Roser Pleixats

DFT Mechanistic Study on Diene Metathesis Catalyzed by Ru‐Based Grubbs–Hoveyda‐Type Carbenes: The Key Role of π‐Electron Density Delocalization in the Hoveyda Ligand

The catalytic activity and catalyst recovery of two heterogenized ruthenium-based precatalysts (H and NO(2)(4)) in diene ring-closing metathesis have been studied by means of density functional calculations at the B3LYP level of theory. For comparison and rationalization of the key factors that lead to higher activities and higher catalyst recoveries, four other Grubbs-Hoveyda complexes have also been investigated. The full catalytic cycle (catalyst formation, propagation, and precatalyst regeneration) has been considered. DFT calculations suggest that either for the homogeneous and heterogenized systems the activity of the catalysts mainly depends on the ability of the precursor to generate the propagating carbene. This ability does not correlate with the traditionally identified key factor, the Ru...O interaction strength. In contrast, precatalysts with lower alkoxy-dissociation energy barriers and lower stabilities compared with the propagating carbene also present larger C1-C2 bond length (i.e., lower pi character of the C-C bond that exists between the metal-carbene (Ru=C) and the phenyl ring of the Hoveyda ligand). Catalyst recovery, regardless of whether a release-return mechanism occurs or not, is also mainly determined by the pi delocalization. Therefore, future Grubbs-Hoveyda-type catalyst development should be based on fine-tuning the pi-electron density of the phenyl moiety, with the subsequent effect on the metalloaromaticity of the ruthenafurane ring, rather than considering the modification of the Ru...O interaction.

Phosphine-Free Perfluoro-Tagged Palladium Nanoparticles Supported on Fluorous Silica Gel: Application to the Heck Reaction†

The immobilization of phosphine-free perfluoro-tagged palladium nanoparticles Pd-1 on fluorous silica gel (FSG) and their utilization in the Heck reaction have been investigated. High yields of vinylic substitution products have been obtained. Recycling studies have shown that the solid-supported palladium catalyst can be readily recovered and reused several times without significant loss of activity. Reactions and recovery of the solid-supported palladium catalyst system can be carried out in the presence of air, without any particular precaution.

Palladium nanoparticles obtained from palladium salts and tributylamine in molten tetrabutylammonium bromide: their use for hydrogenolysis-free hydrogenation of olefins

Carbon-carbon double bonds have been selectively hydrogenated at room temperature in the presence of benzyloxy groups using an atmospheric pressure of hydrogen, toluene or [bmim]PF6 as the solvent and palladium nanoparticles stabilized with tetrabutylammonium bromide. The system [bmim]PF6/palladium nanoparticles can be recycled without noticeable decrease of activity.

Stereospecific preparation of ethyl (E) and (Z)-3-aryl-e-phenylpropenoates by heck reaction

Abstract Ethyl cinnamate reacts with several para -substituted aryl iodides under Jeffery-Larock conditions (Pd(OAc) 2 , NaHCO 3 , n -Bu 4 NBr, DMF, Δ) to give ethyl ( E )-3-aryl-3-phenylpropenoates as major compounds. The reaction of para -substituted ethyl cinnamates with iodobenzene under analogous conditions affords the corresponding Z isomers. The initially stereodefined alkene formed under Heck conditions undergoes a slow isomerization.

Imidazolium-derived organosilicas for catalytic applications

Imidazolium salts are applied in different fields of research and have found a wide variety of applications owing to their distinctive properties. This review exclusively focuses on the various methods (grafting, surfactant-assisted sol–gel synthesis, bridged silsesquioxanes formation) described in recent years for the covalent immobilization of imidazolium salts on silica supports via C–Si bonds and on the catalytic applications of the resulting silica-supported imidazolium salts and NHC complexes. C–C bond formation reactions such as cross-coupling, olefin metathesis or organocatalyzed Knoevenagel condensations as well as functionalization reactions are described. This review is intended to help in inspiring future developments and interest in the growing field of catalysis by supported imidazolium salts and NHC complexes based on organosilicas.

Mechanistic Insights into Ring-Closing Enyne Metathesis with the Second- Generation Grubbs-Hoveyda Catalyst: A DFT Study

The full catalytic process (precatalyst activation, propagating cycle and active-species interconversion) of the ring-closing enyne metathesis (RCEYM) reaction of 1-allyloxy-2-propyne with the Grubbs-Hoveyda complex as catalyst was studied by B3LYP density functional theory. Both the ene-then-yne and yne-then-ene pathways are considered and, for the productive catalytic cycle, the feasibility of the endo-yne-then-ene route is also explored. Calculations predict that the ene-then-yne and yne-then-ene pathways proceed through equivalent steps, the only major difference being the order in which they take place. In this way, all alkene metathesis processes studied here involve four steps: olefin coordination, cycloaddition, cycloreversion and olefin decoordination. Among them, the two more energetically demanding ones are the olefin coordination and decoordination steps. The reaction of the alkyne fragment consists of two steps: alkyne coordination and alkyne skeletal reorganization, the latter of which has the highest Gibbs energy barrier. Comparison between the ene-then-yne and yne-then-ene pathways shows that there is no clear energetic preference for either of the two processes, and thus both should be operative when unsubstituted enynes are involved. In addition, although the endo orientation is computed to be slightly disfavored, it is not ruled out for 1-allyloxy-2-propyne, and thus calculations seem to indicate that the exo versus endo selectivity is strongly influenced by the presence of substituents in the reagent.

Recyclable silica-supported prolinamide organocatalysts for direct asymmetric Aldol reaction in water

Asymmetric organocatalytic materials based on a prolinamide scaffold have been synthesized according to different synthetic routes from a monosilylated precursor: simple or surfactant-assisted co-condensation and grafting on preformed mesostructured silica. The catalytic properties of these materials have been compared for direct asymmetric aldol reactions. The best catalytic material results from a simple co-condensation without structure-directing agent. Simple and green conditions are used for the aldol reaction, the process being performed in water at room temperature, with relatively low amounts of supported organocatalysts and in the absence of an acid co-catalyst. Good recyclabilities are observed without the need for catalyst regeneration, with enantioselectivities (ee up to 92%) higher than that of the parent homogeneous catalysts.

Organic–inorganic hybrid silica materials containing imidazolium and dihydroimidazolium salts as recyclable organocatalysts for Knoevenagel condensations

Organic–inorganic hybrid silica materials containing imidazolium and dihydroimidazolium salts prepared from monosilylated and disilylated monomers by sol–gel methodologies are active and reusable organocatalysts for the Knoevenagel condensation of aromatic aldehydes with malononitrile and ethyl cyanoacetate under solvent-free conditions. Our immobilized systems present higher activities than related homogeneous bis-imidazolium salts, showing the cooperative effect of the matrix surface and the additional advantage of easy recycling. The best performances were obtained with the material derived from polycondensation of a disilylated dihydroimidazolium salt in the absence of tetraethoxysilane (TEOS).

FeCl3-catalyzed conjugate addition of secondary amines, imidazole and pyrazole to methyl 2-acetamidoacrylate. Preparation of β-dialkylamino-αalanine and β-(N-heteroaryi)-α-alanine derivatives

Abstract β-Dialkylamino-α-alanine and β-(N-heteroaryl)-α-alanine derivatives are obtained by conjugate addition of nitrogen based nuclcophiles (cyclic and acyclic secondary amines, imidazole and pirazole) to methyl 2-acetamidoacrylate, 1, under iron(III) chloride catalysis.

Palladium(0) Complexes of a 15-Membered Macrocyclic Triolefin as a Recoverable Catalyst - Monomer- and Polystyrene-Anchored Versions

Preparation of the 15-membered cycle (E,E,E)-1,6,11-tris[(2,4,6-triisopropylphenyl)sulfonyl]-1,6,11-triazacyclopentadeca3,8,13-triene (8) is reported. This cyclic triolefin forms a stable Pd0 complex 9 which catalyzes several cross-coupling reactions and can be recovered. Anchoring to a polystyrene framework affords a solid version of the catalyst, which is recovered by simple filtration and reused without loss of catalytic activity.