Highly anti-Selective Asymmetric Nitro-Mannich Reactions Catalyzed by Bifunctional Amine-Thiourea-Bearing Multiple Hydrogen-Bonding Donors

摘要

The addition of nitroalkanes to imines, known as the nitro-Mannich (or aza-Henry) reaction is a highly valuable carbon-carbon bond-forming process in organic synthesis. Synthetically versatile β-nitro amines formed in this way can be readily converted to 1,2-diamines, α-aminocarbonyl compounds. Since the pioneering work of Shibasaki, much attention has been paid to developing enantioselective catalytic protocols for this reaction over the past decade. Enantio- and diastereoselective nitro-Mannich reactions have been reported using chiral metal complexes and organocata-lysts, such as chiral thiourea, chiral proton catalysts, and chiral phase transfer catalysts. Most recently, Shibasaki and co-workers reported a heterobimetallic Cu-Sm-Shiff base complexes catalyzed nitro-Mannich reactions with excellent .syn-selectivity (>20: 1) and enantioselectivity (83-98%). For anti-selective nitro-Mannich reaction, however, such high diastereoselectivity and enantioselectivity have not been obtained so far. Among numerous literature examples, to our knowledge, there are only few organo-catalytic asymmetric nitro-Mannich reactions reported which can afford both excellent enantioselectivity and high antiselectivity of greater than 10:1 for a broad scope of the reaction partners.~(6b,f) Therefore, there is substantial room for improvement in terms of both selectivity and substrate scope for organocatalytic nitro-Mannich reaction.