From Triazoles to Imidazolines through the Sequential N–H Insertion of α-Imino Rhodium–Carbenes into β-Enamino Esters/Enamine–Imine Tautomerization/Conjugate Addition Cascade

A rhodium(II)-catalyzed denitrogenative coupling of N-sulfonyl-1,2,3-triazoles with ambiphilic β-enamino esters affords 2,5-dihydro-1H-imidazoles (3-imidazolines) with broad functional group tolerance. Mechanistic studies using a deuterium-labeled triazole suggest that the reaction proceeds in a cascade through the N–H insertion of an α-imino rhodium–carbene, followed by enamine–imine tautomerization and conjugate addition. Moreover, the reaction proceeds with high diastereoselectivity for α-substituted β-enamino esters (R3 = Me, Ph) to give a single diastereomer.