A series of bifunctional metalloporphyrins as efficient catalysts for cycloaddition of CO₂ with epoxides

Two porphyrins, named H(2)TEtImP and H(2)DEtImP, with the four and two N-ethylated imidazole groups, respectively, as well as their metalloporphyrins, M-TEtImP (M = Zn, Mn, Co, Mg) and Zn-DEtImP, were synthesized and characterized by ultraviolet-visible (UV-Vis), mass spectrometry (MS), proton nuclear magnetic resonance (H-1 NMR), and Fourier transform infrared (FTIR) successfully. The catalytic activity of these newly synthesized porphyrins with metal centers and halogen ions as variables on the cycloaddition reaction between carbon dioxide (CO2) and epoxides was investigated under solvent-free conditions. The experimental results strongly verified that both halogen ions and metal centers were indispensable active sites. The metal catalytic activity increased in the order Co < Mn < Mg < Zn. The turnover frequency (TOF) of Zn-TEtImP for the cycloaddition reaction of CO2 and epichlorohydrin was as high as 2213 h(-1) (120 degrees C, 1 MPa CO2, 6 h). Moreover, the wide applicability of Zn-TEtImP to this reaction was demonstrated by substrate extension experiments, and the excellent recyclability of Zn-TEtImP was confirmed. Finally, the synergistic catalytic mechanism of Lewis acid (metal center) and nucleophile (Br-) was proposed based on the catalytic results.