Chirally Helical-substituted Polyacetylene: Preparation And Application As Chiral Organocatalyst For Asymmetric Reaction

Helical-substitued polyacetylenes, a type of conjugated polymer, can form stablely helical and predominantly one-handed structures under certain conditions, making the polymers with optical activity. The"synergistic effect" occurring between the helical conjugated backbone and the functional pendants, endows the helical polymers unique properties and they are widely used in mang aspects such as chiral recognition ability, enantioselective separation ability and asymmetric catalysis. Besides, they also show great potential and broad application prospects in biomimetic chemistry, biomedicine, and liquid crystal materials fields. However, some shortcomings of polyacetylenes materials, such as the poor stability, difficulty of dissolving, limit their applications. The preparations of novelly optically active polyacetylenes material have attracted more and more attentions.Ferrocene is composed of a ferrous ion and two cyclopentenyl anions, and ferrous ion is sandwiched between two cyclopentadienyl ligands, which shapes the "sandwich" structure, endowed ferrocene lipophilic, and aromatic property. Its unique stereostructure and electronic effect can be used as chiral catalyst and chiral ligand for asymmetric synthesis.Chiral diamines in catalytic asymmetric synthesis show good catalytic activity and enantioselectivity. A large number of small molecular catalysts derived from chiral diamines play an important role in asymmetric catalytic reactions.The helical structure formed in the (bio)catalysts, e.g. enzymes, is indispensable for them to serve as chiral catalysts. Chirally helical-substituted polyacetylene chains adopted predominantly one-handed helices in some conditions. Aiming to simulate the procedure of enzymic catalytic reaction, we prepared helical polymer bearing ferrocenyl amino-acid and chiral diamine derivatives in pendants as chiral organocatalyst for asymmetric aldol reaction. The major research contents are shown as follows:1. Alanine and threonine as chiral sources were used as substituents to bond with ferrocene groups with lipophilic and space stereocomplexes.Four acetylene acetamides were prepared and structurally identified. The prepared chiral substituted acetylene monomers were copolymerized with a achiral monomer to obtain a series of chiral helical polymeric organocatalysts containing pendant groups of ferrocene derivatives.FT-IR, circular dichroism (CD) and UV-vis absorption spectroscopy measurements show that the optically active helix-substituted polyacetylene macromolecule catalyst has been successfully prepared.The resulting (co)polymers were further used to catalyze aldol reaction between cyclohexanone and p-nitrobenzaldehyde. Only threoninederived copolymers efficiently catalyzed the aldol reaction. A remarkable yield and enantiomeric excess were obtained.2. For the solid-state substituted acetylenic monomer, a co-solvent was introduced, and optically active crosslinked microspheres containing ferrocenyl amino-acid derivative substituent were prepared by suspension polymerization method using 2 wt % polyvinyl alcohol (PVA) deionized water solution as stabilizer. The structures and properties of the resulting optically active crosslinked microspheres were characterized by FT-IR,CD and UV spectroscopy, scanning electron microscopy (SEM), Raman spectroscopy and thermogravimetric analysis (TGA). The results show that the prepared optically active crosslinked microspheres have significant optical activity. The optically active crosslinked microspheres can efficiently catalyze asymmetric aldol reaction of cyclohexanone with p-nitrobenzaldehyde. Moreover, optically active crosslinked microspheres were easily recycled and reused at least three times without significant loss of weight, activity, diastereo- and enantioselectivities.3. We combined (R, R)- and (S, S)-Ts-DPEN with propiolic acid to obtain the novel chiral acetylenic monomers, namely M2R and M2S.Subsequently, M2R and M2S separately underwent copolymerization with other chiral monomes, M3R and M3S, to prepare copolymers.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 copolymers were further used to catalyze aldol reaction between cyclohexanone and p-nitrobenzaldehyde, and the aldol product was formed in medium isolated yield and medium ee%.