The Synthesis Of Helical Polyisocyanides Modified By Proline Or Organic Phosphine And Their Application In Catalytic Asymmetric Reactions

Stimulated by helical structure in biomacromolecules,great efforts have been conducted on artificial helical polymers in recent years.In addition to their interesting helical structure,helical polymers have a range of appealing applications in the fields of chiral recognition,chiral separation,chiral sensing and liquid crystal and so on.Asymmetric catalysis is effective methods for construction chiral centers in molecules and has been widely used in the synthesis of drug intermediates and natural products.Compared with transition metal catalysis and enzyme catalysis,organocatalysis has advantages of toxic heavy metal-free,mild reaction conditions,and broad substrate scope.However,organocatalysis also suffering from some drawbacks,such as higher catalyst loading and difficult to recycle the catalyst.In order to overcome these deficiencies,the strategy that immobilization of these catalysts on macromolecule supporters have been developed by chemists recently.Compared with silicon and traditional polymers,the utilizing of helical polymer as supporter for organocatalysts has many advantages:the stereoselectivity of the reaction can be regulated slightly by the structure of helical polymer,the catalytic efficiency could be improved by the synergistic effect of the helix backbone and the side groups;the helix polymer can be easily separated from the reaction system and recycled due to its large molecular weight.In this paper,two kinds of phenyl isocyanide monomers carrying L-proline or organic phosphines were respectively designed and synthesized.Living polymerization of these monomers using a alkyne-Pd(II)catalyst affording a series of helical polymers with controlled molecular weights(Mns)and narrow molecular weight distributions(Mw/Mns).These optically active helical polyisocyanides bearing L-proline and phosphine pendants can be further used as polymer catalyst in asymmetric Aldol reaction and Rauhut-Currier reaction,respectively.The main contents of this thesis including the following two parts:1.A novel enantiopure phenyl isocyanide(1)with L-proline substituent was designed and synthesized.Living polymerization of 1 using a alkyne-Pd(II)catalyst affording a series of helical poly-lms in high yields with controlled Mns and narrow Mw/Mns bearing L-proline ester as the pendants.Removed the protecting groups on the L-proline pendants lead to the formation of well-defined helical poly-1m-A.Very interestingly,the left-handed helical backbone of poly-lm was reversed to right-handed helix in poly-lm-A as revealed by circular dichroism(CD)and polarimetry investigations.Optically active helical poly-lm-A showed excellent catalytic ability on asymmetric aldol reaction.Comparing to the small molecule(1-A)with similar structure,both the enantioselectivity and the diastereoselectivity of the Aldol reaction were significantly enhanced.The diastereomeric ratio(dr)and enantiomeric excess(ee)values of the aldol reaction are up to>20/1 and 95%,respectively.Moreover,the helical polyisocyanides catalyst can be easily recovered and reused in the Aldol reaction at least four cycles without significant loss of its enantioselectivity and diastereoselectivity.2.A series of enantiopure phenyl isocyanide monomers with BH3-protected phosphine functional group(4a-4d,7,10)were designed and synthesized.Polymerization of these monomers using a alkyne-Pd(II)complex as a catalyst led to the formation of respective helical polyisocyanides in high yields with controlled Mns and narrow Mw/Mns.Removing the protecting BH3 groups afforded helical poly(phenyl isocyanide)s bearing phosphine pendants.Because of the chiral induction of monomer,the isolated helical polyisocyanides showed high optical activity as revealed by circular dichroism(CD)and absorption spectroscopies and polarimetry.The helical structures of these polymers were quite stable in various organic solvents with different polarities,and in a wide temperature range.In addition to the homopolymers,block copolymer were also synthesized in which the monomers(4a,7,10)with the monochiral helical macroinitiator.Moreover,these helical polymers could be used as organocatalysts and showed good performance in enantioselective cross Rauhut-Currier reaction.The ee values of the isolated products 14b of cross Rauhut-Currier reaction could be up to 90%.The polymer organocatalysts could be easily recovered from the reaction mixtures and reused at least five times in the reaction without significant loss of their enantioselectivities and catalytic activities.