The sonoelectrooxidation of thiophene S-oxides

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Abstract

Thiophene-S-oxides (thiophene monoxides) are relatively new compounds, less stable than the better-known thiophene-S-dioxides. They are useful as synthons for a range of applications, including in the production of pharmaceuticals. They have interesting photochemical properties, but in this presentation we contrast the electro-oxidative voltammetry of differently substituted derivatives. We also compare carbocyclic compounds such as tetracyclone, the electro-oxidation of which at relatively high potentials has never been reported in silent or insonated conditions.

Introduction

Thiophene S-oxides constitute a class of compounds that have only become preparatively accessible in the last few years [1]. They are of interest because of their roles in such disparate issues as the study of in vivo toxicology of thiophenes [1], [2] and the development of new optical materials with narrow electronic band gaps [1], [3]. Additionally, some thiophene S-oxides show biological activity towards certain cancer cell types. This might be due to the slow loss of oxygen from these molecules, so in an earlier paper the authors studied possible deoxygenation in the reductive electrochemistry of thiophene S-oxides [4]. This was of further interest because oxygen loss is also promoted photochemically, prompting comparative studies of electrochemical and photochemical reductions [4].

The oxidation of thiophene S-oxides has been less widely studied. There are a few reports regarding the use of chemical oxidants, but very little is known about the electro-oxidative behaviour of thiophene S-oxides [5]. This aspect is of interest as it may also shed light on the metabolism of thiophene containing compounds, which is complicated by the range of available oxidation levels at the sulphur atom. Thus, a thioether function in a natural product such as the amino acid methionine can be oxidised through the respective sulphoxide (monoxide) via a sulphone (dioxide) to a sulphonic acid, the last step also involving bond fission. While the oxidation of the methionine to the sulphoxide is a typical process in vivo which is reversible and regulated, oxidative bond fission is much less common. In vitro, however, oxidation of methionine in proteins by peroxides can lead to the sulphone [6] as well as further oxidation products [7], [8]. Typically, further oxidation steps after bond cleavage are more difficult to predict, and such processes are thought to contribute to irreversible changes during the ageing of an organism.

Oxidation causes loss of aromatic stabilisation and in both thiophene S-oxides and thiophene S-dioxides the sulphur lone pair does not conjugate into the ring. The unsubstituted parent thiophene S,S-dioxide has only recently been isolated [9], [10], while the corresponding unsubstituted thiophene S-oxide has never been prepared, due to spontaneous dimerisation reactions by formal [4+2] cycloaddition (cf., similar behaviour to cyclopentadiene), so these potentially useful synthons remain an elusive target. However, a number of substituted thiophene S-oxides stabilised by functionalisation are now available. These also include fused-ring annelated systems such as dibenzothiophene S-oxides that have been known for a longer time [11].

In this paper we present data for the oxidative voltammetry of tetraphenylthiophene S-oxide, and compare this with tetramethylthiophene S-oxide and dibenzothiophene S-oxide (structures 1–3). To our knowledge none of these compounds have been studied in this potential range. In addition there are interesting contrasts between CO and SO functionalities, and so we also examine the previously unreported electro-oxidative voltammetry of tetracyclone (2,3,4,5-tetraphenylcyclopentadienone, structure 4). Additionally, we have examined the effect of simultaneous ultrasonic irradiation on these systems since insonation is known to produce a variety of benefits in electrochemistry including improved mass transport, increased current, and mitigation of electrode fouling [12], [13], [14]. We note briefly the products so far identified from preparative electrolyses involving tetraphenylthiophene S-oxide and in a forthcoming paper will give a more detailed account of these multipathway and mechanistically complex electrosynthetic reactions.

Section snippets

Experimental

Tetraphenylthiophene S-oxide and tetramethylthiophene S-oxide were prepared according to literature procedures [15], [16], [17], the former by reaction of thionyl chloride with the corresponding zirconacyclopentadiene [17], the latter by controlled peracid oxidation of the corresponding thiophene in the presence of Lewis Acids.[15], [16] Dibenzothiophene S-oxide was also obtained from oxidation of dibenzothiophene. Tetracyclone was synthesised by base-catalysed condensation of benzoin with

Silent voltammetry

We have examined most closely the electrooxidative behaviour of tetraphenylthiophene-S-oxide [Structure 1], where the phenyl rings may limit the complexity of side reactions compared for example to the tetramethyl analogue [Structure 2]. We also evaluate the character of the sulphoxy functionality within the isolated heterocyclic system of a thiophene S-oxide by comparing the voltammogram of dibenzothiophene S-oxide, a molecule in which the sulphoxy group is a bridging function across a

Conclusions

Thiophene S-oxides show oxidative electrochemistry at platinum in MeCN/Bu4NPF6. The tetraphenyl derivative shows two irreversible waves at 1.6 and 2.1 V (vs. SCE) while the tetramethyl analogue shows less well-defined behaviour in this potential region. The areno-annelated derivative dibenzothiophene shows a single irreversible wave at the more forcing potential of 2.4 V. Continuation of the oxidative scan into the reduction regime before repeating the scan cycle produces little loss of

Acknowledgments

J I-V thanks the European Community for a Marie Curie Fellowship. DJW and TT thank EPSRC for a travel grant that made possible this joint experimentation.

References (27)

  • K. Tanaka et al.

    Synth. Metals

    (1989)
  • B. Jiang et al.

    J. Am. Chem. Soc.

    (1999)
  • H. Özyörük et al.

    Electrochim. Acta

    (1987)
  • P. Pouzet et al.

    J. Chem. Soc., Chem. Commun.

    (1995)
  • N. Furukawa et al.

    Heterocycles

    (1998)
  • T. Thiemann et al.

    J. Chem. Res.

    (2002)
  • D. Mansuy et al.

    J. Am. Chem. Soc.

    (1991)
  • T. Thiemann et al.

    J. Phys. Org. Chem.

    (2000)
  • A. Bongini et al.

    J. Chem. Soc., Chem. Commun.

    (2000)
  • S.K. Chowdhury et al.

    Anal. Chem.

    (1995)
  • M. Chiari et al.

    Electrophoresis

    (1991)
  • F.L. Hsu et al.

    J. Org. Chem.

    (1990)
  • J. Nakayama et al.

    J. Am. Chem. Soc.

    (1997)

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