Ruthenium metathesis precatalysts with unsymmetrical Nheterocyclic carbene (NHC) ligands

2016 - 2017Olefin metathesis is one of the most important chemical transformations for the formation of carbon-carbon double bonds. The possibility to build up highly funtionalised alkenes starting from simple olefins makes this reaction indispensable in modern organic synthesis, giving access to a wide range of molecules that would be barely obtained through other synthetic routes. The success of metathesis is due to the development of new and efficient catalysts which can be used in a wide variety of research fields, both in industry and in academia. In this context, the research of the ‘perfect’ metathesis complex still impassions scientists all over the word, and several research papers regarding the development of new catalytic systems are published every year. The group I am part of focuses its attention on the development of new ruthenium metathesis catalysts. Our interest lies in the influence that nature and configuration of substituents on the N-heterocyclic carbene (NHC) ligand could have on the performances of the corresponding metal complexes. In this doctoral thesis, the field of unsymmetrical N-heterocyclic carbene (u-NHC) ruthenium catalysts will be explored. Synthesis and characterisation of several novel complexes will be discussed. Catalytic performances will be evaluated in model metathesis reactions as well as in more attractive metathesis transformations. The relationship between NHC structure and complexes’ behaviours will be investigated using NMR, X-Ray, IR, cyclic voltammetry and DFT calculations. ..[edited by Author]XXX cicl

NHC Ligand Effects on Ru-Catalyzed Cross-Metathesis of Renewable Materials

As petrochemical resources become increasingly scarce and expensive, much attention has been focused on renewable resources from biomass as alternative options for producing basic building blocks for chemical manufacturing. Catalytic olefin metathesis represents a powerful tool to transform biosourced structural motifs in valuable commodity, fine, and specialty chemicals. In that respect, the appropriate choice of the catalyst is the key issue of each metathesis transformation. The current study examines the influence of different N-heterocyclic carbene (NHC) ligands containing one or two N-alkyl substituents on the efficiency of Hoveyda–Grubbs-type catalysts in the cross-metathesis reaction of ethyl oleate with cis-1,4-diacetoxy-2-butene and cross-metathesis of eugenol acetate with cis-1,4-dichloro-2-butene. Interestingly, the introduction of alkyl N-substituents in the NHC ligand was revealed as beneficial for catalytic performances in the examined cross-metathesis (CM) reactions, leading to higher activity and/or selectivity than those observed in the presence of the classical, commercially available Hoveyda–Grubbs second generation catalyst (HGII)

Synthesis and Ring-Opening Metathesis Polymerization of a New Norbornene Dicarboximide with a Pendant Carbazole Moiety

A new norbornene dicarboximide presenting a pendant carbazole moiety linked by a p-methylene benzyl spacer is synthesized. This carbazole-functionalized monomer is polymerized via ring-opening metathesis polymerization using Grubbs third-generation catalyst. Microstructural analysis of resulting polymers performed by Nuclear Magnetic Resonance (NMR) shows that they are stereoirregular. Wide-angle X-ray diffraction (WAXD) and thermal (DSC) analysis indicate that polymers are also amorphous. With respect to the fluorescence analysis, both solution and film polymer samples exhibit only "normal structured" carbazole fluorescence, while excimer formation by overlap of carbazole groups is not detected

Toward More Sustainable Elastomers: Stereoselective Copolymerization of Linear Terpenes with Butadiene

The copolymerization of renewable monomers such as ocimene (O), myrcene (M), and farnesene (F) with butadiene (B), promoted by dichloro{1,4-dithiabutanediyl-2,2′-bis[4,6-bis(2-phenyl-2-propyl)phenoxy]}titanium (1) activated by modified methylalumoxane (m-MAO) under mild reaction conditions, was investigated. Copolymers in a wide range of compositions were obtained through a judicious control of the alimentation feed (up to 85% of terpene incorporated in the case of poly(ocimene-butadiene) (POB)). Analysis of POB, poly(myrcene-butadiene) (PMB), and poly(farnesene-butadiene) (PFB) microstructures revealed the good stereoselectivity of 1, both in the butadiene (up to 95%) and in the terpene (up to 92%, 71%, and 86% for O, M, and F, respectively) insertion. For all these new materials, a complete 13C NMR assignment was performed, revealing a multiblock structure. A sample of POB was also evaluated as a component in a model tread compound leading to improved mechanical properties with respect to the corresponding plain butadiene rubbers

Streptococcus suis-associated neonatal meningitis and sepsis: characterization, antimicrobial resistance, and public health implications

Neonatal meningitis and sepsis were diagnosed in a 32-day-old preterm-born infant. Streptococcus (S.) suis was isolated from cerebrospinal fluid and blood. Next-generation sequencing revealed that the strain was serotype 2 sequence type 1, and contained the ermB and tet(W) genes, which are responsible for resistance to macrolides and tetracycline, along with several pilus-associated genes and 20 virulence factors. High homology was observed with previously identified human and swine strains in the same area. S. suis meningitis and sepsis are mainly reported in adults, related to direct contact with pigs or contaminated pork meat consumption, while it is rarely reported in children. Herein, we describe the first case of S. suis in a newborn associated with meningitis and antimicrobial resistance. The rates of resistance to tetracyclines, lincosamides, and macrolides for this bacterium are increasing and are creating concern worldwide. Altogether, our findings highlight the importance of investigating S. suis in cases of neonatal meningitis, as well as the necessity of assessing the antimicrobial profile to obtain useful information for developing targeted therapies

Enantiopure C1-symmetric N-Heterocyclic Carbene Ligands from Desymmetrized meso-1,2-Diphenylethylenediamine: Application in Ruthenium-Catalyzed Olefin Metathesis

In order to design improved chiral ruthenium catalysts for asymmetric olefin metathesis, enantiomeric catalysts incorporating C1-symmetric N-Heterocyclic carbenes (NHC) ligands with syn-related substituents on the backbone were synthesized starting from meso-1,2-diphenylethylenediamine. The absolute configuration of the enantiomers of the desymmetrized meso diamine was assigned by optical rotation analysis and in silico calculations, and was found to be maintained in their respective ruthenium catalysts by comparison of the relative electronic circular dichroism (ECD) spectra. The catalytic behaviour of the enantiomeric ruthenium complexes was investigated in model asymmetric metathesis transformations and compared to that of analogous complexes bearing C1-symmetric NHC ligands with an anti backbone. Modest enantioselectivities were registered and different catalyst properties depending on the nature of stereochemical relationship of substituents on the backbone were observed

Metal Complexes Bearing Sulfur-Containing Ligands as Catalysts in the Reaction of CO2 with Epoxides

Coupling of CO2 with epoxides is a green emerging alternative for the synthesis of cyclic organic carbonates (COC) and aliphatic polycarbonates (APC). The scope of this work is to provide a comprehensive overview of metal complexes having sulfur-containing ligands as homogeneous catalytic systems able to efficiently promote this transformation with a concise discussion of the most significant results. The crucial role of sulfur as the hemilabile ligand and its influence on the catalytic activity are highlighted as well

Novel Olefin Metathesis Ruthenium Catalysts Bearing Backbone- Substituted Unsymmetrical NHC Ligands

Stable Ru-based catalysts containing unsym- metrical N-heterocyclic carbene (NHC) ligands with phenyl substituents on the backbone in syn and anti stereochemical relationships have been easily prepared and fully characterized. Preliminary investigation revealed that, depending on the backbone configuration, the new Ru complexes displayed different catalytic behaviors in representative olefin metathesis reactions