RATES OF DECARBOXYLATION OF THE RADICAL CATIONS OF INDOL-3-YLACETIC ACIDS AND COMPARISON WITH INDOLIZIN-1-YLACETIC ACIDS

The radical cations of indol-3-ylacetic acid and derivatives were foun d to eliminate CO2 to yield skatolyl radicals with rates in the range ca. 10(2) to > 10(5) s(-1), strongly dependent on substitution. For th e radical cations substituted at nitrogen, the rate of decarboxylation did not vary with pH 4-7.5, but for those unsubstituted at nitrogen, deprotonation caused the rate of decarboxylation to decrease with incr easing pH. The rate of decarboxylation of the radical cations exhibite d a strong dependence on the respective reduction potentials, with a 1 00 mV increase in reduction potential corresponding to a ca. tenfold i ncrease in the rate of decarboxylation. Methylation at the side-chain alpha-position increased the rate of decarboxylation >sixfold, but ins ertion of a methylene group, as in 3-indol-3-ylpropionic acid or trypt ophan, completely inhibited decarboxylation. In contrast, indolizin-1- ylacetic acids, which are isomers of indolylacetic acids in which the heterocyclic nitrogen is the bridgehead, did not decarboxylate on one- electron oxidation.