Représentations semi-stables et modules fortement divisibles

Sans résumé Oblatum 1-IV-1998 &9-VI-1998 / Published online: 14 January 1999     

Sampling from Non-smooth Distributions Through Langevin Diffusion

Methodology and Computing in Applied Probability - In this paper, we propose proximal splitting-type algorithms for sampling from distributions whose densities are not necessarily smooth nor...     

A Stereocontrolled Total Synthesis of Lipoxin B4 and its Biological Activity as a Pro-Resolving Lipid Mediator of Neuroinflammation

Two stereocontrolled, efficient, and modular syntheses of eicosanoid lipoxin B4 (LXB₄) are reported. One features a stereoselective reduction followed by an asymmetric epoxidation sequence to set the vicinal diol stereocentres. The dienyne was installed via a one-pot Wittig olefination and base-mediated epoxide ring opening cascade. The other approach installed the diol through an asymmetric dihydroxylation reaction followed by a Horner-Wadsworth-Emmons olefination to afford the common dienyne intermediate. Finally, a Sonogashira coupling and an alkyne hydrosilylation/proto-desilylation protocol furnished LXB₄ in 25 % overall yield in just 10 steps. For the first time, LXB₄ has been fully characterized spectroscopically with its structure confirmed as previously reported. We have demonstrated that the synthesized LXB₄ showed similar biological activity to commercial sources in a cellular neuroprotection model. This synthetic route can be employed to synthesize large quantities of LXB₄, enable synthesis of new analogs, and chemical probes for receptor and pathway characterization.     

Exploring the Limits of π-Acid Catalysis Using Strongly Electrophilic Main Group Metal Complexes: The Case of Zinc and Aluminium

The catalytic activity of cationic NHC-Zn^II and NHC-Al^III (NHC=N-heterocyclic carbene) complexes in reactions that require the electrophilic activation of soft C−C π bonds has been studied. The former proved able to act as a soft π-Lewis acid in a variety of transformations. The benefit of the bulky IPr NHC ligand was demonstrated by comparison with simple ZnX₂ salts. The tested NHC-Al^III catalyst is not able to activate C−C π bonds but simple AlX₂⁺ ions were found potent in some cases.     

Phosphorescent Cationic Heterodinuclear IrIII/MI Complexes (M=CuI,AuI) with a Hybrid Janus-Type N-Heterocyclic Carbene Bridge

A novel class of phosphorescent cationic heterobimetallic Ir^III/M^I complexes, where M^I=Cu^I ( 4 ) and Au^I ( 5 ), is reported. The two metal centers are connected by the hybrid bridging 1,3-dimesityl-5-acetylimidazol-2-ylidene-4-olate (IMesAcac) ligand that combines both a chelating acetylacetonato-like and a monodentate N-heterocyclic carbene site coordinated onto an Ir^III and a M^I center, respectively. Complexes 4 and 5 have been prepared straightforwardly by a stepwise site-selective metalation with the zwitterionic [(IPr)M^I(IMesAcac)] metalloproligand (IPr=1,3-(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene) and they have been fully characterized by spectroscopic, electrochemical, and computational investigation. Complexes 4 and 5 display intense red emission arising from a low-energy excited state that is located onto the “Ir(C^N)” moiety featuring an admixed triplet ligand-centered/metal-to-ligand charge transfer (³IL/¹MLCT) character. Comparison with the benchmark mononuclear complexes reveals negligible electronic coupling between the two distal metal centers at the electronic ground state. The bimetallic systems display enhanced photophysical properties in comparison with the parental congeners. Noteworthy, similar non-radiative rate constants have been determined along with a two-fold increase of radiative rate, yielding brightly red-emitting cyclometalating Ir^III complexes. This finding is ascribed to the increased MLCT character of the emitting state in complexes 4 and 5 due to the smaller energy gap between the ³IL and ¹MLCT manifolds, which mix via spin–orbit coupling.     

Ion mobility spectrometry – Mass spectrometry coupling for synthetic polymers

This review covers applications of ion mobility spectrometry (IMS) hyphenated to mass spectrometry (MS) in the field of synthetic polymers. MS has become an essential technique in polymer science, but increasingly complex samples produced to provide desirable macroscopic properties of high‐performance materials often require separation of species prior to their mass analysis. Similar to liquid chromatography, the IMS dimension introduces shape selectivity but enables separation at a much faster rate (milliseconds vs minutes). As a post‐ionization technique, IMS can be hyphenated to MS to perform a double separation dimension of gas‐phase ions, first as a function on their mobility (determined by their charge state and collision cross section, CCS), then as a function of their m/z ratio. Implemented with a variety of ionization techniques, such coupling permits the spectral complexity to be reduced, to enhance the dynamic range of detection, or to achieve separation of isobaric ions prior to their activation in MS/MS experiments. Coupling IMS to MS also provides valuable information regarding the 3D structure of polymer ions in the gas phase and regarding how to address the question of how charges are distributed within the structure. Moreover, the ability of IMS to separate multiply charged species generated by electrospray ionization yields typical IMS‐MS 2D maps that permit the conformational dynamics of synthetic polymer chains to be described as a function of their length.     

Silica-Grafted Tris(neopentyl)aluminum: A Monomeric Aluminum Solid Co-catalyst for Efficient Nickel-Catalyzed Ethene Dimerization

A silica-supported monomeric alkylaluminum co-catalyst was prepared via surface organometallic chemistry by contacting tris(neopentyl)aluminum and partially dehydroxylated silica. This system, fully characterized by solid-state ²⁷Al NMR spectroscopy augmented by computational studies, efficiently activates (ⁿBu₃P)₂NiCl₂ towards dimerization of ethene, demonstrating comparable activity to previously reported dimeric diethylaluminum chloride supported on silica. Three types of aluminum surface species have been identified: monografted tetracoordinated Al species as well as two types of bisgrafted Al species—tetra- and pentacoordinated. Of them, only the monografted Al species is proposed to be able to activate the (ⁿBu₃P)₂NiCl₂ complex and generate the active cationic species.