Experimental and theoretical insight into the mechanism of CO2 cycloaddition to epoxides catalyzed by ammonium ferrates

Soluble tetrabutylammonium ferrates, [TBA][FeX3Y] (TBA = nBu4N) were synthetized by treating ferric salts (FeX3) with tetrabuthylammoniom halides. Their activity as a stand-alone catalyst in CO2 cycloaddition reactions to epoxides was assessed under solvent free and quite mild reaction conditions (CO2 pressures between 0.4 and 0.8 MPa) and TOF up to 428 h-1 (T = 150 oC) were observed. Good yields of cyclic organic carbonates were obtained, especially with terminal epoxides, without the need of any Lewis base as co-catalyst, with a broad reaction scope. A scale-up reaction on 5 mL of styrene oxide was performed and the robustness of the catalyst was proved up to three recycles in the case of propylene oxide (TON = 594). To shed light on the reaction mechanism, an extensive set of theoretical calculations has been carried out. Iron salts almost annihilate the barrier for the epoxide ring opening and stabilize the first reaction intermediate. Along the same reaction path, chloride proved to be more effective as nucleophile than bromide, and preferentially attacks on the more hindered carbon atom. On the other hand, when no Lewis acid (LA) is present, the rate determining step of the reaction becomes the ring opening of the epoxide. A tight correlation with experimental results was observed.