Angewandte Chemie International Edition

Chromation and ferration are the latest additions to the concept of alkali-metal-mediated metalation, as described by J. Klett, R. E. Mulvey, and co-workers in their Communication on page 3317 ff. While the more electropositive sodium is essential for the reaction, it is the less electropositive chromium or iron that actually performs deprotonation of benzene. This novel reactivity can be likened to a game of chess in which the queen (Na) holds the king in check, while the knight (Cr, Fe) scores checkm(etal)ate.

Ein Bisoxazolin-Ligand erhöht die Präferenz von Pd^II für π-Systeme. Die direkte Umwandlung von terminalen Alkinen in β-Methoxyacrylate mit Komplexen solcher Bisoxazolin(box)-Liganden wird von K. Kato und Mitarbeitern in der Zuschrift auf S. 3326 ff. beschrieben. Dichtefunktionalrechnungen belegen, dass die π-Orbitale des Liganden in [Pd(box)(TFA)₂] über die Doppelbindungen und die Peripherie delokalisiert sind und energetisch nahe dem LUMO liegen. Die Reaktion kann auch in einer Synthese von Kawain genutzt werden.

“My biggest motivation is to see my students′ progress. When I was eighteen I wanted to be unexcitingly, a chemist…!” This and more about Stefan Mecking can be found on page 3213.

John Wiley & Sons, Hoboken 2008. 260 pp., softcover € 47.90.—ISBN 978-0470060544

A different drummer: The existence of endohedral germanium clusters was predicted earlier by gas-phase experiments. The [Co@Ge₁₀]³⁻ anion now synthesized is surprising, as it breaks with a long line of exclusively deltahedral structures found in the past. Instead, it has a regular pentagonal-prismatic structure (see picture; Co gray, Ge red).

Long-distance service: Finding a selective small-molecule modulator for every domain of every human protein is a Herculean task. Wouldn't it be much easier to control the activity of any protein of choice by simply fusing it with a generally adaptable switch protein and controlling protein activity indirectly?

Adding up: A recent total synthesis of bryostatin 16 is highlighted by two transition-metal-catalyzed addition reactions. The first forges the B ring while establishing its relative stereochemistry. The second closes the macrocycle with a rare isomerization. These reactions neither require substrate activation nor do they generate byproducts. The powerful macrocyclization should be considered alongside more traditional methods.

Mimicking nature synthetically: The successful development of multistep stereoselective syntheses gives access to natural product inspired compound collections having carbo-, oxa-,and azacyclic scaffold structures which promise to provide sources for new reagents in medicinal chemistry and chemical biology research.

Two photons are better than one: This principle applies to a wide range of applications, ranging from engineering to physiology. Recent advances in our understanding of the phenomenon of two-photon absorption (see picture) and in the design of two-photon dyes are rapidly increasing the scope of this field.

One-nucleotide differences in microRNAs (miRNAs) can be discriminated in an assay based on a branched rolling-circle amplification (BRCA) reaction and fluorescence quantification. With the proposed method miRNA can be detected at concentrations as low as 10 fM, and the miRNA in a total RNA sample of a few nanograms can be determined.

Quick on the uptake: Following its identification during a targeted search, the intriguing crystal structure of 3,3′,4,4′-tetra(trimethylsilylethynyl)biphenyl was investigated. Simple removal of the included solvent provides an organic crystal with an open microporous structure that has a striking similarity to that of zeolite A (see picture). Reversible adsorption of nitrogen and hydrogen gases at 77 K confirms that the microporosity is permanent.

Grinding them down: By using a tailor-made additive, even in the absence of racemization in solution, abrasive grinding can yield an enantiopure solid state. This novel chiral resolution technique is based on an asymmetric bifurcation in the crystal size distribution as a result of stereoselective hampered crystal growth. R=o-tolyl.

A trap that closes with a “click”: The copper-catalyzed azide–alkyne cycloaddition can occur in different G-quadruplex structures (see scheme). The species trapped by the click reaction can then be separated and analyzed. By using this approach, a DNA–RNA hybrid-type G-quadruplex structure formed by human telomeric DNA and RNA sequences was detected.

Pass the salt, please! State-of-the-art computations indicate that the stacking complex of a guanine quartet and an adenine quartet (G₄A₄) can function as a potent ditopic receptor for NaCl in aqueous solution (see picture; Na⁺, Cl⁻ yellow, O red, N blue, C black, H white).

Reactivity NOwn: Pyridine N-oxides can be used for the complete regio- and stereoselective synthesis of trans-substituted piperidines. The sequential addition of Grignard reagents and aldehydes or ketones to pyridine N-oxides yields a complete regio- and stereoselective trans 2,3-addition reaction in high yields, and the substituted 2,3-dihydropyridine N-oxide can be reduced to form 2,3-trans-substituted piperidines (see scheme).

Show me your angle: Incorporation of the rigid spin label Ç allows determination of both distance and orientation of two nitroxide spin labels in DNA by PELDOR experiments at common X-band frequencies. The orientational information is obtained by varying the position of the detection pulses over the nitroxide spectrum. Simulation of the set of time traces yields very precise distances and angles.

A window of opportunity: A general copper-catalyzed CH bond-activation path allows arylation of heterocycles with a wide range of aryl bromides (see scheme). The reaction shows excellent regioselectivity and exhibits good functional group tolerance. The 8-aryl xanthines exhibit fluorescence in a variety of solvents and show promise as reagents for biological imaging.

A designer monomeric protein with a βαβ fold—two parallel β strands connected by an α helix (see structure)—was constructed solely from coded amino acids. The high thermal stability of the structure is due to a large extent to tryptophan–tryptophan interactions between the two β strands.

In the nick(el) of time: Bis(μ-oxo) dinickel(III) complexes 2 (see scheme), generated in the reaction of 1 with H₂O₂, are capable of hydroxylating the xylyl linker of the supporting ligand to give 3. Kinetic studies reveal that hydroxylation proceeds by electrophilic aromatic substitution. The lower reactivity than the corresponding μ-η²:η²-peroxo dicopper(II) complexes can be attributed to unfavorable entropy effects.

A new school of thought: Micrometer-sized silver chloride (AgCl) particles (red) in deionized water move under UV illumination by self-diffusiophoresis (see picture). Each AgCl particle secretes ions to which the other particles respond, and they are observed to “school” into regions with higher concentrations of particles. Photo-inactive silica particles (blue) also respond to the chemical secretion by swimming towards and surrounding individual AgCl particles.

A variety of tertiary silanes, even those with functional substituents, undergo an unprecedented iron-catalyzed dehydrogenative coupling (see scheme) in a convenient approach to disilanes, including unsymmetrical disilanes and polymers with SiSi bonds in the backbone. Consideration of the catalytic reaction pathway revealed the intermediacy of a hydrido(disilyl)iron(IV) complex.

Check M(etal)ate: The chessboard and the figures represent a special reaction in which different low-polarity metals can metalate arenes directly when they are brought into the right position. In a combination of queen (sodium) and knight (chromium or iron), it is possible for the knight (usually the weaker piece) to make a direct deadly hit on the king (benzene) in this game of elemental chess.

Activate and reduce: Carbon dioxide was reduced with silane using a stable N-heterocyclic carbene organocatalyst to provide methanol under very mild conditions. Dry air can serve as the feedstock, and the organocatalyst is much more efficient than transition-metal catalysts for this reaction. This approach offers a very promising protocol for chemical CO₂ activation and fixation.

Boxing clever: Direct conversion of a terminal alkyne group into a β-methoxyacrylate is realized with the aid of the bis(oxazoline) ligand (box). Acetyl and ketal protecting groups, free hydroxy groups, and acid-sensitive glycosidic bonds are not affected under the reaction conditions. The one-pot synthesis of (±)-dihydrokawain from the homopropargyl alcohol is also achieved. tfa=trifluoroacetate

Easily disrupted: Micelles of a new amphiphilic block copolymer that bear coumarin groups are sensitive to near infrared light by two-photon absorption of the chromophore. Disruption of the micelles under irradiation at 794 nm results in release of both photocleaved coumarin and encapsulated nile red from the hydrophobic core of micelle into aqueous solution, which results in opposing changes in fluorescence emission intensity.

Chiral 1,2-diols have been prepared from α-aminoxylated aldehydes or cyclohexanone and Grignard reagents with L-proline or its tetrazole derivative as the catalyst. The presence of the ate complex of CeCl₃⋅2 LiCl is essential for the high overall yields and good selectivities (see scheme; DMSO=dimethyl sulfoxide, THF=tetrahydrofuran, Tol=tolyl).

At the crossroads: A unique carbon-bridged annulene motif—dehydroquatyrin—is imprinted into the molecular structure of the tetraazuliporphyrin tetracation (see picture). The macrocycle, which lies at the intersection of annulene, carbocation, and porphyrin chemistry, is obtained by the standard condensation of azulene and arylaldehyde followed by oxidation. The meso positions of the tetracation are susceptible to anionic or weak nucleophilic attack.

Caught in the act: An alkyl alkene Zr^IV complex (see picture; Cp=C₅H₅) has been synthesized and characterized for the first time. The alkene bonding mode is highly asymmetric, and C2 is quite carbocationic. There is also evidence for rotation about the C1C2 bond. This extremely unusual complex provides an exemplar of previously unknown intermediates in Ziegler–Natta and carbocationic polymerization reactions of alkenes.

Taking the plunge: The first example of a Lewis acid catalyzed asymmetric Friedel–Crafts alkylation with olefins in water is described. By using loadings of a DNA-based copper catalyst as low as 0.15 mol %, good yields and excellent enantioselectivities were obtained in the reaction of α,β-unsaturated 2-acyl imidazoles with heteroaromatic π nucleophiles. dmbpy=4,4′-dimethyl-2,2′-bipyridine.

Confined in a molecular corral: A supramolecular network changes the mechanism by which underpotential deposition (UPD) of copper proceeds on a gold electrode modified by a self-assembled monolayer (SAM). Lateral diffusion of Cu adatoms is suppressed between adjacent cells of a network/SAM hybrid structure. Instead, UPD occurs by direct deposition into the SAM filled pores of the network, where the Cu adatoms are confined.

Breaking with convention: A homodinuclear nickel complex derived from a biphenyldiamine-based Schiff base catalyzed an anti-selective Mannich-type reaction of α-ketoanilides (see scheme) to afford unique building blocks for the synthesis of azetidine-2-amides and α-hydroxy γ-amino amides. This approach stands in contrast to conventional Mannich-type reactions for the synthesis of β-amino carbonyl compounds. o-Ns=o-nitrobenzenesulfonyl.

Electron by electron: β-Diketiminato nickel(I) complex fragments are capable of activating N₂ through coordination. The resulting complex can be reduced in two single-electron steps, which further activates the NN bond. The picture shows the structure of the singly reduced complex with μ-η¹:η¹-bound N₂.

All ways lead to Rome? Computer modeling and kinetic measurements identified a distinct residue in Phe/Tyr ammonia lyases (PAL/TAL) which controls whether the Friedel–Crafts or an E₁cB reaction mechanism takes place. Hence, Glu484 in pcPAL favors the Friedel–Crafts reaction (see picture, MIO=4-methylidene imidazol-5-one) whereas an Asn in TAL gives an elimination reaction. These mechanistic investigations also reveal activity of a PAL mutant and a TAL towards an amino alcohol.