From Aryl Iodides to 1,3-Dipoles: Design and Mechanism of a Palladium Catalyzed Multicomponent Synthesis of Pyrroles.

A palladium-catalyzed multicomponent synthetic route to polysubstituted pyrroles from aryl iodides, imines, carbon monoxide, and alkynes is described. To develop this reaction, a series of mechanistic studies on the [Pd(allyl)Cl]2/P(t)Bu3 catalyzed synthesis of imidazolinium carboxylates from aryl iodides, imines, and carbon monoxide were first performed, including model reactions for each individual step in the transformation. These show that this reaction proceeds in a concurrent tandem catalytic fashion, and involves the in situ formation of acid chlorides, N-acyl iminium salts, and ultimately 1,3-dipoles, i.e., Münchnones, for subsequent cycloaddition. By employing a Pd(P(t)Bu3)2/Bu4NCl catalyst, this information was used to design the first four-component synthesis of Münchnones. Coupling the latter with 1,3-dipolar cycloaddition with electron deficient alkynes or alkenes can be used to generate diverse families of highly substituted pyrroles in good yield. This represents a modular and streamlined new approach to this class of heterocycles from readily accessible starting materials.