A recent publication from the Grubbs’ group details an evolved dithiolate-ligated ruthenium precatalyst (Figure 1) that displays enhanced initiation kinetics resulting in decreased reaction times and improved isolated yields. These new precatalysts are presumed to propagate via the same species as the preceding generation; consequently, a similar reaction scope is expected. Likewise, variation of the ortho substituents of the NHC should have effects comparable to those previously observed.
Figure 1: Dithiolate-ligated ruthenium based metathesis precatalyst evolution
Indeed, internal olefins are good substrates for this catalyst family with Z substrates reacting faster than E substrates (kZ/kE ranging from 4 to 200). As witnessed previously, terminal olefins remain challenging and generally result in premature catalyst decomposition. Additionally, reducing the size of the NHC’s ortho substituents (Me/Me → Me/F → F/F) generally afforded faster transformations with E substituents, in agreement with the proposed model (Figure 2).
Figure 2: Proposed model for E-selective olefin metathesis
As always, it will be interesting to follow the progression and adoption of this family of catalysts.