Polyurea-catalyzed cycloaddition of CO2 and epichlorohydrin: a green approach

Abstract

A one-pot solvothermal, polyurea (PU) synthesis was reported using propylene carbonate (PC) and methylene-4,4′-dianiline (MDA), organocatalyzed by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in toluene. The applied method was performed under auxiliary-free conditions (halogen-, metal-, and co-catalyst-free), avoiding the use of the toxic isocyanates. The synthesized PU served as a macro organocatalyst for the cycloaddition of epichlorohydrin (ECH) and CO₂ under the optimized reaction conditions of 130 °C, 24 h and 5 bar CO₂, using 0.5 mL dry DMSO and 150 mg catalyst to produce 1.4 g of 4-chloromethyl-2-oxo-1,3-dioxolane. A catalyst reusability study over five consecutive cycles proved the stability of PU, as no decomposition was detected and it could be recovered by simple washing and filtration. After a few cycles, the amine end group of the catalyst was modified by ECH, decreasing the catalytic activity. In addition, density functional theory (DFT) elucidated the cycloaddition mechanism, together with the energetics of the catalytic cycle. It confirmed the accessibility of PU for the epoxide ring activation from both terminal functionalities, with a lower energy barrier for the amine terminated end compared to the alcohol-terminated one.