Angewandte Chemie International Edition

Enzymatically synthesized symmetrical trehalose diesters self-assemble into fibrous gel networks in a wide range of organic solvents and at very low gelator concentrations. Gels containing acrylate functionalities can be further stabilized through polymerization. In the presence of added water and the same enzyme, the gels degrade into trehalose, as shown in the cover picture. In their Communication on page 4772 ff., J. S. Dordick et al. describe how combining the principles of supramolecular chemistry and the selectivity of biocatalysis represents a powerful strategy to develop defined functional materials.

Symmetry and molecular recognition: The presence of C₃ symmetry in nature has recently inspired the first biomimetic C₃-symmetric catalyst for asymmetric syntheses (see picture). Threefold symmetry also features in a recent approach to modulating the function of the tumor necrosis factor receptor superfamily.

Symmetry and chiral recognition: In contrast to previous arguments, recent reports have demonstrated that C₃-symmetric receptors can bind chiral substrates with high enantioselectivity (see picture: H=host, G=(S) or (R)-configured guest; colored spheres represent different-sized substituents).

Why so slow? Why did it take several decades for the concept of selectivity to develop into a fundamental consideration in organic chemistry? The Essay describes the reception by the scientific community in the first half of the 20th century of the principles of kinetic versus thermodynamic control and explains the physical basis of selectivity (see picture: energy diagram for the reaction of pentamethylenefulvene (A) and maleic anhydride (B)).

Mirror, mirror on the wall, who is the best asymmetric catalyst of all? The field of heterogeneous asymmetric catalysis is multifarious, vibrant, and creates new possibilities. For example, the use of mesoporous solids (see picture) allows selective immobilization in the pores, which results in improved catalytic properties in a number of cases. The most important methods are discussed in the Review.

Face to face: The hexagonal {001} or {00} faces of single crystals of L-cystine were selectively decorated by dipping them into a solution of gold nanoparticles (see picture). The selectivity results from an anisotropic molecular packing of L-cystine in the single crystal.

An inside job: A Lewis acid induced intramolecular Diels–Alder (IMDA) reaction is used as the key step in the formation of symbioimine, an iminium alkaloid. The intermediate octahydronaphthalene is obtained from the IMDA reaction, after which extension of the aldehyde to a nitrile, alkylation of the nitrile, and imine formation allows for an efficient route to the target compound (see scheme; TBS=tert-butyldimethylsilyl).

The long and short of it: A regioselective enzyme-catalyzed acylation of the disaccharide trehalose generated a family of low-molecular-weight gelators with unprecedented properties. The selectivity of enzymatic catalysis enables direct control over gelation properties by simply varying the acyl-chain length to give gelation in solvents ranging from the highly hydrophilic acetonitrile to the highly hydrophobic cyclohexane.

Scaling down the activity: An efficient recyclable palladium-based catalyst has been developed for the aerobic oxidation of alcohols (see scheme). The combination of a substituted bipyridyl ligand and ordered mesoporous channels (in SBA) causes a synergistic effect that results in enhanced activity, the prevention of agglomeration of the palladium nanoparticles, and the generation of a durable catalyst.

DNA deposits with submicrometer features can be prepared by micromolding in capillaries on a mica plate (see picture). Upon evaporation, surface properties of the DNA solution and the support come to the fore. The morphology of the DNA deposit can be controlled simply by adjusting the concentrations of the DNA solution and added salt.

Highly active and selective: A supported Ru₅PtSn nanoparticle cluster (the picture shows an axial projection of a tomogram), prepared from the carbonyl cluster [PtRu₅(CO)₁₅(μ-SnPh₂)(μ₆-C)], is an excellent catalyst in the single-step hydrogenation of dimethyl terephthalate to cyclohexanedimethanol under mild conditions (100 °C, 20 bar H₂).

In groups of three: Monodisperse clusters of WO₃ were prepared on a TiO₂(110) surface through direct sublimation of WO₃ onto the substrate at 300 K followed by annealing to 600 K (see image). Combined evidence from scanning tunneling microscopy, X-ray photoelectron spectroscopy, quartz crystal mass balance, and density functional theory strongly indicates that the clusters are cyclic (WO₃)₃ species.

Sequential decoration of silica spheres by covalent bonding of magnetite nanoparticles (blue) and attachment of functional nanoparticles of Au, CdSe/ZnS, or Pd (red) afforded multifunctional assemblies exhibiting combinations of magnetism with surface plasmon resonance (Au), luminescence (CdSe/ZnS), and catalytic activity (Pd), respectively.

Communicating better over a long distance: The bis(ferrocenylethynyl)ethene 1 undergoes E→Z photoisomerization upon excitation of a charge-transfer band with visible light (546 nm). This structural change leads to a decrease in the “through-bond” mixed-valence interaction between the two ferrocene units.

Brønsted, Brassard, Diels, and Alder: Aldimines 1 undergo aza-Diels–Alder reactions with Brassard's diene 2 in the presence of a chiral cyclic phosphate pyridinium salt 4 as a chiral Brønsted acid to give α,β-unsaturated δ-lactams 3 with excellent enantioselectivity.

Caged up: A family of isopolyoxotungstate clusters of the form [H₄W₁₉O₆₂]⁶⁻ with a cluster cage (blue) identical to that of the Dawson-type heteropolyacids (red) has been discovered. These comprise a single trigonal-prismatic or octahedral {WO₆}⁶⁻ moiety in place of the two heteroanions found in the Dawson structure.

Biofunctionalized ceramic nanowires: A reactive ester polymer has been used to immobilize silicatein, a hydrolytic enzyme involved in the biomineralization of SiO₂, on the surface of TiO₂ nanowires. The surface-bound protein retains its original hydrolytic properties and also acts as a reductant for AuCl₄⁻ in the synthesis of hybrid TiO₂/silicatein/Au nanocomposites.

Deck the walls: The principles of coordination chemistry and multidentate ligand design have been used to functionalize the surface of highly inert MoS₂ nanoparticles. In this approach the tetradentate nitrilotriacetic acid ligand was coupled either to a fluorescent ligand (for detection) or to a reactive polymer, which serves as an anchor to the sulfide and oxide surfaces of inorganic fullerene/MoS₂ and TiO₂ nanorods, respectively (see picture).

Sensitive ELISA: A new enzyme-labeled immunosorbant assay (ELISA) protocol has been developed that makes use of two enzymatic reactions to detect antibody–antigen interactions: 1) the conversion of prothrombin to thrombin by ecarin and 2) the hydrolysis of a nonfluorescent substrate 1 to a fluorescent product 2 by thrombin. The system was used to detect bovine serum albumin, telomerase, and DNA with high sensitivity.

Shiny silver: A superstructure consisting of Ag nanoparticles and CdTe nanowires connected by a streptavidin (SA) and D-biotin (B) affinity pair displays twofold enhancement of the nanowire luminescence. The optical process involved could also be operative in other metamaterials and could serve as a basis for applications in a variety of optoelectronic devices.

Nobody does it better: A comparative study indicates that water, rather than room-temperature ionic liquids (RTILs), is still the solvent of choice for accelerating Diels–Alder reactions. Both the hydrogen-bonding ability and the viscosity of the solvent are thought to play a role.

OR or OOR? The oxygenation of ZntBu₂ in the presence of donor ligands demonstrates the tendency of zinc dialkyls to undergo oxidation of only one ZnC bond under controlled conditions (see scheme). The formation of alkoxide or peroxide species is influenced by the type of donor ligand employed, and these divergent pathways offer a glimpse into the general mechanism of dioxygen activation.

Straightened out: The first binary Group 5 azides were prepared by fluorido–azido exchange starting from MF₅ (M=Nb, Ta) and characterized by vibrational spectroscopy. The crystal structure of the acetonitrile adduct [Nb(N₃)₅(CH₃CN)] (see picture) provides the first experimental evidence for the existence of linear M-N-N coordination for azido complexes.

Ru₂can do it! Substitution of the OH moiety in propargylic alcohols by hydride proceeds smoothly with triethylsilane by catalysis with the thiolate-bridged diruthenium complex 1 (see scheme; Cp*=η⁵-C₅Me₅). This reaction gives the corresponding alkynes in good to high yields with complete selectivity, in contrast to a monoruthenium-catalyzed hydrosilylation of propargylic alcohols with silanes.

Just add a twist: Metallosupramolecular architecture design is used to create anti-cancer agents with high activities in human breast cancer cell lines. Three isomers, which comprise a nonhelical metallocyclophane, an unsaturated double helicate, and a new type of double helicate, bridge the fields of metallosupramolecular architecture and anticancer drug design (picture: double-helical trans/cis isomer; red Ru, green Cl).

Designer elegance: The transannular aldol reaction of a cyclooctene diketone is the key step in this total synthesis of the natural enantiomer of merrilactone A (see scheme). The configuration of the two stereocenters generated in the formation of the central bicyclo[3.3.0]octane framework of the natural product was established using a specially designed bulky protecting group.

Copper makes the difference: An umpolung reactivity of Michael acceptors is observed when alkenyl carbene complexes react with copper ketone enolates and electrophiles. This reaction allows the straightforward synthesis of functionalized three-component coupling products in a completely regio- and diastereoselective way.

Attachment of low-energy electrons to D-ribose triggers a series of complex decomposition reactions associated with the loss of neutral water molecules as well as the excision of C-containing units leading to the degradation of the cyclic structure of the sugar. This excision of C-containing neutral fragments involves C5 exclusively. The sugar unit is thought to play a key role in the mechanism of DNA damage by low-energy electrons.

Help from the neighbors: Palladium-catalyzed allylic alkylations are extremely suitable for the stereoselective introduction of unsaturated side chains to peptides (see scheme; TFA=trifluoroacetate). The chiral information of the peptide can be used to control the formation of the new stereogenic center. In general, S amino acids induce the formation of R-configured amino acids.

Size does matter: When treated with Me₂AlCl, highly soluble bis(aluminate) complexes [Cp*Ln(AlMe₄)₂] (Ln=Y, La, Nd; Cp*=C₅Me₅) undergo intrinsic alkyl/chloro ligand-exchange reactions to give products of variable Ln nuclearity, depending on the size of the Ln^III metal. The reproducible formation of Y₂Al₂, La₆Al₄, and Nd₅Al heterobimetallic alkyl clusters leads to novel AlMe₄ coordination modes (see picture: La₆Al₄ cluster).

Illuminating results: A novel microwave-assisted synthesis in ionic liquids produces highly luminescent and dispersible LaPO₄:Ce,Tb nanocrystals (see picture: ethanol dispersion of the nanocrystals in daylight (A) and with UV excitation (B)). This preparation method offers the possibility of synthesisindependent surface conditioning of the nanocrystals.