Alkene-Chelated Ruthenium Alkylidenes: A Missing Link to New Catalysts

A variety of heteroatom-chelated ruthenium alkylidenes have been developed as metathesis-active catalysts. Alkene-chelated ruthenium alkylidenes, however, have not been considered as a viable alternative because alkene coordination is a necessary step in the catalytic cycle. Relying on common design principles with varying steric and electronic factors, a series of structurally diverse alkene-chelated ruthenium alkylidene complexes were prepared by trapping the intermediates of enyne ring-closing metathesis (RCM) of 1,n-enynes and diynes with a stoichiometric amount of an initiator ruthenium complex. One of the crucial structural elements that promotes the formation of 1,5-alkene-chelates is the exo-Thorpe–Ingold effect, exerted by a gem-dialkyl moiety. These alkene-chelated complexes show a trans relationship between the N-heterocyclic carbene (NHC) ligand and the chelated alkene. On the other hand, η³-vinyl alkylidene complexes were generated from the RCM of ynamide-tethered 1,n-enynes. The presence of an ynamide moiety with a right connectivity is essential for the formation of these rare η³-vinyl alkylidene complexes with a cis relationship between the N-heterocyclic carbene (NHC) ligand and the chelated alkene. The stability and reactivity of these alkene-chelated ruthenium alkylidenes could be finely tuned to show characteristic behaviors in RCM, cross-metathesis (CM), and ring-opening metathesis polymerization (ROMP) reactions.