Pyrrole Macrocyclic Ligands for Cu-Catalyzed Asymmetric Henry Reactions

New chiral perazamacrocycles containing four pyrrole rings have been synthesized by the [2+2] condensation of (R,R)-diaminocyclohexane and 5,5'-(alkane-2,2-diyl)bis(1H-pyrrole-2-carbaldehydes). These macrocycles, differing for the alkyl/aryl meso-substituents, were used as ligands in the copper-catalyzed Henry reactions of aromatic and aliphatic aldehydes with nitroalkanes. In the optimized experimental conditions, the reactions of nitromethane with catalytic amounts of copper diacetate and the methyl-substituted macrocyclic ligand (1:2 ratio) in ethanol at room temperature, high enantiomeric excesses (up to 95% e.e.) were achieved. The positive influence of the pyrrole-containing macrocyclic structure on the efficiency/enantioselectivity of the catalytic system was demonstrated by comparison with the Henry reactions performed using analogous macrocyclic ligands (trianglamines) derived from (R,R)-diaminocyclohexane and aromatic aldehydes and open-chain ligands derived from (R,R)-diaminocyclohexane or 2-pyrrolecarbaldehyde.