Reaction of 5-phenylpenta-2,4-dienoic acid with benzene in trifluoromethanesulfonic acid

Article abstract of DOI:10.1007/s11172-020-2980-7

The reaction of 5-phenylpenta-2,4-dienoic acid with benzene in CF₃SO₃H, depending on the reaction conditions, gives three products, namely, 5,5-diphenylpent-2-enoic acid and tetralone and indanone derivatives. These carbocyclic compo

Full text of DOI:10.1007/s11172-020-2980-7

Russian Chemical Bulletin, International Edition, Vol. 69, No. 10, pp. 1928—1932, October, 2020
1928
Reaction of 5-phenylpenta-2,4-dienoic acid with benzene
in trifluoromethanesulfonic acid
A. R. Ismagilova,а D. N. Zakusilo,а L. V. Osetrova,b and A. V. Vasilyevа,c
аSt. Petersburg State Forest Technical University,
5 Institutskiy per., 194021 St. Petersburg, Russian Federation.
Fax: +7 (812) 670 9390. Е-mail: aleksvasil@mail.ru
bResearch Institute of Synthetic Rubber,
1 Gapsal´skaya ul., 198035 St.-Petersburg, Russian Federation.
Fax: +7 (812) 251 4813
cSt. Petersburg State University, Institute of Chemistry,
7/9 Universitetskaya nab., 199034 St. Petersburg, Russian Federation.
Fax: +7 (812) 428 6733
The reaction of 5-phenylpenta-2,4-dienoic acid with benzene in CF3SO3H, depending on
the reaction conditions, gives three products, namely, 5,5-diphenylpent-2-enoic acid and te-
tralone and indanone derivatives. These carbocyclic compounds are formed through the addi-
tion of two benzene molecules to the starting diene acid and subsequent intramolecular acylation.
Key words: dienoic acids, 5-phenylpenta-2,4-dienoic acid, indanones, 3,4-dihydronaphth-
alen-1(2H)-ones, 5,5-diphenylpent-2-enoic acid, trifluoromethanesulfonic acid, carbocations,
superelectrophilic activation.
Superelectrophilic activation of organic compounds
reaction in the TfOH—CH2Cl2 system (Tf = F3CSO2) at
0 C for 1 h gives products 2—4 in a 55% total yield with
the incomplete conversion of substrate 1 (see Table 1,
entry 1). Raising the temperature to ambient and prolon-
gation the reaction to 1—5 h leads to a significant growth
in the yields of compounds 2—4, with the yield of diphenyl
diene 2 decreasing from 14% to 6% as the reaction time
increases from 1 to 5 h (entries 2—4). Lowering the acid-
ity of the reaction medium by adding pyridine to TfOH
slows down the reaction, and at room temperature it re-
quires 24 h to achieve a complete conversion of starting
compound 1 (entries 5 and 6). Indanone 4 is not formed
in the system with the lowest acidity (TfOH/pyridine =
= 4 : 1 v/v) (entry 6). The reactions under the action of
aluminum halides (entry 8) or in the stronger than TfOH
fluorosulfonic acid FSO3H at –78 C (entry 7) lead to
complex mixtures of products.
The formation of compounds 2—4 indicates that cat-
ionic species with several electrophilic sites are generated
from diene acid 1 in TfOH. A plausible mechanism of
these transformations is presented in Scheme 2. The initial
protonation of the carboxy group of acid 1 gives cation A,
which can be further protonated at the C=C double bonds
under superacidic conditions similarly to the O,C-di-
protonation of conjugated enones.5 In this case, in species A
different double bonds can be protonated: at the C(2) atom
to generate dication B or at the C(4) atom to generate
dication F. The reaction of the latter with benzene gives
under the action of various strong Brønsted and Lewis
acids or acidic zeolites is one of the most efficient organic
methods.1,2 This activation promotes the generation of
multiply charged cations in low nucleophilic media, in
which they can react with poor nucleophiles, for example,
with arenes.2 Some of the best studied objects in this area
are alkenes, the protonation of which in superacids or
coordination with strong Lewis acids gives alkyl cations,
which would undergo further transformations.3—14 Despite
the large amount of data on the superelectrophilic activa-
tion of alkenes, the transformations of conjugated dienes
under these conditions remain poorly studied. The 1,3-buta-
diene systems conjugated with various acceptor substitu-
ents contain several basic centers, such as acceptor hetero-
atoms and carbon atoms of multiple bonds. Their proton-
ation gives cationic species with several electrophilic sites
capable of reacting in different directions.
The purpose of the present work is to study the reac-
tions of 5-phenylpenta-2,4-dienoic acid 1 with benzene
in Brønsted superacids (CF3SO3H, FSO3H) or under the
action of strong Lewis acids AlX3 (X = Cl, Br).
We found that in trifluoromethanesulfonic acid (TfOH),
depending on the conditions, the reaction of phenyl diene
1 with benzene gives diphenylalkene 2, tetralone 3, and
indanone 4 (Scheme 1, Table 1).
The structures of compounds 3 and 4 was confirmed
by X-ray diffraction analysis (Fig. 1). Carrying out this
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1928—1932, October, 2020.
1066-5285/20/6910-1928 © 2020 Springer Science+Business Media LLC

Dienoic acid phenylation
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 10, October, 2020
1929
Scheme 1
Table 1. Reactions of 5-phenylpenta-2,4-dienoic acid 1 with benzene in various acidic systems
leading to compounds 2, 3, and 4
Entry
Reaction conditions
Medium T/C
Product yields (%)
/h
2
3
4
1
2
3
4
5
6
7
8
TfOH, CH2Cl2
0
20
1
1
13
14
12
16
13
59
21
46
48
51
41
31
21
32
26
27
30
—
TfOH
TfOH
TfOH
20
2
20
5
TfOH—pyridine (5 : 1 v/v)
TfOH—pyridine (4 : 1 v/v)
FSO3H
20
24
24
1
20
–78
20
A complex mixture of compounds
A complex mixture of compounds
AlХ3 (X = Cl, Br)
1
species G, the deprotonation of which in a low acidic
system TfOH—pyridine leads to diphenylalkene 2. Further
protonation in the superacid TfOH of the C=C bond in
cation G gives species H, the reaction of which with ben-
zene leads to cation I. Subsequent intramolecular acylation
finalizes the formation of indanone 4. In an alternative
direction, dication B reacts with benzene (a resonance
form B´) to form cation C, which through intermediates
D and E is converted into reaction product 3.
ways (see Scheme 2). The bifurcation of the reaction di-
rections occurs at the stage of formation of dications B
and F. In the first case, the hydrophenylation takes place
at the C(2) and C(5) positions of the pentadienone system
of the starting compound 1 (generation of species C), while
in the second case, the hydrophenylation occurs at the
C(4)=С(5) bond of compound 1 (generation of species
G). In this case, diphenylpentenoic acid 2 lies in the path
of formation of indanone 4. However, indanone 4 is not
formed in the system with the lowest acidity TfOH—
pyridine (4 : 1 v/v) (see Table 1, entry 6). Apparently, this
Based on this reaction scheme, it can be assumed that
indanone 3 and tetralone 4 are formed via different path-
b
a
Fig. 1. The structures of compounds 3 (a) and 4 (b) according to X-ray diffraction data.

Ismagilova et al.
1930
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 10, October, 2020
Scheme 2
1,3,5-Trielectrophile
1,4,5-Trielectrophile
is due to the impossibility of protonation in this system of
the C=C double bond conjugated with a strongly accept-
ing protonated carboxy group, which sharply reduces the
basicity of this double bond. In contrast to this, protonation
of the C=C double bond in cation C having no strong
accepting substituents occurs readily in the TfOH—pyr-
idine system, which finally gives tetralone 3 (see entries 5
and 6).
In the studied transformations leading to compounds
3 and 4, dienoic acid 1, if take into account the formation
of cationic centers upon protonation of the conjugated
pentadiene system of acid 1 in TfOH, can be considered
as a precursor of two three-centered electrophilic species,
namely, J (1,4,5-trielectrophile) and K (1,3,5-trielectro-
phile), respectively (see Scheme 2).
Note that the preparation and study of chemical trans-
formations of such structurally related pentadienoic acids
is of great importance for organic synthesis.15
In conclusion, the reaction of 5-phenylpenta-2,4-di-
enoic acid (1) with benzene in TfOH initially leads to
the products of hydrophenylation of different positions of
the pentadienone system, which are then converted
into benzocarbocyclic derivatives of tetralone 3 and in-
danone 4 series.
Experimental
1Н and 13C NMR spectra were recorded on a Bruker AM-500
instrument (500 and 125 MHz, respectively) in CDCl3. High-
resolution mass spectra were recorded on a Bruker maXis HRMS-
ESI-QTOF instrument. Chromato-mass spectral analysis was
carried out on a G 2570A GC/MSD instrument (Agilent Tech-
nologies 6850c), an HP-5MS capillary column (3 m×0.25 mm),
stationary phase thickness 0.25 μm, carrier gas helium.
5-Phenylpenta-2,4-dienoic acid (1) was synthesized by the
reaction of cinnamaldehyde (5.25 mL, 38 mmol) with malonic
acid (4.7 g, 45 mmol) in pyridine (10 mL) and piperidine
(0.2 mL). The reaction mixture was stirred at 100 C for 4 h and
poured into concentrated hydrochloric acid (30 mL), the formed
precipitate was collected by filtration and recrystallized from
ethanol. The yield was 60%. M.p. 164—167 C (Ref. 16: m.p.
1
165—166 C). H NMR (CDCl3), δ: 6.01 (d, 1 H, J = 15 Hz);
6.88—6.98 (m, 2 H); 7.31—7.58 (m, 4 Н); 7.49 (d, 1 H, J = 7 Hz).
13C NMR (CDCl3), δ: 120.2, 126.0, 127.3, 128.9, 129.3, 135.8,
141.6, 146.9, 171.9. MS (gas chromatography-mass spectrometry
data), m/z (Irel (%)): 174 [М]+ (25), 129 (100), 115 (8), 102 (8),
77 (9), 51 (8).
Reaction of dienoic acid 1 with benzene under the action of
TfOH (general procedure): synthesis of compounds 2, 3, and 4.
Acid 1 (100 mg, 5.7 mmol) was added to a mixture of benzene
(1 mL) and TfOH (1 mL). The mixture was stirred at room

Dienoic acid phenylation
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 10, October, 2020
1931
Table 2. Crystallographic data and parameters of X-ray diffraction experiment for compounds 3
and 4
Parameter
3
4
Molecular formula
C
312.39
293
Monoclinic
P21/n
9.0544(4)
19.1430(8)
10.3358(4)
90
111.570(5)
90
1666.02(13)
4
23H20
O
C23H20O
312.39
100
Monoclinic
P21/c
14.1591(6)
15.6726(8)
7.7444(3)
90
102.226(5)
90
1679.59(14)
4
Molecular weight
T/K
Crystal system
Space group
a/Å
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
Z
dcalc/g cm–3
1.245
1.235
5.198—54.996
2-Range for data collection/deg
Number of reflections
5.552—54.992
collected (Rint
(I 2σ(I))
)
12724
3833
(0.0274)
(0.0277)
217
27562
3855
(0.0316)
(0.0198)
217
(Rint
)
(Rsigma
)
Number of refined parameters
R-factors (I  2σ(I))
R1
0.0406
0.0999
0.0528
0.1118
wR2
R-factors (for all reflections)
R1
wR2
0.0500
0.1074
0.0611
0.1168
temperature for 1 h, poured into water (60 mL), and extracted
with chloroform (3×30 mL). The combined extracts were washed
with water (2×30 mL), dried with Na2SO4, the solvent was
evaporated in vacuo. The reaction products 2, 3, and 4 were
isolated by column chromatography on silica gel, their yields are
given in Table 1. The reactions were carried out similarly in the
systems TfOH—CH2Cl2 and TfOH—pyridine or under the action
of FSO3H or AlX3 (X = Cl, Br) at the corresponding temperature
and the reaction time (see Table 1).
5,5-Diphenylpent-2-enoic acid (2). A crystalline compound,
m.p. 69—70 C. 1H NMR (CDCl3), δ: 2.96 (t, 2 H, J = 7.5 Hz);
4.08 (t, 1 H, J = 7.5 Hz); 5.78 (d, 1 H, J = 15.7 Hz); 6.95 (dt, 1 H,
J = 15.7 Hz, J = 7.5 Hz); 7.16—7.21 (m, 5 Н); 7.25—7.30
(m, 5 Н). 13C NMR (CDCl3), δ: 38.3, 50.1, 122.0, 126.5, 127.8,
128.6, 143.4, 149.8, 152.4. HRMS (ESI), found m/z: 253.1226
[M + H]+; calculated for C17H17O2 253.1223.
4-Diphenylmethyl-3,4-dihydronaphthalen-1(2Н)-one (3).
A crystalline compound, m.p. 149—151 C. 1H NMR (CDCl3),
δ: 2.15—2.22 (m, 2 H); 2.45—2.50 (m, 1 H); 2.72—2.28 (m, 1 H);
3.69—3.72 (m, 1 H); 4.03 (d, 1 H, J = 12 Hz); 6.40 (d, 1 H,
J = 8 Hz); 6.9 (d, 2 H,. J = 8 Hz); 7.05—7.11 (m, 4 H); 7.21—7.24
(m, 2 H); 7.36 (t, 2 H, J = 8 Hz); 7.44 (d, 2 H, J = 8 Hz); 8.01
(d, 1 H, J = 8 Hz). 13C NMR (CDCl3), δ:25.7, 33.5, 43.0,
56.5, 126.4, 126.8, 126.9, 127.3, 128.0, 128.2, 128.4, 128.9,
129.9, 132.2, 142.6, 142.9, 146.2, 160.1, 198.4. MS (gas chrom-
atography-mass spectrometry data), m/z (Irel (%)): 312 [М]+
(8), 167 (100), 152 (20), 115 (9), 103 (10), 77 (10). HRMS
(ESI), found m/z: 313.1592 [M + H]+; calculated for C23H21
O
313.1587.
3-(2,2-Diphenylethyl)indan-1-one (4). A crystalline com-
pound, m.p. 120—121 C. 1H NMR (CDCl3), δ: 2.09—2.15
(m, 1 H); 2.38—2.42 (m, 1 H); 2.72—2.81 (m, 2 H); 3.18—3.23
(m, 1 H); 4.07—4.10 (m, 1 H); 7.16—7.19 (m, 1 H); 7.21—7.23
(m, 1 H); 7.24—7.27 (m, 4 H); 7.32—7.37 (m, 5 H); 7.50 (d, 1 H,
J = 8 Hz); 7.57—7.60 (m, 1 H); 7.71 (d, 1 H, J = 8 Hz). 13C NMR
(CDCl3), δ: 36.5, 42.6, 43.2, 50.0, 123.6, 125.5, 126.4, 126.7,
127.5, 127.6, 128.0, 128.6, 128.8, 134.7, 136.6, 143.52, 144.5,
158.7, 205.9. MS (gas chromatography-mass spectrometry data),
m/z (Irel (%)): 312 [М]+ (8), 167 (100), 152 (19), 115 (7), 103 (8),
77 (7). HRMS (ESI), found m/z: 313.1584 [M + H]+; calcu-
lated for C23H21O 313.1587.
X-ray diffraction analysis of single crystals of compounds 3
and 4 obtained by crystallization from a chloroform—hexane
solvent mixture was carried out on an Xcalibur diffractometer
(graphite monochromator, (Мо-Кα) = 0.71073 Å, temperature
293 K (for compound 3) and 100 K (for compound 4), /2-scan
technique). The crystallographic data and the main refinement
parameters for compounds 3 and 4 are given in Table 2. The
structures were solved using the Superflip17—19 and Olex220
programs and refined using the ShelXL21 program (least squares
method). The structures were deposited with the Cambridge
Crystallographic Data Center, CCDC 1966168 (3) and 1966169

Ismagilova et al.
1932
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 10, October, 2020
(4) at www.ccdc.cam.ac.uk/conts/retrieving.htmL, 12 Union
Road, Cambridge CB2 1EZ, UK; fax: + 44-1223-336-033;
e-mail: deposit@ccdc.cam.ac.uk.
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This work was financially supported by the Russian
Science Foundation (Project No. 18-13-00008 "Conjugated
unsaturated structures (enynes, dienes, diynes, allenes,
etc.) as sources of multisite electrophiles in the synthesis
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Received January 18, 2020;
in revised form March 3, 2020;
accepted March 16, 2020