Electrochemical phosphorylation of coumarins catalyzed by transition metal complexes (Ni—Mn, Co—Mn)

Article abstract of DOI:10.1007/s11172-016-1451-7

A possibility of electrochemical phosphorylation of coumarins (coumarin, 6-methylcoumarin, 7-methylcoumarin) with diethyl phosphite was shown. The approach is based on the oxidation of a mixture of the aromatic compound and diethyl phosphite (1: 1) under mild conditions (room temperature, atmospheric pressure) in the presence of a bimetallic catalytic systems: Mn^IIbipy/Ni^IIbipy and Mn^IIbipy/Co^IIbipy (bipy is the 2,2′-bipyridine). This method allows one to obtain diethyl arylphosphonates in high yields (up to 70%) and 100% conversion of the phosphite.

Full text of DOI:10.1007/s11172-016-1451-7

Russian Chemical Bulletin, International Edition, Vol. 65, No. 5, pp. 1295—1298, May, 2016
1295
Electrochemical phosphorylation of coumarins catalyzed by transition metal
complexes (Ni—Mn, Co—Mn)*
S. O. Strekalova, M. N. Khrizanforov, T. V. Gryaznova, V. V. Khrizanforova, and Yu. H. Budnikova
A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center of the Russian Academy of Sciences,
8
ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Eꢀmail: khrizanforov@gmail.com
A possibility of electrochemical phosphorylation of coumarins (coumarin, 6ꢀmethylcouꢀ
marin, 7ꢀmethylcoumarin) with diethyl phosphite was shown. The approach is based on the
oxidation of a mixture of the aromatic compound and diethyl phosphite (1 : 1) under mild
conditions (room temperature, atmospheric pressure) in the presence of a bimetallic catalytic
II
II
II
II
systems: Mn bipy/Ni bipy and Mn bipy/Co bipy (bipy is the 2,2´ꢀbipyridine). This method
allows one to obtain diethyl arylphosphonates in high yields (up to 70%) and 100% conversion
of the phosphite.
Key words: phosphorylation, oxidation, electrosynthesis, catalyst, coumarin, CHꢀfunctionꢀ
alization.
Coumarins, the structural framework of which is enꢀ
Based on the data obtained in the studies of oxidative
metalꢀinduced phosphorylation of benzene, we have sugꢀ
gested that phosphorylation of coumarin can be effected
2
7
countered in a number of natural compounds and is also
a fragment of compounds possessing pharmacological
activity1 or belonging to fluorescent and laser dyes,
—5
6—9
at room temperature in one step. The pairs MnCl bipy/
2
attract noticeable attention of researchers in the last years.
Thus, it was shown that some 3ꢀphosphorylated coumarins
exhibit cytotoxicity against human leukemia cells in vitro
Ni(BF ) bipy and MnCl bipy/CoCl bipy (bipy is the 2,2´ꢀbiꢀ
4
2
2
2
pyridine) have been chosen as catalytic systems for elecꢀ
trochemical phosphorylation of coumarins 1—3. The
process is distinguished by the equimolar ratio coumꢀ
arin : dialkyl phosphite and room temperature (Scheme 1,
Table. 1).
4
,5
10
and in vivo, as well as a high alkylating activity.
Earlier, 3ꢀphosphonated coumarins were commonly
obtained by the Arbuzov1
1—14
or Knoevenagel
15—17
reacꢀ
tion. These methods, as a rule, give side products, are
frequently based on multiꢀstep sequential reactions, do
not correspond to the atomꢀeconomy principles and,
Scheme 1
therefore, have low efficiency. Until recently, only limited
publications1
8—20
described phosphonation of couꢀ
marins by the radical reaction using excessive amount of
1
8
Mn(OAc) (3 equiv.) and an aliphatic acid as a solvent
3
2
0
19
or noble metal (silver or palladium ) salts. The apꢀ
proaches described in the works1
8—20
have a number of
other disadvantages: high temperature, longꢀtime syntheꢀ
sis, low yield of the final product, and low conversion.
The formation of the C—P bond by direct phosphoryꢀ
lation of a C—H bond in aromatic substrates remains one
of the most important approaches, since it meets generally
accepted principles of "green chemistry", namely, the
atomꢀeconomy, small amount of waste and small number
of steps in the process. In this connection, catalytic reacꢀ
M = Ni, Co
R = H (1, 4), 6ꢀMe (2, 5), 7ꢀMe (3, 6)
bipy = 2,2´ꢀbipyridine
tions, including electrochemical,2
1—26
The phosphorylation does not occur without the cataꢀ
lyst. It is important to note that a 100% conversion of
diethyl phosphite is reached in these syntheses (completely
consumed after passing 2 F of electricity). In contrast to
the reaction conditions of phosphorylation of coumarins
of direct phosphorꢀ
ylation of aromatic compounds become especially attractive.
*
Dedicated to Academician of the Russian Academy of Sciences
O. G. Sinyashin on the occasion of his 60th birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1295—1298, May, 2016.
066ꢀ5285/16/6505ꢀ1295 © 2016 Springer Science+Business Media, Inc.
1

1
296
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 5, May, 2016
Strekalova et al.
Table 1. Electrochemical phosphorylation of coumarins 1—3
with diethyl phosphite in the presence of different catalysts*
heating, i.e., under the conditions causing decomposition
of diethyl phosphite (see, for example, Refs 28 and 29).
31
The P NMR spectrum of compound 3 is reported in the
Entry Substrate
Catalytic system
Product Product
yield (%)
12
work. Therefore, in the Experimental section we report
all physicochemical data for all the isolated products.
2
7
In the preceding study, we found specific features of
electrocatalytic phosphorylation of aromatic compounds
taking benzene as an example. This study included detail
analysis of electrochemical properties of catalyst comꢀ
plexes in the absence and in the presence of diethyl phosꢀ
phite and arene by cyclic voltamperometry. It should be
noted that the earlier published studies do not have data
confirming the mechanism of phosphorylation in the presꢀ
ence of complexes or metal salts (Pd, Ag, Mn). A radical
mechanism is commonly assumed, however, the additives
of TEMPO (2,2,6,6ꢀtetramethylpiperidine Nꢀoxyl), as
a rule, have no influence on the product yield, whereas
organic oxidants capable of generating radicals in the
system, for example, tertꢀbutylhydroperoxide, inhibit the
1
2
3
4
5
6
1
1
2
2
3
3
[MnCl bipy/Ni(BF ) bipy]
4
4
5
5
6
6
30
55
31
61
37
70
2
4 2
[MnCl bipy/CoCl bipy]
2
2
[MnCl bipy/Ni(BF ) bipy]
2
4 2
[MnCl bipy/CoCl bipy]
2
2
[MnCl bipy/Ni(BF ) bipy]
2
4 2
[MnCl bipy/CoCl bipy]
2
2
*
=
Reaction conditions: the ratio of coumarin : diethyl phosphite =
1 : 1, catalyst (0.01 equiv.), 20 °C, Q = 2 F per 1 mole of
diethyl phosphite, galvanostatic regime, MeCN; an anode potenꢀ
tial of 0.7 V relative to Ag/AgCl.
1
9
according to the method described in the work (10%
II
Pd bipy, oxidant K S O (3 equiv.)), under condiꢀ
2
2
8
tions found by us the phosphonate is formed in the presꢀ
19
reaction. This indicated that the reaction can follow not
II
II
ence of a bimetallic catalytic system Mn bipy/Ni bipy or
only the radical mechanism.
II
II
Mn bipy/Co bipy without use of any additional reagents,
with the yields of 3ꢀphosphonated products 4—6 being
higher: in the work19 under the optimum conditions the
highest yield was 48%. As a rule, coumarins with electronꢀ
donating groups 2 and 3 give better yields of the correꢀ
sponding phosphorylation products than unsubstituted
coumarin 1. It is interesting that 7ꢀmethylcoumarin 3 (see
Table 1, entry 6) turned out to be the most suitable subꢀ
strate for phosphorylation at position 3. Apparently, this is
due to the +Iꢀeffect of the methyl group in combination
with the +Mꢀeffect of the heteroatom. In the case when
a methyl group is at position 6 (coumarin 2), the yields of
the CHꢀsubstitution product are lower. The reaction of
unsubstituted coumarin 1 with diethyl phosphite proceeds
also less selective as compared to 7ꢀmethylcoumarin 3.
The use of monometallic systems sharply decreases effiꢀ
ciency of the target reaction and leads to an unseparable
suspension.
27
Earlier, it was shown that the first step of electroꢀ
chemically induced phosphorylation is the formation of
metal phosphonates, which undergo oxidation more easiꢀ
ly than the starting catalyst complexes. Diethyl phosphite
does not undergo oxidation under the studied conditions.
The preparative electrolysis proceeds exactly at potentials
corresponding to the oxidation of phosphonate complexes.
Since nickel and cobalt phosphonate complexes oxidize
earlier (about 0.5 V) than the manganese complexes
27
(
1.0 V) the phosphorylation process is triggered by this
reaction involving cations of Ni or Co. The cations of
manganese coordinates at the carbonyl group of coumarin
and participates in the generation of phosphonyl radicals,
directing the substitution reaction at position 3 of couꢀ
marin under the oxidation conditions. The details of the
mechanism of this complex reaction require further studies.
In conclusion, the electrochemical synthesis of diethyl
phenylphosphonate proceeds under mild conditions (room
temperature, atmospheric pressure, equimolar ratio of reꢀ
agents) and allows one to obtain product in high yield (up
to 70%) and 100% conversion in the presence of the bimeꢀ
It should be noted that all the 3ꢀphosphonylated couꢀ
marins obtained in the present work are described in the
literature, however, strange disagreements are observed in
their physicochemical characteristics and some spectral
data. Thus, for example, in the work2 published in 2015,
diethyl(2ꢀoxoꢀ2Hꢀchromenꢀ3ꢀyl)phosphonate (4) was
described as a viscous liquid, though it is a white powder
with m.p. 65—66 °C (our data, agreeing with the data in
the work18), whereas in the C NMR spectrum of this
compound, the signal at δ 117.9 (d) was by mistake reꢀ
II
II
II
tallic catalytic system Mn bipy/Ni bipy or Mn bipy/
0
II
Co bipy without use of any additional oxidants.
Experimental
13
The NMR spectra were recorded on a Bruker AVANCEꢀ400
1
13
multinuclear spectrometer (400.1 ( H), 100.6 ( C), and 162.0
ported to have the spinꢀspin coupling constant of JP—C
=
31
(
P) MHz) in CDCl . Chemical shifts were recorded relative to
the signals of the deuterated solvent (in the case of H and
C NMR) or phosphoric acid (in the case of P NMR).
3
=
23.2 Hz, whereas according to our data, which agree
1
1
8
with the data in the work, it is J = 196—197 Hz. Note
that in the work,18 the data on the 31P NMR spectra are
absent, which is very important for the monitoring the
reaction progress, which is carried out in acetic acid upon
13
31
Mass spectrometric studies (electrospray ionization) of the
samples was carried out on a Bruker Daltonik GmbH AmazonX
(Germany). Nitrogen was used as a nebulizer gas in the source

Phosphorylation of coumarins
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 5, May, 2016
1297
with a temperature of 220 °C. The source voltage was 4.5 kV.
Solutions of the samples were diluted with acetonitrile to a conꢀ
centration of ∼ 10–3 mg mL . The samples were injected using
an autosampler of an Agilent 1260 Infinity liquid chromatograph
gel (eluent ethyl acetate—hexane). Physicochemical characterꢀ
1
8
istics of the products corresponded to the data given in the work,
–1
31
except for the P NMR spectra and IR spectra, which were not
reported. The product yields are given in Table 1.
(
Agilent Technologies, USA).
Diethyl (2ꢀoxoꢀ2Hꢀchromenꢀ3ꢀyl)phosphonate (3). A white
powder. M.p. 65—66 °C. P NMR, δ: 10.37. H NMR, δ: 8.53
(d, 1 H, C(4)H, 3JPH = 17.2 Hz); 7.66—7.60 (m, 2 H, ArH);
31
1
Coumarin 1 (99%), 6ꢀmethylcoumarin (2) (99%), 7ꢀmethylꢀ
coumarin (3) (98%) (Sigma Aldrich) were used without addiꢀ
tional purification.
7.37—7.33 (m, 2 H, ArH); 4.34—4.22 (m, 4 H, 2 CH ); 1.37 (t, 6 H,
2
13
Acetonitrile served as a basic solvent in the syntheses (extra
pure grade, Acros organics), which was distilled thrice before
experiments: the first distillation was carried out using potassium
permanganate, the second distillation was carried out over phosꢀ
phorus pentoxide, the third distillation was carried out over calꢀ
cium hydride under argon.
2 CH , J = 7.1 Hz). C NMR, δ: 158.3 (d, JP—C = 22.7 Hz);
3
155.3, 153.5 (d, JP—C = 6.4 Hz); 134.4, 129.5, 125.1, 118.0
(d, JP—C = 14.1 Hz); 117.9 (d, 1JP—C = 196.9 Hz); 116.9, 63.5
(d, JP—C = 9.2 Hz); 16.5 (d, J
282.84 [M] . IR (KBr), ν/cm : 2983, 2920, 2875 (—CH3,
= 10.1 Hz). MS (ESI), m/z:
P—C
–1
+
—CH —), 1743 (C=O), 1610, 1562 (Ar—), 1430 (P—C), 1248
2
Diethyl phosphite was obtained according to the described
procedure.30 The purified solvents were stored under an inert
atmosphere in the Schlenk systems.
(P=O), 1055 (P—O). Calculated (%): C, 55.32; H, 5.36; P, 10.97.
C H O P. Found (%): C, 55.31; H, 5.33; P, 10.92.
13
15
5
Diethyl (6ꢀmethylꢀ2ꢀoxoꢀ2Hꢀchromenꢀ3ꢀyl)phosphonate (4).
A yellow powder. M.p. 74—76 °C. 3 P NMR, δ: 11.4. H NMR,
δ: 8.40 (d, 1 H, C(4)H, J = 17.1 Hz); 7.38 (d, 1 H, ArH, J = 8.5 Hz);
7.30 (s, 1 H, ArH); 7.17 (d, 1 H, ArH, J = 8.5 Hz); 4.23—4.17
(m, 4 H, 2 CH ); 2.35 (s, 3 H, C(6)Me); 1.31 (t, 6 H, 2 CH ,
1
1
Salt Et NBF was obtained by mixing 30—35% aqueous soluꢀ
4
4
tion of tetraethylammonium hydroxide Et NOH and HBF until
4
4
the medium became neutral. A white crystalline precipitate was
formed in the course of the reaction, which was collected by
filtration and dried. The resulted powdered salt was recrystalꢀ
lized from ethanol and dried for 2—3 days in a vacuum drying
oven at 55 °C.
2
3
13
J = 7.4 Hz). C NMR, δ: 159.0, 158.9, 153.7, 153.6, 135.6,
135.0, 129.2, 117.9, 117.8, 116.8, 63.6, 20.9, 16.6. MS (ESI),
+
–1
m/z: 296.44 [M] . IR (KBr), ν/cm : 3057, 2963, 2857 (—CH3,
Preparative electrosyntheses were carried out using a B5ꢀ49
source of direct current in a 40ꢀmL threeꢀelectrode cell. The
working electrode potential was detected using a V7ꢀ27 DC
voltmeter relative to the reference electrode Ag/AgCl (C =
—CH —), 1719 (C=O), 1211 (P=O), 1039 (P—O). Calculatꢀ
2
ed (%): C, 56.76; H, 5.78; P, 10.46. C H O P. Found (%):
14
17
5
C, 56.64; H, 5.73; P, 10.41.
Diethyl (7ꢀmethylꢀ2ꢀoxoꢀ2Hꢀchromenꢀ3ꢀyl)phosphonate (5).
=
0.01 mol L–1) in acetonitrile. The surface of the working
A yellow powder. M.p. 62—64 °C. P NMR, δ: 10.9. H NMR,
δ: 8.40 (s, 1 H, C(4)H); 7.39 (d, 1 H, ArH, J = 7.5 Hz); 7.06 (d, 2 H,
31
1
2
platinum (Pt) Uꢀshaped electrode was 48.00 cm . A ceramic
plate with the pore diameter of 900 nm was used as a diaꢀ
phragm. During preparative synthesis, the electrolyte was conꢀ
tinuously stirred with a magnetic stirrer. The electrolysis was
carried out under constant flow of an inert gas, which passed
the system for purification from oxygen and other gaseous imꢀ
purities.
ArH, J = 7.4 Hz); 4.19 (m, 4 H, 2 CH ); 2.40 (s, 3 H, C(7)Me);
2
1
3
1.30 (s, 6 H, 2 CH ). C NMR, δ: 158.9, 155.6, 153.7, 146.4,
3
1
129.3, 126.4, 117.1, 115.8, 106.6 (d,
J
= 197.4 Hz), 63.5,
P—C
+
–1
22.3, 16.6. MS (ESI), m/z: 296.47 [M] . IR (KBr), ν/cm
:
3061, 2966, 2861 (—CH , —CH —), 1721 (C=O), 1210 (P=O),
3
2
1039 (P—O). Calculated (%): C, 56.76; H, 5.78; P, 10.46.
C H O P. Found (%): C, 56.67; H, 5.71; P, 10.42.
Synthesis of metal complexes (general procedure). A solution
of the corresponding ligand (1.83•10–2 mol) in EtOH (30—50 mL)
14
17
5
was slowly added to a solution of metal salt MX (M = Co, Ni)
This work was financially supported by the Russian
Science Foundation (Project No. 14ꢀ23ꢀ00016).
2
–
2
(
1.83•10 mol) in EtOH (100 mL) with stirring. The reaction
mixture was stirred for 3—24 h at a constant temperature (25 °C)
until a crystalline precipitate was formed, which was collected
by filtration under argon and washed with iceꢀcold ethanol. The
resulting complex was dried in a vacuum drying oven for 2—3 days
at a temperature of 25—55 °C. Physicochemical characteristics
of the obtained complexes corresponded to the literature data:
References
1. L. L. Kong, J. F. Hu, N. H. Chen, Chin. Pharmacol. Bull.,
2012, 28, 165.
2. D. Egan, R. O´Kennedy, E. Moran, D. Cox, E. Prosser,
R. D. Thornes, Drug Metab. Rev., 1990, 22, 503.
3. L. Santana, E. Uriarte, F. Roleira, N. Milhazes, F. Borges,
Curr. Med. Chem., 2004, 11, 3239.
4. I. Kostova, Curr. Med. Chem.: AntiꢀCancer Agents, 2005, 5, 29.
5. K. N. Venugopala, V. Rashmi, B. Odhav, Biomed. Res. Int.,
2013, Art. ID 963248; DOI: 10.1155/2013/963248.
6. R. M. Christie, Rev. Prog. Color. Relat. Top., 1993, 23, 1.
7. R. M. Christie, H. Lui, Dyes Pigments, 1999, 42, 85.
8. N. Chandrasekharan, L. Kelly, Spectrum, 2002, 15, 1.
9. M. Fakhafakh, H. Turki, S. A. Rachid El Gharbi, S. Ferryꢀ
Forgues, J. Photochem. Photobiol., 2006, 185, 13.
10. E. Budzisz, E. Brzezinska, U. Krajewska, M. Rozalski, Eur.
J. Med. Chem., 2003, 38, 597.
3
1
32
CoCl bipy and Ni(BF ) bipy.
2
4 2
Electrocatalytic phosphorylation of coumarins (general proꢀ
–
3
cedure). Diethyl phosphite (1.2•10 mol), coumarin 1—3
–
3
(
(
1.2•10 mol), and metal complex MX L (M = Co, Ni)
2
–
5
1.2•10 mol) in MeCN (40 mL) were placed in an electroꢀ
chemical cell. Electrolysis was carried out in an electrochemical
cell with separated anode and cathode compartments at a temꢀ
perature of 23 °C under dry argon at a working electrode potenꢀ
tial of 0.7 V in the galvanostatic regime. The amount of passed
electricity was 2 F per 1 mole of phosphite. After the electrolysis
was completed, the reaction mixture was washed with a saturatꢀ
ed solution of ammonium chloride (3×50 mL) and extracted
with chloroform (3×70 mL). After separation, the organic layer
was dried with magnesium sulfate, the solvent was evaporated.
The residue was purified by column chromatography on silica
11. C. N. Robison, J. F. Addison, J. Org. Chem., 1966, 31, 4325.

1
298
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 5, May, 2016
Strekalova et al.
1
2. R. K. Singh, M. D. Rogers, J. Heterocycl. Chem., 1985,
2, 1713
3. P. Bouyssou, J. Chenault, Tetrahedron Lett., 1991, 32, 5341.
4. N. A. Rodios, A. Bojilova, A. Terzis, C. P. Raptopoulou,
J. Heterocycl. Chem., 1994, 31, 1129.
5. A. Bojilova, R. Nikolova, C. Ivanov, N. A. Rodios, A. Terzis,
C. P. Raptopoulou, Tetrahedron, 1996, 52, 12597.
6. K. Kostka, S. Pastuszko, A. Kotynski, E. Budzisz, Phosphoꢀ
rus, Sulfur Silicon Relat. Elem., 1998, 134, 199.
26. Yu. B. Dudkina, T. V. Gryaznova, O. N. Kataeva, Yu. H.
Budnikova, O. G. Sinyashin, Russ. Chem. Bull. (Int. Ed.),
2014, 63, 2641 [Izv. Akad. Nauk, Ser. Khim., 2014, 2641].
27. M. N. Khrizanforov, S. O. Strekalova, T. V. Gryaznova,
V. V. Khrizanforova, Yu. H. Budnikova, Russ. Chem. Bull.
(Int. Ed.), 2015, 64, 1926 [Izv. Akad. Nauk, Ser. Khim.,
2015, 1926].
28. D. E. C. Corbridge, Phosphorus: Chemistry, Biochemistry and
Technology, 6th ed., CRC Press, Taylor & Francis Group,
Boca Raton—London, New York, 2013, 1473 pp.
29. L. D. Quin, A Guide to Organophosphorus Chemistry, Wileyꢀ
Intersci., New York—Chichester—Weinheim—Brisbane—
Singapore—Toronto, 2000, 394 pp.
30. V. V. Kormachev, M. C. Fedoceev, Preparativnaya Khimiya
Fosfora [Preparative Phosphorus Chemistry], UrO RAN,
Perm´, 1992, p. 69 (in Russian).
31. G. Muller, J. Sales, J. Vinaixa, J. Organomet. Chem., 1984,
273, 255.
32. Y. G. Budnikova, D. I. Tazeev, A. G. Kafiyatullina, D. G.
Yakhvarov, V. I. Morozov, N. K. Gusarova, B. A. Trofimov,
O. G. Sinyashin, Russ. Chem. Bull. (Int. Ed.), 2005, 54, 942
[Izv. Akad. Nauk, Ser. Khim., 2005, 919].
2
1
1
1
1
1
1
1
2
2
2
2
7. Y. Takeuchi, N. Ueda, K. Uesugi, H. Abe, H. Nishioka,
T. Harayama, Heterocycles, 2003, 59, 217.
8. P. Zhou, Y. J. Jiang, J. P. Zou, W. Zhang, Synthesis, 2012,
4
4, 1043.
9. X. Mi, M. Huang, J. Zhang, C. Wang, Y. Wu, Org. Lett.,
013, 15, 6266.
2
0. J.ꢀW. Yuan, Y.ꢀZ. Li, L.ꢀR. Yang, W.ꢀP. Mai, P. Mao,
Y.ꢀM. Xiao, L.ꢀB. Qu, Tetrahedron, 2015, 71, 8178.
1. Yu. H. Budnikova, O. G. Sinyashin, Russ. Chem. Rev., 2015,
8
4, 917.
2. C. Li, T. Yano, N. Ishida, M. Murakami, Angew. Chem.,
Int. Ed., 2013, 52, 9801.
3. Yu. B. Dudkina, T. V. Gryaznova, O. G. Sinyashin, Yu. H.
Budnikova, Russ. Chem. Bull. (Int. Ed.), 2015, 64, 1713 [Izv.
Akad. Nauk, Ser. Khim., 2015, 1713].
2
2
4. T. Gryaznova, Y. Dudkina, M. Khrizanforov, O. Sinyashin,
O. Kataeva, Y. Budnikova, J. Solid State Electrochem., 2015,
1
9, 2665.
5. T. V. Gryaznova, Y. B. Dudkina, D. R. Islamov, O. N. Kaꢀ
taeva, O. G. Sinyashin, D. A. Vicic, Yu. H. Budnikova,
J. Organomet. Chem., 2015, 785, 68.
Received November 11, 2015;
in revised form December 23, 2015