Paul C. J. Kamer

Transition Metal Catalysis Using Functionalized Dendrimers

Dendrimers are well-defined hyperbranched macromolecules with characteristic globular structures for the larger systems. These novel polymers have inspired many chemists to develop new materials and several applications have been explored, catalysis being one of them. The recent impressive strides in synthetic procedures increased the accessibility of functionalized dendrimers, resulting in a rapid development of dendrimer chemistry. The position of the catalytic site(s) as well as the spatial separation of the catalysts appears to be of crucial importance. Dendrimers that are functionalized with transition metals in the core potentially can mimic the properties of enzymes, their efficient natural counterparts, whereas the surface-functionalized systems have been proposed to fill the gap between homogeneous and heterogeneous catalysis. This might yield superior catalysts with novel properties, that is, special reactivity or stability. Both the core and periphery strategies lead to catalysts that are sufficiently larger than most substrates and products, thus separation by modern membrane separation techniques can be applied. These novel homogeneous catalysts can be used in continuous membrane reactors, which will have major advantages particularly for reactions that benefit from low substrate concentrations or suffer from side reactions of the product. Here we review the recent progress and breakthroughs made with these promising novel transition metal functionalized dendrimers that are used as catalysts, and we will discuss the architectural concepts that have been applied.

Chiral Induction Effects in Ruthenium(II) Amino Alcohol Catalysed Asymmetric Transfer Hydrogenation of Ketones: An Experimental and Theoretical Approach

The enantioselective outcome of transfer hydrogenation reactions that are catalysed by ruthenium(II) amino alcohol complexes was studied by means of a systematically varied series of ligands. It was found that both the substituent at the 1-position in the 2-amino-1-alcohol ligand and the substituent at the amine functionality influence the enantioselectivity of the reaction to a large extent: enantioselectivities (ee values) of up to 95% were obtained for the reduction of acetophenone. The catalytic cycle of ruthenium(II) amino alcohol catalysed transfer hydrogenation was examined at the density functional theory level. The formation of a hydrogen bond between the carbonyl functionality of the substrate and the amine proton of the ligand, as well as the formation of an intramolecular H...H bond and a planar H-Ru-N-H moiety are crucially important for the reaction mechanism. The enantioselective outcome of the reaction can be illustrated with the aid of molecular modelling by the visualisation of the steric interactions between the ketone and the ligand backbone in the ruthenium(II) catalysts.

Rhodium catalysed asymmetric hydroformylation with chiral diphosphite ligands

Abstract Chiral diphosphites have been synthesised starting from 1,2:5,6-diisopropylidene- D -mannitol, L -α,α,α,α,-tetramethyl-1,3-dioxolan-4,5-dimethanol and L -diethyltartrate. The diols react in moderate to good yields with 2,2′-bisphenoxyphosphorus chloride and 4,4′,6,6′,-tetra- t -butyl-2,2′-bisphenoxyphosphorus chloride (32–92%) to the corresponding chiral diphosphites. These compounds all exhibit C 2 symmetry and have been used as ligands in the rhodium catalysed asymmetric hydroformylation of styrene. The catalytic activity of the diphosphites strongly depends on the bulkyness of the ligand. With a bulky ligand enantiomeric excesses up till 20% have been obtained under mild reaction conditions (25–40°C, 40 bar syngas). It was found that both enantiomeric excess and regioselectivity to the branched aldehyde strongly depend on the hydroformylation reaction conditions.

Hydroformylation of Internal Olefins to Linear Aldehydes with Novel Rhodium Catalysts

Unprecedented high activities and selectivities were observed in the hydroformylation of internal octenes to linear products using rhodium catalysts with rigid diphosphane ligands. Dibenzophosphole 1 and a phenoxaphosphane analogue with bite angles of 120 and 119°, respectively, are suited for this.

Assembly of Encapsulated Transition Metal Catalysts

Enforced ligand dissociation as a result of steric interactions between ZnII porphyrin units and the N atoms of pyridylphosphane ligands determines the catalytic properties of the encapsulated transition metal complexes. These assemblies show increased catalytic activity in the palladium-catalyzed Heck reaction and rhodium-catalyzed hydroformylation. M=transition metal catalyst.

Phosphorus(III) Ligands in Homogeneous Catalysis: Design and Synthesis: Kamer/Phosphorus(III) Ligands in Homogeneous Catalysis: Design and Synthesis

Over the last 60 years the increasing knowledge of transition metal chemistry has resulted in an enormous advance of homogeneous catalysis as an essential tool in both academic and industrial fields. Remarkably, phosphorus(III) donor ligands have played an important role in several of the acknowledged catalytic reactions. The positive effects of phosphine ligands in transition metal homogeneous catalysis have contributed largely to the evolution of the field into an indispensable tool in organic synthesis and the industrial production of chemicals.

Fast Palladium Catalyzed Arylation of Alkenes Using Bulky Monodentate Phosphorus Ligands

Complex 1b shows an unprecedented high activity in the Heck reaction. Kinetic studies show that in this system not the oxidative addition but the alkene coordination/migratory insertion is the rate determining step.

Versatile Ligands for Palladium‐Catalyzed Asymmetric Allylic Alkylation

Palladium complexes of the chiral diphosphanes 1 and 2 which possess a rigid backbone and a large bite angle catalyze the alkylation of allyl compounds with both high enantioselectivities and reaction rates, particularly with less sterically demanding substrates. 1: R=Me, X=S; 2: R=H, X=C(CH3 )2 .

Übergangsmetallkatalyse mit funktionalisierten Dendrimeren

Dendrimere sind wohldefinierte hochverzweigte Makromolekule, die bei hinreichender Grose charakteristische globulare Strukturen aufweisen. Sie haben viele Chemiker zur Entwicklung von neuen Materialien angeregt, und eine Reihe von Anwendungen, auch in der Katalyse, wurde bereits untersucht. Die jungsten Fortschritte bei den Syntheseverfahren haben den Zugang zu funktionalisierten Dendrimeren sehr vereinfacht, was zu einer raschen Entwicklung der Dendrimerchemie gefuhrt hat. Sowohl die Lage der katalytischen Zentren als auch die raumliche Trennung der Katalysatoren scheinen hier wesentlich zu sein. Dendrimere, die im Kern mit Ubergangsmetallen funktionalisiert sind, sind potentielle Enzymmimetika; die an der Oberflache funktionalisierten hingegen konnten die Lucke zwischen homogener und heterogener Katalyse fullen. Dies konnte vorzugliche Katalysatoren mit neuartigen Eigenschaften – besonderer Reaktivitat oder Stabilitat – liefern. Sowohl die kern- als auch die peripherieorientierte Strategie ergeben Katalysatoren, die die meisten Substrate und Produkte an Grose deutlich ubertreffen, sodass eine Abtrennung durch moderne Membrantrenntechniken moglich ist. Diese neuen homogenen Katalysatoren lassen sich in kontinuierlich betriebenen Membranreaktoren verwenden, was vor allem bei solchen Reaktionen sehr vorteilhaft ware, die von niedrigen Substratkonzentrationen profitieren oder durch Nebenreaktionen des Produkts beeintrachtigt werden. Wir geben hier einen Uberblick uber die jungsten Fortschritte auf dem Gebiet der Ubergangsmetall-funktionalisierten Dendrimere, die als Katalysatoren eingesetzt werden; auch die unterschiedlichen Konzepte fur ihre Architektur werden besprochen.

AN EFFICIENT, PALLADIUM-CATALYSED, AMINATION OF ARYL BROMIDES

Aryl bromides are coupled to primary aliphatic amines, anilines and cyclic secondary amines in an efficient and selective procedure using catalytic amounts of Pd(OAc)2 and the chelating ligand 9,9-dimethyl-4,6-bis(diphenylphosphino)xanthene (Xantphos). This catalyst system accomodating low catalyst loadings (0.5 mol % Pd) allows amination of both electron-poor and electron-rich aryl bromides. Even sterically crowded substrates show good reactivity.