Steric and electronic effects in latent S-chelated olefin metathesis catalysts

In this work, the structure, latency, and activity of twelve sulfur-chelated ruthenium precatalysts were studied by systematically varying their ligand shell. First, the thermal cis-trans isomerization process was monitored, disclosing factors that lead to a larger trans-dihalo ratio. Also, the efficiency of two model ring-closing metathesis reactions, one that produces a methylidene intermediate and another that forms an ethylidene, were investigated under thermal activation. For reactions that involve a methylidene, a bulkier NHC ligand and larger anionic ligands promoted faster reactions, whereas when a more hindered ethylidene intermediate is formed, larger ligands mostly retarded reaction initiation. Thus, different combinations of ligand shells and activation stimuli were used to obtain the best results for each substrate type. Most interestingly, irradiation of complexes disclosed that light irradiation promotes cis/trans or trans/cis-dihalo isomerization, but does not activate otherwise inactive catalysts. Thus, this finding reveals that light activation of S-chelated ruthenium benzylidenes only takes place when the trans-dihalo form is intrinsically active at room temperature. The deeper understanding obtained by this study, and the simplicity of the ligand exchange methodology, make this "mix and match" approach very attractive for selecting the most efficient latent olefin metathesis catalyst depending on the desired type of reaction and substrate.