Highly regio- and stereo-selective copolymerization of CO2 with racemic propylene oxide catalyzed by unsymmetrical (S,S,S)-salenCo(III) complexes

A novel unsymmetrical (S,S,S)-salen ligand bearing a derived chiral-BINOL was synthesized by the reaction of the condensation product of (1S,2S)-1,2-diaminocyclohexane mono(hydrogen chloride) with 3-adamanyl-5-tert-butyl-2-hydroxybenzalde-hyde and 3-formoyl-2-hydroxy-2'-alkyloxy-1,1'-binaphthyl, which originated from (S)-1,1'-bi-2-naphthol via a four-step reaction. The cobalt complexes of this ligand, in conjunction with a nucleophilic cocatalyst, exhibited excellent activity in catalyzing asymmetric, regio- and stereo-selective copolymerization of CO2 and racemic propylene oxide. The highest record of kinetic resolution coefficient (K-rel) was obtained with the use of binary catalyst system consisting of (S,S,S)-salenCo(III) complex 1c and bulky bis(triphenylphosphine)-iminium chloride (PPNCl). The resulting poly(propylene carbonate)s have more than 99% carbonate linkages and more than 98% head-to-tail content.