Polarographic behavior of methyl 4-aryl-2,4-dioxobutanoates

Article abstract of DOI:10.1023/A:1013950903524

Polarographic reduction of methyl 4-aryl-2,4-dioxobutanoates in aqueous 2-propanol involves two cathodic waves and yields methyl 4-aryl-2,4-dihydroxybutanoates.

Full text of DOI:10.1023/A:1013950903524

Russian Journal of General Chemistry, Vol. 71, No. 10, 2001, pp. 1574 1577. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 10, 2001,
pp. 1662 1665.
Original Russian Text Copyright
2001 by Posyagin, Posyagina, Zalesov, Kataev.
Polarographic Behavior of Methyl 4-Aryl-2,4-dioxobutanoates
G. S. Posyagin, E. Yu. Posyagina, V. V. Zalesov, and S. S. Kataev
Perm State University, Perm, Russia
Received April 7, 2000
Abstract Polarographic reduction of methyl 4-aryl-2,4-dioxobutanoates in aqueous 2-propanol involves
two cathodic waves and yields methyl 4-aryl-2,4-dihydroxybutanoates.
Despite the enduring interest in the polarographic
behavior of carbonyl compounds, we found no data on
the polarographic reduction of 4-aryl-2,4-dioxobu-
tanoic acids and their esters, amides, or other deriva-
tives. At the same time, 4-aryl-2,4-dioxobutanoic
acids and their derivatives are interesting and promis-
ing objects. Methyl 4-aryl-2,4-dioxobutanoates Ia Id
can exist in 2,4-diketo form IA containing simulta-
neously -dicarbonyl, -dicarbonyl, and -dicarbonyl
fragments, in -enol form IB, and in -enol form IC.
In the table are given the parameters of the polaro-
graphic waves (i and E ), depolarizer concentrations
d
1/2
(taking into account dilution with buffer solutions),
and limiting current constants:
id = i c m 2/3t 1/6
1
,
d
where c is the concentration (mM); m is the weight of
1
mercury flowing out in unit time (3.22 mg s ); and
t is the dropping period (1.60 s).
R
As seen from the table, the first cathodic wave for
Ia is observed in the entire pH range.
CH
C
O
C COCH3
.O O
The half-wave potentials (E ) of the first wave
H
IC
1/2
monotonically shift toward negative values with in-
creasing pH. The slope dE /dpH in the range of
1
/2
R
R
CH2
pH 2.29 5.15 is 25 mV/pH unit, and in the range
.15 12.06, 82 mV/pH unit. The limiting current of
C
C COCH3
O O
IA
CH
5
O
this wave tends to increase with decreasing acidity of
the medium. At pH 3.20, the second cathodic wave
appears, whose half-wave potentials also shift toward
higher cathodic potentials with increasing pH. The
slopes are also different in different pH ranges. In the
range 3.20 5.15, dE /dpH is practically zero, in the
C
O
C COCH3
O O
H
1
/2
IB
range 5.15 7.31 it is 145 mV/pH unit, and in the
range 7.31 12.06, 20 mV/pH unit.
R = H (a), p-CH3 (b), p-Cl (c), o-OH (d).
The polarographic behavior of Ib and Ic is practi-
cally similar to that of Ia. The polarograms of Id ex-
hibit two irreversible cathodic waves; the second wave
appears earlier and is observed in the entire range of
pH 2 12 with the slope of 51 mV/pH unit. The first
cathodic wave appears at pH > 6.40, and its slope
dE / dp is 85 mV/pH unit. The half-wave potentials
Each of the forms IA IC contains several groups
capable of polarographic reduction; however, previous
studies showed that esters Ia Ic in inert solvents occur
in -enol form IB with a strong intramolecular
hydrogen bond between the -enol hydroxyl and the
-carbonyl group [1 3]. We made an attempt to find
1
/2
which form of Ia Id undergoes polarographic reduction of these waves shift toward higher cathodic potentials
and what is the sequence of the occurring processes. with increasing pH, and pronounced maxima are ob-
served in the curves i = f(pH).
d
The polarographic behavior of Ia Id was studied in
aqueous 2-propanol (20% i-PrOH) at pH 2 12. In the
polarograms we reliably detect one or two irreversible
cathodic waves with the half-wave potentials (E )
The specific features of the polarographic behavior
of Id are probably due to different strength and nature
of the intramolecular hydrogen bond formed between
the phenolic hydroxyl and the -carbonyl group of the
1
/2
and limiting currents (i ) depending on pH.
d
1
070-3632/01/7110-1574$25.00 2001 MAIK Nauka/Interperiodica

POLAROGRAPHIC BEHAVIOR OF METHYL 4-ARYL-2,4-DIOXOBUTANOATES
1575
Characteristics of polarographic waves in solutions of esters Ia Id in 20% aqueous 2-propanol
Ia, c 0.3 M
Ib, c 0.2 M
pH
I wave
II wave
I wave
II wave
id
E1/2
Id
id
E1/2
Id
id
E1/2
Id
id
E1/2
Id
2
3
4
5
6
7
8
9
.29
0.94
0.91
0.75
0.81
0.84
0.65
0.60
0.31
0.40
0.30
0.75
0.77
0.80
0.83
0.92
0.96
1.07
1.19
1.40
1.40
1.33
1.28
1.06
1.14
1.19
0.92
0.85
0.44
0.56
0.42
0.76
0.67
0.77
0.89
0.57
0.38
0.47
0.33
0.13
0.26
0.74
0.78
0.82
0.85
0.92
0.94
1.06
1.19
1.12
1.42
1.61
1.42
1.63
1.89
1.21
0.80
1.00
0.70
0.28
0.55
.20
.26
.15
.40
.31
.19
.52
0.56
0.59
0.50
0.79
0.97
1.09
1.32
1.14
1.13
1.24
1.22
1.24
1.39
1.56
1.57
1.58
1.64
1.64
0.79
0.83
0.71
1.12
1.37
1.54
1.86
1.61
1.60
0.44
0.43
0.64
0.57
0.72
0.75
0.77
1.27
0.75
1.24
1.26
1.26
1.46
1.60
1.62
1.65
1.62
1.68
0.93
0.91
1.36
1.21
1.53
1.59
1.63
2.69
1.59
1
1
0.92
2.06
Ic, c 0.25 M
Id, c 0.4 M
pH
I wave
II wave
I wave
II wave
id
E1/2
Id
id
E1/2
Id
id
E1/2
Id
id
E1/2
Id
2
3
4
5
6
7
8
9
.29
.20
.26
.15
.40
.31
.19
.52
0.29
0.38
0.31
0.39
0.33
0.39
0.27
0.16
0.17
0.59
0.77
0.79
0.84
0.87
0.90
0.94
1.01
1.10
1.19
1.30
0.49
0.64
0.53
0.66
0.56
0.66
0.46
0.27
0.29
1.00
1.26
1.44
1.68
1.71
1.62
1.51
1.42
0.76
0.77
0.73
1.09
1.21
1.26
1.24
1.38
1.43
1.51
1.53
1.45
1.66
1.33
1.53
1.78
1.81
1.72
1.60
1.50
0.80
0.82
0.77
0.55
0.52
0.50
0.43
0.68
0.95
1.15
1.27
0.83
1.19
1.29
1.25
1.31
1.36
1.54
1.54
1.53
1.64
0.93
0.88
0.85
0.73
1.15
1.61
1.95
2.15
1.41
0.81
0.84
1.44
0.79
0.48
1.03
1.08
1.25
1.22
1.43
0.86
0.89
1.52
0.84
0.51
1
1
0.92
2.06
a
1/2
1
2/3
Dimensions: I , A l s mmol mg
; i , A; and
E
, V.
d
d
1/2
ester, as compared to the bond in Ia Ic between the
enol hydroxyl and the -carbonyl group of the ester.
The results of our electrochemical study suggest
the following mechanism of reduction of esters Ia Id.
To study the mechanism of reduction of Ia Id on a
mercury electrode, we determined by controlled-
potential electrolysis the number of electrons (n) taken
up by the depolarizer molecule. The electrolysis prod-
ucts were isolated and identified by TLC, IR spectros-
copy, and mass spectrometry. The studies were per-
formed for Ia in 20% aqueous 2-propanol at pH 1.77,
In acidic and neutral solutions, compound Ia after
protonation is reduced initially to methyl 2-hydroxy-
4-oxo-4-phenylbutanoate (II) with uptake of two elec-
trons and two protons, and then to methyl 2,4-dihy-
droxy-4-phenylbutanoate (III), also with uptake of
two electrons and two protons. In alkaline solutions,
ester Ia is also reduced to III with uptake of four elec-
trons and four protons.
6.42, and 10.31. The potentials of the mercury elec-
trode, at which the electrolysis was performed corre-
sponded to the limiting current plateau of each wave.
For electrolysis in acidic solution (E = 0.9 V), n = 2;
in neutral solution at E = 1.1 V n = 2 and at E =
The final (III, R 0.52) and intermediate (II, R
0.98) electrolysis products have practically identical
IR spectra. The IR spectra of II and III (solutions in
f
f
1
.7 V n = 4; and in alkaline solution at E = 1.8 V
CCl ) contain a broadened band of the ester carbonyl
4
1
n = 4.
at about 1725 cm and a broadened band of the
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 10 2001

1576
POSYAGIN et al.
CH
pH 10.31
1.8 V
pH 1.77
0.9 V
H C
C
O
C COCH3
O O
5
6
E
E
H
Ia
pH 6.427
CH
E 1.1 V
4
4
e ,
H
H C
C
6
C COCH
3
+
5
+
+
OH HO O H
2
2
e ,
H
I wave
+
e , 2H+
2
H C CCH CHCOCH
5
6
2
3
H C CHCH CHCOOCH
5 6 2 3
II wave
O
OH O
OH
OH
II
III
hydroxy group (in II) or groups (in III) at about
EXPERIMENTAL
1
4
3380 cm . The absorption band of the C =O carbo-
nyl group involved in the intramolecular hydrogen
bond with the alcoholic hydroxyl is observed in the
IR spectrum of II in the range 1612 1595 cm , but
The IR spectra were taken on UR-20 and Specord
M-80 spectrometers. Samples were prepared as mulls
1
in mineral oil and solutions in CHCl . The mass spec-
3
tra were taken on an HP-5890 (Series II//HP-5972)
device under the following conditions: ionizing elec-
tron energy 70 eV, injector temperature 250 C,
detector interface temperature 280 C, initial column
it cannot be distinguished among the other bands
observed in this region. The GC MS data for chroma-
tographically pure (TLC) II suggest the presence of
the benzoyl substituent. The chromatogram contains
peaks of four compounds with the retention times of
1
temperature 70 C (2 min), heating rate 20 deg min ,
final column temperature 270 C, HP-5MS column
4
.23, 7.68, 8.00, and 8.65 min. The first three peaks
1
(
25000 0.25 mm), carrier gas helium (1.5 ml min ).
were identified as acetophenone (4.23 min, m/z 120
+
[
2
M] ) and E,Z isomers of methyl 4-oxo-4-phenyl-
The polarographic study was performed in 20%
+
-butenoate (7.68 and 8.00 min, m/z 190 [M] ), which
aqueous 2-propanol in the range of pH 2 12. The
required pH was adjusted with Britton Robinson
buffer solutions. The polarograms were taken with a
PU-1 polarograph.
originate from decomposition of II (8.65 min, m/z 208
+
[
M] ) in the course of chromatographic analysis.
The mass spectrum of II contains a molecular ion
+
peak at m/z 208 [M] and a relative intensity of 1.6%,
To determine the number of electrons consumed
by the depolarizer molecule, electrolysis was per-
formed at controlled potential in a YaSE-2 cell with
separated cathode and anode compartments, with a
platinum gauze as the anode, bottom mercury as the
cathode, and a saturated silver chloride reference elec-
trode. The potential was kept constant with a P-5848
potentiostat. The amount of the passed electricity was
determined with an IPT-1 coulometric integrator.
and also fragment peaks [m/z (relative intensity, %)]:
+
+
1
90 (2.2) [M H O] , 176 (3.0) [M CH OH] , 149
2 3
+
+
(12.4) [M
CH OCO] , 120 (9.1) [H C COCH ] ,
3 5 6 3
+
+
105 (100) [H C CO] , 77 (46.2) [C H ] .
5 6 6 5
The preservation of the -carbonyl group in inter-
mediate ester II suggests that the species subject to the
initial polarographic reduction is other than -enol
form IC. Ester II can form by polarographic reduction
2
Preparative electrolysis was performed in buffer
solutions in 20% aqueous 2-propanol at pH 1.77,
of the C =O carbonyl group in diketo form IA or
2
3
of the C =C bond in -enol form IB. The latter
alternative seems to be more probable, because the
reduction occurs similarly irrespective of pH, and also
in view of increased tendency of Ia Id to enolization.
Thus, we can conclude that the methyl esters of 4-
aryl-2,4-dioxobutanoic acids Ia Id are polarographi-
cally reduced in -enol form IB at any pH to
methyl 4-aryl-2,4-dihydroxybutanoates III, with the
initial reduction of the most labile double bond
6
.42, and 10.31. The electrolysis products were ex-
tracted with CHCl . Chromatography was performed
3
on standard Silufol UV-254 plates in the solvent sys-
tem ether benzene acetone, 10 : 9 : 1, at the solvent
front length of 10 cm. The chromatograms were
developed with iodine vapor.
Methyl 2-hydroxy-4-oxo-4-phenylbutanoate (II).
The solution in aqueous 2-propanol after preparative
electrolysis at pH 1.77 was extracted twice with
2
3
C =C .
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 10 2001

POLAROGRAPHIC BEHAVIOR OF METHYL 4-ARYL-2,4-DIOXOBUTANOATES
1577
equal volume of CHCl . The combined chloroform pound (R 0.52). Found, %: C 63.09; H 6.87.
3
f
extracts were evaporated. The oily residue was re- C H O . Calculated, %: C 62.85; H 6.71.
1
1
14
4
fluxed in ethanol over activated charcoal. After filtra-
tion and solvent evaporation, an oily substance was
obtained, which, according to TLC, was a pure com-
REFERENCES
pound (R 0.98). Found, %: C 63.66; H 5.94.
f
1
2
. Shapet’ko, N.N., Shigorin, D.N., Skoldinov, A.P., Ryab-
chikova, T.S., and Reshetova, L.N., Zh. Strukt. Khim.,
C H O . Calculated, %: C 63.46; H 5.80.
1
1
12
4
Methyl 2,4-dihydroxy-4-phenylbutanoate (III).
The solution in aqueous 2-propanol after preparative
electrolysis at pH 10.31 was extracted twice with
1
965, vol. 6, no. 1, pp. 155 157.
. Kurkovskaya, L.N., Shapet’ko, N.N., Andreichi-
kov, Yu.S., and Saraeva, R.F., Zh. Strukt. Khim., 1972,
vol. 13, no. 6, pp. 1026 1032.
equal volume of CHCl . The combined chloroform
3
extracts were evaporated. The oily residue was re-
fluxed in ethanol over activated charcoal. After filtra-
tion and solvent evaporation, an oily substance was
obtained, which, according to TLC, was a pure com-
3. Andreichikov, Yu.S., Saraeva, R.F., Nalimova, Yu.A.,
Tendryakova, S.P., and Lebedev, N.I., Zh. Org. Khim.,
1978, vol. 14, no. 2, pp. 371 373.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 10 2001