1,4-Dipolar Azo Derivatives of the Breslow Intermediate from N -Aziridinylimines and N-Heterocyclic Carbenes and Their Normal Electron Demand (4 + 2) and (3 + 2) Cycloadditions

The isolation and structural characterization of reactive species derived from N-heterocyclic carbenes (NHCs) have provided valuable insight into their reactivity and the roles they play in catalytic cycles. Notable examples include the Breslow intermediates, their deoxy derivatives also known as N-heterocyclic olefins (NHOs), and aza analogues that are relevant intermediates in the catalytic umpolung of imines. Less is known about the azo derivatives of the Breslow intermediate. Here we report a simple synthesis of these azo derivatives from the reaction of NHCs with Eschenmoser-type N-aziridinylimines, which enabled their isolation and characterization. Unlike previously reported 1,2-diazadienes, the azo derivatives of the Breslow intermediate are nucleophilic 1,4-dipoles that participate in normal electron demand cycloadditions with electron-deficient alkynes. Depending on the dienophile, either (4 + 2)/(2 + 2) cycloaddition cascades leading to spirocyclic dihydropyridazines or formal (3 + 2) cycloadditions yielding pyrazoles or indazoles were observed.