Comparative Investigation of Ruthenium-Based Metathesis Catalysts Bearing N-Heterocyclic Carbene (NHC) Ligands

Abstract

Exchange of one PCy3 unit of the classical Grubbs catalyst 1 by N-heterocyclic carbene (NHC) ligands leads to "second-generation" metathesis catalysts of superior reactivity and increased stability. Several complexes of this type have been prepared and fully characterized, six of them by X-ray crystallography. These include the unique chelate complexes 13 and 14 in which the NHC- and the Ru-CR entities are tethered to form a metallacycle. A particularly favorable design feature is that the reactivity of such catalysts can be easily adjusted by changing the electronic and steric properties of the NHC ligands. The catalytic activity also strongly depends on the solvent used; NMR investigations provide a tentative explanation of this effect. Applications of the "second-generation" catalysts to ring closing alkene metathesis and intramolecular enyne cycloisomerization reactions provide insights into their catalytic performance. From these comparative studies it is deduced that no single catalyst is optimal for different types of applications. The search for the most reactive catalyst for a specific transformation is facilitated by IR thermography allowing a rapid and semi-quantitative ranking among a given set of catalysts.

Improving Grubbs’ II type ruthenium catalysts by appropriately modifying the N-heterocyclic carbene ligand

“…10 Appropriate workup gave complexes 2a-e in good yield. The syn/anti ratio of the free NHCs is maintained during synthesis of the complexes, which are therefore present as 75 isomeric mixtures. Characterization of the new compounds includes crystal structure analyses and the ellipsoid drawings of syn-2b and anti-2a, anti-2d and anti-2e are illustrated in Figure 1.…”

Experimental and numerical analysis of aramid fiber laminates with DCPD resin matrix subjected to impact tests

“…For laminate production 12 layers of fabric were used, where the sample had a thickness of 4.4 mm and a weight of 224 g. Modified dicyclopentadiene (DCPD) was used as the matrix resin, which is a by-product secreted from processing crude oil. The polymerization process was carried out in the presence of a second generation Grubbs catalyst [13].…”

NHC Backbone Configuration in Ruthenium-Catalyzed Olefin Metathesis

“…An important role on the catalytic activity is also played by the bulkiness of N-aryl substituents, as generally reported for Ru complex, even with backbone unsubstituted NHCs [41][42][43][44][45]. Phosphine-containing Ru catalysts bearing N-o-isopropylphenyl groups in place of N-o-tolyl (8-synGII and 8-antiGII, Figure 6) can be prepared in good yields (60%-65%) with the same procedures adopted for 7-synGII and 7-antiGII [27].…”