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

Abstract

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.

Towards Sustainable Catalysis – Highly Efficient Olefin Metathesis in Protic Media Using Phase Labelled Cyclic Alkyl Amino Carbene (CAAC) Ruthenium Catalysts

“…Furthermore, no detectable degradation was observed by monitoring a solution of 12 in non‐degassed acetone containing 20 equivalents of water at RT in the presence of an internal standard with kinetic 1 H NMR for 10 hours. This together with the obtained catalytic robustness in MeOH below indicate that catalysts 10 and 12 are not particularly sensitive for degradation, neither by Ru−H nor by dimer formation …”

“…This together with the obtained catalytic robustness in MeOH below indicate that catalysts 10 and 12 are not particularly sensitive for degradation, neither by RuÀ H nor by dimer formation. [41] The catalytic activity of the complexes 10-12 in protic solvents including MeOH, MeOH/water mixture and isopropanol (IPA) or in neat (Scheme 4, Table 1) were further investigated (9 was not tested as it was insoluble in MeOH at RT). The RCM reaction of 13 with 0.05 mol % catalyst loading revealed 29 % yield at RT using catalyst 10.…”

Specialized Olefin Metathesis Catalysts Featuring Unsymmetrical N-Heterocyclic Carbene Ligands Bearing N-(Fluoren-9-yl) Arm

“…Finally, it shall be stressed that in the above self-CM reaction the general purpose catalysts, such as Ru2 and Ru5, were reported to give very low selectivity, sometimes as low as 20% [24], probably due to extensive catalysts decomposition forming Ru-hydride species [56,57], dimers [29], and possibly other yet undefined species.…”

Acrylate Esters by Ethenolysis of Maleate Esters with Ru Metathesis Catalysts: an HTE and a Technoeconomic Study

“…Achieving high productivity in ethenolysis can be hampered by deactivation of intermediate Ru methylidene species, [Ru=CH 2 ], by a bimolecular coupling or methylidene abstraction, both processes being facilitated by the small size and the high reactivity of the methylidene ligand . In addition, a β ‐hydrogen elimination pathway contributes significantly to decomposition of unsubstituted ruthenacyclobutane intermediates . Ethenolysis of functionalized olefins such as esters is further exacerbated by the presence of functional groups and impurities in ester‐containing feeds .…”