Angewandte Chemie International Edition

“I find in the square … specific settings, divisible areas, number structures, possibilities of time, measure and rhythm as well as the essential form of color which can be placed in the square to interplay with number forms.” This statement by Alfred Jensen (his painting “The River Diagram” is shown in the cover picture) might also hold for research on molecular squares. In their Communication on page 7896 ff., M. Ruben, J.-M. Lehn, and P. Müller et al. describe the spectroscopic dissecting of a single molecular square into regions of different electronic properties by STM/STS techniques.

Tales of the unexpected: A multitude of new reactions have been discovered for nitrones, the chemistry of which was considered well established. Two of them, a formal [3+3] cycloaddition and a pinacol-type coupling (see scheme) are induced by lanthanide derivatives and appear particularly promising and attractive for the synthesis of natural products and complex molecules.

The 100th anniversary of the awarding of the Nobel Prize for Chemistry to Adolf von Baeyer (1835–1912) is drawing to a close. Nevertheless, his scientific work, especially that on organic dyes, C-nitroso compounds, and on hydroaromatic compounds have had and continue to have a profound effect on chemistry and on the chemical industry.

G/C or A/T: that is the question! Nucleotide variations in the human genome, such as single-nucleotide polymorphisms (see picture), can be linked directly to diseases and also to side effects of drugs. Efficient, cost-effective processes that can be conducted in a closed reaction tube allow directly analysis of genomic DNA without preamplification.

The smaller, the better: Metal nanoparticle catalysis is an expanding area at the interface between homogeneous and heterogeneous catalysis. Moreover, its applications in the synthesis of fine organic chemicals, fuel-cell technology, and environmental issues are of growing importance. This Review shows the interdisciplinary character of this catalysis and presents the main recent advances. The picture shows the morphology of an electrosteric stabilized nanoparticle.

More stable than it seems: The binary uranium–nitrogen anion U(N₃)₇³⁻ was synthesized and characterized as the (Bu₄N)₃[U(N₃)₇] salt. The U(N₃)₇³⁻ ion is the first structurally characterized heptaazide and is the first homoleptic azide of an actinide. The crystal structure shows a pentagonal-bipyramidal arrangement of the azide ligands around the central uranium(IV) atom (see structure; N: blue; U: green).

The ligand makes it possible: The asymmetric conjugate addition of diethylzinc to α,β-unsaturated aldehydes and ketones occurs with catalytic amounts of the paracyclophaneketimine ligand 1 without further additives.

Activation of CC bonds in cyclopropanes with a cobalt complex: Methylenecyclopropanes react with one equivalent of [Co₂(CO)₈] under mild conditions to give 2-methylenecyclobutanones in moderate to good yields (up to 85 %). The reaction also proceeds with a catalytic amount of the cobalt complex under an atmosphere of CO (balloon) to give products in good yields (up to 90 %; see scheme).

Inside out: Deiodination of R*₃Si₄I₅ (R*=SitBu₃) with NaR* affords the silicon compound Si₈R*₆. A Si₂ dumbbell(Si1–Si2) is present in this novel cluster framework, embedded as in a sandwich compound between two Si₃R*₃ rings (see structure; the tBu methyl groups are not shown). The Si atoms of the dumbbell are “inverse-tetrahedrally” coordinated by four Si atoms.

Shedding light: Efficient activation of cyclic nucleoside monophosphates (cNMPs) can be achieved upon one- and two-photon flash photolysis of novel photolabile coumarinylmethyl esters of cAMP and cGMP (A=adenosine, G=guanosine) as well as their 8-bromo-substituted derivatives. The phototriggers show high solubility in water and permit space- and time-resolved investigations of the molecular mechanisms of cyclic nucleotide dependent processes.

Simple metal acetates react to give bimetallic heterocubanes of the type [O₄] (see structure, M^1,2=Mn, Co, Ni, Zn), which in turn are precursors for the selective formation of bimetallic oxide nanoparticles with a controlled composition. The size of the particles can be controlled by adjusting the thermolysis temperature.

The controlled deposition of grid-type [2×2] Co^II₄ complexes onto graphite rendered free-standing 0D, 1D, and 2D molecular arrangements, which were examined by STM techniques. Scanning tunneling spectroscopy (STS) investigations enabled the direct addressing of the metal centers at the single-molecule level (see image). The experimentally determined intramolecular positions of the Co^II centers are in agreement with DFT calculations of the molecule.

A ketene comes and goes: The first intermediate in the photoredox reaction of o-nitrobenzaldehyde (1) is identified as ketene 2. In a femtosecond IR experiment a resonance at 2100 cm⁻¹ corresponding to the ketene stretching vibration appears less than one picosecond after photoexcitation. The 90-ps decay of this band is a manifestation of a nucleophilic addition to the CC double bond, which is supported by other spectroscopic experiments (IR and Raman).

Gold in glass: Activating gold silicate glasses with synchrotron X-rays results in the spatial reduction of cationic gold to gold atoms. Subsequent annealing allows the growth of nanoparticles with defined sizes and tailored optical properties (see picture). This opens up new vistas in functionalizing chemically and mechanically stable glasses for integrated nanophotonic applications.

Back and forth like a metronome is how the carbene moiety swings in a dual-site catalyst which produces a predominantly alternating copolymer of norbornene and cyclooctene from a one-pot reaction of appropriate mixtures of the two monomers (see scheme). The alternating sequence of the polymer was confirmed by NMR spectroscopy and gel-permeation gravimetric analysis.

Inside out: Corrosive pitting and etching can be combined in a simple, elegant, and powerful method to transform single-crystal Pd nanocubes into nanoboxes (see SEM image) and nanocages in a one-pot synthesis without the involvement of exotic templates. Emptying the interiors enables the surface plasmon resonance peaks of the Pd nanostructures to be tuned from 410 to 520 nm.

The cytoskyrin cascade: The total syntheses of the bisanthraquinones 2 (+)-2,2′-epi-cytoskyrin A (R²=OMe) and (+)-rugulosin (R²=Me) has been achieved through a cascade sequence from anthradihydroquinone monomeric units 1.

Nonlinear optics: Twisted π-electron system electrooptic (EO) chromophores with exceptional molecular hyperpolarizabilities (−488 000×10⁻⁴⁸ esu at 1907 nm) were designed and synthesized. Crystallographic analysis reveals large twist angles and a highly charge-separated zwitterionic ground state. Experiments on poled host–guest polymers containing these chromophores show them to be promising candidates for EO applications.

Shift work: The identity of an anion complexed to indole-based macrocycles can be determined by the NMR chemical shift of the NH protons (see picture). The difference in the chemical shifts arises from a difference in the strength of the hydrogen-bonding interactions between the macrocycles and the anions.

A self-organized film of boron nitride nanotube–polyaniline (BNNT–PANI) composite is fabricated by using a simple solution method. The BNNTs are fully coated with PANI as a result of strong interactions between these components (see picture). Mechanically tough, BNNTs may have potential as stabilizing additives to PANI or other polymers.

Playing with the band: Long alkyl chains are bound to boron nitride nanotubes (BNNTs) from the reaction of stearoyl chloride with amino groups on the BNNTs; the resulting functionalized BNNTs (see picture) are soluble in many organic solvents. Cathodoluminescence and UV/Vis absorption experiments indicate that the long alkyl chains may induce drastic changes in the band structure of BNNTs.

A simple solution: Simply dissolving three commercially available dyes and two metal salts in a buffer is sufficient to create a sensor which allows the identification of dipeptides with high selectivity by using UV/Vis spectroscopy (see picture for the adaptive behaviour of a dynamic mixture of colored compounds A–H).

Metallomacrocyclic arrays containing eight copper(II) centers (see picture; green Cu, blue N, red O, yellow S) were obtained by using a heptadentate thiocarbohydrazone ligand with a particular conformational and configurational flexibility, as well as an appropriate arrangement of metal-binding sites, coordination ability, and acid–base properties.

Caught in the net: A series of mixed copper(I)–silver(i) cyanide complexes of the form A[Ag₂Cu(CN)₄] (A=K, Rb, Cs) have been found to form interpenetrating layer structures and three-dimensional networks (see picture). Two different forms of the Rb and Cs compounds are observed, which are both topological isomers and polymorphs.

A gentle break: Cleavage of peptides containing the non-natural but proteogenic amino acid azido homoalanine can be accomplished by phosphine- or dithiol-mediated reductive activation of the azide functionality in aqueous buffers (see scheme). The resulting C-terminal lactone of one peptide fragment allows facile further derivatization.

Quality control: UDP-glucose:glycoprotein glucosyltransferase (UGGT) works as the folding sensor in glycoprotein quality control. It glucosylates the Man9GlcNAc2 of misfolded glycoproteins to produce Glc1Man9GlcNAc2, which is a ligand of calnexin and calreticulin. The synthetic substrate Man9GlcNAc2-MTX can be used for the quantitative analysis of UGGT. UDP=uridine 5′-diphosphate, Glc=D-glucose, Man=D-mannose, GlcNAc=N-acetyl-D-glucosamine, MTX=methotrexate.

Complex twists: Following theoretical thermodynamic predictions, it is seen that the tetrametallic triple-stranded lanthanide helicate [Ln₄L₃]¹²⁺ (see structure) dominates the speciation in solution at millimolar concentrations, despite its high positive charge. Isolation of the europium complex in the solid state unambiguously establishes its nanometric triple-helical structure.

It's the crown that does it: The auto-ionization of [TiF₄] has been promoted by complexation with [15]crown-5 to produce [TiF₂([15]crown-5)][Ti₄F₁₈], which contains the new tetrahedral ion [Ti₄F₁₈]²⁻ (see picture). The thermodynamics of the [Ti₄F₁₈]²⁻ ion relative to the [Ti₂F₉]⁻ ion in the gas, solution, and solid states were calculated by density functional methods.

A Smiles rearrangement is the key step in the efficient coupling of primary amines with isocyanides, carbonyl compounds, and electron-deficient substituted phenols to form N-aryl amines (see picture). The presence of a nitro or ester group on the resulting adduct allows applications in heterocyclic synthesis.

Thermoreversible gelation of diblock copolymers of a helical polypeptide, poly(γ-benzyl-L-glutamate) and random-coil polymers such as polyferrocenylsilanes in dilute solution has been discovered (see picture). A new mechanism for self-assembly of block copolymers of helical polypeptides is proposed.

Spring-loaded polymers: The nanohelical structure of poly(L-lactide) (PLLA) in the polystyrene (PS) matrix of chiral PS-PLLA block copolymers displays springlike behavior in response to an applied stimulus. The self-assembled nanohelices are transformed into cylinders by crystallization or shearing (see picture). This thermally reversible phase behavior may be used for manipulating switchable nanohelical structures.

Scaled-down approach: The nitration of phenol is an autocatalytic, strongly exothermic reaction and is thus difficult to handle on an industrial scale. Continuous nitration in a microreactor (see picture) affords higher yields and enhanced process safety, thanks to good heat exchange and mixing properties, as well as rapid radical propagation under sufficiently harsh conditions.

A general approach to the catalytic asymmetric aza-Henry reaction has been developed. The combination of a commercially available phase-transfer catalyst (PTC) with a base is able to promote the in situ formation of N-carbamoyl imines from α-amido sulfones and activate nitromethane towards the asymmetric addition reaction, thus furnishing N-carbamoyl-protected β-nitroamines in good yields and with up to 98 % ee (see scheme; PG=protecting group).

Aluminum-stabilized copper colloids with a size of about 5 nm (see picture) are the key to a quasi-homogeneous version of the heterogeneously catalyzed synthesis of methanol from synthesis gas over Cu/ZnO/Al₂O₃. The catalyst productivity is very high in spite of the absence of ZnO.

Elegant, efficient, and enantiospecific describe the first total synthesis of the quassinoid (−)-samaderine Y (see scheme), which has been shown to display in vitro cytotoxicity and is of interest as a potential antitumor agent. Its synthesis has been accomplished from (S)-(+)-carvone in 21 steps.

From the equator to the pole: The high-temperature, high-yield C₁ isomer of C₇₀(CF₃)₁₀ has an unprecedented structure. The CF₃ groups belong to a para⁷–meta–para ribbon of edge-sharing C₆(CF₃)₂ hexagons, which wraps around the equator of the C₇₀ cage and then climbs up to one of the poles. Calculations demonstrate that this isomer is more stable than the structures observed for C₇₀Br₁₀ and C₇₀(tBuOO)₁₀.

Hydrogen-bonded network: A hydrothermal synthesis delivers the titled compound, which consists of an H-bonded network of discrete [Co(en)₃]³⁺, (Zr₂F₁₂)⁴⁻, and (SiF₆)²⁻ ions. It exhibits an unexpected photoelectronic effect in the near-UV region (see figure).