Facile (3+2) Cycloaddition between an N‐Heterocyclic Olefin and Nitrous Oxide at Ambient Conditions

We report that a gem‐dimethylated N‐heterocyclic olefin (NHO) reacts with N2O at ambient pressure and room temperature to give an imidazolone and an azine formed from intermediately generated 2‐diazopropane. The in situ formation of diazopropane was successfully proven by its engagement in a facile cycloddition with norbornene. According to our studies, the reaction sequence consists of a rate‐determining (3+2) cycloaddition between the NHO and N2O, followed by a cycloreversion that releases the diazoalkane. This mechanistic proposal is supported by DFT calculations. Eyring analysis using temperature‐variable 1H 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.