Oxidation of aromatic compounds: XVI. Radical cations derived from acetylenic compounds with electron-withdrawing groups: Reactions and ESR parameters

Article abstract of DOI:10.1134/S1070428008060031

One-electron oxidation of aryl-substituted acetylenes ArC≡CX where X is an electron-withdrawing group gives different products, depending on the X substituent. Acetylenic substrates with medium-strength electron-withdrawing substituents, X = CO₂

Full text of DOI:10.1134/S1070428008060031

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2008, Vol. 44, No. 6, pp. 791–802. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © A.V. Vasil’ev, S.A. Aristov, G.K. Fukin, K.A. Kozhanov, M.P. Bubnov, V.K. Cherkasov, 2008, published in Zhurnal Organicheskoi
Khimii, 2008, Vol. 44, No. 6, pp. 802–813.
Oxidation of Aromatic Compounds: XVI.* Radical Cations
Derived from Acetylenic Compounds with Electron-Withdrawing
Groups: Reactions and ESR Parameters
A. V. Vasil’eva, S. A. Aristova, G. K. Fukinb, K. A. Kozhanovb,
M. P. Bubnovb, and V. K. Cherkasovb
a St. Petersburg State Academy of Forestry Engineering, Institutskii per. 5, St. Petersburg, 194021 Russia
e-mail: aleksvasil@mail.ru
b Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
ul. Tropinina 49, Nizhnii Novgorod, 603950 Russia
Received November 16, 2007
Abstract—One-electron oxidation of aryl-substituted acetylenes ArC≡CX where X is an electron-withdrawing
group gives different products, depending on the X substituent. Acetylenic substrates with medium-strength
electron-withdrawing substituents, X = CO2R, COAr, COR, PO(OEt)2, give rise to tetrasubstituted ethenes
X(ArCO)C=C(COAr)X. Compounds with strong electron-withdrawing groups (X = COCF3, COCO2R, CN) are
converted into furan derivatives. Probable mechanisms of transformations of ArC≡CX radical cations into
the final products are discussed. Radical cations derived from disubstituted acetylenes were characterized by
ESR spectroscopy.
DOI: 10.1134/S1070428008060031
Organic radical cations are formed as intermediates
in many chemical transformations, and reactions with
participation of radical cations are widely used in the
synthesis of new compounds and materials [2–13].
Studies on the properties and reactivity of radical
cations are very important from both theoretical and
practical viewpoints. Our studies on the oxidation of
acetylene derivatives with intermediate formation of
radical cations open synthetic routes to new classes of
organic compounds [1, 14–21]. The present article
reports on reactions of radical cations derived from
acetylenic compounds having strong electron-with-
drawing groups that are directly attached to the triple-
bonded carbon atom and the results of ESR study on
such radical cations.
a medium-strength electron acceptor [X = CO2R,
COAr, COR, PO(OEt)2, EN = 7.1–7.5, σp ≤ 0.57], the
oxidation of the corresponding substrates I leads to
the formation of tetrasubstituted ethenes II [16–20].
Table 1. Group electronegativities (EN) and Hammett con-
stants σp for electron-withdrawing groups X
Group X
EN [22]
σp [23]
COMe
COPh
CO2Me
PO(OEt)2
COCF3
CF3
7.1
0.38–0.51
0.42–0.51
0.38–0.48
b0.57b
a
–
7.5
a
–
7.6
7.7
7.9
0.80
0.48–0.58
0.63–0.72
c0.71c
One-electron oxidation of arylacetylenes I having
various groups X in the system CF3COOH–CH2Cl2–
PbO2 (Scheme 1) gives two types of products, depend-
ing on the electron-withdrawing power of the X sub-
stituent [16–21]. Table 1 contains such parameters of
different groups X as their electronegativities (EN) and
Hammett constants σp [22–25]. If the substituent X is
CN
a
SO2Ph
SO2Me
SO2CF3
NO2
–
9.0
9.4
9.5
c0.67–0.75c
0.91
0.73–0.93
a
b
c
No data.
Data of [24].
Data of [25].
* For communication XV, see [1].
791

VASIL’EV et al.
792
Scheme 1.
O
X
Ar
1
2'
1'
2
–2e
Ar
X
Ar
O
X
2'
A
1
II
1'
a
2
X
O
O
2'
Ar
F3C
Ar
1
B
1'
X = COCF3
2
–e
Ar
CF3
O
Ar
III
·
X
X
2
C
1
C
I
2'
·
1
Ar
X
Ar
C
C
X
–e
b
1'
2
X
I
A
Ar
C
D
X
·
2'
I
1
1'
c
2
Ar
Ar
II, X = CO2R, COAr, COR, PO(OR)2.
Stronger acceptors (e.g., X = COCF3) give rise to furan
derivatives III [21].
of two intermediate radical cations C and D is possible
as a result of C1–C1′ and C1–C2′ couplings, respectively
(paths b and c in Scheme 1). The unpaired electron in
intermediates C and D is delocalized over different
structural fragments, aryl group and electron-with-
drawing substituent X, respectively.
We proposed in [21] that the key intermediate
formed along the transformation of initial compounds I
into products II and III is dication B which can be
generated in two ways. Among the latter, the most
probable is that involving combination of two radical
cations A (path a in Scheme 1). According to [26],
radical cations like A, in which the electron-with-
drawing (X) and electron-donating groups (alkyl- or
methoxyaryl) are conjugated through double C=C or
triple C≡C bonds, are expected to feature sharp separa-
tion of the unpaired electron and positive charge be-
tween the the C1 and C2 atoms. The radical center on
the C1 atom is essentially stabilized by the electron-
withdrawing group X, while the cationic center on C2
is stabilized by the electron-donating aryl group, as
shown in Scheme 1 for one resonance structure of
radical cation A. This separation makes the C1 atom in
A a reactive vinyl type radical center, and new carbon–
carbon bond is formed with participation of just that
carbon atom in the combination of two species A (path
a in Scheme 1). Such dimerizations are typical for
various radical cations [27–29].
Vinyl radicals are stabilized by aryl and electron-
withdrawing groups in the α-position, and they have
linear structure with the sp-hybridized radical center
[30, 31]. Both electron-donating and electron-with-
drawing substituents increase thermodynamic stability
of radical species [32, 33]. However, attack by vinyl
[34] and trichloromethyl radicals [35] on double C=C
bonds in ethenes having electron-withdrawing groups
follows mainly a path leading to intermediate species
in which the radical center is directly conjugated with
the electron-withdrawing group, i.e., like path c in
Scheme 1. The effect of electron-donating substituents
on delocalization of unpaired electron in these reac-
tions is not significant [35]. The same factors may
determine the stability of radical cation intermediates
A, C, and D and pathways of their subsequent trans-
formations, especially taking into account high degree
of separation of the positive charge and unpaired elec-
tron density therein [26]. Presumably, path c is pre-
ferred in the reaction of radical cation A with initial
Another reaction path of radical cation A is its
reaction with neutral precursor I. In this case formation
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 6 2008

OXIDATION OF AROMATIC COMPOUNDS: XVI.
C2
793
(a)
C1
O4A
O1
C3
O2
C4
P1
O3
C13
C14
C9
C10
C8
C11
C7
C12
C13
C6
O4
O3A
P1A
O2A
O1A
(b)
P1AC
C13E
H13E
C13F
P1AB
H13F
P1C
O1C
O1AB
C13C
P1B
O1AA
H13C
H13B
H13D
C13D
C13A
P1AA
C13B
H13A
O1B
P1A
Fig. 1. (a) Structure of the molecule of tetraethyl 1,4-dioxo-1,4-bis(2,4,6-trimethylphenyl)but-2-ene-2,3-diyldiphosphonate (V) and
(b) packing of molecules V in crystal according to the X-ray diffraction data.
molecule I. This path leads to the formation of radical
cation D in which the radical center is localized on the
C1′ atom directly linked to the acceptor group X. How-
ever, the process does not follow path c, for final prod-
ucts II and III cannot be formed in this case.
CO2R, COAr; Scheme 1) was proved by X-ray anal-
ysis [16, 19]. The formation of trans-(E)-dioxo diphos-
phonates II [X = PO(OEt)2; Scheme 1] [20] was con-
firmed by the X-ray diffraction data for tetraethyl
1,4-dioxo-1,4-bis(2,4,6-trimethylphenyl)but-2-ene-
2,3-diyl]diphosphonate (V) (Fig. 1) obtained by oxida-
tion of diethyl (2,4,6-trimethylphenylethynyl)phos-
phonate (IV).
The contribution of path b with formation of inter-
mediate C (in which the unpaired electron on C2′ is
conjugated with the donor alkyl- or methoxy-substi-
tuted aromatic ring) to the overall reaction pattern of
radical cation A is likely to be insignificant. The
mechanism of subsequent transformations of dication
B into products II and III was discussed in [21]. The
trans (E) structure of tetracarbonyl compounds II (X =
In the present work we examined one-electron
oxidation of acetylenic compounds I having other
electron-withdrawing groups X than those given in
Scheme 1. The oxidation of ethyl 4-(4-methoxy-
phenyl)-2-oxobut-3-ynoate (VI, X = COCO2Et) in the
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 6 2008

VASIL’EV et al.
794
Scheme 2.
O
O
2'
EtO
O
1
OMe
2
1
1'
MeO
O
2
–e
OEt
O
MeO
OEt
VI
VII
Scheme 3.
NC
Ar
1
2
C
1
C
VIII
·
2'
Ar
CN
Ar
C
C
CN
1'
2
–e
·
Ar
CN
VIII
E
F
NC
Ar
1
2'
NC
Ar
1'
2
CF3COOH
1
2'
Ar
CN
–H+, –e
–CF3CO+
1'
·
O
·
2
Ar
CN
O
O
CF3
G
IX
Ar = 4-MeOC6H4.
system CF3CO2H–CH2Cl2–PbO2 gave furan derivative
VII (Scheme 2) via formation of new C1–C1′ bond, as
in the reaction with trifluoromethyl ketones I (X =
COCF3; Scheme 1). Despite the lack of quantitative
data characterizing electron-withdrawing power of the
COCO2Et group, we believe that this group should be
analogous to COCF3, taking into account similar elec-
tronegativities and Hammett constants of the CF3 and
CO2Me groups (Table 1).
path c in Scheme 1) in which the radical center on C1′
is essentially stabilized due to electron-withdrawing
effect of the cyano group (see above). The subsequent
transformations through intermediate species G give
furan IX.
The oxidation of compounds I with X = CO2R,
COAr, COR, and PO(OEt)2 involves in total transfer of
four electrons, leading to products II (Scheme 1). The
oxidation of compounds I, VI, and VIII having much
more powerful electron-withdrawing groups (X =
COCF3, COCO2R, CN; Table 1) is accompanied by
transfer of only two electrons, and the products are
furan derivatives III, VII, and IX (Schemes 1–3) [21].
Further rise in the acceptor power of the substituent
[X = SO2Ph, SO2Me, SO2CF3; EN = 9.0–9.4, σp =
0.67–0.91; Table 1) radically changes the reactivity of
acetylenic substrates in the reaction under study. Acet-
ylenic sulfones Xa–Xd gave no oxidation products
in the heterogeneous system PbO2–CF3CO2H–CH2Cl2
Introduction of an even stronger electron-withdraw-
ing cyano group into acetylenic substrate I (X = CN;
Table 1) gives rise to a radically new mode of carbon–
carbon bonding under conditions of one-electron
oxidation. 3-(4-Methoxyphenyl)propynenitrile (VIII)
in the system CF3CO2H–CH2Cl2–PbO2 is converted
into substituted furan IX (Scheme 3) via formation of
C1–C2′ bond. This reaction path may be interpreted as
follows. Radical cation E reacts with substrate mole-
cule VIII to give intermediate F (in a way similar to
Scheme 4.
F3C
O
O
SO2X
CF3COOH
H2O
SO2X
Ar
SO2X
O
Ar
Ar
XIa–XId
H
Xa–Xd
XIIa–XIId
X = Ph, R = 4-MeC6H4 (a), 2,4-Me2C6H3 (b), 4-MeOC6H4 (c); X = CF3, R = 3,4-Me2C6H3 (d).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 6 2008

OXIDATION OF AROMATIC COMPOUNDS: XVI.
795
Scheme 5.
HSO3F
HSO3F–PbO2
+
·
Ar
XH+
Ar
X
Ar
X
–e
XXVIa, XXVIb, XXVIIa
XXVIId–XXVIIIg, XXVIIIa
XXVIIIb, XXIXb, XXXa–XXXc
XXXI
XIIIa–XIIIc, XIVa–XIVg
XVa, XVb, XVI, XVIIa
XVIIb, XVIIIa–XVIIIc, XIX
XXb, XXc, XXIa–XXIg
XXIIb, XXIII, XXIVa, XXIVb
XXVb
XIII, XX, XXVI, X = COMe, R = 4-MeC6H4 (a), 2,4,6-Me3C6H2 (b), 2,3,5,6-Me4C6H (c), Me5C6 (d); XIV, XXI, XXVII,
X = COPh, R = 2,4,6-Me3C6H2 (a), 2,3,5,6-Me4C6H (b), Me5C6 (c), 2,3,5,6-Me4-4-PhCOC≡CC6 (d), 2,3,5,6-Me4-4-O2NC6 (e),
4-MeOC6H4 (f), 4-MeO-3-O2NC6H3 (g); XV, XXII, XXVIII, X = COCF3, R = 2,3,5,6-Me4-4-CF3COC≡CC6; XVI, XXIII, X =
CO2H, R = 4-MeOC6H4; XVII, XXIV, XXIX, X = CO2Me, R = 4-MeOC6H4 (a); X = CO2Et, R = 4-MeO-3-O2NC6H3 (b); XVIII,
XXV, XXX, X = COCO2Et, R = Ph (a), 2,4,6-Me3C6H2 (b), 4-O2NC6H4 (c); XIX, XXXI, X = SO2Ph, R = Ph.
(20°C, 20 h; the reaction mixture was treated with
water) but were converted into oxo sulfones XIIa–
XIId (Scheme 4). In this case, lead(IV) oxide was not
consumed during the process due to high oxidative
potential of compounds Xa–Xd. Benzoylmethyl sul-
fones XIIa–XIId were also formed from acetylenic
sulfones Xa–Xd on keeping in a mixture of trifluoro-
acetic acid with methylene chloride in the absence of
PbO2, other conditions being equal (20°C, 2 h; treat-
ment with water). Monitoring of the reaction course by
thin-later chromatography confirmed slow transforma-
tion of sulfones Xa–Xd in a mixture of anhydrous
CF3CO2H and CH2Cl2. These data suggest initial addi-
tion of trifluoroacetic acid at the triple bond in mole-
cule Xa–Xd with formation of trifluoroacetate XIa–
XId, followed by hydrolysis to produce final oxo sul-
fone XIIa–XIId (Scheme 4). Such addition reactions
are typical of acetylenic sulfones [36–38]. They may
follow both electrophilic [36] and nucleophilic mech-
anism of addition at a triple carbon–carbon bond
[36–38]. The nucleophilic mechanism seems to be
more probable, taking into account activating effect of
the electron-withdrawing sulfonyl group [36]. Presum-
ably, acetylenic nitro compounds ArC≡CX (X = NO2)
in the system CF3CO2H–CH2Cl2–PbO2 should react in
a way similar to sulfones Xa–Xd since the nitro group
is a powerful electron acceptor (Table 1). However,
such substrates were not studied.
Radical cations derived from acetylenic compounds
were detected and characterized by ESR spectroscopy
in the oxidation with HSO3F–PbO2 at –80°C. Radical
cations XXb, XXc, XXIa–XXIg, XXII, XXIII,
XXIVa, XXIVb, and XXVb were generated from
compounds XIIIb, XIIIc, XIVa–XIVg, XV, XVI,
XVIIa, XVIIb, and XVIIIb, respectively (Scheme 5;
Table 2, Fig. 2). The parameters of radical cations
Table 2. ESR spectra of radical cations XXb, XXc, XXIb, XXIc, XXIe, XXIf, XXIII, XXIVa, XXIVb, XXVb, and
XXXIV, generated from compounds XIIIb, XIIIc, XIVb, XIVc, XIVe, XIVf, XVI, XVIIa, XVIIb, XVIIIb, and XXXIII,
respectively, in the system HSO3F–PbO2 at –80°C
Radical cation
Hyperfine coupling constants,a G
Pattern
g Factor
no.
XXb
aHo-Me = 11.7 (6H), aHm-Me = 10.5 (6H), aHp-H = 0.8 (1H)
aHo-Me = 11.4 (6H), aHm-Me = 10.4 (6H), aHp-Me = 1.0 (3H)
aHo-Me = 11.6 (6H), aHm-Me = 10.4 (6H), aHp-H = 0.8 (1H)
aHo-Me = 11.2 (6H), aHm-Me = 10.2 (6H), aHp-Me = 0.5 (3H)
aHMe = 11.5 (12H)
Multiplet, 13 line groups
Multiplet, 13 line groups
Multiplet, 13 line groups
Multiplet, 13 line groups
Tridecet
2.0025
2.0030
1.9977
2.0029
2.0024
2.0046
2.0038
2.0032
2.0036
2.0031
2.0026
XXc
XXIb
XXIc
XXIe
XXIf
aHOMe = aHm-H = 4.5 (5H)
Sextet
XXIII
XXIVa
XXIVb
XXVb
XXXIV
aHOMe = aHm-H = 4.0 (5H)
Sextet
aHOMe = aHm-H = 4.0 (5H)
Sextet
aHOMe = aHm-H = 4.5 (4H)
Quintet
aHp-Me = 12.6 (3H), aHo-Me = aHm-H = 2.1 (8H)
aHo-Me = 11.6 (6H), aHm-Me = 10.6 (6H), aHp-H = 1.0 (1H), aHCH= = 0.4 (1H)
Quartet of nonets
Multiplet, 13 line groups
a
ortho, meta, and para Positions with respect to the ethynyl group.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 6 2008

VASIL’EV et al.
796
(b)
5 G
(a)
2 G
×20
×10
(c)
5 G
×50
×10
(d)
×40
×10
5 G
Fig. 2. ESR spectra of radical cations (a) XXIVb, (b) XXIe, (c) XXIc, and (d) XXXIV generated by oxidation of compounds XVIIb,
XIVe, XIVc, and XXXIII, respectively, in the system HSO3F–PbO2 at –80°C.
XXIa, XXId, XXIg, and XXII were reported previ-
ously [19, 21].
Fluorosulfonic acid is a superacid capable of pro-
tonating triple-bonded carbon atoms in fairly basic
acetylenic compounds with formation of vinyl type
cations and their subsequent transformation into vinyl
fluorosulfonates or other products [39–43]. Therefore,
we examined the stability of the acetylenic substrates
under study toward fluorosulfonic acid. Compounds
XIIIa, XIIIb, XIVa, XIVd–XIVg, XV, XVIIb,
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OXIDATION OF AROMATIC COMPOUNDS: XVI.
797
Table 3. 1H NMR spectra of cations XXVIa, XXVIb, XXVIIf, XXVIII, XXXa–XXXc, and XXXI generated from
acetylenic compounds XIIIa, XIIIb, XIVf, XV, XVIIIa–XVIIIc, and XIX, respectively, in HSO3F at –80°C via protonation
at the electron-withdrawing group
Ion no.
Chemical shifts δ, ppm
XXVIa
XXVIb
XXVIIe
2.53 s (3H, MeCO), 3.08 s (3H, Me), 7.50 d (2H, Harom, J = 7.6 Hz), 7.93 d (2H, Harom, J = 7.6 Hz)
2.62 s (3H, MeCO), 2.75 s (6H, Me), 2.96 s (6H, Me), 8.60 s (1H, Harom
)
4.33 s (3H, OMe), 7.45 t (2H, m-H, J = 7.4 Hz), 7.85 d (2H, o-H, J = 7.4 Hz), 8.21 d (2H, Harom, J = 6.8 Hz),
8.59 t (1H, p-H, J = 7.4 Hz), 8.71 d (2H, Harom, J = 6.8 Hz)
XXVIII
XXXa
XXXb
2.67 s (Me)
1.55 t (3H, Me, J = 6.0 Hz), 4.82 q (2H, OCH2, J = 6.0 Hz), 7.75–8.94 m (5H, Harom
1.56 t (3H, Me, J = 6.1 Hz), 2.55 s (3H, Me), 2.83 s (6H, Me), 4.82 q (2H, OCH2, J = 6.1 Hz), 7.57 s
(2H, Harom
)
)
XXXc
XXXI
1.75 t (3H, Me, J = 6.0 Hz), 5.33 q (2H, OCH2, J = 6.0 Hz), 8.17 d (2H, Harom, J = 7.9 Hz), 8.67 d (2H, Harom
J = 7.9 Hz)
,
7.44–8.17 m (Ph)
Table 4. 13C NMR spectra of acetylenic compounds XIIIa and XV (CDCl3, 25°C) and the corresponding cationic species
XXVIa and XXVIII formed by protonation at the carbonyl oxygen atom in HSO3F at –80°C
Chemical shifts δC, ppm
Compound
ArCH3
CH3 or CF3
C=Oa
C1
C2
Ci
Co
Cm
Cp
(ion) no.
XIIIa
XXVIa
XV
21.66
22.34
18.13
032.64 q
029.49 q
114.96 q
184.54 q
200.62 q
166.57 q
88.14
94.22
92.04
090.95
113.18
097.83
116.76
139.47
122.40
133.05
131.51
139.78
129.41
138.73
–
141.43
153.28
–
(JCF = 287 Hz) (JCF = 42 Hz)
115.83 q 173.96 q
(JCF = 282 Hz) (JCF = 44 Hz)
XXVIIIb
19.04
99.51
126.64
136.87
146.70
–
–
a
C=OH+ in HSO3F.
Dication.
b
XVIIIa–XVIIIc, and XIX in HSO3F at –80°C under-
went protonation at the electron-withdrawing group to
give cations XXVIa, XXVIb, XXVIIa, XXVIId–
XXVIIg, XXVIII, XXIXb, XXXa–XXXc, and
XXXI, respectively (Scheme 5). Tables 3 and 4 con-
XXVIIf, XXVIII, XXXa–XXXc, and XXXI occur in
equilibrium with their neutral precursors XIIIa, XIIIb,
XIVf, XV, XVIIIa–XVIIIc, and XIX and are stable
under conditions ensuring their “long life” (–80°C).
Unprotonated initial compounds XIIIa–XIIIc, XIVa–
XIVg, XVa, XVb, XVI, XVIIa, XVIIb, XVIIIa–
XVIIIc, and XIX undergo one-electron oxidation to
radical cations XX–XXV by the action of PbO2
[12, 13, 19] (Scheme 5).
1
tain the H and 13C NMR parameters of ions XXVIa,
XXVIb, XXVIIf, XXVIII, XXXa–XXXc, and XXXI
in HSO3F; the corresponding parameters of XXVIIa,
XXVIId, XXVIIe, XXVIIg, and XXIXb were
reported in [39, 40]. For comparison, the 13C NMR
spectra of precursors XIIIa and XV (CDCl3, 25°C) are
given in Table 4.
Radical cations derived from acetylenic compounds
are very unstable and highly reactive intermediates
[44, 45]. We succeeded in detecting relatively stable
radical cation species in the system HSO3F–PbO2
(–80°C) only for substrates XIIIb, XIIIc, XIVa–
XIVg, XV, XVI, XVIIa, XVIIb, and XVIIIb having
methoxy or alkyl groups in the aromatic ring (Table 2,
Fig. 2). The ESR spectra of p-methoxyphenyl-substi-
tuted radical cations XXIf, XXIII, and XXIVa dis-
1
The H NMR spectra of solutions of acetylenic
compounds XIIIa, XIIIb, XIVf, XV, XVIIIa–
XVIIIc, and XIX in HSO3F (Table 3) lacked signals in
the region δ 6.40–6.90 ppm, which are typical of vinyl
protons in the products of alkyne transformations in
HSO3F [40, 41]. Protonated species XXVIa, XXVIb,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 6 2008

VASIL’EV et al.
798
+
·
Mem
Meo
played sextets with hyperfine coupling constants
(HCC) aH of 4–4.5 G (Table 2). The spectral pattern is
determined by interaction of the unpaired electron with
three protons of the methoxy group and two aromatic
protons in the meta positions with the same HCCs (cf.
the ESR spectra of methoxybenzenes [12, 44, 46, 47]).
These radical cations are characterized by SOMO b1
(singly occupied molecular orbital that is the former
highest occupied molecular orbital of the neutral
molecule [2, 3]). According to published data, radical
cations with such SOMO feature considerable spin
density transfer to the C1 atom of the acetylene
fragment [44].
2
1
MeO
Mem
X
Meo
SOMO a2
XXb, XXc, XXIb–XXIe, XXII
X = COMe, COPh, COCF3; R = H, Me, O2N, etc.
methyl ketone XXXII with CF3SO3H at –80°C
[40, 41] (Scheme 6) showed an additional coupling
with the vinyl proton, aH = 0.4 G (Table 2, Fig. 2d).
Scheme 6.
+
·
Hm
Me
Me
Me
Me
2
1
O
OSO2F
HSO3F
MeO
X
CF3
COCF3
Hm
Me
Me
Me
Me
Me H
SOMO b1
XXIf, XXIII, XXIVa
XXXII
XXXIII
+
·
Me
Me
XXIf, X = COPh; XXIII, X = CO2H; XXIVa, X = CO2Me.
OSO2F
HSO3F–PbO2
Replacement of one hydrogen atom in the meta
position of p-methoxyphenyl derivatives XIVg and
XVIIb by a nitro group simplifies the ESR spectra of
the corresponding radical cations XXIg and XXIVb
which show quintet signals (Fig. 2a). The unpaired
electron in radical cations XXIa and XXVb derived
from 2,4,6-trimethylphenylacetylenes also occupies
SOMO b1 [19] (Table 2).
COCF3
Me H
XXXIV
Differences in the spin density distribution (i.e.,
different types of SOMO) in radical cations derived
from acetylenic substrates are responsible for different
paths of transformation of alkynes under conditions of
one-electron oxidation. Substrates giving rise to radi-
cal cations with b1 type of SOMO and reactive carbon
atoms at the triple bond are converted into oxidative
dimerization products II and III (Scheme 1) in good
yields [16–21]. Radical cations having SOMO a2,
where the acetylenic bond contributes little to delocal-
ization of spin density, react along other pathways with
formation of considerable amounts of tarry products,
while the yields of structures like II and III are small
[19, 21].
+
·
Hm
Me
2
1
MeO
X
Hm
Me
SOMO b1
XXIa, XXVb
XXIa, X = COPh; XXVb, X = CO2Et.
The SOMO in radical cations XXb, XXc, XXIb–
XXIe, and XXII having tetra- and pentamethyl-substi-
tuted aromatic rings is characterized by a2 symmetry
with a small contribution of the acetylenic bond
(Table 2; Fig. 2b, c). The HCC for protons in the
ortho-methyl groups is aH = 11.2–11.7 G, and that for
protons in the meta-methyl groups is aH = 10.2–10.5 G
(Table 2; cf. [12, 44, 48, 49]). The ESR spectrum of
radical cation XXXIV derived from fluorosulfonate
XXXIII which was formed by reaction of trifluoro-
Thus the results of our studies [16–21] on the trans-
formations of radical cations derived from acetylenic
compounds with electron-withdrawing groups demon-
strated the possibility of obtaining new synthetically
important polyfunctional compounds via one-electron
oxidation of alkynes. Initially formed radical cations
were characterized by ESR spectroscopy, and mechan-
isms were proposed for their subsequent transforma-
tions into oxidative dimerization products.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 6 2008

OXIDATION OF AROMATIC COMPOUNDS: XVI.
799
EXPERIMENTAL
Harom, J = 7.6 Hz), 7.67 t (1H, Harom, J = 7.6 Hz),
8.07 d (2H, Harom, J = 7.6 Hz).
The IR spectra were recorded from solutions in
chloroform on a Specord 75IR spectrophotometer. The
1H and 13C NMR spectra were measured on a Bruker
AM-500 spectrometer at 500 and 125.76 MHz, respec-
tively, using CDCl3 as solvent and reference (CHCl3,
2-(2,4-Dimethylphenyl)ethynyl phenyl sulfone
(Xb). Yield 56%, mp 85–88°C. H NMR spectrum, δ,
ppm: 2.32 s (3H, CH3), 2.33 s (3H, CH3), 6.97 d (1H,
Harom, J = 7.9 Hz), 7.02 s (1H, Harom), 7.34 d (1H,
1
Harom, J = 7.9 Hz), 7.58 t (2H, Harom, J = 7.6 Hz), 7.67 t
1
δ 7.25 ppm; CDCl3, δC 77.0 ppm). The H and 13C
(1H, Harom, J = 7.6 Hz), 8.07 d (2H, Harom, J = 7.6 Hz).
Found, %: C 71.15; H 5.30. C16H14O2S. Calculated, %:
C 71.08; H 5.22.
NMR spectra of solutions in HSO3F were obtained on
a Bruker Avance 400 instrument at 400 and 100 MHz,
respectively, using methylene chloride as internal ref-
erence (δ 5.32, δC 77.0 ppm). The mass spectra (elec-
tron impact, 70 eV) were recorded on an MKh-1321
mass spectrometer with direct sample admission into
the ion source; batch inlet probe temperature 100–
120°C. X-Ray analysis was performed on a Smart
Apex automatic diffractometer (graphite monochro-
mator, MoKα irradiation, ω–θ scanning). The ESR
spectra of radical cations XXb, XXc, XXIa–XXIg,
XXII, XXIII, XXIVa, XXIVb, and XXVb in the sys-
tem HSO3F–PbO2 were measured at –80°C on a Bruker
spectrometer; the spectra were simulated using
WINEPR SimFonia program. The procedure for gen-
eration of radical cations XXVIa, XXVIb, XXVIIa,
XXVIId–XXVIIg, XXVIII, XXIXb, XXXa–XXXc,
and XXXI in HSO3F was reported previously [39, 40].
2-(4-Methoxyphenyl)ethynyl phenyl sulfone
(Xc). Yield 73%, mp 89–90°C; published data [52]:
mp 103–105°C. IR spectrum: ν 2170 cm–1 (C≡C).
1H NMR spectrum, δ, ppm: 3.82 s (3H, OCH3), 6.86 d
(2H, Harom, J = 8.7 Hz), 7.46 d (2H, Harom, J = 8.7 Hz),
7.58 t (2H, Harom, J = 7.6 Hz), 7.66 t (1H, Harom, J =
7.6 Hz), 8.06 d (2H, Harom, J = 7.6 Hz).
2-(3,4-Dimethylphenyl)ethynyl trifluoromethyl
sulfone (Xd) was synthesized according to the proce-
dure reported in [38]. Yield 35%, oily substance.
1H NMR spectrum, δ, ppm: 2.29 s (3H, CH3), 2.33 s
(3H, CH3), 7.22 d (1H, Harom, J = 7.8 Hz), 7.43 d
(1H, Harom, J = 7.8 Hz), 7.45 s (1H, Harom). Found, %:
C 50.31; H 3.40. C11H9F3O2S. Calculated, %: C 50.38;
H 3.46.
Phenyl phenylethynyl sulfone (XIX). Yield 64%,
The procedures for the synthesis and properties of
diethyl (2,4,6-trimethylphenylethynyl)phosphonate
(IV) [20], 4-arylbut-3-yn-2-ones XIIIa–XIIIc [21, 43],
1,3-diarylprop-2-yn-1-ones XIVa, XIVb, XIVd, and
XIVg [19, 39], 1,1,1-trifluoro-4-[2,3,5,6-tetramethyl-
4-(4,4,4-trifluoro-3-oxobut-1-yn-1-yl)phenyl]but-3-yn-
2-one (XV) [21], 3-arylpropynoic acids and their esters
XVI, XVIIa, and XVIIb [16, 17, 40], and ethyl 4-aryl-
2-oxobut-3-ynoates VI and XVIIIb [43] were de-
scribed previously.
mp 67–68°C; published data [52]: mp 68–69°C. IR
1
spectrum: ν 2190 cm–1 (C≡C). H NMR spectrum, δ,
ppm: 7.36 t (2H, Harom, J = 7.5 Hz), 7.47 t (1H, Harom
,
J = 7.5 Hz), 7.52 d (2H, Harom, J = 7.5 Hz), 7.60 t
(2H, Harom, J = 7.7 Hz), 7.68 t (1H, Harom, J = 7.7 Hz),
8.08 d (2H, Harom, J = 7.7 Hz).
3-(2,3,4,5,6-Pentamethylphenyl)-1-phenylprop-2-
yn-1-one (XIVc) was synthesized according to the
procedure described in [18]. Yield 28%, mp 156–
158°C. IR spectrum, ν, cm–1: 1620 (C=O), 2170
3-(4-Methoxyphenyl)propynenitrile (VIII) was
synthesized according to the procedure described in
[50]. Yield 10%, mp 76–78°C; published data [50]:
mp 78–80°C. IR spectrum, ν, cm–1: 2210 (C≡C), 2270
(C≡N). 1H NMR spectrum, δ, ppm: 3.85 s (3H, OCH3),
1
(C≡C). H NMR spectrum, δ, ppm: 2.23 s (6H, CH3),
2.26 s (6H, CH3), 2.56 s (3H, CH3), 7.22 t (2H, Harom
,
J = 7.1 Hz), 7.62 t (1H, Harom, J = 7.1 Hz), 8.24 d (2H,
Harom, J = 7.1 Hz). Mass spectrum, m/z (Irel, %): 276
[M]+ (84), 261 (58), 246 (15), 233 (29), 218 (10), 199
(20), 141 (20), 105 (100), 77 (55). Found, %: C 87.01;
H 7.25. C20H20O. Calculated, %: C 86.92; H 7.29.
M 276.15.
6.99 d (2H, Harom, J = 8.1 Hz), 7.54 d (2H, Harom
,
J = 8.1 Hz).
1-Aryl-2-(phenylsulfonyl)ethynes Xa–Xc and XIX
were synthesized as described in [51].
Ethyl 4-aryl-2-oxobut-3-ynoates XVIIIa and
XVIIIc were synthesized according to the procedure
described in [53].
2-(4-Methylphenyl)ethynyl phenyl sulfone (Xa).
Yield 60%, mp 87–88°C; published data [52]: mp 87–
88°C. IR spectrum: ν 2190 cm–1 (C≡C). 1H NMR spec-
trum, δ, ppm: 2.37 s (3H, CH3), 7.17 d (2H, Harom, J =
8.0 Hz), 7.41 d (2H, Harom, J = 8.0 Hz), 7.59 t (2H,
Ethyl 2-oxo-4-phenylbut-3-ynoate (XVIIIa).
Yield 25%, mp 31–32°C; published data [53]: oily
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 6 2008

VASIL’EV et al.
800
substance. IR spectrum, ν, cm–1: 1680 (C=O), 1730
(C=O), 2195 (C≡C). H NMR spectrum (CDCl3), δ,
IX, respectively, in the system CF3CO2H–CH2Cl2–
PbO2 was described in [21].
1
ppm: 1.40 t (3H, CH3, J = 7.3 Hz), 4.39 q (2H, OCH2,
J = 7.3 Hz), 7.40 t (2H, Harom, J = 7.8 Hz), 7.50 t
(1H, Harom, J = 7.8 Hz), 7.65 d (2H, Harom, J = 7.8 Hz).
13C NMR spectrum (CDCl3), δC, ppm: 13.91, 63.22,
87.01, 97.88, 119.02, 128.76, 131.82, 133.72, 159.20,
169.55.
Ethyl 4-(2-ethoxy-1,2-dioxoethyl)-3-(4-methoxy-
benzoyl)-5-(4-methoxyphenyl)furan-2-carboxylate
(VII) was obtained by oxidation of 56 mg (0.24 mmol)
of compound VI with 58 mg (0.24 mmol) of PbO2 in
a mixture of 0.1 ml of CF3CO2H and 3 ml of CH2Cl2 at
20°C (reaction time 1.5 h). Yield 14 mg (24%), oily
substance. IR spectrum, ν, cm–1: 1680, 1730 (C=O).
1H NMR spectrum, δ, ppm: 0.98 t (3H, CH3, J =
7.0 Hz), 1.06 t (3H, CH3, J = 7.1 Hz), 3.87 s (6H,
CH3O), 3.93 q (2H, OCH2, J = 7.1 Hz), 4.12 q (2H,
OCH2, J = 7.0 Hz), 6.94 d (2H, Harom, J = 8.7 Hz),
6.99 d (2H, Harom, J = 8.6 Hz), 7.77 d (2H, Harom, J =
8.7 Hz), 7.90 d (2H, Harom, J = 8.6 Hz). 13C NMR spec-
trum, δC, ppm: 13.53 q (J = 126.6 Hz), 13.56 q (J =
127.5 Hz), 55.47 q (J = 144.51 Hz), 55.50 q (J =
144.7 Hz), 61.65 t.q (J = 149.2, 4.5 Hz), 62.54 t.q
(J = 148.3, 4.2 Hz), 113.83 d.d (J = 161.8, 4.5 Hz),
114.21 d.d (J = 161.8, 4.7 Hz), 118.85 s, 120.43 t (J =
7.9 Hz), 130.37 t (J = 7.4 Hz), 130.63 d.d (J = 162.5,
7.0 Hz), 131.70 d.d (J = 161.1, 7.0 Hz), 132.41 s,
139.88 s, 157.39 t (J = 2.7 Hz), 161.82 t (J = 2.5 Hz),
162.21 m, 164.10 m, 162.30 s, 180.52 s, 188.00 t (J =
3.8 Hz). Mass spectrum, m/z (Irel, %): 480 [M]+ (15),
407 (61), 135 (100). Found, %: C 64.85; H 5.08.
C27H24O9. Calculated, %: C 65.00; H 5.03. M 480.14.
Ethyl 4-(4-nitrophenyl)-2-oxobut-3-ynoate
(XVIIIc). Yield 20%, mp 80–82°C. IR spectrum, ν,
cm–1: 1680 (C=O), 1720 (C=O), 2200 (C≡C). 1H NMR
spectrum (CDCl3), δ, ppm: 1.42 t (3H, CH3, J =
7.1 Hz), 4.41 q (2H, OCH2, J = 7.1 Hz), 7.40 d (2H,
Harom, J = 8.3 Hz), 8.27 d (2H, Harom, J = 8.3 Hz).
13C NMR spectrum (CDCl3), δC, ppm: 13.91, 63.60,
89.20, 93.25, 123.84, 125.56, 134.29, 149.03, 158.73,
169.20. Found, %: C 58.51; H 3.65; N 5.75.
C12H9NO5. Calculated, %: C 58.30; H 3.67; N 5.67.
1,1,1-Trifluoro-4-(2,3,5,6-tetramethylphenyl)but-
3-yn-2-one (XXXII) was synthesized according to the
procedure described in [54]. Yield 42%, mp 61–63°C.
1H NMR spectrum, δ, ppm: 2.24 s (6H, CH3), 2.41 s
(6H, CH3), 7.11 s (1H, Harom). Found, %: C 66.09;
H 5.12. C14H13F3O. Calculated, %: C 66.14; H 5.15.
Tetraethyl 1,4-dioxo-1,4-bis(2,4,6-trimethylphen-
yl)but-2-ene-2,3-diyldiphosphonate (V) was synthe-
sized from diethyl 2-(2,4,6-trimethylphenyl)ethynyl-
phosphonate (IV) [20]. A 0.60×0.50×0.40-mm single
crystal of V for X-ray analysis (Fig. 1) was obtained
by slow evaporation of a solution of V in hexane–ethyl
acetate at room temperature over a period of several
days. Monoclinic crystal system; C15H21O4P; unit cell
parameters [at 293(2) K]: a = 10.835(3), b = 16.750(4),
c = 8.631(2) Å; β = 97.378(5)°; V = 1553.4(6) Å3;
Z = 4; space group P2(1)/c; dcalc = 1.267 g/cm3; μ =
0.187 mm–1; 1.90 ≤ θ ≤ 23.99°. Total of 7531 reflec-
tions were measured, 2429 of which were independent
(Rint = 0.0264); divergence factors R1 = 0.0602 [for
reflections with I > 2σ(I)], wR2 = 0.1497 (for all reflec-
tions). The structure was solved by the direct method
and was refined by the least-squares procedure with
respect to F2hkl in anisotropic approximation for non-
hydrogen atoms. Hydrogen atoms were visualized
from the Fourier difference syntheses, and their posi-
tions were refined in isotropic approximation. All cal-
culations were performed using SHELXTL v. 6.10
software package [55].
2,4-Bis(4-methoxyphenyl)furan-3,5-dicarboni-
trile (IX) was obtained by oxidation of 160 mg
(1.0 mmol) of compound VIII with 245 mg
(1.0 mmol) of PbO2 in a mixture of 0.4 ml of
CF3CO2H and 4 ml of CH2Cl2 at 20°C (reaction time
1
2 h). Yield 16 mg (10%), mp 132–134°C. H NMR
spectrum, δ, ppm: 3.81 s (3H, OCH3), 3.86 s (3H,
OCH3), 6.85 d (2H, Harom, J = 8.9 Hz), 6.98 d (2H,
Harom, J = 8.5 Hz), 7.31 d (2H, Harom, J = 8.5 Hz),
7.43 d (2H, Harom, J = 8.9 Hz). 13C NMR spectrum, δC,
ppm: 55.37, 55.39, 109.15, 110.07 (C≡N), 111.49
(C≡N), 114.45, 114.94, 120.02, 120.25, 122.22, 128.50,
128.90, 130.46, 154.86, 160.49, 161.28. Mass spec-
trum, m/z (Irel, %): 330 [M]+ (100), 315 (18), 276 (17),
190 (12), 135 (20). Found, %: C 72.83; H 4.35;
N 8.55. C20H14N2O3. Calculated, %: C 72.72; H 4.27;
N 8.48. M 330.10.
Keto sulfones XIIa–XIId were obtained from
ethynyl sulfones Xa–Xd according to the procedure
analogous to the oxidation of acetylenic compounds
but without PbO2 [21].
The procedure for the oxidation of acetylenic
1-(4-Methylphenyl)-2-(phenylsulfonyl)ethanone
compounds VI and VIII to substituted furans VII and
(XIIa) was obtained from 100 mg (0.39 mmol) of
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 6 2008

OXIDATION OF AROMATIC COMPOUNDS: XVI.
801
compound Xa in a mixture of 0.15 ml of CF3CO2H
and 2 ml of CH2Cl2 at 20°C (reaction time 2 h). Yield
48 mg (45%), mp 125–126°C. IR spectrum: ν(C=O)
Research (project no. 06-03-32728a), and by the Presi-
dent of the Russian Federation (program for support
of leading scientific schools, project no. NSh-4947.-
2006.3).
1
1690 cm–1 . H NMR spectrum, δ, ppm: 2.41 s (3H,
CH3), 4.70 s (2H, CH2), 7.27 d (2H, Harom, J = 8.1 Hz),
7.54 t (2H, Harom, J = 7.6 Hz), 7.65 t (1H, Harom, J =
7.6 Hz), 7.83 d (2H, Harom, J = 8.1 Hz), 7.88 d (2H,
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H
arom, J = 7.6 Hz). Found, %: C 65.65; H 5.10.
C15H14O3S. Calculated, %: C 65.67; H 5.14.
1-(2,4-Dimethyl)phenyl-2-(phenylsulfonyl)etha-
none (XIIb) was obtained from 50 mg (0.19 mmol) of
compound Xb in a mixture of 0.1 ml of CF3CO2H and
2 ml of CH2Cl2 at 20°C (reaction time 2 h. Yield 42 mg
(76%), mp 100–102°C. IR spectrum: ν 1680 cm–1
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(C=O). H NMR spectrum, δ, ppm: 2.35 s (3H, CH3),
2.40 s (3H, CH3), 4.68 s (2H, CH2), 7.05–7.08 m (2H,
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H
H
arom), 7.51–7.54 m (2H, Harom), 7.63–7.66 m (2H,
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1-(4-Methoxyphenyl)-2-(phenylsulfonyl)ethanone
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pound Xc in a mixture of 0.1 ml of CF3CO2H and 2 ml
of CH2Cl2 at 20°C (reaction time 2 h). Yield 46 mg
(80%), mp 108–110°C. IR spectrum: ν 1670 cm–1
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4.67 s (2H, CH2), 6.94 d (2H, Harom, J = 8.9 Hz), 7.54 t
(2H, Harom, J = 7.6 Hz), 7.65 t (1H, Harom, J = 7.6 Hz),
7.88 d (2H, Harom, J = 7.6 Hz), 7.92 d (2H, Harom, J =
8.9 Hz). 13C NMR spectrum, δC, ppm: 55.60, 63.48,
114.09, 128.56, 128.91, 129.14, 131.86, 134.12, 138.85,
164.59, 186.12. Found, %: C 62.12; H 4.90. C15H14O4S.
Calculated, %: C 62.05; H 4.86.
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1
10 h). Yield 50 mg (40%), oily substance. H NMR
spectrum, δ, ppm: 2.34 s (3H, CH3), 2.35 s (3H, CH3),
4.80 s (2H, CH2), 7.29 d (1H, Harom, J = 8.1 Hz), 7.69 d
(1H, Harom, J = 8.1 Hz), 7.73 s (1H, Harom). Mass spec-
trum, m/z (Irel, %): 280 [M]+ (16), 210 (11), 133 (100),
119 (15), 105 (25), 91 (13). Found, %: C 47.21;
H 4.06. C11H11F3O3S. Calculated, %: C 47.14; H 3.96.
M 280.04.
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This study was performed under financial support
by the Federal Goal-Oriented Scientific and Technical
Program “Research and Development in Foreground
Fields of Science and Technics for 2002–2006” (project
no. 1.4.05.06), by the Russian Foundation for Basic
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 6 2008

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802
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