Metal-Free, PPA-Mediated Fisher Indole Synthesis via Tandem Hydroamination–Cyclization Reaction between Simple Alkynes and Arylhydrazines

A new variant of Fisher indole synthesis involving Bronsted acid-catalyzed hydrohydrazination of unactivated terminal and internal acetylenes with arylhydrazines is reported. The use of polyphosphoric acid alone either as the reaction medium or in the presence of a co-solvent appears to provide the required balance for activating the C-C triple bond towards the nucleophilic attack of the hydrazine moiety without unrepairable reactivity loss of the latter due to competing amino group protonation. Additionally, the formal hydration of acetylenes to the corresponding ketones occurs under the same conditions, making it an alternative approach for generating carbonyl groups from alkynes.