Pyrrole-stabilized free carbenes generated from alkynyl imidates or aldimines and electrophilic alkynes give pyrrolone- or pyrrole-containing products

Pyrrole and 2-pyrrolone derivatives are valuable heterocyclic compounds and while classical condensation methods for their synthesis have a long history, many of the recent developments for their preparation involve the use of transition metal catalysis. Here we report a complementary, metal-free strategy for constructing structurally diverse derivatives of these heterocycles. The key feature of the approach is the in situ creation of a reactive intermediate by an initial facile event that simultaneously generates a pyrrole ring bearing a free carbene. This is straightforwardly accessed via a spontaneous, net [3 + 2] cyclization reaction of a linear alkynyl O-silylimidate or alkynyl aldimine with an electrophilic alkyne. The carbene then undergoes either 1,4-silyl migration (to produce 2-pyrrolone derivatives) or C–H insertion, cycloaddition, cyclopropanation or macrocyclization reactions (leading to pyrrole derivatives).