Angewandte Chemie International Edition

DNA can template the formation of different ligand environments that are selective for their binding metals. H. F. Sleiman and co-workers describe in their Communication on page 9919 ff. how placement of a metal in the incorrect DNA-templated environment results in the occurrence of “error-checking”: the metal can spontaneously adjust its redox state, displace another labile metal to form a more-stable complex, or reorganize the coordination site to create a more-favored complex.

The large surface-to-volume ratio of nanomaterials could facilitate efficient doping using purely surface passivation and adsorption. In their Communication on page 9896 ff., R. Q. Zhang, S. T. Lee, and co-workers describe surface passivation and transfer doping in silicon nanowires (SiNWs, see picture) by surface termination with hydrogen atoms and adsorption of ammonia molecules. The approach is an alternative to conventional volume doping to modulate the conductivity of SiNWs or other nanomaterials.

“The biggest challenge facing scientists is the highly effective use of solar energy. If I could be anyone for a day, I would be Michael Jordan …” This and more about Masaki Shimizu can be found on page 9778.

Wiley-VCH, Weinheim 2009. 408 pp., hardcover € 149.00.—ISBN 978-3527319534

Mind over matter: Electronic structure calculations have been used to establish linear energy relations for chemical reactions on solid surfaces which aid in the design of new catalytic materials. Newly discovered accurate linear relations for the hydrogenation of organic molecules on transition metals suggest that the computational design of selective hydrogenation catalysts for complex reactions may soon be feasible.

On the sunny side: Recent examples of visible-light-promoted photoredox catalysis in the presence of [Ru(bpy)₃]²⁺ as an efficient photocatalyst have set new standards for conducting challenging reactions under mild and environmentally benign conditions.

Cutting across: Direct arylations are increasingly viable alternatives to conventional cross-coupling reactions with stoichiometric amounts of organometallic reagents (see Scheme). This Review summarizes recent progress in the rapidly evolving research area of transition-metal-catalyzed CH bond-cleavage reactions for (hetero)aryl syntheses.

Probe it: The new BLW-IGLO approach evaluates the magnetic responses of molecules in which conjugative interactions are “disabled” (see picture, red bonds). The NMR spectroscopy parameters of the localized structures provide ideal non-aromatic olefinic hydrogen δ(¹H) references for the fully conjugated systems.

Passing the baton: A relay cascade process provides cyclohexanes in a direct, efficient, and stereoselective manner. The triple-cascade reaction is efficient, affords high selectivities, and has a broad scope, and different diastereomers are readily accessible by judicious choice of organocatalysts.

Turning a problem to advantage: CO₂, a contributor to global warming, was converted into the valuable resource CH₃OH by adding it to 2,2,6,6-tetramethylpiperidine and B(C₆F₅)₃ in toluene under H₂ (1–2 atm), heating the mixture at 160 °C, and vacuum distillation. CH₃OH was formed via the complex shown (C blue, N purple, O red, B orange, F green) as the sole C₁ product.

A naughty NHC: A stable N-heterocyclic carbene (NHC) reacts with a phosphaalkene not at the expected 2-position, but through the 4-position, which leads to an unprecedented 4-phosphino-substituted NHC that functions as a novel bifunctional ligand for gold(I). The mechanism of this reaction has been investigated using density functional calculations.

It′s finally here: Highly anti-selective Michael reactions of a functionalized aldehyde with nitroolefins have been realized using a primary amine/thiourea catalyst (see scheme; TBS=tert-butyldimethylsilyl). The reaction relies on a conformational strategy based on directing the formation of a Z-configured enamine intermediate.

The CpCo-shuffle: {CpCo} complexes of linear [3]- and [5]phenylene undergo a light-induced haptotropic migration from a more strongly bound cyclobutadiene moiety to a less strongly bound (see scheme). The shift is thermally reversible and proceeds along the periphery of the intervening arene rings. As such, the systems constitute photo–thermal storage systems, as well the first examples of cyclobutadiene-to-cyclobutadiene haptotropism.

A fine fusion: α-fluorosulfoximines readily react with simple nitrones to give monofluoroalkenes with excellent E/Z stereocontrol (see scheme). This novel fluoroolefination method shows promising applications in medicinal chemistry as monofluoroalkenes are potential mimetics for peptide units as protein inhibitors.

A coat to note: An electrochemical immunoassay strategy is developed using phospholipid-coated multiwalled carbon nanotubes (MWNTs) as the electrochemical labels (see scheme; PSA=antigen protein; MB=magnetic bead). This strategy affords great sensitivity and specificity in signal transduction.

I want a new drug dose: External on-demand laser triggering is used in a drug-delivery concept, with on/off ratios in excess of 1000/1. The switching mechanism involves the glass transition of hydrophobic polymers with a large change in diffusivity. Formation of a glassy surface layer of the implant in the off state plays a key role, resulting in negligible off-release. (Picture: data points indicate ibuprofen concentration).

Age shall not weary them: Stereodefective isotactic polybutene samples, synthesized with single-center metallocene catalysts, crystallize from the melt into the stable form I (see picture) and do not undergo phase transformation upon aging at room temperature. Hence, the corresponding physical and mechanical properties of molded objects are not altered by aging.

The postman always rings twice: 1-Alkynyl-3-azabicyclo[3.1.0]hexanes 2 were obtained in good to excellent yields from the stereoselective Bi(OTf)₃-catalyzed dehydrative alkynylcyclopropanation of azaenynols 1, in which the propargyl alcohol motif of 1 acted as a propargyl carbene synthetic equivalent.

How to click with biomolecules: Copper-catalyzed azide–alkyne cycloaddition has been optimized for use with biological molecules. The key development is the addition of two reagents that allow ascorbate to be used as reducing agent whilst eliminating problems caused by copper ascorbate side reactions. The result is a robust, rapid, and convenient procedure for the modification of proteins, DNA, RNA, and other biomolecules (see scheme).

The RTH-type zeolite (see picture) has an attractive pore structure, but its compositional variation is limited. Now, metallosilicates with the RTH-topology can be synthesized by two routes, one of which does not use structure-directing agents. Thus prepared Al-containing RTH-type zeolites exhibited a remarkable catalytic performance for the methanol-to-olefins reaction, producing propene with high selectivity.

Urea as a nitrogen source: The supported ruthenium hydroxide, Ru(OH)_x/TiO₂, acts as an efficient heterogeneous catalyst for the title reaction. The retrieved catalyst after the reaction could be reused without a significant loss of its catalytic performance.

Gold gets a grip: Spectroscopic, structural, and computational results provide evidence for unprecedented Au→Si and Au→Sn interactions in gold(I) complexes derived from diphosphino-substituted silane and stannane ligands (see scheme).

Staying on top: Altering the surface of silicon nanowires (SiNWs) by terminating the surface with different species and/or introducing surface adsorbates can change the electrical properties of the SiNWs. Such easy, nondestructive conductivity modification would expand possible applications of SiNWs.

Nanoscale coordination polymers (NCPs), prepared with metal ions and an iodinated ligand, were synthesized using reverse-phase microemulsion techniques and rapid precipitation procedures. The NCPs carry high payloads of iodine (ca. 63 wt %) and have potential applications as a new class of contrast agents for computed tomography (CT), as demonstrated by phantom studies (see figure).

Freeze the flood: After replacing high boiling-point solvents inside the channels of a metal–organic framework (MOF), benzene is frozen and then removed under vacuum by sublimation. Bypassing the liquid phase eliminates the detrimental effects of surface tension that induce mesopore collapse in MOFs, thereby enhancing their permanent porosity and hydrogen-uptake capacity (see scheme).

It's a flat world: Charge-neutral pincer-type cyclometalated platinum(II) aryl acetylides can be assembled into quasi-two-dimensional nanosheets through bilateral intermolecular noncovalent interactions (see picture). These nanosheets can be further self-organized into layered materials on a flat substrate and exhibit NIR phosphorescence and light-modulated conductivity.

Getting in there: Efficient intracellular delivery of anticancer drugs is achieved by using reversibly cross-linked dextran nanoparticles, which are rapidly destabilized under reductive environments that mimic those of the intracellular compartments. These nanoparticles show high drug loading efficiency and reduction-triggered release of doxorubicin in vitro as well as inside tumor cells, particularly to the cell nucleus (see scheme).

Each to his own: DNA templates the formation of bis(terpyridine), bis(phenanthroline), and terpyridine:phenanthroline ligand environments, which strongly bind Fe^II, Cu^I, and Cu^II (see picture), respectively, to generate highly stable metal–DNA complexes. When a metal is placed in the “incorrect” environment, it can spontaneously adjust its redox state, be displaced by another metal, or reorganize the coordination site to create a more favored complex.

A spectral thermometer: The conformation of molecules on a metallic nanoparticle's surface is sensitive to temperature variations and can be easily monitored in situ by surface-enhanced Raman scattering (SERS). Excitation of the metallic nanoparticle for SERS can concurrently induce a photothermal effect whereby the light absorbed by the nanoparticle is released as heat. From the SERS spectra, the changes in temperature during the photothermal effect can be derived (see picture).

Chilling out: The first 3d/4f cluster based on calix[4]arenes (see picture; purple Mn, brown Gd, red O, blue N) has a high magnetic isotropy and a large number of molecular spin states that are populated even at low temperatures, whereas its ferromagnetic limit is approached only at high applied fields. These results enable the complex to be an excellent magnetic refrigerant for low-temperature applications.

Hot and cold: The high degree of localization in SERS hot spots is demonstrated directly by a selective deposition of Raman analytes inside and outside the field-enhanced regions in a plasmonic cavity (see picture). The localization is demonstrated by measurements of both the absolute SERS intensities and the kinetics for photoinduced degradation at different positions.

Do the twist: The preference of large pentiptycene–pentiptycene dihedral angles and the restricted torsional relaxation for pentiptycene-derived oligo (p-phenyleneethynylene)s in a 2-methyltetrahydrofuran glass at 80 K result in a localized excited state. These results demonstrate not only the concept of torsion-induced localization of excitation but also the efficient intrachain energy hopping processes in conjugated polymers.

In readiness for closure: To ensure optimal convergence in the projected total synthesis of spirastrellolide F, the building block representing the southern hemisphere was prepared with a free carboxylic acid and an enol triflate (Tf) terminus (see picture). This unusual pattern allows the 38-membered macrocyclic core of this potent antimitotic agent to be constructed, while keeping late-stage protecting-group manipulations to a minimum.

Marvel of the sea: A concise and highly convergent total synthesis of the methyl ester of the marine macrolide spirastrellolide F (see picture), which has exquisite antimitotic properties, is reported. In this approach, the northern and the southern hemispheres of this intricate target are stitched together in only two consecutive steps (Suzuki coupling, Yamaguchi lactonization) without any interim protecting-group manipulations.

Well stacked: Organic crystals of small molecules constitute an alternative to common polymeric electrolytes (Nafion 117) and inorganic crystals that are employed as proton exchange membranes in fuel cell systems. A phosphonic acid containing hexaphenylbenzene forms a columnar supramolecular array and exhibits a high and constant intrinsic conductivity of 3.2×10⁻³ S cm⁻¹ from 120 to 180 °C under 1 bar H₂O atmosphere (see picture).

MOF for more gas storage: A new mesoporous metal–organic framework (MOF) with extremely high adsorption capacities for methane, H₂, and n-butane is prepared from the reaction of H₃(btb), H₂(2,6-ndc), and zinc nitrate (btb=benzene-1,3,5-tribenzoate; 2,6ndc=2,6-naphthalenedicarboxylate). The main structural motif is a dodecahedron formed from 12 Zn₄O⁶⁺ clusters, four 2,6-ndc, and eight btb linkers (see picture; green: Zn₄O⁶⁺, gray: C, red: O; the red sphere illustrates the pore volume).

Larger lithium clusters than tetrahedral and octahedral Li_n aggregates can be prepared as cage structures with a central anion. The central chlorine atom (green) serves as a template for the formation of a Li₁₂ icosahedral cage (orange), which is stabilized by 1,4-butanediide “clamps” (C gray).

Let your light shine: A novel state-of-the-art system for light-driven proton reduction with iron catalysts was developed (see scheme; SR=sacrificial reagent). It consists of simple iron carbonyl complexes such as [Fe₃(CO)₁₂], triethylamine as sacrificial reagent (electron donor), and [Ir(bpy)(ppy)₂]PF₆ as photosensitizer (PS). High turnover numbers of more than 3000 for the sensitizer and 400 for the iron catalyst are obtained.

Totally wired: A particular molecular adsorption geometry can be prepared by adsorbing single conjugated polyfluorene chains partially on a clean Au(111) surface and partially on a thin crystalline NaCl film, thus connecting metallic and insulating surface areas. This configuration allows the electronic characterization of one and the same molecular wire as a function of its atomic-scale environment in a planar configuration.

Does it do the twist? High-level theoretical calculations show that the [9]annulene cation exists in two isomers: a twisted 4n electron Möbius aromatic compound, and a non-twisted boat-like Hückel conformation (see scheme). The latter form was detected by laser flash photolysis experiments; an example of a stable, charged, or uncharged parent Möbius annulene is still elusive.

A closer look at α,β-unsaturated hydrazonium ions reveals that they are isoelectronic to the pentadienyl anion, making them suitable substrates to undergo 6π electrocyclizations. The enantioselective catalysis of this transformation is achieved for the first time by asymmetric Brønsted acid catalysis.

How to close a ring: An approach to catalytic asymmetric 6π electrocyclization leads to a highly enantioselective process that was used in the synthesis of chiral indolines (see scheme). Treatment of N-aryl imines under phase transfer conditions in the presence of N-benzyl cinchonidinium chloride generates a delocalized 2-aza-pentadienyl anion system that cyclizes in up to 99 % yield and 98 % ee.