Werner R. Thiel

Magnetically Separable Nanocatalysts: Bridges between Homogeneous and Heterogeneous Catalysis

Recovery and reuse of expensive catalysts after catalytic reactions are important factors for sustainable process management. The aim of this Review is to highlight the progress in the formation and catalytic applications of magnetic nanoparticles and magnetic nanocomposites. Directed functionalization of the surfaces of nanosized magnetic materials is an elegant way to bridge the gap between heterogeneous and homogeneous catalysis. The introduction of magnetic nanoparticles in a variety of solid matrices allows the combination of well-known procedures for catalyst heterogenization with techniques for magnetic separation.

Methyltrioxorhenium/pyrazole—A highly efficient catalyst for the epoxidation of olefins

Abstract A biphasic system consisting of 35% H 2 O 2 and methyltrioxorhenium(VII)/pyrazole in CH 2 Cl 2 catalyzes the epoxidation of a wide range of olefins in excellent yields. Both the reactivity and selectivity of the new catalytic surpasses all known MTO/Lewis base catalysts. The considerable ligand accelerating effect observed here is attributed to the stability of pyrazole against oxidation.

Cooperative acid-base effects with functionalized mesoporous silica nanoparticles: applications in carbon-carbon bond-formation reactions.

Acid-base bifunctional mesoporous silica nanoparticles (MSN) were prepared by a one-step synthesis by co-condensation of tetraethoxysilane (TEOS) and silanes possessing amino and/or sulfonic acid groups. Both the functionality and morphology of the particles can be controlled. The grafted functional groups were characterized by using solid-state (29)Si and (13)C cross-polarization/magic angle spinning (CP/MAS) NMR spectroscopy, thermal analysis, and elemental analysis, whereas the structural and the morphological features of the materials were evaluated by using XRD and N(2) adsorption-desorption analyses, and SEM imaging. The catalytic activities of the mono- and bifunctional mesoporous hybrid materials were evaluated in carbon-carbon coupling reactions like the nitroaldol reaction and the one-pot deacetalization-nitroaldol and deacetalization-aldol reactions. Among all the catalysts evaluated, the bifunctional sample containing amine and sulfonic acid groups (MSN-NNH(2)-SO(3)H) showed excellent catalytic activity, whereas the homogeneous catalysts were unable to initiate the reaction due to their mutual neutralization in solution. Therefore a cooperative acid-base activation is envisaged for the carbon-carbon coupling reactions.

Oxodiperoxo molybdenum modified mesoporous MCM-41 materials for the catalytic epoxidation of cycloocteneElectronic supplementary information (ESI) available: experimental data. See http://www.rsc.org/suppdata/cc/b2/b207645d/

A hybrid heterogeneous catalyst system, which has been synthesized by covalently anchoring oxodiperoxo molybdenum chelate complexes onto the surface of mesoporous MCM-41, is highly active and truly heterogeneous for the liquid-phase epoxidation of cyclooctene with tBuOOH as the oxygen source.

Bifunctional Acid–Base Cooperativity in Heterogeneous Catalytic Reactions: Advances in Silica Supported Organic Functional Groups

Emulating nature is a powerful way to realize selective catalytic reactions and to develop structurally complex organic molecules. A well understood fact in biochemistry is that enzymes are capable of accelerating reactions through cooperative interactions between accurately positioned functional groups present in their active sites. Substrate recognition and activation may proceed through electrostatic, hydrogen bonding, or covalent interactions. Following the concept of ‘site isolation’, significant progress has recently been achieved in the immobilization of mutually destructive catalysts for multistep cascade reactions. ‘Site isolation’ or ‘compartmentalization’ is thus considered as one of the paradigms in heterogeneous catalysis especially in the separation of mutually incompatible catalysts, such as acids and bases. This concept, adopted from nature and through which incompatible sites are spatially separated, aids in avoiding undesired interactions. Thereby tedious work up procedures and purification can be greatly minimized. Cascade reactions—defined as two or more reactions occurring in one reactor—are an emerging strategy in modern synthetic chemistry, as a) several bonds can be formed in a single reaction step in the same reaction vessel without isolation of intermediates, b) they allow a rapid and efficient synthesis of complex molecules from simple starting materials, and c) the overall process is ‘green’ in terms of waste and cost reduction. However, it is not trivial for material chemists to try to mimic biological catalysts by fixing antagonist groups in solid supports for various onestep cooperative catalytic reactions. In a classical concept, a support material serves to handle and recover the catalyst, whereas recent systematic investigations suggest that surfaces will often act cooperatively to stabilize specific catalyst structures. Thus during the last years researchers focused their efforts to precisely position bior multifunctional groups on solid supports for cooperative catalytic reactions. Beyond doubt, this concept of multifunctional heterogeneous or heterogenized catalysts is one way to economic and ecologically benign processes. In this context, organic–inorganic hybrid materials have shown their applicability in a broad variety of heterogeneous catalytic reactions. They show high mechanical, thermal, and chemical stability provided by the inorganic components, while the organic compartments can be independently selected and optimized for specific catalytic applications. Organic modifications of structured inorganic solids, for example mesoporous materials, can be performed by three different ways: by the grafting method, by the co-condensation method, and by the formation of an organosilica material (Scheme 1). By employing one of these methods a series of monoor bifunctional organic groups can be successfully fixed inside the channels of silica supports. Heterogeneous bifunctional catalysts can provide a continuous range of functional groups and offer advantages, such as an enhancement of reactivity and stability of antagonist func-

(Dihalogenmethyl)palladium(II)-Komplexe aus Palladium(O)-Vorstufen des Dibenzylidenacetons: Synthese, Strukturchemie und Reaktivität ag]

Abstract Bis(dibenzylidenacetone)palladium(0) (1) reacts with one equivalent of a chelating phosphine ligand (PP) or two equivalents of a monophosphine ligand (P) to give complexes of the type (PP)Pd(dba) (3a—c; dba = dibenzylidene acetone) and (P)2Pd(dba) (3d,e) respectively, that have been characterized spectroscopically and structurally (X-ray structure analysis). These complexes readily cleave the carbon—halogen bond of chloroform or bromoform. The air- and moisture-stable oxidative addition products (PP)Pd(CHX2)X (4a-d; X  Cl, Br) are thus obtained in 70–85% yield. Their identity was established by a single-crystal X-ray diffraction study of 4a.

Transition Metal Complexes with Organoazide Ligands: Synthesis, Structural Chemistry, and Reactivity

A drastically enhanced stability is observed for organoazides (RN3 ) in the presence of Cu2+ or Pd2+ when the azido group is included in a ligand system chelating the transition metal ions. X-ray structure analysis of such complexes (the structure of a cyclohexaneazide palladium complex is depicted) confirms that the alkylated nitrogen atom of the N3 moiety is coordinated to the transition metal center.

Metal catalyzed oxidations. Part 5. Catalytic olefin epoxidation with seven-coordinate oxobisperoxo molybdenum complexes : a mechanistic study

Abstract Oxobisperoxo molybdenum complexes with chelating pyrazolylpyridine ligands catalyze the epoxidation of olefins by activation of the oxidizing agent ( t -BuOOH or H 2 O 2 ). The reaction mechanism is supported by spectroscopic and kinetic investigations.

New Insights into the Mechanism of Hydroperoxide Activation by Investigation of Dynamic Processes in the Coordination Sphere of Seven-Coordinated Molybdenum Peroxo Complexes

Seven-coordinated molybdenum oxobisperoxo complexes with chelate nitrogen donors like pyrazolylpyridines are catalysts for the epoxidation of olefins. An NMR spectroscopic and quantumchemical study on the fluxionality of the chelate ligand proves that during this process partial ligand dissociation takes place. This gave rise to a detailed theoretical study on the activation of CH3OOH at the model complex (NH3)2MoO(O2)2 including dissociation of one of the ammonia ligands and proton transfer from the hydroperoxide to one of the peroxo ligands.

Contributed paperMetal catalyzed oxidations. Part 5. Catalytic olefin epoxidation with seven-coordinate oxobisperoxo molybdenum complexes: a mechanistic study1

Abstract Oxobisperoxo molybdenum complexes with chelating pyrazolylpyridine ligands catalyze the epoxidation of olefins by activation of the oxidizing agent ( t -BuOOH or H 2 O 2 ). The reaction mechanism is supported by spectroscopic and kinetic investigations.