Coupling reaction between CO2 and cyclohexene oxide: selective control from cyclic carbonate to polycarbonate by ligand design of salen/salalen titanium complexes

Abstract

Based on the mechanistic features of metal salen catalysis systems, titanium(IV) complexes from salen (salen-H₂ = N,N-bis(3,5-di-tert-butylsalicylidene)-1,2-benzenediamine) and its half saturated form salalen have been prepared, which were used as catalysts in conjugation with bis(triphenylphosphino)iminium chloride ([PPN]Cl) for the coupling reaction of CO₂ and cyclohexene oxide (CHO). The salen titanium complex (salen)Ti(IV)Cl₂ showed moderate activity, producing a unique cis-isomer of cyclic carbonate with high conversion up to 100% in 8 h, however, it could not catalyze the copolymerization reaction. Meanwhile, the salalen titanium complex (salalen)Ti(IV)Cl was effective for the copolymerization of CO₂ and CHO, where only one chain grew on Ti during the chain propagation reaction, yielding completely alternating copolymers with –OH and –Cl as terminal groups. Moreover, the nearly complete conversion of CHO indicated that (salalen)Ti(IV)Cl might be used to synthesize multiblock poly(cyclohexene carbonate)s with controllable sequences.