Optically Active Helical Polyacetylene Bearing Ferrocenyl Amino-Acid Derivative in Pendants. Preparation and Application as Chiral Organocatalyst for Asymmetric Aldol Reaction

Abstract

The article reports a novel type of helical polymer-based chiral catalyst for catalyzing asymmetric aldol reactions. Chiral acetylenic monomers containing ferrocenyl amino-acid derivative substituent were synthesized for the first time and structurally identified. The investigated amino acids include alanine and threonine enantiomers. The obtained monomers separately underwent solution homopolymerization and copolymerization with an achirally substituted acetylene monomer in the presence of [Rh­(nbd)­Cl]₂ and Et₃N. Circular dichroism and UV–vis absorption spectra demonstrated that the copolymer chains adopted predominantly one-handed helices, endowing the copolymers with optical activity. The resulting (co)­polymers were further used to catalyze aldol reaction between cyclohexanone and <i>p</i>-nitrobenzaldehyde. Only threonine-derived copolymers efficiently catalyzed the aldol reaction. A remarkable yield (up to 90%) and enantiomeric excess (up to 93%) were obtained. A synergic effect between the helical structures in the copolymer main chains and the pendent catalytic moieties was found to play a crucial role in the asymmetric catalysis