Chemistry - A European Journal

Switching—It Is Universal! Join the highway where, irrespective of the environment, be it solution, self-assembled monolayers, polymer matrices, or molecular switch tunnel junctions—and independently of the bistable molecular switches, be they catenanes or rotaxanes—a single and generic switching mechanism is observed for all bistable mechanically interlocked molecules, as described by J. F. Stoddard, J. R. Heath, J. O. Jeppesen et al. on p. 261 ff. The cover picture is designed from a concept by Amar Flood enriched by Scott Vignon; the artwork for the front cover was produced by Anthony Pease [the cover artwork was produced using POVTree by Gena Obukhov (implementing TomTree by Tom Aust) and stsky.inc by Jaime Vives Piqueres].

The long-standing challenge of transferring chirality from silicon to carbon is accomplished with perfect stereoselectivity in truly incomparable processes: intermolecular hydrosilylation (X=H, Y=CR₂) and intramolecular allylation (X=allyl, Y=O, see scheme). The Concepts article summarizes the targeted development of these reactions, which finally led to the realization of the first Si → C chirality transfer reactions.

Direct nanopatterning of metal, alloys and oxides with 50 nm resolution is made possible by electrodeposition. By using old molding and replication concepts but introducing alkanethiolate self-assembled monolayers (SAM) as anti-adherent layers a high density of three-dimensional nanostructures can be fabricated (see picture). SAMs quality, stability, electrodeposit roughness, grain size and deposition rate must be tuned to obtain pattern transfer at the nanoscale.

The key step in a formal total synthesis of eleutherobin was an unusual kinetically controlled, second-generation Grubbs catalyzed RCM reaction of a densely functionalized diene 3 bearing two PMP-protected allylic alcohols (PMP=p-methoxyphenyl). Subsequent isomerization of the ten-membered enedione (E)-2 to the more stable Z isomer and cleavage of the MOM-protecting group led to the known precursor 1 (MOM=methoxymethyl)

Practical chemistry: Asymmetric conjugate reduction of β,β-disubstituted α,β-unsaturated ketones (see scheme) and esters was accomplished with alkoxyhydrosilanes and 1 mol % of chiral rhodium(2,6-bisoxazolinylphenyl) complexes in high yields and high enantioselectivity (up to 98 %).

Peptide folding dynamics: Numerical simulations of the dynamics of a β-peptide heptamer (see graphic) in methanol suggest two regimes in its response to a lowering of solvent viscosity.

An unprecedentedly short approach to a fully functional surrogate of the “higher sugar” hikosamine is presented, which constitutes the central scaffold of the anthelmintic nucleoside antibiotic hikizimycin. The keys to success are a chromium-induced or chromium-catalyzed Nozaki–Hiyama–Kishi reaction for the crucial fragment coupling, as well as a “Blitz dihydroxylation”/cyclization cascade co-catalyzed by RuO₄ and FeCl₂, which seemingly violates established stereochemical rules and outperforms conventional dihydroxylation protocols using OsO₄ as the oxidant (see scheme).

If cyanide is “nature's trimethylphosphane”, then these new structural analogues for the active site of the Fe-only hydrogenases are attractive models. Like the biological system, the new species feature reactive coordination sites trans to semibridging CO ligands (see examples shown here).

Control of physiologically relevant polyvalent interactions with small molecules comprising a ligand and a self-assembling moiety can be achieved with supramolecular chemistry (see scheme). Self-assembly leads to the formation of noncovalent nanoparticles. These particles—but not the individual molecules—function as highly potent polyvalent inhibitors.

Latex-controlled crystal growth: Latex particles with a corona of functional groups are able to control the morphology of zinc oxide precipitated from aqueous solution. A quantitative relationship is identified that links the shape of the crystals to the chemically defined control parameters, such as charge density in the corona, radius of the latex particle, and overall latex concentration. The correlation of these findings to phenomena seen in biomineralization is addressed.

Equilibrium effects: Mechanistic details on the nature of the equilibrium between oxoiron(IV) porphyrin π-cation radicals and iodosylarene–iron(III) porphyrins in the presence of iodoarenes, the determination of reactive species responsible for olefin epoxidation, and the fast oxygen exchange between the oxoiron(IV) porphyrin π-cation radicals and H₂¹⁸O in the presence of iodoarenes are reported (see scheme).

Threefold symmetry: A new approach to C₃-symmetric molecules based on chiral-base chemistry has been devised and tested. The ruthenium(II) complex illustrated here is composed of two enantiopure C₃-symmetric tripyridine ligands synthesized by this approach.

A nanoporous polymer as a promising luminescent probe (see picture): two polymers based on heterometallic Dy–Mn were constructed by changing the ligand substituent, and the studies of their luminescent properties revealed that they possess good luminescent selectivity for Mg^II ions.

Two fields are better than one: Ten lanthanum(III) coordination compounds have been studied by using ¹³⁹La solid-state NMR spectroscopy at two applied magnetic fields. The experimental (red, see figure) and simulated (blue) spectra of a nonspinning powder sample of trinitratobis(1,10-phenanthroline)lanthanum are examples of the types of lineshapes that were obtained.

High heteroselectivity for chain-end controlled polymerization ofrac-lactide: New group 3 metal complexes bearing tetradentate dianionic bisphenolate ligands have been designed and used as initiators for the ring-opening polymerization of rac-lactide (see picture). The complexes efficiently initiate the formation of poly(lactic acid)s and, in some cases, allow the combination of high activity, productivity and selectivity.

Capped size! A time-resolved study of the solution growth of ZnO nanocrystals and their size distribution in real time in the presence of a capping agent was conducted by using UV-absorption spectroscopy. The optimal conditions for ZnO nanocrystal synthesis with narrowest size distribution were defined (see graph), and non-monotonic dependencies were established, providing a route to rational synthesis.

All shapes and sizes! Various 3d–4f complexes and coordination polymers (see figure) were obtained by using [LCuLn]³⁺ and [(LCu)₂Ln]³⁺ as tectons. The synthetic approach is based on the selective interaction of the spacers with the metal ions: exo-dentate ligands bearing nitrogen-donor atoms connect the copper(II) ions, whereas the spacers with oxygen-donor atoms interact preferentially with the rare-earth cations.

The proton relaxivity of different gadolinium(III)-containing [15]metallacrown-5 complexes with copper(II) as the ring metal (an example of which is shown here) was determined by NMRD measurements. A linear relationship between the molecular mass and the relaxivity was found.

Morphing microspheres! A series of nickel selenides (see figure) has been hydrothermally synthesized. The composition, phase structure, and morphology has been controlled by adjusting the Ni/Se ratio of the raw materials, the pH, and the reaction temperatures and times.

Molecular structure determination of nanospheres and nanotubes for compounds with A/B=1:2 stoichiometry is presented (see figure), by applying the methodology used for SiO₂. Comparisons to crystalline silica with fullerenes and carbon nanotubes suggest that silica nanotubes, with diameters of approximately 1 nm, could be chemically stable.

Complementary catalysis: By combining kinetic resolution (KR) catalyzed by protease subtilisin Carlsberg with in situ racemization catalyzed by complex 1 (see scheme), (S)-selective dynamic KR of sec-alcohols has been achieved. The enzyme stability, selectivity, and activity were improved by using two different surfactants: octyl β-D-glycopyranoside (2) and Brij 56 (3).

By using heterobimetallic cubanes [Sn₃Mg(μ₃-NtBu)₄] and [Sn₃Li(μ₃-NtBu)₄]⁻, a series of chalcogenide derivatives has been prepared and characterised. The highly reactive anionic lithiated cubane [Sn₃Li(μ₃-NtBu)₄]⁻ can be used to prepare trichalcogenides [E₃Sn₃Li(μ₃-NtBu)₄]⁻ (E=Se, Te; see figure) and also acts as an effective chalcogen-transfer reagent.

Don't get cross about substitution: Vinyl-substituted germanes react stereo- and regioselectively with olefins in the presence of complexes containing RuH and RuGe bonds with the formation of functionalized vinylgermanes that cannot be synthesized by olefin cross-metathesis procedures (see scheme). The reaction opens a new catalytic route for preparation of a class of organogermanes that are potent organometallic reagents for organic synthesis, because they show very low toxicity and could replace organotin compounds.

A side-chain chiral member, c₃diPhe, of the family of C^α-tetrasubstituted α-amino acids tends to fold peptides into β-turn and 3₁₀-helix conformations. The S,S enantiomer (bottom) of this residue, unusually lacking asymmetry in the main chain, strongly favours the left-handedness of the turn/helical peptides formed. The opposite is true for the R,R enantiomer (top).

Thermodynamics to the rescue: the nuances of a molecular switch, perched in its lower-conducting ground state on the brink between being ON and OFF, and after WRITING in its fully ON higher-conducting metastable state, lend credence to the postulated mechanism of action of such switches. No surprises: the amplitude of switching in a molecular-switch tunnel junction is controlled by a finely balanced bistable rotaxane (B), fulfilling the role of such an indecisive molecular switch that is extremely sensitive (A) to changes in temperature on READING.

Dispersion energy governs intercalation: Accurate quantum chemical ab initio calculations show a very large stabilization energy of ΔE = −72 kcal mol⁻¹ for the interaction of cationic ethidium with DNA (see figure). The dominant energy contribution stems from the London dispersion forces.

Phosphorus(III) donor ligands cluster in a map of ligand space (shown here) based on new DFT-calculated computational descriptors. This map and a number of predictive models have been derived from a collection of knowledge on ligands in metal complexes (a ligand knowledge base) which reveals novel ligand similarities and indicates the significance of ligand properties that emerge on complexation.

The thermal or microwave-promoted reaction of electron-deficient alkenes with alkoxyalkynyl Fischer carbene complexes (see scheme) represents a straightforward route to a new type of donor–acceptor alkynylcyclopropanes.

Sticky gold: The attachment of ω-alkene-1-thiol-functionalized gold nanoparticles onto hydrogen-terminated silicon surfaces by covalent silicon–carbon bonds is reported. The detailed structural examinations of the modified silicon surface were carried out by using HR-SEM, XPS, and cross-sectional transmission electron microscopy. Cross-sectional TEM images directly indicate that the particles were covalently attached to the silicon surface through SiC bonds, without a native oxide insulating layer (see picture).

Rigid rod-like structures appear to be adopted by a range of polynorbornenes (PNBs) with fused dipolar pendant groups at C-5,6 positions (see picture), which were synthesized by ring-opening metathesis polymerization catalyzed by a ruthenium carbene complex (Grubbs I). Photophysical studies, EFISH measurements, and atomic force microscopy images have been used to investigate the structures and morphology of these polymers.