Metal- and Halide-Free Catalyst for the Synthesis of Cyclic Carbonates from Epoxides and Carbon Dioxide

N,N′-Phenylenebis(5-tert-butylsalicylideneimine) is shown to be an effective single-component catalyst for the metal- and halide-free synthesis of cyclic carbonates from epoxides and carbon dioxide. Using this bis-phenolic catalyst, a series of eight epoxides has been converted into the corresponding cyclic carbonates. Many closely related catalyst structures were found to be catalytically inactive, and the structural features necessary for catalytic activity have been delineated. Unusually, reactions could be carried out under solvent-free conditions or in the green solvents 2-methyl tetrahydrofuran (2-MeTHF) and propylene carbonate. Stereochemical studies showed that carbon dioxide insertion occurs exclusively at the terminal end of the epoxide and does so with loss of stereochemical purity if a 1-deutero-2-alkyl epoxide is used as substrate. On the basis of this evidence, a dual activation mechanism is proposed in which one phenol acts as a Brønsted acid to activate the epoxide, while the other reacts with carbon dioxide to form a carbonic half-ester intermediate which then ring-opens the activated epoxide. Two pathways are subsequently available for cyclization to a cyclic carbonate which have opposite stereochemical consequences. The experimental results suggest that ring-opening of the epoxide by the carbonic half-ester is the rate-determining step of the catalytic cycle.