomputer at Cineca Supercomputing Center (via Magnanelli
6/3, I-40033 Casalecchio di Reno, Bologna, Italy; http://www.
cineca.it/HPSystems). All calculations were carried out by geome-
try optimization at the B3LYP/6-311+G(d,p) level followed by sin-
gle-point frequency calculations at the same level in which imagi-
nary (negative) values were never found, confirming that the com-
puted geometries always refer to a minimum. Unrestricted wave
Conclusions
Nitric oxide and nitrogen dioxide are two radical species
with relevant deleterious effects in biological systems, and
they are involved, both directly and indirectly, in oxidative
and nitrosating stress.[21,22] In this regard, the data here de-
scribed may contribute to a better understanding of the re-
activity of these radicals towards nitroso derivatives, which
can be formed physiologically by oxidation of biological
primary amino groups or as intermediates in the nitration
of tyrosine.[23,24]
Nitrosoarenes cannot be used as spin traps for ·NO2, be-
cause the spin adducts potentially formed are unstable and
cannot be easily detected. In fact, the expected spin adduct
function was used for open-shell species, giving ϽS2Ͼ
0.7501Ϯ0.0001 for spin contamination (after annihilation).
=
·
Macroscale Reactions
reacts further with another NO molecule, which leads to
the formation of diphenylnitroxide, likely at a rate higher
than that of the initial trapping. Diphenylnitroxide is ob-
A – Reaction of Nitrosobenzene with Nitrogen Dioxide: A nitrogen
dioxide benzene solution (0.2 mmol in 5 mL of solvent) was added
to a solution of nitrosobenzene in the same solvent (1 mmol in
10 mL of benzene) whilst stirring at room temperature and in the
presence of oxygen. After 2 h, most of the starting nitrosobenzene
was recovered after evaporation of the solvent. The same reaction
was carried out by using an excess amount of nitrogen dioxide
·
tained in the reaction of nitrosobenzene not only with NO
·
but also with trace amounts of NO2; however, even in this
latter case the reaction involved is between nitrosobenzene
·
and NO (released during the oxidation of nitrosobenzene).
Diphenylnitroxide, which is also the precursor for all the
products isolated from the macroscale reaction between ni-
trosobenzene and nitric oxide, is obtained from the decom-
position of a diarendiazonium nitrate.
·
(2 mmol of NO2 for 1 mmol of nitrosobenzene). After 2 h, nitro-
benzene 2a was almost quantitatively isolated by evaporation of
the solvent, and it was identified by comparison with an authentic
sample.
B – Reaction of Nitrosobenzene with Nitric Oxide in the Presence
of Oxygen: Nitrosobenzene (1 mmol) was dissolved in benzene
(10 mL) and sodium nitrite (3 mmol) was added. Monochloroacetic
acid (3 mmol) was added to this suspension, which was stirred at
room temperature for 2 h. The reaction mixture was poured into
10% aqueous NaHCO3 (40 mL); the organic layer was washed with
water (2ϫ20 mL) and dried with Na2SO4. Evaporation of the sol-
vent gave nitrobenzene 2a in high yield (more than 90%).
Experimental Section
General: Melting points are uncorrected and were measured with
an Electrothermal apparatus. 1H NMR spectra were recorded at
room temperature in CDCl3 solutions with a Varian Gemini 200
spectrometer (TMS was taken as reference peak); J values are given
in Hz. IR spectra were recorded in the solid state with a Perkin–
Elmer MGX1 Spectrophotometer equipped with Spectra Tech.
Mass spectra were recorded with a Carlo Erba QMD 1000 mass
spectrometer in the positive EI mode. ESR spectra were run with
a Bruker EMX EPR spectrometer equipped with an XL microwave
frequency counter, Model 3120 for the determination of g factor
and with a Varian E4 spectrometer. The spectra were recorded with
the following instrumental settings: modulation frequency
100 kHz, modulation amplitude 0.4, sweep width 40 G, microwave
power 5 mW, time constant 1.28 s, receiver gain 5ϫ103. Computer
simulation of ESR spectra were calculated by using the WinSim
program in the NIEHS public ESR software tools package
C – Reaction of Nitrosobenzene with Nitric Oxide in the Absence of
Oxygen: Nitrosobenzene (1 mmol in 5 mL of benzene) was dis-
solved in one arm of a small Y-reactor together with sodium nitrite
(3 mmol), and the other arm contained monochloroacetic acid
(3 mmol) suspended in benzene (4 mL). The solutions were accu-
rately degassed by using the freeze–pump–thaw degassing method,
and the cycles were repeated at least three times. The two solutions
were then mixed and stirred at room temperature for 2 h and
worked up as described above in point B. A small portion of the
benzene solution (0.3 mL) was taken before complete evaporation,
degassed with argon and submitted to ESR spectroscopy: a signal
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FULL PAPER
corresponding to diphenylnitroxide was recorded. A yellowish solid
compound precipitated during evaporation of the solvent, which
was separated by filtration and identified as 4,4Ј-dinitrodiphenyl-
amine (9; 0.08 mmol, 18%). The filtrate was chromatographed on
silica gel (cyclohexane/ethyl acetate, 8:2), and the following prod-
ucts were collected: N-nitrosodiphenylamine 6 (trace), 4-nitro-N-
nitrosodiphenylamine 7 (41%) and 4-nitrodiphenylamine 8 (21%).
θ range = 3.78–70.42°, final R [IϾ2σ(I)]: R1 = 0.065, wR2 = 0.182,
R(all data): R1 = 0.106, wR2 = 0.207.
CCDC-677132 (for 7) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): Bond dissociation energies and geometry optimizations of
·NO2, PhN–N=O·, 10, 16 and 18.
Compound 7: Yellow solid, m.p. 129–131 °C (petroleum ether)
(ref.[31] 128–131 °C). IR: ν = 1599, 1496, 1477, 1341 cm–1.[31] 1H
˜
NMR (200 MHz, CDCl3, 25 °C): δ = 8.99 (d, J = 9.4 Hz, 2 H,
arom.), 7.57 (m, 4 H, arom.), 7.04 (m, 3 H, arom.) ppm.[32] MS
(EI+): m/z (%) = 214 (80), 184 (23), 167 (100); the molecular ion
peak was not visible in the mass spectrum.
Acknowledgments
Compound 8: Orange solid, m.p. 138–140 °C (acetone/petroleum
The authors would like to thank MIUR (Ministero dell’Università
e della Ricerca Scientifica e Tecnologica) and Università Po-
litecnica delle Marche for financial support.
ether 60–80 °C) (ref.[31] 132–135 °C). IR: ν = 3330, 1591, 1512,
˜
1341 cm–1.[31] 1H NMR (200 MHz, CDCl3, 25 °C): δ = 8.12 (d, J
= 9.16 Hz, 2 H, arom.), 7.39 (pseudo t, 2 H, arom.), 7.23 (m, 3 H,
arom.), 6.94 (d, J = 9.16 Hz, 2 H, arom.), 6.26 (br. s, 1 H, NH)
ppm.[33,34] MS (EI+): m/z (%) = 214 (89) [M]+, 167 (100).
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Compound 9: Yellow solid, m.p. 210–212 °C (acetone/petroleum
ether 60–80 °C) (ref.[31] 132–135 °C). IR: ν = 3340, 1614, 1585,
˜
1504, 1323, 1304 cm–1. 1H NMR (200 MHz, CDCl3, 25 °C): δ =
7.21 (d, J = 9.16 Hz, 4 H, arom.), 8.24 (d, J = 9.16 Hz, 4 H, arom.),
6.68 (br. s, 1 H, NH) ppm.[35] MS (EI+): m/z (%) = 259 (100)
[M]+, 229, (24), 183 (23), 167 (64).
·
In order to exclude the presence of NO2 that could be formed
during the decomposition of nitrous acid, the reaction between ni-
·
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Reactions Carried Out in the ESR Cavity
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·NO2 (0.5  in benzene, 0.25 mL) was introduced into the other
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the diarylnitroxide.
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B – Reaction of Nitrosoarenes with Nitric Oxide: Nitrosoarenes
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C – Oxidation of Cupferron: Cupferron (5; 2 mg) was suspended in
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Crystal Data for Compound 7: C12H9N3O3, M = 243.22, mono-
clinic, space group P21/c, a = 8.125(2) Å, b = 6.0212(9) Å, c =
23.490(15) Å, β = 95.93(3)°, V = 1143.0(8) Å3, Z = 4, Dcalcd.
=
1.413 mgm3, F(000) = 504, λ = 1.54178 Å, µ(Cu-Kα) = 0.880 mm–1,
T = 295(2) K, total/unique reflections = 2221/2173 [R(int) = 0.058],
3284
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 3279–3285

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Received: February 25, 2008
Published Online: May 9, 2008
Eur. J. Org. Chem. 2008, 3279–3285
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3285