Highly Selective Ruthenium Metathesis Catalysts for Ethenolysis

Abstract

N-aryl, N-alkyl N-heterocyclic carbene (NHC) ruthenium metathesis catalysts are highly selective toward the ethenolysis of methyl oleate, giving selectivity as high as 95% for the kinetic, ethenolysis products over the thermodynamic, self-metathesis products. The examples described herein represent some of the most selective NHC-based ruthenium catalysts for ethenolysis reactions to date. Furthermore, many of these catalysts show unusual preference and stability toward propagating as a methylidene species, and provide good yields and turnover numbers (TONs) at relatively low catalyst loading (<500 ppm). A catalyst comparison showed that ruthenium complexes bearing sterically hindered NHC substituents afforded greater selectivity and stability, and exhibited longer catalyst lifetime during reactions. Comparative analysis of the catalyst preference for kinetic versus thermodynamic product formation was achieved via evaluation of their steady-state conversion in the cross-metathesis reaction of terminal olefins. These results coincided with the observed ethenolysis selectivities, in which the more selective catalysts reach a steady-state characterized by lower conversion to cross-metathesis products compared to less selective catalysts, which show higher conversion to cross-metathesis products.

Valorisation of vegetable oils via metathesis reactions on solid catalysts: Cross-metathesis of methyl oleate with 1-hexene

“…However, the C6 conversion on HG(0.87%)/SiO 2 remained almost constant (X C6  3%)after 90 min on stream, thereby suggesting a rapid in situ catalyst deactivation.The deactivation of HGbased catalysts in presence of terminal olefins has also been observed by other authors 11 and was attributed to the low stability of methylidene intermediatesleading to hydride species that suppress the metathesis cycle [23,24].…”

“…Most of the work on cross-metathesis of oil-derived unsaturated esters with simple alkenes has been carried out using homogeneous ruthenium-based catalysts. In particular, the metathesis of methyl oleate (MO) with ethylene (ethenolysis) and with 2-butene were investigated on Ru complexes [10,11,12]while the cross-metathesis of MO with 1-hexene was studied on homogeneousWCl 6 /(CH 3 ) 4 Sn catalysts [13]. [14,15],but there are no reportson the use of immobilized Ru complexes for promoting the cross-metathesis of MO with olefins.…”

Exploring the reactivity of a ruthenium complex in the metathesis of biorenewable feedstocks to generate value-added chemicals

“…Our final class of reaction for study was the cross-metathesis of methyl oleate and ethene to yield methyl 9-decenoate (4) and 1-decene (5) [39]. They exhibit high selectivity and catalyst loadings as low as 500 ppm can be used.…”

New catalysts with unsymmetrical N-heterocyclic carbene ligands

“…As an example, Grubbs and coworkers investigated the Ru-based olefin metathesis precatalyst 1, showing high chemoselectivity towards the ethenolysis of methyl oleate (Scheme 1). 5 At catalyst loadings of only 0.05 mol% and 10 bar ethylene, terminal alkenes as the desired products can be obtained in up to 95% selectivity. This represents a valuable method to convert fatty acids, as derived from renewable biomass, to synthetically useful products on a large scale.…”