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 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 N2O, followed by a cycloreversion that releases the diazoalkane. This mechanistic proposal is supported by DFT calculations. Eyring analysis using temperature-variable H-1 NMR spectroscopy allowed us to determine the activation parameters of the initial (3+2) cycloaddition (Delta H-not equal=11.2(4) kcal/mol, Delta S-not equal=-39.8(13) e.u., 293-308 K). Frontier molecular orbital analysis shows that the polarization of the pi-bond of the NHO decisively facilitates the cycloaddition.