Conformational transformations and autooxidation of 5-bromo-2-(2-methylpropyl)-5-nitro-1,3,2-dioxaborinane

Article abstract of DOI:10.1134/S1070428017060185

Conformational study of 5-bromo-2-(2-methylpropyl)-5-nitro-1,3,2-dioxaborinane at the DFT PBE/3ξ level of theory revealed the only sofa–sofa interconversion pathway through a transition state corresponding to 2,5-twist conformer. The barrier to internal rotation of the axial nitro group is several times higher than that for the equatorial nitro group. According to the ¹H, ¹³C, and ¹¹B NMR, IR, and X-ray diffraction data, the main autooxidation products of 5-bromo-2-(2-methylpropyl)-5-nitro-1,3,2-dioxaborinane are 2-bromo-2-nitropropane-1,3-diol and boric acid.

Full text of DOI:10.1134/S1070428017060185

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2017, Vol. 53, No. 6, pp. 926–931. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © O.Yu. Valiakhmetova, T.V. Tyumkina, E.S. Meshcheryakova, L.M. Khalilov, V.V. Kuznetsov, 2017, published in Zhurnal
Organicheskoi Khimii, 2017, Vol. 53, No. 6, pp. 909–915.
Conformational Transformations and Autooxidation
of 5-Bromo-2-(2-methylpropyl)-5-nitro-1,3,2-dioxaborinane
O. Yu. Valiakhmetovaa, T. V. Tyumkinab, E. S. Meshcheryakovab,
L. M. Khalilovb, and V. V. Kuznetsova, c*
a Ufa State Petroleum Technological University, ul. Kosmonavtov 1, Ufa, 450062 Bashkortostan, Russia
b Institute of Petrochemistry and Catalysis, Russian Academy of Sciences,
pr. Oktyabrya 141, Ufa, 450075 Bashkortostan, Russia
c Ufa State Aviation Technical University, ul. K. Marksa 12, Ufa, 450008 Bashkortostan, Russia
*e-mail: kuzmaggy@mail.ru
Received January 11, 2017
Abstract—Conformational study of 5-bromo-2-(2-methylpropyl)-5-nitro-1,3,2-dioxaborinane at the DFT
PBE/3ξ level of theory revealed the only sofa–sofa interconversion pathway through a transition state
corresponding to 2,5-twist conformer. The barrier to internal rotation of the axial nitro group is several times
1
higher than that for the equatorial nitro group. According to the H, 13C, and 11B NMR, IR, and X-ray
diffraction data, the main autooxidation products of 5-bromo-2-(2-methylpropyl)-5-nitro-1,3,2-dioxaborinane
are 2-bromo-2-nitropropane-1,3-diol and boric acid.
DOI: 10.1134/S1070428017060185
Interest in six-membered cyclic boric and boronic
acid esters, 1,3,2-dioxaborinanes, is determined by
importance of these compounds for fine organic syn-
thesis, combination of their practically useful prop-
erties (biologically active compounds, corrosion inhib-
itors, and components of polymeric materials, fuels,
and lubricants), and specific structural features (elec-
tronic and steric intramolecular interactions) [1–3].
2,5,5-Trisubstituted 1,3,2-dioxaborinanes with differ-
ent substituents on C5 characteristically give rise to
conformational equilibrium between sofa invertomers,
which is shifted (partially or completely) toward one
invertomer [4]. Of particular interest is the structure of
cyclic boronic esters with polar substituents on C5,
e.g., with a nitro group. According to the X-ray diffrac-
tion data, 5-methyl-5-nitro-2-phenyl-1,3,2-dioxabori-
nane molecule has a sofa conformation with the axial
shown that 5-bromo-2-(2-methylpropyl)-5-nitro-1,3,2-
dioxaborinane (1) in solution is conformationally
homogeneous. This follows from the signals of
methylene protons on C4 and C6, which appear as
a typical AB quartet with Δδ = 0.47 ppm [4]. In this
work we studied conformational transformations of
cyclic ester 1 in terms of the density functional theory
(DFT) using PBE/3ξ approximation (PRIRODA [6]),
as well as chemical transformations of 1 on prolonged
storage.
The results of computer simulation of conforma-
tional transformations of 1 were consistent with the
experimental data. They indicated almost complete
shift of conformational equilibrium toward conformer
Sax with the axial nitro group. A local minimum (struc-
ture Seq) and transition state (TS) having 2,5-twist con-
formation (2,5-T) were also localized on the potential
energy surface (Scheme 1). The data in Table 1 show
1
nitro group [5]. On the other hand, H NMR study has
Scheme 1.
NO2
O
O2N
Br
B
Bu-i
O2N
O
O
O
B
Bu-i
Br
O
Br
B
Bu-i
O
Sax
2,5-T
Seq
926

CONFORMATIONAL TRANSFORMATIONS AND AUTOOXIDATION
927
Table 1. Calculated energy parameters of minima and transition states on the PES of ester 1 and minima on the PES of diol 3
6
6
O2N
Br
O2N
Br
O
OH
OH
5
5
B
Bu-i
O
4
4
1
3
ΔE0° (ΔE0≠),
kcal/mol
ΔH° (ΔH≠298),
ΔG° (ΔG≠298),
ΔS° (ΔS≠298),
298
298
298
Conformer
1, Sax
–E0,a hartree
kcal/mol
kcal/mol
cal mol–1 K–1
0.0
3228.185057
3228.182939
3228.174630
3228.180674
3228.181432
3228.181947
3046.885254
3046.883239
3046.875849
0.0
1.3
0.0
0.0
1, Seq
1.3
1.3
00.02
(–3.9)–
(–6.0)–
(–5.1)–
1.8
1, 2,5-T
(6.5)
(2.8)
(0.9)
0.6
(6.1)
(2.2)
(0.4)
0.7
(7.2)
(4.0)
(1.9)
0.1
1, NO2 (ax) (TS)
1, NO2 (eq) (TS)b
1, NO2 (eq) (BS)b
3a
0.0
0.0
0.0
0.0
3bc
3cc
1.8
2.0
1.5
2.2
5.9
6.3
5.1
4.1
a
b
c
With correction for zero-point energy.
Relative to Seq.
Relative to 3a.
that the ΔH≠ and ΔG≠ values for the ring intercon-
version approach those found experimentally for most
structurally related compounds [7].
PES also contained a local minimum corresponding
to bisector conformer; the energy difference with
the global minimum does not exceed 0.7 kcal/mol
(Scheme 3; Table 1, Fig. 2).
The preferentially axial orientation of the 5-nitro
group in 1,3,2-dioxaborinanes is accounted for by
stabilizing dipole–dipole and orbital interactions with
the heteroatom part of the ring [1, 3]. This should
affect the barrier to internal rotation of that substituent.
Computer simulation revealed transition states for in-
ternal rotation of the axial and equatorial nitro groups;
the corresponding ΔH≠ and ΔG≠ values are given in
Table 1. The global minimum for conformer Sax cor-
responds to orthogonal arrangement of the nitro group,
and the transition state has bisector structure (BS)
(Scheme 2; Fig. 1).
The energy-degenerate transition states correspond
to the gauche conformation of the nitro group (α =
33.9°). It is seen that the activation parameters for
internal rotation of the axial nitro group are consider-
ably higher (2.1 to 5 times) than those for the equato-
ΔE, kcal/mol
TS
2.8
2.1
1.4
The global energy minimum for Seq is also achieved
with orthogonal orientation of the nitro group. The
Scheme 2.
O
O
O
O
N
N
O
O
Br
Br
0.7
B
Bu-i
B
Bu-i
O
O
Sax
TS (BS)
O
O
N
0.0
–90
–45
0
45
τ, deg
O
Br
B
Bu-i
O
Fig. 1. Energy profile for the rotation of the axial nitro group
in conformer Sax of ester 1 about the C–N bond at 0 K.
Sax
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 53 No. 6 2017

VALIAKHMETOVA et al.
928
Scheme 3.
Br
Br
O
Br
O
O
O
O
O
O
O
N
N
N
B
Bu-i
B
Bu-i
B
Bu-i
O
O
O
O
Seq
TS
Br
BS
Br
Br
O
α
O
O
N
O
N
N
O
O
rial nitro group (Table 1). This suggests appreciable,
most probably dipole–dipole interaction between the
axial substituent and heteroatom moiety of the ring.
The boron atom and oxygen atom of the nitro group in
the transition state approach each other most closely.
It is known that cyclic boronic acid esters are prone
to autooxidation; after prolonged storage (for several
years) even in a closed vessel containing air, 2,2′-oxy-
bis(1,3-dioxa-2-boracycloalkanes) are formed. This
transformation was reported for the first time for
a seven-membered cyclic boronic ester, 3-(2-methyl-
propyl)-1,5-dihydro-2,4,3-benzodioxaborepine [10];
later on, analogous reactions were described for five-
and six-membered analogs [11, 12] (Scheme 4).
O
O
N
2.941 Å
O
Br
B
Bu-i
O
TS
Scheme 4.
( )n
( )n
( )n
O
B
O
B
O
B
O2
Analogous character of internal rotation of the nitro
group was observed for 5,5-dinitro-1,3-dioxane mole-
cule [8]. On the other hand, internal rotation of the
axial nitro group in 2-methyl-5-nitro-1,3,2-dioxabori-
nane is accompanied by conformational rearrangement
of the ring, which additionally increases the activation
barrier of this process [9].
2
R
O
O
O
O
n = 0–2.
A probable autooxidation mechanism was proposed
in [12]. A sample of 1 (which is a high-boiling liquid)
was stored in a closed vessel for more than 12 years
and was gradually converted to almost colorless crys-
talline solid. It may be presumed that the major prod-
uct of autooxidation of 1 would be the corresponding
2,2′-oxybis(dioxaborinane) 5 (Scheme 5).
ΔE, kcal/mol
TS
TS
0.9
0.6
0.3
Scheme 5.
NO2
Br
NO2
Br
O
B
O
B
O2
O
O
BS
i-Bu
O
i-Bu
O
1
NO2
Br
O
·
·
–t-Bu
O
B
O
O
4
O2N
Br
NO2
Br
0.0
–90
1
O
B
O
–45
0
45
τ, deg
·
–t-Bu
B
O
O
O
Fig. 2. Energy profile for the rotation of the equatorial nitro
group in conformer Seq of ester 1 about the C–N bond at 0 K.
5
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 53 No. 6 2017

CONFORMATIONAL TRANSFORMATIONS AND AUTOOXIDATION
929
1
Table 2. H and 13C NMR spectra of 5-bromo-2-(2-methylpropyl)-5-nitro-1,3,2-dioxaborinane (1), crystalline product of its
autooxidation (2), and 2-bromo-2-nitropropane-1,3-diol (3) in D2O
1H NMR spectrum, δ, ppm (J, Hz)
i-Bu
13C NMR spectrum, δC, ppm
C4, C6 C5
i-Bu
Compound
no.
(CH2)2
1
2
3
4.19 q (–12.0) 0.63 d (CH2, 3J = 7.2), 0.82 d (Me, 3J = 6.6), 1.73 m (CH) 61.58 097.07 21.01, 21.33
4.21 q (–13.0)
4.21 q (–13.0)
–
–
64.82 100.42
64.86 100.40
–
–
We analyzed both liquid sample of 1 after pro-
longed storage and colorless crystalline solid formed
under the liquid. The IR spectrum of the liquid phase
contained the expected νas(NO2) band at 1561 cm–1, as
well as C=C, B–H, and O–H stretching bands at 1640,
C–O bonds (Fig. 3, 3a); the tabulated bond lengths of
3 are available from the authors by e-mail. The crys-
tallographic parameters are given in Experimental. The
bond lengths were consistent with those reported
in [18, 19] and found in other aliphatic nitro com-
pounds [22, 23].
1
2524, and 3434 cm–1, respectively. In the H NMR
spectrum of this sample (CDCl3) we observed methy-
lene proton quartet at δ 4.21 ppm due to 4-H and 6-H
protons of 1 [1, 4] and a low-intense multiplet signal at
δ 5.1 ppm assignable to an olefinic proton.
It is reasonable to expect that a structure stabilized
by intramolecular hydrogen bond should predominate
in the gas phase and in solution. The H NMR spec-
1
trum of 3 in D2O showed an AB quartet analogous to
that observed in the spectrum of 1 (2J = –13.0 Hz,
Δδ = 0.13 ppm (Table 2). This means that the structure
with pseudo-chair conformation of the (CH2OH)2 frag-
ment should predominate in conformational equilib-
rium in solution at room temperature. Quantum-
chemical study of the conformational behavior of 3 at
the PBE/3ξ level of theory revealed three main
conformers 3a–3c on the potential energy surface
(Scheme 6). In accordance with their relative energies
(Table 1), the most favorable conformer is 3a with
pseudoaxial nitro group, which is stabilized by intra-
molecular hydrogen bond between the hydroxy groups
(O···H 2.0 Å, Mulliken bond order 0.07).
Presumably, the transformation of cyclic ester 1 on
storage involves radical addition of oxygen molecule
to the boron atom in keeping with the known scheme
for autooxidation of organoboron compounds [13],
followed by elimination of tert-butyl radical (isomer-
ization product of isobutyl radical) with formation of
peroxide 4. A series of secondary reactions leading to
isobutylene, cyclic and acyclic boronic acid esters with
B–H bond, and alkoxyl and hydroxyl radicals are also
possible. By analogy with the known data for auto-
oxidation of some 2-alkyl-1,3,2-dioxaborinanes
[10–12], expected product 5 should reside in the crys-
talline phase.
The crystalline residue (2) began to melt at 122°C
(cf. mp 131°C for pure diol 3) [14]. On the basis of the
1H and 13C NMR data (Table 2), the product was
identified as 2-bromo-2-nitropropane-1,3-diol (3).
Compound 3 (Bronopol) is widely used in cosmetics
as antibacterial agent [15–17]. Its structure was studied
by X-ray analysis [18–20] and IR spectroscopy [21].
Conformational transformations of individual mole-
cules and crystal structure as a whole were observed
when a crystalline sample of 3 was heated to a tem-
perature approaching phase transition. Fedorov et al.
[20] synthesized two samples of 3 from different pre-
cursors, and each sample was studied by X-ray diffrac-
tion. In the present work, X-ray analysis of the crystal-
line product obtained from ester 1 after prolonged
storage showed the presence of diol 3 with a structure
corresponding to that of a sample of 3 described in
[20] with antiperiplanar orientation of the C–C and
Structure 3c has the highest energy despite analo-
gous intramolecular hydrogen bond. Assuming that the
concentration of 3c is negligible, the relative popula-
tions of 3a and 3b were estimated using the known
equation Δ G° = –R T ln(3a/3b). The Δ G° value
O1
Br1
N9
O10
O2
O3
C7
C8
C6
Fig. 3. Structure of the molecule of 2-bromo-2-nitropropane-
1,3-diol (3) according to the X-ray diffraction data.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 53 No. 6 2017

VALIAKHMETOVA et al.
930
Scheme 6.
O
O
Br
HB
N
HB
OH
O
O
Br
NO2
N
HB
Br
HB
O
O
H
H
O
O
H
H
HA
HA
H
H
HA
3c
HA
3a
CH2OH
3b
1.5 kcal/mol for 3b (Table 1) at 298 K corresponds to
the 3a/3b ratio ~93:7. Thus, the population of 3a is
fairly high. These results are consistent with the con-
formational analysis data for propane-1,3-diol which
was also found to prefer pseudo-chair conformation
with intramolecular hydrogen bond [24].
Additional information on the composition of crys-
talline residue 2 was inferred from the 11B NMR
spectra recorded in CDCl3 and D2O, which contained
an intense signal at δB 19.5 (CDCl3) or 18.4 ppm (D2O)
and a weak signal at δB 31.5 (CDCl3) or 33.1 ppm
(D2O). The downfield signal is likely to belong to
boric acid as the final boron-containing autooxidation
product [25], and the upfield signal was assigned to
traces of 1. We failed to obtain single crystals of boric
acid suitable for X-ray analysis. Thus, hydrolysis of
unstable 2,2′-oxybis(dioxa-borinane) 5 gives initial
1,3-diol and boric acid (Scheme 7).
rameters of conformers Sax and Seq (preliminarily cal-
culated at the AM1 level; HyperChem [26]); for this
purpose, the torsion angles OCCC or OBOC were
varied in the range from –50 to +50°; the conformation
of the isobutyl group on the boron atom always corre-
sponded to minimum on the potential energy surface to
eliminate its effect on the ring conformation. The
interconversion pathway and potential barriers were
determined by the transition state search algorithm
implemented in the software used. Internal rotation of
the nitro group in molecule 1 was simulated by scan-
ning the torsion angle ONCBr from 90 to –90°. Analo-
gous calculations were performed for conformers of
diol 3. Stationary points on the potential energy surface
were identified as minima by the absence of imaginary
frequencies in the corresponding Hessian matrix, and
as transition states, by the presence of one imaginary
frequency.
The X-ray diffraction data for diol 3 were obtained
on an XCalibur Eos automated four-circle diffractom-
eter (Mo Kα radiation, λ 0.71073 Å, graphite mono-
chromator, ω-scanning, 2θmax = 62°) using CrysAlisPro
version 1.171.36.20 (Oxford Diffraction Ltd.). The
structure was solved by the direct method and was
refined by full-matrix least-square method in aniso-
tropic approximation for non-hydrogen atoms. Hydro-
gen atoms were localized by Fourier difference syn-
theses and were refined in isotropic approximation. All
calculations were performed using SHELX97 [27].
Crystallographic data and experimental details: tem-
perature 293(2) K; monoclinic crystal system, space
group Cc; unit cell parameters: a = 8.000(2), b =
9.6403(14), c = 8.9416 (16) Å; β = 91.08(2)°; V =
689.5(2) Å3; Z = 2; dcalc = 2.088 g/cm3; μ = 5.923 mm–1;
F(000) = 420.0; scan range 6.62 ≤ θ ≤ 62.36; reflection
indices: –11 ≤ h ≤ 6, –13 ≤ k ≤ 10, –12 ≤ l ≤ 6; number
of independent reflections 923 (Rint = 0.0274); good-
ness of fit 1.254; final divergence factors: R1 = 0.0683
[reflections with Ihkl > 2σ(I)], wR2 = 0.1671; R1 =
0.0751 (all reflections), wR2 = 0.1754; Δρ, min/max:
1.00/–1.10 ēÅ–3. The X-ray diffraction data for diol 3
were deposited to the Cambridge Crystallographic
Data Centre (CCDC entry no. 1040940).
Scheme 7.
Br
H2O
H2O
O
B
NO2
2
B(OH)3 + 3
5
HO
O
Obviously, hydrolytic instability is intrinsic to
cyclic boric and boronic esters derived from diol 3. In
fact, our attempt to synthesize 5-bromo-2-(2-methyl-
propyloxy)-5-nitro-1,3,2-dioxaborinane by reaction of
diol 3 with triisobutyl borate was unsuccessful, and
the only liquid product was the initial boric ester
(Scheme 8). Nevertheless, numerous examples of
stable six-membered cyclic boric esters are known [1].
Scheme 8.
Br
OH
O
NO2
+
B(OBu-i)3
O2N
Br
OH
B
i-BuO
O
EXPERIMENTAL
Conformational behavior of ester 1 was simulated
using PRIRODA software package [6] in the DFT
PBE/3ξ approximation by optimizing geometric pa-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 53 No. 6 2017

CONFORMATIONAL TRANSFORMATIONS AND AUTOOXIDATION
931
1
The H, 13C, and 11B NMR spectra were recorded
9. Valiakhmetova, O.Yu., Bochkor, S.A., and Kuzne-
tsov, V.V., Russ. J. Gen. Chem., 2010, vol. 80, p. 737.
10. Kuznetsov, V.V., Mazepa, A.V., and Spirikhin, L.V.,
Chem. Heterocycl. Compd., 1998, vol. 34, p. 1207.
11. Kuznetsov, V.V. and Mazepa, A.V., Dopov. Nats. Akad.
Nauk Ukr., 1998, no. 11, p. 142.
on a Bruker Avance 400 spectrometer at 400.13,
100.62, and 128.33 MHz, respectively, from solutions
in CDCl3 and D2O with a concentration of 0.01 M.
Diol 3 is poorly soluble in CDCl3; therefore, the data
obtained in D2O were used for comparison. The chem-
ical shifts were measured relative to tetramethylsilane
(1H, 13C) or BF3·Et2O (11B) as internal standard. The
IR spectrum (film) was recorded on a Bruker Vertex
70V spectrometer.
12. Kuznetsov, V.V., Mazepa, A.V., and Spirikhin, L.V.,
Russ. J. Gen. Chem., 2000, vol. 70, p. 1576.
13. Mikhailov, B.M. and Bubnov, Yu.N., Bororganicheskie
soedineniya v organicheskom sinteze (Organoboron
Compounds in Organic Synthesis), Moscow: Nauka,
1977, p. 171.
14. Namba, K., Indzyka, C., and Joneno, M., J. Chem. Soc.
Jpn., Ind. Chem. Sect., 1963, vol. 66, p. 1446.
Ester 1 was described in [4]. A fresh sample of 1
was obtained by reaction of diisobutyl isobutylboro-
nate with an equimolar amount of diol 3 in benzene.
Yield 68%, bp 90–92°C (4 mm).
15. Bryce, D.M., Crosnaw, B., Hall, J.E., Holland, V.R., and
Lessel, B., J. Soc. Cosmet. Chem., 1978, vol. 29, p. 3.
16. Shepherd, J.A., Waigh, R.D., and Gilbert, P., Anti-
microb. Agents Chemother., 1988, vol. 32, p. 1693.
This study was performed under financial support
by the Ministry of Education and Science of the
Russian Federation (project no. 16.1969.2017/PCh).
17. Legin, G.Ya., Pharm. Chem. J., 1996, vol. 30, no. 4,
p. 273.
REFERENCES
18. Gowda, D.S. and Rudman, R., J. Chem. Phys., 1982,
vol. 77, p. 4666.
1. Gren’, A.I. and Kuznetsov, V.V., Khimiya tsiklicheskikh
efirov bornykh kislot (Chemistry of Cyclic Boronic Acid
Esters), Kiev: Naukova Dumka, 1988.
2. Kuznetsov, V.V., Panorama sovremennoi khimii Rossii.
Uspekhi organicheskogo kataliza i khimii geterotsiklov
(Prospect of Modern Chemistry in Russia. Advances in
Organic Catalysis and Chemistry of Heterocycles), Rakh-
mankulov, D.L., Ed., Moscow: Khimiya, 2006, p. 336.
19. Golovina, N.I., Raevskii, A.V., Fedorov, B.S., Gusakov-
skaya, L.G., Trofimova, R.F., and Atovmyan, L.O.,
J. Solid State Chem., 1998, vol. 137, p. 231.
20. Fedorov, B.S., Golovina, N.I., Arakcheeva, V.V., Trofi-
mova, R.F., and Atovmyan, L.O., Russ. Chem. Bull.,
1996, vol. 45, p. 1157.
21. Golovina, N.I., Chukanov, N.V., Raevskii, A.V., and
Atovmyan, L.O., J. Struct. Chem., 2000, vol. 41, p. 238.
3. Kuznetsov, V.V., Russ. J. Org. Chem., 2014, vol. 50,
p. 1227.
22. Nordenson, S., Skramstad, J., and Fløtra, E., Acta Chem.
Scand., Ser. B, 1984, vol. 38, p. 461.
4. Kuznetsov, V.V., Valiakhmetova, O.Yu., and Bochkor, S.A.,
Chem. Heterocycl. Compd., 2007, vol. 43, p. 1577.
23. Trotter, J., Tetrahedron, 1960, vol. 8, p. 13.
5. Kliegel, W., Preu, L., Rettig, S.J., and Trotter, J., Can. J.
Chem., 1986, vol. 64, p. 1855.
6. Laikov, D.N. and Ustynyuk, Yu.A., Russ. Chem. Bull.,
Int. Ed., 2005, vol. 54, p. 820.
24. Shagidullin, Rif.R., Chernova, A.V., Plyamova-
tyi, A.Kh., and Shagidullin, R.R., Russ. Chem. Bull.,
1991, vol. 40, p. 1993.
25. Nöth, H. and Wrackmeyer, B., NMR: Basic Princ.
Prog., 1978, vol. 14, p. 1.
7. Carton, D., Pontier, A., Ponet, M., Soulie, J., and
Cadiot, P., Tetrahedron Lett., 1975, vol. 16, p. 2333.
26. HyperChem 8.0. http://www.hyper.com
8. Khaibullina, G.S., Bochkor, S.A., and Kuznetsov, V.V.,
27. Sheldrick, G.M., Acta Crystallogr., Sect. A, 2008,
Russ. J. Org. Chem., 2014, vol. 50, p. 725.
vol. 64, p. 112.
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