A mechanistic study on coupling of CO2 and epoxide mediated by guanidine/TBAI catalysts

Density functional theory (DFT) calculations at the M062X-D3/def2-TZVP//M062X-D3/def2-SVP level of theory were employed to reveal the mechanism of the reaction between CO2 and styrene oxide for cyclic carbonate, mediated by guanidine and tetrabutylammonium iodide (TBAI) co-catalysts. The noncatalytic reaction occurred via a concerted mechanism, with energy barriers as high as 64.1 and 78.0 kcal mol−1. Three elementary steps were included in the catalytic reaction, and epoxide ring-opening by nucleophilic attack of an iodide anion was predicted to be the rate-determining step (RDS). Guanidine acted as the H-bond donor to activate styrene oxide by (N)H⋯O interaction, facilitating epoxide ring-opening with a low activation barrier (ΔG≠ = 22.2–29.6 kcal mol−1). A good linear correlation between the acidity of the NH group in the guanidine and the energy barrier in the epoxide ring-opening step was observed. The introduction of an amide group could strengthen the hydrogen bonding ability of the guanidine catalyst toward a styrene oxide substrate, decreasing the activation barrier for the cyclic carbonate product. When the guanidine–Cu(I) complex was used as the Lewis acid catalyst, the styrene oxide was activated by O⋯Cu(I) coordination in organometallic catalysis. The energy barriers in the presence of guanidine–Cu(I)/TBAI catalysts could be decreased in contrast to the non-catalytic reaction.