Urs Gellrich

We report that a gem ‐dimethylated N ‐heterocyclic olefin (NHO) reacts with N 2 O at ambient pressure and room temperature to give an imidazolone and an azine formed from intermediately generated 2‐diazopropane. The in situ formation of transient diazopropane was indirectly proven by its engagement in a facile cycloaddition with norbornene. According to our studies, the reaction sequence consists of a rate‐determining (3+2) cycloaddition between the NHO and N 2 O, followed by a cycloreversion that releases the diazoalkane. This mechanistic proposal is supported by DFT calculations. Eyring analysis using temperature‐variable 1 H NMR spectroscopy allowed us to determine the activation parameters of the initial (3+2) cycloaddition (Δ ≠ H =11.2(4) kcal/mol, Δ ≠ S =−39.8(13) e.u., 293–308 K). Frontier molecular orbital analysis shows that the polarization of the π‐bond of the NHO decisively facilitates the cycloaddition.