Showing papers by "Joost N. H. Reek published in 2000"

Ligand Bite Angle Effects in Metal-catalyzed C−C Bond Formation

Abstract: phinoethane) seemed mainly to stabilize intermediates, and often the catalytic reactions were slower when dppe was used instead of the most common monodentate triphenylphosphine. We will briefly review the history of “ligand effects” in catalysis before discussing a range of reactions for which a notable effect has been observed. It has taken quite some time before the positive effect that bidentates can have on selectivities and rates of catalytic reactions was fully recognized. Most of the established examples of bite angle effects involve diphosphine ligands. Therefore, many important catalysts containing a chelate ligand such as bipyridine and diimine will fall outside the scope of this review. The connecting bridge in these bidentates does play a dominant role in the performance of these catalysts, but systematic studies have not been published. The effects of phosphine ligands in catalysis have been known for quite some time. One of the first reports involves the use of triphenylphosphine in the “Reppe” chemistry, the reactions of alkynes, alcohols, and carbon monoxide.1 It was found that formation of acrylic esters was much more efficient using NiBr2(PPh3)2 than NiBr2 without ligand. In the commercial system, though, a phosphine-free catalyst is used. While the reaction was not yet understood mechanistically, the use of phosphines in catalysis attracted the attention of the petrochemical industry worldwide. An early example of a phosphine ligand modified catalytic process is the Shell process for alkene hydroformylation using a cobalt catalyst containing a trialkylphoshine.2 The reaction requires higher temperatures, but it leads to more linear product as compared to the unmodified catalyst. The general mechanism of the hydroformylation reaction has been known for a long time.3 Hydrocyanation as used by Du Pont is another early example of an industrially applied catalytic reaction employing ligands.4 It is a nickel-catalyzed reaction in which aryl phosphite ligands are used for the production of adiponitrile. The development of this process has played a key role in the introduction of the now very common study of “ligand effects” in the field of homogeneous catalysis by organometallic complexes.5 While several industries were working on new homogeneous catalysts, important contributions to the new field were made in academia in the early 1960s with the appearance of the first phosphinemodified hydrogenation catalysts. An early example of a phosphine-free ruthenium catalyst was published by Halpern.6 Triphenylphosphine-modified platinumtin catalysts for the hydrogenation of alkenes were reported by Cramer from Du Pont in 1963.7 In the same year Breslow (Hercules) included a few phosFigure 1. Bite angle: The ligand-metal-ligand angle of bidentate ligands. 2741 Chem. Rev. 2000, 100, 2741−2769

The dynamics of electronic energy transfer in novel multi-porphyrin functionalized dendrimers: A time-resolved fluorescence anisotropy study

Abstract: The dynamics of electronic energy transfer (EET) for a series of spherical porphyrin arrays based on different generations of poly(propylene imine) dendrimers have been investigated using time-resolved fluorescence anisotropy measurements (TRAMS) in a glass environment. The first, third, and fifth generation dendrimers consisting of 4, 16, and 64 porphyrin chromophores, respectively, are investigated in this study. We observe a depolarization of the fluorescence in all three dendrimers as compared to the monoporphyrin model compound, indicating that EET takes place between the chromophores within the dendrimers. The experimental TRAMS results were compared to computationally simulated data obtained from the Pauli master equation. For the first generation dendrimer, we find the rate of energy transfer is well described by Forster theory. Anomalous behavior is observed in the third generation dendrimer where the limiting anisotropy value suggests that energy transfer is confined to only the porphyrins conta...

Accelerated Biphasic Hydroformylation by Vesicle Formation of Amphiphilic Diphosphines

Abstract: The synthesis, aggregation behavior, and catalytic activity of rhodium complexes of a series of Xantphos derivatives with surface-active pendant groups, −4-C6H4O(CH2)nC6H4(SO3Na)− (n = 0, 3, 6) is described. Electron microscopy experiments show that these ligands and their complexes form vesicles in water if the hydrophobic part of the ligand is large enough (n = 3, 6). The formed aggregates are stable at elevated temperatures (90 °C), and their presence leads to a significant enhancement of the solubility of organic substrates in aqueous solution. This enhanced solubility results directly in a higher reaction rate in the rhodium-catalyzed hydroformylation of 1-octene. Furthermore, recycling experiments show that the TOF and the high selectivity toward the more valuable linear aldehyde remains approximately the same in four consecutive runs. This indicates that the aggregates stay intact during the recycling and the active rhodium complex is retained in the water-phase quantitatively.

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

Abstract: 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-NH 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.

Photoinduced energy and electron transfer in bis-porphyrins with quinoxaline Troger's base and biquinoxalinyl spacers.

Abstract: The photophysical characterisation of bis-porphyrins consisting of two porphyrins bridged by either a quinoxaline Troger's base (1 and 2) or a biquinoxalinyl (3) spacer are reported. Efficient intramolecular electronic energy transfer (EET) between the rigidly linked free-base porphyrins in 1 and from the zinc(II) porphyrin to the free-base porphyrin in 2 has been investigated by steady-state absorption and emission spectroscopy, time-resolved fluorescence spectroscopy and semi-empirical calculations. A resonance dipole–dipole mechanism alone cannot account for the rate of EET in both 1 and 2. It is demonstrated that a superexchange mechanism ia the quinoxaline Troger's base linker is responsible for the enhanced energy transfer rates in these systems. Strong quenching of the fluorescence intensity observed in 3 is interpreted as arising from long-range (>18 A) through-biquinoxalinyl bridge mediated photoinduced electron transfer from the free-base porphyrin to the gold(III) porphyrin. These systems provide useful models for the arrangements of the primary donor–acceptor pair in photosynthetic reaction centres, and for elucidating the role of the connecting bridge in electron and energy transfer processes.

Novel developments in hydroformylation

Abstract: The above examples have shown that there is considerable activity in searching for other ways to achieve separation of product and homogeneous catalysts. The improvements needed are the same for all reaction types, also reactions other than hydroformylation. Most of the new techniques have in common that not only the product, but also the starting material and byproducts are separated from the catalyst. This mixture of organic products needs further separation, but as we have seen in Chapter 8 separation of catalyst and byproducts such as heavy ends is an important issue in hydroformylation of simple alkenes. There may be a future for immobilized “drop in“ catalysts as described above in the hydroformylation of fine chemicals.