Chiral salenCo(iii) complexes with bulky substituents as catalysts for stereoselective alternating copolymerization of racemic propylene oxide with carbon dioxide and succinic anhydride

Abstract

A series of new chiral salenCo(III)X complexes [salen = (R,R)-N,N′-bis(3-R-5-tert-butylsalicylidene)-1,2-cyclohexanediamine, X⁻ = OAc⁻, R = CMe₂CHMe₂ (4a), CPhMe₂ (4b), CPh₂Me (4c); X⁻ = 2,4-dinitrophenoxide anion (DNP), R = CMe₂CHMe₂ (5a), CPhMe₂ (5b), CPh₂Me (5c); X⁻ = Br⁻, R = CMe₂CHMe₂ (6a), CPhMe₂ (6b), CPh₂Me (6c)] with bulky substituents at the 3,3′-positions of the salicylidene moiety were synthesized and investigated as catalysts in the copolymerization of CO₂/propylene oxide (PO) and the sequential copolymerization of succinic anhydride/PO and CO₂/PO. The effects of the reaction conditions were examined by using complex 4b as a representative catalyst for the copolymerization of CO₂/PO. It was found that catalysts with different ligands and axial coordinating groups show remarkable differences in their catalytic properties. The catalysts with relatively small R groups show relatively high catalytic activity and produce copolymers with relatively high molecular weights, while the catalysts with relatively bulky R groups show relatively high stereoselectivity. Catalyst systems employing 4-(dimethylamino)pyridine (DMAP) as the cocatalyst show relatively low catalytic activity but high stereoselectivity in comparison with the ones observed in the systems with bis(triphenylphosphoranylidene) ammonium chloride (PPNCl) or PPNDNP as the cocatalyst. With the catalyst system of 5c/DMAP, a high kinetic resolution coefficient (K_rel) of 10.6 for the insertion of (S)-PO over (R)-PO was obtained at 25 °C. High molecular weight PPCs (M_w > 100 kg mol⁻¹) were prepared with 5a/PPNCl and 5a/PPNDNP catalyst systems. Stereoregular block copolymers poly(propylene succinate-block-carbonate) were obtained from sequential copolymerization via catalytic kinetic resolution.