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.