Angewandte Chemie International Edition

Photofixation of CO₂ to produce synthetically valuable molecules can be achieved on a photoelectrode based on silicon nanowires. In their Communication on page 6709 ff., K. L. Tan, D. Wang et al. develop a strategy that uses aromatic ketones to receive the photogenerated electrons. The resulting radicals then react with carbon dioxide to afford α-hydroxy acids, which are precursors for the drugs ibuprofen and naproxen. In close resemblance to natural photosynthesis, their strategy gains reaction specificity by avoiding the direct reduction of CO₂.

The motion of a single water ligand around a peptide bond in acetanilide was probed in real time by time-resolved IR spectroscopy. In their Communication on page 6604 ff., O. Dopfer, M. Fujii et al. observe that triggered by photo-ionization, the H₂O ligand is released from the CO site of the peptide linkage and trapped after a migration time of 5 ps at the NH site of the same peptide bond. The migration occurs via an intermediate in which the H₂O molecule binds neither to the CO nor the NH site.

The reconstitution of the B₁₂ macrocycle by a radical reaction of a blue secocorrinoid, itself a product of B₁₂ degradation, is described by B. Kräutler et al. on page 6780 ff. The epicobalamin obtained is a new type of vitamin B₁₂ derivative, and the first B₁₂ analogue containing the major elements of the B₁₂ structure that is poorly bound by the human B₁₂-binding proteins.

Inorganic nanocrystals play a central role in nanoscience and nanotechnology. In their Communication on page 6608 ff., F. Kleitz, T.-O. Do et al. present the bottom-up, non-aqueous synthesis of uniform titanate nanodisks with diameters of 12–35 nm. These nanodisks have been used as building blocks for a variety of hybrid nanostructures, such as titanate-based mesoporous hybrids with high surface areas and tailored porosity, as well as metal-titanate colloidal hybrids exhibiting enhanced catalytic activity.

”When I was eighteen I wanted to be Bruce Lee, or at least Chuck Norris. My motto is give serendipity a chance! …“ This and more about V. Gevorgyan can be found on page 6562.

John Wiley & Sons, Hoboken, 2011. 256 pp., hardcover, € 69.90.—ISBN 978-0470497401

Taking center stage: Several new features of the title compounds were recently uncovered. P₁-centered cations are employed as reagents for the preparation of unprecedented phosphorus compounds and transition-metal complexes. This represents a remarkable extension of their chemistry, which might make an impact on the field of homogeneous catalysis.

Growth spurt: A recent series of publications promises to pave the way for a polymer approach towards well-defined carbon nanostructures by combining the stepwise organic synthesis of carbon nanotube (CNT) end caps and their use as templates for CNT growth by Diels–Alder addition of nitroethylene as a masked acetylene derivative.

The power of many: The use of self-assembly to create dynamic multivalency (see scheme) is a powerful strategy, with some significant advantages over the use of static multivalent arrays. It mimics processes which occur naturally within cell membranes, and has a wide range of potential applications, both in nanomaterials science and nanomedicine.

Living organisms build special organic–inorganic hybrid materials such as bones, teeth, and shells that are highly sophisticated in terms of their adaptation to function. Chemists and materials scientists mimic these natural strategies to build biominerals and to tune their properties for biomedical and technical applications.

In living color: Aurora kinase A (AKA) was imaged in live cells using a bioorthogonal two-step reaction with a small molecule AKA inhibitor (see scheme) and a fluorescent reporter. The fluorescent molecule was localized to spindle poles and microtubules during metaphase, consistent with the localization of both endogenous and green fluorescent protein tagged AKA. By using this approach, changes in AKA distribution during mitosis were also observed.

A long and winding road: The motion of a single water ligand around a peptide bond in acetanilide was probed in real time by time-resolved IR spectroscopy. Triggered by photoionization, the H₂O ligand is released from the CO site of the peptide linkage and trapped after a migration time of 5 ps at the NH site of the same peptide bond (see picture).

Building with nanobricks: Uniform titanate nanodisks with a controlled diameter of 12–35 nm were synthesized by a non-aqueous method. These nanodisks were used for the design of various titanate-based mesoporous hybrids with a high specific surface area and tailored porosity. They could also be used as efficient stabilizers for the synthesis of small, uniform metal nanoparticles that exhibit enhanced catalytic activity.

More with boron: The title reaction was developed to generate quaternary carbon stereogenic centers through the use of commercially available vinylboron reagents (see scheme, MOM=methoxymethyl, NHC=N-heterocyclic carbene, pin=pinacolato). Application of the method to the two isomeric forms of an intermediate in morphine biosynthesis demonstrates its utility.

Fast, robust, selective: Copper-catalyzed enantioselective additions of homopropargyl groups to a wide range of aldimines proceed readily and with high enantioselectivity. The catalytic method is scalable and practical, the allenylboron reagent is commercially available, and conversion into amines is inexpensive and high-yielding.

Cleanly separated by two photons: The design and photophysical characterization of a new highly stable macrocyclic ytterbium complex featuring a two-photon antenna ligand is described (see picture). The biphotonic sensitization of the near-infrared ytterbium(III) luminescence and the conception of an unconventional near-infrared biphotonic microscope allow performing in-depth imaging of thick tissues.

Tubes from tubes: Microporous organic nanotubes were prepared by Sonogashira coupling between tetrakis(4-ethynylphenyl)methane and N,N′-di(4-iodophenyl)-4,4′-bipyridinium dichloride. These nanotubes were used as a template for secondary inorganic materials, namely Fe₂O₃ nanotubes with a high discharge capacity and excellent stability.

Phase to phase: The in situ hydrolysis of 4-(bromomethyl)benzoic acid to 4-(hydroxymethyl)benzoic acid (HMBA) in the presence of the uranyl cation has resulted in the novel hybrid material (A)[UO₂(HMBA)₃] (A=NH₄⁺ or H₃O⁺), which was characterized by X-ray diffraction. On exposure to ethanol the material undergoes a solid-state phase transformation, accompanied by the loss of a ligand and counterion to form [UO₂(HMBA)₂] (see scheme; O red, C black).

Between the sheets: A unique cross-β-peptide amphiphile assembly positions the acyl chain within the hydrophobic cross-β laminate (see picture; acyl chain: red, peptide: gray, lysine: blue). The atomic-level structure of the self-assembled surface can also be systematically altered, opening the possibility to create a wide range of nanostructured biomaterials.

A porous treasure: Porous aromatic framework PAF-1 (see picture, blue structure) has been lithiated, giving a reduced framework with an increased gas storage capacity compared to native PAF-1 (by 22, 71, and 320 % for H₂, CH₄, and CO₂, respectively). The reduced framework was examined spectroscopically, and the potential hydrogen storage capacity was calculated.

Regioisomers of self-assembling molecules resulted in the evolution of distinct 0D and 1D nanostructures, namely nanorings and nanorods (see picture). J- and H-type excitonic coupling of naphthalene chromophores was found in the nanostructures. The bulk liquid-crystalline states contain hydrogen-bonded rosettes, which explains how the observed regioisomers generate distinct nanostructures with characteristic excitonic interactions.

Monitoring cellular activity: A local redox-cycling-based electrochemical chip device (see picture) has been used to entrap three-dimensional culture cells and evaluate their activity. Deep microwells were incorporated into the chip device for the trapping of embryoid bodies. This chip device is useful for the evaluation of 3D organ tissues.

Spin frustration and the unusual electronic state of Cr⁴⁺ ions are the origins of unconventional colossal magnetoresistance (CMR) in NaCr₂O₄, a new member of the rare mixed-valence chromium oxides. CMR materials have previously been limited to almost only manganese oxides; the discovery of the CMR in a non-manganese-based material opens a new dimension in the quest for novel CMR materials.

Test the water: Calculations have been used to investigate why water deactivates Co₃O₄ to CO oxidation and activates platinum group metals in the same reaction. The significant difference in the potential energy surfaces of OH on metal versus metal oxides is the origin of the different effects water has in these systems.

Tuning to G(O) flat: Photoluminescence in graphene oxide (GO) suspensions can be tuned from red to blue emission (see scheme) by gradually changing the amounts of sp²- and sp³-bonded carbon atoms through reduction of the surface oxide groups. Electron–hole recombination from two different types of excited states is proposed to explain the luminescence in GO at varying degrees of reduction.

AC or DC? Protein-based photoelectrochemical cells that were constructed using a photosynthetic reaction center (left) generated a conventional direct current (DC) output during continuous illumination but an alternating current (AC) during regular light-on/light-off cycles (right). The mechanism of AC generation exploits the ability of nature's reaction centers to store charge as well as to catalyze highly efficient photochemical charge separation.

Dynamic duo: The reversible reaction of boronic acids with alcohols to form boronate esters, coupled to protein mass spectrometry analyses, was used to discover potent oxygenase inhibitors. This dynamic combinatorial mass spectrometry technique could potentially be applied to the identification of other protein inhibitors.

Realizing their potential: The title assay has enabled rapid screening of diverse mixed-metal oxide electrocatalysts composed exclusively of earth-abundant metal ions. Several newly discovered catalyst compositions (e.g., Ni/Al/Fe oxides) exhibit substantially lower overpotentials for water oxidation than known, widely used electrocatalysts.

96 bonds were formed when 62 building blocks (heterotopic ligands with Fe^II and Pt^II ions) self-assembled in a one-pot reaction into a heterometallic Fe₈Pt₆L₂₄ cubic cage (see model, Fe purple, Pt orange, N blue). The dynamic nature of this method also allowed an efficient cage-to-cage conversion: a tetrahedral Fe₄L₆ cage was converted into the Fe₈Pt₆L₂₄ cube, which was subsequently converted into a second tetrahedral Fe₄L′₆ cage.

Variations on an oxidative theme: The precision with which FAD2-type desaturases carry out CH activation reactions on flexible lipidic substrates is astonishing. The conformational space available within the active site of these enzymes has been explored using deuterium-labeled substrates, and evidence for a novel quasi-eclipsed conformer has been uncovered. The scheme shows some prototypical substrate conformations.

A new spin on an old system: The title neutral radicals have been synthesized and characterized for the first time. Thanks to two terminal methoxy groups and three tert-butyl groups, the chiral radicals are configurationally and chemically stable. The three-dimensional π-electron network shows extensive spin delocalization, and the distinct CD properties are attributed to the chirality of the helicene unit (see picture).

Many hands make light work: Small templates working together have directed the formation of a giant π-conjugated macrocycle with a diameter of 10 nm—larger than many enzymes. The 24 porphyrin subunits of the nanoring are well resolved in the STM image. The conformation of the nanoring can be controlled by self-assembly of a stable 2:24 double-strand sandwich complex with 1,4-diazabicyclo[2.2.2]octane (DABCO).

No pressure: A simple monomeric tungstate, [WO₄]²⁻, serves as a highly efficient homogeneous catalyst for various transformations of CO₂ at atmospheric pressure. The tungsten-oxo moiety activates CO₂ and the substrate simultaneously. The catalyst system is high yielding and applicable to a wide range of substrates such as amines (see scheme), 2-aminobenzonitriles, and propargylic alcohols.

In control: By controlling the kinetics of alkynylation over aldolization, the challenging asymmetric catalytic alkynylation of acetaldehyde has been realized. The resulting products are attractive synthons which are produced with good to excellent enantiocontrol, and show broad tolerance and applicability, as demonstrated by the synthesis (+)-tetrahydropyrenophorol.

Lights on: When illuminated, p-type Si nanowires donate photogenerated electrons to aromatic ketones, producing reactive radicals that can harvest CO₂ to yield α-hydroxy acids (see scheme). The reaction scheme closely resembles that of natural photosynthesis and gives up to 98 % yield and selectivity. Products obtained by this reaction include important precursors for nonsteroidal anti-inflammatory drugs, such as ibuprofen and naproxen.

Hitting the right target: Differentiation between two keto moieties was accomplished by a regio- and enantioselective bioamination employing ω-transaminases. Using 1,5-diketones as substrates gave access to the optically pure 2,6-disubstituted piperidine scaffold. The approach allowed the shortest synthesis of the alkaloid dihydropinidine, as well as its enantiomer, by choosing an appropriate ω-transaminase.

By combining DNA probes having different target affinities, but with similar specificity on the same electrode, an extended dynamic response of a biosensor spanning three orders of magnitude in target concentration was obtained. By using a different strategy, the useful dynamic range of an electrochemical DNA sensor was narrowed to only an 8-fold range of target concentrations.

Five in one: Catalytic cascade reactions of dienynes catalyzed by cationic rhodium(I)–binap complexes lead to the formation of 1,2-dihydronaphthalenes, naphthalenes, and 1,4-dihydronaphthalenes. These cascade reactions involve up to five fundamentally different transformations, including the catalytic enantioselective carboformylation of alkenes with aldehydes or the cycloisomerization of enallenes.

Just relax: The dissociation constants (K_D) of various molecular interactions can be determined using a novel competition assay with binding magnetic relaxation nanosensors. In this assay, changes in the magnetic relaxation (MR) of an aqueous suspension of the nanosensors facilitate the fast determination of K_D values by using nanomolar protein concentrations (see picture).

The radiochemical synthesis of [¹⁸F]4-fluorophenols is based on phenol umpolung under oxidative conditions and direct nucleophilic fluorination with [¹⁸F]fluoride (see scheme, TBAF=tetra-n-butylammonium fluoride, TFA=trifluoroacetic acid). Readily available O-unprotected 4-tert-butyl phenols are used as precursors in this one-pot protocol. The reaction is completed in less than 30 minutes at room temperature and can be performed using standard or microfluidic technology.

Distorted germylene: A distorted seven-membered N-heterocyclic germylene, which has a germanium center of relatively high Lewis acidity, reacts with benzaldehyde to give (2+1+1) or (2+2+1) cycloadducts. The type of cycloadduct produced in the reaction is dependent on the ratio of substrates used. It is considered that the reactions involve stereoselective 1,3-dipolar cycloadditions of an in situ generated carbonyl germaylide.

Resistance is futile: A platinum(IV) prodrug conjugated to a gold-nanorod-based delivery agent avoids the type of drug resistance that is associated with cisplatin (see picture). This conjugate is taken up into cells through endocytosis, thus avoiding the resistance-associated uptake mediated by the copper transport protein Ctr1. The platinum(IV) prodrug is more inert than cisplatin to glutathione and metallothionein, which cause deactivation.

Building helicenes: A photochemical cyclization approach affords helicenes in which the fused ring sequence ends with a phosphole unit (see scheme). The stereogenic phosphorus centers of the substrates control the screw sense of helical chirality. The terminal phosphole units undergo photochemical [2+2] annulations to give dimeric helical structures.

Highly active: Surfactant-free Ni nanocatalysts were highly dispersed into the nanoporous carbon support MSC-30 through a dry process, and the obtained Ni@MSC-30 catalyzed the generation of H₂ from the hydrolysis of ammonia borane at room temperature (see picture). The improvement of the performance of surfactant-free non-noble metal nanocatalysts is a promising step toward the application of NH₃BH₃ as an applicable chemical hydrogen storage material.

The tungsten benzylidyne complex [PhCW{OSi(OtBu)₃}₃] efficiently catalyzes the metathesis of conjugated diynes and ring-closing diyne metathesis (see scheme). Although this reaction implies CC single-bond activation, ¹³C labeling studies reveal that it proceeds by classical alkylidyne group exchange and involves cleavage and formation of carbon–carbon triple bonds.

Fixed: Cyclic imines, in which the CN bond is constrained in the Z geometry, have been identified as highly effective substrates for enantioselective rhodium-catalyzed additions of potassium alkenyltrifluoroborates. Not only is the alkene in the products a useful functional handle for subsequent manipulations, products containing aryl sulfamates may be employed in nickel-catalyzed Suzuki–Miyaura and Kumada coupling reactions to generate further compounds of interest.

Coordinative flexibility and ligand bifunctionality play important roles in a biomimetic iron complex of a hexadentate ligand (see picture). This system can bind an auxiliary iodosylbenzene ligand to give the first structurally characterized metal complex of this important oxidizing agent. The complex is a protected precursor for Fe^VO species, and the selective high-yielding catalytic oxygenation of a sulfide is demonstrated.

Non-deformable axles based on paranemic crossover DNAs can serve as components for mechanically stable DNA rotaxanes. The required design strategies provide generally applicable guidelines for the incorporation of mechanically interlocked DNA architectures or even DNA origami structures. DNA rotaxanes with mechanically reinforced axles can serve as precursors for complex molecular machines capable of force transmission.

A novel refinement technique for X-ray diffraction data has been employed to derive SO bond orders in sulfur dioxide experimentally. The results show that ionic SO bonding dominates over hypervalency.

A radical B12 synthesis: A recently described blue 7,8-secocorrinoid, a degradation product of vitamin B₁₂, has a remarkable structural predisposition for ring closure. Reductive reconstitution of the corrin macrocycle by means of a stereospecific radical reaction leads to a 7-epicobalamin. Human B₁₂-binding proteins bind this vitamin B₁₂ analogue significantly less strongly than natural cobalamins.

SiSi activation: Reversible formation of a donor–acceptor complex between an N-heterocyclic carbene and a cyclotrisilene with carbon-based substituents shifts the electron density of the double bond and thus induces strong polarization, as shown by the significantly pyramidal tricoordinate silicon atom.