Direct oxidative carboxylation of olefins into cyclic carbonates at ambient pressure

The direct synthesis of cyclic carbonate from carbon dioxide (CO2) and olefins is attracting much interest in recent years. Herein, 1,5,7-triazabicyclo[4.4.0]dec-5-ene-functionalized phosphonate salts have been synthesized and characterized fully by Nuclear Magnetic Resonance (NMR), Electrospray Ionization Mass Spectrometry (ESI-MS) and Fourier Transform-infrared Spectroscopy (FT-IR) etc. The as-obtained salts have been employed as efficient catalysts for one-pot synthesis of cyclic carbonate from styrene and CO2 through oxidative carboxylation reaction under very mild conditions (0.1 MPa CO2, 80 degrees C) without any other solvents. The yield of cyclic carbonate can reach up to ca. 70 % in the presence of ZnBr2 under the mild reaction condition. Besides, the catalyst can be recycled at least five times without significantly losing its catalytic activity. This approach can be extended to various styrene derivatives and good yields of cyclic carbonates was achieved under ambient conditions. Based on activity evaluation, reaction kinetics and DFT calculation, the six-membered carboxyphosphate species was proposed as a crucial intermediate species and played a crucial role in CO2 activation. Finally, the reaction mechanism is proposed accordingly.