Synthesis and anticorrosive properties of alkylammonium polyfluoro-3-(ethoxycarbonyl)-2-oxo-2h-chromen-4-olates

Article abstract of DOI:10.1134/S1070428014010138

Reactions of ethyl polyfluoro-4-hydroxy-2-oxo-2H-chromene-3-carboxylates with amines under mild conditions afforded alkylammonium polyfluoro-3- (ethoxycarbonyl)-2-oxo-2H-chromen-4-olates which efficiently inhibited hydrochloric acid corrosion of mild stee

Full text of DOI:10.1134/S1070428014010138

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2014, Vol. 50, No. 1, pp. 66–71. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © K.V. Shcherbakov, T.I. Gorbunova, Ya.V. Burgart, V.I. Saloutin, 2014, published in Zhurnal Organicheskoi Khimii, 2014, Vol. 50,
No. 1, pp. 72–77.
Synthesis and Anticorrosive Properties of Alkylammonium
Polyfluoro-3-(ethoxycarbonyl)-2-oxo-2H-chromen-4-olates
K. V. Shcherbakov, T. I. Gorbunova, Ya. V. Burgart, and V. I. Saloutin
Postovskii Institute of Organic Synthesis, Ural Branch, Russian Academy of Sciences,
ul. S. Kovalevskoi/Akademicheskaya 22/20, Yekaterinburg, 620990 Russia
e-mail: shcherbakov@ios.uran.ru; saloutin@ios.uran.ru
Received May 28, 2013
Abstract—Reactions of ethyl polyfluoro-4-hydroxy-2-oxo-2H-chromene-3-carboxylates with amines under
mild conditions afforded alkylammonium polyfluoro-3-(ethoxycarbonyl)-2-oxo-2H-chromen-4-olates which
efficiently inhibited hydrochloric acid corrosion of mild steel at low concentrations (10–4–10–5 M).
DOI: 10.1134/S1070428014010138
Chemical compounds used as metal corrosion in-
hibitors should contain appropriate structural frag-
ments such as heteroatoms (P, S, N, O) in both open
chains and cyclic units and π-bonds favoring stronger
fixing of organic molecules on the metal surface [1].
While developing new metal corrosion inhibitors, in-
creasing attention is now given to ecological aspects
implying reduction of their negative toxic impact on
the environment. In this connection, the recent review
by Gese [2] demonstrated prospects in searching for
metal corrosion inhibitors among medicines of natural
and synthetic origin, which possess necessary struc-
tural elements. As a rule, most medicines undergo bio-
degradation in the environment [3].
reacts with primary amines under mild conditions to
give water-soluble salts IIa and IIb (Scheme 1). The
goal of the present work was to synthesize ammonium
salts of fluorine-containing 4-hydroxycoumarins and
test them as inhibitors of hydrochloric acid corrosion
of mild steel.
New alkylammonium polyfluoro-3-(ethoxycar-
bonyl)-2-oxo-2H-chromen-4-olates IIIa–IIIe were ob-
tained by reacting ethyl polyfluoro-4-hydroxy-2-oxo-
2H-chromene-3-carboxylates Ib and Ic with various
amines (Scheme 1). Salts were formed preferably by
strongly basic amines. As amine component we tried
primary monoamines (methylamine, pKa 10.62; ben-
zylamine, pKa 9.34), secondary amine (1-methylpiper-
azine, pKa 9.14), and diamine (o-phenylenediamine,
pKa 4.47) [7, 8]. Except for o-phenylenediamine,
weakly basic amines (e.g., aniline, pKa 4.58 [7]) did
not form stable salts with coumarins Ib and Ic. The use
of different amines should ensure diversity of counter-
ions in the resulting salts. The reactions were carried
out in ethanol with equimolar amounts of the reactants;
only in the synthesis of methylammonium salt Ia
gaseous methylamine was taken in excess.
Data on anticorrosive properties of a broad spec-
trum of drugs belonging to various chemical classes
have been collected in [2]; in particular, antibiotics
(β-lactams, quinolinones, fluoroquinolinones, tetra-
cyclins, macrolides, aminoglycosides, etc.), sul-
famides, fungicides, antihelminthics, and many others
have been covered. However, there are no published
data on anticorrosive effect of coumarin derivatives,
though coumarin fragment constitutes many natural
and synthetic drugs. For example, 4-hydroxycoumarin
derivatives exhibit anticoagulant activity [4], but their
use as corrosion inhibitors in aqueous media is
practically impossible, for coumarin derivatives are
insoluble in aqueous media and are unstable in alkaline
solution [5].
Although the salt structure of compounds IIa and
IIb synthesized previously from coumarin Ia [9]
(Scheme 1) was proved by a set of spectral methods,
four alternative structures may be proposed theoreti-
cally for salts IIIa–IIIf (Scheme 2). Chroman structure
A results from addition of amine to the C3=C4 bond of
coumarin Ib or Ic; chroman B is the product of amine
addition to the lactone carbonyl group (C2=O); pro-
We previously found [6] that, unlike fluorine-free
analog, 4-hydroxy-5,6,7,8-tetrafluorocoumarin (Ia)
66

SYNTHESIS AND ANTICORROSIVE PROPERTIES OF ALKYLAMMONIUM ...
67
Scheme 1.
O– H2NR2R3
F
R1 = H
F
O
O
X
F
OH
O
R1
O
F
F
F
HNR2R3, EtOH, 25°C
IIa, IIb
X
O– H2NR2R3
COOEt
F
Ia–Ic
R1 = COOEt
F
O
O
F
IIIa–IIIe
I, X = F, R1 = H (a), COOEt (b); X = H, R1 = COOEt (c); II, R2 = H, R3 = Me (a), PhCH2 (b); III, X = H, R2 = H, R3 = Me (a),
PhCH2 (b), 2-H2NC6H4 (c); R2R3N = 4-methylpiperazin-1-yl (d); X = F, R2 = H, R3 = PhCH2 (e).
penoate C could be formed via opening of the α-pyr-
anone ring; and structure D corresponds to the target
ammonium salt. Several tautomeric forms are possible
for structures A–C due to keto–enol transformation.
The probability for the formation of structure C from
coumarins Ib and Ic increases due to the presence of
electron-withdrawing ester group on C3, which raises
partial positive charge on the neighboring lactone car-
bonyl carbon atom (C2). Therefore, reliable determina-
tion of the structure of IIIa–IIIe is necessary.
were compared with the corresponding data for previ-
ously described salts IIa and IIb. The IR spectra of
IIIa–IIIe contained absorption bands typical of
stretching vibrations of lactone (1694–1726 cm–1) and
ester carbonyl groups (1620–1677 cm–1), while the
most characteristic were bands due to stretching vi-
brations of H3N+ (H2N+) group in the region 2609–
1
2731 cm–1. In the H NMR spectra of IIIa–IIIe,
protons in the H3N+ (H2N+) group appeared as a broad-
ened three- or two-proton singlet at δ 7.66–8.45 ppm.
Salt IIIc displayed in the 13C NMR spectrum
(DMSO-d6) only one set of signals, including three
Compounds IIIa–IIIe were assigned salt structure
1
D on the basis of their IR and H NMR spectra which
Scheme 2.
X
X
HO NHR
HO NHR
F
F
COOEt
OH
F
F
COOEt
O
O
O
O
F
X
F
X
A
OH
F
F
COOEt
NHR
F
F
COOEt
NHR
O
O
OH
OH
F
F
B
+
F
F
X
O– H3NR
5
4
F
F
X
F
X
F
F
COOEt
6
7
3
COOEt
NHR
COOEt
2
NHR
F
O
O
8
O
O
O
O
O
O
F
H
H
H
C
D
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 50 No. 1 2014

SHCHERBAKOV et al.
68
downfield signals at δC 159.89, 167.04, and
170.19 ppm (C2, C4, and 3-C=O, respectively, in struc-
ture D). Structures A and B each should give rise to
two downfield signals, whereas four downfield signals
should be observed from propenoate C.
corrosion of mild steel (St3) in hydrochloric acid
medium by a factor of 2–3 at a low concentration. The
inhibitory effect decreases in the series IIIc > IIa ≈
IIId > IIb > IIIa (Fig. 1).
The corrosion protection factor Z was calculated by
the formula
Thus, the formation of salts II and III in the reac-
tions of fluorinated 4-hydroxycoumarins Ia–Ic with
strongly basic amines under mild conditions is the pre-
dominant process regardless of their structure. Obvi-
ously, the presence of three or four electron-withdraw-
ing fluorine atoms in the aromatic moiety of coumarins
Ia–Ic is the main factor determining high acidity of the
4-hydroxy group. This makes the behavior of Ia–Ic
different from the transformations of their fluorine-free
analogs.
Z = (Kn1 – Kn2)/Kn1×100%,
where Kn1 and Kn2 are, respectively, the rates of uni-
form corrosion in the absence and in the presence of
inhibitor; the values of Z for the compounds tested
are as follows: IIIa, 21%; IIb, 37%; IIa, 49%; IIId,
53%; IIIc, 66%.
Analysis of the counterion structure in salts IIa,
IIb, IIIa, IIIc, and IIId shows that 2-aminophenylam-
monium cation in IIIc is the most promising from the
viewpoint of physical adsorption on the electrode
surface. The presence therein of π-electrons favors
stronger coordination to d-metal [1], while the free
amino group possessing a lone electron pair favors
electron density redistribution upon coordination at the
metal surface. Furthermore, possible protonation of the
free amino group in aqueous HCl medium gives rise to
an additional center which may be involved in the
formation of an electrical double layer at the electrode
surface. In any case, these competing processes lead to
a larger reduction of Kn as compared to other cations in
salts IIa, IIb, IIIa, and IIId.
Next, we examined anticorrosive properties of both
previously synthesized salts IIa and IIb and salts IIIa–
IIId obtained in the present work. Preliminarily, it was
found that salts IIa, IIb, IIIa, IIIc, and IIId are poorly
soluble in water; therefore, their use as corrosion in-
hibitors is limited to low concentration range. Benzyl-
ammonium salts IIIb and IIIe turned out to be almost
insoluble in water, and their anticorrosive effect was
not studied.
The inhibitory effect of salts IIa, IIb, IIIa, IIIc,
and IIId is based on electrostatic interaction between
the positively charged ammonium nitrogen atom and
negatively charged electrode [10]. Analysis of the Kn
values revealed fairly similar anticorrosive properties
of the examined compounds. They reduced the rate of
Among salts IIIa, IIIc, and IIId having the same
anion but different cations, the lowest anticorrosive
3500
3000
2500
2000
1500
1000
500
No salt added
IIIa
IIb
IIa
IIId
IIIc
0
0
50
100
150
200
250
300
350
400
450
500
Time, min
Fig. 1. Rate of uniform corrosion of mild steel (St3) in 1 M aqueous HCl; concentration of salts II and III 1×10–5 M.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 50 No. 1 2014

SYNTHESIS AND ANTICORROSIVE PROPERTIES OF ALKYLAMMONIUM ...
69
effect was observed for salt IIIa with methylammoni-
um cation. In order to rationalize higher efficiency of
1-methylpiperazinium salt IIId we made use of the
data of [11], where adsorption of some piperazine
derivatives was studied by semiempirical quantum
chemical calculations. It was shown that under acid
corrosion conditions most piperazine derivatives are
initially protonated at the nitrogen atom (N1–Me). The
subsequent physical adsorption on the metal surface
may involve either one or both amino nitrogen atoms.
In the first case, the piperazine fragment is fixed ortho-
gonally to the surface through the N4 atom possessing
a larger positive charge (vertical adsorption), while in
the second the piperazine ring in a chair conformation
is oriented parallel to the surface (flat adsorption).
In both cases, the adsorption is followed by depro-
tonation.
with 5,6,7,8-tetrafluoro-2-oxo-2H-chromen-4-olate
anion. This may result from combined action of
several factors. Presumably, electrolytic dissociation of
IIa in aqueous HCl eventually leads to orientation of
the cations and anions at the cathode and anode
regions, respectively, and the system reaches an equi-
librium state. In analogous process with compound
IIIa a part of anions are likely to act as chelating agent
–
(by means of the O–C4=C3–C=O fragment) toward
Fe2+ ions [13] which are transferred from the electrode
to acid medium. As a result, compensation processes at
the anode slow down, while polarization resistance of
the system increases, which reduces the inhibitory
effect of salt IIIa as compared to IIa.
Raising the concentration to 10–4 M considerably
increases the inhibitory effect of salt IIb (Z = 73%,
Fig. 2) and, to a lesser extent, the effect of IIIc (Z =
78%, Fig. 2).
The data for alkylammonium 5,6,7,8-tetrafluoro-
2-oxo-2H-chromen-4-olates IIa and IIb revealed ap-
preciably higher inhibitory effect of the methylammo-
nium salt as compared to benzylammonium. The
reduction of Kn in going from IIa to IIb may be
rationalized taking into account steric factor [12]. In
the case of methylammonium salt IIa a larger number
of relatively small coordinating cations per unit surface
area could fit the electrode. In the bulkier benzylam-
monium cation of salt IIb conjugation with the
π-electron system is weakened due to the presence of
methylene spacer, so that its inhibitory effect is lower.
In summary, we have found that the use of readily
accessible alkylammonium polyfluoro-2-oxo-2H-chro-
men-4-olates II and III as metal corrosion inhibitors is
limited by their poor solubility in aqueous media. The
examined compounds at a relatively low concentration
(10–4–10–5 M) considerably reduce the rate of uniform
corrosion of mild steel in aqueous HCl solution, which
favors development of resource-saving technologies.
EXPERIMENTAL
Comparison of the anticorrosive properties of salts
IIa and IIIa having the same cation but different
anions shows a stronger inhibitory effect of salt IIa
The melting points were measured in open capil-
laries on a Stuart SMP3 melting point apparatus and
are uncorrected. The IR spectra were recorded on
3500
3000
2500
2000
1500
1000
500
No salt added
IIb
IIIc
0
0
50
100
150
200
250
300
350
400
450
500
Time, min
Fig. 2. Rate of uniform corrosion of mild steel (St3) in 1 M aqueous HCl; concentration of salts IIb and IIIc 1×10–4 M.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 50 No. 1 2014

SHCHERBAKOV et al.
70
1
a Perkin Elmer Spectrum One spectrometer from
1033, 999 (C–F). H NMR spectrum, δ, ppm: 1.20 t
1
samples dispersed in mineral oil. The H, 13C, and 19F
(3H, CH2CH3, J = 7.1 Hz), 4.05 s (2H, CH2Ph), 4.06 q
(2H, CH2CH3, J = 7.1 Hz), 7.38–7.52 m (6H, Ph, 5-H),
8.15 br.s (3H, H3N+). 19F NMR spectrum, δF, ppm:
5.47 m, 7.78 m, 19.45 m (1F each). Found, %:
C 57.77; H 4.18; F 14.27; N 3.69. C19H16F3NO5. Cal-
culated, %: C 57.72; H 4.08; F 14.42; N 3.54.
NMR spectra were measured in DMSO-d6 on a Bruker
DRX-400 spectrometer at 400, 100, and 376.4 MHz,
respectively, using tetramethylsilane (1H, 13C) or C6F6
(19F) as reference. The elemental compositions were
determined on Perkin Elmer PE 2400 Series II and
Thermo Scientific EA 1108 analyzers.
2-Aminoanilinium 3-(ethoxycarbonyl)-5,6,7,8-
tetrafluoro-2-oxo-2H-chromen-4-olate (IIIc). Yield
64%, white powder, mp 174°C (from EtOH). IR spec-
trum, ν, cm–1: 3422, 3352, 3248 (NH2, C–H), 3024,
2611 (H3N+), 1720 (C2=O), 1677, 1662, 1632 (C=O,
ester), 1540, 1522, 1500, 1476 (δNH, C=C), 1047,
The rates of uniform corrosion (Kn) of mild steel
(St3) and corrosion inhibition efficiencies of com-
pounds IIa, IIb, IIIa, IIIc, and IIId were determined
at room temperature by measuring polarization resis-
tance with the aid of an Ekspert-004 universal auto-
matic corrosion meter and a two-electrode probe
(Ekoniks-Ekspert Ltd., Moscow). The electrodes, each
of 0.00078 m2 surface area, were made of St3 mild
steel (Cu, 0.05–0.15%; Mn, 0.40–0.65%; P ≤ 0.04%,
Cr ≤ 0.30%, S ≤ 0.05). Each Kn value given in Figs. 1
and 2 was determined as average of four parallel
measurements, the mean-square deviation being no
more than 3%.
1
1014 (C–F). H NMR spectrum, δ, ppm: 1.20 t (3H,
CH2CH3, J = 7.1 Hz), 4.08 q (2H, CH2CH3, J =
7.1 Hz), 6.84–7.00 m (4H, C6H4), 7.51 d.d.d (1H, 5-H,
J = 10.6, 8.5, 2.3 Hz), 7.66 br.s (5H, H2N, H3N+).
13C NMR spectrum, δC, ppm: 14.27 s (CH2CH3), 58.79
s (CH2CH3), 93.58 s (C3), 106.03 d.d (C4a, J = 19.0,
3.4 Hz), 118.73 br.s (C1′), 119.98 s (C3′), 122.55 s (C4′),
129.55 br.s (C2′), 138.11 d.d.d (C5, J = 250.0, 12.5,
3.1 Hz), 139.15–139.81 m (C8a, C8), 140.78 d.d.d (C6,
J = 250.8, 17.5, 12.8 Hz), 145.33 d.d.d (C7, J = 243.0,
10.6, 1.3 Hz), 159.89 s (C2), 167.04 s (C4), 170.19 s
(3-CO). 19F NMR spectrum, δF, ppm: 5.89 m, 7.90 m,
19.71 m (1F each). Found, %: C 54.51; H 3.97;
F 14.27; N 6.92. C18H15F3N2O5. Calculated, %:
C 54.55; H 3.82; F 14.38; N 7.07.
Initial coumarins Ia–Ic were synthesized according
to the procedures reported in [14], and salts IIa and
IIb were prepared as described in [6].
Ammonium polyfluoro-3-(ethoxycarbonyl)-2-
oxo-2H-chromen-4-olates IIIa–IIIe (general proce-
dure). Coumarin Ia–Ic, 1 mmol, was dissolved in
20 mL of ethanol, 1 mmol of the corresponding amine
was added, and the mixture was stirred for 25°C until
the reaction was complete (TLC). The solvent was
removed, and the solid residue was recrystallized from
appropriate solvent.
1-Methylpiperazinium 3-(ethoxycarbonyl)-6,7,8-
trifluoro-2-oxo-2H-chromen-4-olate (IIId). Yield
69%, white powder, mp 141°C (from Et2O). IR spec-
trum, ν, cm–1: 3085, 2980, 2907 2806 (C–H, NCH3,
H2N+), 1726 (C2=O), 1677, 1646 (C=O, ester), 1554,
1
Methylammonium 3-(ethoxycarbonyl)-6,7,8-tri-
fluoro-2-oxo-2H-chromen-4-olate (IIIa). Yield 92%,
yellow powder, mp >350°C (from Et2O). IR spectrum,
ν, cm–1: 3084, 2985 (CH, H3N+), 1707 (C2=O), 1664,
1647 (C=O, ester), 1575, 1521, 1449, 1412 (C=C),
1512, 1478 (C=C), 1046, 1032 (C–F). H NMR spec-
trum, δ, ppm: 1.21 t (3H, CH2CH3, J = 7.1 Hz), 2.22 s
(3H, CH3), 2.80 m (8H, NCH2CH2N), 4.08 q (2H,
CH2CH3, J = 7.1 Hz), 7.52 d.d.d (1H, 5-H, J = 10.7,
8.6, 2.3 Hz), 8.45 br.s (2H, H2N+). 19F NMR spectrum,
δF, ppm: 5.62 m, 7.81 m, 19.53 m (1F each). Found,
%: C 52.75; H 5.10; F 14.74; N 7.09. C17H19F3N2O5.
Calculated, %: C 52.58; H 4.93; F 14.68; N 7.21.
1
1050, 1030, 996 (C–F). H NMR spectrum, δ, ppm:
1.20 t (3H, CH2CH3, J = 7.1 Hz), 2.38 s (3H, NCH3),
4.07 q (2H, CH2CH3, J = 7.1 Hz), 7.50 d.d.d (1H, 5-H,
J = 10.7, 8.6, 2.2 Hz). 19F NMR spectrum, δF, ppm:
5.54 m, 7.79 m, 19.47 m (1F each). Found, %:
C 49.75; H 4.10; F 17.74; N 4.09. C13H12F3NO5. Cal-
culated, %: C 48.91; H 3.79; F 17.85; N 4.39.
Benzylammonium 3-(ethoxycarbonyl)-5,6,7,8-
tetrafluoro-2-oxo-2H-chromen-4-olate (IIIe). Yield
68%, white powder, mp 176°C (from CCl4). IR spec-
trum, ν, cm–1: 3035, 2609 (C–H, H3N+), 1694 (C2=O),
1647, 1620 (C=O, ester), 1580, 1518, 1488 (C=C),
Benzylammonium 3-(ethoxycarbonyl)-6,7,8-tri-
fluoro-2-oxo-2H-chromen-4-olate (IIIb). Yield 81%,
white powder, mp 169°C (from CCl4). IR spectrum, ν,
cm–1: 3098, 2986, 2902 2731 (C–H, H3N+), 1700
(C2=O), 1637 (C=O, ester), 1574, 1519, 1486 (C=C),
1
1028, 996 (C–F). H NMR spectrum, δ, ppm: 1.20 t
(3H, CH2CH3, J = 7.1 Hz), 4.05 s (2H, CH2Ph), 4.06 q
(2H, CH2CH3, J = 7.1 Hz), 7.37–7.47 m (5H, Ph),
8.16 br.s (3H, H3N+). 19F NMR spectrum, δF, ppm:
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SYNTHESIS AND ANTICORROSIVE PROPERTIES OF ALKYLAMMONIUM ...
71
chev, N.D., Eur. J. Med. Chem., 2006, vol. 41, p. 882;
Chen, D.-U., Kuo, P.-Y., and Yang, D.-Y., Bioorg. Med.
Chem. Lett., 2005, vol. 15, p. 2665.
–5.32 m, 0.96 m, 6.56 m, 14.20 m (1F each). Found,
%: C 55.24; H 3.65; F 18.29; N 3.13. C19H15F4NO5.
Calculated, %: C 55.21; H 3.66; F 18.39; N 3.39.
5. Sethna, S.M. and Shah, N.M., J. Indian Chem. Soc.,
1940, vol. 17, p. 211; Ashraf Ali, S., Desai, R.D., and
Shroff, H.P., Proc. Indian Acad. Sci., Sect. A, 1941,
vol. 13, p. 184; Fries, K. and Klostermann, W., Ber.,
1906, vol. 39, p. 871; Perkin, W.H., Justus Liebigs Ann.
Chem., 1869, vol. 150, p. 81.
This study was performed under financial support
by the Council for Grants at the President of the
Russian Federation (project no. NSh-3656.2014.3) and
by the Ural Branch of the Russian Academy of
Sciences (project no. 12-M-123-2045).
6. Shcherbakov, K.V., Burgart, Ya.V., Saloutin, V.I., and
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