Abstract

Abstract

Abstract

Abstract

Abstract

A series of chiral (R)-binaphthol-based diphosphite ligands with different substituents was prepared and applied in the asymmetric nickel-catalyzed hydrocyanation of styrene and 1,3-cyclohexadiene to investigate the influence of their steric properties. The optimum steric properties for the hydrocyanation reaction lie within a narrow window. With the optimized ligand, hydrocyanation of styrene gave full conversion (Subs/Ni=100) with 49 % ee, the TON was determined to be 600. Hydrocyanation of 1,3-cyclohexadiene gave 50 % conversion (Subs/Ni=500) with an excellent ee of 86 %. This demonstrates that high ees are not only accessible for vinylarenes but also for conjugated dienes in the asymmetric nickel-catalyzed hydrocyanation.

The chiral diphosphonite ligand (11bR,11′bR)-4,4′-(9,9-dimethyl-9H-xanthene-4,5-diyl)bis[dinaphtho[2,1-d:1′,2′-f][1,3,2]dioxaphosphepin] ((R,R)-XantBino; (R)-1), based on a rigid xanthene backbone, was applied in the Pt/Sn-catalyzed hydroformylation of styrene (4a), 4-methylstyrene (4b), vinyl acetate (4c), and allyl acetate (4d), by using a Pt/Sn ratio of 1 : 1. High ee of up to 80% were observed, along with good regioselectivities towards the desired branched aldehydes. For styrene, an interesting inversion in the stereoselection process was observed at elevated temperatures, and a mechanism is proposed considering the temperature dependence of the regioselectivity. The complex [PtCl₂{(S,S)-XantBino}] ((S)-2) was characterized by X-ray crystal-structure analysis, revealing an unusual out-of-plane ligand coordination of the metal fragment. The complex [PtCl(SnCl₃){(R,R)-XantBino}] ((R)-3) was characterized by means of ³¹P-NMR spectroscopy.

Eine elegante Methode zur wässrigen Zweiphasenhydroformylierung von 1-Octen mit einem wasserlöslichen Rh-Katalysator und amphiphilen Latexpartikeln als Phasentransfervermittler wurde entwickelt (siehe Schema). Es wurden beträchtliche Olefinkonzentrationen in der wässrigen katalysatorhaltigen Phase erreicht und damit praktikable Wechselzahlen unter milden Reaktionsbedingungen erzielt.

An elegant method for the aqueous-phase hydroformylation of 1-octene with a water-soluble Rh catalyst and amphiphilic latex particles as phase-transfer agents has been developed (see scheme). In this way, substantial concentrations of the olefin in the aqueous catalyst-containing phase were reached, and thus reasonable turnover frequencies under mild reaction conditions could be achieved.

A simple method for the effective immobilization of homogeneous catalysts on polystyrene colloids via non-covalent binding is demonstrated. Stable latices with sufficiently high loading of accessible borate anions are prepared via emulsion polymerization. Incorporation of cationic rhodium complexes, supported via their borate counter-anion is efficient, and these supported homogeneous catalysts maintain constant catalytic activity for CC hydrogenation during several recycles, with very low metal leaching.

This review article illustrates possible aspects for molecular-weight enlargement and immobilisation of transition-metal complexes that have been developed for bridging the gap between homogeneous and heterogeneous catalysis. The recycling of homogeneous catalysts can be performed using different types of supports, such as dendrimers, hyperbranched polymers, nanostructured materials or stabilized nanoparticles in combination with suitable filtration methods and equipments. Continuous homogeneous catalysis is ideally performed in a continuously operated membrane reactor. A general overview of molecular-weight enlargement along with selected examples and recent developments in continuous homogeneous catalysis is provided. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

The cover picture shows the concept of operating a homogeneous catalyst under continuous operation conditions: The recycling of molecular-weight-enlarged transition-metal complexes by suitable filtration methods and equipment in order to combine the advantages of both classical homogeneous and heterogeneous catalysis. Various aspects of molecular-weight enlargement and immobilization of catalysts, and recent developments, are reported in the Microreview by D. Vogt et al. on p. 4011 ff.

Silsesquioxanes are employed as ligand backbones for the synthesis of novel phosphite compounds with 3,3′-5,5′-tetrakis(tert-butyl)-2,2′-dioxa-1,1′-biphenyl substituents. Both mono- and bidentate phosphites are prepared in good yields. Two types of silsesquioxanes are employed as starting materials. The monophosphinite 1 and the monophosphite 2 are prepared from the thallium silsesquioxide derived from a completely condensed silsesquioxane framework (c-C₅H₉)₇Si₇O₁₂SiOTl. The diphosphite 3 is synthesized starting with the incompletely condensed monosilylated disilanol (c-C₅H₉)₇Si₇O₉(OSiMePh₂)(OH)₂. For monophosphite 2, the corresponding trans-[PtCl₂(2)] complex 4 is characterized by NMR spectroscopy as well as by X-ray crystallography, as the first example of a completely condensed oxo-functionalized silsesquioxane framework. The coordination of the bidentate ligand 3 towards Pd, Mo and Rh is studied, both by NMR spectroscopy as well as by X-ray crystallography. Various modes of coordination are shown to be possible. The molecular structures for the complexes trans-[PdCl₂(3)] (5), cis-[Mo(CO)₄(3)] (6) and the dinuclear complex [{Rh(μ-Cl)(CO)}₂(κ²-3)] (7) have been determined. In the rhodium-catalyzed hydroformylation of 1-octene high activities, with turnover frequencies of up to 6800 h⁻¹, are obtained with these new nanosized phosphorus ligands.

The synthesis of a novel class of sterically demanding diphosphonites 1–8, based on rigid backbones, is described. The starting materials are all commercially available and the methodology allows for a modular approach. All ligands have been fully characterized, including an X-ray crystal structure for compound 1, 4,5-bis{di[(2-tert-butyl)phenyl]phosphonito}-9,9-dimethylxanthene. The coordination of these diphosphonite ligands towards Ni(II) and Ni(0) precursors is investigated, both by NMR spectroscopy as well as X-ray crystallography and compared with the behaviour of diphosphine ligands such as Xantphos. The molecular structure for complex 9, trans-[NiBr₂(1)] is described in detail. The nickel-catalyzed isomerization of 2-methyl-3-butenenitrile to 3-pentenenitrile is studied, a relevant step in the industrially important hydrocyanation of butadiene (the DuPont adiponitrile process). Good activities and selectivities to the desired 3-pentenenitrile are obtained in this reversible CC bond activation reaction.

The improved synthesis of the chiral diphosphonite, XantBino (1), based on a xanthene backbone and bearing chiral binaphthyl groups on both P-atoms is described together with its Pd^II and Rh^I complexes. The ³¹P NMR spectra of both complexes point out that the two phosphorus atoms are chemically inequivalent. The complex cis-[PdCl₂(1)] (2) is structurally characterized by NMR spectroscopy and X-ray crystallography. The molecular structure reveals an unusually small bite angle for this member of the xantphos family of only 100°. The rhodium-catalyzed asymmetric hydroformylation of styrene and vinyl acetate as well as the asymmetric hydrogenation of methyl (Z)-2-acetamidocinnamate, applying this chiral diphosphonite 1, are described. Low enantiomeric excesses are obtained in the asymmetric hydroformylation, while use of the catalyst precursor [Rh(cod)(1)]BF₄ (3) results in a promising enantiomeric excess in the Rh-catalyzed asymmetric hydrogenation. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

A shape-persistent nanosize dodecakis(NCN-Pd^II-aqua) complex (4b) was applied as a homogeneous catalyst in the double Michael reaction between methyl vinyl ketone and ethyl α-cyanoacetate under continuous reaction conditions in a nanofiltration membrane reactor. Due to its macromolecular dimensions, the catalyst is retained in the reactor (R=99.5% determined by ICP-AAS) during catalysis. In addition, the catalyst was found to be stable under the continuous reaction conditions as a constant activity was obtained at prolonged reaction times (26 h, 65 exchanged reactor volumes). The turnover number of the catalyst was thus increased by a factor greater than 40 from 80 (batch) up to >3000 mol/mol Pd. Further development of this technology will allow an increase of the number of (industrial) catalytic processes in which homogeneous catalysts are applied.

The continuous recovery and recycling of soluble metal nanoparticles by means of ultrafiltration is described, employing hybrids of palladium nanoparticles with highly branched amphiphilic polyglycerol as a catalyst for cyclohexene hydrogenation as a model reaction. In a continuously operated membrane reactor a productivity of 29000 TO over 30 exchanged reactor volumes was observed for nanoparticles of 2.2 nm size, with a maximum rate of 1200 TO h⁻¹. Catalysis by soluble metal complexes can be excluded. After 30 hours of operation, some decrease in activity is observed which is due to deposition of palladium on the ultrafiltration membrane, however this material does not contribute to catalytic activity.

A series of easily accessible diphosphane compounds 1−4, based on bisphenol A derived backbones, has been prepared. A straightforward two-step synthetic route has been employed to obtain these new ligands in good yields from cheap starting materials. Molecular structures for both compound 2 and its oxidized form (5·H₂O) have been determined and are discussed in detail. The coordination of ligand 2 to palladium, platinum, and rhodium precursors has been studied by NMR spectroscopy as well as X-ray crystallography. The molecular structures of the face-to-face dimeric species [{PdCl(CH₃)(2)}₂] (6), [{PtCl₂(2)}₂] (7), and [{RhCl(CO)(2)}₂] (8) are described in detail. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

New homochiral xantphos-type diphosphonite ligands with binaphthoxy substituents have been prepared and characterized by NMR spectroscopy. These ligands have been applied in the nickel-catalyzed hydrocyanation of styrene and other vinylarenes. Enantioselectivities up to 63 % ee have been obtained by using 4-isobutylstyrene as a substrate. Addition of an excess of ligand strongly inhibits the hydrocyanation reaction since the bis-chelate nickel complexes formed are highly stable and catalytically inactive.

New chiral aminophosphine phosphinite ligands with a stereogenic center at the aminophosphine phosphorus atom were prepared based on (R,S)-ephedrine as the chiral auxiliary and backbone. Substituents at the chiral aminophosphine as well as at the phosphinite phosphorus atom were varied. These new ligands were applied to the rhodium-catalyzed asymmetric hydroformylation of vinyl arenes. The enantiomeric excess reached up to 77 %. ¹H and ³¹P NMR studies of the Rh complexes under syngas pressure reveal that [HRh(CO)₂(P∧P)] complexes with the NP* moiety in an axial position are responsible for enantioselectivity.

Kontinuierlich kann die Codimerisierung von Styrol und Ethen mit dendritischen Pd-Komplexen wie 1 in einem Nanofiltrations-Membranreaktor durchgeführt werden. Die Selektivität der Reaktion wird unter kontinuierlichen Bedingungen deutlich erhöht. Monomere Modellkomplexe und dendritische Katalysatoren werden im Satzbetrieb hinsichtlich Aktivität und Selektivität verglichen.

A codimerization of styrene and ethene can be carried out continuously in a nanofiltration membrane reactor with dendritic Pd complexes such as 1. The selectivity of the reaction is increased considerably under continuous conditions. The activity and selectivity of monomeric model complexes and the dendritic catalysts were compared in batch reactions.