Heterocyclic ions from N-oxides of N-(4-nitrobenzylidene)-2-cyclopropylaniline and N-(4-nitrobenzylidene)-2-cyclopropylmethylaniline: Formation, isomerization, and conversions

Article abstract of DOI:10.1023/A:1023720525095

The N-oxide of N-(4-nitrobenzylidene)-2-cyclopropylaniline is converted by the action of strong acids into the corresponding 2,1-benzoxazinium derivative. Under identical conditions the N-oxide of N-(4-nitrobenzylidene)-2-cyclopropylmethylaniline forms the corresponding 2,1-benzoxazinium and 2,1-benzoxazepinium salts in a ratio of 1: 2. The 2,1-benzoxazepinium ions are thermodynamically less stable and are isomerized with time into 2,1-benzoxazinium ions. Treatment of 2,1-benzoxazinium salts with hydrobromic acid solution and subsequent neutralization leads to o-(2-hydroxyalkyl)anilines and p-nitrobenzaldehyde. The effect of the nature of the ortho substituent on the direction of the conversions of the corresponding arylcyclopropanes is discussed.

Full text of DOI:10.1023/A:1023720525095

Chemistry of Heterocyclic Compounds, Vol. 39, No. 2, 2003
HETEROCYCLIC IONS FROM N-OXIDES OF
N-(4-NITROBENZYLIDENE)-2-CYCLOPROPYLANILINE
AND N-(4-NITROBENZYLIDENE)-2-CYCLOPROPYLMETHYL-
ANILINE: FORMATION, ISOMERIZATION, AND CONVERSIONS
E. V. Trofimova, A. N. Fedotov, R. A. Gazzaeva, S. S. Mochalov, Yu. S. Shabarov,
and N. S. Zefirov
The N-oxide of N-(4-nitrobenzylidene)-2-cyclopropylaniline is converted by the action of strong acids
into the corresponding 2,1-benzoxazinium derivative. Under identical conditions the N-oxide of
N-(4-nitrobenzylidene)-2-cyclopropylmethylaniline forms the corresponding 2,1-benzoxazinium and
2,1-benzoxazepinium salts in a ratio of 1 : 2. The 2,1-benzoxazepinium ions are thermodynamically less
stable and are isomerized with time into 2,1-benzoxazinium ions. Treatment of 2,1-benzoxazinium salts
with hydrobromic acid solution and subsequent neutralization leads to o-(2-hydroxyalkyl)anilines and
p-nitrobenzaldehyde. The effect of the nature of the ortho substituent on the direction of the conversions
of the corresponding arylcyclopropanes is discussed.
Keywords: o-(2-hydroxyalkyl)anilines, heterocyclic ions, N-oxides of N-(4-nitrobenzylidene)-2-
cyclopropylanilines, 2,1-benzoxazinium salts, 2,1-benzoxazepinium salts, isomerization.
The intramolecular acid-catalyzed reaction of o-nitro substituted phenyl- (1) and benzylcyclopropanes
(2) in the kinetically controlled stages of the conversion is completed with the formation of only the heterocyclic
N-oxo-3-ethylbenzisoxazolinium (3) and N-oxo-3-ethyl-2,1-benzoxazinium (4) ions. This is essentially the
result of attack of the internal nucleophile (nitro group) on the carbenium ions formed directly from the
cyclopropane fragments [1,2]. The regioselectivity of the processes of forming the heterocyclic ions 3 and 4 is
caused, as considered by the authors of [1,2], by the nucleophilic cooperation of the nitro group with
corresponding fission of the cyclopropane rings.
In progressing the investigations linked with clarifying the role of substituents capable of participating
in the intramolecular stabilization of the carbenium ions arising under acid-catalyzed conditions of breaking the
three-carbon ring in appropriate arylcyclopropanes, in the present work we have studied the conversion of the
N-oxides of N-(4-nitrobenzylidene)-2-cyclopropylaniline (7) and N-(4-nitrobenzylidene)-2-cyclopropyl-
methylaniline (8) under the action of fluorosulfonic acid and conc. H2SO4. The required N-oxides 7 and 8 were
synthesized from the corresponding o-cyclopropyl substituted nitrobenzenes 1 and 2 according to the following
scheme.
__________________________________________________________________________________________
Moscow M. V. Lomonosov State University, Moscow 119899, Russia; e-mail: fed@org.chem.msu.ru.
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 234-242, February, 2003. Original article
submitted June 20, 2000.
0009-3122/03/3902-0205$25.00©2003 Plenum Publishing Corporation
205

NO2
HC
O
R
R
Zn
NH4Cl
EtOH
NO2
NHOH
1, 2
5, 6
R
NO2
CH2
N
O
CH
7, 8
1, 5, 7 R =
2, 6, 8
;
R =
According to the structure of compounds 7 and 8 the N-oxide fragment in them [–N(O)=CH–] is
isoelectronic with the nitro group [–N(O)=O]. This forms a basis for proposing that the behavior of N-oxides 7
and 8 under standard conditions must not differ significantly from the behavior of nitro compounds 1 and 2.
However we have successfully established that the character of the conversions of compounds 7 and 8 under the
conditions taken differs from that which is typical for nitro compounds 1 and 2 [1,2].
+
+
H
_
_
+
O
+
O
N
N
NO2
CH
NO2
CH
9
7
+
+
1. –H
2. +H
+
_
OSO2F
_
+
O
+
O
N
N
NO2
CH
NO2
CH
10
11
O
+
N
NO2
CH
12
It turned out that, unlike 2-nitrophenylcyclopropane (1), compound 7 directly after mixing with
fluorosulfonic acid, was converted not into the expected analog of isoxazolinium ion 3 but into the six-
membered heterocyclic ion 12. Formally ion 12 is the result of intramolecular nucleophilic stabilization of a
carbenium ion of the homobenzyl type (10), the formation of which directly from the cyclopropane fragment of
the initial compound 7 is impossible.
It is important to note that the 2,1-benzoxazinium derivative 12 formed is so stable that on leaving it for
24 h in acid even at +20°C the formation of no significant quantity of any isomeric ions was observed.
Its analog N-oxide 8 also behaves anomalously compared with 2-nitrobenzylcyclopropane 2. Under the
action of fluorosulfonic acid this compound directly after mixing the reactants forms not only the expected
cyclic ions of the 2,1-benzoxazinium 15 type, but also the seven-membered cyclic ions 16, which does not
206

correspond to direct opening of the cyclopropane ring. Compound 16 belongs to the little known class of
4,5-dihydro-2,1-benzoxazepines. The quantity of the latter from the time of mixing the reactants and plotting the
1H NMR spectrum of the reaction mixture exceeded by twofold the amount of the six-membered ions 15. It
turned out later that depending on the duration of storing the solution of ions 15 and 16 in acid at 20°C (with
1
periodic plotting of H NMR spectra) the amount of 2,1-benzoxazepinium ions 16 fell but that of
2,1-benzoxazinium ions 15 increased and 2 h after the beginning of the reaction practically only proton signals
1
belonging to the structure of the six-membered ion 15 were identified in the H NMR spectrum of the reaction
mixture.
OSO2F
_
+
O
+
N
_
CH
NO2
NO2
+
O
18
16
N
CH
NO2
13
+
O
N
CH
+
1. –H
+
+
2. +H
1. –H
+
2. +H
OSO2F
_
+
O
N
+
O
_
+
CH
NO2
NO2
17
15
N
CH
NO2
14
+
O
N
+
H
CH
_
+
16
67%
15
O
N
0.5 h
2 h
33%
CH
NO2
98%
2%
8
The structures of the cyclic ions 12, 15, and 16 were confirmed by 1H and 13C NMR spectral data and by
comparison of the characteristic parameters of these spectra with the parameters of the spectra of the analogous
ions 4 and 19-21 described previously (see Tables 1 and 2).
10
7
7
8
4
5
8
3
4a
10
6
7
9
Cl
9
Cl
+
+
O
+
+
O
O
O
8a
N
N
N
N
8
O
O
O
O
20
21
4
19
207

1
TABLE 1. H and 13C NMR Spectra of the Aliphatic Fragments of the
Cyclic Ions 12, 15, and 16 and Their Known Analogs
1Н NMR spectrum, δ, ppm
13С NMR spectrum, δ, ppm
Ion
Literature
Н-(7)
Н-(8)
Н-(9)
Н-(10)
С(7)
С(8)
С(9)
С(10)
3.22 d
5.04 qt
1.68 d
—
31.73
29.75
29.42
83.96
96.06
88.65
7.44
—
—
[4]
—
12
19
15
(2Н)
(1Н)
(3Н)
3.68 m
6.06 m
1.95 d
—
16.90
25.05
—
(2Н)
(1Н)
(3Н)
3.61 d
(1Н);
3.65 s
(1Н)
5.21 qt
2.46 m
1.51 t
7.20
(1Н)
(2Н)
(3Н)
3.62 m
5.86 m
2.27 m
1.18 t
27.76
32.91
29.90
21.90
100.24 24.95
6.52
16.92
20.75
[5]
—
4
(2Н)
(1Н)
(2Н)
(3Н)
3.48 m
2.58 m
5.24 m
1.98 d
25.05
47.66
86.94
98.78
16
20
21
(2Н)
(2Н)
(1Н)
(3Н)
4.32 m
2.03 m
2.18 s
[6]
[6]
(2Н)
(2Н)
(3Н)
4.32 m
3.09 m
7.31 t
2.01 d
32.47 106.71 20.46
(2Н)
(1Н)
(1Н)
(3Н)
It is evident from the data given in Table 1 that the character of the spin-spin couplings and the values of
13
the chemical shifts of the corresponding 1H and C atoms of the aliphatic fragments of both the six-membered
ions 12 and 15 and the seven-membered ion 16 correlate satisfactorily with the parameters of the cyclic ions
similar to them formed, as was demonstrated in [3-5], from the corresponding o-nitro substituted
arylcyclopropanes under the conditions taken.
It is interesting to consider one further aspect confirming indirectly the structure of the heterocyclic ions
1
12, 15, and 16 obtained by us. It was discovered previously in [6] that in the H NMR spectrum of 3-ethyl-3-
methyl-N-(4-nitrophenyl)methylidene-2,1-benzisoxazolinium ions (23) formed on dissolving 2-(1-methyl-
cyclopropyl)-N-(4-nitrobenzylidene)aniline N-oxide (22) in trifluoroacetic acid, the signal of the azomethine
proton was displayed 1.47 ppm further towards low field than the signal of this proton in the spectrum of the
initial compound 22 in CCl4. The chemical shifts of all the protons of the p-nitro substituted benzene ring have
1
the same value, but in the H NMR spectrum of the initial N-oxide 22 in CCl4 the hydrogen atoms of the
nitrophenyl fragment are displayed as two doublets characteristic for 1,4-disubstituted benzenes containing
nonequivalent substituents (Table 2).
TABLE 2. Chemical Shifts of Protons of the N-Oxoazomethine Group and
1
p-Nitrophenyl Fragments in the H NMR Spectra of Compounds 7, 8, 12,
15, 16, 22, and 23
Chemical shifts, δ, ppm (J, Hz)
Compound or ion
+
Ar–H
(s, 1H)
O
N=CH–
7.82
7.93
7.91
9.17
9.43
9.06
9.38
8.25 d (2Н); 8.48 d (2Н); Jо = 9
8.36 d (2Н); 8.69 d (2Н); Jо = 8.8
8.19 d (2Н); 8.47 s (2Н); Jо = 9
8.71 s (4Н)
8.97 s (4Н)
8.94 s (4Н)
7
8
22*
12
15
16
23*
8.58 s (4Н)
_______
* Data of [6].
208

Such a difference in the multiplicity of proton signals of identical fragments and in their chemical shifts
is explained by the authors of [6] by the fact that the N-oxide fragment participates in the formation of the
heterocyclic ion 23. As a result of this there is delocalization of the positive charge on the nitrogen atom of the
heterocycle with the participation of both the azomethine double bond (displacing the proton signal towards low
field) and also of the p-substituted benzene ring (varying the proton chemical shifts of the latter).
1
As is evident from the data of Table 2 the changes of the characteristics of the H NMR spectra of the
initial N-oxides 7 and 8 and the ions 12, 15, and 16 formed from them have the same character, as for
compounds 22 and 23. This may indicate only that in our case the N-oxide fragment participates in the
formation of the heterocyclic ions from compounds 7 and 8 under conditions of the acid-catalyzed fission of the
cyclopropane rings in them.
In principle it was impossible also to exclude the probability of stabilizing the carbenium ions formed
from the cyclopropane rings of compounds 7 and 8 not by intra- but by intermolecular attack of nucleophile
which should lead to a covalently bonded fluorosulfonate of the type of 11, 17, and 18. How as far is it possible
to judge from NMR spectral data, the character of the multiplicity of the signals of the corresponding atoms
(1H or 13C) and their chemical shifts may correspond both with the cyclic ions 12, 15, and 16 and also with the
covalently bonded fluorosulfonates 11, 17, and 18. Considering that the covalently bonded fluorosulfonates
formed from aryl cyclopropanes containing strong electron-withdrawing groups in the aromatic ring are capable
of existing only at temperatures between -70 and -30°C [7], and also that, in our view, the form of the fragments
1
of the H NMR spectra of fluorosulfonates 11, 17, and 18 belonging to the p-nitrobenzylidene substituent (see
Table 2) must be practically identical to the corresponding portion of the spectra of the initial N-oxides, we
consider that the stable reaction products resulting from compounds 7 and 8 are not covalently bonded
fluorosulfonates but the cyclic ions 12, 15, and 16.
It is interesting that the chemical behavior of the cyclic ions 12 and 15 formed from N-oxides 7 and 8
also differed from the behavior of their six-membered analogs obtained from the corresponding
o-nitrophenylcyclopropanes [1,2]. As follows from the data of [8,9] treatment of a solution of 3-ethyl-2,1-
benzoxazinium N-oxide salt (4) with ice water or cooled hydrobromic acid leads in quantitative yield either to
1-(2-nitrophenyl)-2-butanol [8] or to 2-bromo-1-(2-nitrophenyl)butane [9]. On the other hand, under the action
of 40% HBr the 2,1-benzoxazinium ion 15 is converted not into a bromo substituted N-oxide of type 24, as
might have been expected, but into 1-(2-aminophenyl)-2-butanol (25). The second reaction product was
p-nitrobenzaldehyde in this case.
Br
HBr
_
1. 40% HBr
2. NH3
OH
NH2
+
O
15
N
CH
NO2
25, 63%
24
We assume that since the formation of 1-(2-nitrophenyl)-2-butanol was not observed in the reaction of
ion 4 with 40% HBr [8], the formation of the hydroxy group in compound 25 occurs not as a result of
nucleophilic attack by a water molecule at the carbon atom bound in heterocycle 15 with an oxygen atom, but in
all probability is initiated by attack of bromine anion at the benzyl carbon atom of the nitrobenzylidene
fragment. The result of this is the formation of N-(α-bromo-p-nitrobenzyl)-3-ethyl-2,1-benzoxazinium salt (26),
which may in principle correspond to the formation of p-nitrobenzaldehyde and 3-ethyl-2,1-benzoxazine (27),
capable under the reaction conditions of being converted into 2-aminophenyl-2-butanol (25).
It is however not excluded that intermediate 26 is converted into the same final products through the
preliminary fission of the N–O bond in it.
209

In order to explain the relatively rapid formation of the cyclic ions 12 and 16, which do not correspond
with the direct fission of the cyclopropane ring in the initial N-oxides 7 and 8 under the action of acid, it is
obviously necessary to take two factors into consideration which may influence the process of forming the
structures indicated. These are the nucleophilic activity of the ortho substituent participitating in the
heterocyclization, and the stability of the open type carbenium ions formed on breaking the cyclopropane
fragments. In reality, under the action of strong proton acids carbenium ions of the benzyl (secondary) type must
be formed from the three-carbon rings of compounds 1 and 7 (with or without nucleophilic assistance). Ions of
such a type actively interact with a nucleophilic oxygen atom of the nitro group, giving cyclic
benzisoxazolinium ions 3 [1]. In difference to this the nucleophilic activity of the N-oxide oxygen atom is
probably significantly less (for example, due to the reduction of the negative charge on the nucleophilic oxygen
atom as a result of its delocalization along the p-nitrobenzylidene substituent system) and a carbenium ion of
benzyl type 9 is isomerized to the more stable homobenzyl carbocation 10 before effecting heterocyclization.
The latter also interacts with the ortho substituent giving ion 12. In favor of this for example are the data on the
ions of benzyl type formed from p-nitrophenylcyclopropane by the action of fluorosulfonic acid. These are
capable even at -70°C of rapidly isomerizing into ions of the homobenzyl type [7] . Also in favor are the tertiary
cations of the benzyl type generated from the cyclopropane ring of N-oxide 22, which are not capable under the
reaction conditions of being rapidly isomerized into the corresponding ions of a homobenzyl type (secondary),
they readily form heterocyclic ions 23 (Table 2 [6]) by intramolecular interaction with the N-oxide fragment.
In all probability the same factors influence the course of the acid catalyzed conversion of compound 8,
and, as in the case of compound 7, in the kinetically controlled stage of the reaction a determining role is played
by the stability of the carbenium ions formed. Actually it is possible to explain the predominant formation of
heterocyclic ions of seven-membered structure 16 at the initial stage of the conversion if it is accepted that the
ions of homobenzyl type 14, formed directly on breakdown of the three-carbon ring in compound 8, are capable
of rapid isomerization into ions of γ-type 13. That such an assumption has a basis is indicated by the data on the
study of the behavior of p-nitrobenzylcyclopropane in the Ritter reaction [10], from which it follows that in
spite of the absence of the possibility of the direct formation of γ-carbenium ions from the initial substrate,
adducts corresponding to it are formed in predominant amount.
A determining role in the thermodynamically controlled stage of the reaction of compound 8 with
fluorosulfonic acid is evidently played by the different stability of the heterocyclic ions 15 and 16. Analysis of
spatial models of the cyclic ions 15 and 16 and their relative energies calculated by MNDO and AM1*
quantum-chemical methods indicate the higher thermodynamic stability of ion 15, which also causes
isomerization of ions 16 into ions of the six-membered structure 15 through the reversible generation of their
open form ions 13 and 14 similarly to that described in [8].
EXPERIMENTAL
1
The H NMR spectra were obtained on a Varian XL 100 (100 MHz) and VXR 400 (400 MHz)
spectrometers in CCl4 and CDCl3 (internal standard was TMS), and the IR spectra on a UR 20 instrument (in
liquid films or nujol). A check on the purity of compounds was effected by GLC on a Tsvet-104 instrument.
Carrier gas was nitrogen, column was 5% SE-30 on Chromaton N-AW-DMCS (3000 × 4 mm). Column
chromatography was carried out on aluminum oxide of activity grade II, eluent was ether–pentane, 2:1 or ether.
2-Nitrophenylcyclopropane 1 was obtained according to [11], yield 85%; bp 106-107°C (5 mm Hg),
n20D 1.5613.
_______
* The corresponding material is being prepared for publication.
210

2-Nitrobenzylcyclopropane 2 was synthesized according to [12], yield 48%; bp 103-104°C (1 mm Hg),
n20D 1.5452.
N-Oxides of N-(4-Nitrobenzylidene)-2-cyclopropylaniline (7) and N-(4-Nitrobenzylidene)-2-
cyclopropylmethylaniline (8). Zinc dust (3.24 g) was introduced portionwise into a mixture of the appropriate
o-nitroarylcyclopropane (20 mmol) and a solution of ammonium chloride (1.32 g) in water (77 mmol)
maintaining the temperature of the reaction mixture at 40-45°C. The mixture was stirred for 1 h, the precipitated
solid was filtered off, and washed with hot water (50 ml). The filtrate was cooled to room temperature, extracted
with ether (3 × 50 ml), and the extract dried with magnesium sulfate. The solvent was evaporated, and the
residue stirred with a solution of p-nitrobenzaldehyde (3.24 g, 0.02 mmol) in alcohol (50 ml), and left overnight.
After evaporating the solvent the residue was recrystallized from ethanol.
Nitrone 7 (1.81 g, 32%) of mp 127-128°C was obtained from compound 1. IR spectrum, ν, cm-1: 1400
1
(N=O), 1620 (C=N). H NMR spectrum, δ, ppm (J, Hz): 0.78 and 1.0 (4H, two m, 2CH2 c-Pr); 2.2 (1H, m,
CH c-Pr); 7.00-7.80 (4H, m, Harom); 7.8 [1H, s, N(O)=CH]; 8.30-8.55 (4H, dd, J = 8, Harom). Found, %: C 68.08;
H 5.03; N 9.84. C16H14N2O3. Calculated, %: C 68.08; H 4.96; N 9.93.
Nitrone 8 (2.94 g, 46%) of mp 124-125°C was obtained from compound 2. IR spectrum, ν, cm-1:
1
1460 (N–O), 1610 (C=N). H NMR spectrum, δ, ppm (J, Hz): 0.18-0.87 (4H, m, 2CH2 c-Pr); 0.9-1.45 (1H, m,
CH c-Pr); 2.75 (2H, d, benzyl protons); 7.37-7.74 (4H, m, Harom); 8.36 (2H, d); 8.69 (2H, d, Jo = 8.8, Harom); 7.90
(1H, s, N=CH). Found, %: C 69.08; H 5.49; N 9.71. C17H16N2O3. Calculated, %: C 68.91; H 5.44; N 9.45.
Heterocyclic Ions 12, 15, and 16 from N-Oxides 7 and 8 (General Procedure). The appropriate
N-oxide (1.2 mmol) was added gradually with stirring to fluorosulfonic acid (2 ml) cooled to -50°C. The
reaction mixture was stirred until complete solution of the starting material, the temperature was raised to 20°C,
and the composition of the products analyzed by 1H and 13C NMR.
Conversions of the N-(4-Nitrophenyl)methylidene-3-ethyl-2,1-benzoxazinium Ion (15) under the
Action of Hydrobromic Acid. Nitrone 8 (10 mmol) was added in small portions with constant stirring to
conc. H2SO4 (10 ml) cooled to -30°C. The reaction mixture was maintained at the same temperature for 1 h, then
poured into hydrobromic acid (20 ml), and left for 12 h. The reaction mixture was then extracted with
chloroform (4 × 50 ml), the aqueous solution after extraction was neutralized with ammonium hydroxide, and
extracted once again with chloroform (3 × 50 ml). The chloroform extracts from the neutralized solution were
dried, the solvent evaporated, and 2-(2-hydroxybutyl)aniline (25) (1.04 g, 63%) was obtained as a yellow oil.
IR spectrum, ν, cm-1: 1630, 3380, 3480 (NH2), 3645, 3250 (OH). 1H NMR spectrum, δ, ppm (J, Hz): 0.83 (3H, t,
J = 7.0, CH3); 1.18-1.71 (2H, m, –CH2–CH3); 2.49 (2H, d, J = 5.2, Ar–CH2); 3.31-3.80 (1H, m, CH); 3.98 (3H,
s, –NH2 and –OH); 6.25-7.36 (4H, m, Harom). Found, %: C 72.78; H 9.21; N 8.53. C10H15NO. Calculated, %:
C 72.73; H 9.11; N 8.49.
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