Bioelectrochemistry and Bioenergetics, 15 (1986) 125-135 A section of J. Electroanal. Chem., and constituting Vol. 211 (1986) Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands 125 zyxwvutsrqpo 827-ELECTROCHEMICAL REDUCTION MECHANISM OF 5(lH)-PYRROMETHENONES IN DIMETHYLFORMAMIDE ON A MERCURY ELECTRODE JOSEP CLARET l , JOAN M. FELIU and CARLOS MULLER Departament de Quimica FGca, Fact&at de Quimica, Universitat the Barcelona, Au. Diagonal 647, 08028 Barcelona (Catalunya) (Spain) JOSEP M. RIBC) and XAVIER SERRA Departament de Quimica Orgbnica, Vniversitat de Barcelona, Au. Diagonal 647, 08028 Barcelona (Catalunya) (Spain) (Revised manuscript received August 1st 1985) SUMMARY The mechanism of the electrochemical reduction of some S(lH)-pyrromethenones and their aryl analogs (5-arylmethylene-3,4-dimethyl-3-pyrrolin-2-ones) in dimethylformamide on a mercury electrode, using LiClO, as supporting electrolyte, has been investigated. Polarographic and voltammetric experimen- tal data show that 5(1H)-pyrromethenones undergo a reductive electrodimerization only through an EC mechanism, whereas their aryl analogs can also be reduced through an ECE mechanism. This different behaviour gives information about the reactivity of the bridge carbon atom of these compounds. INTRODUCTION zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA The reduction of bile pigments (1,19-bilindiones) can be involved in many processes of biological significance, e.g., in the mammals, bilirubin IXcll is reduced, after its excretion in the gut, to urobilinoids [l]. Although photoxidation processes of the chromophore group of biliproteins (phytochrome and phycocyanobilins) have been proposed in order to explain their biological mechanisms at molecular level [2], we think that the knowledge of the reductive behaviour of bile pigments is also essential to fully understand their redox properties. In the chemical study of bile pigments, the previous investigation of model compounds that represent partial structures of them, e.g., pyrromethenes, pyr- romethenones and 5-yliden-3-pyrrolin-2-ones, has been successful:, this approach has already been used to study the isomerization, tautomerization and conformational behaviour of bile pigments [3] and can also be utilized to investigate their chemical reactivity [4]. l To whom correspondence should be addressed. 0302-4598/86/$03.50 Q 1986 Elsevier Sequoia S.A.