A new electrochemical procedure for preparing 2,4,6-triphenylselenopyran from 1,5-diketones under conditions of acid catalysis

Article abstract of DOI:10.1134/S1070427208030208

A new electrochemical procedure was developed for preparing 2,4,6-triphenylselenopyran from 1,5-diketones under conditions of acid catalysis at oxidative electrochemical activation of hydrogen selenide in the presence of a one-electron oxidizing agent in a nonaqueous solution. The electrolysis products were analyzed by gas chromatography with a mass-selective detector.

Full text of DOI:10.1134/S1070427208030208

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2008, Vol. 81, No. 3, pp. 448 451. Pleiades Publishing, Ltd., 2008.
Original Russian Text T.G. Dmitrienko, S.S. Popova, 2008, published in Zhurnal Prikladnoi Khimii, 2008, Vol. 81, No. 3, pp. 463 466.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
A New Electrochemical Procedure for Preparing
2,4,6-Triphenylselenopyran from 1,5-Diketones
under Conditions of Acid Catalysis
T. G. Dmitrienko and S. S. Popova
Technological Institute, Saratov State Technical University, Engels, Saratov oblast, Russia
Saratov Military Institute of Chemical, Biological, and Radiation Protection, Saratov, Russia
Received February 9, 2007
Abstract A new electrochemical procedure was developed for preparing 2,4,6-triphenylselenopyran from
1,5-diketones under conditions of acid catalysis at oxidative electrochemical activation of hydrogen selenide
in the presence of a one-electron oxidizing agent in a nonaqueous solution. The electrolysis products were
analyzed by gas chromatography with a mass-selective detector.
DOI: 10.1134/S1070427208030208
Selenium-containing heterocyclic compounds are
widely used for preparing pharmaceuticals, odorants
in perfumery, photomaterials, photosensitizers, dyes,
and polymer fibers [1, 2]. High biological activity of
low-toxic 4H-selenopyrans was revealed recently.
Therefore, development of new convenient methods
for preparing these compounds becomes an urgent
problem [1]. Reaction of 1,5-diketones with hydrogen
sulfide in the presence of acids yields thiopyrans or
their disproportionation products, thiopyrylium salts,
thiacyclohexanes, or dihydrothiopyrans [3]. Seleno-
pyrans and selenopyrylium salts can be prepared
similarly by acid-catalyzed reaction with hydrogen
selenide [4].
As chemical synthesis of selenopyrans has already
been developed [1], the aim of this study was to de-
velop an electrochemical synthesis of selenopyrans
and selenopyrylium salts. In this study we developed
a new procedure for preparing 4H-selenopyrans and
examined some features of their reactions. For this
purpose, we studied reaction of 1,5-diketones with
electrolytically generated hydrogen selenide under
conditions of acid catalysis and oxidative activation of
the reagent. Hydrogen selenide formed by electrolysis
on a selenium electrode reacts with a 1,5-diketone in
ethanol to give 4H-selenopyran as one of the products.
EXPERIMENTAL
Most of methods for preparing arylaliphactic
4H-selenopyrans are based on reactions of gaseous
hydrogen selenide with appropriate 1,5-diketones or
reactions of selenopyrylium salts with metal hydrides
[4]. The most convenient reactions of arylaliphatic
1,5-diketones with hydrogen selenide can be per-
formed only with polysubstituted diketones. The main
products of these reactions were mixtures of seleno-
pyrylium salts and selenacyclohexanes.
Electrochemical synthesis was performed in a glass
three-electrode electrochemical cell with nonseparated
anode and cathode compartments in a nonaqueous
solution at a fixed temperature with stirring. A plati-
num wire 0.25 mm in diameter was used as an auxili-
ary electrode (anode). Electrodes prepared by applica-
tion of selenium to the surface of different metals
or selenium electrodes prepared by different methods
were used as working electrodes (cathodes). The first
step was the development of the electrode design.
Selenium was applied to nickel, copper, and platinum
electrodes of different configurations. The nickel elec-
Electrochemical synthesis of some organoselenium
compounds and electrochemical behavior of some
selenium-containing heterocyclic compounds is a rela-
tively new field of selenium electrochemistry. There-
fore, development of new procedures for preparing
selenium-containing heterocyclic compounds and
study of their redox properties are urgent problems.
2
trode was a 5 cm plate cut from Ni foil; the copper
electrode was a copper wire 1.5 and 2.5 mm in diam-
eter and 5 cm long; the platinum electrode was plati-
num wire 0.25 mm in diameter and 10 mm long,
448

A NEW ELECTROCHEMICAL PROCEDURE FOR PREPARING 2,4,6-TRIPHENYLSELENOPYRAN 449
which was sealed in a glass tube. Selenium was ap-
plied as follows. Selenium powder in a ceramic
crucible was placed in a muffle furnace and was
melted at 273 C. An appropriate metallic electrode
was immersed in the melt and kept there for 5 s.
The electrode was taken out from the melt and cooled
in air until the selenium completely solidified.
tions is an open question. Cathodic dissolution of
an element is a paradoxical fact itself.
The majority of researchers believe [5, 6] that
organoelement compounds are formed by the radical
mechanism by the reaction of a radical formed at the
cathode with the electrode material. However, not all
the cathodic reactions of formation of organoelement
compounds can be described within the framework of
this concept. These facts suggest that radicals formed
in the course of electrolysis are present in the form
of an intermediate complex with the cathode material,
rather than in the form of kinetically independent
species. Under favorable conditions, this complex
abstracts an atom from the crystal lattice of the cath-
ode to form an organoelement compound. In some
cases, an organoelement compound is formed by
a pathway other than a direct electrochemical reaction.
The electrolyte was prepared as follows: PCl
3
(3.1 ml) was added from a dropping funnel to ethanol
(22 ml) placed in a conical flask. The flask was
cooled on an ice bath, and its contents were stirred
with a magnetic stirrer. Then a weighed portion of
1,3,5-triphenylpentane-1,5-dione was dissolved in
the flask. The reaction course and the purity of the
reaction products were monitored by thin-layer chro-
matography (TLC) using a hexane ether chloroform
mixture (3 : 1 : 1) as an eluent (the chromatogram was
developed with iodine vapor) and gas chromatography
with mass-selective detector (GC MSD).
Tomilov et al. [6] showed that the formation mech-
anism of organoelement compounds by cathodic reac-
tions strongly depends on the type of the organic
compound and cathode and should be considered
separately in each particular case. The electrochemical
behavior of the most widely known organoselenium
compounds was examined in [7, 8]. Not only HCl but
also HBr, CH COOH, and Lewis acids (ZnCl and
The selenium electrode potential controlled with
the aid of a P-5848 potentiostat was 0.4 V. The
organoselenium salt was electrochemically identified
by measuring its voltametric properties with a sta-
tionary platinum electrode with a surface area of
2
3.48 mm as the working electrode and a saturated
3
2
silver chloride electrode with a waterproof diaphragm
ZnBr ) can be used to produce an acid media. It
2
2
should be noted that chemical and electrochemical
one-electron irreversible oxidation of hydrogen sele-
nide in nonaqueous solutions yields a highly reactive
radical cation which exhibits strong acid properties
and can be fragmented in nonaqueous solution with
proton elimination.
as the reference electrode. A 70 mm platinum plate
was used as an auxiliary electrode. The supporting
electrolyte was 0.20 M NBu ClO . The electro-
4
4
chemically prepared salt was identified on the back-
ground of 2,4,6-triphenylselenopyrylium perchlorate
prepared by the chemical procedure described in [1].
Exhaustive microelectrolysis of 2,4,6-triphenylseleno-
Electrolysis of 1,5-diketones in 4 N HCl solution
in ethanol gives the following most probable products:
pyran was performed at a constant potential E =
a
1.0 V. Microelectrolysis of 2,4,6-triphenylseleno-
pyrylium perchlorate was performed at a cathode
R
1
R
R1
Ar
R2
Ar
R2
Ar
R1
Ar
potential E = 0.8 V for 3 h.
c
H Se/HX
2
(1)
It is known [5, 6] that electrochemical preparation
of different organoselenium compounds involves for-
mation of organoelement compounds in the course of
both anodic and cathodic processes. However, the
electrochemical method, based on these reactions, for
preparing compounds with carbon element bonds is
poorly understood.
O O
Se
H Se/HX
2
R
R1
Ar
R2
2
,
Ar
HSe
Se
HSe
Usually an organic compound present in an electro-
lyte can be reduced at the cathode during electrolysis
of this solution. However, in some case another cath-
odic process involving dissolution of the cathode is
observed. This reaction is mainly typical for electro-
reduction of aldehydes, ketones, some unsaturated
compounds, and alkyl halides. The formation mech-
anism of organoelement compounds by cathodic reac-
1
2
where Ar, R = Ph; R , R = H; X = Cl, Br.
4H-Selenopyran formed at an excess of hydrogen
selenide can add the HSe group to the double bonds
of the selenopyran ring to form hydroselenium deriva-
tives. The hydroselenium derivatives can be formed
directly from 1,5-diketone by scheme (1.2).
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 81 No. 3 2008

450
DMITRIENKO, POPOVA
The GC MSD analysis of the electrolysis products
(10 mA) and voltage. Formation of 2,4,6-triphenyl-
selenopyran was observed in 1 h after starting the
electrolysis. The reaction mixture was analyzed at 2-h
intervals. Hydroselenium derivatives [2,6-(dihydro-
seleno)-2,4,6-triphenylselenacyclohexane] and the cor-
responding diselenides (hydroseleno derivatives are
oxidized with atmospheric oxygen to diselenides)
were detected by GC MSD in 5 h after starting the
electrolysis. The fact that only a very small amount of
the initial 1,5-diketone in the reaction mixture was
determined by GC suggests that the reaction goes
virtually to completion.
shows that 2,4,6-triphenylselenopyran is formed even
at a low current (6 mA) after electrolysis for 1 h.
The amount of 2,4,6-triphenylselenopyran significant-
ly increases and the amount of the initial 1,3,5-tri-
phenylpentane-1,5-dione decreases after electrolysis
for 1.5 h at this current. Further analysis performed
at 2-h intervals showed formation and gradual ac-
cumulation of 2,4,6-triphenylselenopyran in the solu-
tion and a decrease in the amount of the initial
1,5-diketone up to its complete disappearance in 8 h
after the reaction start. A flaky precipitate of 2,4,6-tri-
phenylselenopyran was obtained after completion of
the electrolysis in 4 N HCl in C H OH. However, in
A change in the electrolysis parameters (J =
15 mA, = 7 h) has no effect on the quantitative
composition of the system but accelerates formation
of the above compounds and increases their yields.
Thus, the reaction can be described by the following
scheme:
2
5
this case formation of selenopyrylium salts and the
corresponding selenacyclohexanes by disproportiona-
tion of 4H-selenopyran is also possible:
R
R
R
R2
Ar
R2
R1
Ar
R1
Ar
R1
Ar
R2
Ar
R2
R
R1
(2)
+
1
,
R
R2
R1
Ar
R
Ar
Ar
+
[O]
Se
Ar
Se
Se
Se
A
Se
2
R1
Ar
HSe
Se
Ar
Se
R
Se
2
HSe
where A is anion; Ar, R = Ph; R , R = H; X = Cl,
Br.
HSe
HSe
1
Ar
(3)
2
where Ar, R = Ph; R , R = H; X = Cl, Br.
Taking into account that fact that selenopyrylium
salts are readily soluble in alcohol, we suggest that
the precipitate is 2,4,6-triphenylselenacyclohexane
which is poorly soluble in this solvent.
Electrochemical synthesis of heterocyclic selenium-
containing compounds under conditions of acid catal-
ysis in a nonaqueous solution yields a mixture of
products and can be described by the following
scheme:
Electrolysis on a selenium-coated nickel plate as
the cathode was performed for 13 h at constant current
R
R2
Ar
R1
Ar
1
Se
R
R
R2
Ar
R1
Ar
R2
R1
Ar
H Se/HX
2
H Se/HX
2
3
(4)
Ar
SeH
HX
O O
Se
R
HSe
R
R2
R1
Ar
R2
Ar
R1
Ar
2
+
+
Se
Ar
Se
A
1
2
where A is anion; Ar, R = Ph; R , R = H; X = Cl,
Br.
atmosphere along three pathways which differ in the
extent of transformation. Reaction (4.1) yields aryl-
aliphatic 4H-selenopyrans; reaction (4.2) yields dis-
proportionation products of 4H-selenopyrans; and
Arylaliphatic 1,5-diketones react with hydrogen
selenide under conditions of acid catalysis in an inert
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 81 No. 3 2008

A NEW ELECTROCHEMICAL PROCEDURE FOR PREPARING 2,4,6-TRIPHENYLSELENOPYRAN 451
reaction (4.3) is characterized by addition of hydrogen
selenide to the double bonds of the selenopyran ring
[scheme (4)].
(3) Procedures for preparing selenium electrodes
were developed.
It should be noted that the cathode material is con-
sumed in the course of electrolysis, which is probably
due to occurrence of multistep reactions and free
radical formation. Two sharp waves at 0.93 and
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p
5
introduction of 2 10
M 2,4,6-triphenylseleno-
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(1) An electrochemical procedure was developed
for preparing 2,4,6-triphenylselenopyran and seleno-
pyrylium salts from 1,5-diketones under conditions of
acid catalysis in a nonaqueous solution on a selenium
cathode. The 2,4,6-triphenylselenopyran yield was
67%.
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RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 81 No. 3 2008