Electroreductive coupling of aromatic acyl bromides to 1,2-diketones

Article abstract of DOI:10.1246/bcsj.74.755

The direct electroreduction of aromatic acyl bromides gave aromatic 1,2-diketones. The best result was obtained using a Pb cathode in acetonitrile containing Bu₄NClO₄ as a supporting electrolyte. Aromatic acyl bromides substituted by an electron-donating group afforded 1,2-diketones in moderate-to-good yields, whereas acylated endiols were formed exclusively from aromatic acyl bromides substituted by an electron-withdrawing group.

Full text of DOI:10.1246/bcsj.74.755

c. Jpn.
Bull. Chem. Soc. Jpn., 74, 755—756 (2001)
755
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Short Articles
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Table 1.
Run
Constant Current Electrolysis of Benzoyl Bromide
Electroreductive Coupling of Aro-
matic Acyl Bromides to 1,2-Dike-
tones
Solvent
Cathode
Electrolyte
Yielda) of
Yielda) of
N. Kise et al
1a/%
2a/%
01
5
6
SJA8
09-2673
0040
tober
1
2
3
4
5
6
7
8
9
AN
acetone
THF
DMF
AN
AN
AN
AN
AN
Pb
Pb
Pb
Pb
Sn
Cd
Zn
Pt
Bu4NClO4
Bu4NClO4
Bu4NClO4
Bu4NClO4
Bu4NClO4
Bu4NClO4
Bu4NClO4
Bu4NClO4
Bu4NClO4
Bu4NBr
65
43
3
0
0
6
12
9
24
36
10
0
78
71
52
68
67
67
56
70
Naoki Kise* and Nasuo Ueda
Department of Biotechnology, Faculty of Engineering,
Tottori University, Tottori 680-8552
(Received October 25, 2000)
C
3
00
10
11
12
AN
AN
AN
Pb
Pb
Pb
31
24
12
The direct electroreduction of aromatic acyl bromides
gave aromatic 1,2-diketones. The best result was obtained
using a Pb cathode in acetonitrile containing Bu4NClO4 as a
supporting electrolyte. Aromatic acyl bromides substituted
by an electron-donating group afforded 1,2-diketones in mod-
erate-to-good yields, whereas acylated endiols were formed
exclusively from aromatic acyl bromides substituted by an
electron-withdrawing group.
Bu4NBF4
LiClO4
00
0
a) Isolated yields.
porting electrolyte (run 1). The yield of 1a decreased in ace-
tone (run 2). Using THF or DMF as a solvent, poor results
were obtained due to the formation of complex mixtures (runs
3, 4). Only a Pb cathode gave good yields of 1a. The other
cathode materials (Sn, Cd, Zn, Pt, and graphite) yielded α,α′-
stilbenediol dibenzoate (2a) as a main product (runs 5–9). As
a supporting electrolyte, Bu4NClO4 afforded a better result
than the other tetrabutylammonium salts (runs 10, 11). The
use of LiClO4 considerably increased the yield of 2a (run 12).
It was predictable that the presence of oxygen influenced the
result of the electrolysis, since it has been reported that benzil
is decomposed to benzoic acid by electroreduction in the pres-
ence of oxygen.15 In fact, electrolysis without bubbling nitro-
gen gas lowered the yields of the products (1a: 49%, 2a: 24%).
Therefore, electrolysis needed to be carried out under a nitro-
gen atmosphere. The current density also affected the yield of
the products. Good results were obtained over the range of 2
mA cmꢀ2 (0.05 A, 1a: 63%, 2a: 25%) to 4 mA cmꢀ2 (0.1 A).
Electrolysis at 8 mA cmꢀ2 (0.2 A) brought about a decrease in
the yield of 1a (32%) and an increase in that of 2a (57%).
Since we could not observe the reduction potential of benzoyl
bromide, as previously reported,16 the cathode potential was
measured during the constant-current electrolysis (0.1 A) of
benzoyl bromide (Fig. 1). Initially, the cathode potential
dropped rapidly, and then became stable at ꢀ0.85 V vs. SCE.
After that, the potential gradually rose to ꢀ0.87 V vs. SCE at
the end of electrolysis. The electrolysis of benzoyl bromide at
a constant potential of ꢀ0.85 V vs. SCE gave a slightly better
result (1a: 68%, 2a: 14%) than did constant-current electroly-
sis (run 1 in Table 1), although a constant-current electrolysis
is much more convenient than a constant-potential electrolysis.
Table 2 presents the results of electroreductive coupling of
several aromatic acyl bromides and benzoyl halides under the
same conditions as run 1 in Table 1. Aromatic acyl bromides
substituted by an electron-donating group, such as a methoxy
or methyl group, afforded 1,2-diketones 1b–d in moderate-to-
good yields (runs 2–4). On the other hand, it can be presumed
that electron-withdrawing substituents lower the reduction po-
tential of the 1,2-diketone and, therefore, facilitate the reduc-
The reductive coupling of benzoyl chloride using metal re-
ducing agents1–3 or by electroreduction4,5 has been reported to
give α,α′-stilbenediol dibenzoate (Scheme 1). In this reaction,
benzil initially formed by dimerization of a benzoyl radical is
reduced to benzoin dianion, which reacts instantly with ben-
zoyl chloride.2 Therefore, half amounts of benzoyl chloride
were wasted in the dimer formation. In order to obtain benzil
from benzoyl chloride, reduction with activate lead,6 SmI2,7 or
Li under sonication8 has been reported. Other methods for the
preparation of aromatic 1,2-diketones by the reductive cou-
pling of aromatic acid derivatives include the reduction of aro-
matic acids with excess lithium,9 indirect electroreduction of
aromatic esters using a consumable Mg anode in the
presence10 or absence of SmCl3,11,12 the reductive coupling of
2-aryl-2-oxazolinium salts with Zn or by electroreduction,13
and the reduction of aromatic acyl cyanides with SmI2 or
ZnI2.14 In this paper, we report that the direct electroreduction
of aromatic acyl bromides afforded aromatic 1,2-diketones.
This reaction provides an alternative method for the synthesis
of aromatic 1,2-diketones without using metal reducing agents.
The results of the constant-current (0.1 A) electrolysis of
benzoyl bromide using a divided cell are summarized in Table
1. The best yield of benzil (1a: 65%) was obtained using a Pb
cathode in acetonitrile (AN) containing Bu4NClO4 as a sup-
Scheme 1.

756 Bull. Chem. Soc. Jpn., 74, No. 4 (2001)
© 2001 The Chemical Society of Japan
Bu4NClO4 (4.1 g, 12 mmol) in dry acetonitrile (40 mL) was put
into a divided cell of a 50 mL beaker (4.5 cm diameter, 6 cm
height) equipped with an Pb cathode (5ꢁ10 cm2), a Pt anode
(2ꢁ2 cm2), and a cylindrical ceramic diaphragm (2.5 cm diameter,
7 cm height). A stream of dry nitrogen gas was bubbled into the
catholyte (outside the diaphragm) for 10 min. To the catholyte
was added freshly distilled aromatic acyl bromide (3 mmol).
Electrolysis was carried out at a constant current of 0.1 A under
continuous bubbling of dry nitrogen gas until 290 C of electricity
(1 F/mol) had passed. The electrolyte was poured into water and
extracted with Et2O. The products were isolated by column chro-
matography on silica gel (hexane–AcOEt).
Determination of Products. 1,2-Diketones 1b, 1c, 1d, 1g,
and 1i were confirmed by a comparison of their spectroscopic data
with those of authentic samples prepared by reported methods.6,8
Acylated endiols 2 were determined by a comparison of their
spectroscopic data with reported ones. 1,2
Fig. 1.
ity in the electrolysis of benzoyl bromide (3 mmol) at 0.1 A
using a Pb cathode in 0.3 mol dmꢀ3 Bu4NClO4/AN.
Correlation between cathode potential and electric-
1h: Rf 0.5 (hexane–ethyl acetate, 5 : 1). IR (KBr) 1680, 1610,
Table 2.
Constant Current Electrolysis of Acyl Halidesa)
1600, 1580, 1455, 760, 730, 695 cmꢀ1
.
1H NMR (200 MHz,
CDCl3) δ 7.34–7.46 (m, 6 H), 7.46 (d, 2 H, J ꢂ 15.0 Hz), 7.57–
7.70 (m, 4 H), 7.85 (d, 2 H, J ꢂ 15.0 Hz). 13C NMR (50 MHz,
CDCl3) δ 120.08, 129.47, 129.54, 131.91, 134.91, 148.40, 189.75.
Anal. Calcd for C18H14O2: C, 82.42; H, 5.38%. Found: C, 82.35;
H, 5.41%.
Run
R
X
Yieldb) of 1/%
Yieldb) of 2/%
1
2
3
4
5
6
7
8
9
Ph
4-MeOC6H4
4-MeC6H4
2-MeC6H4
4-ClC6H4
4-MeO2CC6H4
1-naphthyl
PhCHꢂCH
Ph
Br
Br
Br
Br
Br
Br
Br
Br
Cl
I
1a
1b
1c
1d
65
52
70
40
0
2a
2b
2c
2d
2e
2f
24
26
12
8
83
70
32
18
78
51
42
55
References
1
2
L. Horner and K. Dickerhof, Chem. Ber., 116, 1603 (1983).
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0
248, 221 (1983).
1g
1h
1a
1a
1i
29
14
6
30
10
0
2g
2h
2a
2a
2i
3
T.-C. Wu and R. D. Rieke, J. Org. Chem., 53, 2381 (1988)
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4
A. Guirado, F. Barba, C. Manzanera, and M. D. Velasco, J.
10
11
12
Ph
C2H5
C2H5
Org. Chem., 47, 142 (1982).
Br
Cl
5
G. T. Cheek and P. A. Horine, J. Electrochem. Soc., 131,
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1796 (1984).
a) Electrolysis was carried out under the same conditions as run 1
in Table 1. b) Isolated yields.
6
7
L. Mészáros, Tetrahedron Lett., 1967, 4951.
P. Girard, R. Couffignal, and H. B. Kagan, Tetrahedron
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tion of the 1,2-diketones to the benzoin dianion. Actually, the
substitution of a chloro or methoxycarbonyl group brought
about the exclusive formation of acylated endiols 2e, f (runs 5,
6). The electroreduction of cinnamoyl chloride at a constant
potential of ꢀ1.45 V vs. SCE with a mercury pool cathode in
acetone containing LiClO4 has been reported to give 2,4-
dibenzyl-1,3-cyclobutanedione.17 Under our conditions, 1,2-
diketone 1h and acylated endiol 2h were obtained from cin-
namoyl bromide (run 8). The preferential formation of benzil
(1a) is specific for benzoyl bromide; that is, the electrolysis of
benzoyl chloride or iodide gave 2a as a major product (run 9,
10). It has been reported that the reduction potenial of benzoyl
chloride is more negative than that of 1a.4 Although the reduc-
tion potential of benzoyl iodide is unknown, this result shows
that 1a is more subject to reduction than benzoyl iodide under
our conditions. Aliphatic acyl halides also yielded mainly acy-
lated endiols (runs 11, 12).
8
B. Hee and P. Boudjouk, Tetrahedron Lett., 22, 2757
(1981).
9
R. Karaman and J. L. Fry, Tetrahedron Lett., 30, 6267
(1989).
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587 (1984).
Experimental
Starting Materials. Benzoyl bromide was commercially
available. The other acyl bromides were prepared from aromatic
acids by the reported methods.18,19 Benzoyl iodide was obtained
by the treatment of benzoyl chloride with NaI.20
18 J. M. Aizpurua and C. Palomo, Synthesis, 1982, 684.
19 von H.-J. Bestmann and L. Mott, Justus Liebigs Ann.
Chem., 693, 132 (1966).
20 D. W. Theobald and T. C. Smith, Chem. Ind. (London),
1958, 1008.
General Procedure for Electroreduction.
A
solution of