Catalytic oxidation of furan and hydrofuran compounds 8. Synthesis of 5-ethoxycarbonyl-4-hydroxy-3-oxo-2(3H)-furanone by the oxidation of furfural in the system aqueous H2O2-VOSO4-ethanol

Article abstract of DOI:10.1007/s10593-008-0043-3

The special features of the oxidation of furfural by aqueous hydrogen peroxide in the presence of vanadyl sulfate and ethanol has been studied for the first time. It has been established that this reaction proceeds with the formation of previously unknown

Full text of DOI:10.1007/s10593-008-0043-3

Chemistry of Heterocyclic Compounds, Vol. 44, No. 3, 2008
CATALYTIC OXIDATION OF FURAN
AND HYDROFURAN COMPOUNDS
8
*. SYNTHESIS OF 5-ETHOXYCARBONYL-
-HYDROXY-3-OXO-2(3H)-FURANONE
4
BY THE OXIDATION OF FURFURAL
IN THE SYSTEM AQUEOUS H
ETHANOL
2 2 4
O –VOSO –
V. V. Poskonin
The special features of the oxidation of furfural by aqueous hydrogen peroxide in the presence of
vanadyl sulfate and ethanol has been studied for the first time. It has been established that this reaction
proceeds with the formation of previously unknown oxidation products of furan compounds, one of
which was isolated from the reaction mixture as the ethyl ester. It was established by spectral methods
and qualitative reactions that this product is 5-ethoxycarbonyl-4-hydroxy-3-oxo-2(3H)-furanone.
Keywords: hydrogen peroxide, furfural, vanadium catalyst, oxidation.
It was established previously that the oxidation of furan compounds by aqueous hydrogen peroxide in
the presence of vanadium catalyst is a complex multidirectional process [2, 3]. The main factors influencing the
composition of the resulting products are the nature of the substituent in the furan nucleus and the type of
organic solvent used [2, 4-6]. On studying the joint action of these and other factors conditions emerged for the
oxidation of furfural 1 and furan 2 leading to the preferential formation of known poorly available products, viz.
5
2
-hydroxy-2(5H)-furanone (3) and its derivatives [7-9], succinic acid [2], furan-2-carboxylic acid amide [2], and
,5-diethoxy-2,5-dihydrofuran (4) [10, 11]. It turned out that addition of ethanol to the system for oxidizing
compound 2 assisted acceleration of the process and of the accumulation of its intermediate products,
dialdehydes, polyacetals, and carboxylic acids, as a result of their partial binding into the corresponding acetals
and esters.
It seemed of interest in this connection to study the possibility of broadening the synthetic possibilities
of oxidizing aldehyde 1 with aqueous hydrogen peroxide in the presence of a compound of vanadium (IV, V) by
introducing ethanol or butanol into the reaction medium. Reactions of furfural with H O in systems
2
2
simultaneously containing metal-containing catalysts and alcohol have not been studied previously.
_
*
______
For Communication 7 see [1].
_
________________________________________________________________________________________
Kuban State Technological University, Krasnodar 350072, Russia; e-mail: k-obh@kubstu.ru. Translated
from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 386-392, March, 2008. Original article submitted
February 24, 2005; revision submitted January 15, 2008.
0
009-3122/08/4403-0295©2008 Springer Science+Business Media, Inc.
295

TABLE 1. Results of the Oxidation of Furfural 1 with Aqueous H
2 2
O in
the Presence of VOSO in Aqueous and Aqueous-alcoholic Media at
4
6
0°C (1:H
2
O
2
:VOSO
4
= 1: 3.2:0.005; [1]init = 1.2 M)
Time for
Products, %*
Acids
Extent
Expt.
τ
½
,
complete
Cosolvent of furfural
conversion, %
Furanone
No.
min
consumption
5 + 6 7 + 8
9
12
10
2 2
of Н О , h
1
2
3
4
–
100
87
100*
40
42
29
–*
6.5
2.5
3
40
21
15
9
6
5
12
10
8
< 1
13
11
9
8
8
6
EtOH
EtOH
n-BuOH
2
3
37
6
< 1
< 1
16
10
_
*
*
*
______
Of reacted furfural.
2
Reached in the presence of 3.5 mol H O per mol furfural.
Not determined.
2
2
3
The oxidation of aldehyde 1 in aqueous alcoholic media was carried out in comparison with its
previously studied reaction in water. Vanadyl sulfate was used as catalyst, existing in aqueous solutions of H O
2
2
as peroxy complexes of vanadium (V). The catalyst was taken in an amount previously selected for the
oxidation of compound 1 with aqueous hydrogen peroxide. The reaction conditions are given in Table 1.
In experiments 2-4 in place of the calculated amount of water the same volume of ethanol or 1-butanol
was introduced into the reaction mixture.
It was discovered that the catalytic oxidation of furfural in aqueous solutions of alcohols differs
substantially from its reaction in water. For complete conversion of aldehyde 1 (1 mol) in water (expt. 1)
3
.2 mol H O was required, while in the presence of alcohols (expt. 2 and 4) at this ratio the extent of
2 2
conversion was significantly lower. For complete conversion of 1 mol furfural in aqueous ethanol (expt. 3)
.5 mol H
It is characteristic that on introducing ethanol a significant acceleration of the process was observed,
3
2 2
O was required.
while in the presence of 1-butanol this acceleration was insignificant.
The chief products of oxidation of furfural in the system aqueous H O –VOSO were C carboxylic
2
2
4
4
acids (Table 1, expt. 1). The main product was cis-β-formylacrylic acid (5), isolated as the tautomeric furanone
by the procedure developed by us previously [2, 8, 12]. The trans-isomer of β-formylacrylic acid (6), maleic
3
and fumaric acids (7) and (8), succinic acid (9), 2(5H)-furanone (10) accumulated in lesser amounts. Yields of
these compounds were established by known chromatographic and polarographic methods [2, 8, 12].
On adding alcohols the overall yield of these products was reduced from 70 to 35-45% (expt. 2-4), and
on increasing further the amount of organic solvent approached zero. This occurred primarily as a result of a
sharp fall in the yield of acid 7. Notably, a fall was also observed in the yield of an important side product of
aldehyde 1 oxidation, formic acid (11), formed as a result of oxidative cleavage of the C−C bond between the
heterocycle and the substituent [2, 13, 14]. Simultaneously, as might have been expected on the basis of the
results of [13, 14], the yield of furan-2-carboxylic acid (12) was significantly increased in expt. 2-4.
On the basis of studies [15-17] it was assumed that alcohols in the H O –vanadium compound system
2
2
would be oxidized to the corresponding aldehydes and carboxylic acids. It was established by GLC and
qualitative reactions that these compounds are in fact formed in experiments 2-4, however their yields did not
exceed 2-5%.
It was established by TLC that the main conversion products of furfural in the system H O –VOSO –
2
2
4
alcohol are new compounds not formed under other conditions of oxidizing furans. A mixture of these products
2
96

TABLE 2. Spectral Characteristics of Furanone 13
1
Com-
pound
IR spectrum,
ν, cm
Н NMR spectrum,
δ, ppm (J, Hz )
Mass-spectrum,
m/z (I, %)
-
1
+
1
3а + 13b
1160–1260 (С–О–С),
1.22 (3Н, t, J = 7, СН
4.08 (2.5–2.8 Н, m,
710, 1715 (С=Оketone), ОСН + Н-5);
3
);
186 [М] (0.3), 144 (11),
1
1
1
1
3
630 (С=С),
130 (29), 129 (11), 128 (13),
127 (21), 101 (52), 100 (41),
99 (23), 83 (15), 73 (21)
2
730 (С=Оester),
770, 1745 (С=Оlact),
500 (ОН)
6.20–7.10 (0.2–0.5 Н,
very br., ОН)
was obtained by concentrating the oxidation products from experiment 3 and was a very viscous substance
crystallizing with difficulty. It readily resinified under conditions of distillation under reduced pressure. On
interacting with NaOH or Na CO solution the new products formed salts, which enabled them to be assigned to
2
3
the class of carboxylic acids.
The acids obtained by the oxidation of aldehyde 1 in aqueous ethanol were therefore converted to their
esters. For this, the water was removed from the alcoholic oxidation products by azeotropic distillation, ethanol
was added, and esterification of the acid products was carried out by the usual procedure. The main component
of the mixture formed was obtained in a pure state by distillation under reduced pressure. The spectral
characteristics of this compound are given in Table 2.
The spectral data form a basis for proposing that the product isolated is 5-ethoxycarbonyl-4-hydroxy-
3
-oxo-2(3H)-furanone (13) existing in the tautomeric forms 13a and 13b:
O
OH
O
O
H
COOEt
O
O
O
COOEt
O
1
3a
1
3b
-
1
A series of strong bands was present at 1710-1770 cm in the IR spectrum of product 13 indicating the
presence in its molecule of ketone, ester, and lactone oxogroups. The presence of a hydroxyl group and a C=C
bond was also confirmed by absorption maxima (Table 2). The presence of bands corresponding to the C–O–C
grouping indicates the cyclic form of the molecule of ester 13.
1
Signals were present in the H NMR spectrum of product 13 caused by the resonance of protons of
methyl and hydroxyl groups. A multiplet observed at 4.08 ppm is evidently caused by the overlap of signals of
CH group protons and the proton at position 5 of furanone 13b. The ratio of the intensities of the signals of this
2
multiplet and of the OH group changes depending on the concentration of compound 13 and the type of solvent
(
CCl , CHCl , CO(CD ) , which may be explained by the change in intensity of the H-5 signal as a result of
4 3 3 2)
displacement of the equilibrium between the keto–enol tautomers of ester 13. This indicates that 20-50%
molecules of the obtained ester may be in the enolic form 13a.
A molecular ion of mass 186 m.u. was present in the mass spectrum of compound 13. The ion of m/z 29
had maximum intensity. The other most stable fragment ions are given in Table 2.
O
OEt
OEt
O
O
1
4
2
97

Among the theoretically possible oxidation products of aldehyde 1 only compounds 13 and 14 may
have a molecular ion of mass 186 a.m.u., however the structure of the latter substance does not correspond to
the data of elemental analysis, IR and NMR spectroscopy.
Additional proof in favor of the existence of the isolated product in the form of structures 13a and 13b was
obtained with the aid of qualitative reactions. An ethanolic solution of compound 13 instantly acquired an intense
dark yellow-orange color on adding a 1% solution of FeCl , which indicated the presence of the enolic group. The
3
presence of a 1,2-diketone grouping was confirmed with the aid of known procedures [18] (positive reaction with
hydroxylamine and NiCl in ammonia vapor, the formation of colored complexes with thiophene). The presence of a
2
C=C bond was confirmed by the instant decolorization of bromine water on adding an alcoholic solution of product
1
3 to it. Ester 13 did not interact with an aqueous solution of AgNO , which indicates the absence of an enediol
3
grouping in its tautomeric forms. However this reaction proceeds rapidly in alkaline medium, which may be
explained by the facile hydrolysis and fission of the lactone ring of compound 13 at pH >7.
O
OH
O
OH
H O, NaOH
2
–
+
–
COO
· 2Na
OOC
–
EtOH
COOEt
O
O
OH
13a
The predominance of tautomeric form 13a may be explained by the presence in the compound 13
molecule of electron-withdrawing substituents stabilizing the enolic grouping [19, 20].
The structure considered above for ester 13 enables the suggestion that one of the main products of furfural
oxidation in the system aqueous H O –VOSO –ethanol is 5-carboxy-4-hydroxy-3-oxo-2(3H)-furanone (15).
2
2
4
The change in the direction of furfural oxidation under the studied conditions in comparison with water
may be linked with the ability of alcohols to facilitate cleavage of bonds in peroxide molecules and to increase
the effective charge of vanadium(V) ions [21. 22].
It is noteworthy that acid 12 is oxidized significantly more slowly than furfural in aqueous alcoholic
media. This permits the suggestion that it is not the main intermediate in the process of forming product 15.
EXPERIMENTAL
The IR spectrum of compound 13 was taken on a Specord-40 M spectrometer (Germany) in nujol (NaCl
1
and KBr prisms in the region of 800-3800 cm¯), the H NMR spectrum on a Bruker WM 200 (250 MHz)
instrument in CCl , internal standard was TMS, and the mass spectrum on a CH-6 instrument with direct
4
insertion of substance into the ion source at an ionization energy of 70 eV, emission current 10 mA, and
o
temperature 120 C. TLC of reaction mixtures and of the isolated product 13 were obtained on Silufol UV-254
plates in chloroform or in the system chloroform–acetone, 3:1. Visualization was with acidified KMnO solution
4
and 2,4-dinitrophenylhydrazine, with an alcoholic solution of bromophenol blue, and with iodine vapor.
The yields of carboxylic acids 5-9 (as their ethyl esters) and furanone 10 were determined by GLC.
Esters of acids were obtained by boiling concentrates of reaction mixtures in the system ethanol–chloroform–
benzenesulfonic acid (see procedure for the synthesis of compound 13). Analysis of esterification products was
carried out on a Chrom-4 instrument (Czechoslovakia) with a flame-ionization detector, and a glass column
(
300×0.3 cm) containing 15% polymethylphenylsiloxane oil (PMPS-4) on chromaton N-AW DMCS. Column,
evaporator, and detector temperatures were 150, 220, and 220°C respectively. Carrier gas was nitrogen at
2
98

3
0 ml/min. The retention times of furanone 10, 5-ethoxy-2(5H)-furanone (product of interaction of acid 5 with
ethanol), diethyl esters of fumaric and maleic acids, were 2.62-2.75, 4.15-4.30, 5.30-5.50, and 6.35-6.50 min
respectively. The same chromatographic conditions were used for determining the yields of the oxidation
products of ethanol and 1-butanol.
The contents of isomers of β-formylacrylic acid 5 and 6, maleic and fumaric acids 7 and 8, in reaction
mixtures were determined by polarography. A sample was taken from the reaction mixture (0.5 ml) obtained in
experiments 1-4 and was dissolved in background electrolyte (0.1 M HClO or pH 7.4 phosphate buffer
4
solution) made up to volume (50 ml), and stirred. Samples were analyzed in 1 day on a LP-70 polarograph in a
thermostatted cell with a dropping mercury electrode (m = 1.59 mg/sec; t = 3.66 sec at a potential of -1.0 V, and
2
5 ± 0.2°C), the reference electrode was a normal calomel electrode. Half wave potentials for the reduction of
compounds 5-8 in 0.1 M HClO were 0.38-0.40 (for isomers 5 and 6) and 0.67 V (for isomers 7 and 8). The half
4
wave potential for the reduction of compounds 7 and 8 in phosphate buffer mixture was 1.45-1.50 V. The
concentrations of the substances being determined in the samples were established with a calibration graph.
Procedure for the Oxidation of Furfural. Experiment 1. Furfural (5 ml, 0.060 mol) was added to a
-
4
solution of VOSO 3H O (0.064 g, 3.10 mol) in water (30.3 ml) and 38.5% H O (14.7 ml, 0.192 mol), and
4
2
2
2
stirred at 60°C until complete conversion of H O and the resulting organic peroxides (absence of color reaction
2
2
with KI solution).
Experiments 2-4 were carried out by the procedure of experiment 1, in place of the indicated amount of
water the same volume of ethanol or 1-butanol was added.
Synthesis of 5-Ethoxycarbonyl-4-hydroxy-3-oxo-2(3H)-furanone (13). A solution of furfural (15 ml,
-
4
0
.18 mol) and VOSO 3H O (0.192 g, 9.1·10 mol) in 37.5% H O (135 ml, 1.56 mol) and ethanol (300 ml) was
4 2 2 2
stirred at 60°C for 12 h. The solvent was removed, ethanol (30 ml), chloroform (60 ml), and benzenesulfonic
acid (0.5 g) were added to the residue. The mixture was boiled with azeotropic distillation of water for 10-12 h,
evaporated, extracted with ether, washed with NaHCO solution, and with water, dried over Na SO , distilled
3
2
4
under reduced pressure, and ester 13 obtained in 23% yield (from the initial furfural). Bp 167-172° (18 mm Hg).
Found, %: C 45.16; H 3.22. C H O . Calculated, %: C 45.23; H 3.11.
7
6
6
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