ELECTROCHEMICAL METHOXYLATION OF ALKENYLDICHLOROANISOLES
681
The table shows that introduction of two Cl atoms
shifts the oxidation potential of alkenylanisoles to-
ward more positive values.
Cl
Cl
Cl
Cl
OMe
CH
OMe
OMe
+
OMe
OMe
CH=CH Me
It is interesting that chlorinated alkenylphenol is
oxidized considerably more readily, which is appar-
ently due to the possible formation of a quinoid deriv-
ative (Eox = 1.23 V) in the anodic region; this deriva-
tive is reduced to hydroquinone (Ered = 1.05 V) in
the cathodic region. Oxidation of anisoles is irreversi-
ble, as indicated by the absence of cathodic peaks in
the cyclic voltammogram at reverse potential scanning.
Cl
Cl
Trimethyl 2,5-dichloro-6-methoxyorthobenzoate
was obtained as by-product in 17% yield. We failed
to increase the yield of the acetal by varying the elec-
trolysis conditions.
EXPERIMENTAL
Preliminary experiments performed under the con-
ditions favorable for the cleavage of the C=C bond
(anhydrous methanol, p-toluenesulfonic acid support-
ing electrolyte [2, 3]) failed. Better results were ob-
tained with NaF as supporting electrolyte. In this case,
noticeable oxidation of methanol starts at an anode
potential of +0.8 V, and hence the transformation of
alkenyldichloroanisoles will occur by the mechanism
of indirect electrochemical oxidation:
Voltammetric measurements were performed on a
PI-50-1 potentiostat with a rotating platinum disk elec-
trode. The supporting electrolyte was 0.5 M LiClO4
in dry acetonitrile. The potential scanning rate was
50 mV s , and the electrode rotation rate, 3000 rpm.
The reference electrode was Ag/0.1 M AgNO3.
1
The starting compounds were purified by vacuum
distillation (2 mm Hg) before measurements.
.
MeOH
MeO + H+,
Syntheses were performed in a diaphragmless cell
equipped with a graphite anode (S = 26.6 cm2), a pla-
tinum cathode, a thermometer, and a reflux condenser.
The solution was stirred with a propeller stirrer.
e
C=C + MeO
C C OMe,
MeO
The electrolyzer was charged with 30 ml of abso-
lute methanol and 0.12 g of KF. The electrolysis was
performed with a current of 2.65 A (current density
OMe
OMe
MeO
2MeOH
2e
C C OMe
C C OMe
C
2
0.1 A cm ). Immediately after switching on the cur-
Preparative electrolyses showed that, at continuous
supply of the substrate so as to maintain its minimal
concentration in the solution, the methoxylation of
2-allyl-3,6-dichloroanisole occurs without formation
of substituted benzaldehyde acetal, and two products
are formed:
rent, 10 mmol of a substrate [2-(propen-1 -yl)-3,6-di-
chloroanisole or 2-allyl-3,6-dichloroanisole] dissolved
in 8 ml of methanol was added; its amount corre-
sponded to 8 F of electricity per mole of the substrate.
After the whole amount of the substrate was added,
the electrolysis was continued for a certain period, so
that the total amount of electricity was 10 F per mole
of the substrate. The electrolysis was performed at
55 60 C.
Cl
Cl
OMe
OMe
OMe
OMe
CH2 CH
CH2 CH=CH2
After the electrolysis completion, methanol was
removed on a rotary evaporator, the bottoms were
extracted with benzene to separate the supporting
electrolyte, and the extract was analyzed chromato-
graphically.
Cl
Cl
Cl
OMe
+
OMe
OMe
CH CH
OMe
Cl
CONCLUSIONS
However, in the methoxylation of 2-(propen-1 -yl)-
3,6-dichloroanisole under the same conditions, the
C=C bond is cleaved, and 2,5-dichloro-6-methoxy-
benzaldehyde dimethyl acetal is formed in a yield of
about 50% by the reaction
(1) Introduction of chlorine atoms into alkenyl-
anisole molecule appreciably shifts the oxidation poten-
tial toward positive values. An increase in the oxida-
tion potential complicates cleavage of the C=C bond.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 4 2006