Rearrangements and Tautomeric Transformations of Heterocyclic Compounds in Homogeneous Reaction Systems Furfural–Н2О2–Solvent

Article abstract of DOI:10.1134/S1070363218080030

General information on the reactions of furfurals with hydrogen peroxide is given. We have discussed the Baeyer–Villiger rearrangement of furan 2-hydroxyhydroperoxides and tautomeric transformations with proton transfer of 2-hydroxyfuran and β-formylacrylic acid formed in a homogeneous reaction system furfural–Н₂О₂–solvent under the catalysis with the formed acids. The factors affecting these rearrangements and tautomeric transformations as well as their specificity in comparison with benzene type compounds, and the pathway of the reactions of furan aldehydes with Н₂О₂ in water have been analyzed. Ketoenol tautomerism of cyclic hemiacetal form of β-formylacrylic acid leading to its transformation into succinic anhydride has been described for the first time.

Full text of DOI:10.1134/S1070363218080030

ISSN 1070-3632, Russian Journal of General Chemistry, 2018, Vol. 88, No. 8, pp. 1568–1579. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © L.A. Badovskaya, V.V. Poskonin, 2018, published in Zhurnal Obshchei Khimii, 2018, Vol. 88, No. 8, pp. 1245–1257.
Rearrangements and Tautomeric Transformations
of Heterocyclic Compounds in Homogeneous Reaction
Systems Furfural–Н2О2–Solvent
L. A. Badovskayaa and V. V. Poskonina*
a Kuban State Technological University, ul. Moskovskaya 2, Krasnodar, 350072 Russia
*e-mail: vposkonin@mail.ru
Received March 29, 2018
Abstract—General information on the reactions of furfurals with hydrogen peroxide is given. We have
discussed the Baeyer–Villiger rearrangement of furan 2-hydroxyhydroperoxides and tautomeric trans-
formations with proton transfer of 2-hydroxyfuran and β-formylacrylic acid formed in a homogeneous reaction
system furfural–Н2О2–solvent under the catalysis with the formed acids. The factors affecting these
rearrangements and tautomeric transformations as well as their specificity in comparison with benzene type
compounds, and the pathway of the reactions of furan aldehydes with Н2О2 in water have been analyzed. Keto-
enol tautomerism of cyclic hemiacetal form of β-formylacrylic acid leading to its transformation into succinic
anhydride has been described for the first time.
Keywords: furan aldehydes, hydrogen peroxide, tautomerism, the Baeyer–Villiger reaction, peroxides,
molecular rearrangement
DOI: 10.1134/S1070363218080030
Development of ideas on furan aldehydes reactions
with hydrogen peroxide. A series of rearrangements
and tautomeric transformations of furan and
hydrofuran compounds may occur in a homogeneous
furfural–Н2О2–solvent reaction mixture, determining
the direction of processes in the mixture. Review [1]
gives a historical overview of the research on furfural
reactions with hydrogen peroxide. Even though these
reactions have been studied since 1899, their
mechanism have long been controversial and not
coinciding with the data on the Н2О2 reactions with
other aromatic oxo compounds. It is known [1–3] that
the oxidation of benzene-type aldehydes and ketones
occurs through the formation of their α-hydroxyhydro-
peroxides. These intermediates undergo an intra-
molecular Baeyer–Villiger rearrangement affording
either the corresponding benzoic acids (esters) or
formylphenols which are further hydrolyzed into phenols
and formic acid. It is notable that the oxidation of
benzaldehyde yields mainly benzoic acid, evidencing
the ease of the hydrogen atom transfer to the electron-
deficient α-oxygen atom of the peroxide moiety in
comparison with phenyl radicals during rearrange-
ments of the α-hydroxyhydroperoxides.
The data on formation of furan-type α-hydroxy-
hydroperoxides and their transformations have
remained unknown until publication of our studies [1].
Such peroxides have been discovered in the reaction
mixtures during oxidation of furfural 1 and 5-sub-
stituted (СН3, NO2) furan aldehydes with hydrogen
peroxide [4].
Further studies [5‒8] have discussed the formation
and transformations of α-hydroxyhydroperoxides of
furfural, 5-methylfurfural, and 5-nitrofurfural in their
reactions with Н2О2 in water, ethanol, n-butanol, and
dioxane. Individual peroxides of furfural and 5-nitro-
furfural have been isolated [6].
The oxidation of aldehyde 1 with 30% aqueous
solution of Н2О2 has given a mixture containing the
product of rearrangement of furfural α-hydroxyhydro-
peroxide 2: 2-formyloxyfuran 3 [9]. Hydrolysis of the
latter affords formic (6) and β-formylacrylic (8a+8b)
acids, along with an unidentified compound.
The final products of furfural oxidation in different
solvents are listed in Table 1. Distinct features of these
reactions are as follows: the stability of the formed
1568

REARRANGEMENTS AND TAUTOMERIC TRANSFORMATIONS
1569
Table 1. Composition of products of the reaction between furfural 1 and 30% Н2О2 in different solventsa
Yield, mol% (with respect to furfural)
Compound
water
180 min
10.8
6.6
ethanol
210 min
70.1
0.8
dioxane
440 min
Furfural α-hydroxyhydroperoxide 2
β-Formylacrylic acid (8a and 8b)
Maleic acid (9)
98.0
Traces
10.8
0.2
3.1
–
Fumaric acid (10)
0.6
–
β-Formylpropionic acid (14)
Succinic acid (15)
4.0
1.0
–
20.0
1.3
3.0
–
2-Furoic acid (4)
8.1
–
Traces
–
Formic acid (6)
86
20
2(3Н)-Furanone (11) and 2(5Н)-furanone (12)
40
12
a [1] : [Н2О2] = 1 : 2, 60±1°С, conversion of furfural at the given reaction duration 70%.
peroxides in dioxane, the predominance of 2-furoic
acid 4 in the products in ethanol, and the formation of
2(3Н)- and 2(5Н)-furanones (11, 12) along with
succinic acid (15) as the major products in water [5, 7,
8, 10]. Based on these data, furanone 12 and acid 15
have been prepared simultaneously with 30 and 35%
yield, respectively, via oxidation of furfural 1 with
15% aqueous hydrogen peroxide (furfural–Н2О2
1 : 2.3 mol/mol, 4 h, 70±1°С) [11, 12].
The final products of oxidation of aldehyde 16 are
methyl-substituted homologs of hydrofuranones 11
and 12 [5-methyl-2(3Н)-furanone 20 and 5-methyl-
2(5Н)-furanone 21] and methyl-substituted structural
analogs of acids 14 and 8b [acetylpropionic (levulinic)
acid 22 and β-acetylacrylic acid 23] (Scheme 1)
[8, 10]. The reaction of aldehyde 16 with aqueous
Н2О2 does not yield 5-methyl-2-furoic acid 24.
The oxidation of aldehyde 17 with 28% aqueous
hydrogen peroxide diluted with dioxane gives
exclusively 5-nitro-2-furoic acid 25 in a quantitative
yield [13].
Scheme 1 has been suggested for the reactions
occurring in a homogeneous aldehyde 1–Н2О2–water
system in view of the available data. Most of the
products and stages in this scheme have been experi-
mentally confirmed [7–11].
Besides the considered oxidation processes,
homogeneous reactions of furfural with Н2О2 in the
presence of compounds of V and VI group elements
have been described [14–20].
It is notable that the pH of reaction mixture is
reduced to 1–2 during oxidation of furfural, due to the
formation of the acids (Scheme 1) which act as acid
catalysts in most of the involved stages. Formic acid 6
formed at the beginning of the oxidation is of special
importance. Hence, the considered reaction is acid-
autocatalytic.
The addition of catalytic amount of a soluble com-
pound of V+5, Nb+2, Nb+5, Mo+6, Cr+6, or Se+4 to the
aldehyde 1–Н2О2–water reaction system immediately
affords the peroxo complexes of these elements, as
confirmed in the study of the model systems by means
of spectroscopy and chromatography. These com-
plexes vigorously react with the carbonyl group of
furfural with the formation of the corresponding furyl-
containing organometal ozonides which are likely
transformed into 2-formyloxyfuran 3. Furfural α-hydroxy-
hydroperoxide 2 is not formed in these reactions.
The study of the furan aldehydes containing sub-
stituents which do not react with aqueous Н2О2 (5-methyl-
furfural 16 and 5-nitrofurfural 17) has allowed
elucidation of the role of electron-donor (СН3) and
electron-acceptor (NO2) groups [8, 10, 13]. These
reactions occur through the formation of α-hydroxy-
peroxides of 5-methylfurfural (18) and 5-nitrofurfural
(19).
The reactions are the fastest in the presence of Se+4
or V+5 compounds, being the slowest in the presence of
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 8 2018

1570
BADOVSKAYA, POSKONIN
Scheme 1.
OH
C__OOH
H2O2
O
H
~
O__C
_H2O
CHO
O
O
2
O
3
H
1
_HCOOH
~
H2O
_H2O
6
H2O2
COOH
O
4
O
OH
OH
HOO
O
O
O
5
7
12
_H2O
H2O
COOH
[HO__CH=CH__CH2COOH]
OHC
O
OH
O
O
O
O
O
O
8b
8c
13
8a
11
H2O
H2O2
COOH
H2O2
HOOC__CH2__CH2__COOH
OHC__CH2__CH2__COOH
CH=CH
CH=CH
HOOC
14
15
HOOC COOH
9
10
niobium compounds [15, 20]. The rates of the reaction
in the presence of Nb+2 and Nb+5 were equal and close
to that in the absence of any catalyst. This observation
can be explained by the negative oxidation potential of
niobium in its compounds, regardless of the oxidation
state. The final composition of the products of the
catalytic oxidation of furfural 1 in the presence of V+5,
Mo+6, Cr+6, or Se+4 differs from that under the acid
autocatalysis conditions [20]. In the presence of V+5, β-
formylacrylic acid (as a mixture of two tautomeric
forms 8a and 8b) is predominantly formed, its yield
being insignificant in the absence of the catalyst. The
reactions in the presence of Cr+6 give mainly acids 8
and 15, whereas the presence of Mo+6 triggers the
formation of furanone 12 and hydroxyacids (tartaric 26
and malic 27). Niobium compounds do not signi-
ficantly affect the composition of the major products of
the furfural oxidation as compared to the catalyst-free
process (Table 2).
The oxidation of furfural with 5% Н2О2 at 50±1°С
in the presence of SeO2 (furfural:Н2О2:catalyst molar
Table 2. Composition of major products formed in a homogeneous reaction mixture furfural–Н2О2–water–catalyst upon
complete decomposition of peroxide compoundsa
Yield, %b
Catalyst
8a+8b
9
8
12
22
29
34
8
15
48
17
14
15
10
11
26
–
27
1
Acid autocatalysis
Nb2O5
4
1
12
8
–
–
Nb(CH3COO)2
1
–
–
c
NaVO3
33
22
7
9
3
4
Na2Cr2O7
9
25
33
4
1
Na2MoO4
1
8
27
a
c
[1] : [Н2О2] : [catalyst] = 1 : 3.2 : 0.05, 60±1°С. b A mixture of β-formylpropionic, malonic, and oxalic acids is also formed (overall yield
20% in the presence of Nb+2 and Nb+5, 10% in the presence of V+5); 1–6% of malonic acid is formed in the presence of Mo+6 and Cr+6.
The catalyst loading was reduced tenfold due to the vigorous reaction.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 8 2018

REARRANGEMENTS AND TAUTOMERIC TRANSFORMATIONS
1571
ratio 1 : 3 : 0.05) leads to the formation of β-formyl-
acrylic acid 8 (as a mixture of tautomers 8a and 8b,
yield 60%) along with maleic (9) and fumaric (20)
acids (total yield 22%), succinic (15) and β-formyl-
propionic (14) acids (total yield 8%), and 2(5Н)-
furanone 12 (yield 8%) [14–16].
our studies [9–11]. Peroxyformic acid has been con-
sidered the oxidant in an organic phase. Alternatively,
splitting of the cycle in a molecule of aldehyde 1
assisted by water in acidic medium has been
suggested. This seems unlikely in view of stability of
furan cycle in furfural owing to the conjugation with
the carbonyl group [1].
In all the mentioned cases, formic acid 6 is formed
with a quantitative yield.
The oxidation of furfural 1 with hydrogen peroxide
in the presence of homogeneous acid catalysts in two-
phase aqueous-organic systems has been performed
[23]. The conditions favoring predominant formation
of furanone 12 have been elaborated; it has been
further hydrogenated in the presence of heterogeneous
metal complex catalysts to afford γ-butyrolactone. The
formation of intermediate furanone 12 and acids 9, 15
has been found coinciding with the scheme in [11].
The well-known epoxidizing ability of molyb-
denum peroxo complexes and characteristic trans-
formation of vanadium peroxo complexes into the
complexes with singlet oxygen V+5(O2) determined the
differences in the pathway and mechanism of the stage
the ester 3 and hydroxyfuran 5 oxidation as compared
to the acid autocatalysis conditions, which leads to the
formation of compounds 8, 26, and 27 in significant
amounts (Table 2) [20]. A procedure to prepare acid 8
in up to 60% yield [16] has been developed basing on
the reaction of furfural with Н2О2 in the presence of
V2O5.
Strongly acidic ion-exchange resin (Amberlist-15)
has been used as solid catalyst in oxidation of furfural
and other furan derivatives with hydrogen peroxide
[24, 25]. The use of such catalyst has directed the
reaction towards predominant formation of the acid 15,
its yield being up to 74% under the optimized condi-
tions. It has been suggested [25] that π,π-interaction
between the tosyl cycle of the catalyst and that of the
furan compound occurs in the first stage, leading to the
formation of an intermediate containing five-
membered cycle built of carbon and oxygen atoms of
the carbonyl group of the furan compound as well as
sulfur and oxygen atoms of the sulfonic group of the
catalyst. Under the action of H2O2, this intermediate is
further transformed into 2-formyloxyfuran 3, earlier
isolated as a product of homogeneous oxidation of
aldehyde 1 under acid autocatalysis conditions [9]. The
mechanism of formation of this ester during oxidation
in the presence of Amberlist-15 has not been con-
sidered by the authors. The transformation of ester 3
into furanone 11 and further into acid 15 has been
shown in the schemes in [9, 11].
Since the yield of 2(5Н)-furanone 12 is the highest
in the presence of niobium compounds (Table 2), the
optimal conditions of the furfural reaction in the Н2О2–
Н2О–Nb(CH3COO)2 system affording this lactone in
up to 64% yield have been elaborated [19].
Besides the above-described homogeneous reac-
tions of furfural with Н2О2, heterogeneous oxidation of
aldehyde 1 with hydrogen peroxide has been described
[21–28]. These studies aimed to optimize the
conditions favoring the increase in the yield of target
oxidation products. The oxidation with 30% hydrogen
peroxide in a water–1,2-dichloroethane two-phase
system in the presence of Na2SO4 at 70°С, followed by
boiling in the organic phase has given a mixture of
furanones 11 and 12 (Scheme 1). Heating of these
mixtures with triethylamine resulted in 67% yield of
furanone 12, total reaction duration being 14 h [21].
The process intermediates have not been studied, and
the reaction mechanism has been explained using the
scheme from [10].
Catalytic oxidation of furfural with hydrogen
peroxide in an aqueous medium in the presence of
titanium silicalite (TS-1) as catalyst (50°C, aldehyde 1
to Н2О2 molar ratio 1 : 7.5, 24 h) has afforded acid 9 in
78% yield [26]. Sequential use of these catalysts (TS-1
and Amberlist-70) at different stages of the process has
increased the yield of acid 9 to 80%, the process
duration being 28 h, and the H2O2 excess being
reduced to 4.4. The suggested reaction mechanism
includes as the first stage the epoxidation of one of the
double bonds of the furan ring of furfural with
Comparative study of oxidation of aldehyde 1 with
Н2О2 in water–1,2-dichloroethane and water–ethyl
acetate two-phase systems in the presence of formic
acid (1 : 0.8 molar ratio with respect to furfural) has
been reported [22]. The yield of furanone 12 has been
up to 60–62%, and the total yield of acids 9 and 15 has
been 15–20%. The suggested mechanism of their
formation has been based on the schemes elaborated in
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 8 2018

1572
BADOVSKAYA, POSKONIN
Scheme 2.
Fur
. . .
Fur
H
fast
. . .
HOOH
C=O
HOOH +
C=O + HOOH
HOO
H
(1)
(2)
(3)
H
1
A
Fur
Fur
OH
slow
. . .
. . .
HOO
H
HOOH
+ H2O2
C
C=O
H
OOH
H
A
2
Fur
Fur
H
Fur
H
OH
OH HO
1
+
C
C
C
O
OOH
O
H
2a
Fur = furyl.
hydrogen peroxide [26]. This seems unlikely, since
epoxidation of the furan cycle is known for furan and
its derivatives containing electron-donor substituents
[1] but is not possible for furfural, due to the reduced
electron density in the cycle of aldehyde 1 and
disturbance of the cycle conjugation with the carbonyl
group upon epoxidation.
α-hydroxyhydroperoxides, hydrofurans, and aldehyde-
acids. This has allowed the analysis of rearrangements
and tautomerism of a series of heterocyclic compounds
in the mentioned systems.
Formation and rearrangements of furan α-hyd-
roxyperoxides. Pure furfural α-hydroxyhydroperoxide
2 has been isolated upon oxidation of furfural with
concentrated Н2О2 on cooling (–15°С). Peroxide 2 is
very unstable and explodes at room temperature in
pure form [6]. In view of this, peroxide 2 has been
prepared and studied in the form of a solution in n-
butanol [17, 29]. Under these conditions oxidation of
aldehyde 1 is fast enough, and the peroxide com-
pounds are exclusively accumulated in the solution at
certain moment. Besides peroxide 2, furfural α,α'-
dihydroxyperoxide 2a, a product of the reaction
between aldehyde 1 and peroxide 2, is also formed
(Scheme 2).
It has been shown that oxidation of furfural with
Н2О2 in the presence of polystyrene sulfonic acid as
the acid catalyst affords acids 9 and 15 (yield 46 and
28%, respectively) [27]. The oxidation mechanism has
not been discussed.
Another procedure of furfural oxidation with 10%
hydrogen peroxide in methanol has been performed
in the presence of photogenerated iron catalyst
[FeCp(C6H5R)]PF6 at 20°С [28]: the mixture has been
irradiated with visible light during 2 h, and then the
process has been carried out in dark over 4 days with
addition of Н2О2 by portions [28]. The oxidation has
given methyl 2-furanoate (yield 90%). Advantages of
this method include the high yield of the ester and the
possibility of regeneration of the catalysis for repeated
use. At the same time, the process is quite time
consuming and involves toxic methanol. Mechanism
of the reaction has not been discussed.
Kinetics of oxidation of furfural in n-butanol with
30% hydrogen peroxide prepared by dilution of 99%
Н2О2 with n-butanol [17] has revealed the pseudo first
rate order with respect to furfural and the second rate
order with respect to Н2О2, i. e. overall the third
reaction rate order. This has allowed description of the
formation of peroxides 2 and 2a by Scheme 2.
As seen from the presented data, rearrangements of
furan hydroxyperoxides and tautomeric transforma-
tions of the products of deep oxidation of furan
aldehydes with hydrogen peroxides have not been
considered so far. At the same time, the experimental
data are available on homogeneous oxidation in the
furfural–Н2О2–water systems, including mechanism of
formation and products of transformations of furan
In view of the kinetic data, the rate of the process in
Scheme 2 is determined by stage 2. The formation of
complex A (stage 1) is in agreement with the
activation parameters of the reaction between furfural
and Н2О2 in n-butanol [29] and suggest the association
between the carbonyl group of furfural and Н2О2
molecules [30]. The third reaction rate order in the
reaction of nucleophilic addition at the carbonyl group
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 8 2018

REARRANGEMENTS AND TAUTOMERIC TRANSFORMATIONS
1573
Table 3. Parameters of transformation of furfural α-hydroxyhydroperoxide 2 into 2-furoic acid 4 in basic medium of amine
(сstart = 0.1 mol/L)
Solvent
Pyridine
Triethylamine
Т, °С
24
k×10–2, s–1
Ea, kcal/mol
10.0
А×10–5, s–1
ΔSa, e. u.
–26
1.65
3.7
3.9
25
1.75
10.0
–25
for synchronous addition of the nucleophile and
proton, which is the case of the considered process as
well [31].
The study of transformations of α-hydroxyhydro-
peroxide 2 in pyridine and triethylamine at 24–25°С
has revealed that it is completely consumed during 15–
20 min with quantitative formation of acid 4. The
decomposition of peroxide 2 follows the first-order
kinetics, and the rate as well as activation parameters
in both solvents are almost equal (Table 3). The
activation parameters point at the formation of a highly
polar transition state during the transformation of
peroxide 2 into acid 4. Hence, the transformation of
furfural and 5-methylfurfural α-hydroxyhydroper-
oxides into the corresponding 2-furoic acids is the
major pathway in the basic solvents.
It is notable that the formation of α-hydroxy-
hydroperoxides of furan aldehydes during oxidation of
these aldehydes with hydrogen peroxide in dioxane is
accelerated by electron-donor (СН3) as well as electron-
acceptor (NO2) substituents [8], which is in line with
the reaction through the formation of complex A
(Scheme 2).
The oxidation of furfural in water gives exclusively
peroxide 2, whereas peroxide 2a is formed in trace
amount [7]. In this case, the formation of peroxide 2 is
accompanied by its decomposition into furanones and
acids (Scheme 1 and Table 1). At the same time, the
third reaction rate order is preserved: the first with
respect to furfural, the first with respect to Н2О2, and
the first with respect to formic acid formed via
hydrolysis of ester 3 and acting as a protogen in the
formation of complex A (Scheme 2) [8]. This fact
confirms that the formation of α-hydroxyhydro-
peroxide 2 limits the overall rate of the multistage
reaction between furfural and Н2О2 in an aqueous medium.
In contrast to peroxide 2, 5-nitrofurfural α-hydroxy-
hydroperoxide 19 is relatively stable. It has been
prepared via oxidation of 5-nitrofurfural 17 with 35%
Н2О2 in a water–dioxane medium at 45°C in 95% yield
[6]. Peroxide 19 is a stable crystalline compound of mp
79–80°С. Upon 2 h heating in water at 70°С it is
quantitatively transformed into 5-nitro-2-furoic acid 25
[6, 13]. It is notable that the oxidation of aldehyde 17
with 30% Н2О2 gives peroxide 19 as the only
intermediate, which is further transformed into acid 25
in 98% yield [13].
The products of α-hydroxyhydroperoxide 2 trans-
formations in water have been compared with these
obtained upon addition of water to its solutions in n-
butanol, ethanol, and dioxane. The prepared aqueous-
organic solutions containing 25% of water have been
heated at 60°С until complete decomposition of the
peroxides [7, 8]. A mixture of acids and hydrofura-
nones is formed in water (Table 1). In the case of the
alcohols and dioxane with addition of water, quail-
tative composition of the products is the same but
2-furoic acid 4 predominates among the acids (yield up
to 60%), the yield of formic acid being reduced.
Comparison of the reactions of Н2О2 with
aldehydes 1, 16, furan ketones 28, and 5-methyl-2-
acetylfuran 29 has shown the identity of their products
and oxidation mechanisms [33]. Chromatographic
analysis of the reaction mixture has revealed the
presence of α-hydroxyhydroperoxides with structure
and properties analogous to these of peroxides 2 and
18 formed from furfural and 5-methylfurfural, respec-
tively. The products of deep oxidation of ketones have
been isolated as well: 2(3Н)-furanone 11, 2(5Н)-
furanone 12, 5-methyl-2(3Н)-furanone 20, and 5-
methyl-2(5Н)-furanone 21; β-formylacrylic 8а, maleic
9, succinic 15, β-acetylacrylic 23, and β-acetyl-
propionic 22 acids. The furanones 20 and 21 are major
products of the ketones oxidation. The oxidation of
furan ketones is accompanied by quantitative forma-
tion of acetic acid. Esters of 2-furoic or 5-methyl-2-
furoic acid are not formed.
It is notable that the oxidation of 5-methylfurfural
16 with aqueous hydrogen peroxide does not give 5-
methyl-2-furoic acid 24. At the same time, the
oxidation of aldehyde 16 in a more basic solvent
(water : triethylamine = 1 : 1) yields acid 24 as the
major product [32].
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 8 2018

1574
BADOVSKAYA, POSKONIN
Scheme 3.
HA
.
β
O
α
O
.
.
Н2О2
H
O
1, 16, 17
C
H
.
.
.
O
H
X
B
X = H (2), CH3 (8), NO2 (19).
НА: Н2О2, forming acids
HA
HA
.
.
.
.
.
.
O
O
O
O
H
O
H
C
H
H
.
C
.
.
.
.
.
O
O
O
H
H
X
X
D
C
Baeyer−Villiger rearrangement
_H2O,
_HA
_H2O,
_HA
~
~
O
H
O__C
X
X
COOH
O
3
O
4, 24, 25
products of hydrolysis
and oxidation
5_15, 20_23
X = H (4), CH3 (14, in alcohols, dioxane, pyridine, or triethylamine); X = NO2 (25, in water); X = Н, CH3 (5–15, 20–23, in
water).
The oxidation of furan ketones is slower in
comparison with furan aldehydes, as expected in view
of the effect of the methyl group increasing the
electron density at the carbon atom of the carbonyl
group and sterically hindering the nucleophilic attack
of this atom by hydrogen peroxide.
are protonated (by hydrogen peroxide and then with
the formed acids) at the α-oxygen atom of the peroxide
fragment (Scheme 3). This enhances the polarization
of the peroxide bond and additionally reduces the
electron density at its α-oxygen atom. The formation of
an intramolecular hydrogen bond between the
hydroxyl and peroxide groups with the formation of a
sufficiently stable five-membered cycle is also
probable (complex B, Scheme 3).
The discussed data have suggested the concept of
the rearrangements of furan α-hydroxyhydroperoxides
in homogeneous reaction mixtures containing the furan
aldehyde, Н2О2, and solvent. The formation of such
hydroxyhydroperoxides occurs through the formation
of type A complexes (Scheme 2), with hydrogen
peroxide acting simultaneously as nucleophile and
electrophile. In the case of reactions of furfural and 5-
methylfurfural in water, the forming organic acids
(mainly formic one) begin to act as protogen in
complexes A (Scheme 2). α-Hydroxyhydroperoxides
It is to be seen that the Baeyer–Villiger rearrange-
ments of α-hydroxyhydroperoxides of furfural, furan
aldehydes with electron-donor substituents in the furan
ring (as exemplified by 5-methylfurfural), and furan
ketones 28 and 29 with an alkyl substituent or without
other substituents in the ring proceeds with the
migration of furyl radicals to the electron-deficient α-
oxygen atom of the peroxide bond (Scheme 3). This is
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 8 2018

REARRANGEMENTS AND TAUTOMERIC TRANSFORMATIONS
1575
Scheme 4.
δ_
δ_
OH_
OH
A
H
H
HO_
HO_
H
H
Х
Х
A
OH
OOH
δ_
δ_
Х
A
H
A
Х
H
OH _
OOH
C
C
O
O
H
H
E
F
(А–Х) basic solvent (alcohols, dioxane, pyridine, or triethylamine).
a
distinct feature of these rearrangements in
adjacent C–H σ-bond act as donors, and 5-nitro-2-
furoic acid 25 is the product of the Baeyer–Villiger
rearrangement. This pathway of rearrangement of
peroxide 2 during oxidation of furfural 1 in an aqueous
medium is insufficient, being absent for the reaction of
5-methyl-furfural. However, 2-furoic (4) and 5-methyl-
2-furoic acids (24) are the major products of oxidation
in the reactions of these aldehydes with Н2О2 in more
basic organic media (alcohols, pyridine, or triethyl-
amine) [32].
comparison with the rearrangements of similar
peroxides of benzaldehyde and benzene ketones. This
fact can be explained by the enhanced electron density
in the furan ring of furan peroxides owing to the
conjugation with the heteroatom as well as lability of
the π-electron system [34–36] not typical of the
benzene cycle. Hence, rearrangements of the listed
furan aldehydes involve the overlapping of the p-
orbital of the electron-deficient α-oxygen atom of the
peroxide bond in the molecule of peroxides 2 and 18
with the σ-orbital of the adjacent С–С bond (furyl),
bonding electrons of which act as donor (transition
state C, Scheme 3).
рН of the reaction medium also affects the direction
of the rearrangement of furfural α-hydroxyhydro-
peroxide 2 [37]. As shown in Table 4, the reaction of
aldehyde 1 with Н2О2 in an aqueous medium, the
formation of acid 4 starts only at pH 5, being the most
pronounced at рН 7. In acid aqueous medium (рН 0–4)
furanone 12 and acid 15 are mainly formed, as shown
above. The presence of protons first favors the
formation of peroxide 2 and then induces the polariza-
tion of the peroxide bond reducing the electron density
at its α-oxygen atom. This, on the one hand, facilitates
If the furan ring contains an electron-acceptor
group (as in the case of 5-nitrofurfural), the electron
density in the cycle is sufficiently reduced, and the
formation of such transition state is impossible. In this
case, hydrogen atom migrates to the electron-deficient
atom of peroxide 19 through transition state D
(Scheme 3), in which the bonding electrons of the
Table 4. The effect of the medium pH on the formation of 2(5Н)-furanone 12 and carboxylic acids 4 and 15 in the furfural–
Н2О2–Н2О systema
Yield of oxidation products, %
рН
Furfural conversion, %
12
38
50
40
22
13
–
4
–
15
40
40
40
43
45
50
40
20
4
0
1
2
3
4
5
6
7
100
100
98
–
–
80
–
65
–
52
30
50
65
45
48
–
50
–
7.5
58
–
a [Furfural] : [Н2О2] = 1 : 10, 60°С, 4 h.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 8 2018

1576
BADOVSKAYA, POSKONIN
the rearrangement of peroxide 2 and, on the other
hand, favors the formation of formyloxyfuran 3. In an
aqueous medium, the latter is hydrolyzed into
hydroxyfuran 5, which undergoes further transforma-
tions (Scheme 1).
Products of hydrolysis of ester 3 are acid 6,
lactones 11 and 12, and semialdehyde of succinic acid
14 found in the reaction mixture upon oxidation of
furfural (Scheme 1). Compound 14 is formed from
lactone 9 [10]. The hydrolysis is relatively fast (k =
13.0×10–3 min–1). Lactone 12 has been isolated in a
yield up to 40% during oxidation of furfural 1 with
hydrogen peroxide in an aqueous medium [10, 11].
The freshly prepared furanone 12 contains up to 20%
of furanone 11 as the admixture; the latter is
transformed into lactone 12 upon heating or 2 months
storage at room temperature [10]. High-resolution
NMR spectrum recorded for the admixtures in lactone
12 contains relatively weak signals at δ 4.17, 5.80,
6.58, and 6.95 ppm [10]. They are attributed to the
presence of 2-hydroxyfuran 5, the tautomeric form of
furanone 12. This has been further confirmed by the
observation of a weak band at 3500 cm–1 in the IR
spectrum of lactone 12, typical of the OH group of
aromatic enols. The enol 5 has been observed also in
the products of the oxidation by means of chromato–
mass spectrometry [39].
The predominant formation of 2-furoic acid 4 via
rearrangement of furfural α-hydroxyhydroperoxide 2
observed in basic organic media can be explained by
the formation of hydrogen bonds between the
molecules of the organic solvent and protons of the
furan ring of the peroxide. The formation of such
bonds upon dissolution of furfural in dioxane and
1
DMSO has been confirmed by means of Н NMR
spectroscopy [38]. Similarly, the formation of
complexes E and F involving peroxide 2 (Scheme 4)
during the reaction of furfural with Н2О2 in basic
media can be assumed. The formation of dioxane–5-
nitrofurfural α-hydroxyhydroperoxide 19 complexes
has been observed during the oxidation of 5-nitro-
furfural with hydrogen peroxide in dioxane [38].
The formation of such bulky structures sterically
hinders the migration of furan ring to the electron-
deficient α-oxygen atom of the peroxide bond through
complex C (Scheme 3) and, hence, the formation of
formyloxyfuran 3. In this case, migration of proton
through complex D and formation of acid 4 are
favorable.
Tautomeric and isomeric transformations of
products of furfural oxidation with hydrogen
peroxide. The available data have allowed the
explanation of the mechanism of the lactones and acids
formation in the furfural–Н2О2–Н2О reaction system.
The readily proceeding hydrolysis of ester 3, in
contrast to this of formyloxybenzene, is due to the
formation of unstable enol 5 from ester 3.
The discussed data show that the Baeyer–Villiger
rearrangement of a series of furan α-hydroxyhydro-
peroxides (which have been isolated in the reactions of
furan aldehydes with Н2О2) exhibits specific features
in comparison with the rearrangements of similar
benzene-type peroxides. This occurs owing to the
differences in the structure and properties of the furan
and benzene rings [1].
Positive effect of the hydroxyl group conjugation
with the π-system of the furan ring occurs in the
molecule of hydroxyfuran 5. As a result, the electron
density in the cycle is enhanced, its aromaticity is
reduced, and the diene system reactions become
characteristic of it. This leads to the easy mobility of
the hydroxyl group hydrogen to the position 3 of the
furan cycle (adjacent to the hydroxyl group) and to the
opposite position 5, explaining the tautomeric trans-
formation of enol 5 into lactones 11 and 12 (Scheme 1).
Further transformations of formyloxyfuran 3 and
formyloxybenzene formed via rearrangement of the
corresponding α-hydroxyhydroperoxides are significantly
different as well. The products of hydrolysis of formyl-
oxybenzene (formed via the oxidation of benzaldehyde
with hydrogen peroxide) are formic acid 6 and phenol
[1]. Hydrolysis of formyloxyfuran 3 yields acid 6 and
2-hydroxyfuran 5 (Scheme 1), electronic structure and
properties of the furan ring of the latter strongly
differing from these of the benzene ring. This is the
reason for other differences in the transformations of
formyloxybenzene and formyloxyfuran in the reaction
systems where these are formed via the Baeyer–
Villiger rearrangement.
The equilibrium in this keto-enol tautomerism is
shifted towards the lactone forms, because the total
energy of the bonds in lactones is higher than in
hydroxyfuran 5, and the conjugated system is partially
preserved in the molecules of unsaturated lactones 11
and 12. Such transformations have been described in
the literature [40]. At the same time, as mentioned
above, lactone 12 contains minor amount of the enol 5.
This is a typical case when the enol double bond is
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 8 2018

REARRANGEMENTS AND TAUTOMERIC TRANSFORMATIONS
1577
Scheme 5.
OHC
COO_
OHC
COOH
O
HO
O
8a
8d
8b
conjugated with other double bonds [41], as in the
mole-cule of intermediate 5.
due to aromaticity of the benzene ring [41]. This fact
rules out any further transformations of phenol
analogous to these shown in Scheme 1 for furan
aldehydes. Hence, it is the keto-enol tautomerism of 2-
hydroxyfuran 5, intermediate of furfural oxidation with
hydrogen peroxide, which determines the dramatic
difference of this process from benzaldehyde oxidation.
Hydrofuranone 11 is readily hydrolyzed into acid
13 which exists in the stable tautomeric form of
aldehyde-acid 14 (Scheme 1). The formation of acid
14 from 2(3Н)-furanone 11 has been confirmed
experimentally [10, 42]. The aldehyde-acid 14 is
oxidized with hydrogen peroxide in the reaction
mixture into succinic acid 15, which has been isolated
from the products of the furfural reaction with Н2О2 in
up to 40% yield [11].
In the furfural–Н2О2–solvent reaction system the
tautomeric transformations involving the proton
transfer are observed also for the products of
hydroxyfuran 5 oxidation (Scheme 1). β-Formylacrylic
acid 8 has been isolated from the reaction medium in
two forms: open carbonyl (8b) and cyclic hemiacetal
(8a) ones, their ratio being changed in the course of the
reaction. Initially, when the reaction medium pH is
close to neutral, the carbonyl form 8b is predominant.
As the medium becomes more acidic, the ratio is
changed in favor of the cyclic form 8a which is
prevailing in the final products [18]. pH of the reaction
medium is reduced from 6.5 to 1 due to the formation
of acids 6, 8b, and 15 (Scheme 1). To explain the
observations, the equilibrium between compounds 8a
and 8b has been studied by means of polarography
[43]. The reduction has been performed using a
dropping mercury electrode with m 2.01 mg/s and t1
3.41 s. Only the 8b form can be reduced under the
experiment conditions, which allows monitoring the
ratio of the forms by measuring the current of
reduction.
In contrast to lactone 11, compound 12 is stable in
acidic aqueous media, including the conditions of
furfural oxidation with hydrogen peroxide in water. It
does not undergo acid hydrolysis and is not oxidized
by hydrogen peroxide in acid media [11].
The product of oxidation of tautomer 5 is β-formyl-
acrylic acid 8 existing in the reaction medium in two
tautomer forms 8a and 8b. It is formed via the addition
of Н2О2 at the diene system of 2-hydroxy-furan 5
through the formation of hydroperoxide 7 (Scheme 1).
Its yield is low upon oxidation of furfurol 1 with
hydrogen peroxide in water in the absence of a catalyst
(Table 1). However, the oxidative ability of the
medium is enhanced when the reaction of aldehyde 1
with Н2О2 is performed in the presence of selenium,
vanadium, molybdenum, or niobium compounds, and
the oxidation of hydroxyfuran 5 becomes competitive
to its tautomeric transformations into lactones 11 and
12. Under these conditions, acid 8 is formed in
significant yield [15, 16, 18, 20].
It has been found that β-formylacrylic acid exists in
almost exclusively open neutral 8b and anionic 8d
forms in basic to weakly acidic media (Scheme 5). At
рН 0–4, 30–60% of it is in the cyclic form 8a. The
trans-form of acid 8 has not been observed.
Acid 8b is further oxidized into maleic acid 9,
which it slightly isomerized into fumaric acid 10. The
ease of transformation of hydroxyfuran 5 into stable
products 8a, 8b, 12 and further 9 and 15 favors the
shift of the equilibrium of hydrolysis of ester 3 towards
the intermediate 5 and the products of its tautomeric
and oxidative transformations. The formed acids 6, 8b,
9, and 15 act as catalysts at all the stages of the reac-
tion in an aqueous medium shown in Scheme 1.
The obtained data on the pH effect on the cycle-
chain tautomerism of acid 8 are in agreement with the
behavior of other acylacrylic acids [44].
The observed partial transformation of the
hemiacetal form 8a into succinic acid 15 [45] has not
been known before. Heating of a neutral aqueous
solution of compound 8a (с = 1.2 mol/L) at 60°C has
afforded acid 15 with 3% yield after 3 h and 13% yield
after 24 h. At рН 9–10 and 70–80°С, hemiacetal 8a is
Oxidation of benzene with hydrogen peroxide stops
at the formation of phenol, the phenol–ketone tauto-
meric equilibrium of which is shifted towards phenol
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 8 2018

1578
BADOVSKAYA, POSKONIN
Scheme 6.
H2O
O
HOOC
COOH
HO
O
HO
O
OH
O
8e
O
8c
O
8a
15
7. Badovskaya, L.A., Kaklyugina, T.Ya., and Kul’nevich, V.G.,
Chem. Heterocycl. Compd., 1977, vol. 13, no. 5, p. 484.
doi 10.1007/BF00473189
completely transformed into acid 15 within minutes;
the latter has been isolated as such and in the form of
diethyl ester [45]. Scheme 6 of the transformations has
been suggested.
8. Badovskaya, L.A., Doctoral (Chem.) Dissertation,
Krasnodar, 1983.
Hemiacetal 8a containing a carbonyl group is in
equilibrium with the enol form 8e which can have a
keto form 8c, succinic anhydride. Hydrolysis of the
latter gives acid 15. In the basic media the equilibrium
is shifted from the form 8a towards form 8c due to the
hydrolysis of the latter into acid 15. Hence, it can be
suggested that acid 15 is partially formed from
hemiacetal 8a in the furfural–Н2О2–Н2О system
(Scheme 1) when the medium acidity has not yet
significantly increased.
9. Kul’nevich, V.G., Badovskaya, L.A., and Muzychenko, G.F.,
Chem. Heterocycl. Compd., 1970, vol. 6, no. 5, p. 535.
doi 10.1007/BF00500664
10. Badovskaya, L.A., Chem. Heterocycl. Compd., 1978,
vol. 14, no. 10, p. 1062. doi 10.1007/BF00469941
11. Badovskaya, L.A., Latashko, V.M., Poskonin, V.V.,
Grunskaya, E.P., Tyukhteneva, Z.I., Rudakova, S.P.,
Pestunova, S.A., and Sarkisyan, A.V., Chem.
Heterocycl. Compd., 2002, vol. 38, no. 9, p. 1040. doi
10.1023/A:1021288711593
12. Badovskaya, L.A., Muzychenko, G.F., Abramyants, S.V.,
Latashko, V.M., and Kul’nevich, V.G., USSR Author’s
Certificate 470516, 1975; Byull. Izobret., 1975, no. 18.
13. Krapivin, G.D., Badovskaya, L.A., and Kul’nevich, V.G.,
Chem. Heterocycl. Compd., 1975, vol. 11, no. 9,
p. 1018. doi 10.1007/BF00471285
14. Gavrilova, S.P., Badovskaya, L.A., and Kul’nevich, V.G.,
Kinet. Katal., 1979, no. 5, p. 1338.
15. Gavrilova, S.P., Candidate Sci. (Chem.) Dissertation,
The above discussion has revealed that most of the
stages of the reactions between furan aldehydes with
hydrogen peroxides occur through rearrangements and
tautomeric transformations of furan and hydrofuran
compounds 2, 5, 8a, 9, and 12. The predominant
formation of 2-furoic acids or hydrofuranones 8a, 9,
12, and acid 15 in this complex oxidation process is
directed by the substitution in the furan cycle, nature
and basicity of the solvent, and acid-base properties of
the medium.
Krasnodar, 1980.
16. Poskonin, V.V., Badovskaya, L.A., Gavrilova, S.P., and
Kul’nevich, V.G., Zh. Org. Khim., 1989, vol. 25,
p. 1701.
17. Grunskaya, E.P., Badovskaya, L.A., Kaklyugina, T.Ya.,
and Poskonin, V.V., Kinet. Catal., 2000, vol. 41, no. 4,
p. 447. doi 10.1007/BF02756060.
CONFLICT OF INTERESTS
No conflict of interest was declared by the authors.
REFERENCES
18. Poskonin, V.V., Doctoral (Chem.) Dissertation,
Krasnodar, 2001.
1
Kul’nevich, V.G. and Badovskaya, L.A., Russ. Chem.
Rev., 1975, vol. 44, p. 574.
19. Poskonin, V.V., Chem. Heterocycl. Compd., 2009, vol.
45, no. 10, p. 1177. doi 10.1007/s10593-010-0404-6
2. Belov, V.N., Kheifits, A.A., and Virezub, S.I., Reaktsii i
metody issledovaniya organicheskikh soedinenii (Reac-
tions and Methods for Studying Organic Compounds),
no. 10, Moscow: Khim. Literatura, 1961, p. 7.
20. Badovskaya, L.A. and Poskonin, V.V., Kinet. Catal.,
2015, vol. 56, no. 2, p. 164. doi 10.1134/
S0023158415020019
21. Cao, R., Lio Ch.,and Lin, L., Org. Prep. Proc. Int.,
1996, vol. 28, no. 2, p. 215. doi 10.1080/
00304949609356524
3. Syrkin, Ya.K. and Moiseev, I.I., Usp. Khim., 1960,
vol. 29, p. 425.
4. Badovskaya, L.A., Candidate Sci. (Chem.) Dissertation,
22. Li, X., Lan, X., and Wang, T., Catalysis Today, 2016,
Krasnodar, 1967.
vol. 276, p. 97. 10.1016/j.cattod.2015.11.036
5. Badovskaya, L.A., Trudy KPI, 1975, no. 66, p. 3.
23. Li, X., Lan, X., and Wang, T., Green Chem., 2016,
vol. 18, p. 638. doi 10.1039/C5GC02411K
24. Choudhary, H., Nishimura, S., and Ebitane, K., Chem.
Lett., 2012, vol. 41, p. 409. doi 10.1246/cl.2012.409
6. Badovskaya, L.A., Krapivin, G.D., Kaklyugina, T.Ya.,
Kul’nevich, V.G., and Muzychenko, G.F., Zh. Org.
Khim., 1975, vol. 11, p. 2446.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 8 2018

REARRANGEMENTS AND TAUTOMERIC TRANSFORMATIONS
1579
25. Hemant Choudhary, Shum Nishimura, and Kohki
Ebitane, Appl. Catal., 2013, vol. 458, p. 55. doi
10.1016/j.apcata.2013.03.033
26. Alonso-Fagundez, N., Agirrezabal-Telleria, J., Arias, P.L.,
Fierro, J.L., Mariscal, R., and Lopez Granados, M., RSC
Adv., 2014, vol. 4, p. 54960. doi 10.1039/C4RA11563E
27. Alonso-Fagundez, N., Laserna, V., Alba-Rubio, A.C.,
Mengibar, M., Heras, A., Mariscal, R., and Lopez
Granados, M., Catalysis Today, 2014, vol. 234, p. 285.
doi 10.1016/j.cattod.2014.01.041
28. Moulinez, F., Ruiz, H., and Astruc, D., J. Organomet.
Chem., 1988, vol. 340, no. 2, p. 13. doi 10.1016/0022-
328X(88)80082-2
36. Bleidelis, Ya.Ya., Uspekhi khimii furana (Advances in
Furan Chemistry), Riga: Zinatne, 1978, p. 7.
37. Badovskaya, L.A., Poskonin, V.V., and Ponomarenko, R.I.,
Russ. J. Gen. Chem., 2014, vol. 84, no. 6, p. 1133. doi
10.1134/S1070363214060140
38. Krapivin, G.D., Yablonskii, O.P., Chkalova, E.G.,
Badovskaya, L.A., and Kul’nevich, V.G., Zh. Fiz.
Khim., 1976, vol. 1, no. 6, p. 1399.
39. Badovskaja L.A., Poskonin V.V., and Povarova L.V.,
Fundamental’nye problemy khimicheskoi nauki
(Fundamental Problems of Chemical Science),
Yekaterinburg: Ural. Otd. Ross. Akad. Nauk, 2016,
vol. 1, p. 398.
29. Kakluygina, Т.Ya, Candidate Sci. (Chem.) Dissertation,
40. Katritdkii, A. and Lagovskaya, Dzh., Khimiya
geterotsiklicheskikh soedinenii (Chemistry of Hetero-
cyclic Compounds), Moscow: Inostrannaua Literatura,
1963, p. 195.
Krasnodar, 1974.
30. Muzychenko, G.F., Kaklyugina, T.Ya., Badovskaya, L.A.,
Karyakin, A.V., and Kul’nevich, V.G., Zh. Fiz. Khim.,
1972, vol. 46, no. 9, p. 2391.
41. March, J., Problems in Advanced Organic Chemistry,
M. Dekker, 1971.
42. Tsybina, N.M., Protopopova, T.V., and Skoldinov, A.P.,
31. Jencks, W.P., Mechanism and Catalysis of Simple
Carbonyl Group Reactions, Moscow: Mir, 1967, p. 342.
Zh. Org. Chem., 1970, vol. 6, p. 269.
32. Badovskaya, L.A., Poskonin, V.V., and Povarova, L.V.,
Russ. Chem. Bull., 2017, vol. 66, no. 4, p. 593. doi
10.1007/s11172-017-1778-8
33. Tsygankova, L.V. and Kul’nevich, V.G., Chem.
Heterocycl. Compd., 1972, vol. 8, no. 4, p. 404. doi
10.1007/BF00477407
43. Strizhov, N.K., Poskonin, V.V., Badovskaya, L.A., and
Kupina, E.P., Russ. J. Org. Chem., 2002, vol. 38, no. 2,
p. 251. doi 10.1023/A:1015578003374
44. Valter, R.E. Kol’chato-tsepnaya izomeriya v organi-
cheskoi khimii (Ring-Chain Isomerism in Organic
Chemistry), Riga: Zinatne, 1978, p. 39.
34. Popelis, Yu.Yu., Uspekhi khimii furana (Advances in
Furan Chemistry), Riga: Zinatne, 1978, p. 19.
35. Marino, D., Uspekhi khimii geterotsiklov (Advances in
45. Poskonin, V.V. and Badovskaya, L.A., Chem.
Heterocycl. Compd., 2003, vol. 39, no. 5, p. 594. doi
10.1023/A:1025137914137
Heterocyclic Chemistry) , Riga: Zinatne, 1976, p. 21.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 8 2018