Optically active helical polyisocyanides bearing chiral phosphine pendants: Facile synthesis and application in enantioselective Rauhut-Currier reaction

Abstract

Three novel enantiopure phenyl isocyanide monomers with BH3-protected phosphine functional group 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 molecular weights (Mns) and narrow molecular weight distributions (Mw/Mns). Removing the protecting BH3 groups afforded helical poly(phenyl isocyanide)s bearing phosphine pendants. Thanks to 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. Moreover, these helical polymers could be used as organocatalysts and showed good performance in enantioselective cross Rauhut-Currier reaction. The enantiomeric excess (ee) values of the isolated products 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.