Synthesis and structure of products of hydroxylamine acylation with 3-carboxy-2,2,5,5-tetramethylpyrrolinoxyl derivatives

Article abstract of DOI:10.1134/S1070428009080132

The reaction of NH₂OH with the derivatives of 2,2,5,5-tetramethylpyrrolin-1-oxyl-3-carboxylic acid in all events led to the formation of a mixture of the corresponding nitroxylhydroxamic acid with a stable O-acylhydroxylamine. The ratio between

Full text of DOI:10.1134/S1070428009080132

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2009, Vol. 45, No. 8, pp. 1189−1199. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © V.D. Sen’, G.V. Shilov, V.A. Golubev, 2009, published in Zhurnal Organicheskoi Khimii, 2009, Vol. 45, No. 8, pp. 1199−
1208.
Synthesis and Structure of Products of Hydroxylamine Acylation
with 3-Carboxy-2,2,5,5-tetramethylpyrrolinoxyl Derivatives
V. D. Sen’, G. V. Shilov, and V. A. Golubev
Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka,
Moscow oblast,142432 Russia
e-mail: senvd@icp.ac.ru
Received November 5, 2008
Abstract—The reaction of NH2OH with the derivatives of 2,2,5,5-tetramethylpyrrolin-1-oxyl-3-carboxylic acid in
all events led to the formation of a mixture of the corresponding nitroxylhydroxamic acid with a stable
O-acylhydroxylamine. The ratio between the products depends on the nature of the acylating agent and under the
studied conditions varies from ~5.5 : 1 to 1 : 3 indicating the comparable nucleophilicity in this reaction of N and
O atoms in the hydroxylamine. The most active chloride of the mentioned acid alongside the indicated products
afforded in a considerable yield N,O-diacylhydroxylamine and the triacylated hydroxylamine, 3-{[(2,2,5,5-
tetramethyl-1-oxylopyrrolin-3-yl)carbonyloxyimino][(2,2,5,5-tetramethyl-1-oxylopyrrolin-3-yl)carbonyloxy]-
methyl}-2,2,5,5-tetramethylpyrrolin-1-oxyl. The structure of both latter compounds was established by XRD
analysis.
DOI: 10.1134/S1070428009080132
In the preceding study [1] we developed a preparation
procedure for nitroxylhydroxamic acids that might
potentially combine the wide range of biological action
of hydroxamic acids [2] and nitroxyl radicals [3] and
besides were interesting as a research tool at the use of
ESR method. In the developed method the carboxy group
was activated by converting it into acid chloride. Inasmuch
as it was possible to obtain acid chlorides only for a few
five- and six-membered cyclic nitroxylcarboxylic acids,
procedure [1] was of limited applicability. The other
known methods of hydroxamic acids synthesis are
N-acylations of hydroxylamine or its O-protected
derivatives with carboxylic acids esters and with mixed
anhydrides with carbonic acid esters [4]. The O-protect-
ed derivatives of hydroxylamine are hardly suitable for
the synthesis of nitroxylhydroxamic acids because the
nitroxyl radicals are unstable under the conditions of acid
hydrolysis [5] or hydrogenolysis [6] of the protective
groups. In the first case the nitroxyl radicals suffer acid
disproportionation [7], and in the second event they are
reduced to hydroxylamines [8]. The disadvantage of the
acylation of unprotected hydroxylamine consists in the
formation of a mixture of products of mono-, di-, and
triacylation. To our knowledge no systematic studies were
performed concerning the effect of carboxylic acids
activation on the selectivity of NH2OH acylation [9–11].
The di- and triacyl derivatives of NH2OH, have their
proper interest, for they may be present as isomers.
Isomers of di- and triacyl derivatives of NH2OH may
form at alternative N-/O-acylation of NH2OH monoacyl
derivatives, and also due to the hydroxame-hydroxime
tautomerism in the hydroxamic acids and their derivatives.
By few examples of NH2OH N,O-diacyl derivatives it
was shown [12–15] that unambiguous proof of their
structure could be obtained only by XRD method.
In this study we investigated in detail the reaction of
the hydroxylamine with derivatives of the most available
nitroxylcarboxylic acid, 3-carboxy-2,2,5,5-tetramethyl-
pyrrolin-1-oxyl (Ia): its acid chloride Ib, methyl ester Ic,
N-hydroxysuccinimide ester Id, mixed anhydride Ie, and
also we performed the condensation of acid Ia with
NH2OH using dicyclohexylcarbodiimide (DCC). The
effect of the method of acid Ia activation on the yield of
reaction products was evaluated. The structures of
products were established from spectral data and by XRD
analysis.
The reaction products obtained from NH2OH with
1189

SEN’ et al.
R
1190
OH
NOH
HO
R
N
O
+
O
O
R
HO
N
O
R
R
IVc
IIb
IVd
O
R
O
R
R
R
O
X
R
Ià_Ie
O
R
HN
O
O
R
O
N
H2NOH
ONH2
NHOH
+
+
+
+
+
Ia
O
N
R
O
O
R
O
R
R
O
OH
IIa
IVa
III
V
IVb
O
R
O
O
O
O
O
O
R
+
+
+
N
R
O
R
N
O
OH
VII
VI
VIII
O
(d), IC(O)OEt (e).
; X = OH (a), Cl (b), OMe (c),
R =
O N
N
O
O
from ~5.5:1 to 1:3 (Table 1) indicating the comparable
nucleophilicity in this reaction of N and O atoms in the
hydroxylamine. One more reaction proceeds simulta-
neously: reduction with hydroxylamine of nitroxyl groups
of radicals Ia–Ie. This reaction was relatively slow, and
the reduction product VII was identified only in the case
of a weak acylating agent, methyl ester Ic. At the use of
active acylating agents Ib, Id, and Ie or of the system
Ia–DCC and slight excess of NH2OH the products of
the nitroxyl group reduction were not formed in consider-
able amounts.
acid Ia and its derivatives and the products yields are
presented on the scheme and in Table 1.
An important feature of reactions between compounds
Ia–Ie and NH2OH is the obligatory formation of the
mixture of products of N- and O-acylation, hydroxamic
acid IIa and O-acylhydroxylamine III respectively.
Compound III is stable and unlike the majority of known
examples [9] does not undergo isomerization when treated
with excess NH2OH into the more thermodynamically
feasible hydroxamic acid IIa. The ratio between the yields
of compounds IIa and III depends on the nature of the
acylating agent and varies under the studied conditions
Pyrrolinecarboxylic acid Ia with DCC and acid chloride
Table 1. Yields of products of reaction of acid Ia and its derivatives with NH2OH
a Found by HPLC with respect to the initial compound; preparative yield is given in parentheses.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 8 2009

SYNTHESIS AND STRUCTURE OF PRODUCTS OF HYDROXYLAMINE ACYLATION
1191
compound IVa was only 1%. The reaction was carried
out under anhydrous conditions. and the only possible way
to understand the high yield of anhydride VI was to
assume a formal O-acylation of ester III along an unclear
mechanism. When the base used in the reaction was
NaHCO3 the N-acylation of compound III prevailed, and
the ratio of yields of compounds IVa and VI was 5:1. In
the acylation of ester III with mixed anhydride Ie
compound IVa also formed as the main product, the ratio
of yields of compounds IVa and VI was 19:1.
Ib with Et3N acylated NH2OH predominantly at the
nitrogen atom and therefore they are the most efficient
for the preparative synthesis of hydroxamic acid IIa. In
contrast, the prevailing reaction with activated ester Id
was O-acylation of hydroxylamine, and therefore this
procedure is the most suitable for preparation of
compound III. Hydroxylamine attached both carbonyl
groups of the mixed anhydride Ie providing a difficultly
separable mixture of compounds Ia, IIa, and III.
Therefore reagent Ie is unsuitable for preparative
procedures. Besides compounds Ib and Ie partially
acylate the primarily formed compounds IIa and III to
diacyl derivatives IV thus additionally complicating the
separation of products. The employing of the most active
acylating mixture of acid chloride Ib with pyridine resulted
in the formation of all possible acylation products of
NH2OH.
O
NO2
NO2
R
O
O N
+
III
IX
The structure of hydroxylamine O-ester III was also
confirmed by its reaction with m-nitrobenzaldehyde that
afforded in 93%-νûμ yield the expected azomethine IX.
In order to establish the formation routes of di- and
triacyl derivatives of NH2OH and also to understand the
formation of anhydride VI in the process we investigated
the reactions of acid IIa, ester III, and N,O-diacyl-
hydroxylamine IVa with acid chloride Ib. In reaction of
compound IIa with an equimolar quantity of acid chloride
Ib in the presence of Et3N formed biradical IVa in ~90%
yield (by HPLC data). Another product of this reaction
(yield ~5%) was also a biracial that was not preparatively
isolated. The ESR spectrum of the latter radical in benzene
(~10–4 mol l−1) contains five lines with the amplitudes
ratio 100:19:121:17:90 and splitting constant aN 1.47 mT.
This spectrum is characteristic of nitroxyl biradicals where
the energy of exchange interaction J and aN value are
related by an expression J ≈ 10aN [16]. The UV spectrum
of this biradical in the eluent B (see EXPERIMENTAL)
contains a single symmetric band with λmax 217 nm
characteristic of RC(O)-derivatives of NH2OH lacking
C=N bond (see below). These data are consistent with
the structure of biradical IVb and disagree with the
structures of the hydroximic acid derivatives IVc and
IVd. Hence under the conditions we studied hydroxamic
acid IIa underwent both O- and N-acylation with the
rates ratio ~20 : 1 respectively. The presumable
hydroximic tautomer IIb was not detected in the solution
by IR spectroscopy and did not notably yield its acylation
products IVc and IVd.
Triradical V formed in a high yield both in the reaction
of acid IIa with a double quantity of acid chloride Ib and
in the reaction of biradical IVa with an equimolar amount
of compound Ib (yields 91 and 83% respectively). In
both cases in an yield ~7% another isomer of triradical
was obtained whose ESR spectrum in eluent B contained
7 lines with the amplitudes ratio 100 : 79 : 89 : 132 : 126 :
63 : 89 and a splitting constant aN 1.55 mT. This spectrum
is characteristic of nitroxyl triradicals where the exchange
interaction of unpaired electrons with the nitrogen nuclei
is modulated by the intramolecular motion of the atoms
[17]. The UV spectrum of this triradical in the eluent
contains a single absorption band with λmax 224 nm. It is
presumable proceeding from the spectral data that the
minor product of these reactions is either one of the eight
possible stereoisomers of triradical VIII or one of sixteen
possible steric isomers of triradical V.
The structure of acid IIa was established in the
preceding communication [1]. The new pyrrolinoyl
hydroxylamine derivatives III, IVa, and V are high-
melting crystalline substances of yellow color. Their
structure was established from elemental analysis, IK,
ESR, UV, and mass spectra, and compounds IVa and V
were besides subjected to XRD analysis.
IR spectrum of the chloroform solution of compound
III contains an absorption band of an endocyclic C=C
bond at 1629 cm–1 and bands characteristic of hydroxyl-
amine O-acyl derivatives, 1729 cm–1 of the stretching
vibrations of C=O group, 1553 cm–1 of the bending
The expectable main product in the reaction of ester
III with acid chloride Ib would be derivative IVa.
However the HPLC data showed that at equimolar
reagents ratio in the presence of Et3N anhydride VI was
obtained in 92% yield, and the yield of the expected
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 8 2009

SEN’ et al.
1192
vibrations of NH2, 3229 and 3319 cm–1 of the symmetric
and asymmetric vibrations of NH2 [9, 18]. The carbonyl
groups in the N- and O-acylhydroxylamines are known
[9–11] to have absorption in different range of the IR
spectrum providing a reliable tool for distinguishing these
compounds. In N-acylhydroxylamines this amide band
appears at 1640–1670 cm–1 (for acid IIa 1663 cm–1) and
in O-acylhydroxylamines the carbonyl is a part of the
ester group and its absorption band is observed at 1720–
1760 cm–1 (for compound III 1722 and 1729 cm–1 in
solid state and in solution respectively).
UV spectra of compounds III and IVa in the region
200–350 nm contain a single band with λmax 208–
216 nm. The molar extinction factor in this band for
monoradical III is ε ~104 l mol–1 cm–1, and for biradical
IVa it is twice larger. In the spectrum of triradical V the
absorption maximum is shifted to the longwave region
(λmax 223 nm) compared to the spectra of compounds
III and IVa. This shift is evidently due to the overlapping
of the absorption by the fragments C=C–C=N and C=C–
C=O. The absorption band of the C=C–C=O group in
compounds III, IVa, and V shields the weaker π > π*
band of the nitroxyl group. The latter is observed as
a shoulder at 244 nm in the spectrum of compound V.
The yellow color of compounds obtained originates from
the weak π > π* absorption of the nitroxyl group
(~400 nm), ε ~5 l mol–1 cm–1 [21].
The ESR spectra of the dilute solutions of compound
III contain three lines each due to the splitting on 14N. In
water solution at ~20°C the constant aN is 1.63 mT, the
value characteristic of the nitroxyl radical of pyrroline
series [16]. The ESR spectrum of the dilute solution of
IVa in benzene at ~20°C is composed of 9 lines with the
amplitudes ratio (38 : 100 : 75) : (37 : 88 : 30) : (71 : 92 :
27), aN 1.42 mT. This spectrum is consistent with the
theoretical spectrum of nitroxyl biradicals with J/aN ≈ 0.2
[16]. The spectrum of triradical V in a benzene solution
at ~20°C contains 7 lines with the amplitudes ratio 49 :
63 : 68 : 100 : 58 : 55 : 45 and is characheristic of nitroxyl
triradicals [16]. The amplitudes ratio is different from
the theoretical 1 : 3 : 6 : 7 : 6 : 3 : 1 for triradicals with
J >> aN because of broadening of all spectral lines save
the extreme ones due to the modulation of the exchange
interaction of three unpaired electrons by the intra-
molecular motions of atoms [17].
Taking in consideration the existence in the solution
of a tautomeric equilibrium between IIa and IIb, the
acylation of the primarily formed mixture of compounds
IIa, IIb, and III might provide four isomers of NH2OH
diacyl derivatives of structures represented by formulas
IVa–IVd. The IR spectrum of the solution of isolated
isomer IV in CHCl3 contained the stretching vibrations
bands at 1629 (C=C), 1705 (O=CN), 1769 (O=CO), and
3200 (N–H) cm–1. Three latter bands are characteristic
of the N,O-diacyl hydroxylamine derivatives [10, 14]; they
are consistent with structure IVa and disagree with
structures of isomers IVb–IVd. The triacyl derivatives
may exist as isomers V and VIII whose benzoyl analogs
have been described in [19, 20] like β- and α-isomers of
tri-benzoylhydroxylamine respectively. The IR spectrum
of the solution of isolated isomer of triradical in CHCl3
contains the stretching vibrations bands at 1602 (C=N),
1631 (C=C), and 1760 (O=CO) cm–1 in agreement with
structure V.
In mass spectra of compounds III, IVa, and V peaks
of negative ions [M – H]– are observed whose masses
correspond to the calculated values. In the spectra of all
compounds strong peaks of fragment ions are present
with m/z 168 and 183 originating from the splitting of CH3
and N=O from [C9H14N2O3]–. The mass spectra of
compounds III and IIa [1] differ only in the intensity
ratio of peaks [M – H]– and fragment ions.
B
The reported spectral data of compounds IVa and V
completely correspond to the results of their XRD
investigation (Figs. 1, 2). For N,O-diacylhydroxylamine
IVa four s-cis-trans-isomers are possible. The bond
lengths and bond angles in the obtained isomer IVa are
compiled in Tables 2, 3. In this isomer the carbonyl group
A
Fig. 1. Molecular structure of biradical IVa.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 8 2009

SYNTHESIS AND STRUCTURE OF PRODUCTS OF HYDROXYLAMINE ACYLATION
Table 2. Bond lengths in molecule IVa
1193
and C=C bond in the N-acyl A are located in the s-
trans-position, and in the O-acyl B, in s-cis-position with
respect to each other. Both pyrroline rings A and B are
planar, and the angle between the planes is 156.5°. The
structures of the pyrroline ring in hydroxamic acid IIa
[1] and A ring of molecule IVa differ insignificantly. The
comparison in pairs of the bond lengths of double bonds
C7–C10 and C2–C3, and also of ordinary bonds C7–C8
and C3–C4 in A and B rings shows that the corresponding
bonds in B ring are longer by ~0.03 . The lengths of
the other bonds differ insignificantly. The longer
endocyclic double bond in B ring suggests a stronger
conjugation in the s-cis-fragment O1C5C3C2 of the O-
acyl as compared to the s-trans-fragment O3C6C7C10 of
the N-acyl. Besides the bond C3–C5 in the O-acyl is
shorter than the bond C6–C7 in the N-acyl owing to the
stronger conjugation. In agreement with the above stated
the dihedral angle O3C6C7C10 in the N-acyl equals 57.2°,
whereas the dihedral angle O1C5C3C2 in the O-acyl is
only 3.7°. Dihedral angles O3C6N2O2 (0.1°), C6N2O2C5
(74.9°), and N2O2C5O1 (169.8°) are close to the values
published for the other N,O-diacyl hydroxylamine
derivatives [12, 14, 15]. For the dihedral angle C6N2O2C5
of N,O-dibenzoylhydroxylamine in solution the estimated
value was 70° [22]. This fact indicates the sufficiently
rigid conformation of this fragment and its weak
deformation under the effect of the intermolecular forces
in the crystal. The fragment O2C5O1C3 is flat and is turned
relative to the plane C7C6N2 by 100.3°.
Bond
d, Å
Bond
d, Å
O1–C5
O2–N2
O4–N1
N1–C1
N2–C6
N3–C9
C1–C11
C2–C3
C3–C4
C4–C13
C7–C10
C8–C15
C9–C10
C9–C17
1.188(6)
1.413(4)
1.276(5)
1.483(5)
1.346(5)
1.480(5)
1.514(6)
1.339(5)
1.516(6)
1.543(6)
1.309(5)
1.513(9)
1.478(6)
1.511(7)
O2–C5
O3–C6
O5–N3
N1–C4
N3–C8
C1–C2
C1–C12
C3–C5
C4–C14
C6–C7
C7–C8
C8–C16
C9–C18
1.372(5)
1.195(6)
1.258(5)
1.481(5)
1.472(5)
1.486(6)
1.531(7)
1.478(6)
1.516(7)
1.488(6)
1.495(6)
1.533(8)
1.524(8)
hydroxylamine III against the action of excess NH2OH
is also underlain by the shielding of the carbonyl group
and its conjugation with the C=C bond of the pyrroline
ring. The stability of O-acyl derivatives of NH2OH may
also originate from the decrease in the electrophilicity of
carbonyl groups under the effect of electron-donor
substituents. For instance, in reaction of isatoic anhydride
with NH2OH a stable O-(2-aminobenzoyl)hydroxylamine
[24] was obtained in 60% yield: Here the electrophilicity
of the carbonyl carbon atom was reduced by the effect
of the ortho-amino group, and also owing to the
conjugation with the benzene ring.
It is known [9, 18, 23] that in the acylation of NH2OH
a mixture of products of N- and O-monoacylation initially
always formed. However the carbonyl group of the
O-acyl derivatives in most cases reacts with excess
NH2OH, and the O-acylhydroxylamine converts into
thermodynamically more stable hydroxamic acid. The
latter is also valid for N,O-diacyl hydroxylamine deriv-
atives that in reaction with excess NH2OH give a double
amount of hydroxamic acid [18]. However in our case
the reaction products III and IVa do not react with excess
NH2OH at a notable rate (ratio III or IVa–NH2OH 1 : 2,
20°C, 3 h). The XRD data for compound IVa make clear
the reason of this unusual stability. The fragment O1=C5–
C3=C2 of the O-acyl in IVa is virtually planar, and the
electrophilicity of the carbonyl group is reduced by the
conjugation with the C2=C3 bond. Besides, as seen from
Fig. 1, the attack of the electrophilic atom C5 with
hydroxylamine is hampered by contiguous methyl groups
C13 and C14. Presumably the stability of O-acyl-
Triradical V may exist in the form of two isomers with
different spatial arrangement of substituents at the C=N
bond. In its turn, each of these isomers may have eight
s-cis-trans-configurations of three conjugated double
bonds. The molecular structure of triradical V we have
isolated is presented in Fig. 2 and the bond distances and
bond angles in this molecule are compiled in Tables 4, 5.
Triradical V is a Z-isomer since its atoms O2 and O3 are
located in a cis-position with respect to C6=N2 bond. The
carbonyl groups in acyl fragments B and C are present
in the s-trans-position relative to the double bonds of the
pyrroline rings. The double bond C6=N2 is in a s-trans-
position with respect to the double bond of the pyrroline
ring A. Dihedral angles are C10C7C6N2 (2.4°), C2C3C5O1
(16.3°), and C21C20C19O6 (19.1°). The interatomic
distances in the three flat pyrroline rings A–C are identical
within 3σ and are close to those of fragment B in molecule
IVa. This fact reflects the approximately equal extent of
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 8 2009

SEN’ et al.
Table 4. Bond lengths in molecule V
1194
B
Bond
O2–C5
d, Å
Bond
O2–N2
d, Å
1.369(4)
1.415(3)
O3–C6
O6–C19
O7–N4
N2–C6
N3–C8
N4–C22
C8–C7
C8–C15
C20–C19
N1–C4
C6–C7
1.379(4)
1.194(4)
1.271(3)
1.274(4)
1.480(4)
1.462(5)
1.511(5)
1.529(5)
1.461(4)
1.486(4)
1.454(4)
1.314(4)
1.523(5)
1.488(5)
1.529(4)
1.515(4)
O3–C19
O1–C5
O5–N3
O4–N1
N3–C9
N4–C23
C8–C16
C20–C21
C20–C23
N1–C1
C7–C10
C2–C1
1.382(4)
1.189(4)
1.269(4)
1.270(3)
1.474(5)
1.485(4)
1.526(4)
1.325(4)
1.505(5)
1.482(4)
1.328(4)
1.499(4)
1.532(5)
1.518(5)
1.473(4)
1.523(6)
C
A
C2–C3
C23–C27
C22–C21
C22–C25
C3–C4
C23–C26
C22–C24
C5–C3
Fig. 2. Molecular structure of triradical V.
Table 3. Bond angles in molecule IVa
C1–C12
C1–C11
C4–C13
C9–C18
1.515(5)
1.525(5)
1.513(6)
C4–C14
C10–C9
C9–C17
1.518(6)
1.499(5)
1.549(5)
Angle
ω, deg
Angle
ω, deg
C5O2N2
114.1(3)
O4N1C1
122.1(3)
O4N1C4
C6N2O2
O5N3C9
N1C1C2
C2C1C11
C2C1C12
C3C2C1
C2C3C4
N1C4C3
C3C4C14
C3C4C13
O1C5O2
O2C5C3
O3C6C7
C10C7C8
C8C7C6
N3C8C15
N3C8C16
C15C8C16
C10C9C18
C10C9C17
C18C9C17
122.0(3)
115.9(3)
122.5(4)
99.4(3)
C1N1C4
O5N3C8
C8N3C9
N1C1C11
N1C1C12
C11C1C12
C2C3C5
C5C3C4
N1C4C14
N1C4C13
C14C4C13
O1C5C3
O3C6N2
N2C6C7
C10C7C6
N3C8C7
C7C8C15
C7C8C16
C10C9N3
N3C9C18
N3C9C17
C7C10C9
115.7(3)
122.3(4)
115.2(3)
110.1(3)
109.3(4)
110.9(4)
121.9(4)
125.8(3)
110.0(4)
108.7(3)
112.4(4)
126.6(4)
123.2(4)
112.2(4)
127.2(4)
99.1(3)
conjugation of the double bonds C6=N2, C5=O1, and
C19=O6 with the double bonds of rings A, B, and C
respectively. The angles between the rings A, B, and C
and virtually planar fragments C7C6N2O3, C3C5O1O2,
and C20C19O3O6 amount respectively to 11.2, 15.8, and
16.8°.
113.7(4)
112.8(3)
113.7(4)
112.2(3)
98.9(3)
The linear part of the molecule is characterized by
dihedral angles C3C5O2N2 (11.1°), O1C5O2N2 (10.8°),
C5O2N2C6 (10.8°), O2N2C6C7 (172.8°), C7C6O3C19
(57.2°), C6O3C19O6 (18.3°), and O3C19C20C21 (18.7°).
The fragment C7C6O3N2O2 is practically flat, the
minimum deviation from the mean plane is 0.0035 for
atom O3, and maximum, 0.0429 for atom C6. The length
of the double bond C6=N2 equals 1.274(4)
1.346(5) for analogous ordinary bond in molecule
IVa. The bonds C5=O1 and C19=O6 are double bonds
(Table 2), and all other bonds in chains C7 C3 and C6 C20
are ordinary. According to our information, the structure
of a tryacyl hydroxylamine derivative was established in
this study for the first time.
112.8(3)
113.2(4)
123.4(4)
110.1(4)
124.6(4)
112.7(3)
120.1(4)
109.5(5)
110.2(4)
112.8(4)
112.5(4)
113.5(5)
110.5(4)
against
111.2(4)
113.1(5)
98.9(3)
...
...
109.9(5)
111.0(4)
114.2(4)
The formation of triradical V at the acylation of
compound IVa suggests that in the solution of compound
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 8 2009

SYNTHESIS AND STRUCTURE OF PRODUCTS OF HYDROXYLAMINE ACYLATION
1195
Table 5. Bond angles in molecule V
IVa a certain equilibrium concentration of tautomer IVc
is present. Although its fraction in the equilibrium is
insignificant and it has been impossible to detect it in the
solution by IR spectroscopy, presumably the high
nucleophilicity of its OH group ensures the high velocity
and selectivity of the compound IVa acylation into
triradical V. The hydroximic structure of IVc type failed
to be detected by Schraml et al. [14] for the other N,O-
diacyl derivatives, yet their 1H NMR data indicated an
exchange process in the NH group that might be caused
by tautomerism.
Angle
ω, deg
Angle
ω, deg
C5O2N2
C6N2O2
O5N3C9
O7N4C22
C22N4C23
N3C8C16
N3C8C15
C16C8C15
C21C20C23
O4N1C4
C4N1C1
N2C6C7
C10C7C6
C6C7C8
N4C23C27
C27C23C20
112.6(2)
109.8(2)
121.9(3)
123.0(3)
114.9(2)
110.3(3)
109.1(3)
110.8(3)
112.4(3)
122.3(3)
115.0(2)
120.7(3)
124.2(3)
123.6(3)
109.9(3)
113.6(3)
C6O3C19
O5N3C8
C8N3C9
O7N4C23
N3C8C7
C7C8C16
C7C8C15
C21C20C19
C19C20C23
O4N1C1
N2C6O3
O3C6C7
117.7(2)
122.3(3)
115.8(3)
121.8(3)
99.0(2)
112.9(3)
114.0(3)
125.3(3)
122.1(3)
122.7(3)
123.8(2)
115.2(3)
112.2(3)
113.3(3)
99.2(3)
EXPERIMENTAL
IR spectra were recorded in the range 400–4000 cm–1
on a spectrophotometer Specord 75IR from mulls in
mineral oil or solutions in CHCl3, UV spectra, on
a spectrophotometer Specord UV-VIS from solutions in
ethanol. ESR spectra were taken at room temperature
on an EPA-2M instrument. Mass spectra were measured
on a LTQFT instrument in the mode of negative ions
registering by electrospray procedure, the voltage on the
emitter needle 3 kV, charge rate 1 μl min–1. The samples
were dissolved in 50% aqueous MeCN. Determination
accuracy ±0.01 mass unit. Electron impact mass spectra
were registered on a Finnigan GC-MS instrument. The
melting points were measured on a heating block RNMK.
HPLC was performed on a chromatograph Milikhrom
[column 2 × 64 mm, Separon C18 (5 μm), detection at
240 nm], eluent MeCN–0.1 Μ water solution of KH2PO4,
40 : 60 (A), 60:40 (B). Retention volumes of substances,
μl, in eluent A: (Ia) 150, (Ib) 995, (Ic) 570, (Id) 430, (Ie)
980, (IIa) 200, (III) 270, (IVa) 530, (IVb) 390, (V) 3500,
(VI) 1350, (VII) 440, (VIII) 2800; in eluent B: (Ie) 320,
(IIa) 160, (III) 200, (IVa) 240, (IVb) 200, (V) 550, (VI)
370, (VIII) 500, (IX) 380. TLC was carried out on Silufol
UV-254 plates.
C10C7C8
C3C2C1
N4C23C20
N4C23C26
108.2(3)
C27C23C26
N4C22C21
C21C22C24
C21C22C25
O1C5O2
112.0(3)
100.1(2)
113.2(3)
112.3(3)
123.6(3)
C20C23C26
N4C22C24
N4C22C25
C24C22C25
O1C5C3
113.1(3)
111.4(3)
109.6(3)
109.9(3)
126.4(3)
O2C5C3
C2C3C5
C5C3C4
N1C1C12
110.0(3)
126.2(3)
120.6(3)
109.5(3)
C20C21C22
C2C3C4
N1C1C2
C2C1C12
113.1(3)
113.1(3)
99.4(3)
114.4(3)
N1C1C11
C12C1C11
N1C4C13
110.9(3)
110.4(3)
109.8(3)
C2C1C11
N1C4C14
C14C4C13
111.7(3)
109.4(3)
112.3(3)
N1C4C3
C13C4C3
O6C19C20
98.8(2)
112.0(3)
127.1(3)
C14C4C3
O6C19O3
O3C19C20
113.6(3)
122.1(3)
110.8(3)
Initial 2,2,5,5-tetramethyl-1-oxylopyrroline-3-carb-
oxylic acid (Ia) and its ester Ic were prepared by
procedure [25]. Acid chloride Ib, activated ester Id, and
mixed anhydride Ie was obtained as described in [26–
28] respectively. Hydroxylamine hydrochloride was
recrystallized from methanol. The alcoholic solution of
NH2OH was prepared by procedure [29]. Triethylamine
and acetonitrile were dried and distilled over KOH and
P2O5 respectively.
C7C10C9
N3C9C18
N3C9C17
C18C9C17
113.7(3)
111.0(3)
N3C9C10
C10C9C18
C10C9C17
99.0(2)
113.2(3)
113.4(3)
108.8(3)
110.9(4)
scanning]. In the experiment plate single crystals were
used of compounds IVa and V of the size 0.15 × 0.1 ×
0.05 mm. The parameters of unit cell were determined
and refined by 35 reflections measured in the angle range
X-ray diffraction study was carried out on an automatic
four-circle diffractometer Bruker P-4 [graphite
monochromator, λ(MoKα) 0.71073 , 293 K, θ/2θ-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 8 2009

SEN’ et al.
1196
θ 10–15°. The experimental reflections array was obtain-
ed in the angle range θ 2.46–24.99°. The structures were
solved by the direct method. The positions and thermal
parameters of the nonhydrogen atoms were refined in
isotropic and further in anisotropic approximation by the
full-matrix least-squares method.Atomic coordinates and
equivalent thermal parameters are available from the
authors. Hydrogen atoms were revealed from the differ-
ence Fourier synthesis and were not taken into the
refinement. All calculations were carried out with the
use of SHELXTL software [30].
(6 mmol) of acid chloride Ib, 0.63 g (9 mmol) of
NH2OH·HCl, and 1.67 ml (12 mmol) of Et3N in 12 ml of
MeCN was performed as described in [1]. The
composition of reaction products determined by HPLC
is reported in Table 1. Acid IIa was isolated from the
reaction mixture in 54% yield
Reaction of acid chloride Ib with NH2OH in the
presence of pyridine. To a solution of 405 mg (2 mmol)
of acid chloride Ib in 4 ml of MeCN at ~20°C was added
while stirring in succession 78 mg (1.1 mmol) of
NH2OH·HCl and 237 mg (3 mmol) of pyridine. After
20 min the reaction mixture was diluted with 10 ml of
anhydrous ether. The solution was separated from the
formed precipitate and was evaporated in a vacuum. The
residual oily substance was extracted with ether (10 ml),
and the extract was evaporated to give 360 mg of products
mixture whose yields are compiled in Table 1. The
chromatographic separation of the mixture on silica gel
provided a fraction (150 mg) containing mainly biradical
IVa and triradical V. By crystallization of this mixture
from benzene and toluene we obtained 50 mg of biradical
IVa, fine yellow prismatic crystals, mp156–158°C. For
the isolation of triradical V the benzene mother liquor of
the previous crystallization was evaporated to dryness.
The residue was recrystallized in succession from
cyclohexane and MeCN to obtain 15 mg of pure com-
pound V, yellow needle crystals, mp 187–189°C.
For biradical IVa overall number of independent
reflections 1802, 1425 with intensity I > 2σ(I). The main
crystallographic parameters: a 8.151(2), b 11.287(2),
3
c 11.014(2) , α = γ = 90°, β 91.86(3)°, V 1012.8(4)
,
d
calc 1.198 g/cm3, space group P21, Z 2. Final divergence
factors: R1 0.0439 for reflections with I > 2σ(I), R2 0.0584
for all reflections. GOOF 1.027.
For triradical V overall number of independent
reflections 2925, 2526 with intensity I > 2σ(I). The main
crystallographic parameters: a 11.544(2), b 13.448(3),
3
c 19.896(4) , α = β = γ = 90°, V 3088.7(11)
,
dcalc 1.143 g/cm3, space group P212121, Z 4. Final
divergence factors: R1 0.0376 for reflections with I >
2σ(I), R2 0.0465 for all reflections. GOOF 0.996.
Reaction of 3-carboxy-2,2,5,5-tetramethylpyr-
rolin-1-oxyl (Ia) with hydroxylamine. Carbodiimide
procedure. To a mixture of 203 mg (1 mmol) of acid Ia
and 85 mg (1.2 mmol) of NH2OH·HCl in 4 ml of MeCN
at cooling with ice was added while stirring in succession
251 mg of dicyclohexylcarbodiimide and 0.17 ml of Et3N.
After 10 min the cooling was removed, and the stirring
was continued for 4 h more at ~20°C. Further to the
reaction mixture 4 ml of ether was added, the mixture
was left standing for 30 min, the precipitate was filtered
off and washed on the filter by a mixture MeCN–ether,
1 : 1. Yields of the reaction products present in the mixture
are reported in Table 1. The yellow solution obtained was
subjected to chromatography on a column 30 × 60 mm
packed with silica gel preliminary washed with with dilute
HCl to remove iron-containing impurities. The elution was
carried out first with ether, than with ethyl acetate to
obtain a fraction with pure acid IIa. On distilling off the
solvent we obtained in the residue 85 mg (39%) of
compound IIa, yellow prismatic crystals, mp 180–181.5°C
(decomp.) [1].
Reaction of methyl ester Ic with NH2OH. To
210 mg (1 mmol) of compound Ic under an argon
atmosphere at ~20°C was added 1 ml of freshly prepared
2 Μ solution of NH2OH in EtOH. For monitoring with
HPLC the samples of reaction mixture were diluted with
MeOH to a concentration of ~2 mg/ml, and chromato-
grams were registered under above mentioned conditions.
In 20 h ~65% of initial compound Ic was consumed, and
the ratio of yields of compounds IIa, III, and VII was 3 :
1 : 60. Reaction product VII was identified by comparison
of UV spectra and retention volume with an authentic
sample prepared by the reduction of compound Ic with
methanol solution of HCl [31] (20 mg of Ic, 0.1 ml of 2N
HCl, MeOH, ~20°C, 1 h).
Reaction of activated ester Id with NH2OH. To
a suspension of 281 mg (1 mmol) of ester Id in 2 ml of
MeOH at ~20°C was added dropwise within 5 min while
stirring 1.3 ml of freshly prepared 1 Μ solution of NH2OH
in MeOH. The reaction was monitored by TLC and
HPLC. In 15 min after the start of the reaction we found
by TLC (eluent CHCl3–MeCN, 5 : 2) in the reac-
tion mixture compounds IIa (Rf 0.08), III (Rf 0.75), and
Reaction of acid chloride Ib with NH2OH in the
presence of Et3N. The reaction between 1.22 g
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SYNTHESIS AND STRUCTURE OF PRODUCTS OF HYDROXYLAMINE ACYLATION
1197
N-hydroxysuccinimide (Rf 0.15). Yields of the products
are given in Table 1. After 30 min the reaction mixture
was evaporated to obtain yellow oily substance. By flash-
chromatography on silica gel (eluent CHCl3–MeCN, 10 :
1) we isolated from this residue 142 mg (71%) of 3-
(aminooxycarbonyl)-2,2,5,5-tetramethylpyrrolin-1-
oxyl (III), yellow crystals, mp 95–98°C (decomp.). UV
spectrum (EtOH), λmax, nm (å, l mol–1 cm–1): 214 (1.13 ×
104). IR spectrum, ν, cm–1 (mineral oil): 3077 (H–C=C),
1722 (C=O), 1631 (C=C), 3270, 3195 1572 (NH2);
(CHCl3): 1729 (C=O), 1629 (C=C), 3319, 3229, 1553
(NH2). ESR spectrum (H2O): 3 lines, aN 1.63 μΤ. Mass
spectrum, m/z (Irel, %): 198.09 (3) [M – H]– (calculated
[M – H]– 198.1004), 197.09 (5) [M – H2]–, 183.27 (100)
[M – CH4]–, 168,18 (6) [M – HNO]–, 124.18 (2), 113.09
(6). Found, %: C 54.05; H 7.48; N 13.97. C9H15N2O3.
Calculated, %: C 54.26; H 7.59; N 14.06., M 199.23.
(100) [C8H11N2O3]–, 168.18 (7) [C9H14NO2]–, 124.09 (2).
Found, %: C 59.37; H 7.51; N 11.40. C18H27N3O5.
Calculated, %: 59.16; H 7.45; N 11.50. M 365.43.
After isolation of compound IVa in the water mother
liquor accumulated the isomeric biradical with spectral
characteristics consitent with the structure of compound
IVb. UV spectrum of this biradical in eluent A contains
a single band, λmax 218 nm, and ESR spectrum, three
lines with aN 1.60 μT. ESR spectrum of the benzene
solution of the presumed compound IVb contained five
lines with the amplitudes ratio 100 : 19 : 121 : 17 : 90,
aN 1.47 μT.
b. Amixture of 100 mg (0.50 mmol) of compound III
and 131 mg (0.51 mmol) of anhydride Ie was dissolved
in 0.7 ml of MeCN and left standing for 4 days at ~20°C.
As shown by HPLC, the reaction product contained 75%
of biradical IVa, 4% of anhydride VI, and 5% of
presumed triradical VIII. The obtained yellow solution
was worked up similarly to the preceding experiment to
get 106 mg (58%) of compound IVa, mp 156–158°
(toluene).
Reaction of mixed anhydride Ie with NH2OH. To
a solution of 26 mg (0.1 mmol) of compound Ie in 0.3 ml
of solvent indicated in Table 1 was added 0.08 ml of
freshly prepared 2 Μ solution of NH2OH in anhydrous
EtOH. In 15 min after the start of the reaction we found
by TLC (eluent CHCl3–MeCN, 5 : 1) in the reaction
mixture compounds Ia (Rf 0.29), IIa (Rf 0.04), III
(Rf 0.47), IVa (Rf 0.57). Yields of products according to
HPLC data are given in Table 1.
3-{[(2,2,5,5-Tetramethyl-1-oxylopyrrolin-3-
yl)carbonyloxyimino][(2,2,5,5-tetramethyl-1-
oxylopyrrolin-3-yl)carbonyloxy]methyl}-2,2,5,5-
tetramethylpyrrolin-1-oxyl (V). (a) A solution of
100 mg (0.5 mmol) of hydroxamic acid IIa and 56 mg
(0.55 mmol) of Et3N in 0.4 ml of MeCN was added while
stirring and cooling with cold water to a solution of
203 mg (1 mmol) of acid chloride Ib in 0.4 ml of MeCN.
The stirring at ~20°C was continued for 1 h. Then at
stirring the mixture was treated with 0.8 ml of water and
left standing till the end of crystallization. The precipitated
reaction product was filtered off, washed with 50%
aqueous MeCN, and dried. Yield 242 mg (91%), yellow
prismatic crystals, mp 187–189°C (aqueous MeCN). UV
spectrum (EtOH), λmax, nm (ε, l mol–1 cm–1): 222 (3.04 ×
104), 244 (2.43 × 104). IR spectrum (CHCl3), ν, cm–1:
1760 (C=O), 1631 (C=C), 1602 (C=N). ESR spectrum
(benzene, ~25°C): 7 lines with the amplitudes ratio 49 :
63:68:100:58 : 55 : 45, aN 1.48 μT. Mass spectrum, m/z
(Irel, %): 530.26 (1) [M – H]– (calculated [M – H]–
530.2740), 183.18 (100) [C8H11N2O3]–, 168.14 (3)
[C9H14NO2]–, 143.09 (3), 113.09 (4). Found, %: C 61.25;
H 7.43; N 10.30. C27H39N4O7. Calculated, %: C 61.00;
H 7.39; N 10.54. Μ 531.63.
N,O-Bis[(2,2,5,5-tetramethyl-1-oxylopyrrolin-3-
yl)carbonyl]hydroxylamine (IVa). (a) To a solution of
203 mg (1.02 mmol) of compound IIa and 117 mg
(1.16 mmol) of Et3N in 1 ml of MeCN at cooling with ice
was added at stirring within 10 min a solution of 203 mg
(1 mmol) of acid chloride Ib in 0.5 ml of MeCN. The
mixture was stirred for 10 min at cooling and then 1.5 h
at ~20°C. According to HPLC the yields of reaction
products in the mixture were as follows (%): Ia 2, IIa 2,
IVa 88, IVb 4, V 5, VI 1. The separated precipitate of
Et3N·HCl with admixture of triradical V was filtered off,
the yellow solution obtained was evaporated in a vacuum,
the residue was treated with 1 ml of water. The formed
crystals were filtered off, washed with water, and dried.
Yield of compound IVa 242 mg (66%), yellow prismatic
crystals, mp 156–158° (toluene). UV spectrum (EtOH),
λ
max, nm (ε, l mol–1 cm–1): 213 (2.21 × 104). IR spectrum
(CHCl3), ν, cm–1: 3200 (NH), 1769 (OC=O), 1705
(NC=O), 1629 (C=C). ESR spectrum (benzene, ~20°C):
triplet of triplets with amplitudes ratio (38:100:75):(37 :
88 : 30) : (71 : 92 : 27), aN 1.42 μÒ. Mass spectrum, m/z
(Irel, %): 364.18 (60) [M – H]– (calculated [M – H]–
364.1872), 223.18 (4), 198.18 (5) [C9H14N2O3]–, 183.18
b. A solution of 61 mg (0.3 mmol) of acid chloride Ib
in 0.8 ml of MeCN was added within 10 min at stirring to
a mixture cooled with ice of 100 mg (0.27 mmol) of
biradical IVa and 36 mg (0.36 mmol) of Et3N in 0.8 ml
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 8 2009

SEN’ et al.
1198
MeCN. The mixture was stirred for 15 min at cooling,
then 4 h at ~20°C. Then the solvent was distilled off at
a reduced pressure, and the residue was treated with
1 ml of water. The precipitated reaction product was
filtered off, washed with water, and dried. Yield 121 mg
(83%), mp 187–189°C (aqueous MeCN).
2. Marmion, C.J., Griffith, D., and Nolan, K.B. Eur. J. Inorg.
Chem., 2004, p. 3003.
3. Soule, B.P., Hyodo, F., Matsumoto, K., Simone, N.L.,
Cook, J.A., Krishna, M.C., and Mitchell, J.B., Free Rad.
Biol. Med., 2007, vol. 42, p. 1632.
4. Giacomelli, G., Porcheddu, A., and Salaris, M., Org. Lett.,
2003, p. 2715.
2,2,5,5-Tetramethyl-1-oxylopyrrolin-3-carbox-
anhydride (VI). To a solution of 100 mg (0.5 mmol) of
ester III and 61 mg (0.6 mmol) of Et3N in 0.4 ml of
anhydrous MeCN at cooling with ice while stirring was
added within 10 min a solution of 104 mg (0.51 mmol) of
acid chloride Ib in 0.4 ml MeCN. The mixture was stirred
for 10 min at cooling, then 45 min at ~20°C. By HPLC
data the yields of reaction products in the mixture were
as follows, %: Ia 2, IVa 1, V 4, VI 92, VIII 1. The
separated precipitate composed of Et3N·HCl and
triradical V was filtered off. The obtained yellow solution
was concentrated in a vacuum and worked up as
described above.. Yield 143 mg (81%), yellow plate
crystals, mp 142–144°C (cyclohexane) (141.5–143°C
[26]). IR spectrum (CHCl3), ν, cm–1: 1783 and 1727
(C=O), 1624 (C=C), 1125 and 997 (O=C–O–C=O). ESR
spectrum (benzene, ~25°C): 9 lines with amplitudes ratio
2:100:22:15:105:9:24:99:2, aN 1.43 μÒ.
5. Barlaam, B., Hamon, A., and Maudet, M., Tetrahedron
Lett., 1998, vol. 39, p. 7865.
6. Pirrung, M.C. and Chau, J.H.-L., J. Org. Chem., 1995,
vol. 60, p. 8084.
7. Sen’, V.D. and Golubev, V.A., J. Phys. Org. Chem., 2009,
vol. 22, p. 138.
8. Litvin, E.F., Kozlova, L.M., Shapiro,A.B., Rozantsev, E.G.,
and Freidlin, L.Kh., Izv. Akad. Nauk SSSR, Ser. Khim., 1975,
p. 1353.
9. Bauer, L. and Exner, O., Angew Chem., Int. Ed., 1974,
vol. 13, p. 376.
10. Zhungietu, G.I. and Artemenko, A.I., Gidroksamovye
kisloty i ikh proizvodnye (Hydroxamic Acids and Its
Derivatives), Kishinev: Shtinitsa, 1986, p. 9.
11. Pilipenko, A.T. and Zul’figarov, O.S., Gidroksamovye
kisloty (HydroxamicAcids), Moscow: Nauka, 1989, p. 25.
12. Goettlicher, S. and Ochsenreiter, P., Chem. Ber., 1974,
vol. 107, p. 398.
3-(3-Nitrobenzylideneaminooxycarbonyl)-2,2,5,5-
tetramethylpyrrolin-1-oxyl (IX). To a solution of
100 mg (0.5 mmol) of ester III and 114 mg (0.75 mmol)
of m-nitrobenzaldehyde in 0.5 ml of methanol was added
90 mg (1.5 mmol) of acetic acid. The reaction mixture
was brought to boiling and then left standing for 5 h at
~20°C. The separated precipitate was filtered off, wash-
ed with MeOH, and dried in a vacuum. Yield 155 mg
(93%), yellow needle crystals, mp 156–157° (MeOH).
UV spectrum (EtOH), λmax, nm (ε, l mol–1 cm–1): 301 sh
(1.48 × 103), 251 (2.49 × 104), 225 sh (1.63 × 104). IR
spectrum (mineral oil), ν, cm–1: 3097, 3080, 3068, 3060
(Ph and H–C=C), 1745 (C=O), 1630, 1619, 1611 (Ph
and C=C), 1572 (C=N), 1532, 1354 (NO2). ESR
spectrum (benzene, ~25°C): 3 lines, aN 1.48 μT. Mass
spectrum (electron impact, 70 eV), m/z (Irel, %): 332 (46)
[M]+, 318 (3), 302 (1) [M – NO]+, 184 (11) [RCO2H]+,
167 (96) [RCO]+, 154 (42), 153 (69), 152 (71), 150 (33),
148 (17), 139 (38), 137 (39), 125 (20), 109 (100). Found,
%: C 57.91; H 5.31; N 12.73. C16H18N3O5. Calculated,
%: C 57.83; H 5.46; N 12.64. Μ 332.
13. Schraml, J., Mindl, J., Roithova, J., Blechta, V., Sykora, J.,
Soukupova, L., Karban, J., Bartlova, M., and Exner, O.,
Organometallics, 2004, vol. 23, p. 2157.
14. Schraml, J., Sykora, J., Fiedler, P., Roithova, J., Mindl, J.,
Blechta, V., Cnsarova, I., and Exner, O., Org. Biomol. Chem.,
2004, vol. 2, p. 2311.
15. Grassi, G., Cordaro, M., Bruno, G., and Nicolo, F., Helv.
Chim. Acta, 2002, vol. 85, p. 196.
16. Buchachenko, A.L. and Vasserman, A.M., Stabil’nye
radikaly (Stable Radicals), Moscow: Khimiya, 1973,
408 p.
17. Hudson, A. and Luckhurst, G., Chem. Rev., 1969, vol. 69,
p. 191.
18. Jencks, W.P., J. Am. Chem. Soc., 1958, vol. 80, p. 4581.
19. Usova, E.M. and Voronin, E.M., Dokl. Akad. Nauk SSSR,
1957, vol. 113, p. 1306.
20. Exner, O., Coll. Czech. Chem. Commun., 1962, vol. 27,
p. 2284.
21. Rozantsev, E.G. and Sholle, V.D., Organicheskaya khimiya
svobodnykh radikalov (Organic Chemistry of Free
Radicals), Moscow: Khimiya, 1979, p. 195.
22. Artemenko,A.I., Tikunova, I.V.,Anufriev, E.K., Ehlicka, V.,
and Exner, O., Coll. Czech. Chem. Commun., 1981, vol. 46,
p. 729.
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