Fe and Ni-catalyzed electrochemical perfluoroalkylation of C—H bonds of coumarins

Article abstract of DOI:10.1007/s11172-017-1906-5

A new method for the preparation of perfluoroalkylcoumarins in a single step is developed. The compounds are prepared via electrocatalytic reduction of a 1 : 1 mixture of an aromatic compound (coumarin, 6-methylcoumarin, and 7-methylcoumarin) and a fluoroalkylating reagent under mild conditions (room temperature, normal pressure) using [bipyFe^II] or [bipyNi^II] complexes as catalysts. The developed method makes it possible to obtain perfluoroalkylcoumarins in high yields and 100% conversion of the fluoroalkylating reagent.

Full text of DOI:10.1007/s11172-017-1906-5

1
446
Russian Chemical Bulletin, International Edition, Vol. 66, No. 8, pp. 1446—1449, August, 2017
Fe and Niꢀcatalyzed electrochemical perfluoroalkylation
of С—Н bonds of coumarins*
M. N. Khrizanforov, S. O. Strekalova, V. V. Grinenko, V. V. Khrizanforova, T. V. Gryaznova, 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 new method for the preparation of perfluoroalkylcoumarins in a single step is develꢀ
oped. The compounds are prepared via electrocatalytic reduction of a 1 : 1 mixture of an
aromatic compound (coumarin, 6ꢀmethylcoumarin, and 7ꢀmethylcoumarin) and a fluoroꢀ
II
alkylating reagent under mild conditions (room temperature, normal pressure) using [bipyFe ]
II
or [bipyNi ] complexes as catalysts. The developed method makes it possible to obꢀ
tain perfluoroalkylcoumarins in high yields and 100% conversion of the fluoroalkylating
reagent.
Key words: coumarins, perfluoroalkylation, catalysis.
Nowadays, fluorine and fluoroalkylꢀsubstituted aroꢀ
can be used as anticancer agents, fluorescent markers,
and optical molecular sensors.1
2,13
matic compounds attract the attention of the researchers
in the pharmacy, agrochemistry, and other areas.1
Among these classes of compounds, fluoroalkylated couꢀ
—12
Despite considerable progress in the investigation of
the fluoroalkylation reactions, catalytic fluoroalkylation
1
2,13
14—18
marins occupy a particular place.
It is known that
reactions have been little studied as yet.
The reacꢀ
coumarin and its derivatives belong to the most imporꢀ
tant heterocyclic compounds and are found in a variety of
natural substances and pharmaceutically active moleꢀ
tions of direct fluoroalkylation of С—Н bonds are even
less known,1
9—22
although the latter approach is interestꢀ
ing because of its atomꢀeconomy level is high and it is
3
23
cules. Many of coumarin derivatives exhibit anticoaguꢀ
connected with lowꢀwaste technologies.
lant, antitumor, antiviral, antiꢀinflammatory, antioxiꢀ
Among the catalysts used for CHꢀfluoroalkylation,
4
,5
24,25
26
27
dant, antimicrobial, and inhibitory properties. Besides,
they are useful synthetic building blocks in organic and
medicinal chemistry. It is known that the introduction of
fluorine or fluorineꢀcontaining fragments into organic
molecules can lead to appreciable changes in their bioꢀ
copper,
iridium or ruthenium, palladium, ytterꢀ
bium28 are most used, and nickel and iron30 are used less
often. Taking into account the trend to introduce inexꢀ
pensive catalysts, it is important to develop catalytic systems
for the functionalization of C—H bonds based on more
29
logical properties.1 Thus, it was found that fluorinatꢀ
ed heterocycles show more pronounced antibacterial
activity in a comparison with their nonꢀfluorinated anaꢀ
logs. The fluorinated compounds often have specific bioꢀ
logical activity, good solubility, and lipophilicity. These
lead to an increase in the permeability of cell membranes
and better bioavailability of fluorinated compounds
compared to their nonꢀfluorinated analogs. In addiꢀ
tion, fluorinated compounds are stable in the course of
,2
6,7
affordable and cheap metals such as Fe, Ni, and Co.
Metal complexes both in high (Ni /Ni and
PdIII/Pd ),
23
III
II
II 21,31—33
II 30
I 22
0
and in low (Fe , Ni , Ni (see
Refs 18, 34)) oxidation states are used for the electroꢀ
chemical fluoroalkylation.
Note that there is practically no description of the
methods for obtaining fluoroalkyl coumarins by electroꢀ
catalytic reactions. Synthesis of 3ꢀtrifluoroalkylated
coumarin from bis(trifluoroacetyl) peroxide was reported
in 1991 by Matsui and coꢀworkers.3 Despite the relativeꢀ
ly short reaction time (4 h), the preparative yield was
33%. Sometime later the reaction of Langlois reagent
5
metabolism, since they are more resistant to the oxidaꢀ
tion.8
—10
The introduction of a perfluoroalkyl group into
potentially useful organic molecules can significantly
improve physical, chemical, and their biological properꢀ
(R SO Na) with unsubstituted coumarins in the presꢀ
F
2
1
0,11
III
36
ties.
Coumarins containing the perfluoroalkyl group
ence of Mn was described. The reaction resulted in
the formation of 3ꢀperfluoroalkylꢀ2Hꢀchromenꢀ2ꢀones in
moderate yields (60—65%). However, for the reaction to
proceed successfully a high temperature (80 °C) and a twoꢀ
fold excess of the fluoroalkylating reagent were required.
*
Based on the Materials of the XX Mendeleev Congress on
General and Applied Chemistry (September 26—30, 2016,
Ekaterinburg, Russia).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1446—1449, August, 2017.
066ꢀ5285/17/6608ꢀ1446 © 2017 Springer Science+Business Media, Inc.
1

Perfluoroalkylation of coumarins
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 8, August, 2017
1447
In the 21 century the baton to study the fluoroalkylation
reaction of coumarins was passed on the Domowski group.
They reported the preparation of 3ꢀ(trifluoromethyl)ꢀ
coumarin and 3ꢀ(trifluoromethyl)ꢀ6ꢀnitrocoumarin by
treating the corresponding commercially available couꢀ
marinꢀ3ꢀcarboxylic acid with sulfur tetrafluoride.37 The
reaction proceeded within 3—6 h at high temperatures
giving the product yield not exceeding 36%. Quite reꢀ
Scheme 1
3
8
cently, in 2014, Zu and coꢀworkers proposed modificaꢀ
tion of the reaction described in Ref. 36: i.e., introducꢀ
tion of the trifluoromethyl group by direct radical Csp —H
2
substitution in coumarins. The reaction proceeds under
mild conditions to give 3ꢀ(trifluoromethyl)coumarins in
moderate yields (up to 56% in the presence of excess
1
—3: R1 = R2 = H (1), R1 = Me, R2 = H (2); R1 = H, R2 = Me (3)
X = I, SO Cl, M = Fe, Ni
2
3
8
Mn(OAc) in acetic acid). In the same year Ding and
3
Compound R1 R2
RF
Compound R1 R2
RF
coꢀworkers presented the reaction of alkynes with Togni
and Umemoto reagents, catalyzed by copper salts (aceꢀ
tates, oxides, and halides) leading to 3ꢀ(trifluoromethyl)ꢀ
coumarins in moderate yields (up to 60%).3 The reacꢀ
4
5
6
H
H
Me
H
H
H
C6F13
CF3
C6F13
7
8
9
Me
H
H
H
CF3
Me C6F13
Me CF3
9
tion proceeds in the presence of excess K CO upon heating.
2
3
ity. In addition, we have previously found that these comꢀ
plexes in a mixture with fluoroorganic reagents are reꢀ
duced at lower negative potentials (in average by 300 mV
to the anodic side) compared with the fluoroalkylating
agents. Distinctive features of the reaction are the equimoꢀ
lar ratio of coumarin : fluoroalkylating agent and room
temperature of the reaction (Scheme 1, Table 1). In the
absence of a catalyst, the fluoroalkylation proceeds very
slowly and unselectively. As a result different isomeric
coumarin derivatives were isolated in the yield not exꢀ
ceeding 25%.
Taking into account the method of metalꢀinduced perꢀ
fluoroalkylation of benzene and caffeine that we proposed
earlier, we attempted to realize a single step electrocataꢀ
lytic fluoroalkylation of the С—Н bonds of coumarins
under mild conditions at room temperature.
Results and Discussion
II
The complexes of iron [bipyFe Cl ] and nickel
2
I
[
bipyNi (BF ) ] were selected as catalysts for the fluoroꢀ
4
2
alkylation of coumarins. The choice of these complexes
is due to the values of their redox potentials and availabilꢀ
It should be noted that under conditions of electroꢀ
catalysis, after passing 2 F of electricity, a 100% converꢀ
Тable 1. Electrocatalytic fluoroalkylation of coumarins 1—3a
F
–E/Vb
Yieldc
%)
Isomer ratiod
Coumarin
R X
Catalyst
Product
(
С(3)RF С(6)RF С(7)RF
II
1
1
1
1
2
2
2
2
3
3
3
3
C F I
[(bipy)Fe ]
0.9
1.0
0.3
0.5
0.9
1.0
0.3
0.5
0.9
1.0
0.3
0.5
4
4
5
5
6
6
7
7
8
8
9
9
73 (61)
65 (55)
67 (56)
45 (34)
78 (64)
66 (54)
63 (53)
57 (45)
67 (52)
62 (52)
65 (54)
59 (49)
25
35
22
30
25
30
30
11
20
30
35
11
1
1
1
2
2
1
1
1
1
1
0
—
—
—
—
6
13
II
C F I
[(bipy)Ni ]
6
13
II
CF SO Cl
[(bipy)Fe ]
3
2
II
CF SO Cl
[(bipy)Ni ]
1
3
2
II
C F I
[(bipy)Fe ]
—
—
—
—
1
1
1
0
6
13
II
C F I
[(bipy)Ni ]
6
13
II
CF SO Cl
[(bipy)Fe ]
3
2
II
CF SO Cl
[(bipy)Ni ]
3
2
II
C F I
[(bipy)Fe ]
6
13
II
C F I
[(bipy)Ni ]
6
13
II
CF SO Cl
[(bipy)Fe ]
3
2
II
CF SO Cl
[(bipy)Ni ]
3
2
a
Reaction conditions: coumarin : RF = 1 : 1, DMF, argon, 23 °C, galvanostatic mode, Q = 2 F per 1 mole R X,
F
Znꢀanode, Ptꢀcathode.
b
In respect to Ag/AgCl.
c
The yield was determined from the integrated intensity of the signals in the 19F NMR spectra of the reaction mixture.
The preparative yield of the products is indicated in parenthesis. The current output coincides with the preparative yield
with accuracy of ±0.3%.
d
The ratio of isomers was determined from the intensity of the signals in the 19F NMR spectra.

1
448
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 8, August, 2017
Khrizanforov et al.
sion of the fluoroalkylating reagent is achieved. It should
be emphasized that electrochemistry allows one to avoid
the use of any additional chemical reagents (bases, oxiꢀ
dants or reducing agents). This is especially important
from the point of view of possible upscaling of the develꢀ
oped method.
An analysis of the isomer ratio of fluoroalkylcoumarins
formed in the presence of different metals (see Table 1)
shows that the reactions in each case probably follows its
own specific mechanism. This is in good agreement with
slowly under stirring. Reaction mixture was stirred at constant
temperature (25 °С) for 3—24 h till formation of crystalline
precipitate. The precipitate was filtered under an argon atmoꢀ
sphere and washed with cold (0 °C) ethanol. The obtained comꢀ
3
1
22
plexes [bipyFeCl2] and [bipyNi(BF4)2] were dried for
—3 days in vacuum oven at 25—55 °С. Physicochemical charꢀ
2
acteristics of complexes correspond to the literature data.
Electrocatalytic fluoroalkylation of coumarins (general proꢀ
cedure). Fluoroalkylating reagent (1.2 mmol), heterocyclic
compound 1—3 (1.2 mmol), metal complex MX L (0.12 mmol),
2
and DMF (40 mL) were placed in the electrochemical cell.
The electrolysis was carried out in the electrochemical cell
with separated anodic and cathodic compartments at 23 °С
under the atmosphere of dry argon in a galvanostatic mode.
The potentials of the working electrode are indicated in Table 1.
The amount of electricity transmitted was 2 F per 1 mol of the
fluoroalkylating agent. After completion of the electrolysis, the
reaction mixture was washed with a saturated solution of amꢀ
monium chloride (3×50 mL) and extracted with chloroform
(3×70 mL). After separation, the organic layer was dried over
magnesium sulfate and the solvent was evaporated. The residue
was purified by column chromatography on silica gel (eluent
was ethyl acetate—hexane, 10 : 1). The yields of products 4—9
are shown in Table 1.
the previously published experimental data obtained by
our group.21,30
The mechanisms of the observed transformaꢀ
tions will be discussed in detail in our next publications.
Thus, the electrocatalytic synthesis of fluoroalkylcouꢀ
marins proceeds under mild conditions (room temperature,
normal pressure) with an equimolar ratio of the reagents
and allows the preparation of products in high yield (up to
7
8%) and 100% conversion of the fluoroalkylating reagent
I
II
by using [(bipy)Ni ] or [(bipy)Fe ] as the catalysts.
Experimental
3
ꢀ(Perfluorohexyl)ꢀ2Hꢀchromenꢀ2ꢀone (4). Light crystals,
–
l
Preparative electrosynthesis was carried out using a direct
m.p. 115—118 °C. IR, ν/cm : 3063, 1740, 1629, 1611, 1515,
1461, 1201. 1H NMR (600 MHz, CDСl ), δ: 8.15 (s, 1 H,
H(4)); 7.71—7.23 (m, 3 H). С (100.6 MHz, CDСl ), δ: 156.1,
current source B5ꢀ49 in a 40 mL threeꢀelectrode cell. The valꢀ
3
2
13
ue of the potential of the working Pt electrode (S = 48 cm ) was
3
fixed using a DC voltmeter B7ꢀ27 relative to the Ag/AgCl refꢀ
erence electrode (C = 0.01 M) in MeCN. The anode was a zinc
rod with a diameter of 1 cm. During electrolysis, the electrolyte
was constantly stirred by means of a magnetic stirrer at a conꢀ
stant flow of an inert gas that passed through a purification
system (to remove oxygen and other impurity gases).
154.6, 146.2, 143.1, 129.1, 126.4, 121.5, 117.6, 116.4, 114.4,
22.0. 19F NMR (376 MHz, CDCl ), δ: –81.03 (t, 3 F); –111.32
3
(t, 2 F); –120.8 (br.s, 2 F); –121.3 (br.s, 2 F); –122.47 (br.s,
+
2 F); –126.35 (br.s, 2 F). MS, m/z: 464 [M] . Found (%):
C, 38.71; H, 1.03. C H F O . Calculated (%): C, 38.81; H, 1.09.
15
5
13
2
Physicochemical characteristics of complexes correspond to
3
6
Coumarin (99%), 6ꢀmethylcoumarin (99%), 7ꢀmethylꢀ
the literature data.
3ꢀ(Trifluoromethyl)ꢀ2Hꢀchromenꢀ2ꢀone (5). White crystals,
coumarin (98%), CF SO Cl (all SigmaꢀAldrich), and C F13I
3
2
6
1
(
P&M Invest, Russia) were used as received.
Dimethylformamide (extra pure, Acros Organics) served as
m.p. 130—133 °C. H NMR (600 MHz, CDCl ), δ: 8.17 (s, 1 H,
3
C(4)H); 7.69 (t, 1 H, ArH, J = 7.9 Hz); 7.63 (d, 1 H, ArH,
J = 7.9 Hz); 7.42—7.37 (m, 2 H, ArH). 1 C NMR (100.6 MHz,
3
a solvent in the synthesis; DMF was distilled over calcium
hydride under argon atmosphere before experiment.
CDCl ), δ: 155.9, 154.6, 143.4, 134.4, 129.5, 125.3, 121.3,
3
1
9
Tetraethylammonium tetrafluoroborate (Et NBF ) was preꢀ
117.6, 116.7, 115.9. F NMR (376.5 MHz, CDCl ), δ: –66.3
(s, 3 F). MS, m/z: 214.56 [M] . Found (%): C, 56.12; H, 2.31.
4
4
3
+
pared by mixing a 30—35% aqueous solution of tetraethylꢀ
ammonium hydroxide, Et NOH, and HBF until neutral pH
C H F O . Calculated (%): C, 56.09; H, 2.35. Physicochemical
4
4
10
5
3
2
3
8
was reached. Precipitated white crystalline solid was filtrated
and dried. Obtained powder salt was recrystallized from ethaꢀ
nol and dried 2—3 days in vacuum oven at 55 °С.
characteristics of complexes correspond to the literature data.
6ꢀMethylꢀ3ꢀ(perfluorohexyl)ꢀ2Hꢀchromenꢀ2ꢀone (6). Lightꢀ
beige crystals, m.p. 144—146 °C. IR, ν/cm : 3063, 1608, 1505,
1461, 1199. H NMR (600 MHz, CDCl ), δ: 8.54 (br.s, 1 H,
H(4)); 7.61—7.21 (m, 3 H); 2.41 (s, 3 H). С NMR (100.6
–
l
1
NMR spectra were recorded using Bruker AVANCEꢀ400
3
1
13
multinuclear spectrometer (400.1 MHz ( Н) and 162.0 MHz
3
1
(
Р)). Chemical shifts are represented against internal stanꢀ
MHz, CDСl ), δ: 156.4, 152.7, 143.3, 135.5, 135.1, 129.2,
3
1
19
19
dard, deuterated solvent ( Н) and C F ( F).
121.6, 117.4, 116.8, 116.5, 20.7. F NMR (376 MHz, CDCl ),
6
6
3
Mass spectra were recorded using AmazonX spectrometer
Bruker Daltonik GmbH, Germany). Electrospray was used as
ionization method. Nitrogen at 22 °С was used as nebulizer gas
δ: –82.00 (t, 3 F, J = 9.8 Hz); –106.01 (t, 2 F, J = 14.5 Hz);
(
–120.53 (br.s, 2 F); –122.03 (br.s, 2 F); –122.72 (br.s, 2 F);
+
–126.05 (br.s, 2 F). MS, m/z: 478 [M] . Found (%): C, 40.16;
in the source. Source voltage was 4.5 кV. The solutions of the
H, 1.40. C16H F O . Calculated (%) C, 40.19; H, 1.48.
7 13 2
–
3
–1
samples were diluted with acetonitrile to ~10 mg mL . The
introduction of the samples was performed using autosampler
of the liquid chromatograph Agilent 1260 Infinity (Agilent
Technologies, USA).
6ꢀMethylꢀ3ꢀ(trifluoromethyl)ꢀ2Hꢀchromenꢀ2ꢀone (7). White
1
crystals, m.p. 140—143 °C. H NMR (600 MHz, CDCl ), δ:
3
8.09 (s, 1 H, C(4)H); 7.48 (d, 1 H, ArH, J = 8.5 Hz); 7.39 (s, 1 H,
ArH); 7.29 (d, 1 H, ArH, J = 8.5 Hz); 2.44 (s, 3 H, C(6)Me).
1
3
Synthesis of metal complex (general procedure). To the
С NMR (100.6 MHz, CDСl ), δ: 156.2, 152.8, 143.3, 135.5,
3
1
9
solution of metal salt MX (1.83 mol) in ethanol (100 mL), 2,2´ꢀbiꢀ
135.2, 129.1, 121.4, 117.5, 116.8, 116.5, 20.7. F NMR
2
–
2
+
pyridine (1.83•10 mol) in ethanol (30—50 mL) was added
(376 MHz, CDCl ), δ: –65.3 (s, 3 F). MS, m/z: 228.98 [M] .
3

Perfluoroalkylation of coumarins
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 8, August, 2017
1449
Found (%): C, 57.93; H, 3.02. C H F O . Calculated (%):
C, 57.90; H, 3.09. Physicochemical characteristics of comꢀ
plexes correspond to the literature data.38
17. M. Khrizanforov, T. Gryaznova, O. Sinyashin, Y. Budnikꢀ
ova, J. Organomet. Chem., 2012, 718, 101.
18. M. N. Khrizanforov, T. V. Gryaznova, D. Yu. Mikhailov,
Yu. H. Budnikova, O. G. Sinyashin, Russ. Chem. Bull., 2012,
61, 1560.
19. G. Landelle, A. Panossian, S. Pazenok, J. P. Vors, F. R.
Leroux, Beilstein J. Org. Chem., 2013, 9, 2476
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Khrizanforov, Yu. H. Budnikova, Russ. Chem. Bull., 2016,
65, 1798.
21. M. N. Khrizanforov, S. V. Fedorenko, S. O. Strekalova,
K. V. Kholin, A. R. Mustafina, M. Ye. Zhilkin, V. V. Khriꢀ
zanforova, Y. N. Osin, V. V. Salnikov, T. V. Gryaznova,
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Khrizanphorov, Sh. Latypov, O. Kataeva, D. A. Vicic, O. G.
Sinyashin, Yu. H. Budnikova, Dalton Trans., 2012, 41, 165.
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Ed., 2012, 51, 3944.
1
1
8
3
2
7
ꢀMethylꢀ3ꢀ(perfluorohexyl)ꢀ2Hꢀchromenꢀ2ꢀone (8). White
1
crystals with a pink tint, m.p. 123—125 °C. H NMR (600 MHz,
CDСl ), δ: 8.34 (s, 1 H, H(4)); 7.74—7.20 (m, 3 H); 2.43 (s, 3 H).
С NMR (100.6 MHz, CDСl ), δ: 156.3, 154.8, 146.3, 143.2,
3
1
3
3
19
1
29.1, 126.5, 121.5, 117.2, 116.4, 114.4, 22.2. F NMR (376 MHz,
CDCl ), δ: –81.33 (t, 3 F, J = 9.8 Hz); –105.32 (t, 2 F,
3
J = 14.5 Hz); –120.19 (br.s, 2 F); –121.93 (br.s, 2 F); –122.47
–
l
(
1
br.s, 2 F); –126.05 (br.s, 2 F). IR, ν/cm : 3072, 1604, 1510,
+
462, 1195. MS, m/z: 478 [M] . Found (%): C, 40.17; H, 1.41.
C16H7F13O . Calculated (%): C, 40.19; H, 1.48.
2
7
ꢀMethylꢀ3ꢀ(trifluoromethyl)ꢀ2Hꢀchromenꢀ2ꢀone (9). White
1
crystals, m.p. 120—122 °C. H NMR (600 MHz, CDСl ), δ:
3
8
.11 (s, 1 H, C(4)H); 7.49 (d, 1 H, ArH, J = 8.0 Hz); 7.18 (d, 2 H,
1
3
ArH, J = 7.6 Hz); 2.5 (s, 3 H, C(7)Me). С NMR (100.6 MHz,
CDСl ), δ: 156.2, 154.8, 146.4, 143.2, 129.1, 126.5, 121.5,
1
3
1
9
17.5, 116.4, 114.4, 22.0. F NMR (376 МHz, CDCl ), δ:
3
+
–
65.5 (s, 3 F). MS, m/z: 229.05 [M] . Found (%): C, 57.95;
H, 3.00. C H F O . Calculated (%): C, 57.90; H, 3.09. Physiꢀ
1
1
8
3
2
cochemical characteristics of complexes correspond to the litꢀ
erature data.
25. K. Fujikawa, Y. Fujioka, A. Kobayashi, H. Amii, Org. Lett.,
2011, 13, 5560.
3
8
2
6. N. Iqbal, J. Jung, S. Park, E. J. Cho, Angew. Chem., 2014,
126, 549.
This work was financially supported by the Russian
Science Foundation (Project No. 14ꢀ23ꢀ00016).
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son, S. L. Buchwald, Science, 2010, 328, 1679.
2
2
3
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4, 1321.
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Received March 31, 2017;
in revised form July 10, 2017