Mild formation of cyclic carbonates using Zn(II) complexes based on N2S2-chelating ligands

We have prepared a series of Zn(II) complexes (1-3) based on a versatile N2S2-chelating ligand abbreviated as btsc [btsc = bis-(thiosemicarbazonato)] derived from simple and accessible building blocks. These complexes comprise a Lewis acidic Zn(II) center useful for substrate activation, and we have investigated the potential of these compounds in the cyclo-addition reaction of carbon dioxide to various epoxides yielding cyclic carbonate structures. Initial screening studies with complexes 1-3 showed that complex 3 is most suited for this CO2 fixation reaction under particularly mild conditions (45 °C, pCO2 = 10 bar) and low catalyst loadings (1 mol%). Furthermore, upon examination of the substrate scope, complex 3 shows appreciable catalytic turnover for a range of terminal epoxides, while for the sterically more challenging epoxides almost no conversion was achieved under comparable conditions. Additional experiments indicated that higher yields of cyclic carbonates may be realized by simply increasing the (co)catalyst loading up to 3%, while maintaining mild reaction conditions. The use of a relatively non-toxic and abundant metal and an environmentally benign solvent system (MEK, methyl ethyl ketone) mark this protocol as an attractive way for organic carbonate production