Loss and Reformation of Ruthenium Alkylidene: Connecting Olefin Metathesis, Catalyst Deactivation, Regeneration, and Isomerization

Ruthenium-based
olefin metathesis catalysts are used in laboratory-scale
organic synthesis across chemistry, largely thanks to their ease of
handling and functional group tolerance. In spite of this robustness,
these catalysts readily decompose, via little-understood pathways,
to species that promote double-bond migration (isomerization) in both
the 1-alkene reagents and the internal-alkene products. We have studied,
using density functional theory (DFT), the reactivity of the Hoveyda–Grubbs
second-generation catalyst 2 with allylbenzene, and discovered
a facile new decomposition pathway. In this pathway, the alkylidene
ligand is lost, via ring expansion of the metallacyclobutane intermediate,
leading to the spin-triplet 12-electron complex (SIMes)­RuCl2 (3R21, SIMes = 1,3-bis­(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene).
DFT calculations predict 3R21 to be a very active alkene isomerization initiator, either operating
as a catalyst itself, via a η3-allyl mechanism, or,
after spin inversion to give R21 and formation of a cyclometalated
Ru-hydride complex, via a hydride mechanism. The calculations also
suggest that the alkylidene-free ruthenium complexes may regenerate
alkylidene via dinuclear ruthenium activation of alkene. The predicted
capacity to initiate isomerization is confirmed in catalytic tests
using p-cymene-stabilized R21 (5), which promotes isomerization in particular under conditions
favoring dissociation of p-cymene and disfavoring
formation of aggregates of 5. The same qualitative trends
in the relative metathesis and isomerization selectivities are observed
in identical tests of 2, indicating that 5 and 2 share the same catalytic cycles for both metathesis
and isomerization, consistent with the calculated reaction network
covering metathesis, alkylidene loss, isomerization, and alkylidene
regeneration.