Angewandte Chemie International Edition

Enantioselective intramolecular crossed-benzoin reactions can be effected by using chiral N-heterocyclic carbene catalysts. The tetracyclic chiral carbene catalyst shown in the cover picture enables the reaction to proceed with formation of only one enantiomer of the α-hydroxy-substituted tetralones. D. Enders and co-workers disclose the development of the quaternary stereocenters in the organocatalytic reaction in their Communication on page 1463 ff.

New carbenes open up new vistas. The P-heterocyclic carbene 1 was obtained after planarization of the P atoms in the heterocycle with sterically demanding substituents. The cyclohexyl substituent in the cyclic alkylaminocarbene 2 can be locked by proper substitution in the most sterically demanding conformation such that a low-coordinate metal center, as in complex 3, is stabilized.

Shedding light on DNA structures: Investigation of local DNA conformational changes associated with biological events is essential for understanding the function of DNA. A modern approach to this is the use of 5-halouracil as a probe molecule. Upon irradiation with UV light, a uracil-5-yl radical is formed and initiates a follow-up reaction in the DNA that depends on the local conformation of the DNA (see picture).

A vancomycin mimic: Synthetic macrocycle 1 shows promising antibiotic activity towards Gram-positive bacterial strands owing to its ability to bind the (R)-alanyl-(R)-alanine terminus of peptoglycans. But how useful are other amino acid derived macrocycles?

Light moves: Azobenzene liquid-crystalline elastomer (LCE) films with completely different types of mesogen alignment have been prepared by in situ photopolymerization. Upon irradiation with UV light, an LCE film with homogeneous (parallel) alignment bends toward the light source, whereas an LCE film with homeotropic (normal) alignment bends away from it.

Imperative irradiation: A large enantiomeric excess in a liquid-crystal phase can be induced by circularly polarized light (CPL) as an external stimulus, and an imbalance in the two chiral domains becomes remarkable in the B_X phase (see pictures), which is similar to the B₄ phase made up by classical bent-core mesogens.

When push comes to shove: A switchable [2]rotaxane has been developed in which the binding of a metal ion (e.g. Cd²⁺, see scheme) to a bis(picolyl)amine fragment that acts as one of the stoppers induces a shift of the macrocyclic component along the thread from one binding site (green) to another weaker binding site (orange).

Filling the pores: A zinc-based metal–organic framework (MOF) can be transformed reversibly from an open (a) to a dense (b) configuration. The microporous solid is the first example of a MOF that is highly selective in the gas-chromatographic separation of alkanes.

C-substituted zwitterionic 6π+6π benzoquinonemonoimines are obtained by a CC bond formation reaction. A selective, formal insertion of tetracyanoethylene (TCNE) or 7,7′,8,8′-tetracyanoquinodimethane (TCNQ) into a CH bond of a quinonoid ring provides a new, general synthetic method in quinonoid chemistry.

Swapping structures and cations: In an acid-induced 3D-to-3D transformation at room temperature, K₆Cd₄Sn₃Se₁₃ is converted into K_14−xH_xCd₁₅Sn₁₂Se₄₆. The [Cd₁₅Sn₁₂Se₄₆]¹⁴⁻ framework, which contains a tunnel network of accessible cages of labyrinthine complexity (see picture), is highly robust, acid resistant, and has excellent ion-exchange properties.

In the red: The synthesis and optical properties of homologous ladder-type near-infrared-absorbing chromophores, namely, two pentarylenebis(dicarboximide)s (green traces; see picture) as well as a hexarylenebis(dicarboximide) (yellow trace) have been studied. A strong bathochromic shift of the absorption maximum is noted with an increasing degree of annulation as is evident from the other traces.

In best order: Nanoaggregates of chromophores formed in a controlled fashion in self-assembled functional hybrid nanocomposites enhance fluorescence (see scheme). These hybrids minimize the problem of solid-state quenching and are thus applicable in the fabrication of electroluminescence devices.

A sticky situation: A site-specific immobilization of proteins onto a glass surface may allow the creation of protein microarrays. This approach relies on the Staudinger ligation between azide-modified proteins and a phosphane-functionalized glass surface.

Well mixed: A 1:1 mixture of two different binaphthol-derived monophosphonite ligands (L) is employed in the Rh-catalyzed hydrogenation of prochiral olefins (see scheme; cod=cyclooctadiene). Variation of the enantiopurity of the ligands results in unusual nonlinear effects; furthermore, >92 % ee can be achieved even if one component is racemic.

Predictable solubilities! Reagents supported on a tetraarylphosphonium group have solubilities that can be predicted. The reagents and their by-products can be readily removed, recovered, and recycled after reaction by a precipitation/filtration sequence initiated by the addition of a low-polarity solvent, such as diethyl ether. Furthermore, the supported reagents are robust and exhibit reactivities similar to their parent compounds.

Back to the elements: The activity of Pt/K_0.95Ta_0.92Zr_0.08O₃ for the photocatalytic splitting of water into H₂ and O₂ is greatly improved by dye sensitization (see picture). Among the organic dyes examined, the highest photocatalytic activity is obtained with cyanocobalamin (vitamin B₁₂) as a sensitizer.

Single charged bioparticles such as viruses, bacteria, and mammalian whole cells have been successfully desorbed by laser-induced acoustic waves and analyzed with a quadrupole ion-trap mass spectrometer (see picture). This approach offers a high sensitivity of detection and good accuracy of the mass measurement. The capability to measure the mass of an entire cell has broad potential in biomedical applications.

A phosphepine catalyst mediates enantioselective [3+2] cycloadditions of allenes with a variety of β-substituted α,β-unsaturated enones to produce highly functionalized cyclopentenes that contain two contiguous stereocenters (see scheme). Surprisingly, opposite regioisomers are produced as compared to reactions with β-unsubstituted enones.

Optically active epoxides are prepared by the highly enantioselective epoxidation of benzylidenecyclobutanes using a glucose-derived ketone as the catalyst and oxone as the oxidant. A subsequent Lewis acid catalyzed rearrangement of the resulting epoxides with Et₂AlCl or LiI provides α-aryl substituted cyclopentanones in high enantiomeric excess (see scheme; Tol= tolyl).

Lewis basic, metal-free pyridyloxazolines catalyze the reduction of prochiral aromatic ketones and ketimines with Cl₃SiH in good enantioselectivity (up to 94 % ee). Arene–arene interactions between the substrate and the catalyst are likely to play a role in the enantiodifferentiation process.

Twice poor equals potent: HIV-1 Protease assembles its own potent inhibitor through formation of the triazole linkage from azide- and alkyne-containing fragments that are themselves poor binders.

More fused rings for fullerenes: Fuller-1,6-enynes are a peculiar class of 1,6-enynes that involve a highly strained double-bond component as well as a curved molecular surface. They undergo an unprecedented thermal [2+2] cycloaddition to afford products with three neighboring fused rings on one six-membered ring of the C₆₀ core as shown in the scheme.

Origin and generality of the halogen effect in Diels–Alder reactions were explored with high-accuracy CBS–QB3 computations. Halogen substitution increases cycloaddition exergonicities and decreases activation barriers. This effect is attributed to the energetic preference for electronegative halogen substituents to be attached to a more highly alkylated, and therefore more electropositive, carbon framework.

Soft lenses: Reversible antibody–antigen cross-links at the surface of a hydrogel microlens allow focal tuning of the microlens in response to soluble antigens (see picture; PBS=phosphate-buffered saline) to form the basis of a label-free biosensor/bioassay technique, whose sensitivity can be tuned by controlling the number of cross-links required to effect a specific swelling response.

Selective incorporation of CdSe quantum dots (QDs) in the poly(4-vinylpyridine) nanodomains of a poly(styrene-b-4-vinylpyridine) diblock copolymer thin film (see structure) results in much higher conductivity and faster electron tunneling than in a random distribution of the same QDs in poly(4-vinylpyridine) homopolymer.

The strongest nitrogen base is 1,8-bis(dimethylamino)naphthalene-2,7-diolate according to experimental data and DFT calculations. Its proton affinity (PA) is comparable to those of simple ions such as OH⁻, NH₂⁻, and H⁻. The diaminonaphthalene diolate 2 is protonated at the neutral amino groups with extremely high gas-phase and solution proton affinity to form highly stable H-bonded conjugate acid 1. DMSO=dimethyl sulfoxide.

Changes in polyhedral shapes around Co and N atoms in a polyhedral-interconversion reaction coupled to a proton-transfer process are followed in a theoretical study through a generalized polyhedral-interconversion coordinate within the framework of the Avnir continuous shape measures.

Electrolytically generated four-membered-ring radical-cation intermediates are involved in the anodic olefin cross-metathesis of enol ethers and olefins in a lithium perchlorate/nitromethane electrolyte solution (see scheme). The reaction mechanism was investigated through deuterium-labeling studies.

Getting cross: Excellent asymmetric inductions and very good yields are achieved in the generation of a quaternary stereocenter in α-hydroxy-substituted tetralones by using chiral N-heterocyclic carbene catalysts in an enantioselective intramolecular crossed-benzoin reaction. The synthesis of the corresponding α-hydroxyindanones with good ee values is also possible by this route.

Ironing out the kinks: A highly regioselective allylic alkylation can be performed in the presence of catalytic amounts of an iron(−II) complex and triphenylphosphane (see scheme; EWG=electron-withdrawing group). Allyl carbonates and pronucleophiles are coupled in high yields with a high regioselectivity comparable only with that obtained with Rh catalysts. The reaction is broadly applicable and does not require external base.

Over the top: Electrochemically induced oxygen spillover on a Pt film has been investigated by photoelectron emission microscopy. Oxygen ions migrate through the YSZ substrate to the triple-phase boundary, where they are converted into adsorbed oxygen atoms that can oxidize the carbon layer on the Pt surface (see scheme; YSZ=yttria-stabilized zirconia).

Watch your step! The individual steps during the chlorination of [Al₁₃]⁻ clusters via [Al₁₁]⁻, [Al₉]⁻, and [Al₇]⁻ intermediates (see picture) can be “visualized” by means of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The reaction can be interpreted as a model reaction for the chlorination of bulk aluminum.