Hydrogen transfer reactions of supported metallocene catalysts

The evolution of methane from methylaluminoxane (MAO) solutions is enhanced in the presence of homogeneous metallocenes. This reaction serves as a model for the deactivation of metallocene catalysts. By supporting different metallocenes on a silica/MAO carrier the deactivation reaction by alpha-hydrogen transfer among metallocene active sites and aluminum alkyls can be suppressed. The suppression of alpha-hydrogen transfer is proven for different Al/Zr ratios and by near independence of the polymerization activity on the catalyst aging time, after reaching maximum activity. Aluminum alkyls and MAO leach Cp₂ZrCl₂ from the carrier, the leached metallocene is only active in polymerization if MAO is present.     

Electron transfer reactions in photoelectron emission spectroscopy of aqueous solutions

Lowering of the emission yield of anions by certain cations (e.g. H⁺, Fe²⁺, Ba²⁺) is interpreted as the result of electron transfer between anions and cations. An equation is derived which predicts the exponential dependence of the yield on concentration of electron acceptor at constant emitter concentration. This equation is verified by experiment.     

Organosilanols as catalysts in asymmetric aryl transfer reactions

[reaction: see text] Various ferrocene-based organosilanols have been synthesized in four steps starting from achiral ferrocene carboxylic acid. Applying these novel planar-chiral ferrocenes as catalysts in asymmetric phenyl transfer reactions to substituted benzaldehydes afforded products with high enantiomeric excesses. The best result (91% ee) was achieved in the addition to p-chlorobenzaldehyde with organosilanol 2b, which has a tert-butyl substituent on the oxazoline ring and an isopropyl group on the silanol fragment.     

A new type of B-podand catalysts for solid-liquid phase transfer reactions

Boron podands 1-4 (B-podands) were studied as strong complexing agents of alkali metal cations and very powerful catalysts in typical anion promoted reactions under solid-liquid conditions, even in chlorobenzene and acetonitrile. The results were comparable with the catalytic activity of classical phase transfer catalysts: crown ethers, polyethylene glycols (PEG) and previously studied Si-podands.     

Modular chiral thiazolidine catalysts in asymmetric aryl transfer reactions

Modular chiral thiazolidine derivatives were synthesized in a single step from inexpensive and commercially available starting materials. These ligands catalyzed enantioselective arylation of different aldehydes using aryl boronic acids as a source of transferable aryl groups. The products were obtained in excellent yields and good enantioselectivities.     

Tertiary amine oxides as phase transfer catalysts for substitution and dichlorocyclopropanation reactions

Tertiary amine oxides catalyze a variety of phase transfer reactions, primarily by $\text{in}$$\text{situ}$ deoxygenation to form the tertiary amine or derived catalyst.     

Polymer hydrogel confined palladium nanoparticles as recyclable catalysts for Suzuki and Heck cross-coupling reactions

Reusable palladium nanoparticles highly dispersed in porous and hydrophilic interpenetrating polymer networks(IPN),i.e.,Pd@IPN hybrid gels,are employed for catalysis of Suzuki and Heck coupling reactions.Good yields are obtained with high turnover frequencies when the reactions are run with very low Pdloadings.The use of IPN gives better recyclability than that of crosslinked polyvinyl alcohol alone.The polymer networks allow the reactants to have easy access to the Pd metals.The catalysts combine high activity with the reusability offered by the heterogeneous system,without the need for strong coordination or chelating ligands.     

Cu–iminopyridine complexes as catalysts for carbene and nitrene transfer reactions

Copper complexes with chiral iminopyridine ligands were screened for their catalytic efficiency in carbene (cyclopropanation) and nitrene transfer reactions (aziridination, C? H amidation). Both pre‐formed and in situ formed complexes were considered. The results highlighted the poor catalytic efficiency of these complexes in cyclopropanation reactions employing methyl phenyldiazoacetate as the carbene source, whereas better results were obtained in nitrene transfer reactions, particularly in the amidation of C? H bonds, albeit the enantioselectivity of the reactions was negligible in nearly all cases. Finally, copper complexes were also found to promote an interesting oxidative functionalization of alkynes with PhI(OAc)₂ at room temperature. Copyright © 2014 John Wiley & Sons, Ltd.     

Recent topics in enantioselective acyl transfer reactions with dialkylaminopyridine-based nucleophilic catalysts

This digest focuses on recent topics in intra- and intermolecular enantioselective acyl transfer reactions with dialkylaminopyridine-based catalysts over the past decade (2007–2017). The structures of the catalysts are divided into four categories according to the location of the chiral environment (C2, C3, or C4 position of pyridine, and miscellaneous). The role of the structure in the enantioselective acyl transfer reaction and the reaction mechanism will be discussed briefly.     

Polymer viologen as a stabilizing agent of colloidal platinum for photoinduced electron transfer reactions

Photoinduced electron transfer and hydrogen evolution were investigated using colloidal platinum stabilized by viologen-pendant Nylon (Pt—PV²⁺). From laser flash photolysis experiments, the Pt—PV²⁺ was found to accept directly an electron from excited ZnTMPyP⁴⁺, and the reverse electron transfer was retarded by the positive charge of the protecting polymer, while no net electron transfer was observed for colloidal platinum stabilized by poly(vinyl alcohol) (Pt—PVA). The rate of hydrogen evolution was measured via steady-state irradiation experiments in the presence of a sacrificial electron donor. The quantum yield of hydrogen production with a Pt—PV²⁺ catalyst was higher than that with Pt—PVA by a factor of 7 and reached 0.38.     

Polymer fibers as carriers for homogeneous catalysts

This paper describes a polymer fiber-based approach for the immobilization of homogeneous catalysts. The goal is to generate products that are free of catalysts which would be of great importance for the development of optoelectronic or pharmaceutical compounds. Electrospinning was employed to prepare the non-woven fiber assembly composed of polystyrene. The homogeneous catalyst scandium triflate was immobilized on the polystyrene fibers during electrospinning and on corresponding core shell fibers using a fiber template approach. An imino aldol and an aza-Diels-Alder model reaction were carried out with each fibrous catalytic system. This resulted in the immobilization of homogeneous catalysts in a polymer environment without loss of their catalytic activity and may even be enhanced when compared with reactions carried out in homogeneous solutions.     

Assembled enantioselective catalysts for carbonyl-ene reactions

A new strategy for generating assembled polymeric chiral titanium complexes using linked bis-BINOL ligands have been developed, and the assembled catalysts showed excellent enantioselectivity (up to 96.5% ee) for carbonyl-ene reaction under heterogeneous conditions to afford corresponding α-hydroxy esters in high yield. The linkers between two BINOL units of the ligands in the assembled catalysts were found to have significant impact on the enantioselectivity of reaction, which demonstrated the importance of the supramolecular structures of the assemblies for their catalytic behaviours.     

Studies on ruthenium(II) Schiff base complexes as catalysts for transfer hydrogenation reactions

The reactions of [RuHCl(CO)(B)(EPh₃)₂] (B = EPh₃ or Py; E = P or As) and Schiff bases in 1:1 molar ratio led to the formation of [RuCl(CO)(EPh₃)(B)(L)] (E = P or As; B = PPh₃, AsPh₃ or Py; L = Schiff base ligand). The new complexes have been characterized by analytical and spectroscopic (IR, electronic and ¹H NMR) data. They have been assigned an octahedral structure. The new complexes were found to catalyse the transfer hydrogenation of ketones.     

Polymer supported palladium N-heterocyclic carbene complexes: long lived recyclable catalysts for cross coupling reactions

Polymer supported palladium complexes, containing a pyridyl bis N-heterocyclic carbene ligand system derived from isonicotinic acid, provide highly stable catalysts for Heck and Suzuki cross coupling reactions. These heterogenous catalysts can be recycled ?14 times with no loss of activity.     

Photochemical reactions of aromatic compounds XLV: controlling factors for reactivities and mechanisms in photoelectron transfer reactions of some selected diarylcyclobutanes with electron acceptors

We have attempted to explore mechanistic aspects of the photosensitized ring-cleavage reactions of cis-1,2-diphenylcyclobutane (1), cis-transoid-cis-cyclobutal[1,2-a:4,3-a′] diindene (2) and r-1,c-2-dimethyl-t-3,t-4-di(4-methoxyphenyl)cyclobutane (3) by electron acceptors (A) in acetonitrile. The experimental results demonstrate that the ring cleavage of 1 and 2 occurs as a consequence of the rapid geminate recombination of ion-radical pairs occurring at a rate of well over 10⁹ s⁻¹ without ionic dissociation. In the case of 3, however, the photoreactions proceed by way of a chain-reaction mechanism involving the free cation radical of 3 which undergoes ring cleavage at much less than 10⁷ s⁻¹. The rapid ring cleavage of 1⁺ and 2⁺ is attributed to significant perturbations of the cyclobutane ring by the population of positive charge on the orbital array of the two π-electron systems and the cyclobutane-ring σ framework because of strong through-bond couplings. It is presumed that the cyclobutane ring of 3⁺ is much less distorted since the positive charge is mostly localized on the aryl group. The rapid geminate recombination of the A⁻⁻−1⁺ and A⁻⁻−2⁺ pairs is discussed in terms of a very efficient transition from the “distorted” and “ring-opened” minima of the A⁻⁻−D⁺ surface to the A–D surface. In the case of 3, this mechanism cannot be expected to operate in the geminate recombination.     

Ruthenium catalysts bearing N-heterocyclic carbene ligands in atom transfer radical reactions

The catalytic activity of ruthenium-p-cymene complexes bearing N-heterocyclic carbene ligands in atom transfer radical addition (ATRA) or polymerisation (ATRP) strongly depends on the substituents of the carbene ligand, thereby providing a nice illustration of the importance of organometallic engineering and ligand fine tuning in homogeneous catalysis.     

Polymer-supported cryptands as catalysts for phase-transfer reactions

Polymer-supported cryptands were prepared by the reaction of hydroxyl-containing cryptands and chloromethylated polystyrene resin. The polymer-supported cryptands were effective catalysts for aqueous organic two-phase reactions.