Reactivity regulation for olefin metathesis-catalyzing ruthenium complexes with sulfur atoms at the terminal of 2-alkoxybenzylidene ligands

For regulating the olefin metathesis (OM) activity of the Hoveyda-Grubbs second-generation complex (HG-II), the structural modification of the benzylidene ligand is a useful strategy. This paper reports the effect of a chalcogen atom placed at the end of the benzylidene group on the catalytic properties of HG-II derivatives, using complexes with a thioether or ether component in the benzylidene ligand ( ortho -Me-E-(CH 2 ) 2 O-styrene; E = S, O). Nuclear magnetic resonance and X-ray crystallographic analyses of the complex with a thioether moiety (E = S) proved the ( O , S )-bidentate and trans -dichlorido coordination for the complex. A stoichiometric ligand exchange between HG-II and the benzylidene ligand (E = S) produced the corresponding complex with an 86% yield, confirming higher stability of the complex (E = S) than that of HG-II. Despite the bidentate chelation, the complex (E = S) exhibited OM catalytic activity, indicating the exchangeability of the S -chelating ligand with an olefinic substrate. The green solution color, a characteristic of HG-II derivatives, was retained after the complex (E = S)-mediated OM reactions, indicating high catalyst durability. Conversely, the complex (E = O) rapidly initiated OM reactions; however, it showed low catalyst durability. In the OM reactions conducted in the presence of methanol, the complex (E = S) exhibited higher yields than the complex (E = O) and HG-II: the S -coordination increased the catalyst tolerance to methanol. A coordinative atom (such as sulfur) placed at the terminal of the benzylidene ligand can precisely regulate the reactivity of HG-II derivatives. A Hoveyda-Grubbs-type complex with a sulfur-containing benzylidene ligand has high stability and tolerance to methanol and retains olefin metathesis activity.