240
Chemistry Letters 2001
Evidence for Ionic Interaction between Cationic Surfactant and Anionic Intermediate Generated
in Cathodic Reduction of Acetophenone
Yorimitsu Kodama,* Akihito Fujiwara, Hideki Kawamoto, Nobuaki Ohta, Akira Kitani,† and Sotaro Ito
Department of Applied Physics and Chemistry, Faculty of Engineering, Hiroshima University, 1-4-1 Kagamiyama,
Higashi-Hiroshima 739-8527
†Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527
(Received November 27, 2000; CL-001075)
The cathodic reduction of acetophenone in the presence of
a chiral cationic surfactant in aqueous media gave S- or R-1-
phenylethanol with 8–12 % ee. The observed enantioselectivity
clearly suggests the interaction between the cationic surfactants
and the anionic intermediate generated from the one-electron
reduction of acetophenone.
The electrolytic reactions of organic compounds have been
usually carried out in organic solvents such as acetonitrile,
dichloromethane and DMF.1 The use of water instead of organ-
ic solvents is strongly desired from the viewpoint of environ-
mental safety and economical cost. The electrolyses of organic
compounds in aqueous media are, however, very limited,
because of their low current efficiency and low selectivity.
Recently, we have reported that the cathodic reduction of ace-
tophenone 1 in the presence of cationic surfactants forms 1-
phenylethanol 2 selectively in aqueous solutions.2,3 The high
selectivity of 2 observed in aqueous solutions is unusual,
because the selective electroreduction of 1 to 2 is known to be
achieved only in organic solvents. The selective formation of 2
was explained in terms of the electrostatic interaction between
the negative charge on the intermediate anion derived from ace-
tophenone and the positive charge on the micelle formed from
cationic surfactants, though the direct evidence for this explana-
tion was not present.2,3 In this study, we would like to report
the enantioselective reduction of 1 in the presence of chiral
cationic surfactants. The present observations will hardly
explained without postulating the electrostatic interaction
between the cationic surfactants and the anionic intermediate
generated from the one-electron reduction of acetophenone.
The electrolytic apparatus and conditions were essentially
the same as the previous ones.2,3 Acetophenone (2.2 mmol) in
100 mL of an aqueous sodium sulfate solution (0.05 M (1 M =
1 mol dm–3)) containing 10 mL of acetonitrile and 2.7 mmol of
surfactant was electrolyzed on Pb cathode (12 cm2) at room
temperature under the constant-potential conditions at –2.1 V vs
SCE. The catholyte was stirred in nitrogen atmosphere during
the electrolysis. After passing 5 mF (1.2 F mol–1 (1 F =
96484.56 C)) of electric charge, the catholyte was subjected to
HPLC analysis. HPLC analysis conditions were: Chiralcel OB
(250 × φ 4.6 mm) column (Daicel Chemical Industries, LTD.),
an eluent with 9:1 hexane/2-propanol at 1.0 mL/min.
The additive effect of the surfactants on the product distri-
bution in the electroreduction of 1 was summarized in Table 1.
In the presence of the cationic surfactants 4 and 5, the selectivi-
ty of 2 was increased up to the ratio observed in the presence of
hexadecyltrimethylammonium bromide (CTAB), a typical
cationic surfactant commercially available. Likewise, the addi-
tion of 6 and 7 to the catholyte accelerated the formation of 2.
From the results in Table 1, it is understood that the additives
4–7 act as a cationic surfactant and favor the formation of 2, as
CTAB does.
The enantioselectivity of 2 in the electroreduction of 1 was
summarized in Table 2. In the absence of surfactant and in the
presence of achiral surfactants, the enantiomer excess (ee) was
essentially zero. The addition of the chiral surfactants S-4, S-5,
R-6 and R-7 to the catholyte, on the contrary caused an asym-
metric reaction to give R-2 on the ee values of 3.4–12 %. It is
also noteworthy that if the S-surfactant accelerates the formation
of S-2, the corresponding R-surfactant favors the R-isomer, and
vice versa. Successful examples of the enantioselective conver-
sions in electroreduction are quite few. On electroreduction of
ketones in the presence of chiral additive such as ephedrine
derivatives,7,8 alkaloids,7,9 and poly(pyrroles),10 the correspon-
ding alcohols were obtained enantioselectively with 20, 20, and
17 % ee, respectively. Shäfer studied the electroreduction of 4-
methylcoumarin in the presence of chiral alkaloid, yohimbin, to
get (+)-R-4-methyl-3,4-dihydrocoumarin with 47 % ee.11
It is well known that the electroreduction of carbonyl com-
pounds proceeds in two-step one-electron transfer, as illustrated
in Scheme 1.
Four chiral cationic surfactants 4–7, which have an asym-
metric carbon near the ammonium nitrogen with a positive
charge, were synthesized4–6 and the additive effect of the chiral
surfactants on the product distribution in cathodic reduction of
1 was examined.
Copyright © 2001 The Chemical Society of Japan
Kodama, Yorimitsu
