Mechanistic aspects of the cycloaddition of CO2 with propylene oxide using the trans-Ru(py)4Cl2 catalyst

The mechanism of the cycloaddition of CO2 with propylene oxide to afford propylene carbonate catalyzed by a highly active trans-dichlorotetrapyridineruthenium [trans-Ru(py)4Cl2] complex and tetrabutylammonium chloride (TBAC) has been studied by means of electrospray ionization mass spectrometry (ESI–MS), structural characterization of trans-Ru(py)4Cl2, catalyst activity tests and so on. Further experiments demonstrated that the tributylamine formed in situ was involved in the catalysis and that addition of butyl chloride to re-convert the tributylamine into TBAC resulted in the inhibition of the reaction. The mechanistic study explains the reported early experimental observations well and provides a clear profile for the cycloaddition of carbon dioxide with propylene oxide using trans-Ru(py)4Cl2 as catalyst. The mechanism also fully explains the role of the TBAC by providing a role for the in situ generated tributylamine in activating the carbon dioxide.