Synthesis And Properties Of Helical Poly(Phenylacetylene)s Bearing L-proline Derivatives As Chiral Catalysts

Helical poly?phenylacetylene?s have attracted much attention due to the conjugated structure,and have been widely used in asymmetric catalysis,chiral resolution,molecular recognition and other fields.Inspired by the enzyme,researchers have associated the substituted polyacetylenes with helical structure to an asymmetric catalytic reaction.The polymer may also have the catalytic properties by introducing a group having a chiral catalysis in the side groups of the substituted polyacetylenes.In this paper,four helical poly?phenylacetylene?s bearing proline derivatives were synthesized via a series of reactions.Two of the monomers and the corresponding polymers were used to catalyze asymmetric reactions,and the factors influencing the asymmetric catalytic performance were also discussed.The phenylacetylene monomers PAA-Fmoc-Pro-OH and PAA-Fmoc-Pro-4-NO₂-BC were synthesized via Sonogashira coupling reaction,off-protecting group TMS,amidation reaction and derivatization reaction.Polymers were synthesized by the rhodium-catalyzed and polymerization of PAA-Fmoc-Pro-OH and PAA-Fmoc-Pro-4-NO₂-BC respectively.The monomer PAA-Pro-OH,PAA-Pro-4-NO₂-BC and the corresponding polymer PPAA-Pro-OH,PPAA-Pro-4-NO₂-BC with free secondary amino groups were synthesized via deprotection reaction.The structure of monomers and polymers were confirmed by ¹H NMR and IR.The results of the optical rotation and the CD spectra showed that the polymers adopted dynamic helical conformation,and their optical activity can be regulated by changing external factors.The asymmetric aldol reaction of acetone with 4-nitrobenzaldehyde and the Michael addition reaction of cyclohexanone with trans-?-nitrostyrene were carried out in the presence of the monomers PAA-Pro-OH,PAA-Pro-4-NO₂-BC or the polymers PPAA-Pro-OH,PPAA-Pro-4-NO₂-BC.When the polymers were used as a catalyst,both the main chain helical conformation and the side chain proline derivatives played an important role in the asymmetric catalytic reaction.The catalytic effect of poly?phenylacetylene?s derivative PPAA-Pro-OH as a chiral catalyst for asymmetric Michael addition was better than that of asymmetric aldol reaction.In the asymmetric Michael addition reaction,the catalytic effect of the polymer was better than that of the corresponding monomer.By optimizing the reaction conditions,the enantioselectivity and diastereoselectivity of asymmetric Michael addition products had been greatly improved,the ee value was up to 96%,the syn/anti value was up to 99/1,and the catalyst can be recycled.