Oxidation of aromatic compounds: XV. Oxidation of arylacetylenes in a system HF-PbO2

Article abstract of DOI:10.1134/S1070428007100065

The oxidation of symmetrically substituted diarylacetylenes ArC≡CAr (Ar=C₆H₄R) containing strong electron-withdrawing groups R = 4-COMe, 4-CO₂Me, 3-CO₂Et, and 4-NO₂ in a system HF-PbO₂ at -

Full text of DOI:10.1134/S1070428007100065

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2007, Vol. 43, No. 10, pp. 1446−1450. © Pleiades Publishing, Ltd., 2007.
Original Russian Text © A.O. Shchukin, A.V. Vasil’ev, G.K. Fukin, A.P. Rudenko, 2007, published in Zhurnal Organicheskoi Khimii, 2007, Vol. 43, No. 10,
pp. 1455−1459.
Oxidation of Aromatic Compounds: XV.* Oxidation
of Arylacetylenes in a System HF−PbO2
A. O. Shchukina, A. V. Vasil’eva, G. K. Fukinb, and A. P. Rudenkoa
aSt. Petersburg Academy of Forestry Engineering, St. Petersburg, 194021 Russia
e-mail: aleksvasil@mail.ru
bRazuvaev Institute of Organometallic Chemistry, Russian Academy of Scienses, Nizhnii Novgorod, Russia
Received December 20, 2006; in final form April 06, 2007
Abstract—The oxidation of symmetrically substituted diarylacetylenesArC≡CAr (Ar=C6H4R) containing strong
electron-withdrawing groups R = 4-COMe, 4-CO2Me, 3-CO2Et, and 4-NO2 in a system HF–PbO2 at –10÷–20°C
led within 0.5–3 h to the formation of Z,Z-1,2,3,4-tetrakis(aryl)-1,4-difluorobuta-1,3-dienes ArFC=C(Ar)–
(Ar)C=CFAr. The butadiene structures obtained exist in solutions as s-cis- and s-trans-conformers and in the
crystalline state are present in the stable s-cis-form.
DOI: 10.1134/S1070428007100065
Alkynes oxidation is widely employed in the organic
synthesis [2]. Single-electron transfer reactions involving
acetylene derivatives make it possible to prepare quite a
number of synthetically valuable multifunctional com-
pounds [1, 3–7].
fluoride at –10÷–20°C for 0.5–3 h to give (Z,Z)-1,2,3,4-
tetrakis(aryl)-1,4-difluorobuta-1,3-dienes IIa–IId
through a stage of formation of cation radicals Aa–Ad.
The oxidation under the same conditions (–20°C, 3 h) of
1,2-diphenylacetylene (tolan) (Ie) according to the data
1
of H and 19F NMR spectra provided a mixture of
We formerly demonstrated that the oxidation of
diarylacetylenes in a system CF3CO2H–CH2Cl2–PbO2
proceeding through an intermediate formation of
diarylacetylenes cation radicals resulted in the products
of oxidative dimerization, 1,2,3,4-tetraarylbut-2-ene-1,4-
diones [4–6]. The essential drawback of this system was
insufficient oxidative potential of the lead dioxide for
efficient activation of diarylacetylenes into the cation
radical state when the diarylacetylenes contained two
strong electron-withdrawing groups COMe, CO2Me,
NO2, CN etc. [4–6]. The potential of PbO2 is enhanced
in the presence of acids stronger than CF3CO2H, namely,
of HSO3F and HF [8, 9]. The chemical [10] and
electrochemical [11] oxidation of organic compounds in
an anhydeous liquid hydrogen fluoride is of special
importance for the preparation of fluorinated derivatives.
polyfluorinated oligomeric products. The process carried
out in the presence of a nucleophilic additive , sodium
fluoride (molar ratio tolan:NaF:HF = 1:2:50) also resulted
in oligomeric compounds. The analogous reaction of
diarylacetylene Id in the system HF–NaF–PbO2 did not
increase the yield of the target product IId as compared
with the oxidation in the system HF–PbO2 (see
EXPERIMENTAL).
Scheme 1.
The target of the present study was investigation of
reactions of diarylacetylenes with strong electron-
withdrawing groups in the system HF–PbO2.
Scheme 1 represents the oxidation of compounds Ia–
Id with lead dioxide in anhydrous liquid hydrogen
Ar = C6H4R; R = 4-COMe (a), 4-CO2Me (b), 3-CO2Et (c),
4-NO2 (d).
* For Communication XIV, see [1].
1446

OXIDATION OF AROMATIC COMPOUNDS: XV.
1447
(a)
Table 1. Ratio of s-cis- and s-trans-isomers of compounds
IIa–IId in CDCl3 at 20°C
C26
C27
C23
C18
C14
O2
Isomer content, %
s-trans
O3
Compd. no.
s-cis
C17
C15
C16
C24
C22
IIa
IIb
IIc
IId
74
73
75
77
26
27
25
23
C13
C12
C11
C25
C20
C19
C21
C10
C28
C29
F2
C1
Table 2. 19F NMR spectra and the ratio of s-cis- and s-trans-
isomers in solution of (Z,Z)-1,2,3,4-tetrakis(4-methylcarbonyl-
phenyl)-1,4-difluorobuta-1,3-diene (IIa) in (CD3)2SO at
various temperatures
F1
C34
C3
C2
C33
C32
C30
C7
C6
C4
Isomer content, % (δ, ppm)
T, °C
C31
C35
C36
C5
s-cis
s-trans
O4
C8
C9
20
60
90
74 (–93.15)
67 (–93.56)
56 (–93.15)
26 (–145.38)
33 (–145.98)
44 (–145.38)
O1
(b)
C9
C8
X-ray diffraction analysis revealed that compound IIa
existed in the crystalline state solely in the s-cis-con-
formation (see the figure). However the 1H and 19F NMR
spectra of solutions of compounds IIa–IId in CDCl3
contain additional signals due to the presence in the
solutions of dienes IIa–IId alongside the prevailing s-
cis- conformers also s-trans- (minor) conformers [12]
(Scheme 1). The ratio of s-cis- and s-trans-forms of
compounds IIa–IId estimated from the areas of the
O1
C5
C4
C6
C7
C3
F1
C2
C1
C10
1
corresponding signals in the H and 19F NMR spectra
C31
C32
C28
C29
C36
C19
(Table 1) was virtually independent of the substituents.
C35
D4
C30
In solutions the s-cis- and s-trans-isomers are
involved in a reversible temperature equilibrium.As seen
from Table 2, at growing temperature the fraction of the
s-trans-conformer IIa increased.
F2
C33 C34
Molecular structure of (Z,Z)-1,2,3,4-tetrakis(4-methylcarb-
onylphenyl)-1,4-difluorobuta-1,3-diene (IIa) (a). A fragment
of IIa molecule demonstrating the mutual position of aryl rings
C2-7 and C29-34 (b). Main bond lengths in the butadiene
fragment of structure IIa: F1–C1 1.365(2), F2–C28 1.360(2),
C1–C10 1.329(2), C10–C19 1.493(2), C19–C28 1.335(2) Å.
The presence in solution of s-cis- and s-trans-
conformers simultaneously is due to the hindered rotation
around the central C–C bond of the butadiene system in
compounds IIa–IId caused by the four streically bulky
aryl substituents contained in the structure. Only the
s-cis-form quantitatively crystallizes from solution which
is probably stabilized by the π−π interaction [13] between
aromatic systems in positions 1 and 4 of the butadiene
system. The analysis of the structure of molecule IIa
shows that the aryl rings C2-7 and C29-34 are partially
located over each other (see the figure). The dihedral
angle between the planes of these rings is 30.1 deg. The
shortest distances between the center of C29-34 ring and
atoms C7, H7 of the other aromatic ring are respectively
3.887 and 3.89 Å; according to [13] this fact may
evidence an additional H-π interaction. This is indirectly
confirmed by the deviation of the fluorine atoms from
the corresponding planes of the aromatic rings. Torsion
angles F1C1C2C7 and F1C28C29C34 are respectively –32.1
and 31.2 deg indicating the turn of the aryl fragments
with respect to the C1–C2 and C28–C29 bonds. Diene
fragment C1C10C19C28 does not lie in a common plane
due to the significant spatial dimensions of the four aryl
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 10 2007

SHCHUKIN et al.
1448
Scheme 2.
groups. Therefore the C–C distances within this fragment
correspond to a system with an incomplete conjugation.
HF_PbO2
.
+
A large difference in the chemical shifts of fluorine
signals (about 50 ppm) in the 19F NMR spectra of s-cis-
and s-trans-isomers IIa–IId stands out (see EXPERI-
MENTAL and Table 2). These findings are consistent
with published data [14] demonstrating that depending
on intramolecular surrounding of fluorine atoms in the
terminal positions of various conjugated butadiene
systems their signals appear in the 19F NMR spectra in
a wide range of chemical shifts (δ –92÷–143 ppm).
Ar
Ar
Ar
Ar
_e
А
I
Ar
Ar
Ar
+
Ar
I
2 HF
_2H+, _e
.
F
F
Ar
Ar
Ar Ar
II
B
a 10.7972(6), b 11.3357(7), c 12.1028(7) Å,
α 90.056(1), β 102.155(1), γ 97.440(1) deg,
V 1435.30(15) Å3, Z 2, space group P-1, dcalc 1.302 g/
cm3, μ 0.093 mm–1, 1.72 ≤ θ ≤ 26.00°, 12432 reflections
were measured, among them 5615 independent reflec-
tions (Rint 0.0245), R1 0.0460 [I > 2σ(I)], wR2 0.1047
(for all reflections). The structure was solved by the direct
The presumable mechanism of formation of diene
structure II (Scheme 2) involves the interaction of the
primary cation radical A with the molecule of the initial
substrate resulting in generation of a new carbon-carbon
bond between the acetylene carbon atoms [1]. The
intermediate cation radical of B type suffers further a
single-electron oxidation and a reaction with HF
molecules to give the final difluoride II. Therewith the
overall pathway of initial compound I transformation into
the product of the oxidative dimerization II includes a
transfer of two electrons (Scheme 2) (cf. with data of
[1]). Further oxidation of dienes IIa–IId in the system
HF–PbO2 is hampered by the deactivating effect of four
aryl fragments with strong electron-withdrawing groups.
method and refined by the least-squares method for F2
hkl
in anisotropic approximation for all nonhydrogen atoms.
The hydrogen atoms were obtained from the difference
Fourier synthesis and were refined isotropically. All
calculations were carried out with the use of SHELXTL
v. 6.10 software [15].
The preparation procedures and properties of 1,2-
bis(4-methylcarbonylphenyl)acetylene (Ia), 1,2-bis(4-
methoxycarbonylphenyl)acetylene (Ib), 1,2-bis(3-
ethoxycarbonylphenyl)acetylene (Ic), 1,2-bis(4-nitro-
phenyl)acetylene (Id), and 1,2-diphenylacetylene (Ie) we
published before [4, 5].
EXPERIMENTAL
1H and 19F NMR spectra were registered on a spec-
trometer Bruker AM-500 at operating frequencies 500
and 470.7 MHz respectively in solutions of CDCl3,
(CD3)2SO, or (CD3)2CO. As internal references in
1H NMR spectra residual signals were used of CHCl3 (δ
7.25 ppm), (CD3)(CD2H)SO (δ 2.50 ppm), and
(CD3)(CD2H)CO (δ 2.05 ppm), in 19F NMR spectrum,
signal of CFCl3 (δ 0.0 ppm). IR spectra were recorded
on a spectrophotometer Specord 75IR from solutions of
compounds in CHCl3. Mass spectra were measured on
MKh-1321 instrument, ionizing electrons energy 70 eV,
direct admission in the ion source at the heater tempera-
ture 100–120°C.
Diarylacetylenes Ia–Id oxidation in a system HF–
PbO2. General procedure. In 2.5–10 ml of anhydrous
liquid hydrogen fluoride at –10÷–20°C was dissolved
0.2–0.75 mmol of acetylene compound Ia–Id, 0.2–
0.75 mmol of PbO2 was added, and the reaction mixture
was stirred for 0.5–3 h at –10÷–20°C. On completion of
the reaction the mixture was kept in an open vessel under
a strong hood at 25°C to evaporate HF. The residue was
treated with a saturated NaHCO3 solution (100 ml) and
extracted with chloroform (3×50 ml). The extract was
washed with water solution of NaHCO3 and with water,
dried over Na2SO4, the solvent was distilled off under a
vacuum of a water-jet pump. The solid residue was
subjected to separation by preparative TLC on silica gel,
eluent hexane–chloroform. Yield of compounds IIa–IId
was estimated from the weight of fractions obtained by
chromatography.
X-ray study of the single crystal of compound IIa
was carried out on an automatic diffractometer Smart
APEX (graphite monochromator, MoKα radiation, ω−θ
scanning). A single crystal of compound IIa of dimen-
sions 0.32×0.18×0.10 mm for X-ray study (see the figure)
was obtained by keeping at room temperature for several
days a solution of compound IIa in DMSO saturated at
50–60°C. Crystals of C36H28F2O4 at 293(2) K triclinic,
(Z,Z)-1,2,3,4-Tetrakis(4-methylcarbonylphenyl)-
1,4-difluorobuta-1,3-diene (IIa) was obtained by
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 10 2007

OXIDATION OF AROMATIC COMPOUNDS: XV.
1449
1
oxidation of 53 mg (0.2 mmol) of diarylacetylene Ia
with 48 mg (0.2 mmol) of PbO2 in 2.5 ml HF at –20°C
within 0.5 h. Yield 20 mg (36%), mp 248–250°C.
s-cis-Form. H NMR spectrum (CDCl3), δ, ppm: 1.32
t (6H, 2Me, J 7.0 Hz), 1.37 t (6H, 2Me, J 7.1 Hz), 4.29 q
(4H, 2CH2, J 7.0 Hz), 4.37 q (4H, 2CH2, J 7.1 Hz), 7.13–
8.61 m (16Harom).
Here and hereinafter the spectra of s-cis- and s-trans-
forms were extracted from the spectra of mixtures.
s-cis-Form. 1H NMR spectrum (CDCl3), δ, ppm:
2.55 s (6H, 2COMe), 2.56 s (6H, 2COMe), 7.24 d
1
s-trans-Form. H NMR spectrum (CDCl3), δ, ppm:
1.33 t (6H, 2Me, J 7.0 Hz), 1.38 t (6H, 2Me, J 7.1 Hz),
4.30 q (4H, 2CH2, J 7.0 Hz), 4.38 q (4H, 2CH2, J 7.1
Hz), 7.13–8.61 m (16Harom). Mass spectrum: m/z 682
[M]+. Found, %: C 70.12; H 5.42. C40H36F2O8. Calculat-
ed, %: C 70.37; H 5.32. M 682.24.
(4Harom, J 8.4 Hz), 7.70 d (4Harom, J 8.4 Hz), 7.73 (4Harom
,
1
J 8.3 Hz), 7.90 (4Harom, J 8.3 Hz). H NMR spectrum
[(CD3)2SO], δ, ppm: 2.54 s (6H, 2COMe), 2.55 s (6H,
2COMe), 7.22 d (4Harom, J 8.3 Hz), 7.79 d (4Harom
,
(Z,Z)-1,2,3,4-Tetrakis(4-nitrophenyl)-1,4-difluoro-
buta-1,3-diene (IId) was obtained by oxidation of
200 mg (0.75 mmol) of diarylacetylene Id with 180 mg
(0.75 mmol) of PbO2 in 10 ml HF at –10°C within 3 h.
Yield 45 mg (21%), mp 170–173°C. s-cis-Form. 1H NMR
spectrum [(CD3)2CO], δ, ppm: 7.56 d (4Harom, J 8.7 Hz),
7.65 d (4Harom, J 8.2 Hz), 8.00 d (4Harom, J 8.7 Hz),
8.35 d (4Harom, J 8.2 Hz). 19F NMR spectrum [(CD3)2CO],
δ, ppm: –91.54 s.
J 8.3 Hz), 7.85 (4Harom, J 8.3 Hz), 7.94 (4Harom, J 8.3 Hz).
19F NMR spectrum (CDCl3), δ, ppm: –91.17 s.
1
s-trans-Form. H NMR spectrum (CDCl3), δ, ppm:
2.58 s (6H, 2COMe), 2.61 s (6H, 2COMe), 7.59 d (4Harom
,
,
J 8.5 Hz), 7.71 d (4Harom, J 8.4 Hz), 7.94 d (4Harom
1
J 8.5 Hz), 8.00 d (4Harom, J 8.4 Hz). H [(CD3)2SO], δ,
ppm: 2.58 s (6H, 2COMe), 2.60 s (6H, 2COMe), 7.65 d
(4Harom, J 8.4 Hz), 7.73 d (4Harom, J 8.6 Hz), 8.02 d
(4Harom, J 8.6 Hz), 8.05 d (4Harom, J 8.4 Hz). 19F NMR
spectrum (CDCl3), δ, ppm: –145.92 s. Mass spectrum:
m/z 562 [M]+. Found, %: C 76.98; H 4.90. C36H28F2O4.
Calculated, %: C 76.85; H 5.02. M 562.20.
1
s-trans-Form. H NMR spectrum [(CD3)2CO], δ,
ppm: 7.42 d (4Harom, J 8.7 Hz), 7.91 d (4Harom, J 8.6 Hz),
8.10 d (4Harom, J 8.7 Hz), 8.25 d (4Harom, J 8.2 Hz).
19F NMR spectrum [(CD3)2CO], δ, ppm: –145.95 s. Mass
spectrum: m/z 574 [M]+. Found, %: C 57.98; H 3.03;
N 10.02. C28H16F2N4O8. Calculated, %: C 58.54; H 2.81;
N 9.75. M 574.09.
(Z,Z)-1,2,3,4-Tetrakis(4-methoxycarbonylphenyl)-
1,4-difluorobuta-1,3-diene (IIb) was obtained by
oxidation of 136 mg (0.46 mmol) of diarylacetylene Ib
with 111 mg (0.46 mmol) of PbO2 in 5 ml HF at –20°C
within 1 h. Yield 26 mg (18%), mp 198–201°C. IR
spectrum, ν, cm–1 (conformers mixture):1710 (C=O),
1600 (C=C).
The study was carried out under a financial support
of the Federal Center of Scientific and Technical Progress
“Research and Development on the Priority Directions in
Science and Technology” for 2002–2006 (NIR ¹ 1.4.05.06,
IMKh RAN and SPbGLTA) and of the Russian Founda-
tion for Basic Research (grant 06-03-32728-a).
1
s-cis-Form. H NMR spectrum (CDCl3), δ, ppm:
3.88 s (6H, 2CO2Me), 3.90 s (6H, 2CO2Me), 7.20 d
(4Harom, J 8.4 Hz), 7.70 d (4Harom, J 8.2 Hz), 7.79 d
(4Harom, J 8.4 Hz), 7.98 d (4Harom, J 8.2 Hz). 19F NMR
spectrum (CDCl3), δ, ppm: –91.30 s.
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