Angewandte Chemie International Edition

Highly twisted permethoxylated hexa-peri-hexabenzocoronene forms inclusion complexes with hexafluorobenzene and fullerene guest molecules. In addition to the “double-concave” surface which allows its coordination to suitable guest molecules on both sides, the flexible methoxy groups at the periphery of this host molecule adjust their orientation to the size and shape of the guest molecules. Further details are reported in the Communication by K. Müllen et al. on page 1247 ff.

Out of the blue: Photoinduced magnetism in polymeric “Prussian Blue” analogues is a well understood phenomenon that can even occur near room temperature. The lessons learned are now being transferred to the synthesis of molecular cyanide-bridged single-molecule magnets (see picture; Mo yellow, C black, N blue, Cu purple) whose magnetic properties change drastically on light irradiation.

A question of scale: The interaction and recognition processes of DNA have been traditionally described on the Ångström scale. Embedded in DNA are also informational codes that control these processes on the nanoscale (see picture). Nanoscience can take advantage of these codes to control the self-assembly of biological and nonbiological molecules or their orientation on surfaces.

Five catalyst–substrate complexes of the type [Rh(chiral ligand)(β-dehydroamino acid derivative)]BF₄ were characterized for the first time by X-ray analysis (see example). Low-temperature NMR spectroscopy proved that three of these complexes are “major” substrate complexes, which lead to the main product in asymmetric hydrogenation, contrary to the known classical examples with analogous α-dehydroamino acid derivatives.

In great shape: Star-shaped telechelic oligomers can be coupled with a junction unit to yield amorphous copolyester-urethane networks, which are not only biodegradable but can also be fixed in a new, temporary shape after they have been processed into a permanent shape. The permanent shape can be recovered by heating (see picture). Such polymer networks may find application in ophthalmology.

Building bridges: The combination of the Lewis acidic dichloroboryl complex [(η⁵-C₅Me₅)Fe(CO)₂BCl₂] and a Lewis basic [Pd(PR₃)] fragment gives the first dinuclear monoboryl complex with a symmetric μ, η² bridging BR₂ ligand (see structure). The dative nature of the palladium–boron bonding interaction makes the dinuclear compound a rare example of a transition-metal borane complex.

Large jumps in pH on the nanosecond timescale can be achieved upon one- and two-photon flash photolysis of novel coumarinylmethyl esters, which are phototriggers for H⁺ (see scheme, F=pH-sensitive fluorescence indicator). The compounds are excellent tools for the study of rapid H⁺-triggered processes, for example, proton migration along membrane surfaces.

Two birds were killed with one stone by the synthesis of the trisubstituted pyridine (R,R)-1 and its diastereoisomer. In a short synthetic sequence for the construction of the GE 2270 thiazolyl peptides, 2,3,6-tribromopyridine was converted into pyridine 1 in only four steps (Bn=benzyl). In addition, the absolute and relative configuration of a major degradation product of GE 2270A was proven.

Tapping nature's biosynthetic potential: Formation of the rare p-nitro benzoate precursor of aureothin, a mixed polyketide metabolite of Streptomyces thioluteus, requires two key enzymes: a p-amino benzoate synthase and an N-oxygenase. Feeding of surrogates to engineered block mutants (e.g. S. lividans ZX1::pHJ97) reveals that the iterative modular aureothin polyketide also accepts p-cyano benzoate as the starter, which is subsequently processed to the nitrile analogue of aureothin, aureonitrile, which has improved cytostatic properties.

Cutting corners: The perovskite-like ternary nitrides MFe₃N of the iron- and platinum-group metals are theoretically investigated. All eight compounds are predicted as being ferromagnetic, with saturation moments between 7.1 and 9.2 μ_B per formula unit, and with larger M atoms preferentially occupying the corner position 1a (see picture). Two ternary nitrides, yet to be made, are predicted as lucrative synthetic goals.

One small step for a catalyst: A biomimetic asymmetric synthesis of carbohydrates was accomplished by a proline-catalyzed aldol reaction of 2,2-dimethyl-1,3-dioxan-5-one as the methylene component with various aldehydes. This new organocatalytic C₃+C_n strategy leads directly to selectively protected simple sugars and amino sugars.

A simple model of zeolite growth and dissolution has been constructed. Lattice-energy calculations suggest that both growth and dissolution, progressing through attachment or leaching of monomeric units, involves a number of exothermic and endothermic steps. Oligomeric units may play a key role in overcoming the most endothermic (rate-determining) steps of crystal growth associated with the pictured high-energy structure.

It hinges on the ligand: The electron-donor ability of NiN₂S₂ ligands is compared to that of traditional diphosphine and diimine ligands. Structural comparison of NiN₂S₂ and o-phenanthroline (phen) complexes of {W(CO)₄} and {PdX₂} units shows asymmetry imposed by the hinge bonding of the NiN₂S₂ S donors (see superimposed structures) which favors stereo- and regioselective substrate addition.

Intramolecular cycloaddition reactions are central in the total syntheses of colombiasin A (1) and elisapterosin B (2) described, which use (−)-dihydrocarvone as the starting material. A Moore rearrangement of a vinylcyclobutene is used to initiate the cycloaddition.

A convergent and enantioselective total synthesis of (+)-leucascandrolide (1) was accomplished in 17 steps. Central to this synthesis is the rapid and efficient integration of the bispyran moiety into 1 using two consecutive [4+2] annulation reactions between an aldehyde and the chiral silanes 2 and 3 (TMS=trimethylsilyl, Mes=mesityl=2,4,6-trimethylphenyl).

Pipe-dream realized: The as-made nanocomposite derived from a block-copolymer-directed sol–gel synthesis consists of silica networks embedded in an organic matrix. Calcination of the structure at high temperatures gives a final skeletal silica network consistent with the morphology called plumber's nightmare (point group Imm; see small-angle X-ray scattering (SAXS) pattern of an inverse form of this morphology).

Bonding out of the blue: Strong evidence of the occurrence of specific hydrogen bonds, the so called blue-shifting hydrogen bonds, between hydrogen atoms of CH₃ in 9-methyltriptycenes and halogen substituents in the peri position (see scheme) comes from low-temperature ¹H NMR measurements that yield the ¹J(C,H) values for the individual CH₃ protons, as well as from natural bond orbital analysis and DFT calculations of the geometries and J couplings.

Photochemical cleavage of ArO bonds in phenyl esters substituted with electron-donating groups offers a convenient method for the arylation of alkenes and arenes. The reaction proceeds by an S_N1 mechanism via a triplet phenyl cation to form allylbenzene or biphenyl derivatives under mild conditions (see scheme).

Coupling drives the rotation: A theoretical study of proton-transfer dynamics of 5-methyltropolone (5MTR) has identified a long-range mechanical interaction. Methyl group rotation can be driven by proton transfer because of coupling between hyperconjugation and tautomerization within 5MTR (see HOMO–LUMO interaction). A quantum-mechanical mechanism is presented in which this molecule undergoes a conformational change.

Poles apart: The relative orientation of a pair of functional groups—amides in this case—can be controlled by the choice of the substituent that lies between them. Groups which are effectively cylindrically symmetrical (Me, Cl) allow direct interaction between the dipoles of the amides and induce an anti orientation, whereas polar groups (OCOR, OSO₂R, SO₂R) interact with the dipole of both amides to yield a syn orientation (see picture).

More than 1 g of the dizinc compound [Zn₂(η⁵-C₅Me₅)₂] can be readily obtained in a single preparation. Density functional calculations reveal that the ZnZn bond is relatively strong and derives mainly from interaction between the Zn 4 s orbitals (see picture).

Extremely nonplanar permethoxylated hexa-peri-hexabenzocoronene can be prepared by using a facile ferric chloride cyclodehydrogenation procedure. The combination of a rigid “double-concave” aromatic core with 18 flexible methoxy groups at the periphery makes it a remarkable host molecule, as revealed by single-crystal analysis of its crystalline inclusion complexes with hexafluorobenzene and fullerene guest molecules (see crystal structure).

Sweet solution: The P2₁/n (Z=64) structure of the commercial sweetener sodium saccharinate, which has an unusually large unit cell of 15.6 nm³, is an occupational and displacive modulation of an idealized C2/m (Z=8) structure that results from a structural misfit. This determination (see picture) shows—after more than 150 years of use of the compound as a food additive—that it is better regarded as a 1.875 hydrate than a dihydrate.

Covalent ties: The polyoxometalate (nBu₄N)₃[(MeO)TiW₅O₁₈] is easily anchored to alkanol-functionalized surfaces of porous and single-crystal silicon (see picture, polyhedra: polyoxometalate, dark blue: Si surface). Scanning tunneling microscopy reveals the growth of 35–50 nm diameter islands on functionalized Si(111), which suggests that the immobilized polyoxometalates act as nucleation sites for electrostatic aggregation.

On the starting block: Polystyrene-block-poly(4-vinylpyridine) and a [60]fullerene carboxylic acid combine to form spherical supramolecular aggregates (see picture), the size and morphology of which can be controlled by changing the block composition ratio in the copolymer. The method is generally applicable to the construction of organic nanoparticles.

Hi ho silver! Coordination polymer chains assembled from a Ni^II hexaazamacrocycle and sodium 4,4′-biphenyldicarboxylate can be packed to form a porous framework (see picture). Nanocomposites of silver particles and matrix can be prepared by immersing the framework in AgNO₃ solution, and the matrix-free silver nanoparticles (≈3 nm) isolated by treatment of the solid with boiling dioctyl ether containing oleic acid.

Potential feedback: Metallic electrodes with controlled gap widths ranging from about 10 nm down to several angstroms (as determined by SEM measurements) can be fabricated electrochemically by using a simple potential feedback system with a unique electrode configuration (see picture). The working principle is based on the potential distribution in the electric double-layer. The process is simple, controllable, and reproducible.

Self-assembly of hexagonal 3–5-nm ZnS nanocrystals can be used for the large-scale production of zinc sulfide nanoporous nanoparticles (NPNPs). The uniform, spherical NPNPs are monodisperse and have surface areas on the order of 156 m² g⁻¹. The ZnS NPNPs are more effective photocatalysts than Degussa P25 titania or ZnS nanocrystals in the photodegradation of eosin B at ambient temperature (see picture).