ELECTROCHEMICAL OXIDATION OF INDOLE-3-ACETIC-ACID - MECHANISMS AND PRODUCTS FORMED IN ACIDIC MEDIUM

The electrochemical oxidation of indole-3-acetic acid (1) has been inv estigated in acidic aqueous solution. The first voltammetric oxidation peak I-a of 1 corresponds to an initial two-electron one-proton react ion to give a cation of 3-methyleneindolenine carboxylic acid. Very ra pid decarboxylation of the latter intermediate gives a 3-methyleneindo lenine 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 dim er is the major initial product but is more easily oxidized than 1, gi ving dimer 5 as its major product along with smaller amounts of 2-4. R eaction pathways are proposed to explain the formation of 2-6 from 1 a nd 2-5 from 6. A spontaneous dimerization reaction of 3-methylene-2-ox indole has been discovered which results in the formation of diastereo mers of spiro[benz[c,d]indole-5(2H), 3'-[3H]indole]-2,2'-(1'H)-dione, 1, 2a, 3,4-tetrahydroindole (7 and 8).