Origin of the Breakthrough Productivity of Ruthenium–Cyclic Alkyl Amino Carbene Catalysts in Olefin Metathesis

Examined herein is the basis for the outstanding metathesis productivity of leading cyclic alkyl amino carbene (CAAC) catalysts relative to their important N-heterocyclic carbene (NHC) predecessors, as recently demonstrated in the topical contexts of metathesis macrocyclization and the ethenolysis of renewable oils. The difference is traced to the stability to decomposition of the metallacyclobutane (MCB) intermediate. The CAAC catalysts are shown to undergo little to no β-H elimination of the MCB ring, a pathway to which the H2IMes catalysts are highly susceptible. Unexpectedly, however, the CAAC catalysts are found to be more susceptible to bimolecular coupling of the key intermediate RuCl2(CAAC)(═CH2), a reaction that culminates in elimination of the methylidene ligand as ethylene. Thus, an NMR study of transiently stabilized RuCl2(L)(py)(═CH2) complexes (L = CAAC or H2IMes) revealed bimolecular decomposition of the CAAC derivative within 5 min at RT, as compared to a time scale of hours for the H2IMes analogue. The remarkable productivity of the CAAC catalysts is thus due to their resistance to β-elimination, which enables their use at part per million loadings, and to the retarding effect of these low catalyst concentrations on bimolecular decomposition.