Nele Ledoux

Improved ruthenium catalysts for the modified Friedlaender quinoline synthesis

Herein we describe an improved ruthenium catalyst for the oxidative cyclization of 2-aminobenzylalcohol with ketones, leading to quinolines via a modified Friedlaender synthesis.

Rational design and convenient synthesis of a novel family of ruthenium complexes with O,N-bidentate ligands

A two step procedure for the synthesis of a novel family of homogeneous and immobilized Ru-complexes containing Schiff bases as O,N-bidentate ligands is described. The new Ru-complexes have been structurally characterized by IR, Raman,1H-,13C-NMR spectroscopy. The Schiff bases were associated with a diversity of inorganic and organic ligands such as chloride, phosphane, arenes, various carbenes (alkylidene, vinylidene, indenylidene and allenylidene as well as N-heterocyclic carbenes) and cyclodienes. By choosing a selective range of substituents for the Schiff base, useful physical and chemical properties of the prepared Rucomplexes can be induced. This synthetic approach is promising in creating a valuable and diverse selection of Ru-complexes, valuable for future applications.

Exploring new synthetic strategies in the development of a chemically activated Ru-based olefin metathesis catalyst

The objective of this work was to develop an industrially relevant olefin metathesis initiator, which circumvents the expensive, patent protected, often cumbersome preparative routes via Grubbs benzylidene complexes. Upon coordination of a Schiff base ligand to a second-generation ruthenium allenylidene complex, the formation of three catalyst isomers was observed. The major isomer was successfully isolated, and tested in a few olefin metathesis reactions. Acids such as HCl and HSiCl(3) were found to boost the metathesis reaction but the in situ formation of a neutral Ru carbyne complex restricted the catalytic capacity. Using the Lewis acid PhSiCl(3), the formation of a carbyne species was avoided, and turnover numbers up to 30,000 were reached in the ring-opening metathesis polymerisation of cycloocta-1,5-diene.

New N-Heterocyclic Carbene Ligands in Grubbs and Hoveyda–Grubbs Catalysts

A series of N-heterocyclic carbene (NHC) ligands bearing aliphatic amino side groups were synthesized and reacted with the Grubbs first generation catalyst. Reactions involving symmetrical, aliphatic NHCs did not allow the isolation of any pure NHC substituted complexes due to their instability. Unsymmetrical NHCs having a planar mesityl group on one amino side reacted with Grubbs catalyst in a favorable manner, and the resulting complexes were stable enough to be isolated. X-ray crystallographic analysis demonstrated that the mesityl group is co-planar with the phenyl ring of the benzylidene, which indicates that a π–π interaction between the mesityl arm and the benzylidene moiety might constitute an important structural element. Catalysts substituted with an NHC derived from a primary or secondary amino-group were found to surpass the parent-complex for the ROMP of cycloocta-1,5-diene. The catalyst substituted with an NHC derived from tBu-NH2 was considerably less metathesis active. Also new N-alkyl-N′-(2,6-diisopropylphenyl) heterocyclic carbenes were synthesized. These NHC ligands revealed a different reactivity towards Grubbs complexes than the hitherto reported imidazolinylidenes: (i) facile bis(NHC) coordination was found, and (ii) both NHCs on the bis(NHC) complexes can be exchanged with a phosphine, thereupon regenerating the Grubbs first generation complex. Furthermore, a comparison between the classical Hoveyda–Grubbs complexes and complexes substituted with N-alkyl-N′-(aryl) heterocyclic carbenes demonstrates that the introduction of one aliphatic group into the NHC framework does not improve the catalytic activity in any of the tested metathesis reactions. The introduction of two aliphatic amino side groups enhances the reactivity in the ROMP reaction while the increase of steric interactions lowers the CM activity. The lower activity of the N-alkyl-N′-(2,6-diisopropylphenyl) heterocyclic carbene complexes compared with the N-alkyl-N′-mesityl heterocyclic carbene complexes, may analogously be attributed to a more demanding steric environment. While small differences in donor capacities might cause a significantly different catalytic behavior, it is thus plausible that subtle steric differences exert a more determining influence on the activity of the catalysts. In addition, the obtained results confirm that the NHCs amino side groups play a pivotal role in determining the reactivity, selectivity as well as the stability of the corresponding catalysts.

Olefin Metathesis Mediated By: - Schiff Base Ru-Alkylidenes -Ru-Alkylidenes Bearing Unsymmetrical NH Ligands

The classic Grubbs second-generation complex 2 was modified through 1. The introduction of a bidentate Schiff base ligand 2. Changes in the amino side groups of the NHC ligand Representative olefin metathesis test reactions show the effects induced by the ligand modifications and demonstrate some interesting new properties of the described catalysts. catalysts.