Rates of reductive elimination of substituted nitrophenols from the (indol-3-yl)methyl position of indolequinones

A series of indolequinones bearing substituted nitrophenols on the (indol-3-yl)methyl position was synthesised. The nitrophenol leaving groups were appropriately substituted to give a wide range (4 units) in phenolic pKa value. The rate of reductive elimination of phenoxide anions from the (indol-3-yl)methyl position of semiquinone radicals was dependent upon this pKa, with a decrease in 3.8 pK units shortening the half-life from 28 to 1.5 ms. Only 2,4-dinitrophenol (pKa = 3.9) was eliminated from an unsubstituted (indol-3-yl)methyl position at a rate that would compete with reoxidation of the radical by oxygen. A nitrothiophenol leaving group was eliminated comparatively slowly and only from the hydroquinone. These studies demonstrate the dependence upon leaving group pKa of the rate of reductive elimination from the (indol-3-yl)methyl position of indolequinones.