l-Proline derived arylmethanamine ligands and their application in the copper-catalyzed asymmetric Henry reaction: a rare example of a Cu-complex with a dicopper tetraacetate core

Synthesis of chiral salalen ligands and their in-situ generated Cu-complexes for asymmetric Henry reaction

Chiral salalen ligands derived from (S)-proline and derivatives of salicyaldehydes were synthesized, and their in-situ generated Cu (II) complexes were evaluated in the asymmetric Henry reaction. Salalen ligand of different substituents on the phenyl moiety showed remarkable effect on the enantioselectivity of nitro-aldol product of 4-nitrobenzaldehyde and nitromethane. Cu (II) complex generated in situ with (S)-2-(tert-butyl)-6-((2-(((2-hydroxy-3-methylbenzylidene)amino)methyl)pyrrolidin-1-yl)methyl) phenol (10 mol%) and Cu (OAc)₂ .H₂ O (10 mol%), found to be better catalyst for nitro-aldol reaction between 4-nitrobenzaldehyde and nitromethane, gave corresponding product in 85% yield and 88% enantiomeric excess (ee) in isopropanol at 35°C after 40 hours. The catalyst also used for the Henry reaction with different substituted benzaldehydes and corresponding products were obtained in 22% to 99% yields with 66% to 92% ee. Henry reaction of 4-nitrobenzaldehyde and prochiral nitroethane gave anti-selective product (dr = 79/21; anti/syn) in a 91% yield with 80% ee.

Role of Configuration at C6 in Catalytic Activity of l-Proline-Derived Bifunctional Organocatalysts

l-Proline-derived chiral bifunctional (thio)urea organocatalysts epi-PTU and epi-PU were newly synthesized, and their catalytic performances were compared with their C6 epimeric catalysts PTU and PU in various Michael reactions of nitrostyrene in terms of reactivities and stereoselectivities. The experimental results indicate that a proper relative stereochemistry at C2 and C6 in l-proline-derived bifunctional organocatalysts is important for successful catalysis and that catalysts (PTU and PU) with the 2S,6R configuration are much more efficient.