Reaction of Carbodiimides with trifluoromethanesulfonic acid

Article abstract of DOI:10.1134/S1070363213100095

Carbodiimides RN=C=NR (R = i-Pr, c-C₆H₁₁, Ph) react with trifluoromethanesulfonic acid with successive formation of O-triflyl isoureas RNHC(OTf)=NR, the isomeric N-triflyl ureas RN(Tf)C(O)NHR, and symmetrically substituted ureas RNHC

Full text of DOI:10.1134/S1070363213100095

ISSN 1070-3632, Russian Journal of General Chemistry, 2013, Vol. 83, No. 10, pp. 1853–1858. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © L.L. Tolstikova, B.A. Shainyan, N.N. Chipanina, 2013, published in Zhurnal Obshchei Khimii, 2013, Vol. 83, No. 10, pp. 1643–
1648.
Reaction of Carbodiimides with Trifluoromethanesulfonic Acid
L. L. Tolstikova, B. A. Shainyan, and N. N. Chipanina
А.Е. Favorsky Irkutsk Institute of Chemistry, Siberian Branch, Russian Academy of Sciences,
ul. Favorskogo 1, Irkutsk, 664033 Russia
e-mail: bagrat@irioch.irk.ru
Received April 23, 2013
Abstract—Carbodiimides RN=C=NR (R = i-Pr, c-C6H11, Ph) react with trifluoromethanesulfonic acid with
successive formation of О-triflyl isoureas RNHC(OTf)=NR, the isomeric N-triflyl ureas RN(Tf)C(O)NHR, and
symmetrically substituted ureas RNНC(О)NНR. Carbodiimide Me3SiN=C=NSiMe3 in the reaction with TfOH
undergoes desilylation to afford ester CF3SO2OSiMe3 and unsubstituted carbodiimide (cyanamide), which
gives the products of di- and trimerization and urea.
DOI: 10.1134/S1070363213100095
Carbodiimides RN=C=NR are highly electrophilic
compounds, which actively react with various proton
donors to form either the adducts or the products of
their dehydration being converted to ureas (RNH)2CO.
In particular, carboxylic acids react with carbodiimides
with the initial formation of acylimidocarbamates (О-
acylisoureas), which further either rearrange to N-
acylureas or react with the second molecule of the acid
to give the corresponding urea and the acid anhydride
[1, 2].
Ac
C
O
C
O
RNH
NR
(1)
RN
C
NR
+
AcOH
C
O
RNH
NR
NHR
+
Ac2O
RNH
Ac
The structure of N-acylurea was proved, in parti-
cular, by XRD analysis for the adduct of polyfluoro-
carboxylic acid C6F13(CH2)9COOH with dicyclohexyl
carbodiimide [3].
N-trifluoromethylsulfonylcarbodiimides
(N-triflyl-
carbodiimides) CF3SO2N=C=NR (R = H, SiMe3). The
latter are obtained by the reaction of trifluoromethane-
sulfonic anhydride with bis(trimethylsilyl)carbodi-
imide [9], or by the aza-Curtius rearrangement in the
reaction of N-(triflyl)carboxyimidoyl chloride with
sodium azide [8], or by the rearrangement of O-tri-
methylsilyl- and O-tosylsubstituted triflylimides of
arenehydroxamic acids [10]. The third methos is the
hydrolysis of N,N'-bis(triflyl)carbamoyl chloride [11].
Finally, as the most available method for the synthesis
of perfluoroalkylsulfonyl ureas, the reaction of sodium
salts of perfluoroalkanesulfonamides with isocyanates
in yields from 49 to 84% was claimed [5].
Unlike the reaction with carboxylic acids, in the
reaction with sulfonic acids carbodiimides act only as
dehydrating reagents resulting in the formation of
ureas and sulfonic acid anhydrides in good yields [1].
There are no data in the literature on the formation of
intermediate O-alkyl(aryl)sulfonyl isoureas or N-alkyl
(aryl)sulfonyl ureas, including the recent monograph
[4].
At the same time, N-sulfonyl ureas show high
biological and herbicidal activity [5]. As to perfluoro-
alkanesulfonyl ureas, several methods of their prepara-
tion are known. The first one was the reaction of trifluoro-
methylsulfonyl isocyanate with aniline [6] and other
amines [7, 8]. The second method is the hydrolysis of
As follows from Eqs. (2), all these methods suggest
the involvement in the process of different derivatives
of triflamide CF3SO2NH2 or its analogs. With this in
mind and aiming at the elucidation of the possibility of
1853

1854
TOLSTIKOVA et al.
CF3SO2N
C
NR
NHR
C
H2O
NHSO2CF3
Cl
RNH2
H2O
CF3SO2N
C
O
RFSO2NH
C
O
C
CF3SO2N
(2)
RN
O
RFSO2NHNa
formation of O-triflyl isoureas and(or) N-triflyl ureas
similar to O-acyl isoureas and N-acyl ureas shown in
Scheme (1) as well as elaboration of the method of
preparation of N-triflyl ureas based on the most ac-
cessible triflate starting materials, we have studied the
reaction of carbodiimides RN=C=NR (I) [R = i-Pr (а),
c-C6H11 (b), Ph (c), Me3Si (d)] and cyanamide H2NCN
(II) with trifluoromethanesulfonic acid (TfOH).
Carbodiimides Ia–Ic exothermically react with
TfOH in methylene chloride solution at room tempera-
ture according to reaction (3) with the formation of
high-melting solid products representing the mixtures
of the isomeric adducts, N-triflyl ureas (III) and
O-triflyl isoureas (IV), which were identified by IR
spectroscopy.
+
C
N
+
R
C
NHR
R
N
C
NHR
N
R
TfOH
R
N
(3)
Tf
O
III
OTf
IV
I
In the IR spectra of N-triflyl ureas (IIIa, IIIb) in
KBr the ν(С=О) stretching vibrations band at 1694 cm–1
is present, which coincides with the ν(С=О) band in
compounds RFSO2NHC(O)NHR [5, 6]. The band has a
low-frequency shoulder at 1678 (IIIa) or 1667 cm–1
IIIb, which, on the basis of the literature data [12] was
assigned to the stretching vibrations ν(С=N) in
O-triflyl ureas (IV). In the IR spectra of the products
the stretching vibration bands ν(NH) at 3370 (IIIa) or
3440 cm–1 IIIb and the bending vibrations δ(NH) at
1537 and 1282 cm–1 (IIIа) or 1527 and 1264 cm–1
(IIIb) are also present.
effect of the triflyl group decreasing the polarization of
this bond.
The IR monitoring of the reaction was performed
by the example of the reaction with carbodiimide Ib.
In the spectrum of the precipitate separated in a small
amount from the reaction mixture, the major band is ν
(С=О) band of product IIIb (see Fig. 1a). In the
spectrum of the main product obtained after
evaporation of the filtrate, the ν(С=N) band of product
IVb and ν(С=О) bands of products IIIb and Vb with
close peak intensities are present (see Fig. 1b). Finally,
after the treatment of this product with water,
practically only the low frequency band belonging to
urea Vb remains (see Fig. 1c). All this points to the
order of transformations I → IV → III → V, which is
similar to that described in review [2] for the reaction
of carbodiimides with carboxylic acids. According to
these data, the formation of N-acyl ureas is favored by
the higher temperature and the duration of the process
[2]. We observed the same temperature dependence for
the reaction of carbodiimides Ia, Ib with TfOH. When
carrying out the reaction of compound Ia with TfOH at
–30 to –40°С for 2 days, the precipitate was not
formed even on the next day, and the IR spectrum after
removal of the solvent contains the ν(С=О) band of
IIIa only as a shoulder on the high-frequency wing of
the ν(С=N) band of IVa.
On addition of TfOH to the solution of diphenyl-
carbodiimide Ic in methylene chloride the solution
instantly turns bright-yellow. For the reaction products
IIIc and IVc the corresponding bands in the IR spectra
are notably shifted to low frequencies and appear at
1674 (IIIc) and 1637 cm–1 (IVc). The observed
ν(С=О) and ν(С=N) frequency values for compounds
IIIa–IIIc and IVa–IVc are substantially higher than
the ν(С=О) frequencies in the symmetrically
substituted ureas RNH–C(O)–NHR (V), equal to 1629,
1626 and 1612 cm–1 for R = i-Pr, c-C6H11 and Ph,
respectively. This is indicative of a larger double-
bonding character of the C=N and С=О bonds in
compounds III and IV as compared to the С=О bond
in compounds V due to strong electron-withdrawing
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 83 No. 10 2013

1855
REACTION OF CARBODIIMIDES WITH TRIFLUOROMETHANESULFONIC ACID
It is known that in low polar solvents like CH2Cl2,
(a)
isoureas from carbodiimides and carboxylic acids are
formed instantly and in the absence of a nucleophile or
base they can exist during several hours [13]. In our
case, isourea IVb stabilized by the triflyl substituent
undergoes rearrangement to compound IIIb and
hydrolysis to urea Vb only upon boiling in water.
For the synthesis of N-acyl ureas not only the
reaction with carboxylic acids but the transfer of the
acyl group from their ammonium salts to carbodiimides
is employed [3]. However, this technique cannot be
applied to the triflate salts: the triethyl amine or
pyridine triflates do not react with carbodiimide Ib
even upon prolonged reflux in CHCl3.
1H and 13С NMR spectra of the products of the
reaction of trifluoromethanesulfonic acid with carbo-
diimides Ia–Ic are complicated not only due to
overlapping of the signals of the two isomeric forms
III and IV, but also due to nonequivalence of
substituents R. Note that such nonequivalence was
observed even in the formally symmetrical isoelec-
tronic analogs of O-triflyl ureas (IV), N-triflyl guani-
dines (RNH)2C=NTf [14, 15].
(b)
(c)
Reaction of carbodiimide Id with one equivalent of
TfOH proceeds with the formation of the products of
exchange of one trimethylsilyl group with the triflyl
group, N-triflyl-N'-trimethylsilylcarbodiimide VI and
silanol VII. Close analog of reaction (4) is the de-
silylation of carbodiimide Id under the action of
trifluoroacetic anhydride or trifluoroacetyl chloride
[16].
(d)
+
C
Id
Me3Si
N
N
SiMe3 TfOH
(4)
+
TfN
C
VI
N
SiMe3
Me3SiOH
VII
ν, cm–1
IR spectra of the products of TfOH reaction with
carbodiimides in the range 1550–1750 cm–1. (a) The initial
precipitate from the reaction with carbodiimide Ib; (b) he
precipitate after removal of solvent; (c) after treatment with
water; and (d) product of the reaction with carbodiimide Iа
at –30 to –40°С.
This is indicated by the shift of the ν(N=C=N) band
from the value of 2203 cm–1 characteristic of com-
pound Id to 2230 cm–1 coinciding with the literature
19
data for compound (VI) [9]. The only signal in the F
NMR spectrum of the product of reaction (4) (–79.3 ppm)
also coincides with that known for compound VI
(–79.5 ppm) [8]. Compound VI is presented in the
carbodiimide form, although the possibility of the
carbodiimide–cyanamide isomerization was studied
[17, 18]. The formed trimethylsilanol VII is converted
into hexamethyldisiloxane, as proved by the presence
of the intense ν(SiOSi) band at 1056 cm–1 in the IR
spectrum, and a signal at 7.7 ppm in the 29Si NMR
spectrum that is in the region typical for siloxanes. The
product of the reaction is a liquid that solidified upon
storage. The IR spectrum has broadened bands in the
range 3190–3360, 2220–2230, and 1550–1660 cm–1,
which become wider and merge in time. Apparently,
this is the result of the processes of dimerization and
trimerization typical for trifluoromethylsulfonyl carbo-
diimides [4] as well as of hydrolysis resulting in N-
sulfonyl-substituted ureas [19].
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 83 No. 10 2013

1856
TOLSTIKOVA et al.
Reaction of carbodiimide Id with two equivalents
of TfOH leads to a complete desilylation and a
formation of trimethylsilyl trifluoromethanesulfonate
CF3SO2OSiMe3 VIII, as is unequivocally proved by a
large downfield chemical shift of the Si NMR signal
equal to 43.2 ppm (δSi 43.5 ppm [20]).
29
+
[HN C
NH
NH2]
2TfOH
2TfOSiMe3
N
C
Id +
2TfOH
VIII
(5)
2+
+
H2N
H2N
H2N
H2N
TfOH
−TfOH
C
C
N
C
N
C
NH TfO−
2TfO−
NH2
IX
X
The formed carbodiimide existing in equilibrium
with the more stable cyanamide tautomer is converted
into the dimer, which can exist either as a double salt
IX or as the mono-salt Х, as was shown earlier [21].
From the data of elemental analysis, the product of the
reaction is mono-salt Х containing about 20% of salt
IX. The presence of a wide ν(NH) band in the IR
spectrum with several maxima in the range 3230–
3430 cm–1 is consistent with the formation of salts IX, X.
standard the signals of residual protons (for 1H spectra)
or carbon atoms (for 13С spectra) of the solvent were
used, the chemical shifts are given relative to TMS
(1Н, 13С, 29Si) and CCl3F (19F).
N,N-Diisopropylcarbodiimide, N,N-dicyclohexyl-
carbodiimide, and N,N-bis(trimethylsilyl)carbodiimide
were commercial reagents used without further
purification. N,N-Diphenylcarbodiimide was prepared
from N,N'-diphenylurea by the known procedure [22].
According to reaction (5), the reaction of trifluoro-
methanesulfonic acid with cyanamide II must be
similar to its reaction with carbodiimide Id in the 2:1
ratio. Indeed, the reaction proceeds with the complete
conversion of cyanamide and the formation of double
salt IX and mono-salt Х, as well as of urea, the product
of hydrolysis of cyanamide.
N-Trifluoromethylsulfonyl-N,N'-diisopropyl urea
(IIIа). To the suspension of 0.25 g (2.0 mmol) of
carbodiimide Ia in 5 ml of CH2Cl2 0.30 g (2.0 mmol,
18 ml) of TfOH was added at vigorous stirring and
cooling with cold water, the reaction mixture was
stirred for 4 h at room temperature and left overnight.
The precipitate formed was filtered off and dried in a
vacuum. Yield 0.39 g (71%), mp 78°С. IR spectrum
(KBr), ν, cm–1: 3372, 2991, 1698, 1662, 1538, 1283,
Therefore, in the reaction of carbodiimides with
TfOH we did not observe the formation of the product
of dehydration, trifluoromethanesulfonic anhydride
with the characteristic 19F NMR signal at –73.6 ppm.
In the 19F NMR spectrum of the product of reaction
with carbodiimide Ib one signal at –78.4 ppm is
observed, which is characteristic of substituted amides
TfNHR. This is consistent with the order of trans-
formations I → IV → III → V found by IR moni-
toring, and with the described in the literature O→N
migration of acyl group in the reaction of carbodiimide
Ib with carboxylic acids [2].
1
1241, 1178, 1033, 637. Н NMR spectrum, δ, ppm:
5.73 br.s (1H, NH), 3.80–3.70 m (2H, NСH), 1.22 d
(12H, CH3, J 6.4 Hz). 13С NMR spectrum, δC, ppm:
1
153.08 (С=O), 120.36 q (CF3, JCF 322.0 Hz), 45.31,
45.15 (NCH), 22.50, 22.47 (CH3). 19F NMR spectrum:
δF –79.08 ppm. Found, %: С 34.56; Н 5.81; N 10.33; S
11.48. C8H15F3N2O3S. Calculated, %: С 34.78; Н 5.47;
N 10.14; S 11.61.
N-Trifluoromethylsulfonyl-N,N'-dicyclohexyl
urea (IIIb). To the suspension of 0.413 g (2.0 mmol)
of carbodiimide Ib in 5 ml of CH2Cl2 0.30 g (2.0 mmol,
18 ml) of TfOH was added at vigorous stirring and
cooling with cold water, the reaction mixture was
stirred for 6 h at room temperature and left overnight.
The precipitate formed was filtered off and dried in a
vacuum. Yield of product IIIb with the isomeric IVb
0.27 g (39%), mp 127°С. IR spectrum (KBr), ν, cm–1:
3440, 3042, 2948, 2866, 1695, 1353, 1283, 1238, 1162,
EXPERIMENTAL
IR spectra were recorded on a Bruker Vertex 70
instrument in KBr and in solutions in CCl4, CHCl3,
and CH2Cl2. NMR spectra were registered on a Bruker
DPX-400 spectrometer with working frequencies 400
(1Н), 100 (13С), 376 (19F), and 80 MHz (29Si) in
CD3CN (if not stated otherwise). As an internal
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 83 No. 10 2013

1857
REACTION OF CARBODIIMIDES WITH TRIFLUOROMETHANESULFONIC ACID
1
1030, 636, 516. Н NMR spectrum, δ, ppm: 7.38 d
(1H, NH, J 7.2 Hz), 3.80–3.70 m (0.5H, CHNTf),
3.70–3.50 m (1.5H, СHNН), 1.87–1.07 m (20H, СH2).
13С NMR spectrum, δC, ppm: 154.16 and 153.99
(С=O), 121.64 q (CF3, J 319.8 Hz), 58.75 (TfNC),
51.71 (CNH), 33.61, 32.43, 32.10, 26.31, 26.18, 26.02,
25.84, 25.65, 25.41. 19F NMR spectrum: δF –78.78 ppm.
Found, %: С 45.91; Н 6.50; F 15.92; N 7.77; S 8.84.
C14H23F3N2O3S. Calculated, %: С 47.18; Н 6.50; F
15.99; N 7.86; S 9.00.
NH). 13С NMR spectrum (DMSO-d6), δC, ppm:
1
155.74, 154.70, 120.92 q (CF3, JCF 321.6 Hz). 19F
NMR spectrum: δF –77.85 ppm. Found, %: С 14.63; Н
2.10; F 25.63; N 21.72; S 12.07. Calculated for the
IX:X = 4:1 mixture of salts, %: С 14.81; Н 2.03; F
25.40; N 22.06; S 14.29.
Reaction of cyanamide (II) with TfOH was
carried out as above described for the reaction of Id
with one equivalent of TfOH (b). Conversion of cyan-
amide 100%. IR specrum (KBr), ν, cm–1: 3422, 3364,
3227, 1734, 1696, 1590, 1280, 1251, 1173, 1028, 640,
N-Trifluoromethylsulfonyl-N,N'-diphenyl urea
(IIIc). To the suspension of 0.39 g (2.0 mmol) of
carbodiimide Ic in 3 ml of CH2Cl2 0.30 g (2.0 mmol,
18 ml) of TfOH was added at vigorous stirring and
cooling with cold water, the reaction mixture was
stirred for 3 h at room temperature and left overnight.
Yield of product IIIc after evaporation of solvent 0.69 g
(100%), yellow crystals, mp 120°С. IR spectrum
(KBr), ν, cm–1: 3432, 1674, 1637, 1612, 1587, 1492,
1285, 1242, 1227, 1172, 1030, 756, 691, 636. 1Н NMR
spectrum, δ, ppm: 10.52 br.s (1H, NH), 8.15–7.00 m
(10H, Ph). 13С NMR spectrum, δC, ppm: 149.91
(С=O), 140.57, 135.42, 131.96, 131.80, 130.06, 128.77,
127.19, 125.43. 19F NMR spectrum: δF –79.32 ppm.
Found, %: С 48.39; Н 3.11; N 7.47; S 8.84.
C14H11F3N2O3S. Calculated, %: С 48.84; Н 3.22; F
16.55; N 8.14; S 9.31.
1
518. Н NMR spectrum (DMSO-d6), δ, ppm: 7.97 br.s
(2Н, NH), 7.1 br.s (1Н, NH). 13С NMR spectrum
(DMSO-d6), δC, ppm: 162.98, 161.87, 155.77, 154.68,
1
19
120.94 q (CF3, JCF 321.9 Hz). F NMR spectrum: δF
–77.84 ppm.
ACKNOWLEDGMENTS
This work was performed with the financial support
from the Russian Foundation for Basic Research (grant
no. 13-03-00055).
REFERENCES
1. Khorana, H.G., Chem. Rev., 1953, vol. 53, no. 2, p. 145.
2. Bakibaev, А.А. and Shtrykova, V.V., Russ. Chem. Rev.,
1995, vol. 64, no. 10, p. 929.
3. Buchanan, G.W., Rastegar, M.F., and Enright, G.,
J. Fluorine Chem., 2007, vol. 128, p. 1026.
Reaction of bis(trimethylsilyl)carbodiimide (Id)
with TfOH. а. To 0.37 g (2.0 mmol) of carbodiimide
Id in 5 ml of CH2Cl2 with stirring and cooling with
cold water 0.30 g (2.0 mmol, 0.18 ml) of TfOH was
added, the mixture was stirred for 2 h and left
overnight. After removal of the solvent 0.4 g of white
viscous residue was obtained, which solidified upon
storage. IR spectrum (film), ν, cm–1: 2962, 2230, 1658,
1555, 1388, 1259, 1224, 1172, 1041, 846, 768, 640,
519. 13С NMR spectrum (DMSO-d6), δC: 156.89 ppm.
19F and 29Si NMR spectra see in the text.
4. Ulrich, H., Chemistry and Technology of Carbodi-
imides, Chichester: Wiley, 2007.
5. Benfodda, Z., Delon, L., Guillen, F., and Blancou, H.,
J. Fluorine Chem., 2007, vol. 128, p. 1353.
6. Behrend, E. and Haas, A., J. Fluorine Chem., 1974,
vol. 4, p. 83.
7. Gerstenberger, M.R.C., Haas, A., and Pauling, H., Helv.
Chim. Acta, 1982, vol. 65, p. 490.
8. Yagupolskii, L.M., Shelyazhenko, S.V., Maletina, I.I.,
Petrik, V.N., Rusanov, E.B., and Chernega, A.N., Eur.
J. Org. Chem., 2001, p. 1225.
b. To 0.37 g (2.0 mmol) of carbodiimide Id in 5 ml
of CH2Cl2 0.60 g (4.0 mmol, 0.36 ml) of TfOH was
added at stirring and cooling with cold water, the
reaction mixture was stirred for 2.5 h and left
overbight. The formed fine-dispersed precipitate was
filtered off, dried, washed with hot ethyl acetate, and
dried to obtain 0.36 g of white fine-dispersed powder,
mp 202–204°С. IR spectrum (KBr), ν, cm–1: 3429,
3385, 3348, 3242, 1735, 1696, 1592, 1278, 1244,
1228, 1171, 1031, 640, 518. 1Н NMR spectrum
(DMSO-d6), δ, ppm: 8.0 br.s (2Н, NH), 7.1 br.s (1Н,
9. Roesky, H.W. and Giere, H.H., Z. Naturforsch. B, 1970,
vol. 25, p. 773.
10. Yagupolskii, L.M., Shelyazhenko, S.V., Maletina, I.I.,
Sokolenko, L.V., Chernega, A.N., Rusanov, E.B., and
Tsymbal, I.F., J. Fluorine Chem., 2007, vol. 128, p. 515.
11. Petrik, V.N., Kondratenko, N.V., and Yagupolskii, L.M.,
J. Fluorine Chem., 2003, vol. 124, p. 151.
12. Liu, Y., Synlett., 2009, p. 1353.
13. Klausner, Y.S. and Bodansky, M., Synthesis, 1972,
p. 453.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 83 No. 10 2013

1858
TOLSTIKOVA et al.
14. Tolstikova, L.L., Chipanina, N.N., Oznobikhina, L.P.,
and Shainyan, B.A., Russ. J. Org. Chem., 2011, vol. 47,
no. 9, p. 1278.
18. Lichtenberger, O., Woltersdorf, J., and Riedel, R.,
Z. Anorg. Allg. Chem., 2002, vol. 628, p. 596.
19. Kurzer, F. and Douraghi-Zadeh, K., Chem. Rev., 1962,
15. Shainyan, B.A., Tolstikova, L.L., and Schilde, U.,
vol. 67, p. 107.
J. Fluorine Chem., 2012, vol. 135, p. 261.
20. Olah, G.A., Laali, K., and Farooq, O., Organometallics,
16. Romanov, D.V., Strelkova, T.V., Strelenko, Yu.A., and
Vasil’ev, N.V., Russ. Chem. Bull., 1999, vol. 48, no. 4,
p. 771.
17. Gordetsov, A.S., Kozyukov, V.P., Vostokov, I.A.,
Sheludyakova, S.V., Dergunov, Yu.I., and Mironov, V.F.,
Russ. Chem. Rev., 1982, vol. 51, no. 5, p. 485.
1984, vol. 3, p. 1337.
21. Mushkin, Yu.I. and Finkelstein, A.I., Zh. Obshch.
Khim., 1965, vol. 35, no. 10, p. 1937.
22. Ohshiro, J., Mori, Y., Minami, T., and Agawa, T.,
J. Org. Chem., 1970, vol. 36, no. 6, p. 2076.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 83 No. 10 2013