A study of the reaction mechanism of 3-nitro-4-R-furoxans formation by nitrosation of dipotassium salts of 1-hydroxyimino-2,2-dinitro-1-R-ethanes

Article abstract of DOI:10.1007/s11172-011-0134-7

The mechanism proposed earlier for explanation of the furoxan ring formation in the nitrosation of dipotassium salts of 1-hydroxyimino-2,2-dinitro- 1-R-ethanes with NaNO₂/AcOH was confirmed experimentally by determining the ionization constants of the dinitromethyl and oxime fragments in the starting dipotassium salt and by examining ¹H, ¹³C, ¹⁴N, and ¹⁵N NMR and mass spectra of isomeric 3(4)-nitro-4(3)-R-furoxans with the ¹⁵N(5) and ¹⁵N(2) ring atoms, respectively, and 3,4-diarylfuroxan with both ¹⁵N-labeled ring atoms. A comparative analysis of the IR and Raman spectra of the ¹⁵N-labeled furoxan derivatives obtained and their unlabeled analogs allowed refinement of the literature data on interpretation of the vibrational spectra of furoxan derivatives.

Full text of DOI:10.1007/s11172-011-0134-7

Russian Chemical Bulletin, International Edition, Vol. 60, No. 5, pp. 855—860, May, 2011
855
A study of the reaction mechanism of 3ꢀnitroꢀ4ꢀRꢀfuroxans formation
by nitrosation of dipotassium salts of 1ꢀhydroxyiminoꢀ2,2ꢀdinitroꢀ1ꢀRꢀethanes*
I. V. Ovchinnikov, Yu. A. Strelenko, N. A. Popov, A. O. Finogenov, and N. N. Makhova
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5328. Eꢀmail: mnn@ioc.ac.ru
The mechanism proposed earlier for explanation of the furoxan ring formation in the
nitrosation of dipotassium salts of 1ꢀhydroxyiminoꢀ2,2ꢀdinitroꢀ1ꢀRꢀethanes with NaNO2/AcOH
was confirmed experimentally by determining the ionization constants of the dinitromethyl and
oxime fragments in the starting dipotassium salt and by examining 1H, 13C, 14N, and 15N NMR
and mass spectra of isomeric 3(4)ꢀnitroꢀ4(3)ꢀRꢀfuroxans with the 15N(5) and 15N(2) ring atoms,
respectively, and 3,4ꢀdiarylfuroxan with both 15Nꢀlabeled ring atoms. A comparative analysis
of the IR and Raman spectra of the 15Nꢀlabeled furoxan derivatives obtained and their unlaꢀ
beled analogs allowed refinement of the literature data on interpretation of the vibrational
spectra of furoxan derivatives.
Key words: mechanism, nitrosation, dipotassium salts of 1ꢀhydroxyiminoꢀ2,2ꢀdinitroꢀ1ꢀRꢀ
ethanes, ionization constants, isomeric 15Nꢀlabeled 3(4)ꢀnitroꢀ4(3)ꢀRꢀfuroxans, 15Nꢀlabeled
1
3,4ꢀdiarylfuroxan, H, 13C, 14N, and 15N NMR spectra, IR spectroscopy, mass spectrometry,
13C—15N coupling constants.
Earlier,1,2 a novel method for the synthesis of 3ꢀnitroꢀ
Scheme 1
4ꢀRꢀfuroxans 1 has been developed at the laboratory of
nitrogenꢀcontaining compounds of the N. D. Zelinsky Inꢀ
stitute of Organic Chemistry (Russian Academy of Sciꢀ
ences). The method involves nitrosation of dipotassium
salts of 1ꢀhydroxyiminoꢀ2,2ꢀdinitroꢀ1ꢀRꢀethanes 2 with
NaNO2/AcOH. The starting compounds 2 were prepared
from hydroximoyl chlorides 3 and a sodium salt of diniꢀ
tromethane. Thermodynamically more stable 4ꢀnitroꢀ3ꢀ
Rꢀfuroxans 4 were obtained by thermal isomerization of
3ꢀnitro isomers 1 in boiling toluene. The aforesaid syntheꢀ
sis of compounds 1 offers a novel method of furoxan ring
formation. The mechanism proposed for this reaction inꢀ
volves (1) nitrosation of the anion of the dinitromethyl
fragment in compounds 2, (2) an intramolecular attack of
the oxime anion in intermediate 5 on the N atom of the
nitroso group, and (3) elimination of NO2 anion, which
leads to furoxan 1 (Scheme 1).
Here we present experimental data that confirm the
earlier proposed mechanism. The data were obtained
using two approaches: (1) determination of the ionization
constants of the dinitromethyl and oxime fragments
in dipotassium salts 2 and (2) synthesis of isomeric
3(4)ꢀnitroꢀ4(3)ꢀRꢀfuroxans 1´ and 4´ with the 15N(5)
and 15N(2) ring atoms, respectively, and 3,4ꢀR2ꢀfurꢀ
R = 4ꢀMeOꢀ3,5ꢀ(NO2)2C6H2 (a), Ph (b), 2ꢀNO2C6H4 (c),
3ꢀNO2C6H4 (d), 4ꢀNO2C6H4 (e), 4ꢀBrC6H4 (f),
3,4,5ꢀ(NO2)3C6H2 (g), COMe (h), CO2Et (i)
oxan 6´ with both 15Nꢀlabeled ring atoms and examinaꢀ
1
tion of their H, 13C, 14N, and 15N NMR and mass
* Dedicated to Academician V. N. Charushin on the occasion of
his 60th birthday.
spectra.aa
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 835—840, May, 2011.
1066ꢀ5285/11/6005ꢀ855 © 2011 Springer Science+Business Media, Inc.

856
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 5, May, 2011
Ovchinnikov et al.
Optimization of the conditions for this reaction has
where εB, ε, and εBH+ are the molar absorption coefficients
of the fully ionized, halfꢀprotonated, and fully protonated
forms, respectively; HX is the Hammett acidity function,
we graphically determined the pKa values of the oxime
(pKa = 4.45) and dinitromethyl fragments (pKa = 2.23)
revealed1,2 that addition of an equimolar amount of AcOK
to the reaction mixture prior to the nitrosation of dipotasꢀ
sium salts 2 increases the yields of furoxans 1 from 50—55
to 68—80%. This fact is indirect evidence for the particiꢀ
pation of the oxime anion in the cyclization. However, to
verify this conjecture, as well as to estimate the participaꢀ
tion of the anion of the dinitromethyl fragment in the first
reaction step (nitrosation), we should experimentally deꢀ
termine the acidity constants of both the fragments. These
constants were estimated spectrophotometrically on
the example of dipotassium salt 2a (R = 4ꢀMeOꢀ3,5ꢀ
(NO2)2C6H2). The spectrum contains two absorption
peaks with λmax = 250 (weak) and 370 nm (intense). It is
known3a,b that the intense band with λmax = 350 nm corꢀ
responds to the π—π*ꢀtransition of the dinitromethyl
group. Since the dinitromethyl group in salt 2a is conjuꢀ
gated with the oxime group, the band with λmax = 370 nm
in its UV spectrum can most likely be attributed to the
π—π*ꢀtransition of the oximinoꢀdinitromethyl fragment,
while the weak band with λmax = 250 nm is probably due
to the π—π*ꢀtransition of the benzene fragment.
1
2
(see Experimental).
The molar absorption coefficient of the band with
λmax = 250 nm changed only slightly during the experiꢀ
ment and did not reach zero upon the increase in c, which
confirms the correct assignment of this band to the benzꢀ
ene fragment of the molecule.
The data obtained provide much evidence for the mechꢀ
anism proposed. Indeed, the first reaction step should inꢀ
volve nitrosation of the anion of the dinitromethyl fragꢀ
ment. Insofar as the oxime fragment has a lower (though
insignificantly) pKa value (4.43) than the pKa of AcOH
1
(4.74), addition of AcOK actually increases the concenꢀ
tration of this fragment and, consequently, the yield of the
final product 4ꢀ(4ꢀmethoxyꢀ3,5ꢀdinitrophenyl)ꢀ3ꢀnitroꢀ
furoxan (1a).
As a second approach to proving the mechanism of the
reaction discovered, we obtained isomeric 3(4)ꢀnitroꢀ4(3)ꢀ
Rꢀfuroxans 1´ and 4´ containing 15Nꢀlabeled atoms in the
furoxan ring (N(5) and N(2), respectively, in compounds
1´f and 4´f as examples. For this purpose, we obtained
from 4ꢀbromobenzaldehyde and 15Nꢀlabeled hydroxylꢀ
amine (96.7%ꢀenriched 15NH2OH•HCl) compound 3´f
(R = 4ꢀBrC6H4) and then transformed it into furoxans 1´f
and 4´f through intermediates 2´f and 5´f. In addition,
dehydrochlorination of 15Nꢀlabeled acid chloride 3´f gave
nitrile oxide 7, which undergoes cyclodimerization into
diarylfuroxan 6´ containing two 15N ring atoms (Scheme 2).
Spectrophotometric determination of acidity constants
is based on the dependence of the molar absorption coeffiꢀ
cients on the relative contents of the fully protonated,
halfꢀprotonated, and ionized forms in acidꢀbase equilibriꢀ
um. For salt 2a, the UV spectra of its solutions in water
and in 0.1 M NaOH are identical, indicating that this salt
is fully ionized in water. As an aqueous solution of salt 2a
is gradually acidified with H2SO4, the molar absorption
coefficient of the band with λmax = 370 nm decreases to
reach zero at c = 44% (c is the concentration of H2SO4 in
solution); i.e., salt 2a is fully protonated (a nondissociated
acid form) under these conditions. Note that at c = 0.50,
0.91, and 2.00%, the molar absorption coefficients of
the band with λmax = 370 nm are equal. Obviously, salt 2a
is monoprotonated in this pH range. Using the stanꢀ
dard formula4,5
1
The H, 13C, 14N, and 15N NMR spectra of all the
15Nꢀlabeled furoxans obtained are given in Table 1.
According to the data in Table 1, the labeled N atoms
are included in the furoxan ring: the chemical shifts for the
15N(5) atoms in compounds 1´f and 6´ are identical and
the chemical shifts for the 15N(2) atoms in compounds 4´f
and 6´ have close values. The positions of the labeled 15
atoms in the compounds studied are evident from the coupꢀ
N
= pK – HX
,
(1)
ling constants J13
N: the nearer the corresponding
15
C—
Table 1. 1H, 13C, 14N, and 15N NMR spectra of 15Nꢀlabeled furoxans 1´f, 4´f, and 6´
Furoxan
δ1
δ14
δ13 , J13
N/Hz
15
H
N
C
C—
[δ14N, Δν1/2/Hz]
1´f
4´f
7.57, 7.70 (both d, –12.0 (N(5)) [–39.0, 122.4 (C(1), Ar, 2J = 6.2); 126.1 (C(3), furoxan); 127.3 (C(4), Ar, 5J = 0.6);
3J = 8.2) 130.5 (C(2), Ar, 3J = 2.0); 132.5 (C(3), Ar); 150.5 (C(4), furoxan, 1J = 2.6)
NO2, Δν1/2 = 11]
7.47, 7.68 (both d, –16.2, (N(2)) [–35.4, 108.7 (C(3), furoxan, 1J = 24.5); 118.4 (C(1), Ar, 2J = 1.0); 126.7 (C(4),
3J = 8.36)
NO2, Δν1/2 = 36]
Ar, 5J = 0.5); 130.3 (C(2), Ar, 3J = 1.5); 132.6 (C(3), Ar); 157.7 (C(4),
furoxan, 2J = 1.0)
6´
7.38 (br.d,
3J = 8.27);
7.57 (m)
–12.20 (N(5)),
–25.38 (N(2))
113.3 (C(3), furoxan, 1J = 24.56, 2J = 1.33); 121.6 (C(1), Ar, 2J = 1.19);
125.3 (C(4), Ar); 125.4 (C(1´), Ar, 2J = 6.62, 3J = 1.65); 126.0 (C(4´), Ar);
129.8 (C(2), Ar, 3J = 1.98); 130.1 (C(2´), Ar, 3J = 1.39); 132.5 (C(3´), Ar);
132.6 (C(3), Ar); 155.1 (C(4), furoxan, 1J = 6.3, 2J = 1.86)

Mechanism of formation of 3ꢀnitroꢀ4ꢀRꢀfuroxans
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 5, May, 2011
857
C atom to the labeled 15N atom, the higher the coupling
for the corresponding fragmentation ions (see Table 2).
The fragmentation character fully agrees with known data
on the fragmentation of diarylfuroxans and nitrofurꢀ
oxans.6a EIꢀinduced fragmentation of a furoxan ring is
characterized by successive elimination of two NO fragꢀ
ments; nitrofuroxans lose the NO2 group and the NO fragꢀ
ment of the ONO group into which the NO2 group isomerꢀ
izes under electron impact. A similar pattern was observed
in the mass spectra of the compounds studied.
constant. The coupling constants J13
in the spectra
15
C—
N
of compounds 1´f and 4´f approximate to those in the
spectrum of furoxan 6´. Some differences in the chemical
shifts of the corresponding C atoms, the labeled N atoms,
and the coupling constants in the spectra of compounds
1´f, 4´f, and 6´ are due to the influence of the nitro group
in the first two compounds.
Thus, the determination of the ionization constants of
the dinitromethyl and oxime fragments in the starting
dipotassium salts 2 (with salt 2a as an example), as well as
the synthesis of 15Nꢀlabeled furoxans (with compounds
1´f, 4´f, and 6´ as examples) and comparative analysis of
Scheme 2
1
their H, 13C, and 15N NMR and mass spectra, provides
experimental evidence that the mechanism proposed earlier
for the novel synthesis of 3ꢀnitroꢀ4ꢀRꢀfuroxans is correct.
The synthesis of 15Nꢀlabeled furoxans 1´f, 4´f, and 6´
allowed some refinements to the literature data on the IR
and Raman spectra of the furoxan ring using the spectra of
the unlabeled isomeric furoxans 4ꢀ(4ꢀbromophenyl)ꢀ3ꢀ
nitrofuroxan (1f), 3ꢀ(4ꢀbromophenyl)ꢀ4ꢀnitrofuroxan (4f),
and 3,4ꢀbis(4ꢀbromophenyl)furoxan 6 and their 15Nꢀlaꢀ
beled analogs 1´f, 4´f, and 6´ (Tables 3—5).
As expected, 15Nꢀlabeling of the furoxan ring causes
noticeable shifts of the vibrational frequencies of the ring
bonds to the lower frequencies (see Tables 3—5). In addiꢀ
tion, analogous shifts are experienced by some of the abꢀ
sorption bands of the benzene ring. This makes it someꢀ
what more difficult to interpret the spectral patterns, esꢀ
pecially for compounds 6 and 6´. The lowerꢀfrequency
shifts in the spectra of compound 6´ (by 35 (IR) and
14 cm–1 (Raman)) and in the IR spectrum of compound
4´f (by 19 cm–1) compared to the corresponding spectra
of their unlabeled analogs 6 and 4f are observed for a band
at 1600 cm–1 (stretching vibrations of the furoxan
ring). Insofar as 15Nꢀlabeling of the N(5) atom of the
furoxan ring (compound 1´f) does not change the freꢀ
quency of these vibrations against those in compound 1f,
it is obvious that they are mainly contributed by the
fragment C=N→O. This conclusion agrees with the literꢀ
ature data.6b
The small lowerꢀfrequency shifts (by 3—4 cm–1) of the
absorption bands at 1500—1550 cm–1 in the IR spectra of
15Nꢀlabeled compounds 1´f, 4´f, and 6´ compared to their
unlabeled analogs 1f, 4f, and 6 suggest that these vibraꢀ
tions are mainly contributed by the fragments containing
no labeled atoms; for this reason, we assigned them to the
C—C stretching vibrations in the furoxan ring. For comꢀ
pound 1´f, the lowerꢀfrequency shift of the weak band at
1480 cm–1 (by 9 cm–1) compared to compound 1f (see
Table 3) can be attributed to a substantial contribution of the
N(5)ꢀcontaining fragment of the furoxan ring (C=N—O)
to these vibrations. In the Raman spectrum (compounds 6
and 6´, Table 5), this vibration (1466 cm–1) is of medium
Fragmentation of all three compounds 1´f, 4´f, and 6´
in the mass spectra also agrees with the 15Nꢀlabeled strucꢀ
tures proposed. Because bromine exists as a mixture of
the 79Br and 81Br isotopes in equal amounts, the fragmenꢀ
tation data for compounds 1´f and 4´f are given separateꢀ
ly for either isotope (79Brꢀ1´f,81Brꢀ1´f, 79Brꢀ4´f, and
81Brꢀ4´f; Table 2). These four compounds undergo identiꢀ
cal fragmentation with different relative peak intensities

858
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Ovchinnikov et al.
Table 2. Mass spectra of 15Nꢀlabeled furoxans 1´f, 4´f, and 6´
Compound
MS, m/z (Irel (%))
286 [M]+ (28), 255 [M – 15NO]+ (16), 240 [M – NO2]+ (23), 225 [M – 15NO – 14NO]+ (44),
209 [M – 15NO – NO2]+ (98), 195 [M – 15NO – 2 14NO]+ (6), 179 [M – 15NO – 14NO – NO2]+ (12),
167 [Br + C6H4 + C]+ (29), 155 [Br + C6H4]+ (9), 130 [M – BrC6H4]+ (8), 114 [M – BrC6H4 – O]+ (9),
99 [M – BrC6H4 – 15NO]+ (32)
79Brꢀ1´f
81Brꢀ1´f
79Brꢀ4´f
81Brꢀ4´f
6´
288 [M]+ (29), 257 [M – 15NO]+ (16), 242 [M – NO2]+ (28), 227 [M – 15NO – 14NO]+ (39),
211 [M – 15NO – NO2]+ (90), 197 [M – 15NO – 2 14NO]+ (6), 181 [M – 15NO – 14NO – NO2]+ (13),
169 [BrC6H4 + C]+ (23), 157 [BrC6H4]+ (7), 131 [M – BrC6H4 – 1]+ (7), 102 [M – BrC6H4 – 15NO – 1]+ (100),
101 [M – BrC6H4 – 14NO – 1]+ (9), 100 [M – BrC6H4 – 15NO]+ (55)
286 [M]+ (28), 255 [M – 15NO]+ (23), 240 [M – NO2]+ (17), 225 [M – 15NO – 14NO]+ (69),
209 [M – 15NO – NO2]+ (55), 195 [M – 15NO – 2 14NO]+ (7), 179 [M – 15NO – 14NO – NO2]+ (10),
167 [BrC6H4 + C]+ (54), 155 [BrC6H4]+ (9)], 130 [M – BrC6H4]+ (5), 114 [M – BrC6H4 – O]+ (22),
99 [M – BrC6H4 – 15NO]+ (49)
288 [M]+ (27), 257 [M – 15NO] (22), 242 [M – NO2]+ (16), 227 [M – 15NO – 14NO]+ (72),
211 [M – 15NO – NO2]+ (54), 197 [M – 15NO – 2 14NO]+ (6), 181 [M – 15NO – 14NO – NO2]+ (11),
169 [BrC6H4 + C]+ (53), 157 [BrC6H4]+ (7), 131 [M – BrC6H4 – 1]+ (4), 102 [M – BrC6H4 – 15NO – 1]+ (100),
101 [M – BrC6H4 – 14NO – 1]+ (6), 100 [M – BrC6H4 – 15NO]+ (57)
396 [M]+ (for two 79Br) (6), 398 [M]+ (for 79Br and 81Br) (12), 400 [M]+ (for two 81Br) (6), 334 (52), 336 (100),
338 [M – 2 15NO]+ (50), 177 [M – 2 15NO – 2 81Br]+ (13), 176 [M – 2 15NO – 2 81Br – 1]+ (82),
175 [M – 2 15NO – 2 81Br]+ (14), 174 [M – 2 15NO – 2 81Br – 1]+ (10)
intensity and the corresponding band is shifted by 4 cm–1
to the lower frequencies.
oxan ring in the synthesis of 3ꢀnitroꢀ4ꢀRꢀfuroxans 1 by
nitrosation of dipotassium salts of 1ꢀhydroxyiminoꢀ2,2ꢀ
Two absorption bands in the 1300—1450 cm–1 range
(the lowerꢀfrequency band at 1300—1360 cm–1 and the
higherꢀfrequency band at 1380—1450 cm–1) in the specꢀ
tra of all the compounds studied (1f, 4f, and 6 and their
15Nꢀlabeled analogs 1´f, 4´f, and 6´) were assigned to the
O—N→O stretching vibrations in the furoxan ring; for the
isomeric arylnitrofuroxans, the lowerꢀfrequency band
(1300—1360 cm–1) is masked by the band νs(NO2). The
inꢀplane bending vibrations in the furoxan ring absorb at
Table 3. IR spectra of compounds 1f and 1´f
ν/cm–1
Δν/cm–1
Assignment
1f
1´f
716
732
757
793
828
855
988
1012
1020
1077
1198
1261
1281
1338
1480
1518
1549
1578
1598
1620
1630
712
729
753
789
828
845
982
1008
1020
1075
1196
1257
1280
1332
1471
1514
1548
1578
1598
1620
1630
4
3
4
4
0
10
6
4
0
2
2
4
1
6
9
4
1
0
0
0
0
ρ
Ph; βfur; ν
C—Br
—«—
—«—
—«—
σ
σ
—«—
—«—
σ
800—1150 cm–1
.
NO2
On the whole, the band assignments for the furoxan
ring we made from our experimental data agree with the
results6b obtained by solving vibrational problems and inꢀ
terpreting the IR spectra of various furoxan derivatives.
However, the comprehensive set of our experimental data
allow some refinements to the previous interpretation of
the vibrational spectra of furoxans. For instance, the bands
at 1500—1550 cm–1, which have been assigned6b to the
C=N—O vibrations, we assigned to the C—C stretching
vibrations in the ring and the C=N—O fragment probably
absorbs at 1410—1475 cm–1. The stretching vibrations of
the fragment O—N→O, which is an "embedded" nitro
group, seem to be manifested at 1300—1450 cm–1 rather
than at 1410—1475 cm–1 as has been reported earlier;6b in
addition, the fragment O—N→O absorbs at two frequenꢀ
cies: 1380—1450 (νs(O—N→O)) and 1300—1360 cm–1
(νas(O—N→O)).
fur
Ph
—«—
—«—
σ
fur
σ
Ph
s
ν
; ν
NO2 fur
ν
fur
—«—
as
ν
NO2
ν
Ph
—«—
ν
fur
—«—
Note. Here and in Tables 4 and 5, the symbols ν, σ, and
β denote stretching, inꢀplane bending, and bending vibraꢀ
tions, respectively; the superscripts s and as refer to symꢀ
metric and antisymmetric vibrational modes, respectively.
To sum up, we obtained experimental data in favor of
the earlier proposed mechanism of formation of the furꢀ

Mechanism of formation of 3ꢀnitroꢀ4ꢀRꢀfuroxans
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 5, May, 2011
859
Table 4. IR spectra of compounds 4f and 4´f
dinitroꢀ1ꢀRꢀethanes 2 with NaNO2 in acetic acid. In adꢀ
dition, our comparative analysis of the IR and Raman
spectra of three 15Nꢀlabeled furoxans and their unlabeled
analogs allowed some refinements to the known assignꢀ
ments of the absorption bands in the vibrational spectra of
furoxans.
ν/cm–1
Δν/cm–1
Assignment
4f
4´f
710
749
760
802
831
940
955
990
709
740
758
799
828
955
965
990
1
9
2
3
3
—
—
0
β
β
β
Ph
fur
Ph; βfur; ν
C—Br
Experimental
—«—
—«—
Dipotassium salt 2a, 4(3)ꢀ(4ꢀbromophenyl)ꢀ3(4)ꢀnitrofurꢀ
oxans 1f and 4f, and 15Nꢀlabeled samples 1´f and 4´f were preꢀ
pared according to known procedures.2 The melting point of
compound 1f (1´f) is 80.5—81 °C (cf. Ref. 2: m.p. 80—81 °C);
the melting point of compound 4f (4´f) is 57—58 °C (cf. Ref. 2:
m.p. 56—57 °C). The synthesis of 3,4ꢀbis(4ꢀbromophenyl)ꢀ
furoxan 6 and its 15Nꢀlabeled analog 6´ was carried out as
described earlier.7 The melting point of compound 6 (6´) is
163—164 °C (cf. Ref. 7: m.p. 164 °C).
σ
Ph
1019
1075
1126
1190
1228
1271
1290
1309
1367
1400
1442
1488
1512
1579
1620
1019
1072
1119
1190
1230
1270
1289
1309
1362
1400
1435
1488
1509
1570
1601
0
3
7
0
—
1
1
0
5
0
7
0
3
9
19
—«—
σfur; σ
—«—
Ph
σ
Ph
σ
Ph
σPh; σ
fur
NMR spectra were recorded on Bruker WMꢀ250 (1H,
250 MHz) and Bruker AMꢀ300 spectrometers (13C, 75.5 MHz;
14N and 15N, 21.5 MHz) in DMSOꢀd6. Chemical shifts δ are
referenced to Me4Si (1H, 13C) as the internal standard and to
—«—
s
ν
; ν
NO2 fur
ν
ν
ν
ν
ν
ν
Ph
MeNO2 (
14N and 15N) as the external standard. Mass spectra
fur
were measured on a Varian MAT CH 6 instrument (EI, 70 eV).
The UV spectra of solutions of dipotassium salt 2a were reꢀ
corded on a Specord UVꢀVIS instrument for pH = 13.0—3.0 and
on a Specord UVꢀVIS M40 instrument for moderately acidic
media (CH2SO4 = 0.2—45%). In both cases, quartz 10ꢀmmꢀthick
cells were used.
Ph
fur; ν
Ph
; ν
as
NO2 fur
fur
IR spectra were recorded on an IFS 113 v FTIR spectroꢀ
meter (KBr pellets). Raman spectra were recorded on Ramanor
HGꢀ2S, Ramalos, and U 1000 spectrometers with an argon laser
as an excitation source.
Determination of the ionization constants of the dipotassium
salt of 1ꢀhydroxyiminoꢀ1ꢀ(4ꢀmethoxyꢀ3,5ꢀdinitrophenyl)ꢀ2,2ꢀ
dinitroethane (2a). Dipotassium salt 2a was dissolved in disꢀ
tilled water to produce a solution with the concentration
C = 0.82•10–3 mol L–1. Its aliquots (1 mL) were placed in voluꢀ
metric flasks (50 mL) and working solutions were prepared by
adding water, 0.1 M NaOH, buffer solutions (Table 6), or sulfuꢀ
ric acid solutions with precise concentrations (Table 7) to a total
Table 5. IR and Raman spectra* of compounds 6 and 6´
ν/cm–1
Δν/cm–1
Assignment
6
6´
725 [726]
738
0722 [722]
0731
03 [4]
07
β
ρ
Ph; ν
C—Br
fur
752 [746]
830 [826]
840
0746 [742]
0820 [820]
0832
06 [4]
10 [6]
08
—«—
σfur, σ
Ph
—«—
968
0958
10
σ
fur
995 [990]
1018 [1012]
1072 [1070]
1112 [1114]
0991 [986]
1018 [1008]
1072 [1070]
1102 [1102]
1118
04 [4]
00 [4]
00 [0]
10 [12]
—«—
Table 6. Processing of the UV spectroscopic data for determiꢀ
nation of pKa1 of compound 2a
σ
Ph
—«—
σ
fur
Entry
pH
Optical density, D
log
1180 [1174]
1189
1180 [1174]
1187
00 [0]
02
σ
Ph
—«—
1
2
3
4
5
6
7
8
9
6.0
13.0
5.0
2.5
3.0
4.0
4.5
4.25
0.65
0.65
0.65
0.18
0.23
0.32
0.38
0.34
0.44
—
—
—
0.90
0.64
0.20
0.02
0.15
0.18
1280
1280
00
—«—
1305 [1300]
1333 [1326]
1398 [1394]
1448 [1466]
1505 [1500]
1574 [1514]
1596 [1570]
1602 [1594]
1301 [1298]
1322 [1320]
1398 [1392]
1448 [1462]
1502
1582 [1514]
1561 [1556]
1602 [1594]
04 [2]
11 [6]
00 [2]
00 [4]
03
— [0]
35 [14]
00 [0]
ν
fur
—«—
ν
fur; ν
Ph
—«—
—«—
ν
Ph
ν
fur
ν
4.75
Ph
* C2a = 0.04•10–6 mol L–1
.
* The Raman spectra are given in square brackets.

860
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 5, May, 2011
Ovchinnikov et al.
Table 7. Processing of the experimental data for determination
log
of pKa of compound 2a
2
Entry CH SO
Optical
density, D
Acidity
function
log
2
4
1
2
(%)
0.5
0
H0
HA
1
2
3
4
5
6
7
0.20
0.60
8.30
24.50
30.25
35.13
44.67
0.1233
0.1245
0.1177
0.0645
0.0302
0.0166
—
—
—
—
—
—
—
–0.5
–1.0
–1.5 –2.0
–2.5
Hx
–0.697 —0.996 —1.353
–1.324 —1.807 —0.480
–1.737 —2.577
–2.069 —2.894
–0.5
–1.0
0.480
0.804
—
—
—
* C2a = 0.04•10–6 mol L–1
.
volume of 50 mL. The concentrations of the H2SO4 solutions
were taken from tables relating concentration to specific graꢀ
vity. The ionization constants of the oxime and dinitromethyl
fragments were determined graphically according to standard
formulas.4,5
Fig. 2. Determination of the ionization constant pKa of the
dinitromethyl fragment in compound 2a.
2
log[(ε + – ε)/(ε – ε +)] = –2.23 + 1.10HA and hence pKa2 is
BH
BH
2
The pKa value of the oxime fragment was determined from
1
2.23 (see Fig. 2, line 2).
the slope of a straight line governed by Eq. (1) (see Table 6). The
Xꢀintercept corresponds to pKa = 4.43 (Fig. 1).
Since the dinitromethyl group in salts 2 is protonated
at much lower pH values than the oxime group, its pKa
1
This work was financially supported in part by the Diviꢀ
sion of Chemistry and Materials Science of the Russian
Academy of Sciences (Program "Development of Scienꢀ
tific Foundations for the Preparation of a New Generaꢀ
tion of HighꢀEnergy Compounds").
2
was determined by titrating the working solutions of salt 2a
with sulfuric acid solutions of moderate concentrations
(CH SO < 45%; Table 7).
2
4
In Fig. 2, log[(ε
– ε)/(ε – ε +)] is plotted versus Hx.
+
BH
BH
The tangent of the slope of the straigh2t line for H0 (see Fig. 2,
line 1) largely differs from unity; log[(ε – ε)/(ε – ε +)] =
References
+
BH
BH
2
= –1.58 + 1.74H0). Therefore, the acidity function of the
dinitromethyl fragment is not H0. For HA, the equation is
1. V. G. Dubonos, I. V. Ovchinnikov, N. N. Makhova, L. I.
Khmel´nitskii, Mendeleev Commun., 2002, 12, 120.
2. I. V. Ovchinnikov, A. O. Finogenov, M. A. Epishina, A. S.
Kulikov, Yu. A. Strelenko, N. N. Makhova, Izv. Akad. Nauk,
Ser. Khim., 2009, 2072 [Russ. Chem. Bull., Int. Ed., 2009,
58, 2137].
log
3. (a) M. Kamlet, D. Glover, Tetrahedron Lett., 1960, 17;
(b) M. Kamlet, D. Glover, J. Org. Chem., 1962, 27, 537.
4. L. Hammett, Physical Organic Chemistry: Reaction Rates, Equiꢀ
libria, and Mechanisms, McGrawꢀHill, New York, 1970, 420 pp.
5. A. Albert, E. P. Serjeant, Ionization Constants of Acids and
Bases: a Laboratory Manual, Methuen, London, 1962, 180 pp.
6. L. I. Khmel´nitskii, S. S. Novikov, T. I. Godovikova, Khimiya
furoksanov: stroenie i sintez [The Chemistry of Furoxans: Strucꢀ
tures and Synthesis], 2nd ed., Nauka, Moscow, 1996, 382 pp.:
(a) p. 95; (b) p. 78 (in Russian).
0.8
0.6
0.4
0.2
0
7. P. Gruenanger, Atti Accad. Naz. Lincei Cl. Sci. Fis. Mat. Nat.
Rend., 1954, 16, 726.
2.5
3.0
3.5
4.0
4.5
pH
–0.2
Fig. 1. Determination of the ionization constant pKa of the
Received June 21, 2010;
1
oxime fragment in compound 2a.
in revised form January 11, 2011