Piet W. N. M. van Leeuwen

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.

Bite angle effects of diphosphines in C–C and C–X bond forming cross coupling reactions

Catalytic reactions of C-C and C-X bond formation are discussed in this critical review with particular emphasis on cross coupling reactions catalyzed by palladium and wide bite angle bidentate diphosphine ligands. Especially those studies have been collected that allow comparison of the ligand bite angles for the selected ligands: dppp, BINAP, dppf, DPEphos and Xantphos. Similarities with hydrocyanation and CO/ethene/MeOH reactions have been highlighted, while rhodium hydroformylation has been mentioned as a contrasting example, in which predictability is high and steric and electronic effects follow smooth trends. In palladium catalysis wide bite angles and bulkiness of the ligands facilitate generally the reductive elimination thus giving more efficient cross coupling catalysis (174 references).

Bite angle effects in diphosphine metal catalysts: steric or electronic?

The effects of wide bite angles of bidentate phosphine ligands on three catalytic reactions are reviewed: rhodium catalysed hydroformylation, nickel catalysed hydrocyanation, and palladium catalysed reactions of ethene, carbon monoxide and methanol leading to polyketone or methyl propanoate. The P–M–P bite angle plays a crucial role in determining the selectivity and rate in all three reactions. In this review an attempt is made to separate the mode of action into a steric and an electronic one. The regioselectivity of hydroformylation seems to be governed by steric factors, while the rate of reaction is determined by the electronic influence of the bite angle. The rates in hydrocyanation and polyketone formation were previously thought to be determined by orbital effects, but that should be questioned. Selectivity in the palladium carbonylation reaction is mainly due to steric factors.

Phosphite-containing ligands for asymmetric catalysis.

Phosphite-Containing Ligands for Asymmetric Catalysis PietW. N.M. van Leeuwen, Paul C. J. Kamer,Carmen Claver,Oscar P amies,* andMontserrat Di eguez* Institute of Chemical Research of Catalonia, Avinguda Països Catalans 16, 43007 Tarragona, Spain University of St. Andrews, EaStCHEM, School of Chemistry, St. Andrews, Fife KY16 9ST, United Kingdom Universitat Rovira i Virgili, Departament de Química Física i Inorg anica, C/Marcel 3 lí Domingo s/n, 43007 Tarragona, Spain

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.

New directions in supramolecular transition metal catalysis

Supramolecular chemistry has grown into a major scientific field over the last thirty years and has fueled numerous developments at the interfaces with biology and physics, clearly demonstrating its potential at a multidisciplinary level. Simultaneously, organometallic chemistry and transition metal catalysis have matured in an incredible manner, broadening the pallet of tools available for chemical conversions. The interface between supramolecular chemistry and transition metal catalysis has received surprisingly little attention. It provides, however, novel and elegant strategies that could lead to new tools in the search for effective catalysts, as well as the possibility of novel conversions induced by metal centres that are in unusual environments. This perspective describes new approaches to transition metal catalyst development that evolve from a combination of supramolecular strategies and rational ligand design, which may offer transition metal catalysts for future applications.

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.

Decomposition pathways of homogeneous catalysts.

Homogeneous catalysts can decompose in a variety of ways: metal deposition, ligand decomposition, reaction with impurities, dimer formation, and reaction of the metal center with the ligand. Sometimes these side reactions lead to temporary deactivation only and the catalyst activity can be restored. Without attempting to be complete, I have collected a range of examples of catalyst decomposition. Our examples will concentrate on homogeneous reactions of commercial interest, such as polymerization, oxidation, oligomerization, hydroformylation, carbonylation, hydrocyanation, and cross-coupling reactions.