Ruthenium Olefin Metathesis Catalysts with N-Heterocyclic Carbene Ligands Bearing N-Naphthyl Side Chains

A series of second-generation ruthenium-based olefin metathesis catalysts bearing N-naphthyl-substituted N-heterocyclic carbene (NHC) ligands have been prepared and fully characterized. By reaction with the appropriate NHC, these complexes are readily accessible in one synthesis step from the commercially available first-generation precursors [RuCl2(═CHPh)(PCy3)2] (Grubbs I, GI) or [RuCl2(═CH-o-iPrO-Ph)(PCy3)] (Hoveyda−Grubbs I, HGI) by simple exchange of one phosphine ligand with the free NHCs. Time-dependent conversions in the ring-closing metathesis (RCM) of standard substrates leading to di- as well as trisubstituted olefins have been measured for these catalysts. When benchmarked against the parent SIMes-containing the Grubbs II precatalyst (GII), most of these new NHC structures show enhanced reactivity in RCM. From these comparative studies, valuable information was gathered which shows that the alkyl substitution on the naphthyl side chains can enhance or lower the catalytic performance, depending on the bulk and the position of these alkyl groups. The behavior of the best performing precatalysts has been investigated in the RCM of a series of representative substrates, in enyne metathesis reactions as well as in cross-metathesis (CM).