Zur Kenntnis der Indolreaktion nach Fischer, I. Thermische und Säurekatalysierte Reaktionen von N, N′‐Dimethyl‐N‐phenyl‐N′‐alkenylhydrazinen

N,N′‐dimethyl‐N‐phenyl‐N′‐alkenylhydrazines 9a–9f (Scheme 2) have been prepared by three different routes. These compounds yield N‐methyl‐indols 14a–14f (Scheme 3) upon thermolysis at 110° or upon treatment with dilute acid in anhydrous solvents at room temperature. The rate of the acid catalysed reaction varies by several orders of magnitude within the range of the examples studied. The observed structure‐reactivity relationship can be rationalized as reflecting the different tendency of the alkenylhydrazines 9a–9f for protonation on carbon vs. protonation on nitrogen (Scheme 4), the nitrogen protonated tautomer undergoing a rapid charge induced [3,3]‐sigmatropic rearrangement leading ultimately to indol.