Electrochemical reduction of the synthetic pyrethroid insecticides tralomethrin and tralocythrin at glassy carbon and mercury electrodes

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Abstract

Tralomethrin and tralocythrin may be electrochemically reduced in an initial irreversible two-electron process in the solvents acetonitrile and methanol at glassy carbon and mercury electrodes. The reduction potentials at mercury electrodes are between 600–900 mV less negative than at glassy carbon electrodes. Despite this difference, under both voltammetric and controlled potential electrolysis (CPE) conditions at either electrode surface, the overall reduction process for both compounds involves the elimination of two molecules of bromide ion to yield quantitatively the synthetic pyrethroid compounds deltamethrin (from tralomethrin) and cypermethrin (from tralocythrin) as products. The mechanism of bromide elimination therefore is highly dependent upon the electrode surface. At a glassy carbon electrode, reduction of the pyrethroid compounds is a concerted process involving bromide elimination. In contrast, the presence of the mercury electrode introduces the possibility of chemically reducing tralomethrin (or tralocythrin) to give deltamethrin (or cypermethrin) and a mercury bromide species at the electrode surface. The reaction provides a catalytic pathway for reduction and under voltammetric or CPE conditions the reduction process at the mercury electrode, therefore, is actually associated with the reduction of the mercury bromide species, present at the electrode surface, to elemental mercury and free bromide ion.

Introduction

Tralomethrin and tralocythrin, Fig. 1a, are synthetic pyrethroid insecticides synthesised by bromination across the double bonds of the dihalovinyl substituent of the pyrethroids deltamethrin [1]and the (S)-α-(1R)-cis isomer of cypermethrin [2], Fig. 1b, respectively. The compounds are produced as a mixture of two stereoisomers as the bromination reaction introduces a further chiral centre into deltamethrin (a single stereoisomer) and the cypermethrin stereoisomer.

In a previous publication [3]the reduction behaviour of a range of pyrethroids based upon esters of α-cyano-3-phenoxybenzyl alcohol was detailed. The initial reduction process led to cleavage of both the ester and the cyanide bond. In this paper we report the reduction behaviour of tralomethrin and tralocythrin in the non-aqueous solvents acetonitrile and methanol. Although these compounds are based on α-cyano-3-phenoxybenzyl alcohol, the initial reduction behaviours follow a different and specific reaction mechanism which is fully elucidated.

Section snippets

Pyrethroid compounds

Tralomethrin (93.8%), tralocythrin (94%) (Roussell Uclaf Australia Pty., Pennant Hills, N.S.W., Australia), cypermethrin (99.3%) (Shell Chemical Pty., Melbourne, Vic., Australia) and deltamethrin (99%) (CSIRO Division of Wool Technology, Belmont, Vic., Australia) were gifts of the organisations named and were used without further purification. The IUPAC names of the pyrethroid compounds used in this study are presented in Appendix A.

Instrumentation

Electrochemical, HPLC and mass spectrometric instrumentation,

Cyclic voltammetry

Reduction (Epred) and oxidation (Epox) peak potential data obtained for tralomethrin and tralocythrin are summarised in Table 1.

Discussion

The results of voltammetric and CPE experiments indicate that at both mercury and glassy carbon electrodes the overall reduction mechanism of tralomethrin and tralocythrin involves reductive debromination to give one mole of deltamethrin and cypermethrin, respectively, and two moles of bromide ion. The debromination reactions are independent of the proton donating ability of the solvent.

Under conditions of reductive CPE, despite the fact that very different potentials are employed, identical

Acknowledgements

Funding for this study was provided by the Australian Wool Research and Development Corporation through the award of postgraduate scholarship UDG002 to D.C.C. Mass spectra were obtained by Mr Gary Franklin of Deakin University. Much of the research was undertaken at La Trobe Univeristy when A.M.B. was Professor of Chemistry at that institution. The donation of pyrethroid standards by the organisations listed in the text is gratefully acknowledged.

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Present address: Department of Chemistry, Monash University, Clayton, Victoria 3168, Australia.

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