Tetrahedron Letters
Reductive electrochemical formation of 6H-dibenzo[b,d]pyran-6-one
and 2-benzopyran-1(1H)-one
b
c
Belen Batanero a, Fructuoso Barba a, , Isidoro Barba , Michail N. Elinson
⇑
a Department of Organic Chemistry, University of Alcala, 28871 Alcalá de Henares (Madrid), Spain
b Department of Organic Chemistry, University of Alicante, Ap. 99, 03080 Alicante, Spain
c N.D. Zelinsky Institute of Organic Chemistry, Leninsky Prospect 47, 119991 Moscow, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present Letter several carbolactones (oxidative products) are obtained under aprotic cathodic con-
ditions in the preparative scaled electrolysis of 1,2-quinones in a divided electrochemical cell and in the
presence of oxygen. When 9,10-phenanthrenequinone is reduced 6H-dibenzo[b,d]pyran-6-one and [1,10-
biphenyl]-2,20-dicarboxylic acid are obtained as major products. In the reduction of 1,2-naphthoquinone,
2-benzopyran-1(1H)-one, and 2-(2-carboxyethenyl)-benzoic acid were formed as main products. The
proposed mechanism to explain the formation of these and other products, that involves an electron-
transfer reaction to the oxygen in air, is now discussed.
Received 23 September 2013
Revised 22 October 2013
Accepted 25 October 2013
Available online 7 November 2013
Keywords:
1,2-Quinones cathodic reduction
Electrochemical Baeyer–Villiger/Dakin-type
reaction
Ó 2013 Elsevier Ltd. All rights reserved.
Electron-transfer
Superoxide anion
Aromatic carbolactones
Introduction
peroxide, had already been described under aqueous solution by
the use of glassy-carbon modified cathodes.3,4
In a previous paper1 we have described the formation of ben-
zoic acid in the cathodic reduction of benzil, performed under an
argon atmosphere, with dry dichloromethane as the solvent. The
mechanistic proposal of this process involved an electron-transfer
reaction during the work-up (in the presence of O2) between the
electrogenerated benzil dianion and the oxygen of the air, produc-
ing the anion radical of benzil and a superoxide anion. Subsequent
coupling reaction of both, with further intramolecular nucleophylic
attack to the carbonyl group, afforded a dioxetane intermediate
that decomposed to benzoic acid.
The present communication deals with the cathodic reduction
of 9,10 phenanthrenequinone (1a) and 1,2-naphtoquinone (1b) in
the presence of oxygen, using nominally anhydrous acetonitrile/
LiClO4 as solvent-supporting electrolyte system (SSE), to its colored
anion radical. The indirect formation of superoxide anion at the
cathodic solution makes possible a further coupling with the elec-
trogenerated anion radicals to produce aromatic carbolactones 2
and carboxylic acids. Discussion and mechanism proposals to ex-
plain their formation are given.
Recently we have also described2 that the electrochemical
reduction, in acetonitrile and other aprotic solvents, of 1,2-qui-
nones at the first reduction potential of these systems (closely to
ꢀ0.5 V, vs Ag/Ag+) produces, in the absence of oxygen, the expected
and colored anion radical after a charge consumption correspond-
ing to a 1e-/substrate molecule process. However, the starting qui-
none was completely recovered, after an electron-transfer to O2,
when this anion radical solution was elaborated.
Results and discussion
Cathodic reduction of 9,10-phenanthrenequinone (1a)
The experimental results in the cathodic reduction of 9,10-
phenanthrenequinone (1a), in the presence of O2, and employing
nominally anhydrous acetonitrile/LiClO4, were as follows: in the
organic phase 30% yield of 6H-dibenzo[b,d]pyran-6-one (2a) was
obtained. In the aqueous phase: [1,10-biphenyl]-2,20-dicarboxylic
acid (3a, 31% yield), 20-hydroxy-[1,10-biphenyl]-2-carboxylic acid
(4a, 8% yield) and 21% yield of 2-phenylbenzoic acid (5a) were ob-
tained. It should be indicated that the carboxylic acids 3a, 4a, and
5a were isolated after the aqueous phase was acidified and ether
extracted.
This electron-transfer reaction from the anion radical of a qui-
none to oxygen molecules, as an indirect way to produce hydrogen
⇑
Corresponding author.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.