Stéphane Bellemin-Laponnaz

Chiral N-Heterocyclic Carbenes as Stereodirecting Ligands in Asymmetric Catalysis

In recent years, N-heterocyclic carbenes (NHC) have proved to be a versatile class of spectator ligands in homogeneous catalysis. Being robust anchoring functions for late transition metals, their ligand donor capacity and their molecular shape is readily modified by variation of the substituents at the N-atoms and the structure of the cyclic backbone. After the first attempts to use chiral NHC ligands in asymmetric catalysis in the late 1990s, which initially met with limited success, several novel structural concepts have emerged during the past two years which have led literally to an explosion of the field. With a significant number of highly selective chiral catalysts based on chiral NHCs having been reported very recently, several general trends in the design of new NHC-containing molecular catalysts for stereoselective transformations in organic synthesis emerge.

Group 1 and 2 and Early Transition Metal Complexes Bearing N-Heterocyclic Carbene Ligands: Coordination Chemistry, Reactivity, and Applications

N‐Heterocyclic Carbene Ligands: Coordination Chemistry, Reactivity, and Applications Steṕhane Bellemin-Laponnaz*,†,‡ and Samuel Dagorne* †IPCMS (Institut de Physique et Chimie des Mateŕiaux de Strasbourg), CNRS-Universite ́ de Strasbourg, 23 rue du Loess BP 43, F-67034 Strasbourg, France ‡University of Strasbourg Institute for Advanced Study (USIAS), 5 alleé du Geńeŕal Rouvillois, F-67083 Strasbourg, France Institut de Chimie de Strasbourg, CNRS-Universite ́ de Strasbourg, 1 rue Blaise Pascal, F-67000 Strasbourg, France

Metal Silylenes Generated by Double Silicon–Hydrogen Activation: Key Intermediates in the Rhodium-Catalyzed Hydrosilylation of Ketones†

Rhodium silylenes, which are generated by double Si-H activation at the metal, are involved in a low-activation-barrier mechanism of the hydrosilylation of ketones with R(2)SiH(2). A DFT-based study of reaction mechanisms accounts for the experimental observations, notably the rate enhancement for R(2)SiH(2) over R(3)SiH and an inverse kinetic isotope effect.

“Catalysis in a Tea Bag”: Synthesis, Catalytic Performance and Recycling of Dendrimer-Immobilised Bis- and Trisoxazoline Copper Catalysts

Bis- and trisoxazolines (BOX and trisox), containing a linker unit in the ligand backbone that allows their covalent attachment to carbosilane dendrimers, have been employed as polyfunctional ligands for recyclable Cu(II) Lewis acid catalysts that were immobilised in a membrane bag. The oxazolines contained an alkynyl unit attached to their backbone that was deprotonated with LDA or BuLi and then reacted with the chlorosilyl termini of zeroth-, first- and second-generation carbosilane dendrimers in the presence of TlPF(6). The functionalised dendritic systems were subsequently separated from excess ligand by way of dialysis. The general catalytic potential of these systems was assessed by studying two benchmark reactions, the alpha-hydrazination of a beta-keto ester as well as the Henry reaction of 2-nitrobenzaldehyde with nitromethane. For both reactions the bisoxazoline-based catalysts displayed superior selectivity and, in particular, catalyst activity. The latter was interpreted as being due to the hindered decoordination of the third oxazoline unit, the key step in the generation of the active catalyst, in the immobilised trisox-copper complexes. Solutions of the second-generation dendrimer catalysts were placed in membrane bags, fabricated from commercially available dialysis membranes, with the purpose of catalyst recycling based on dialysis. Overall, the supported BOX catalyst gave good and highly reproducible results throughout the study, whereas the performance of the trisox dendrimer system decreased monotonically. The reason for the different behaviour is the markedly lower activity of trisox-based catalysts relative to those based on the BOX ligand. This necessitated an increased reaction time for each cycle of the trisox derivatives, resulting in higher levels of catalyst leaching, which was attributed to a modification of the structure of the membrane by its exposure to the solvent trifluoroethanol at 40 degrees C.

Light-Powered Self-Healable Metallosupramolecular Soft Actuators

Supramolecular functional materials able to respond to external stimuli have several advantages over their classical covalent counterparts. The preparation of soft actuators with the ability to respond to external stimuli in a spatiotemporal fashion, to self-repair, and to show directional motion, is currently one of the most challenging research goals. Herein, we report a series of metallopolymers based on zinc(II)-terpyridine coordination nodes and bearing photoisomerizable diazobenzene units and/or solubilizing luminescent phenylene-ethynylene moieties. These supramolecular polymers act as powerful gelating agents at low critical gelation concentrations. The resulting multiresponsive organogels display light-triggered mechanical actuation and luminescent properties. Furthermore, owing to the presence of dynamic coordinating bonds, they show self-healing abilities.

Synthesis and Coordination of the New Chiral Tridentate O,N,O Ligand 2,6-Bis[(1S,2S,5R)-(−)-menthyl]pyridine to Molybdenum(VI) and Vanadium(V) Oxo Complexes: Crystal Structures of [(2,6-Bis{(−)-menthyl}pyridine)MoO2] and [(2,6-Bis{(−)-menthyl}pyridine)VO]2(μ-O)

The novel tridentate chiral ligand 2,6-bis{(−)-menthyl}pyridine (1), was readily prepared from the reaction of 2,6-dilithiopyridine with (−)-menthone. Reaction of 1 with VO(OiPr)3 and [MoO2(acac)2] resulted in the formation of the new metal-oxo complexes [VO(ONO)]2(μ-O) (2) and [MoO2(ONO)] (3) [ONO = (1− 2 H)]. Both metal-oxo compounds 2 and 3 have demonstrated the ability to catalyze the asymmetric oxidation of prochiral olefins with tBuOOH as the oxidant. The compounds 1−3 have been fully characterized by 1H, 13C and 51V (where appropriate) NMR spectroscopy, mass spectrometry, microanalysis and IR spectroscopy. Furthermore, the molecular structures of 2 and 3 have been determined by single-crystal X-ray diffraction.

Co-ordination of the chiral N,O-ligand 2-[(1S, 2S, 5R)(−)-menthol]-pyridine to molybdenum(VI) and vanadium(IV) oxo complexes: Crystal structures of [MoO2{2-(−)-menthol-pyridine}2] and [VO{2-(−)-menthol-pyridine}2]

Abstract Reaction of the chiral 1-pyridin-2-yl-menthol ligand (1) with [VO(acac)2] and [MoO2(acac)2] results in the formation of the metal–oxo complexes [VO(N–O)2] (2) and [MoO2(N–O)2] (3), where N–O=2-[(−)-menthol]-pyridine. The molecular structures of (2) and (3) have been determined by single-crystal X-ray diffraction, which revealed the expected square-pyramidal geometry for (2) with the pyridine ring nitrogens mutually trans, and the expected slightly distorted octahedral arrangement of (3) with cis di-oxo ligands. Both metal–oxo compounds (2) and (3) have demonstrated the ability to catalyse the asymmetric oxidation of prochiral olefins and sulfides with H2O2 or tBuOOH as the oxidant.

Synthesis and biological assays on cancer cells of dinuclear gold complexes with novel functionalised di(N-heterocyclic carbene) ligands.

New dinuclear di(N-heterocyclic carbene) silver(I), gold(I) and gold(III) complexes have been synthesised and their antiproliferative effects towards various cancer cell lines have been screened. The di(N-heterocyclic carbene) ligands have a propylene linker between the carbene moieties and the imidazole backbone has been functionalised with a 1-benzyl- or 1-PEG-1,2,3-triazole ring (PEG=poly(ethylene glycol)) via a CuAAC (copper azido alkyne cycloaddition) reaction. The resulting gold(I) and gold(III) complexes display an antiproliferative activity superior to that of the unfunctionalised pristine complexes together with a higher selectivity towards cancerous cells with respect to healthy cells.

High tacticity control in organolanthanide polymerization catalysis: formation of isotactic poly(α-alkenes) with a chiral C3-symmetric thulium complex

The thulium complexes [Tm((i)Pr-trisox)(CH(2)SiMe(2)R)(3)] (R = Me , Ph ) were synthesized from the thulium trialkyl precursors [Tm(CH(2)SiMe(2)R)(3)(thf)(2)]; reaction of with two equivalents of [Ph(3)C][B(C(6)F(5))(4)] gave a cationic complex 1c, which was found to polymerize 1-hexene, 1-heptene and 1-octene to give the corresponding polyolefins with moderate to good activities and with minimum isotacticity of 90%, 83% and 95%, respectively.

Isomerization of allylic silyl ethers catalyzed by ReO3(OSiR3) complexes

Abstract Efficient isomerization of allylic alcohols and allylic ethers at room temperature by rhenium (VII) oxo complexes is described.