Ruthenium Complexes Bearing Thiophene-Based Unsymmetrical N-Heterocyclic Carbene Ligands as Selective Catalysts for Olefin Metathesis in Toluene and Environmentally Friendly 2-Methyltetrahydrofuran.

Three mono-N-heterocyclic carbene (NHC) ruthenium 2-isopropoxybenzylidene (10 a-c) and one bis(NHC) indenylidene complex (8) bearing an unsymmetrical N-heterocyclic carbene ligand were synthesized and structurally characterized by single-crystal X-ray diffraction. The catalytic activity of the newly obtained complexes were evaluated in ring-closing metathesis (RCM) and ene-yne (RCEYM) reactions in toluene and environmentally friendly 2-MeTHF under air. The results confirmed that although all tested reactions can be successfully mediated by catalysts 10 a-c, their general reactivity is lower than the benchmark all-purpose Ru catalysts with symmetrical NHC ligands. However, the latter cannot compete with specialized ruthenium complex 10 a in industrially relevant self-CM of terminal olefins in neat conditions.