Effect of the bulkiness of indenylidene moieties on the catalytic initiation and efficiency of second-generation ruthenium-based olefin metathesis catalysts

We report on the synthesis and characterization of a second generation of ruthenium catalysts bearing different sized indenylidene ligands, denoted as RuCl2(3-(2,6-xylyl)-1-indenylidene)(SIMes)(PCy3) 5a, RuCl2(4-methyl-3-(o-tolyl)-1-indenylidene)(SIMes)(PCy3) 5b, RuCl2(3-1-naphthyl-1-indenylidene)(SIMes)(PCy3) 5c and RuCl2(3-(p-fluorophenyl)-1-indenylidene)(SIMes)(PCy3) 5d. These complexes were characterized by NMR, IR, HRMS and elemental analysis. Moreover, the configurations of complexes 5a, 5b and 5d were confirmed by single-crystal X-ray diffraction analysis. The catalytic performances of complexes 5a–d were evaluated in several olefin metathesis reactions: the RCM of substrates 6 and 7, the RCEYM of substrate 8 and the ROMP of COD, in comparison with commercially available catalyst 3a. Careful analysis of the catalytic performances and single-crystal X-ray diffraction data of complexes 5a–5d reveal that the steric modification on the 3-phenyl group has a significant influence on the ligand congestion around the ruthenium center, altering the catalytic activity in metathesis reactions. In addition, the better-performing complex 5a was further investigated in the RCM of substrate 6 in comparison with complex 3c and benchmark complexes 1b, 2 and 3a.