N. Gabriel Lemcoff

The ruthenium alkylidene catalyzed ring‐closing metathesis (RCM) of 5‐membered rings is an intramolecular reaction that is highly favored due to entropic and enthalpic contributions. Counterintuitively, by using trifluoromethyl sulfur‐chelated ruthenium benzylidene ( Ru−SCF 3 −I ), RCM of a diene with a hindered dimethyl terminus, diethyl 2‐allyl‐2‐(3‐methylbut‐2‐en‐1‐yl) malonate ( S1 ), could be achieved at high concentrations but not at low concentrations. This finding led to the discovery that several additives, including ethyl acetate, significantly enhance the latent catalyst's activity. DFT computations and NMR control experiments suggest that additive coordination influences the catalytic cycle. Moreover, the “ethyl acetate effect” was studied in ring‐opening metathesis polymerizations (ROMP), providing a more efficient initiation when the reactions are run in environmentally friendly ethyl acetate as the solvent. The strong influence of simple coordinating additives (sometimes present in the substrate itself) on the activation of latent catalysts provides important insights into the reaction mechanism and opens a doorway towards improving the efficiency of these types of catalysts.