Angewandte Chemie International Edition

Bleaching catalysts accelerate the removal of a variety of unwanted chromophores present in stains or wood pulp. The cover picture shows the structures of lignin residues from wood pulp and of lycopene which is found in tomato stains. Peroxo–metal and oxo–metal species are proposed as intermediates in the various catalytic cycles. In their Review on page 206 ff. R. Hage and A. Lienke show that a large variety of transition-metal complexes enhance the activity of H₂O₂ or O₂ towards stain or wood-pulp bleaching.

Doubling up: Converging results from theory and experiments on model systems as well as the nitrogenase active site have recently given new clues as to how it is possible to split the NN bond under ambient conditions during the conversion of nitrogen into ammonia. Quantum chemical methods have been shown to be key to studying reaction mechanisms and unstable intermediates (see graph) which are difficult to characterize experimentally.

The key step in the gold-catalyzed cycloisomerization of enynes is the nucleophilic attack at the Au-coordinated CC triple bonds to form a vinylic gold intermediate (see example). The homogeneous gold-catalyzed reaction of enynes under mild conditions has wide scope and high efficiency and gives access to complex or otherwise inaccessible products.

Stain removal: The bleaching of textiles and paper are amongst the most important industrial oxidation processes. Chlorine-based bleaching agents, however, can damage the fibers and lead to chlorinated waste products, and H₂O₂ is expensive and less efficient. Transition-metal catalysts based on Fe, Mn, or Co, which activate H₂O₂ or O₂ (see picture), offer an alternative. In this way, it should be possible to achieve savings both in terms of the amount of chemicals and the energy required for bleaching processes.

Vive la différence! The binding of small molecules of pharmaceutical interest to DNA can be screened by differential-frequency saturation transfer difference spectroscopy (DF-STD). The technique allows the probing of DNA recognition phenomena, such as those of base-pair intercalators, minor groove binders, and external backbone binders (see picture).

Carbon-assisted thermal coevaporation of In₂O₃ and GeO₂ generates monocrystalline, sub-micrometer In₂Ge₂O₇ tubes with a laminar crystal structure (see figure). Within an In₂Ge₂O₇ tube, the {InO₆} octahedron laminae are parallel to the [100] growth direction. These structures may find applications in waveguides or microlasers since their diameters are close to the wavelength and mean free path of appropriate photons.

Making the right substitution: A remarkable acceleration effect of phenyl-substituted silyl groups in hydroalumination and carbolithiation reactions of propargylic alcohols (see formula) is described. The results show that an aryl silyl group can act not only as a protecting group but also as an activating group of the alkynyl moiety. This work provides an efficient group-selective approach to multifunctionalized alkenes.

Inorganic cluster anions (Zintl ions) have been cross-linked by transition metals in the presence of alkyl ammonium surfactants and gold substrates to produce nanostructured thin films (see picture). These films display a variety of phases, including hexagonal, cubic, lamellar, and wormlike. Optical measurements show that the films are semiconductors, and I–V measurements indicate rectifying behavior.

In a twist: Encapsulation of a nine-residue peptide (Trp-Ala-Glu-Ala-Ala-Ala-Glu-Ala-Trp) within a bowl-shaped coordination host in water induces and stabilizes the α-helical conformation. The α-helical peptide is recognized through two types of host–guest interactions: a hydrophobic interaction with both terminal Trp residues and electrostatic interactions between the Glu residues and the high positive charge of the host (12+).

Copper crowned: A trigonal-bipyramidal [Cu(H₂O)₅]²⁺ ion, which is inserted between two crown ether molecules ([18]crown-6) to form a unique supramolecular sandwich, [Cu(H₂O)₅([18]crown-6)₂]²⁺ (see picture), has been characterized unambiguously by single-crystal X-ray diffraction. Isopolyanions, [Mo₆O₁₉]²⁻, facilitate the formation of this new structure.

Absolutely without auxiliaries: The enantioselective syntheses of both enantiomers of the dimeric pyrrole–imidazole alkaloids sceptrin and ageliferin have been achieved by a non-auxiliary-based route, which allows assignment of the absolute configuration of natural ageliferin. The “programming” of the oxaquadricyclane fragmentation leading to enantiopure tetrasubstituted cyclobutanes is the crucial step in the synthesis.

From aldehydes to allenic alcohols: Studies into the asymmetric catalysis of the Nozaki–Hiyama allenylation are described. Ligand 1 a is effective for the enantioselective allenylations of various aldehydes, with the generation of products with ee values ranging from 72–83 %. The products can be easily desilylated or used as allenylsilanes. DIPEA=diisopropylethylamine, DMI=1,3-dimethyl-2-imidazolidinone, DMS=dimethylsilyl, TMS=trimethylsilyl.

A radical change: α-Phenylselenyl-β-ketoesters (e.g., 1) undergo radical cyclization in the presence of a chiral Lewis acid complex Mg(ClO₄)₂–3 with Et₃B/O₂ as the radical initiator. Monocyclic compounds, such as 2 (or bicyclic products from diene precursors), were obtained in good yields (33–82 %) with 65–97 % ee in this first enantioselective PhSe-group-transfer radical cyclization.

A matter of concentration: Cyclic pseudorotaxanes are formed by the addition of copper(I) ions to a ligand consisting of a ring containing a chelating unit and, appended to the ring, a rigid fragment containing another chelating unit. The tetramer is formed as the major product in concentrated solution, and the analogous trimer prevails in more dilute solution; both species are connected through a concentration-dependent slow equilibrium.

A nonaqueous approach that involves the reaction of tungsten isopropoxide with benzyl alcohol leads to tungsten oxide nanowire bundles without the use of any additional structure-directing templates. The bundles consist of crystalline nanowires with highly uniform diameters of about 1 nm and aspect ratios exceeding 500 (see picture). Gas-sensing tests show a high sensitivity to NO₂.

In three steps to ruthenium-based heterogeneous catalysts: The central element of this novel and simple route is the immobilization of Ru⁰ nanoparticles on montmorillonite (see picture; the arrows indicate a Ru nanoparticle intercalated in the interlaminar space of montmorillonite) with the help of an ionic liquid. The resulting catalyst is highly efficient for the hydrogenation of benzene.

Read the fine print: Biomolecular binding events can be counted with a readout system based on compact disc (CD) pickup technology. An immunoassay of C-reactive protein was performed by microcontact printing of antibodies onto a support, autometallography, and imaging by a CD reader mounted on an optical microscope (see picture). The dynamic range was four orders of magnitude with a detection limit of 1 pM.

All sorts of aldehydes 2 (R=aryl, vinyl, branched, and linear alkyl) undergo the highly enantioselective addition of diphenylzinc (1) in the presence of catalytic (S)-4 to give α-substituted benzyl alcohols 3 in high yields. In contrast to previous catalysts, no additive is required, the reaction is carried out at room temperature, and the reaction is equally effective with linear aldehydes.

Free trade: The electron exchange of cytochrome c freely diffusing to gold electrodes modified by hydroxy-terminated alkanethiol self-assembled monolayers (SAMs; see picture) is examined by high-pressure kinetic studies. The SAM thickness, solution viscosity, and hydrostatic pressure influence a changeover in the charge-transfer mechanism and the corresponding kinetic response.

Zero- and one-dimensional metal carboxylates are shown to transform into higher-dimensional structures (see figure), thereby indicating the presence of a progressive building-up process.

A double-barreled approach: The addition of ferrocene to an antiestrogen drug skeleton was found to induce cytotoxicity towards breast-cancer cells that are resistant to the common antiestrogen drug. The efficacy of this drug has been related to the proton-coupled electron transfer observed in the presence of pyridine as a base (see picture).

Free to rotate: The rotational spectrum of unlabeled and ¹³C-labeled benzene species reveals that the benzene–trifluoromethane complex is stabilized by a CH⋅⋅⋅π weak hydrogen bond and undergoes an almost-free rotation of the two subunits with respect to each other (see picture). The estimated dissociation energy is 8.4 kJ mol⁻¹.

From radical to biradical: The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of the C₅H₅ radical is recorded after direct excitation from the X̃ ² ground state and two-photon excitation via the à ² electronically excited state. The intensity distributions of the spectra enables the characterization of the energetics, symmetry, and structure of the lowest two electronic states of the cation (see picture).

Mild attachment: Proteins can be ligated chemoselectively and immobilized on functionalized glass surfaces by means of Diels–Alder cycloadditions (see scheme).

Seeing the light: The use of a coupled assay with bacterial luciferase results in a common light-emission screen for a variety of potential biotransformations. Screening the E. coli proteome for the oxidation of simple alcohols to good and bad luciferase substrates reveals examples of substrate ambiguity and catalytic promiscuity. Reactions leading to aldehydes can easily be screened by this method.

Mission accomplished. The fungal metabolite diversonol has been synthesized for the first time. A key step in the total synthesis was the domino oxa-Michael–aldol condensation of salicylic aldehyde 1 and 4-hydroxycyclohexenone (2) that afforded the tricyclic scaffold of the target compound.

No reagents and very mild conditions are required for the transformation of isolable precursors 3 into highly strained cycloalkynes 4 (cycloheptyne, cyclohexyne, norbornyne), which can be trapped by cycloaddition reactions.

Three in one: Ordered nanostructures of SnSe, SnO₂, and elemental tin (see SEM image) were prepared by template-assisted synthesis in porous alumina and macroporous silicon. The use of the templates as chemical reactants allowed the preparation of all three target materials from the same single-source precursor, Sn(SePh)₄.

Loaded dice: Copper cubanes 1 and ent-1 were generated in a one-pot reaction by diastereoselective self-assembly, starting from ligands derived from L- and D-tartaric acid, respectively. Both structures were confirmed by X-ray structure analysis.

An independent jump: Excess electrons move through DNA by using a hopping-type mechanism in which the pyrimidine bases dT and dC act as ‘stepping stones’. It was shown that GC base pairs, in contrast to AT base pairs, lower the efficiency of the excess-electron transfer through the duplex. Through the use of the shown DNA hairpins, three different electron acceptors (

) were investigated with the electron donor, reduced flavin (

).