Operation of the Boomerang Mechanism in Olefin Metathesis Reactions Promoted by the Second-Generation Hoveyda Catalyst

Abstract

A long-standing question in olefin metathesis centers on whether the "release−return" (boomerang) mechanism contributes to the productivity of Hoveyda-class catalysts. According to this mechanism, a molecule of oisopropoxystyrene (A) is liberated during catalyst initiation, but recaptures the active catalyst following metathesis. The relevance of this pathway for the second-generation Hoveyda catalyst HII was assessed in metathesis of 1,1-and 1,2disubstituted olefins. Crossover studies with 13 C-labeled A*, as well as competition experiments involving ring-closing or cross metathesis (RCM, CM) in the presence of A (equimolar with HII) indicated rapid reuptake of styrenyl ether. The crossover studies indicated highly efficient catalyst initiation, with the entire catalyst charge being activated before metathesis was complete. In a comparative study involving CM of anethole with methyl acrylate, sustained activity was shown for HII, whereas the secondgeneration Grubbs catalyst GII was rapidly deactivated. These data demonstrate that the release−return mechanism is indeed operative for HII in these demanding metathesis reactions, and that facile shuttling from a protected recapture cycle into the productive metathesis cycle contributes to the superior performance of HII relative to GII.

Molecular and Silica‐Supported Mo and W d0 Imido‐Methoxybenzylidene Complexes: Structure and Metathesis Activity

“…of cis ‐4‐nonene in C 6 D 6 , no new alkylidene signal could be observed in 1 H‐NMR while the olefin was fully metathesized (after decoordination of the methoxy group the newly formed alkylidene should have usual syn configuration and thus should be clearly distinguishable from the parent anti methoxybenzylidene). These observations imply that either only a very small fraction of the complex is activated and responsible for catalysis (consistent with the increased stability of the alkylidene moiety in methoxybenzylidene complexes) or that the liberated (and presumably strongly adsorbed on silica) o‐ methoxystyrene initiation product can be recaptured after the removal of the substrate analogously to what is proposed for Ru Hoveyda−Grubbs catalysts (‘boomerang’ mechanism) …”

Continuous Nanofiltration and Recycle of a Metathesis Catalyst in a Microflow System

“…Although still debated in the literature, strong evidence from Grela et al 59. and more recently Bates et al 60. suggests that, under our conditions, the (non‐deactivated) catalyst in our postreaction mixture in the nanofiltration cell is likely to consist of 4 as the catalyst resting state.…”

Metal-catalyzed organic reactions using mechanochemistry

“…3% conversion into the expected dihydropyrrole. 62 The development of an efficient mechanochemical methodology to conduct olefin metathesis reactions was very recently reported by Do and co-workers, 63 using ruthenium-based 64 Hoveyda-Grubbs catalysts. The methodology, that is applicable to liquid and solid olefins, was developed for conducting cross metathesis and ring-closing metathesis reactions, affording the expected products in >90% isolated yield.…”

“…It is, therefore, clear that the solvent-intense methodologies for product isolation by recrystallization and chromatography remain highly popular, effectively diminishing the environmentally-friendly value of a catalytic, solvent-free reaction. Such state of affairs may be gradually changing, based on recent reports 62, 64,66 in which product purification is achieved by washing with water and/or minimum amounts of organic solvent. Sublimation may hold particular promise in that context, and was recently investigated as a completely solvent-free method for product isolation in mechanochemical organic and organometallic reactions.…”