Pyrrole without Life: Reaction of Aminomethylene with the Propargyl Radical

Carbenes are reactive species found across gas-phase environments, from combustion to planetary atmospheres and interstellar space. Their reactions with radicals represent a compelling path to increasing chemical complexity, in which the formation of the first aromatic ring is a foundational step. To date, no selective gas-phase bottom-up route to the smallest nitrogen-bearing aromatic ring, pyrrole, is known. We investigated the reaction of the simplest aminocarbene, aminomethylene, with the prototypical resonance stabilized propargyl radical. Photoelectron photoion coincidence spectroscopy and semiautomated electronic structure calculations reveal a barrierless, addition-elimination mechanism producing pyrrole + H. The reaction path depends on the orientation of propargyl during the association, in which the allenyl resonance form (H₂C═C═CH^•) of propargyl leads to pyrrole formation. This selective pathway highlights the promise of radical chemistry to fill important gaps in chemical reaction networks.