Electrochemical methoxylation of cinnamic acid and its derivatives

Article abstract of DOI:10.1023/A:1013812321096

Electrochemical methoxylation of cinnamic acid is accompanied by decarboxylation and yields 1,1,2-trimethoxy-2-phenylethane through formation of γ-truxillic acid in the initial stage. Under the same conditions methyl cinnamate and cinnamamide give rise to

Full text of DOI:10.1023/A:1013812321096

Russian Journal of Organic Chemistry, Vol. 37, No. 11, 2001, pp. 1592 1595. Translated from Zhurnal Organicheskoi Khimii, Vol. 37, No. 11, 2001,
pp. 1666 1668.
Original Russian Text Copyright
2001 by Il’chibaeva, Kagan, Tomilov.
Electrochemical Methoxylation of Cinnamic Acid
and Its Derivatives
I. B. Il’chibaeva1, E. Sh. Kagan1, and A. P. Tomilov2
1
South Russian State Technical University, ul. Prosveshcheniya 132, Novocherkassk, 346430 Russia
e-mail: fkoh@novoch.ru
2
State Research Institute of Organic Chemistry and Technology, Moscow Russia
Received July 10, 2000
Abstract Electrochemical methoxylation of cinnamic acid is accompanied by decarboxylation and yields
1,1,2-trimethoxy-2-phenylethane through formation of -truxillic acid in the initial stage. Under the same
conditions methyl cinnamate and cinnamamide give rise to hydrodimerization and reduction products. The
results show that introduction of electron-acceptor substituents into the -position of the aliphatic chain
of styrene hinders methoxylation.
In the recent time a considerable attention is given
to electrochemical methoxylation since methoxy
derivatives obtained in such a way are key compounds
in numerous organic syntheses [1]. Methoxylation of
styrene (I) initially gives 1,2-dimethoxy-1-phenyl-
ethane (II), and the subsequent electrolysis results
in cleavage of the C C bond with formation of benz-
aldehyde dimethyl acetal (III) as final product [2]
(Scheme 1).
(yield 96%, according to the GLC data); further elec-
trolysis led to formation of benzaldehyde dimethyl
acetal (III) (4%, GLC; Scheme 2).
Scheme 2.
Scheme 1.
After passing 6.76 105 C/mol of electricity, the
ratio of 1,1,2-trimethoxy-2-phenylethane (V) and
benzaldehyde dimethyl acetal (III) was 54:46 (GLC).
Cinnamic acid, like diphenylacrylic acid [3], is
classed with so-called anomalous carboxylic acids
which lose two electrons at an anode, yielding
carbocation A. Methoxide ions B are generated at
a cathode. These species combine in the bulk solution
to give methoxystyrene C. Methoxylation of the latter
at the double C C bond, as in the case of unsubsti-
tuted styrene, leads to formation of the primary
product, 1,1,2-trimethoxy-2-phenylethane (V). The
reaction mechanism can be illustrated by Scheme 3
[4, 5]. It should be noted that at a graphite anode
-truxillic acid (VI) is formed as intermediate product
(Scheme 4); it is known [6] that no dimerization
occurs in solution. After passing 0.96 105 C/mol
The goal of the present work was to examine the
effect of substituents in the aliphatic chain of styrene
on the methoxylation process. We have performed
electrochemical methoxylation of cinnamic acid and
its methyl ester and amide under conditions reported
in [2] for methoxylation of styrene. The electrolysis
was carried out in a diaphragmless electrolyzer in
the galvanostatic mode at 60 65 C using a 0.1 M
solution of KF in methanol as supporting electrolyte.
In the electrolysis of cinnamic acid (IV), after
passing 1.93 105 C/mol of electricity, the major
product was 1,1,2-trimethoxy-2-phenylethane (V)
1070-4280/01/3711-1592$25.00 2001 MAIK Nauka/Interperiodica

ELECTROCHEMICAL METHOXYLATION OF CINNAMIC ACID
1593
Scheme 3.
of electricity, the yield of -truxillic acid was 63%,
calculated on the reacted cinnamic acid. -Truxillic
After passing 1.93 105 C/mol of electricity, the
yield of dimeric product IX was 23% at a stainless
acid gradually disappears, and it is not detected by steel cathode, 16% at a zinc cathode, and 8% at a cop-
the end of the process. Acid VI is converted into
cinnamic acid on heating, and the latter undergoes
further transformations.
per cathode. By electrolysis of cinnamamide (X) we
obtained only product XI as a result of reduction of
the double bond (Scheme 6).
Scheme 6.
Scheme 4.
The yield of the reduction and dimerization prod-
ucts descreases when the reaction is carried out in
a diaphragm electrolyzer.
Our results led us to conclude that introduction of
electron-acceptor groups into the -position of the
side chain of styrene molecule hampers methoxylation
of the double C C bond. This conclusion is consist-
ent with the assumption that electrolytic methoxyla-
tion of cinnamic acid begins only after elimination
of the carboxy group which is a fairly strong electron
acceptor.
Methyl cinnamate and cinnamamide showed a dif-
ferent behavior under analogous conditions. In both
cases no methoxylation products were obtained, but
reduction of the double bond occurred with formation
of monomeric and dimeric products. The electrolysis
of methyl cinnamate VII afforded 61% of ester VIII
and 23% of diester IX (Scheme 5).
Scheme 5.
EXPERIMENTAL
The electrolysis was carried out in a 200-ml dia-
phragmless sealed cell equipped with an electrode
assembly (platinum or graphite anode as a 50 15-mm
plate and two cathodes each as a 50 15-mm stainless
steel plate; anode-to-cathode distance 3 mm), reflux
condenser, and magnetic stirrer. GLC analysis was
performed on an LKhM-8MD chromatograph with
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 11 2001

1594
IL’CHIBAEVA et al.
1
a heat-conductivity detector. The H NMR spectra
were recorded on a Varian VXR-300 spectrometer (VII). a. In a diaphragmless cell. The electrolytic
Electrochemical reduction of methyl cinnamate
(300 MHz) in deuterated solvents. The IR spectra
were obtained on a Specord 75IR spectrometer in
mineral oil.
Cinnamic acid was of pure grade; KF of pure grade
was dried in a desiccator over P2O5 prior to use.
Methanol (from NZSP) contained 99.5% of the main
substance and was used without additional purifica-
tion. Methyl cinnamate was synthesized by the proce-
dure reported in [7].
cell was charged with 6.5 g (0.04 mol) of methyl
cinnamate, 0.43 g (0.005 mol) of KF, and 125 ml of
methanol. The conditions were the same as above.
When 1.93 105 C/mol (2F/mol) of electricity was
passed, most part of the solvent was distilled off.
Crystalline product IX was filtered off and recrystal-
lized from methanol. The yield of dimer IX was 2 g
(23%), colorless crystals with mp 174 175 C. IR
1
spectrum, , cm : 1720, 1600, 1450. 1H NMR
spectrum (CDCl3), , ppm): 2.4 m (4H, CH2), 3.25 t
(2H, CH), 3.62 s (6H, OCH3), 7.28 m (5H, Harom).
Electrochemical methoxylation of cinnamic acid
(IV). The electrolytic cell was charged with 6.9 g
(0.04 mol) of cinnamic acid, 0.43 g (0.005 mol) of
KF, and 125 ml of methanol. The electrolysis was
carried out at 60 65 C under vigorous stirring at
The residue was extracted with benzene, the extract
was evaporated, and the residue was distilled under
reduced pressure to isolate methyl 3-phenylpropionate
(VIII). Yield 3.5 g (61%), bp 118 120 C (6 mm),
a current density of 0.1 A/cm2 (0.5 A); 1.93 105 C
n2D0 = 1.4991. IR spectrum, , cm : 3600 3400, 1720,
1
1
mol (2F/mol) of electricity was passed. When the
1
1650, 1600, 1450. H NMR spectrum (CDCl3),
,
reaction was complete, the solvent was distilled off
under reduced pressure (water-jet pump), and the
residue was treated with a 10% solution of KOH to
remove unchanged cinnamic acid. The aqueous layer
was extracted with benzene (2 30 ml), and the extract
was acidified with hydrochloric acid to isolate 5 g
of unreacted cinnamic acid. The benzene extract was
evaporated under reduced pressure to obtain 2 g
(80% on the reacted cinnamic acid or 22% on the
total amount of cinnamic acid) of 1,1,2-trimethoxy-
ppm: 2.5 t (2H, CH2), 2.9 t (2H, CH2), 3.25 d (1H,
CH), 3.65 s (3H, OCH3), 7.28 m (5H, Harom).
b. In a diaphragm cell. Methyl cinnamate, 4 g
(0.025 mol), and LiClO4, 1 g (0.01 mol), were dis-
solved in 150 ml of methanol. A 120-ml portion of
that solution was placed in the cathode space, and
the remaining 30-ml portion, in the anode space. The
electrolysis was carried out at a current strength of
0.75 A (cathode current density 0.1 A/cm2) and was
terminated when 1.93 105 C/mol (2F/mol) of elec-
tricity was passed. From the catholyte we isolated
1.3 g (32%) of methyl 3-phenylpropionate (VIII) and
0.5 g (7%) of dimer IX. We failed to perform elec-
trolysis using potassium fluoride as supporting elec-
trolyte because of too high resistance of the system.
Electrochemical reduction of cinnamamide (X).
The procedure was the same as described above for
reduction of methyl cinnamate (VII). From 6 g
(0.04 mol) of compound X we obtained 3.5 g (60%)
of 3-phenylpropionamide (XI) as colorless crystals
with mp 103 105 C (from water) [9].
2-phenylethane (V), bp 98 100 C (6 mm), nD20
=
1
1
1.4912. IR spectrum, , cm : 1720, 1620, 1450. H
NMR spectrum (CDCl3), , ppm: 3.18 s (3H, OCH3),
3.25 s (3H, OCH3), 3.45 s (3H, OCH3), 4.15 d (1H,
CH), 4.35 d (1H, CH), 7.28 m (5H, Harom). Only the
1H NMR and mass spectra of V were given in [2, 8].
After passing 6.76 105 C/mol (7F/mol) of elec-
tricity, the conversion of cinnamic acid was 100%.
According to the GLC data, a mixture of 1,1,2-tri-
methoxy-2-phenylethane (V, 54%) and benzaldehyde
dimethyl acetal (III, 46%) was formed. After approp-
riate treatment, we isolated 2.9 g (33%) of compound
V and 2.45 g (28%) of III, bp 80 82 C (6 mm), nD20
=
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-Truxillic acid was separated by filtration; yield
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 11 2001

ELECTROCHEMICAL METHOXYLATION OF CINNAMIC ACID
1595
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