External Reductant-Free Palladium-Catalyzed Reductive Insertion of Isocyanide: Synthesis of Polysubstituted Pyrroles and Its Applications as a Cysteine Probe.

In this work, an unprecedented route is described for the synthesis of 2-amino-4-cyanopyrrole derivatives via palladium-catalyzed reductive isocyanide insertion of alkynyl imines. In the reactions, no external reductant is added and isocyanide plays a dual role as both a C1 synthon for imidoylation and a cyano source for cyanation. Mechanism studies suggest a [4 + 1] cycloaddition, an isocyanide insertion, β-carbon elimination, and palladium hydride-based reduction are involved. Moreover, the application of 2-amine-4-cyanopyrroles as a cysteine probe is realized to detect cysteine.