One Pot Electrooxidative Conversion of Aromatic 1,2-Diketones into Methyl Carboxylates

Article abstract of DOI:10.1081/SCC-120025186

One-pot conversion of several aromatic1,2-diketones into the corresponding methyl carboxylates was performed by indirect electrooxidation using iodide ion as an electron carrier in the presence of catalytic amount of sodium cyanide.

Full text of DOI:10.1081/SCC-120025186

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Õ
SYNTHETIC COMMUNICATIONS
Vol. 33, No. 21, pp. 3771–3776, 2003
One Pot Electrooxidative Conversion of Aromatic
1,2-Diketones into Methyl Carboxylates
Mitsuhiro Okimoto,* Yuji Nagata, Satoru Sueda, and
Yukio Takahashi
Department of Applied Chemistry and Circumstance
Engineering, Kitami Institute of Technology,
Kitami, Japan
ABSTRACT
One-pot conversion of several aromatic1,2-diketones into the
corresponding methyl carboxylates was performed by indirect
electrooxidation using iodide ion as an electron carrier in the
presence of catalytic amount of sodium cyanide.
Key Words: Electrooxidation; Benzil; Methyl carboxylate; Iodide
ion; Cleavage.
*Correspondence: Mitsuhiro Okimoto, Department of Applied Chemistry and
Circumstance Engineering, Kitami Institute of Technology, Koen-cho 165,
Kitami 090-8507, Japan; E-mail: oki@chem.kitami-it.ac.jp.
3
771
DOI: 10.1081/SCC-120025186
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3772
Okimoto et al.
Several studies describing the cleavage of benzil in the presence
of cyanide ions have been reported. Trisler and Frey have obtained
stilbenediol dibenzoate from benzil by treatment with sodium cyanide
[
1]
in dimethyl sulfoxide.
with cyanide ions in alcoholic solutions to afford cleavage products,
Kwart and Baevsky have treated benzil
[
2]
benzaldehyde, and the corresponding benzoate. Furthermore, Dakin
and Harington have found that benzamide and benzaldehyde
are formed as main products by the cleavage of benzil in alcoholic
[
3]
ammonium cyanide solutions. Although cleavage of benzil by cyanide
ions has been studied extensively, and is well-known, as mentioned
above, complete transformation of 1,2-diketones through electro-
oxidation into a single product has yet to be reported, to the best of
our knowledge. As a note, we have previously reported on the effective
electrooxidative conversion of benzaldehydes into the corresponding
methyl carboxylates, in the presence of catalytic amount of sodium
[
4]
[5]
cyanide or KI.
As a continuation in our series of studies using electrochemical
methods, we have carried out one-pot conversion of 1,2-diketones into
two equivalent amounts of methyl carboxylates. During the beginning
stages of the experiment, we have confirmed that the utilization of one
tenth equivalent of sodium cyanide relative to benzil in MeOH was
sufficient to nearly complete the cleavage of benzil into benzaldehyde
and methyl benzoate. Moreover, this cleavage reaction occurred within
a few minutes at room temperature. In our studies, several 1,2-diketones
were cleaved, between the two carbonyl groups, and subsequently
electrooxidized to afford the corresponding methyl carboxylates in
good to excellent isolated yields (Table 1).
Unexpectedly, the yields of the corresponding methyl carboxylates
0
(
4
2e, 2k) from 2,2 -dimethoxy benzil (1e) and 2-pyrizil (1k) were only
6 and 43%, respectively. In the former case, cleavage reaction by the
ꢀ
cyanide ions was not as efficient, even at elevated temperatures (60 C),
and/or with a longer reaction period (6 h). In contrast, in the latter case,
although the cleavage reaction proceeded effectively, the subsequent
electrooxidation process of the corresponding aldehyde into the methyl
carboxylate did not proceed smoothly. Other reaction conditions of the
electrooxidation, such as electrode material, halogen ion source, amount
of added KI, and reaction temperature were examined using benzil (1a)
as the standard substrate. To study the electrode material, substituting
the platinum net with four pieces of carbon poles (diameter, 5 mm;
height, 80 mm) as the anode material resulted in the decrease in the yields
of methyl benzoate, from 95 to 82% (yields were determined using GLC
analysis), in the case of a lead plate (width, 30 mm; height, 50 mm)

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One Pot Conversion of Aromatic 1,2-Diketones
3773
a
Table 1. One pot conversion of 1,2-diketones into methyl carboxylates.
Oxidation
R C C R
2 R C OMe
O
O
KI, NaCN / MeOH
O
2
1
b
Yield of 2
(%)
Current passed
ꢂ1
Substrate R
(F mol
)
1
1
1
1
1
1
1
1
1
1
1
a
b
c
d
e
f
Ph
4-CH
4.0
4.0
5.7
6.5
7.8
3.7
5.5
5.0
4.0
4.0
4.9
80 (95)
82 (92)
81 (88)
83 (88)
46 (53)
93 (98)
85 (94)
94 (98)
86 (93)
83 (93)
43 (55)
3
C
6
H
4
2-CH C H
3
3
3
6
4
4-CH
2-CH
OC
6
H
4
OC
6
H
4
4-ClC H4
6
g
h
i
6 4
2-ClC H
4-tert-Bu-C
2-Furyl
6
H
4
j
2-Thienyl
2-Pyridil
k
a
Substrate: 2.5 mmol; KI: 2.0 mmol; NaCN: 0.25 mmol, 0.5 mmol(1c),
ꢀ
1
b
.0 mmol(1d), 4.0 mmol(1e); MeOH: 40 mL; temperature: ca.15 C.
Isolated yield. Values in parentheses are GLC yield.
as the anode, the yield of the methyl benzoate was only 58%, and
benzaldehyde remained unreacted (30%). Reactions using alternate
halogen sources, such as NaI, I , KBr, and KCl, were investigated,
2
resulting in methyl benzoate yields of 90, 88, 47, and 46%, respectively.
These results suggested that iodide ions were effective as electron carrier
in this oxidation system, relative to the bromide and chloride ions.
Investigations have shown that the utilization of 0.8 equiv. of KI for
1a was sufficient for this indirect electrooxidation. In terms of the
reaction temperature, we did not observe significant differences in
the yields of methyl benzoate (ꢁ4%) between the temperature range
ꢀ
of 0–50 C.
Possible reaction pathways of this cleavage reaction and the
subsequent oxidation are illustrated in Schs. 1–3. Presumably, the 1,2-
diketone molecule was cleaved into the corresponding methyl carboxylate
and aldehyde by attack of the cyanide ion at the first step of the reaction,
as shown in Sch. 1, theoretically, methyl carboxylate was already formed
in 50% yield in this first reaction step. As the second reaction step, the
subsequent electrooxidation of the aldehyde proceeded gradually. In

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3774
Okimoto et al.
-
CN
CN
R
C
O
C
O
R
R
C
C
R
O
O-
CN
O- CN
O
C
R
R
C
C
R
R
R
C
-
O
O
CN
CH
+
MeOH
-CN-
R
C
H
R
C
OMe +
O
O
-O
Scheme 1.
MeOH
OMe
-
H+
-H+
R
C
H
R
C
H
-
I-
O
O
Route 1
+
I
I
-
-
2e
R
C
OMe
Anode
-
O
-
2e
MeOH
CN
CN-
CN
NC
C+
OMe
H+
+
- CN
-
-
-H
R
C
H
R
C-
O
R
R
C
R
C
CN
O
O
O
O-
H
H
Route 2
Scheme 2.
the presence of either iodide or cyanide ions, we proposed two reaction
pathways (Sch. 2, Routes 1 and 2, respectively), as based on our previous
[4,5]
studies.
Route 1 is the indirect electrooxidation of the aldehyde by
the action of the iodine cationic species, which were formed through a
two-electron transfer of the iodide ion on the anode. Route 2 is the direct
oxidation of cyanohydrin derivative, which was formed by the reaction of
the aldehyde with cyanide ions to afford benzoyl nitrile through a
two-electron transfer on the anode. We had previously conformed that
benzoyl nitrile readily reacted with MeOH to give methyl benzoate, even

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One Pot Conversion of Aromatic 1,2-Diketones
3775
-
+
-
2e , + MeOH, -2H
R
C
H
R
C
OMe
O
O
Scheme 3.
[6]
at room temperature. On the other hand, we also observed that some
aromatic aldehydes could be converted into the corresponding methyl
carboxylates in ca. 85% yield after passing 2.3 F/mol of electricity
under the same reaction conditions employed in this study.
In our opinion, Route 1 is strongly preferred over Route 2, since the
oxidation potential of iodide ions is much lower than that of cyanohydrin
[7]
derivative, and in fact, in the absence of KI, methyl carboxylates could
not be obtained in such high yields. Shortened versions of both reaction
routes presented in Sch. 2 are illustrated in Sch. 3. In conclusion, we have
established a one-pot conversion of 1,2-diketones into the corresponding
methyl carboxylates under very mild conditions by utilizing the electro-
chemical technique with catalytic amounts of cyanide and iodide ions.
EXPERIMENTAL
All electrooxidation products (2a–2k) were identified by comparison
of their physical and spectral data with those of authentic samples.
[
8]
The substrates were prepared by the usual oxidation reactions of the
corresponding acyloines, which were provided by the typical benzoin
[9]
condensations
of the corresponding aldehydes. Other reagents
were obtained from commercial suppliers and used without further
purifications. Preparative-scale electrooxidations were carried out in a
tall 50 mL beaker equipped with a fine frit cup as the cathode compart-
ment, a cylindrical platinum net anode (diameter, 33 mm; height, 40 mm),
and a nickel coil cathode. 1,2-Diketones 1a–1k were oxidized under
conditions as follow: a solution of substrate 1a–1k (2.5 mmol), KI
(2.0 mmol), and NaCN (0.25 mmol, 0.5 mmol for 1c, 1.0 mmol for 1d)
in MeOH (40 mL) was electrooxidized under a constant current (0.2 A).
During the course of the electrooxidation, the anolyte was magnetically
stirred, and the temperature of the cell was maintained at approximately
ꢀ
1
treated using a three fold amount of the anolyte solution. After concen-
5 C. After completion of the electrooxidation, the reaction mixture was
ꢀ
trating the combined anolyte solution in vacuo at approximately 30 C to
roughly one-fifth of its original volume, the resulting residue was treated

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3776
Okimoto et al.
with water (60 mL), and the oily layer was extracted with ether
(
3 ꢃ 40 mL). The combined ether extracts were washed with aqueous
sodium thiosulfate solution (20 wt%, 20 mL), and dried over anhydrous
magnesium sulfate. After removal of the solvent, the residue was purified
by distillation or by column chromatography (diameter, 3.5 cm; height,
2
5 cm) on silicagel using ether as the eluent.
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1
2
3
4
. Trisler, J.C.; Frye, J.L. J. Org. Chem. 1964, 30, 306.
. Kwart, H.; Baevsky, M.M. J. Am. Chem. Soc. 1958, 80, 580.
. Dakin, H.D.; Harington, C.R. J. Biol. Chem. 1923, 55, 487.
. Chiba, T.; Okimoto, M.; Nagai, H.; Takata, Y. Bull. Chem. Soc. Jpn.
1
982, 55, 335.
5. Okimoto, M.; Chiba, T. J. Org. Chem. 1988, 53, 218.
6. Okimoto, M.; Chiba, T. J. Org. Chem. 1996, 61, 4835.
7. Shono, T.; Matsumura, Y.; Hayashi, J.; Mizoguchi, M. Tetrahedron
Lett. 1979, 165.
8
. Clarke, H.T.; Dreger, E.E. Organic Syntheses, 2nd Ed.; Gilman, H.,
Ed.; John Wiley and Sons Inc.: New York, 1956; Vol. 1, 87.
. Adams, R.; Marvel, C.S. Organic Syntheses. 2nd Ed.; Gilman, H.,
Ed.; John Wiley and Sons, Inc.: New York, 1956; Vol.1, 94.
9
Received in Japan April 9, 2003