Rapid Decomposition of Olefin Metathesis Catalysts by a Truncated N‑Heterocyclic Carbene: Efficient Catalyst Quenching and N‑Heterocyclic Carbene Vinylation

The Grubbs methylidene RuCl2(PCy3)2(CH2) GIm is immediately decomposed by the small N-heterocyclic carbene (NHC) tetramethylimidazol-2-ylidene (IMe4), even at −80 °C. Labeling experiments with RuCl2(PCy3)2(13CH2) reveal capture of the methylidene ligand as ethylimidazolium chloride [13CH313CH2–IMe4]­Cl, A*. Density functional calculations indicate that IMe4 first abstracts methylidene from five-coordinate GIm (a pathway unavailable to bulkier NHCs), generating the N-heterocyclic olefin (NHO) H2CIMe4. The latter attacks a second GIm; protonolysis of highly basic [Ru]–CH2CH2IMe4 then liberates A. Identical, slightly slower, reactivity is seen for RuCl2(H2IMes)­(PCy3)­(CH2), GIIm. The high efficiency of this process opens up opportunities for tandem metathesis enabled by sterically accessible NHCs and NHOs.