Free radicals: XXVIII. Reaction of 2,4,6-triphenylpyranyl with (diacetoxy-λ3-iodanyl)benzene

Article abstract of DOI:10.1134/S1070428011030201

The major product of the reaction of 2,4,6-triphenylpyranyl with (diacetoxy-λ³-iodanyl)benzene in acetonitrile and acetone was 2,4,6-triphenylpyrylium acetate. Analogous reaction in isopropyl alcohol resulted in the formation of methane, carbon dioxide, 1,3,5-triphenylpent-2-ene- 1,5-dione, 2-benzoyl-3,5-diphenylfuran, 1,3,5-triphenylpenta-2,4-dien-1-one, and a small amount of 2,4,6-triphenylpyrilium acetate.

Full text of DOI:10.1134/S1070428011030201

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2011, Vol. 47, No. 3, pp. 442–445. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © B.S. Tanaseichuk, M.K. Pryanichnikova, A.A. Burtasov, A.I. Lisina, A.V. Dolganov, 2011, published in Zhurnal Organicheskoi
Khimii, 2011, Vol. 47, No. 3, pp. 447–450.
Free Radicals: XXVIII.* Reaction of 2,4,6-Triphenylpyranyl
with (Diacetoxy-λ3-iodanyl)benzene
B. S. Tanaseichuka, M. K. Pryanichnikovaa, A. A. Burtasova, A. I. Lisinaa, and A. V. Dolganovb
a Ogarev Mordovian State University, ul. Bol’shevistskaya 64, Saransk, 430000 Russia
e-mail: mordorgchem@mail.ru
b Nesmeyanov Institute of Organometallic Compounds, Russian Academy of Sciences,
ul. Vavilova 28, Moscow, 119991 Russia
Received March 12, 2010
Abstract—The major product of the reaction of 2,4,6-triphenylpyranyl with (diacetoxy-λ3-iodanyl)benzene in
acetonitrile and acetone was 2,4,6-triphenylpyrylium acetate. Analogous reaction in isopropyl alcohol resulted
in the formation of methane, carbon dioxide, 1,3,5-triphenylpent-2-ene-1,5-dione, 2-benzoyl-3,5-diphenylfuran,
1,3,5-triphenylpenta-2,4-dien-1-one, and a small amount of 2,4,6-triphenylpyrilium acetate.
DOI: 10.1134/S1070428011030201
Chemistry of peroxy compounds, in particular
peroxycarboxylic acids, attracts considerable interest
due to their use as radical initiators [2]. At present, the
main procedure for the synthesis of acetoxyl radicals
is thermal decomposition of peroxy acids. However,
decomposition of acetoxyl radical with liberation of
carbon dioxide under these conditions is a very fast
process, so that a number of authors [3] believe that
such radicals cannot exist outside solvent cage. In this
connection, search for new methods for generation of
acetoxyl radicals seems to be quite important. A prob-
able procedure is that involving electron-rich [4] 2,4,6-
triphenylpyranyl (I) as electron donor and (diacetoxy-
λ3-iodanyl)benzene (II) as acceptor (the latter is widely
used as oxidant) [5–7].
nitrile, and propan-2-ol. The reactions in acetone and
acetonitrile were carried out at slightly elevated tem-
perature (~35°C) in a carbon dioxide atmosphere,
taking into account that 2,4,6-triphenylpyranyl (I) can
be oxidized with atmospheric oxygen [8]. The end of
the process was determined by disappearance of the
red color typical of the initial radical. In fact, pyranyl
radical I reacted with (diacetoxy-λ3-iodanyl)benzene
(II) in acetone and acetonitrile as shown in Scheme 1,
and acetoxyl radical thus generated reacted with initial
radical I to form pyrylium acetate III. At a I-to-II ratio
of 2:1, the major product was pyrylium salt III (80–
90%), and the reaction was accompanied by insignif-
icant gas evolution (less than 0.1 mol of a CO2–CH4
mixture per mole of compound II).
We presumed that the reaction of radical I with
compound II could give rise to acetoxyl radical IV
(Scheme 1) and that the latter could participate in
various reactions, including reactions with both radical
I and solvent. As solvents we tried acetone, aceto-
The reaction of radical I with compound II with
propan-2-ol was performed under carbon dioxide at
60°C. The reaction was accompanied by vigorous gas
evolution, and termination of the latter process indicat-
ed that the reaction was complete. In this case,
Scheme 1.
Ph
Ph
·
AcO–
Ph
+
PhI
+
AcO
·
+
PhI(OAc)2
Ph
O
Ph
Ph
O
I
II
III
IV
* For communication XXVII, see [1].
442

FREE RADICALS: XXVIII.
443
ν, mmol
pyrylium acetate III was formed in an insignificant
amount; therefore, the reaction of triphenylpyranyl I
with compound II was studied in more detail at a con-
stant concentration of the former and different concen-
trations of the latter. At a II-to-I molar ratio of larger
than or equal to unity, the reaction solutions lost their
red color due to pyranyl I when the reaction was com-
plete (gas evolution ceased), whereas the reaction solu-
tions remained red at II-to-I molar ratio of less than
unity.
A solid separated from the solution was identified
as 1,3,5-triphenylpent-2-ene-1,5-dione (V). According
to the GLC and TLC data, the mother liquor contained
triphenylpyrylium acetate (III), acetone, acetic acid,
iodobenzene, and a number of products resulting from
transformations of 2,4,6-triphenylpyranyl (I); among
the latter, we isolated and identified (by IR and 1H and
13C NMR spectra and comparison with authentic
samples) 1,3,5-triphenylpent-2-ene-1,5-dione (V),
1,3,5-triphenylpenta-2,4-dien-1-one (VI), and 2-ben-
zoyl-3,5-diphenylfuran (VII).
0.4
0.3
0.2
0.1
0.0
AcOH
V
0.0
0.5
1.0
Ratio II:I
Fig. 1. Dependence of the amounts of acetic acid and
1,3,5-triphenylpent-2-ene-1,5-dione (V) on the reactant ratio
II:I (vI = 0.667 mmol).
estimate temperature effect on the product composition
(Fig. 2). As follows from the above relations (Fig. 1),
the reaction in propan-2-ol is complete at equimolar
amounts of the reactants (compounds I and II). There-
fore, we concluded that this reaction leads to the forma-
tion of compounds III and IV and that compounds V
and VII are formed as a result of further transforma-
tions of 2,4,6-triphenylpyrylium acetate (III).
Ph
Ph
While considering solvent effect in the reaction
under study, it should be noted that solvents charac-
terized by low viscosity, such as acetone (η = 0.30 cP)
and acetonitrile (η = 0.33 cP), facilitate escape of
acetoxyl (IV) from solvent cage, and radical IV reacts
with pyranyl I to produce pyrylium acetate III. Quite
different pattern is observed with more viscous
propan-2-ol (η = 1.77 cP) as solvent; it may hamper
escape of acetoxyl (IV) from solvent cage. Acetoxyl
radical IV can either undergo decarboxylation to give
Ph
Ph
Ph
Ph
O
O
O
V
VI
Ph
Ph
Ph
O
O
VII
1
The amount of liberated gaseous products depends
on the ratio of pyranyl I and compound II; it increases
in parallel with the amount of I and remains constant
when the ratio I:II reaches 1:1. Analogous depend-
ence is observed for acetic acid (Fig. 1). Excess com-
pound II over I after attainment of their equimolar
ratio does not affect the amount and composition of the
gaseous products (methane and carbon dioxide).**
However, the amount of pentenedione V which could
be formed only from pyranyl I [8–10] decreases as the
reactant ratio increases and remains constant when the
reactant ratio becomes equimolar (Fig. 1).
0.8
2
0.6
3
4
0.4
5
0.2
0.0
30
40
50
60
Temperature, °C
We also examined the reaction of pyranyl radical I
with compound II at a reactant ratio of 0.67 : 1
(mmol/mmol) at different temperatures with a view to
Fig. 2. Temperature dependences of (1) overall amount of
acetic acid and carbon dioxide, (2) overall amount of
methane and acetic avid, (3) amount of carbon dioxide,
(4) amount of methane, and (5) amount of acetic acid formed
in the reaction of 2,4,6-triphenylpyranyl (I) with (diacetoxy-
λ3-iodanyl)benzene (II) in propan-2-ol; ratio I:II = 0.667:1.
** These compounds are formed as the major products in thermal
decomposition of acetyl peroxide in isopropyl alcohol [9].
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 3 2011

TANASEICHUK et al.
444
methyl radical and carbon dioxide or abstract a hydro-
gen atom from isopropyl alcohol with formation of
acetic acid and isopropoxyl radical (VIII). Methyl
radical is also capable of abstracting hydrogen from
isopropyl alcohol, yielding methane and radical VIII
(Scheme 2).
propyl alcohol. The major product of reactions with
nucleophiles is usually 1,3,5-triphenylpent-2-ene-1,5-
dione (V) [10, 12, 13]. For instance, compound V was
formed together with VI and VII in the reaction of
2,4,6-triphenylpyrylium tetrachloroferrate with sodium
nitrite in isopropyl alcohol [13], and the yield of V
changed from 14 to 60%, depending on the conditions.
The same compound was formed in analogous reac-
tions performed using different alcohols as solvents
[13]. 2-Benzoyl-3,5-diphenylfuran (VII) is likely to be
formed as a result of mild oxidation of V [13]. Struc-
turally related 2-acetyl-3,5-dimethylfuran was obtained
by oxidation of 2,4,6-trimethylpyrylium perchlorate
with hydrogen peroxide [14].
Scheme 2.
·
·
+
AcO
Me
CO2
·
·
AcO
+
Me2CHOH
AcOH
+
Me2COH
VIII
VIII
·
Me
+
Me2CHOH
CH4 +
The reaction direction depends on the temperature
(Fig. 2). At low temperature predominant formation of
acetic acid is observed, whereas at elevated tempera-
ture the main process is decarboxylation of acetoxyl
(IV) with formation of methane.
EXPERIMENTAL
Gas chromatographic analysis was performed on
Chrom-42 and Kristall LYuKS 2000 chromatographs
equipped, respectively, with nitrogen phosphorus and
flame ionization detectors; glass columns, 3000×3.5
and 1500×3.5 mm; stationary phase 3% of OV-17 on
Chromaton N-Super (0.16–0.20 mm); oven tempera-
ture 40, 170, and 250°C; injector temperature 100,
240, and 300°C; carrier gas nitrogen, flow rate 40 ml×
The sum of the mole fractions of methane and ace-
tic acid remains almost constant (~0.65–0.68 mmol) at
different temperatures (Fig. 2); therefore, we presumed
that both these products originate mainly from radical
IV. Apart from transformation into pyrylium acetate
III, triphenylpyranyl I can be involved in other reac-
tions. In particular, isopropoxyl (VIII) can reduce pyr-
anyl radical I to 1,3,5-triphenylpenta-2,4-dien-1-one
(VI) which is also formed via reduction of pyrylium
salts [11] (Scheme 3).
min–1. The H and 13C NMR spectra were recorded
1
from solutions in CD2Cl2 on a Bruker AMX-400 spec-
trometer at 400 and 100 MHz, respectively. TLC
analysis was performed on Silufol UV-254 plates using
toluene, petroleum ether, benzene, chloroform, or
CCl4–benzene (1:1) as eluent; spots were detected by
treatment with iodine vapor and UV irradiation.
Scheme 3.
Ph
Compounds I [15] and II [16] were synthesized by
known methods. Authentic samples of V, VI, and VII
were prepared as described in [17, 12, 18], respec-
tively. Solvents were purified according to standard
procedures [19].
·
·
+
Me2COH
Ph
O
Ph
I
Ph
Ph
Reaction of 2,4,6-triphenylpyranyl (I) with (di-
acetoxy-λ3-iodanyl)benzene (II). The reaction was
carried out in a setup analogous to the Zerewitinoff–
Hölscher setup for microdetermination of active hydro-
gen [20]. One arm of the setup was charged with
a mixture of compounds I and II at a required ratio,
an the other arm was charged with 5 ml of the corre-
sponding solvent. The reaction vessel was adjusted to
35°C (with acetone or acetonitrile as solvent) or 60°C
(propan-2-ol), and dry carbon dioxide was bubbled
through the solvent over a period of 30 min. The sol-
vent was then transferred into the arm containing the
reactants. When the reaction was complete (red color
disappeared), the volume of the evolved gas was meas-
–Me2CO
Ph
Ph
O
Ph
O
Ph
H
VI
Pyrylium acetate III (which was expected to be the
major product of the reaction of radical I with com-
pound II in propan-2-ol) was formed in a very small
amount, and its yield decreased as the temperature
rose: ~7% at 35°C, ~4% at 40°C, ~2% at 50°C, and
~1% at 60°C. These data may be rationalized only
assuming ready reaction of pyrylium salts with nucleo-
philes [10–13]; the latter may be acetate ion and iso-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 3 2011

FREE RADICALS: XXVIII.
5. Wirth, T. and Hirt, U.H., Synthesis, 1999, p. 1271.
445
ured, and carbon dioxide was absorbed by alkali. The
remaining gaseous product was methane.
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Analysis of the reaction mixture (in acetone or
acetonitrile) by GLC showed the presence of acetic
acid (9 and 7%, respectively). The mole fraction of
acetic acid was calculated with respect to iodobenzene
whose yield was arbitrarily assumed to be 100%. After
evaporation of the reaction mixture, the residue was
almost pure 2,4,6-triphenylpyrylium acetate (III; 87
and 90%, respectively).
When the reaction was carried out in propan-2-ol,
the mixture was kept for 24 h on exposure to air, and
the precipitate of 1,3,5-triphenylpenta-2,4-dien-1-one
(VI) was filtered off. The filtrate contained (GLC)
iodobenzene, acetic acid, and acetone. The fraction of
acetic acid was calculated with respect to iodobenzene
whose yield was arbitrarily assumed to be 100%. The
filtrate was evaporated to dryness, and the residue was
treated with diethyl ether. The undissolved material
was 2,4,6-triphenylpyrylium acetate (III). The ether
extract was evaporated under reduced pressure, and the
residue was subjected to chromatography on silica gel
(40–100 μm) using benzene as eluent. We isolated
three fractions: 1,3,5-triphenylpent-2-ene-1,5-dione
(V), mp 120–122°C [12], Rf 0.45; 1,3,5-triphenyl-
penta-2,4-dien-1-one (VI), mp 124–126°C [17],
Rf 0.60; and 2-benzoyl-3,4-diphenylfuran (VII),
mp 117°C [18], Rf = 0.70.
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