Angewandte Chemie International Edition

The composition of Stradivari's varnish has raised numerous hypotheses and controversies for the past two centuries, although a clear understanding of the materials could not be reached. In their Communication on page 197 ff., J.-P. Echard, L. Bertrand et al. describe the chemical stratigraphy of the varnishes from five representative Stradivari instruments by using a wide array of analytical techniques. In particular, Stradivari used several red pigments, and may have sought a variety of tints to give his instruments their beautiful appearance.

Redox-active monolayers form the basis of a solid-state setup that is able to mimic the input/output characteristics of electronic circuits. In their Communication on page 169 ff., M. E. van der Boom and co-workers demonstrate sequential logic operations controlled by multiple chemical inputs, with one circuit equivalent to a set–reset latch—one of the fundamental parts in random access memory. The kind of logic (combinatorial or sequential) can be controlled by keeping the current state static or dynamic.

Wiley-VCH, Weinheim 2009. 283 pp., hardcover € 32.90.—ISBN 978-3527325467

“The secret of being a successful scientist is to have co-workers who are better than you ever were. My favorite subject at school was chemistry …” This and more about Anthony P. Davis can be found on page 26.

A fruitful endeavor: Bent-core or banana mesogens may form chiral liquid-crystalline phases even though the molecules themselves are achiral. In the examples provided, short-range orientational and positional order alone caused the formation of chiral isotropic liquids from achiral molecules (see AFM image of nanofilaments in one such liquid crystal).

Clicking along nicely: Few methods have been reported to be an efficient entry to trisubstituted triazoles with high regioselectivity. This challenge has inspired a search for new reactivity and the development of new chemical approaches. The development of an efficient, robust, one-pot procedure as a route to highly decorated trisubstituted triazoles will be an added bonus to the range of click reactions (see scheme).

More than meets the eye: The use of chiral olefin metathesis catalysts should not be viewed as relevant only to cases that require control of absolute stereochemistry: such chiral catalysts often offer levels of efficiency, product selectivity, and E/Z stereoselectivity that are unattainable with the achiral variants (see example; TBS=tert-butyldimethylsilyl).

Born of frustration: Sterically encumbered Lewis acid and Lewis base combinations do not form “classical” Lewis acid/base adducts. Rather, the unquenched Lewis acidity and basicity of such sterically “frustrated Lewis pairs (FLPs)” is available to heterolytically activate hydrogen (see picture) and can be subsequently utilized for hydrogenation catalysis. FLPs also react with a variety of other small molecules.

A periodic dependence on the Au segment length is observed for the electric field at the nanogap of long-segment Au nanostructures. An optimized geometry of these platforms leads to an intense surface-enhanced Raman scattering (SERS) signal at the nanogap (see picture). Information about molecular transport and vibrational spectra may therefore be simultaneously obtained.

For V′s a jolly good fellow: A one-pot reaction of aniline derivatives with VO(OiPr)₃ in the presence of NaH affords either trinuclear arylimidovanadium(V) triisopropoxide with a tridendritic centrosymmetric structure (see picture, left) or axially chiral binuclear arylimidovanadium(V) triisopropoxide (right), wherein self-assembly creates a highly ordered molecular arrangement in the solid state.

Micro sandwiches: The new technique of hydrodynamic focusing lithography (HFL) utilizes stacked microfluidic flows for polymer microparticle synthesis. The method can improve the throughput of flow lithography for multifunctional particles and produce more complex chemically patterned particles (see examples).

Room for accommodation: A novel porphyrin-related macrocycle was constructed by replacement of the dipyrromethene unit with a 1,10-phenanthroline moiety. This macrocycle is capable of complexation and fluorescent detection of Mg²⁺ with high selectivity over other physiologically relevant metal ions such as Na⁺, K⁺, and Ca²⁺. It functions well as a fluorescent sensor for Mg²⁺ even in HEPES buffered aqueous DMSO solution (pH 7.4).

Detection of gamma rays is shown using a non-scintillating organic-based sensor composed of poly(olefin sulfone)/carbon nanotube blends. Functionalization of the polymers can be performed after polymerization to tailor their structure with different pyrene and bismuth-containing moieties not accessible by copolymerization, and a systematic improvement in sensitivity is achieved in this way.

Enantiopure metallahelicenes have been prepared by cyclometalation of 2-pyridyl-substituted benzophenanthrenes followed by resolution using chiral HPLC. They are red phosphors at room temperature and their chiroptical properties can be modulated by oxidation of the metal center to the oxidation state IV.

More than one job: Quantum dots (QDs) conjugated with thiol-modified small interfering RNA (siRNA) were functionalized with thiol-modified RGD and HIV-Tat peptides. These multifunctional QDs were used for the targeted delivery and tracking of siRNA molecules for the knockdown of the EGFRvIII gene, which led to the down-regulation of the PI3K-Akt signaling pathway and the apoptosis of malignant brain cancer cells.

J-aggregates respond dynamically to vortexes created by stirring. The CD signal inverts with stirring sense and its intensity increases. Prolonged stirring leads to deposition of chiral aggregates on the cuvette wall, the chirality of the deposits depends on the stirring sense. Stirring shifts the equilibrium of a racemic mixture towards the side chosen (and favored) by the stirring sense (see picture, CW=clockwise, CCW=counter clockwise stirring).

Big, bigger, biggest: Polyoxoniobate anions [HNb₂₇O₇₆]¹⁶⁻ and [H₁₀Nb₃₁O₉₂(CO₃)]²³⁻ incorporate pentagonal Nb(Nb)₅ building blocks; the central Nb ion is seven-coordinate within the clusters. The Nb₂₇ species was observed using ESI-MS, thus demonstrating some solution stability; the Nb₃₁ species is chiral and incorporates a carbonate ligand in the outer section of the cluster. The two species are the largest polyoxoniobates reported to date.

Getting the better of troublemakers: C-terminal peptide alcohols cannot be synthesized by conventional solid-phase peptide synthesis (SPPS) because of the absence of a free carboxylic group to attach to the resin. This problem was circumvented by anchoring a β-amino alcohol residue to the resin to provide a starting point for SPPS. An intramolecular O–N acyl shift completed the synthesis of the desired peptides (see scheme).

New donors—new products: Threonine aldolases (L-TA, D-TA) have now been found to accept donors other than glycine. In a simple asymmetric biocatalytic aldol reaction alanine, serine, and cysteine reacted with a range of simple acceptor aldehydes to yielded α-substituted serine derivatives (see scheme; PLP=pyridoxal phosphate).

The entry of the gladiators: Two effects oppose each other for control of the intermetallic interactions within polynuclear (supra)molecular helicates in solution (see picture). Coulombic interactions produce large intermetallic repulsion at short distance, and solvation effects result in a large intermetallic attraction for small pseudo-spherical ions with short intermetallic separations d.

What's the potential? An anodic oxidation/organocatalytic α-arylation of aldehydes using substituted electron-rich aromatic compounds has been developed. The method gives access to meta-substituted anilines and dihydrobenzofurans in good yields and excellent enantioselectivity (see scheme; Pg=protecting group). This method is an example of a new concept combining organocatalysis with electrochemistry.

Burning an ethanolic solution of vanadyl(IV) acetylacetonate in a glass beaker affords monoclinic VO₂ [VO₂(M)], and thus brings this formerly expensive oxide into the realm of conventional laboratory synthesis. Differential scanning calorimetry (DSC) showed consistent heating and cooling curves (see picture) for 50 reversible transitions between VO₂(M) and the higher-temperature rutile phase of VO₂.

Interior decorating in nanospace: One or two rod-shaped bissulfonate bridging ligands were incorporated into a self-assembled Hg^II capsule by the site-selective replacement of inner TfO⁻ ligands. TsO⁻ ligands were arranged inside the resulting capsule, in which the bissulfonate ligand(s) connected two opposite Hg^II vertices, by ligand exchange of the remaining inner TfO⁻ ligands (see picture). Tf=trifluoromethanesulfonyl, Ts=p-toluenesulfonyl.

Unwringing the ring: Methylenecyclopropanes reacted with vinyl and aryl Grignard reagents in the presence of a nickel catalyst to afford vinyl- and arylmagnesation products, respectively, through a selective CC bond cleavage (see scheme). The reaction provides a new method for the preparation of substituted homoallyl and allyl Grignard reagents.

Better than Elmer's glue: Three necessary conditions for enhancement of solid interfacial interactions guide materials design to create strong, stable composites for interfacial adhesion. Ab initio molecular dynamics simulations were used to study copper adhesion on TaN(111) surfaces with a variety of intervening metals to enhance adhesion (see picture). The predicted adhesion phenomena agree well with experimental observations.

Michael's a square: An easily prepared squaramide catalyst that promotes the highly enantioselective Michael addition reaction of diphenyl phosphite to a range of nitroalkenes is described. This method leads to chiral β-nitro phosphonates, which are precursors to biologically active β-amino phosphonic acids.

Laser-ablated titanium, zirconium, and hafnium atoms react with BF₃ during condensation in excess argon to form the first group 4 borylene complexes, FBMF₂ (see structure). The products are identified from matrix infrared spectra and characterized by density functional and CASSCF/CASPT2 methods.

Two rings to rule them all: A versatile method has been developed for the room-temperature synthesis of diquinanes from acyclic precursors, thereby generating two rings, three stereocenters, and a double bond with high selectivity. The products of the double cyclization can be derivatized with excellent diastereoselection into an array of highly functionalized compounds.

Two by two, hurrah! Hurrah! Dimers of Ag nanospheres with a range of sizes were generated by etching Ag nanocubes with Fe(NO₃)₃ in ethanol in the presence of poly(vinyl pyrrolidone) (PVP). These well-defined dimers (see TEM image) enable systematic study of the hot-spot phenomenon in surface-enhanced Raman scattering (SERS).

Having a selective memory: Osmium(II)-based monolayers on glass substrates are versatile platforms for the generation of several sequential logic circuits with multiple inputs which are able to display random access memory (RAM) functionality in the form of a set/reset latch. Additionally, the type of logic displayed, for example, sequential or combinatorial, can be controlled by keeping the current state static or dynamic.

Pretty vacancy: The formation energy of Al vacancies in aluminum nitride is decreased by doping with nonmagnetic scandium ions. These vacancies are shown to be the cause of the room-temperature ferromagnetism in the resulting 1D hexagonal nanoprisms of AlN:Sc (see micrograph), a result that is confirmed by first-principles calculations. The doping approach provides a new route to dilute magnetic semiconductor materials.

Cleaning up polymerase chain reactions: DNA polymerases are found that copy two additional nucleotide letters (Z and P) in an expanded DNA alphabet to support six-letter polymerase chain reactions (PCR). Incorporated into external primers in a threefold multiplexed PCR, primers containing Z and P gave much cleaner results than standard multiplexed PCR.

Silver service: Two simple compounds, H₂O⋅⋅⋅AgCl and H₂S⋅⋅⋅AgCl (see picture), formed by interaction of either H₂O or H₂S with AgCl, are detected by rotational spectroscopy. AgCl is produced by laser ablation of silver in the presence of CCl₄ and then picks up H₂O or H₂S. H₂O⋅⋅⋅AgCl is isomorphic with its hydrogen- and halogen-bonded analogues H₂O⋅⋅⋅HCl and H₂O⋅⋅⋅ClF, while H₂S⋅⋅⋅AgCl is similarly related to H₂S⋅⋅⋅HCl and H₂S⋅⋅⋅ClF.

Two in one: A periodic mesoporous phenylene silica (PMO) with acidic framework walls and basic pore channels was obtained by using protecting-group techniques. This approach allows location of the acidic groups in the hydrophobic benzene layers and of the amine groups in the hydrophilic silica layers. The bifunctional material is an efficient catalyst for the tandem conversion of benzaldehyde dimethyl acetal into 2-nitrovinylbenzene (see scheme).

One at a time: Stepwise opening and final removal of the η¹-bound Cp* substituents in the bridged pentelidene complexes [Cp*E{W(CO)₅}₂] (E=P, As) occurs by the reaction with primary phosphines. Not only novel diphospha- and arsaphosphanorbornenes are obtained, but also diastereomerically pure complexed triphosphines. All reaction steps were monitored by NMR spectroscopy, and compounds structurally characterized.

Demolition with dysprosium: Aberration-corrected transmission electron microscopy allows chemical transformations to be observed at the atomic scale. Formation of dysprosium clusters inside carbon nanotubes, rupture of nanotube sidewalls, and formation of end-caps were observed in situ.

What is Stradivari's “secret”? The composition of the mythical varnish that coats Stradivari's violins has raised controversial assumptions for the past two centuries. By using a complementary array of analytical tools, the chemical microstratigraphy of these varnishes has been established. The results provide information on the materials and techniques that were used by the Master, with a detailed characterization of the varnish.

(+)- or (−)-pinoresinol: that is the question. Which of the two enantiomeric lignans is formed during laccase-catalyzed phenol coupling of (E)-coniferyl alcohol (1) depends on the dirigent protein. In the presence of the first enantiocomplementary dirigent protein AtDIR6, (−)-2 is formed (78 % ee). Preferential formation of (+)-2 is observed in the presence of the dirigent protein FiDIR1, whereas only racemic 2 is formed in the absence of dirigent proteins.

Hide and seek! The reactive intermediate in the title reaction has eluded conventional structure determination for years. With the help of residual dipolar couplings the conformation in solution could be determined (see picture). The sensitive intermediate was aligned in a liquid-crystalline phase containing poly(γ-benzyl-L-glutamate), and the orientation of the ligands was determined through local order tensors.

Removing the core: A new approach for the construction of nanotubes is based on a photocatalytic effect. When TiO₂–ZnO core–shell nanowires in aqueous solution are irradiated with UV light, TiO₂ nanotubes with uniform, tunable walls form (see picture).

Trimesic acid amide serves as a scaffold for a lipophilic cyclophilin inhibitor, a fluorescent rhodamine dye (TAMRA), and a (D-Glu)₆ oligopeptide residue. Although the affinity of 1 for intracellular cyclophilin A (CypA) is very high, fluorescence measurements indicate complete exclusion from the cell. CypA-induced chemotaxis of lymphocytes is inhibited by 1 since extracellular cyclophilins are responsible for the physiological signal.