Electrochemical oxidation of indole-3-acetic acid: Mechanisms and products formed in acidic medium

Abstract The electrochemical oxidation of indole-3-acetic acid ( 1 ) has been investigated in acidic aqueous solution. The first voltammetric oxidation peak I a of 1 corresponds to an initial two-electron one-proton reaction to give a cation of 3-methyleneindolenine carboxylic acid. Very rapid decarboxylation of the latter intermediate gives a 3-methyleneindolenine cation which undergoes a complex series of reactions yielding 3-hydroxy-2-oxindole ( 2 ), indole-3-aldehyde ( 3 ), 3-methylene-2-oxindole ( 4 ), 3-(3′-methylene indole)-3-hydroxymethyl-2-oxindole ( 5 ) and 2-(3′-methyleneindole)-indole-3-acetic acid ( 6 ) as products. The latter dimer is the major initial product but is more easily oxidized than 1 , giving dimer 5 as its major product along with smaller amounts of 2–4 . Reaction pathways are proposed to explain the formation of 2–6 from 1 and 2–5 from 6 . A spontaneous dimerization reaction of 3-methylene-2-oxindole has been discovered which results in the formation of diastereomers of spiro[benz[ c , d ]indole-5(2 H ), 3′-[3 H ]indole]-2,2′-(1′ H )-dione, 1, 2a, 3,4-tetrahydroindole ( 7 and 8 ).