Heavy fluorous phosphine-free ruthenium catalysts for alkene metathesis

Second generation Grubbs catalyst was modified stepwise with perfluoroalkylated isopropoxystyrene and two perfluoroalkanoate or perfluoropolyoxaalkanoate ligands to afford heavy fluorous phosphine-free ruthenium complexes, which displayed high activity in model ring-closing metathesis reactions. Surprisingly, substitution with linear perfluoropolyether chains led to the complexes of higher activity and fluorophilicity compared to perfluoroalkyl chains, while the use of branched perfluoropolyether ponytails resulted in significantly inferior activity probably due to increased steric hindrance around the active ruthenium centre. Similar reactivity pattern with slightly lower activity was observed for the second generation Hoveyda–Grubbs catalysts bearing perfluoroalkanoate or perfluoropolyoxaalkanoate groups and non-fluorinated isopropoxybenzylidene ligand. Depending on the reaction system, unactivated precatalysts can be recycled by heavy fluorous extraction with perfluoro(methylcyclohexane).