Rates of reductive elimination of substituted nitrophenols from the (indol-3-yl)methyl position of indolequinones
作者:Elizabeth Swann、Christopher J. Moody、Michael R. L. Stratford、Kantilal B. Patel、Matthew A. Naylor、Borivoj Vojnovic、Peter Wardman、Steven A. Everett
DOI:10.1039/b101842f
日期:——
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.
一系列带有取代硝基苯酚的吲哚醌类化合物在(吲哚-3-基)甲基位置被合成。硝基苯酚离去基团被适当取代,使得酚pKa值范围广泛(4个单位)。从半醌自由基的(吲哚-3-基)甲基位置上,羟基阴离子的还原消除速率依赖于这个pKa值,pKa值每降低3.8个单位,半衰期从28毫秒缩短到1.5毫秒。只有2,4-二硝基苯酚(pKa = 3.9)能够以与自由基被氧再氧化竞争的速率从无取代(吲哚-3-基)甲基位置上消除。硝硫苯酚离去基团消除相对缓慢,并且仅从对苯二酚中消除。这些研究表明,从吲哚醌类的(吲哚-3-基)甲基位置上还原消除的速率依赖于离去基团的pKa值。