Metal-catalyzed enantioselective allylation, which involves the substitution of allylic metal intermediates with a diverse range of different nucleophiles or S(N)2'-type allylic substitution, leads to the formation of C-H, -C, -O, -N, -S, and other bonds with very high levels of asymmetric induction. The reaction may tolerate a broad range of functional groups and has been applied successfully to the synthesis of many natural products and new chiral compounds.

Metal-catalyzed enantioselective allylation in asymmetric synthesis.

Multivalent interactions can be applied universally for a targeted strengthening of an interaction between different interfaces or molecules. The binding partners form cooperative, multiple receptor-ligand interactions that are based on individually weak, noncovalent bonds and are thus generally reversible. Hence, multi- and polyvalent interactions play a decisive role in biological systems for recognition, adhesion, and signal processes. The scientific and practical realization of this principle will be demonstrated by the development of simple artificial and theoretical models, from natural systems to functional, application-oriented systems. In a systematic review of scaffold architectures, the underlying effects and control options will be demonstrated, and suggestions will be given for designing effective multivalent binding systems, as well as for polyvalent therapeutics.

Multivalency as a chemical organization and action principle.

The Engineering and Physical Sciences Research Council and the European Research Council are acknowledged for financial support.

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Stimuli-Responsive Metal–Ligand Assemblies

This critical review describes recent developments in the field of supramolecular chemistry of terpyridine–metal complexes. The synthesis and characteristics of single as well as multiple homo- and heterometallic complexes is discussed. Furthermore, complexes containing fullerenes, biological building blocks, extended aggregates of different architectures as well as rings are presented. A special emphasis is placed upon the properties (e.g. redox properties, luminescence etc.) of functional systems. Potential applications in optical nano-devices, molecular storage units, molecular switches and solar cells are discussed.

Recent developments in the supramolecular chemistry of terpyridine–metal complexes

Recent advances in supramolecular coordination chemistry have allowed chemists to synthesize macromolecular complexes that exhibit various properties intrinsic to enzymes. This Review focuses on structures inspired by properties and functions observed in enzymes rather than precise models of enzyme active sites. These structures are synthesized using convergent, modular, and high-yielding coordination-chemistry-based methods, which allow one to tailor the size, shape, and properties of the resulting complexes. Many of the structures discussed exhibit reactivity and specificity reminiscent of natural systems, and some of them have functions that exceed the natural systems which provided the inspiration for initially making them.

Enzyme Mimics Based Upon Supramolecular Coordination Chemistry

The binding of lead(II) cations to the terpyridine-type subunits of the helical ligand 1 leads to the self-assembly of different polynuclear metallosupramolecular architectures of nanometric size. Three different entities are generated and may be interconverted as a function of metal/ligand stoichiometry: a [4 x 4]Pb(16)(II) grid-type array 2, a [4 # 4]Pb(12)(II) double-cross species 4, and an intermediate complex 3. The structures of 2 and 4 have been confirmed by X-ray crystallography; that of 3 is based on NMR spectral data. The interconversion of the three species generates dynamic diversity and represents an expression of constitutional dynamic chemistry. In the course of ion binding, the helical molecules of ligand 1 unwrap to yield fully extended strands arranged in perpendicular fashion in the architectures 2-4 generated. This process amounts to molecular motions in two directions which confer to the present systems characteristics of two-dimensional nanomechanical devices, capable of performing 2D-contraction/extension motions. The triple features of self-organization, dynamic interconversion, and potential addressability displayed by the processes described trace a self-fabrication approach to nanoscience and nanotechnology.

Self-assembly, structure, and dynamic interconversion of metallosupramolecular architectures generated by Pb(II) binding-induced unfolding of a helical ligand.

The interaction of appropriate metal ions (Pb(II), Zn(II)) with helical ligand strands, obtained by hydrazone polycondensation, generates polymetallic supramolecular architectures of rack and grid types, by uncoiling of the ligand. The interconversion between the helical free ligand and the linearly extended ligand in the complexes produces reversible ion-induced, nanomechanical molecular motions of large amplitude. It has been integrated in an acid-base neutralisation fuelled process, which links the extension/contraction of the ligand strands to alternating changes in pH.

Formation of RACK- and grid-type metallosupramolecular architectures and generation of molecular motion by reversible uncoiling of helical ligand strands.

Mechanistic features, cooperativity, and robustness in the self-assembly of multicomponent silver(I) grid-type metalloarchitectures.

Reaction of the lithium triamidostannate [MeSi{SiMe 2 N(p-Tol)} 3 -SnLi(OEt 2 )] (1) with 0.5 molar equivalents of MCl 2 (M=Zn, Cd, Hg) in toluene afforded the corresponding heterodimetallic complexes [MeSi(Si-Me 2 N(p-Tol)} 3 Sn] 2 M {M=Hg (2), Cd (3), and Zn (4)}. The molecular structures of the mercury and cadmium complexes were determined by X-ray diffraction and found to adopt a linear Sn-M-Sn metal-metal bonded array (d(Sn-Hg) 2.6495(2), d(Sn-Cd) 2.6758(1) A), these being the first Hg-Sn and Cd-Sn bonds to be characterized by X-ray diffraction. That the Hg-Sn bonds are shorter than the Cd-Sn bonds in the isomorphous complexes is attributed to relativistic effects in the mercury system. In contrast, the structure of the Zn analogue is unsymmetrical with one stannate unit being Sn-Zn bonded (d(Sn(1)-Zn) 2.5782(4) A), while the Zn" atom bridges two amido functions of the second stannate cage, thus representing a second isomeric form of these complexes. The different degree of metal-metal bond polarity is reflected in the 1 1 9 Sn NMR chemical shifts of the three complexes. Variable-temperature NMR studies and a series of 1 H ROESY experiments of the cadmium complex 3 in solution revealed a dynamic exchange between the two isomers.

Metal–Metal Bonds between Group 12 Metals and Tin: Structural Characterization of the Complete Series of Sn‐M‐Sn (M=Zn, Cd, Hg) Heterodimetallic Complexes

The chiral diphospholane ligands Duphos (1) and Duxantphos (2), which can be differentiated by their bite angle, were applied to palladium-catalysed asymmetric reactions, essentially allylic alkylations with symmetrically (3a−d) or unsymmetrically (5a,b) substituted allylic acetates as substrates. The most interesting results were found with the first series of substrates in which 2 was more reactive and/or selective than 1. The model complexes of the catalytic intermediates [Pd{(S,S)-1}(η3-cyclohexenyl)](BF4) (10) and exo-[Pd{(R,R)-2}(η3-cyclohexenyl)](SbF6) (11) could be characterised by X-ray crystallography. 1H and 31P NMR spectroscopic analyses of [Pd{(R,R)-2}(η3-diphenylallyl)](SbF6) (12) and [Pd{(R,R)-2}(η3-allyl)](SbF6) (13) revealed the presence of exchanging isomers in solution. Structural data provided by these techniques combined with the preferential rotation model led to a satisfactory interpretation of the catalytic allylic alkylation results. It appears that the bite angle plays a crucial role in the positioning of the proximal methyl groups, which determines the distribution of allyl isomeric intermediates and the balance between clockwise and anticlockwise rotations in the enantioselective step. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Roland Graff

Papers

Self-assembly, structure, and dynamic interconversion of metallosupramolecular architectures generated by Pb(II) binding-induced unfolding of a helical ligand.

Formation of RACK- and grid-type metallosupramolecular architectures and generation of molecular motion by reversible uncoiling of helical ligand strands.

Mechanistic features, cooperativity, and robustness in the self-assembly of multicomponent silver(I) grid-type metalloarchitectures.

Metal–Metal Bonds between Group 12 Metals and Tin: Structural Characterization of the Complete Series of Sn‐M‐Sn (M=Zn, Cd, Hg) Heterodimetallic Complexes

Palladium Complexes with Chiral Diphospholane Ligands: Comparative Catalytic Properties and Analysis of (η3‐Allyl)palladium Species