Facile synthesis of cyclic carbonates from CO₂ and epoxides with cobalt(II)/onium salt based catalysts

Efficient catalytic systems based on the combination of cobalt(II)/onium salts for the coupling reaction of CO₂ and epoxides are reported. The catalytic systems work under mild reaction conditions (e.g. 10 bar and 120 °C) and produce cyclic carbonates with high turnover frequencies (TOF) and selectivity. From a series of onium salts, bis(triphenylphosphoranylidene)ammonium chloride proved to be the most active cocatalyst (TOF = 2314), while tetrabutyl ammonium chloride (TOF = 2223) and 1,3-bis(2,4,6trimethylphenyl)imidazolium chloride (TOF = 2174) provided only slightly reduced activities under similar conditions. The influence of various reaction parameters including CO₂ pressure, reaction temperature and loading of onium salt was determined. In order to obtain an insight into the catalytic process, the reaction was followed by electrospray ionisation-mass spectrometry. Additionally, in situ FT-IR measurements show that the catalyst is stable even with low loading (0.025 mol%) and as a result, the reaction proceeds steadily over several hours.