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 ap propriately substituted to give a wide range (4 units) in phenolic pK(a) va lue. The rate of reductive elimination of phenoxide anions from the (indol- 3-yl)methyl position of semiquinone radicals was dependent upon this pK(a), with a decrease in 3.8 pK units shortening the half-life from 28 to 1.5 ms . Only 2,4-dinitrophenol (pK(a)=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 comp aratively slowly and only from the hydroquinone. These studies demonstrate the dependence upon leaving group pK(a) of the rate of reductive eliminatio n from the (indol-3-yl)methyl position of indolequinones.