Acid-Promoted Cyclization of N,N-Disubstituted Malononitriles with Substituted Propiolaldehydes: A Metal-Free Strategy for the Construction of 3-Dicyanovinylindoles

Reported herein is a reaction paradigm wherein benzoic acid mediates the 1,4-addition of N,N-disubstituted aminomalononitriles to substituted propiolaldehydes, triggering a cascade cyclization that proceeds via a five-membered cyclic quaternary ammonium intermediate. This transient species undergoes sequential C-N bond cleavage, electron reorganization, and intramolecular Friedel-Crafts alkylation, culminating in dehydration-aromatization to furnish 3-dicyanovinylindoles. This methodology pioneers a one-pot, transition-metal-free strategy for assembling structurally diverse 3-dicyanovinylindoles in good yields, circumventing traditional dependencies on Fischer indole synthesis or metal-catalyzed cyclization.