Phosphine-Catalyzed Asymmetric Cycloaddition Reaction of Diazenes: Enantioselective Synthesis of Chiral Dihydropyrazoles.

Although carbon-carbon, carbon-nitrogen, and carbon-oxygen double bonds or their combinations have extensively been applied in phosphine-catalyzed asymmetric cycloaddition, a nitrogen-nitrogen double bond has never been investigated in chiral phosphine catalysis. In this paper, we present phosphine-catalyzed asymmetric [3+2] cycloaddition of diazenes with Morita-Baylis-Hillman (MBH) carbonates to give chiral dihydropyrazoles in high yields with excellent enantioselectivities. Various MBH carbonates and diazenes worked well under the mild reaction conditions.