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A series of new monodentate phosphane ligands 2have been evaluated in the Mizoroki–Heck arylation reaction of iodobenzene and styrene and compared with our previously reported ligands, 1, 3and 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 3bprovides an optimum in both properties within the systems studied, resulting in a rate limiting migratory insertion step.

The coordination geometry around a platinum atom can be controlled by a diphosphane capsule composed of a tetracationic xantphos‐type ligand and a tetraanionic calix[4]arene. Reaction of the diphosphane capsule with the platinum precursor [PtCl2(MeCN)2] yields the bisligated biscalix[4]arene trans‐platinum capsule, whereas the same diphosphane in the absence of calix[4]arene prefers the formation of the monoligated cis‐platinum complex, as indicated by 31P{1H} NMR and ESI‐MS. The two calix[4]arenes stabilize the kinetic product (trans‐Pt), and slow the formation of the thermodynamic product (cis‐Pt).

Three phosphanes containing azobenzene groups were synthesized, and the photochemical behaviour of their platinum complexes was studied by using the lightinduced EZ isomerization as a switch to change ligand properties.

The reaction of [CpFeη5C5H4–CH=N–CH2n–C4H3S] Cp = η5C5H5 and n= 1 1b or 2 1c with Na2[PdCl4] and NaAcO·3H2O gave [CpFeμη5Feη1PdC5H3–CH=N–CH2n–C4H3SCl] n= 1 2b or 2 2c with a [Csp2,ferrocene,N,Sthienyl]–ligand. The study of the reactivity of 2bor 2cwith PPh3under different experimental conditions has afforded the isolation of [CpFeμη5Feη1PdC5H3–CH=N–CH2n–C4H3SClPPh3] n= 1 3b or 2 3c and[CpFeμη5Feη1PdC5H3–CH=N–CH2n–C4H3SPPh3][BF4] n= 1 4b or 2 4c where 1bor 1cbehave as a bidentate [Csp2,ferrocene,N]–in 3band 3c or as a terdentate [Csp2,ferrocene,N,Sthienyl]–in 4band 4c ligand. The crystal structures of 3b·12CH2Cl2, 3c·2H2O and 4c·CH2Cl2confirm the mode of binding of the ligands. The results obtained from these studies and the solution behaviour of 4band 4creveal that the σPd–S bond of 2b, 2c, 4band 4cis more labile that in [CpFeμη5Feη1PdC5H3–CH=N–C6H42SMeCl] or [CpFeμη5Feη1PdC5H3–CH=N–C6H42SMePPh3][BF4] with a [Csp2,ferrocene,N,Sthioether]–ligand.

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 pyridinephosphane ligands for catalysis. Therefore, we synthesized ligands 1a–d, which differ in the bulky aryl substituent at the pyridyl moiety. The palladium complexes 6a–dof these ligands are insoluble and have been characterized by various techniques, including solidstate NMR and for 6a, b, and d singlecrystal Xray 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 transconfiguration of the methyl and chloride anions. With intermediately sized pyridyl substituents, complex 6bis formed, having one cissurrounded and one transsurrounded palladium center in the molecule. This kind of complexation has not been observed before. With the least bulky ligand, complex 6ais formed. Depending on the synthetic conditions employed, the methyl and the chloride in this complex can be in a cisconfiguration at both palladium atoms, or show the unique cistranscoordination.© WileyVCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008

Combinatorial chemistry in combination with highthroughput screening technologies is an important way of finding new successful catalyst systems. The design of ligand libraries of bidentate phosphorus ligands and the application of their transitionmetal complexes in homogeneous asymmetric catalysis reactions will be described in this review. Till now three different approaches were developed to arrive at such libraries of bidentate phosphorus ligands: 1 modular synthesis of bidentate ligands 2 the solid support synthesis of bidentate ligands and 3 the selfassembly of ligand building blocks into bidentate ligands. The scope and limitations of these strategies will be discussed on the basis of a limited number of articles that dealt with the synthesis of at least 15 bidentate phosphorus ligands. © WileyVCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008

The synthesis and resolution of chiral phenoxaphosphane 3, with the stereogenic center at the phosphorus atom, is described. Compound 3 has been synthesized following a well-known procedure for trapping a phosphorus atom within a six-membered ring. The resolution of the racemic mixture of 3 was achieved through separation of its diastereomeric palladacycle derivatives 7a,b and 9a,b. The absolute configuration of enantiopure phosphanes 3a,b was assigned unequivocally by means of X-ray crystal structure determination for complex 9a and by combination of NOE(1H–1H)/COSY(1H,1H) spectroscopy and DFT calculations for complexes 7a,b, which in both cases led to identical results. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Chiral calixarene-based diphosphite ligands 3a–d have been obtained via lower-rim functionalisation of the p-tert-butylcalix[4]arene core. High enantiomeric excesses (up to 94 %) and good activities were obtained in the rhodium-catalyzed asymmetric hydrogenation of prochiral olefins withTADDOL-containing diphosphites 3c,d. This is the firstexample of chiral calix[4]arene-modified ligands that induce high enantioselectivity in metal-catalysed asymmetric reactions.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Coordinative bonds have been used to prepare supramolecular ligands leading to well-defined catalysts formed by assembly. The construction of these ligands is based on selective metal–ligand interactions between nitrogen donor atoms of phosphorus-nitrogen building blocks and various zinc(II) porphyrins. The major advantage of this supramolecular approach of catalyst preparation is the simplification of ligand variation enabling straightforward modification of steric, electronic and chiral properties of the supramolecular ligand. A large number of new ligands becomes accessible by this modular variation of the building blocks. The ligand assembly based on pyridyl phosphites and zinc(II) porphyrin with electron-withdrawing substituents led to a twelve-fold increase in activity and an increase in enantioselectivity from 17 to 50 % in the rhodium-catalyzed hydrogenation of dimethyl itaconate. The first examples of assemblies based on non-chiral ligands and chiral zinc(II) porphyrin template molecules show, as proof of principle, an enantiomeric excess up to 18 % in the asymmetric palladium-catalyzed allylic alkylation. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

We present a review concerning the mechanism involved in the hydroxycarbonylation of styrene. We discuss the intermediates proposed in the literature, the effect of the acidic medium, the reactivity and kinetics, as well as the results of theoretical calculations on hydroxycarbonylation and related reactions. We have performed an in situ high-pressure NMR study to obtain more insight into the mechanisms that control the regioselectivity and have assessed how this is affected by the phosphorus ligand and the catalyst precursor. We observed palladium hydride and palladium acyl species and we suggest a catalytic cycle involving palladium hydride, palladium alkyl, and palladium acyl intermediates.

The bite angle of bidentate phosphane ligands has a pronounced influence on the degree of retention of the double bond geometry of the allylic substrate in the allylic alkylation reaction. To study the effect of the ligand on the regioselectivity, (Z)‐ and (E)‐pent‐2‐enyl acetate were used as substrates. The alkylation of substrates with an (E) conformation of the double bond results in the preferential formation of the linear (E) product. A larger bite angle of the ligand results in an increase of the regioselectivity to >98% for the Sixantphos ligand. Analogously, the alkylation of (Z) substrates results in the formation of the linear (Z) product. Remarkably, for (Z) substrates, a larger bite angle of the ligand leads to an increased regioselectivity for the formation of the branched product instead of the linear product, up to 47.5% for Sixantphos. The observed regioselectivities are rationalized in terms of: a) a competition between syn‐antiisomerization and alkylation, and b) a combination of steric and electronic effects in the transition state of the reaction. For all ligands tested, the reaction is faster for the (E) than for the (Z) substrate. However, competition experiments using the Sixantphos ligand show a relatively fast reaction rate for the (Z) substrate, which indicates that the coordination of the substrate to palladium is the discriminating, but not the rate‐determining, step when both substrates are present.

The cover picture showsthe lightinduced isomerization of E,E,Etrisazobenzene?phosphane to the Z,E,E, Z,Z,E and Z,Z,Z conformers. This phototriggered skeletal change has been illustrated by the natural umbrellalike shape and movements of jellyfish. Details of the synthesis and switchability of this type of ligands and their platinum complexes are discussed in the Short Communication by Z. Freixa et al. on p. 2075 ff.

The cover picture showsstructures that represent three different ways of making phosphorusbased ligand libraries: modular synthesis, solidphase synthesis and formation of ligands by assembly of smaller building blocks. These ligand libraries are important for the rapid identification of new catalysts for asymmetric conversions. In this Microreview on p. 2939 ff J. N. H. Reek et al. show examples that illustrate these strategies.