Electrochemical reduction of quarternized 2-pyridinio-1,3-indandiones on mercury electrode—II. Electroreduction in protogenic medium

Abstract

The electroreduction mechanism of 2-pyridinio- and 2-quinolynio-1,3-indandiones (pyrophthalones, quinophthalones, N-ylides of 1,3-indandione) was established using methods of classical polarography, cyclic voltammetry and controlled-potential electrolysis (CPE) on mercury electrode in protogenic medium (20% aqueous DMF buffer). In all cases it is the 1,3-indandionyl moiety which undergoes electroreduction, whilst the pyridinium (dihydropyridine) part is not reduced within the potential range accessible to polarographic study. The products of preparative electrolysis were identified by means of mass and PMR spectrometry. The role of surface protonization of bipolar ions in the process of electroreduction was ascertained and the degree of adsorption of N-methylphthalone molecules on the mercury electrodes assessed (Γ ∞=1.2×10 −10 mole cm −2). The estimate of C and O protonization separately was also made. it was found that electrochemical reduction of onium betaines of 2- N-pyridinium-1,3-indandione ( N-ylides) leads to cleavage of the CN + bond at the second stage of electroreduction. Similarity between electrochemical reduction of N-pyridinium indandiones and their reduction by various chemical agents (NaBH 4, Zn, H 2, H 2/PtO 2) is also discussed.