Chiral Atropisomeric-NHC Catechodithiolate Ruthenium Complexes for Z-Selective Asymmetric Ring-Opening Cross Metathesis of Exo-Norbornenes.

A set of 16 chiral ruthenium complexes containing atropisomerically stable N-Heterocyclic Carbene (NHC) ligands was synthesized from prochiral NHC precursors. After a rapid screening in asymmetric ring-opening-cross metathesis (AROCM), the most effective chiral atrop BIAN-NHC Ru-catalyst (up to 97 : 3 er) was then converted to a Z-selective catechodithiolate complex. The latter proved to be highly efficient in Z-selective AROCM of exo-norbornenes affording valuable trans-cyclopentanes with excellent Z-selectivity (>98 %) and high enantioselectivity (up to 96.5 : 3.5 er).

Transition metal complexes bearing atropisomeric saturated NHC ligands.

From achiral imidazolinium salts, chiral transition metal complexes containing an N-heterocyclic carbene (NHC) ligand were prepared (metal = palladium, copper, silver, gold, rhodium). Axial chirality in these complexes results from the formation of the metal-carbene bond leading to the restriction of rotation of dissymmetric N-aryl substituents about the C-N bond. When these complexes exhibited a sufficient configurational stability, a resolution by chiral high-performance liquid chromatography (HPLC) on preparative scale enabled isolation of enantiomers with excellent enantiopurities (>99% ee) and good yields. A study of the enantiomerization barriers revealed the effect of the backbone nature as well as the type of transition metal on its values. Nevertheless, the evaluation of palladium-based complexes in asymmetric intramolecular α-arylation of amides demonstrated that the ability to induce an enantioselectivity cannot be correlated to the configurational stability of the precatalysts.

From Prochiral N-Heterocyclic Carbenes to Optically Pure Metal Complexes: New Opportunities in Asymmetric Catalysis.

Well-defined optically pure transition metal (TM) complexes bearing C1- and C2-symmetric N-heterocyclic carbene (NHC) ligands were prepared from prochiral NHC precursors. As predicted by DFT calculations, our strategy capitalizes on the formation of a metal-carbene bond which induces an axis of chirality. Configurationally stable atropisomers of various NHC-containing TM complexes were isolated by preparative HPLC on a chiral stationary phase in good yields and excellent optical purities (up to 99.5% ee). The carbene transfer from an optically pure Cu complex to a gold or palladium center reveals, for the first time, a full stereoretentivity, supporting the hypothesis of an associative mechanism for the transmetalation. The potential of these new chiral TM complexes was illustrated in asymmetric catalysis with up to 98% ee.

Exploring Phosphine Electronic Effects on Molybdenum Complexes: A Combined Photoelectron Spectroscopy and Energy Decomposition Analysis Study.

In organometallic chemistry, especially in the catalysis area, accessing the finest tuning of a catalytic reaction pathway requires a detailed knowledge of the steric and electronic influences of the ligands bound to the metal center. Usually, the M-L bond between a ligand and metal is depicted by the Dewar-Chatt-Duncanson model involving two opposite interactions, σ-donor and π-acceptor effects of the ligand. The experimental evaluation of these effects is essential and complementary to in-depth theoretical approaches that are able to provide a detailed description of the M-L bond. In this work, we present a study of LMo(CO)5 complexes with L being various tertiary phosphine ligands by means of mass-selected high-resolution photoelectron spectroscopy (PES) performed with synchrotron radiation, DFT, and energy decomposition analyses (EDA) combined with the natural orbitals for chemical valence (NOCV) analysis. These methods enable a separated access of the σ-donor and π-acceptor effects of ligands by probing either the electronic configuration of the complex (PES) or the interaction of the ligand with the metal (EDA). Three series of PR3 ligands with various electronic influences are investigated: the strong donating alkyl substituents (PMe3, PEt3, and PiPr3), the intermediate PPhxMe(3-x) (x = 0-3) set, and the PPhxPyrl(3-x) set (x = 0-3 with Pyrl being the strong electron withdrawing pyrrolyl group C4H4N). For each complex, their adiabatic and vertical ionization energies (IEs) could be determined with a 0.03 eV precision. Experiment and theory show an excellent agreement, either for the IE determination or electronic effect analysis. The ability to interpret the spectra is shown to depend on the character of the ligand. "Innocent" ligands provide the spectra that are the most straightforward to analyze, whereas the "non-innocent" ligands (which are ionized prior to the metal center) render the analysis more difficult due to an increased number of molecular orbitals in the energy range considered. A very good linear correlation is finally found between the measured adiabatic ionization energies and the interaction energy term obtained by EDA for each of these two types of ligands, which opens interesting perspective for the prediction of ligand characters.

[2+1] Cycloaddition Affording Methylene- and Vinylidenecyclopropane Derivatives: A Journey around the Reactivity of Metal-Phosphinito-Phosphinous Acid Complexes.

Metal-phosphinito-phosphinous acid complexes are interesting catalysts exhibiting unique reactivities. In this account, we intend to provide a clear overview of palladium- and platinum-phosphinito-phosphinous acid complexes, their preparation from secondary phosphine oxides, and their applications in catalysis. They have been mainly used to develop [2+1] cycloadditions to afford methylenecyclopropane derivatives using norbornenes and various alkynes as partners. As a function of the catalyst, the reaction conditions, or the nature of the reagents, different synthetic transformations have been observed: [2+1] cycloadditions, giving rise to either alkylidenecyclopropanes or vinylidenecyclopropanes; tandem [2+1]/[3+2] cycloadditions, and so forth. The mechanisms of these reactions have been studied to rationalize the different reactivities observed.

Preparation of Vinylcyclopropanes by Sodium Mediated Reductive Isomerization of Methylenecyclopropanes.

We disclosed therein a new reaction of reductive isomerization of methylenecyclopropanes (MCPs) to vinylcyclopropanes (VCPs). On treatment with sodium metal in liquid ammonia, MCPs bearing a C-O bond at allylic position undergo both a reductive cleavage of the C-O bond and an isomerization of the C-C double bond giving rise to VCPs. The scope of the reductive isomerization was investigated and showed a broad applicability since various functional groups are tolerated. MCP substrates were straightforwardly prepared by a palladium-promoted [2 + 1] cycloaddition between norbornene derivatives with alkynes.

C 2-Symmetric atropisomeric N-heterocyclic carbene-palladium(II) complexes: synthesis, chiral resolution, and application in the enantioselective α-arylation of amides.

The concept of atropisomeric N-heterocyclic carbene (NHC)-metal complexes was extended to NHCs possessing a C2-symmetry and implemented to prepare palladium-based complexes. An in-depth study of the NHC precursors and the screening of various NHC ligands enabled us to circumvent the issue associated with the formation of meso complexes. A set of 8 atropisomeric NHC-palladium complexes were prepared and then obtained with high enantiopurities, thanks to an efficient resolution by chiral HPLC at the preparative scale. These complexes displayed good activity in the intramolecular α-arylation of amides and various cyclic products were isolated with excellent enantioselectivities (up to 98% ee).

Palladium-mediated phosphine-dependent chemoselective bisallylic alkylation leading to spirocarbocycles.

Cycles everywhere: The selectivity in the transformations of 1,3-diones to carbocycles by palladium-catalyzed bisallylic alkylations is strongly dependent on the phosphine that is employed. Moreover, synthesized vinylcyclopentenes can be easily transformed into cycloheptadiene derivatives through a carbon-carbon allylic bond cleavage.

Olefin metathesis featuring ruthenium indenylidene complexes with a sterically demanding NHC ligand.

The synthesis and characterization of two new ruthenium indenylidene complexes [RuCl(2)(SIPr)(Py)(Ind)] 6 and [RuCl(2)(SIPr)(3-BrPy)(Ind)] 7 featuring the sterically demanding N-heterocyclic carbene 1,3-bis(2,6-di isopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) are reported. Remarkable activity was observed with these complexes in ring closing, enyne, and cross metathesis of olefins at low catalyst loadings. The performance of SIPr-bearing complexes 6 and 7 as well as [RuCl(2)(SIPr)(PCy(3))(Ind)] 5 in ring opening metathesis polymerization is also disclosed. This work highlights the enormous influence of the neutral "spectator" ligands on catalyst activity and stability.

Chemoselective olefin metathesis transformations mediated by ruthenium complexes.

Over the past decade, ruthenium-mediated metathesis transformations, including polymerization reactions, cross-metathesis, ring-closing metathesis, enyne metathesis, ring-rearrangement metathesis, and also tandem processes, represent one of the most studied families of organic reactions. This has translated into the development of a large number of structurally diverse catalysts. Whereas most of these investigations are focused on determining catalytic performance, only rare examples of studies dealing with chemoselectivity have been reported to date. Usually, variations are observed in product conversions but rarely in product distributions. In this critical review, we provide an overview of the stereochemistry of newly formed C=C bonds either in ring-closing or cross-metathesis as a function of the catalyst structure. A discussion of disparities encountered in macrocyclisation reactions leading (or not) to the formation of dimeric products is also presented. Since distinctive metathesis products could be isolated as a function of the ligand borne by the ruthenium centre--phosphine or N-heterocyclic carbene in the dissymetrization of trienes, enyne metathesis and ring rearrangements, these topics are also discussed (72 references).

The influence of phosphane ligands on the versatility of ruthenium-indenylidene complexes in metathesis.

The aim of the present study is to develop readily available and stable pre-catalysts that could be easily prepared on large scale from simple starting materials. Based on the hypothesis that substitution of classical PCy(3) with phosphanes of varying electron-donating properties could be a straightforward manner to improve catalytic activity, a methodical study dealing with the effect of phosphane fine-tuning in ruthenium-indenylidene catalysts was performed. Challenged to establish how the electronic properties of para-substituted phosphane ligands translate into catalyst activity, the versatile behaviour of these new ruthenium-indenylidene complexes was investigated for a number of metathesis reactions.

Backbone tuning in indenylidene-ruthenium complexes bearing an unsaturated N-heterocyclic carbene.

The steric and electronic influence of backbone substitution in IMes-based (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) N-heterocyclic carbenes (NHC) was probed by synthesizing the [RhCl(CO)2(NHC)] series of complexes to quantify experimentally the Tolman electronic parameter (electronic) and the percent buried volume (%V(bur), steric) parameters. The corresponding ruthenium-indenylidene complexes were also synthesized and tested in benchmark metathesis transformations to establish possible correlations between reactivity and NHC electronic and steric parameters.

Chemodivergent metathesis of dienynes catalyzed by ruthenium-indenylidene complexes: an experimental and computational study.

A study on the enyne metathesis reaction leading to the formation cyclic compounds using ruthenium-indenylidene complexes is presented. Several 1,11-dien-6-ynes have been subjected to ruthenium metathesis cyclization by using ruthenium-indenylidene complexes bearing various phosphine and N-heterocyclic carbene (NHC) ligands. Interestingly, for some substrates chemodivergent metathesis occurs and is a function of the catalyst employed. This led us to investigate the competing "ene-then-yne" or "yne-then-ene" reaction pathways apparently at play in these systems using both experimental observations and DFT calculations. Experimental and computational studies were found in good agreement and permit to conclude that for phosphine-containing catalysts, the "ene-then-yne" pathway is exclusively adopted. On the other hand, for catalysts bearing NHC ligands, both pathways are possible.

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

The introduction of N-heterocyclic carbene ligands that incorporate correctly substituted naphthyl side chains leads to increased activity and stability in second generation ruthenium metathesis catalysts.

Ruthenium-indenylidene complexes: powerful tools for metathesis transformations.

Ruthenium-indenylidene complexes represent a class of robust and efficient pre-catalysts for olefin metathesis reactions. In this feature article, we provide an overview of the various complexes belonging to this family and summarise their use in various applications. The relation between the nature of ancillary ligands around the metal coordination sphere of these complexes and their catalytic activity is also discussed.

Bond dissociation energies of carbonyl gold complexes: a new descriptor of ligand effects in gold(i) complexes?

Ligand electronic effects in gold(i) chemistry have been evaluated by means of the experimental determination of M-CO bond dissociation energies for 16 [L-Au-CO]+ complexes, bearing L ligands widely used in gold catalysis. Energy-resolved analyses have been made using tandem mass spectrometry with collision-induced dissociation. Coupled with DFT calculations, this approach enables the quantification of ligand effects based on the LAu-CO bond strength. A further energy decomposition analysis gives access to detailed insights into this bond's characteristics. Whereas small differences are observed between phosphine- and phosphite-containing gold complexes, carbene ligands are shown to stabilize the gold-carbonyl bond much more efficiently.

Sustainable concepts in olefin metathesis.

Ruthenium-catalyzed olefin metathesis reactions represent an attractive and powerful transformation for the formation of new carbon-carbon double bonds. This area is now quite familiar to most chemists as numerous catalysts are available that enable a plethora of olefin metathesis reactions. Nevertheless, with the exception of uses in polymerization reactions, only a limited number of industrial processes use olefin metathesis. This is mainly due to difficulties associated with removing ruthenium from the final products. In this context, a number of studies have been carried out to develop procedures for the removal of the catalyst or the products of catalyst decomposition, however, none are universally attractive so far. This situation has resulted in tremendous activity in the area dealing with supported or tagged versions of homogeneous catalysts. This Review summarizes the numerous studies focused on developing cleaner ruthenium-catalyzed metathesis processes.

[RuCl2(η6-p-cymene)] complexes bearing phosphinous acid ligands: preparation, application in C-H bond functionalization and mechanistic investigations.

A series of [RuCl2(η(6)-p-cymene)] complexes bearing phosphinous acid (PA) ligands have been straightforwardly prepared from the dimer [RuCl2(p-cymene)]2 and secondary phosphine oxides (SPOs) and fully characterized. The steric parameter quantification of PAs, other L ligands and η(6)-p-cymene allowed a better comprehension of the coordination chemistry of these types of complexes and explained the absence of coordination in the case of bulky SPOs such as Ad2P(O)H. These complexes were tested in the C-H activation/functionalization of 2-phenylpyridine and a good activity was obtained at 80 °C for the complex exhibiting the highest steric bulk. A study on halide effects, either on the ruthenium complex or for the aryl halide partner, has also been carried out showing drastic differences. Further investigations on halide effects were performed notably by using a cationic ruthenacycle which was found to be an intermediate for the reaction. In order to rationalize the role played by the phosphinous acid, a mechanism involving a concerted metallation deprotonation favored by a phosphinito species has been proposed.