Experimental and theoretical study of Hoveyda–Grubbs catalysts modified by perfluorohexyl ponytail in the alkoxybenzylidene ligand

The alkoxybenzylidene ligand of Hoveyda–Grubbs 1st and 2nd generation catalysts was modified with one or two perfluorohexyl groups by Ullmann reaction with the aim to improve both the fluorophilicity and activity of the catalyst. While bis(perfluorohexylation) resulted in inability of the ligand to exchange tricyclohexylphosphine ligand of parent Grubbs catalysts, mono(perfluorohexylation) and subsequent ligand exchange resulted in the formation of complexes of light fluorous properties and substantially higher activity in model RCM reactions. Modification of the mesityl group of the unsaturated NHC ligand by polyfluoroalkyl ponytails resulted in the formation of ruthenium precatalyst furnishing active catalytic intermediate with light fluorous properties. DFT computations of the model initiation reaction of ethene with Hoveyda–Grubbs 2nd generation catalyst or its pentafluoroethylated counterpart revealed that in the latter, the intermediate ruthenacyclobutane can form and decompose with significantly lower energies, thus explaining its higher activity.