1,4-Bis(alkylsulfonyl)benzenes. A new and unexpected cathodic cleavage leading to the corresponding phenoxy ions

Article abstract of DOI:10.1016/S0040-4039(00)00049-6

1,4-Bis(alkylsulfonyl)benzenes 2 exhibit the formation of a fairly stable anion radical under cathodic electron transfer. Its fragmentation leads both to 1-alkylsulfonyl-4-alkyl benzenes (ipso substitution) and 4- alkylsulfonylphenoxy ions, the presence of the latter suggesting a new mode of cleavage for strongly activated sulfones. (C) 2000 Published by Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00049-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1763–1765
1,4-Bis(alkylsulfonyl)benzenes. A new and unexpected cathodic
cleavage leading to the corresponding phenoxy ions
Jacques Simonet,∗ Olivier Fourets and Jean-François Pilard
Laboratoire d’Electrochimie Moléculaire et Macromoléculaire, UMR 6510, Beaulieu, Université de Rennes 1, 35042 Rennes,
France
Received 2 December 1999; accepted 5 January 2000
Abstract
1,4-Bis(alkylsulfonyl)benzenes 2 exhibit the formation of a fairly stable anion radical under cathodic
electron transfer. Its fragmentation leads both to 1-alkylsulfonyl-4-alkyl benzenes (ipso substitution) and
4-alkylsulfonylphenoxy ions, the presence of the latter suggesting a new mode of cleavage for strongly activated
sulfones. © 2000 Published by Elsevier Science Ltd. All rights reserved.
Keywords: cathodic cleavage; aromatic sulfones.
It now appears that attempts to classify and normalize to a certain extent the modes (and mechanisms)
of reduction of monoalkyl sulfonyl and poly(alkylsulfonyl) benzenes is a difficult task. In fact, reactions
involving 1,3,5-tris and 1,2,4,5-tetraalkyl (and aryl) sulfonyl benzenes showed complex mechanisms
but, in most cases, coupling of radical anions to strongly stabilized dianions could be considered as the
main primary pathway.1 Coupling reactions were not found to occur for more substituted benzenes,
and hexa(alkylsulfonyl)benzenes, for example, exhibited conformational changes at the stage of the
radical anion owing to the strong crowding and mutual interactions of sulfonyl groups.2 Concerning
disulfones 1 (R=alkyl group), the two-electron reduction in aprotic solvents mainly afforded substitution
compounds 1a (mixture of ortho — minor — and meta — major — alkylated derivatives) and sulfinates
1b.3 The ratios 1a/1b were found to be roughly equal to 1. Under such conditions, it was strongly
expected that the title compounds 2 would follow a very similar course and a study of their cathodic
reduction was, for some time, postponed owing to its assumed lack of originality.
∗
Corresponding author. E-mail: jacques.simonet@univ-rennes1.fr (J. Simonet)
0040-4039/00/$ - see front matter © 2000 Published by Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00049-6
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This short preliminary note aims to show that the behaviour of compounds 2 could not be easily
foreseen. The cleavage reaction of 2·¯ (a first order reaction within a concentration range between
10−4 to 3×10−3 mol L−1) was found to be solvent-independent. Thus, dimethylformamide (DMF),
dimethylsulfoxide (DMSO) and acetonitrile were successfully used and all permit phenoxy ion forma-
tion. Electrolyses were performed on a mercury pool or amalgamated copper grid, a platinum sheet
as well as a glassy carbon electrode. Electrolytes were in all cases tetraalkylammonium salts, mainly
tetrafluoroborates. Thus, a certain number of disulfones 2 were reduced and Table 1 shows the principal
preliminary data. It is worth noting that after total completion of electrolyses (about 2 mol of electron per
mol of 2), an electrophile such as methyl iodide was added in the cathodic compartment before work-up
and contact with air. It must also be pointed out that all electrolyses were achieved under an inert argon
atmosphere and in very dry solvents by treatment of the solution with activated neutral alumina (heated
to 350°C for several hours under vacuum).
Table 1
Fixed potential electrolyses of 2 (1 mmol) at Cu/Hg electrode (area: 15 cm2) in DMF+nBu4NBF4 0.1
mol L−1
Anode: carbon rod. Volume of catholyte: 100 ml (area: 20 cm2). Applied potential: that of E° (standard
potential of the first peak) obtained by voltammetry. Reference electrode: Ag/AgI/I− 0.1 mol L−1 in
DMF.
The concomitant obtention of compounds 2a and 2b appears surprising. It strongly depends both on the
relative stability of R·, possibly produced during the reduction process, and on substrate concentration.
Compound 2b would come from an ipso substitution explained by a radical coupling of 2·¯ and R·
normally issued by the cleavage of 2·¯ in the bulk (the transient anion would suffer an elimination leading
to the loss of RSO2−). However, the formation of 2a is more intriguing since it is strongly expected
that the oxygen atom can come neither from dioxygen nor from solvent moisture. Disulfones were
also shown to be totally unreactive in the presence of strong bases like Bu4NOH and, therefore, an

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aromatic substitution process cannot be considered. A new mode of fragmentation involving a radical
anion possessing a sulfonyl group should then occur.
References
1. Benaskar, M.; Simonet, J. Tetrahedron Lett. 1994, 35, 1727–1730.
2. Ghanimi, A.; Simonet, J. J. Electroanal. Chem. 1997, 425, 217–220. Ghanimi, A.; Fabre, B.; Simonet, J. New J. Chem. 1998,
831–837.
3. Belkasmioui, A.; Benaskar, M.; Simonet, J. Tetrahedron Lett. 1991, 22, 2481–2484. Cauliez, P.; Benaskar, M.; Simonet, J.
Electrochim. Acta. 1997, 42, 2191–2199. Cauliez, P.; Benaskar, M.; Ghanimi, A.; Simonet, J. New J. Chem. 1998, 253–256.