The catalytic reduction of esters by using H₂ is a sustainable alternative to the use of classic stoichiometric reagents. Nowadays, a wide range of highly efficient homogeneous catalysts are known, but these suffer from poor catalyst recoverability. Therefore, most industrial applications are based on heterogeneous catalysts, but these processes are generally operated under harsh conditions (>200 °C, >10.0 MPa). Herein, we describe the first catalytic system that combines the activity of a homogeneous catalyst with the recoverability of a heterogeneous catalyst. The presented system is capable of hydrogenating esters under very mild conditions (25 °C, 5.0 MPa) and can easily be recovered. The catalyst is based on phosphorus ligands covalently attached to a polymeric support and is readily obtained by a facile solid-phase synthetic protocol in high yields with minimal workup.

Effective and modular synthetic approaches toward phosphine–phosphite ligands and phosphine–phosphonite ligands featuring a diphenyl ether backbone have been developed. The phosphine–phosphite ligands are obtained by a two-step protocol from 2-bromo-2′-methoxydiphenyl ether. The phosphine–phosphonite ligands are prepared in a four-step synthetic protocol that involves a novel, unsymmetrical diphenyl ether derived phosphine–phosphorusdiamide as key building block. Structural studies on Pt^II complexes with either phosphine–phosphite or phosphine–phosphonite ligands indicate strict cis coordination for these ligand systems. High-pressure NMR spectroscopy studies of Rh complexes under syngas indicate the presence of two ea isomers for Rh(H)(CO)₂(PP). The existence of this mixture is further supported by high-pressure IR spectroscopy studies. In order to benchmark the activity and selectivity of these novel, wide-bite-angle, mixed-donor ligands, they were screened in Pd-catalyzed asymmetric allylic alkylation as well as Rh-catalyzed hydrogenation and hydroformylation reactions. The ligands give 100 % conversion and low-to-moderate enantioselectivity in the allylic alkylation of 1,3-diphenyl-2-propenyl acetate and cyclohexyl-2-enyl acetate with dimethyl malonate. In the hydroformylation of styrene, good conversion and regioselectivities are achieved but only moderate enantioselectivity. The ligands give good conversions in asymmetric hydrogenation of typical substrates, with good-to-excellent enantioselectivities of up to 97 % depending on the substrate.

The development of artificial copper enzymes from sterol carrier protein type 2 like domain (SCP-2L) for the use in asymmetric catalysis was explored. For this purpose, proteins were modified with various nitrogen donor ligands. Maleimide-containing ligands were found most suitable for selective cysteine bio-conjugation. Fluorescence spectroscopy was used to confirm copper binding to an introduced phenanthroline ligand, which was introduced in two unique cysteine containing SCP-2L mutants. Copper adducts of several modified SCP-2L templates were applied in asymmetric Diels–Alder reactions. A clear influence of both the protein environment and the introduced ligand was found in the asymmetric Diels–Alder reaction between azachalcone and cyclopentadiene. A promising enantioselectivity of 25 % ee was obtained by using SCP-2L V83C modified with phenanthroline–maleimide ligand. Good endo selectivity was observed for SCP-2L modified with the dipicolylamine-based nitrogen donor ligand. These artificial metalloenzymes provide a suitable starting point for the implementation of various available techniques to optimise the performance of this system.

A series of new monodentate phosphane ligands 2 have been evaluated in the Mizoroki–Heck arylation reaction of iodobenzene and styrene and compared with our previously reported ligands, 1, 3 and 4. The concept of rational ligand design is discussed, and we describe how the performance of this new ligand family could be predicted. Employing our best ligand, 3,3′-di-tert-butyl-5,5′-dimethoxybiphenyl-2,2′-diyl diisopropylphosphoramidite (3b), we explored the scope of the reaction with regards to solvent and the substrate. We also investigated the electronic dependence of the reaction by analysing the relationship between the rate and Hammett constant. Sufficient steric bulk is required to enforce the catalytic reaction to proceed through the mono-coordinated palladium species, thereby increasing its reactivity. The electronic properties determine the concentration of the active species from the monomer dimer equilibrium and their intrinsic reactivity. The cyclic phosphoramidite 3b provides an optimum in both properties within the systems studied, resulting in a rate limiting migratory insertion step.

Many bioinspired transition-metal catalysts have been developed over the recent years. In this review the progress in the design and application of ligand systems based on peptides and DNA and the development of artificial metalloenzymes are reviewed with a particular emphasis on the combination of phosphane ligands with powerful molecular recognition and shape selectivity of biomolecules. The various approaches for the assembly of these catalytic systems will be highlighted, and the possibilities that the use of the building blocks of Nature provide for catalyst optimisation strategies are discussed.

The cover picture shows the formation of a 15-base-long DNA strand from individual nucleosides. The light-blue nucleoside has been modified so that it can form an amide bond with a phosphanylcarboxylic acid, once the strand has been prepared. In this way, artificial “metallo-DNAzymes” can be developed by complex formation of transition metals with phosphane-modified oligonucleotides, bridging the cap between homogeneous and biocatalysis. This is symbolised in the background by the famous Swilcan Bridge at the 18th hole of the Old Course of St Andrews, Scotland. Details are discussed in the article by P. C. J. Kamer et al. on p. 3229 ff.

Seven phosphane-functionalized deoxyuridines have been prepared by amide bond formation between aminodeoxyuridines and phosphanylcarboxylic acids. X-ray crystal structures for two of these new modified nucleosides have been obtained. The same coupling method has been extended to oligonucleotides. The phosphane containing strands have been purified and characterized by MALDI-TOF and, for the first time, ³¹P NMR spectrometry. Coordination of a phosphane-modified 15-mer to a [PdCl(η³-allyl)] moiety has been confirmed by ³¹P NMR spectroscopy.

A family of monodentate polystyrene-supported phosphites, phosphoramidites and phosphanes has been prepared and evaluated as ligands in rhodium-catalysed asymmetric hydrogenation and palladium-catalysed asymmetric allylic alkylation. The supported ligands yielded active and enantioselective catalysts, which in selected cases match the performance of the nonsupported counterparts. As expected, the performance of the supported ligands in the rhodium-catalysed hydrogenation depends on the nature of the ligand, the type of polymeric support, as well as on the substrate. Additionally, the supported ligands have been applied in the monodentate ligand combination approach, by combining them with nonsupported monodentate ligands. The partially supported heteroligand combinations possess different catalytic properties than the related nonsupported combinations. The heteroligand species, however, are not formed selectively, and nonsupported homoleptic complexes also contribute to the overall activity. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The coordination mode in a metal complex is critically dependent on the ligands surrounding the metal, on the precursors used, and on the conditions during synthesis. Our goal was to obtain palladium compounds with pyridine-phosphane ligands for catalysis. Therefore, we synthesized ligands 1a–d, which differ in the bulky aryl substituent at the pyridyl moiety. The palladium complexes 6a–d of these ligands are insoluble and have been characterized by various techniques, including solid-state NMR and (for 6a, b, and d) single-crystal X-ray diffraction, showing that bimetallic complexes are formed in which two ligands span two palladium centers. The configuration around these centers is determined by the steric bulk of the ligand. In complexes 6c,d, with the ligands bearing the largest steric groups, both centers have a trans configuration of the methyl and chloride anions. With intermediately sized pyridyl substituents, complex 6b is formed, having one cis surrounded and one trans-surrounded palladium center in the molecule. This kind of complexation has not been observed before. With the least bulky ligand, complex 6a is formed. Depending on the synthetic conditions employed, the methyl and the chloride in this complex can be in a cis configuration at both palladium atoms, or show the unique cis-trans coordination.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)