Electrochemical oxidation of benzoins to benzils in the presence of a catalytic amount of KI in basic media

Article abstract of DOI:10.1055/s-2005-861809

Various benzoins were transformed into the corresponding benzils via an electrochemical method. Although the strategy of oxidizing benzoins to the corresponding benzils is generally chosen depending on the nature of the aromatic moiety, the electrochemical technique reported herein allows oxidation that is widely applicable, and without the use of environmentally undesirable oxidants.

Full text of DOI:10.1055/s-2005-861809

SHORT PAPER
705
Electrochemical Oxidation of Benzoins to Benzils in the Presence of a Catalytic
Amount of KI in Basic Media
Electrochemical
O
xida
i
tionof
t
B
enz
o
s
n
u
zils hiro Okimoto,* Yukio Takahashi, Yuji Nagata, Gaku Sasaki, Kaori Numata
Department of Applied and Environmental Chemistry, Kitami Institute of Technology, Koencyo, 165 Kitami, Hokkaido 090-8507, Japan
Fax +81(157)247719; E-mail: oki@chem.kitami-it.ac.jp
Received 27 September 2004; revised 18 November 2004
Table 1 Electrochemical Oxidation of Benzoin (1a) to Benzil (2a)a
Abstract: Various benzoins were transformed into the correspond-
Ph
Ph CH
OH
C
O
Ph
Ph C
O
C
O
Oxidation
ing benzils via an electrochemical method. Although the strategy of
oxidizing benzoins to the corresponding benzils is generally chosen
depending on the nature of the aromatic moiety, the electrochemical
technique reported herein allows oxidation that is widely applica-
ble, and without the use of environmentally undesirable oxidants.
1a
2a
Run
Halogen Ion Source Base (5 mmol)
(mmol)
Yield (%)b
Key words: alcohols, ketones, dehydrogenations, iodine, electron
transfer
1
2
3
4
5
6
7
8
none
NaOMe
NaOMe
NaOMe
NaOMe
none
51
60
84
82
36
49
60
73
KI (0.1)
KI (1.0)
NaI (1.0)
KI (5.0)
KBr (1.0)
KCl (1.0)
KI (1.0)
The oxidation of benzoins is an important synthetic reac-
tion in affording the corresponding benzils, which further
serve as useful organic compounds.1 Well-known oxida-
tive methodologies of benzoins into benzils involve oxi-
dants such as nitric acid,2 copper sulfate,3 and chromium
oxide.4 These oxidants, however, are environmentally un-
desirable and/or unsuitable for the oxidation of benzoins
that are sensitive to further oxidative cleavage of carbon–
carbon bonds. Consequently, several novel methods for
the oxidation of benzoins to benzils have been recently in-
NaOMe
NaOMe
NaOH
a 1a: 8 mmol; MeOH: 40 mL; current passed: 2.1 F/mol; reaction
vestigated, and although each method claims certain ad- temp: ca. 15 °C.
b Determined by GC analysis.
vantages in obtaining benzils from benzoins, most of
these methods require stoichiometric or excessive
amounts of toxic metal oxidants5 or expensive catalysts6
with molecular oxygen and/or long reaction periods at el-
evated temperatures. In view of minimizing toxic waste,
we focused our studies on the oxidation of benzoins using
electrochemical procedure. Electric current is one of the
cleanest tools in the oxidation of organic compounds,7 and
accordingly, several electrochemical oxidation methods
have provided a variety of useful and/or novel organic
compounds, without the use of any environmentally unde-
sirable oxidants. In continuation of our series on the elec-
trochemical preparation of organic compounds,8 a wide
variety of benzoins were transformed into the correspond-
ing benzils using an indirect electrochemical method with
iodide ion as an electron carrier.
In the case where NaOMe was used as the lone supporting
electrolyte, the yield of 2a was nearly 50% (run 1). In con-
trast, when KI was used alone, the yield was merely 36%
(run 5). Subsequently, it was determined that for 8 mmol
of 1a, the use of 1 mmol of KI or NaI and 5 mmol of
NaOMe was favorable in affording higher yields of 2a
(runs 3, 4). In the cases involving KBr or KCl instead of
KI, the yields of 2a were 49% (run 6) and 60% (run 7), re-
spectively. These results suggest that 1) 1a was oxidized
indirectly by the cationic iodide species that formed on the
anode, and 2) the presence of base was necessary to ad-
vance the reaction.
The electrochemical oxidation of benzoins 1a–n was car-
ried out under optimal conditions, based on the yields list-
ed in Table 1, and the results are shown in Table 2.
Our investigations on the electrooxidation of benzoins be-
gan with the optimization of the reaction conditions. The
reactions of benzoin (1a) in the presence of various halide
ion sources and bases, along with the resulting yields of
benzil (2a), are listed in Table 1.
During the course of electrooxidation, the composition of
the reaction mixture was carefully monitored using silica
gel TLC analysis (diethyl ether–hexane, 1:1). For each re-
action, electrolytic current was applied until the substrate
was nearly completely consumed. Except in the case of
1h, the starting benzoins were converted into the corre-
sponding benzils after the consumption of 2.1 (1a) to 2.8
(1e, 1i) F/mol of electricity. In some cases, because of the
low solubility of the substrates in MeOH, the electrooxi-
SYNTHESIS 2005, No. 5, pp 0705–0707
x
x
.
x
x
.2
0
0
5
Advanced online publication: 14.02.2005
DOI: 10.1055/s-2005-861809; Art ID: F13804SS
© Georg Thieme Verlag Stuttgart · New York

706
M. Okimoto et al.
SHORT PAPER
Table 2 Electrochemical Oxidation of Benzoins 1 to the Corresponding Benzils 2a
R1
Oxidation
R1
R2
CH
OH
C
O
R2
C
O
C
O
KI, NaOMe in MeOH
2
1
Benzoin
1
Current Passed
(F/mol)
Yield (%)b
Mp (°C) or (bp, °C/mbar)
R1
Ph
R2
Ph
Found
Reportedc
95
1a
1b
1c
1d
1e
1f
2.1
2.2
2.5
2.7
2.8
2.2
2.5
2.4
2.8
2.5
2.7
2.7
2.5
2.4
78
81
80
86
72
72
75
62
79
75
72
84
76
78
92–94
4-MeC6H4
2-MeC6H4
4-MeOC6H4
2-MeOC6H4
4-ClC6H4
2-furyl
2-pyridyl
Ph
4-MeC6H4
2-MeC6H4
4-MeOC6H4
2-MeOC6H4
4-ClC6H4
102–105
104–105
92–94
131–133
127
89–91
130–131
127–129
195–197
197–199
162
1g
1hd
1i
2-furyl
159–161
2-pyridyl
151–153
154–155
72–74
60–61
115–116
105–106
41
4-ClC6H4
72–74 (163–165/1.3)
58–60 (120–122/1.3)
114–116
1j
Ph
4-MeOC6H4
4-(Me)2NC6H4
4-MeOC6H4
2-furyl
1k
1l
Ph
2-ClC6H4
Ph
102–104
1m
1n
39–41 (143–145/2.6)
57–59 (159–161/2.6)
Ph
2-thienyl
–
a Compound 1 (8 mmol) was oxidized in MeOH (40 mL) containing KI (1 mmol) and NaOMe (5 mmol) under the constant current (0.3 A) at
a reaction temperature of 15 °C, except for 1d, 1l (30 °C), and 1k (50 °C).
b Isolated yield based on 1.
c See ref. 5j for 2i,j,l and ref. 9 for 2a–h,k,m.
d KCl (2 mmol) was used instead of KI.
Anode
dation was carried out at elevated temperatures of 30 °C
(1d, 1l) and 50 °C (1k). In the case of a-pyridoin (1h) us-
-2e
ing KI, the corresponding a-pyridil (2h) was not obtained;
in fact, a small amount of unreacted 1h was recovered
from the tar-like material. Subsequently, it was found that
2h could be obtained in 62% yield by substituting KCl for
KI.
+ I+ - H+
NaOMe
Ar C CH Ar
- I- - H+
NaOMe
Ar
C C
Ar
Ar C CH Ar
O O
O
O O
I
OH
Although details of the reaction mechanism remain un-
clear, it can be proposed that the iodide ion plays an im-
portant role as an electron carrier in this reaction system.
Specifically, the iodide ion undergoes a two-electron oxi-
dation process to form an iodonium ion, which then reacts
with the substrate to give an intermediate as illustrated in
the structure within the square brackets in Scheme 1.
2NaOMe + H2
2Na+
2MeOH
2Na
+2e
Cathode
In this electrolysis system, the presence of a base such as
NaOMe presumably facilitates the deprotonations of both
the substrate and the intermediate to form the final prod-
uct. Theoretically, as shown in the upper pathway of
Scheme 1, at least two equivalents of base (NaOMe) are
required for each substrate. Experimentally, however,
only about 0.6 equivalent of base was sufficient for the re-
Scheme 1
Synthesis 2005, No. 5, 705–707 © Thieme Stuttgart · New York

SHORT PAPER
Electrochemical Oxidation of Benzoins to Benzils
707
1H NMR (CDCl3): d = 7.0–7.2 (m, 1 H, thienyl), 7.4–7.7 (m, 3 H,
action. Presumably, this is due to the cathodic reaction in
which sodium ions are continuously reduced to sodium
C6H5), 7.7–7.9 (m, 2 H, C6H5), 7.9–8.1 (m, 2 H, thienyl).
that immediately reacts with methanol to give NaOMe 13C NMR (CDCl3): d = 128.88 (CH), 129.78 (CH), 130.10 (CH),
132.63 (C), 134.83 (CH), 136.66 (CH), 136.86 (CH), 139.79 (C),
185.56 (C=O), 192.04 (C=O).
MS: m/z (%) = 216 (M+, 10), 111 (C4H3SCO+, 50), 105 (C6H5CO+,
100), 77 (43), 51 (20), 39(20).
during the course of the electrooxidation, as illustrated in
the lower pathway of Scheme1. The optimal amount of
NaOMe (5 mmol for 8 mmol of benzoins) used herein was
determined separately. Using excessive amounts of
NaOMe (16 mmol or 32 mmol for 8 mmol of benzoins)
did not increase the yield of the products. Furthermore,
the base can also function as an efficient supporting elec-
trolyte (the terminal voltage of the electrolytic cell was al-
ways maintained at less than 10 V).
HRMS: m/z calcd for C12H8O2S: 216.0245; found: 216.0253.
References
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In conclusion, by means of an electrochemical method,
the oxidation of benzoins was achieved under very mild
conditions, and without the use of environmentally unde-
sirable reagents. The corresponding benzils were obtained
in moderate to good yields with reasonable current effi-
ciencies in a range of 51% (1e) to 74% (1a). Moreover, the
isolation of the resulting benzils was relatively simple.
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reported values (listed in Table 2). Compound 2n was identified us-
ing its spectral data as described below. The substrates were synthe-
sized according to methods given in the literature.10 All other
reagents were obtained from commercial suppliers and were used
without further purifications.
Electrooxidation of Benzoins 1 to Benzils 2; General Procedure
Preparative-scale electrooxidations were carried out in a tall 50 mL
beaker, equipped with a fine frit cup as the cathode compartment,
an insert cylindrical platinum net (diameter: 32 mm; height: 35 mm;
55 mesh) as the anode, and a nickel coil cathode. A solution of sub-
strate 1a–n (8 mmol), KI (166 mg, 1 mmol), and NaOMe (270 mg,
5 mmol) in MeOH (40 mL) was electrooxidized under a constant
current (0.3 A). During the course of the electrooxidation, the
anolyte was magnetically stirred while maintaining the temperature
of the cell at approximately 15 °C, except for 1d and 1l (30 °C) and
1k (50 °C). After completion of the electrooxidation, the reaction
mixture was concentrated in vacuo at approximately 40 °C to
roughly 1/5th of its original volume, and the resulting solid was col-
lected by filtration and washed with small portions of cold MeOH.
In the cases of 1i,j,m,n, the product was somewhat difficult to so-
lidify, and therefore, the resulting residue was treated with H2O (30
mL), and then extracted with Et2O (3 × 40 mL). The combined ether
extracts were washed with an aq Na2S2O3 solution (20% w/w, 20
mL), and dried (MgSO4). After removal of the solvent, the residue
was purified by distillation under reduced pressure or by recrystal-
lization from small amounts of EtOH.
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1-Phenyl-2-(2-thienyl)ethanedione (2n)
Yield: 78%; bp 159–161 °C/2.6 mbar; mp 57–59 °C; fine plates
(EtOH).
IR (KBr): 1670, 1647, 1593, 1408, 1234, 1221, 746, 739, 719, 646
cm–1.
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Synthesis 2005, No. 5, 705–707 © Thieme Stuttgart · New York