Abstract
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.
Highlights
Olefin metathesis is the most versatile tool known for the formation of carbon−carbon double bonds.[1]
The ruthenium-based catalysts, such as the Grubbs second generation catalyst 1 (Chart 1)[2] and its phosphine-free congener known as the Hoveyda−Grubbs second generation catalyst 2,3 have become widely used in organic synthesis[1] and are to an increasing extent being adopted in industrial
Using allylbenzene as a model 1-alkene substrate in density functional theory (DFT) explorations of decomposition mechanisms, we discovered a surprisingly facile, stepwise 1,2-shift leading to a breakdown of the metallacyclobutane and loss of methylidene analogous to that triggered by ethylene
Summary
Olefin metathesis is the most versatile tool known for the formation of carbon−carbon double bonds.[1]. Deuterium from 3 was observed in the isomerization products, suggesting that an unknown, active Ru-D species is formed via C−D activation of the CD3 groups.[18] Activation of N-heterocyclic carbene (NHC) aryl C−H bonds of ruthenium olefin metathesis catalysts is a well-known catalyst deactivation reaction,[19,20] but so far, no such C−H activation products, hydride or other, with appreciable isomerization activity have been identified. Identifying such isomerization-active decomposition products and establishing a decomposition−isomerization reaction sequence consistent with the above three guidelines was the goal of this work. The isomerization activity of the alkylidene-free complex was confirmed by synthesizing and testing a donor-stabilized version of this compound
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