Electron transfer in the cathodic reduction of α-dicarbonyl compounds

Article abstract of DOI:10.1016/j.tet.2007.11.102

Electron transfer processes take place during the cathodic reduction, under an argon atmosphere, of different α-dicarbonyl substrates. Carboxylic acids or methylene diesters are obtained from benzil or furil after electron transfer to the oxygen in the air, during the workup, or after electron transfer to the solvent. Involving an electron transfer to dichloromethane, 2-hydroxy-2-hydroxymethyl-2H-acenaphtylen-1-one or benzo[1,3]dioxin-8-one are formed when acenaphthenequinone or 1,2-cyclohexanedione are, respectively, reduced.

Full text of DOI:10.1016/j.tet.2007.11.102

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 1834e1838
www.elsevier.com/locate/tet
Electron transfer in the cathodic reduction of a-dicarbonyl compounds
*
´
Belen Batanero, Fructuoso Barba
Department of Organic Chemistry, University of Alcala´, 28871 Alcala´ de Henares, Madrid, Spain
Received 19 October 2007; received in revised form 26 November 2007; accepted 28 November 2007
Available online 3 December 2007
Abstract
Electron transfer processes take place during the cathodic reduction, under an argon atmosphere, of different a-dicarbonyl substrates.
Carboxylic acids or methylene diesters are obtained from benzil or furil after electron transfer to the oxygen in the air, during the workup,
or after electron transfer to the solvent. Involving an electron transfer to dichloromethane, 2-hydroxy-2-hydroxymethyl-2H-acenaphtylen-1-
one or benzo[1,3]dioxin-8-one are formed when acenaphthenequinone or 1,2-cyclohexanedione are, respectively, reduced.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Cathodic reduction; Electron transfer; Dichloromethane; 1,2-Dicarbonyl compounds
1. Introduction
stronger or weaker nucleophiles, but rather it depends on the
formal potential E0 (taken as the average of the cathodic
0
The electrochemical reduction of a-dicarbonyl substrates
has already been studied, from the synthetic point of view,
in diaryl-1,2-diketones such as benzil, both, under protic con-
ditions to prepare benzoine1 or in aprotic media, but in the
presence of electrophiles to get dialkylated2 or diacylated3
compounds, as well as enediol iminocarbonates.4
However, the preparative scaled cathodic reduction of other
a-dicarbonyl compounds has been poorly studied. An example
is the acylation of 1,2-acenaphthenedione that affords, depend-
ing on the employed acylating reagent, the diacylated deriva-
tive or a diketopinacol diester.5 The enediol diester formation
from 9,10-phenanthrenequinone or 1,2-acenaphthenequinone
has also been performed.6
and the anodic peak potentials Epc and Epa) of the second
reduction peak of the quinone. The more negative E0 value
0
of the quinone, the higher yield of dioxole was obtained.
In the present work 1,2-dicarbonyl substrates: benzil (1a),
a-furil(1b), acenaphthenequinone (1c), and1,2-cyclohexanedione
(1d) have been electrolyzed under aprotic CH2Cl2/Et4NCl and po-
tentiostatic conditions to afford, in some cases, non-dioxole prod-
ucts whose formation is rationalized throughanion radical species.
2. Results and discussion
2.1. Benzil and a-furil
Recently, an easy conversion of ortho-quinones into 1,3-
dioxoles has been described7 as a single-step process by
cathodic reduction in dichloromethane. In this paper it was
demonstrated that the formation of the dioxole did not occur
through a nucleophilic substitution in the solvent by the elec-
trogenerated dianion, but through an electron transfer. The
yield of the formed 1,3-dioxole did not depend on the use of
Cyclic voltammetry of benzil (1a) and a-furil (1b) in
dichloromethane/Et4NCl shows a reversible two-electron reduc-
tion peak (see Table 1) with Epc and Epa values of ꢀ1.07 and
ꢀ0.87 V (vs Ag/Agþ), respectively.
Preparativeelectrolysisof1aafforded,afterachargeconsump-
tion corresponding to a 2eꢀ/substrate molecule process, benzoic
acid (3a) (30% yield) together with the corresponding methylene
diester (60% yield). Similar results were obtained with 1b.
The electroreduction of benzil to benzoic acid has been,
under different experimental conditions, already described in
the literature. Some authors8 assumed that, under open-air
* Corresponding author. Fax: þ34 91 8854686.
E-mail address: fructuoso.barba@uah.es (F. Barba).
0040-4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.11.102

B. Batanero, F. Barba / Tetrahedron 64 (2008) 1834e1838
1835
Table 1
Peak potentials E (V vs Ag/Agþ) of a-dicarbonyl compounds 1(aed) in
dichloromethane/Et4NCl as SSE (solvent-supporting-electrolyte system)
_
_
+ 2e-
Ar-CO-CO-Ar
Ar-CO-CO-Ar
Ar-CO-CO-Ar
_
_
.
_
_
1
Substrate
Epc1
Epc2
Epa1
Epa2
.
Ar-CO-CO-Ar
+
CH2Cl + Cl
+
CH2Cl2
a
b
c
Benzil
a-Furil
ꢀ1.07
ꢀ0.87
ꢀ0.77
ꢀ1.67
ꢀ0.96
ꢀ0.77
ꢀ0.74
Acenaphthenequinone
1,2-Cyclohexanedione
ꢀ1.49
ꢀ1.36
O-
O-
Ar
Ar
Ar
.
O
Ar
+
CH2Cl
d
.
.
O
O-CH2Cl
Ar
O
Ar
Scan rate: 50 mV/s, cathode: Pt, anode: Pt.
E.T.
workup
_
O2
+ 2e-
_
Ar-CO-CO-Ar
Ar-CO-CO-Ar
Ar-CO-CO-Ar
Ar-CO-CO-Ar
.
.
_
_
_
_
.
_
O
Ar
Ar-CO-CO-Ar
O- O-
+ O2
+
O2
+ O2
.
Ar
O-CH2Cl
.
_
.
_
2
Ar-COO-
+ O2
Ar
Ar
O O
_ O-CH2Cl
O
O
Scheme 1.
O
O
SN
Ar
Ar
Ar
Ar
Ar-COO-CH2-OOC-Ar
_
- Cl
O
O
O
atmosphere, the O2 is cathodically reduced to superoxide an-
ion Oꢀ2 , which reacts further with benzil. However, this postu-
ꢁ
Scheme 2.
late is ruled out in our process because we have carried out the
electrolysis under an argon atmosphere.
Agþ) as Epc1 and Epc2, respectiv0ely (see Table 1). The corres-
ponding formal potentials E0 are ꢀ0.75 and ꢀ1.42 V,
respectively.
When preparative electrolysis of 1c was carried out at the
constant potential value of the first voltammetric peak, a charge
consumption corresponding to a 1 Faraday/mol process is
achieved by coulometry. The main obtained product, after
treatment of the catholyte, was the starting material, however,
some amount (10%) of naphthalene-1,8-dicarboxylic acid (3c)
was also formed.
Electrolysis of 1c, carried out at the constant potential value
of the second reduction peak, afforded, after a charge consump-
tion corresponding to a 2 Faraday/mol process, 2-hydroxy-2-
hydroxymethyl-2H-acenaphthylen-1-one (2c, 44% yield) and
naphthalene-1,8-dicarboxylic acid (3c, 47% yield).
Once the reduction was finished, the cathodic solution was
elaborated, under open-air atmosphere. Under these conditions,
the oxygen transforms the electrogenerated dianion into the cor-
respondinganionradicalbyelectrontransfer(thereductionpoten-
tial of O2 is ꢀ0.85 V versus SCE,9 less negative than ꢀ1.7 V10 of
CH2Cl2), at the same time superoxide anion is evolved. Both im-
mediately react to give a dioxetane intermediate that decomposes
to the dicarboxylate, as it is indicated in Scheme 1.
The formation of non-isolable dioxetane intermediate has
already been suggested11 in the literature by reduction of the
diketone to the corresponding anion radical, at the same
time O2 is electrochemically reduced to superoxide anion
ꢁ
Oꢀ2 , leading to the dioxetane after coupling of both anion radi-
cals. This possibility is also discarded in our case because we
are working under argon atmosphere.
These results can be explained taking into account that the
reduction of 1c at the first voltammetric peak potential affords
an anion radical that was not able to transfer one electron to
the solvent, CH2Cl2. The reason is the formal potential value
Concerning the formation of the methylene diesters, the
following way, as shown in Scheme 2, is proposed.
0This proposal is supported by the fact that formal potentials
E0 (taken as the average of the cathodic and the anodic peak
potentials, Epc and Epa) of benzyl and a-furil are ꢀ1.01 and
ꢀ0.82 V (vs Ag/Agþ), respectively, in both cases higher than
ꢀ0.8 V and subsequently an electron transfer to the solvent,
dichloromethane, can take place.7 On the other hand, the eno-
late will acquire the more stable trans-disposition, hindering
the formation of the expected 1,3-dioxole.
E0 ¼ꢀ0.75 V. In a previous work7 we described that E0
0
0
1
values less negative than ꢀ0.8 V (vs SCE) do not mediate in
the reduction of dichloromethane. However, when this ca-
thodic solution is elaborated, under open-air atmosphere, the
main anion radical molecules transfer an electron to the oxy-
gen with a reduction potential considerably less negative
than that of CH2Cl2. At this time superoxide anion is formed
and further coupled with some acenaphthenequinone anion
radical molecules, still present in solution, to afford the dicarb-
oxylic acid 3c (Scheme 3), similar to the previously described
procedure for benzil and a-furil.
The mentioned methylene diesters have been chemically
obtained by reaction of tetraethylammonium carboxylates
with dichloromethane after 4-days refluxing conditions,12 or
by using PEG-600 as catalyst.13
Preparative electrolysis of 1c at the second voltammetric
peak potential affords the corresponding dianion which
evolves to 2-hydroxy-2-hydroxymethyl-2H-acenaphthylen-1-
one (2c), instead of the expected7 1,3-dioxole. The explanation
of this result (Scheme 4) is based on the different contribution
to the resonance hybrid of the radical anion forms, (i) and (ii).
2.2. Acenaphthenequinone
Cyclic voltammetry of acenaphthenequinone (1c) shows, un-
der anhydrous dichloromethane/tetraethylammonium chloride
as SSE, two reversible peaks at ꢀ0.77 and ꢀ1.49 V (vs Ag/

1836
B. Batanero, F. Barba / Tetrahedron 64 (2008) 1834e1838
2.3. 1,2-Cyclohexanedione
O–
O
O
O
O
O
.
+ 1 e-
.
workup
_
+
O2
Cyclic voltammetry of 1,2-ciclohexanedione (1d) shows,
under anhydrous dichloromethane/tetraethylammonium chlo-
ride, an irreversible peak at ꢀ1.67 V (vs Ag/Agþ). This vol-
tammogram is in agreement with the hydrogen evolution
experimentally observed in the cathodic compartment when
preparative electrolysis of 1d was carried out at the potential
of ꢀ1.7 V.
O2
electron-
transfer
.
_
+ O2
O–
O–
COO-COO-
COOHCOOH
O O
H+
3c
The first electron transfer to 1d affords an anion radical that
evolves to hydrogen and the enolate as shown in Scheme 7.
This anion reacts with another substrate molecule, adsorbed
at the electrode surface, to give after condensation a new
anion, which can be immediately reduced to the dianion
Scheme 3.
.
O–
O
O–
O–
O
O–
O
O
.
+ 2 e-
CH2Cl2
O
electron-
transfer
-Cl-
O
O
O–
O
O
+ 1 e-
.
+ H
(i)
(ii)
-
.
+
CH2Cl
Scheme 4.
Scheme 7.
The contribution of resonance forms in o-quinone anion
radicals, as 9,10-phenanthrenequinone (Scheme 5) is well de-
fined. The form (ii) contributes considerably less than i to the
resonance hybrid, because the aromatization of the ring is not
modified in the first one.
(Scheme 8) or can be desorbed, due to its negative charge,
from the cathode to the solution, as a tetraethylammonium
salt, subsequently protonated during the workup to give 10-hy-
droxy-bicyclohexyl-2,3,20-trione (3d), obtained from the aque-
ous phase as a secondary product (32%). In solution, 3d is
stabilized as the 1,3-diol tautomer form.
However, in acenaphthenequinone the resonance keto-form
(ii) contributes higher to the stability of the anion radical than
the keto-form in o-quinones, due to the fact that now an aro-
matic ring is not destroyed. The coupling reaction between
An electron transfer from the dianion to the solvent,
CH2Cl2, allows the formation of the anion radical, chloro-
methyl radical CH2Cl, and chloride anion, as shown in
ꢁ
ꢁ
the anion radical and the chloromethyl radical CH2Cl, with
Scheme 9. Final coupling of radicals with further intramole-
subsequent nucleophilic attack and further ring opening of
the spiro epoxide, affords 2c, as indicated in Scheme 6.
The formation of naphthalene-1,8-dicarboxylic acid takes
place because the electron transfer of the dianion to the solvent
is very slow, and some dianion molecules, still present in the
workup, transfer the electron to the oxygen of the air. It is sup-
ported by the fact that, whether the catholyte is immediately
elaborated under open-air atmosphere, or it is allowed to
stay for 2e3 h under stirring and argon atmosphere, the yield
of 2c is considerably improved with time, to 3c expense. The
given yields correspond to crudes elaborated after stirring.
O
O
O
O
O
O–
O
O
+ 1 e-
O
O
-
+
.
-H
-O
-O
O
O
workup
+ H+
O
HO
3d
Scheme 8.
O–
O–
O–
O
.
O–
O
.
O
O
+ 2 e-
CH2Cl2
electron-
transfer
-Cl-
(ii)
(i)
.
+
CH2Cl
Scheme 5.
OH
O–
O
O
O–
O
O
O
OH
Cl
.
- Cl-
work-up
H2O/H+
.
+
CH2Cl
2c
Scheme 6.

B. Batanero, F. Barba / Tetrahedron 64 (2008) 1834e1838
1837
O
O
O
O–
.
O
-Cl-
O
O
O
+ CH2Cl2
.
+
CH2Cl
O
O
-
O
O
-
2d
Scheme 9.
cular nucleophilic substitution afforded a benzo[1,3]dioxin-8-
one (2d) in 42% yield.
NMR (300 MHz, CDCl3) d: 2.6 (br s, 2H), 3.92 (s, 2H), 7.65e
7.76 (m, 3H), 7.87e7.92 (m, 1H), 7.95 (d, 1H, J¼7.0 Hz),
8.13 (d, 1H, J¼8.2 Hz). 13C NMR (75.4 MHz, CDCl3) d: 66.7,
79.2, 121.1, 122.5, 125.9, 128.4, 128.8, 130.5, 130.6, 132.3,
137.0, 141.9, 205.1. MS m/e (relative intensity) EI: 196
(Mþꢀ18, 100), 183 (53), 168 (11), 155 (19), 139 (23), 127
(79), 115 (7), 101 (9), 74 (20), 63 (24), 51 (15). MS m/e (relative
intensity) IQ: 215 (Mþþ1, 8), 197 (Mþþ1ꢀ18, 57), 196 (100),
184 (49), 169 (26), 155 (27), 139 (12), 127 (31). Anal. Calcd for
C13H10O3: C, 72.90; H, 4.67. Found: C, 73.07; H, 4.60.
In this case, the electrolysis was finished (the current
decreased from 120 to 30 mA) after a charge consumption
corresponding to 0.8 Faraday/mol was circulated through the
cell, which is in agreement with the yields of the obtained prod-
ucts 2d and 3d and with the mechanism proposed to explain their
formation.
3. Experimental section
3.1.2. Benzo[1,3]dioxin-8-one (2d)
Compounds 1aee are commercially available and have
been used without purification.
(297 mg, 42% yield), oil. IR (KBr) n¼3416, 2922, 1714,
1693, 1454 cmꢀ1. H NMR (300 MHz, CDCl3) d: 1.62e2.58
1
3.1. General electrochemical procedure
(m, 13H), 2.8 (td, 1H, J1¼13.6 Hz, J2¼6.1 Hz), 4.8 (d, 1H
J¼6.1 Hz), 5.2 (d, 1H J¼6.1 Hz). 13C NMR (75.4 MHz,
CDCl3) d: 20.1, 22.1, 25.7, 27.1, 36.4, 38.1, 38.2, 80.9, 87.4,
132.6, 145.2, 192.4, 207.6. MS m/e (relative intensity) EI: 237
(Mþþ1, 9), 236 (Mþ, 67), 208 (8), 192 (9), 180 (12), 179
(35), 150 (100), 121 (20), 107 (15), 84 (35), 79 (53), 66 (41),
55 (66). MS m/e (relative intensity) IQ: 277 (Mþþ41, 5), 265
(Mþþ29, 15), 237 (Mþþ1, 59), 207 (100), 179 (17). Anal.
Calcd for C13H16O4: C, 66.10; H, 6.78. Found: C, 65.87; H, 6.88.
The electrochemical reductions were performed under po-
tentiostatic conditions in a concentric cell with two compart-
ments separated by a porous (D4) glass frit diaphragm and
equipped with a magnetic stirrer. A platinum plate was used
as the cathode (9 cm2) and as the anode (4 cm2), and a Ag/
AgCl electrode as the reference. The SSE was nominally an-
hydrous dichloromethane containing 0.05 M tetraethylammo-
nium chloride.
A solution of the electroactive a-dicarbonyl compound
(3.0 mmol in 60 ml of SSE) was electrolyzed under argon at-
mosphere (the solution was deaerated by Argon bubbling for
15 min) at a constant potential corresponding to the single re-
duction peak (1a, 1b, 1d) or second reduction peak (1c). Once
the reduction was finished the solvent in the cathodic solution
was removed under reduced pressure. The residue was ex-
tracted with ether/water and the organic phase dried over
Na2SO4 and concentrated by evaporation. The resulting solids
were chromatographed on silica gel (22ꢂ3 cm) column, using
CH2Cl2, CH2Cl2/hex or CH2Cl2/EtOH as eluents. Spectro-
scopic description of the new compounds is given below.
Compound 2c was characterized as 2-hydroxyketone and 2d
as benzo[1,3]dioxin-8-one. The aqueous phase was acidulated
with HCl (5%) and further extracted with ether, dried over
Na2SO4, and concentrated by evaporation. From the aqueous
solutions were isolated the dicarboxylic acids (3a, 3b, and
3c) and the 10-hydroxy-bicyclohexyl-2,3,20-trione (3d).
3.1.3. 10-Hydroxy-bicyclohexyl-2,3,20-trione (3d)
(216 mg, 32% yield), oil. IR (KBr) n¼3392, 2927, 1674,
1408, 1310, 1110 cmꢀ1 1H NMR (300 MHz, CDCl3) d:
.
1.1e2.60 (m, 14H), 15.0 (br s, 1H). 13C NMR (75.4 MHz,
CDCl3) d: 178.2, 174.0, 164.1, 122.5, 80.1, 34.4, 33.0, 26.4,
26.2, 23.1, 22.7, 22.2. MS m/e (relative intensity) EI: 224
(Mþ, 5), 206 (38), 178 (37), 150 (100), 122 (31), 79 (44),
55 (62). Anal. Calcd for C12H16O4: C, 64.29; H, 7.14. Found:
C, 64.11; H, 6.97.
Acknowledgements
This study was financed by the Spanish Ministry of Science
and Education CTQ2007-62612/BQU. B.B. thanks the Span-
ish Ministry of Science and Technology for the ‘Ramon y
Cajal’ contract and the CAM-UAH (CCG06-UAH/PPQ-0447)
financial support.
3.1.1. 2-Hydroxy-2-hydroxymethyl-2H-acenaphthylen-
1-one (2c)
References and notes
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3030, 2923, 1720, 1604, 1493, 1180, 1059, 835, 782 cmꢀ1. 1H

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