Synthesis of ruthenium catalysts. The corresponding Hoveyda-Grubbs complexes were obtained by displacement of the tricyclohexylphosphine ligand in ruthenium precursor Ru18 by the free NHCs generated using KOC(CF 3 )CH 3 as a base (Scheme 2A). Complex Ru17 was isolated using column chromatography, whereas complex Ru16 was purified by simple crystallisation from a tetrahydrofuran-pentane mixture. Both complexes (brown-orange solids) are stable for months when stored in a protective atmosphere. It is noteworthy that upon the synthesis of known Ru5 (see Supporting Information for details), we noticed its fragility when column chromatography was performed with regular solvents. Even with carefully degassed dry solvents, complex Ru5 was obtained in only 19% yield, which is considerably lower than in the case of Ru16 and Ru17, 72% and 43% respectively. The molecular structures of complexes Ru16 and Ru17 were confirmed by X-ray diffraction experiments on single crystals grown by layering dichloromethane complex solutions of Ru16 and Ru17 with pentane (respectively, Scheme 2B and C, Figure , Table , and Supporting Information for details). Scheme 2. A) Synthesis of Ru16 and Ru17; conditions A: i) 1.75 equiv. of 5, 1.75 equiv. KOC(CF 3 )CH 3 , benzene, rt, 30 min., ii) 1 equiv. Ru18, 60 °C, 1 h; conditions B: i) 1.75 equiv. of 8, 2 equiv. KOC(CF 3 )CH 3 , benzene, rt, 15 min., ii) 1 equiv. Ru18, 60 °C, 30 min. B) XRD structure of Ru16, ellipsoids are drawn at 50% probability. C) XRD structure of Ru17, ellipsoids are drawn at 50% probability.