Organocatalytic Nucleophilic Addition of Hydrazones to Imines: Synthesis of Enantioenriched Vicinal Diamines

In the presence of a catalytic amount of chiral phosphoric acid, nucleophilic addition of N-monosubstituted hydrazones to N-Boc imines affords differentially protected vicinal diamines in the form of beta-amino N, N'-dialkyldiazenes in excellent yields with high chemo-, diastereo-, and enantio-selectivity. This catalytic asymmetric aza-Mannich reaction represents the first example in which N-alkyl hydrazones serve as alpha-azo carbanion equivalents to provide vicinal diamines with control of the two contiguous stereocenters. The adducts are readily converted into the monoprotected or free diamines and derivatives thereof.