Angewandte Chemie International Edition

Bind the gap …… The cover picture shows an idealized rendering of a DNA minicircle (green) with a 21-nucleotide-long single-stranded region, or gap. M. Famulok and co-workers show in their Communication on page 967 ff. how these circles can be generated and visualized by atomic force microscopy. Any complementary oligonucleotide (red) can hybridize to the gap so that synthetic functional groups can be incorporated. In a second study on page 971 ff. they show that RNA molecules can also hybridize to form a heteroduplex with the gap, enabling the incorporation of nucleic acid motifs in DNA minicircles.

Two complementary approaches to tuning emission colors are based on a single source of YVO₄ nanoparticles doped with Ln and P ions, as described by X. Liu and co-workers in the Communication on page 906 ff. The emission wavelengths and intensity ratios of the emission bands can be precisely controlled by the choice of dopant (Ln=Eu, Dy, Sm) and control of dopant concentration. In this manner, the color of emitted light can be easily tuned under single-wavelength excitation.

Cleaning up without enzymes: Alkyl mercury cations are poorly degradable. Protonolysis of HgC bonds by bacteria takes place with the aid of organomercury lyase, and functional models based on dihydroimidazolethione ligands have been recently reported. Relatively high metal coordination numbers (e.g. 4, see figure) are the key to bond activation and thus to non-enzymatic detoxification of the neurotoxic methylmercury cation.

Unlocking the cage: New methods for the activation of white phosphorus have demonstrated that transition-metal complexes may facilitate some remarkable new transformations of P₄. Furthermore, main-group species such as carbenes and silylenes may also promote the formation of synthetically important phosphorus-rich compounds.

Lateral thinking: Molecular solids with tunable color and excellent emission efficiencies or increased electron affinities have been obtained by lateral boryl substitution on a π-delocalized framework (see picture, D=donor group, B=boryl group). This approach gives rise to a new design principle for organic optoelectronic materials.

Olé to olefins: Olefins as components of transition-metal-catalyzed reactions can influence the outcome of a reaction through increased activity, stability, or selectivity. This Review outlines the interaction of olefins with transition metals and documents examples of olefins that influence catalytic reactions; a particular focus is on cross-coupling reactions.

Starting gate: A molecular logic gate is based on chemosensors that change their fluorescence intensity in response to various inputs (pH, metal ions) in a microfluidic system. A combinatorial circuit, such as a half adder (see picture), uses fluorescein (green) and rhodamine B (red) derivatives that respond to pH. A logic gate with a protein and Cu²⁺ ions as inputs is also described.

Molecular design: Metal-tunable octahedral nanocages that act as molecular chemosensors are generated by a new strategy (the metals can be transition metals or lanthanides). Amide groups in the supramolecular structures act as guest-accessible functional sites. These interact with glucosamine molecules and function as efficient communicators to convert the recognition information into metal-tunable chromogenic and fluorogenic responses (see picture).

Greater than the sum of its parts: Hydrogen storage in complex hydrides is accelerated by using the ternary composite 2 LiNH₂/LiBH₄/MgH₂, which exhibits a “self-catalyzing” reaction pathway that results in faster H₂ desorption, lower desorption temperatures, and suppression of NH₃ release in comparison to the constituent binary composites (see diagram). The enhanced properties arise from the incorporation of an ionic liquid phase (Li₄BH₃H₁₀) and from ancillary-reaction seeding of a reversible H₂ storage reaction.

Two for the price of one: A Pd/dppf-based catalyst provides access to the title compounds from 1,2-dihalogenated aromatic compounds and allylic amines in a single reaction flask. The initial aryl amination step occurs with excellent selectivity for the aryl iodide to ensure the formation of a single indole regioisomer, which can be functionalized in situ by N-arylation (see scheme). dba=dibenzylideneacetone, dppf=1,1′-bis(diphenylphospanyl)ferrocene.

A stereochemical twist in the tale: The treatment of the propargylic acetal 1 with one equivalent of n-butyllithium provided the dihydrobenzofuran 2 with an exocyclic E double bond. The results of DFT calculations suggest that strong coordination between the lithium cation and one oxygen atom of the acetal group drives the triple bond toward an E alkene. (A representation of the electron distribution near the transition state is shown on the right.)

Mix and match: A self-assembled ligand library (SAL XY) affords a wide range of R/S ratios in Rh-catalyzed asymmetric hydroboration (see scheme; nbd=2,5-norbornadiene, R* is a chiral substituent). Ligand-scaffold optimization reveals “substrate-tailored” ligands that afford high regio- and enantioselectivity for a variety of ortho-substituted styrene derivatives.

Binding by the light: A photoactive ruthenium(II)–pterin wire designed to target the tetrahydrobiopterin (H₄B) binding site of inducible nitric oxide synthase (iNOS_heme) binds to iNOS_heme in a competitive fashion with the natural cofactor and other small molecules; thus it can be used to screen potential inhibitors. Rapid charge injection to the heme occurs upon 450-nm excitation of an iNOS_heme/wire conjugate.

Film noir: A centimeter-scale foam film of nonionic surfactant can be prepared by using the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]). On transferring to a substrate with submillimeter holes, the black films on the holes spontaneously become thinner to give reversed bilayers solvated with the ionic liquid (see picture). They are stable under ultrahigh vacuum conditions and at temperatures higher than 150 °C.

Chemical chameleon: Two complementary approaches to emission color modulation are based on (Ln, P)-doped YVO₄ nanoparticle systems. The emission wavelengths and intensity ratios can be controlled using different host–activator systems and dopant concentrations. This approach allows access to a wide range of colors (see picture) using single-wavelength excitation.

Complex polycycles made simple: A practical synthesis of the simplest cross-conjugated pentaene, [5]dendralene, is reported. The hydrocarbon undergoes controllable and atom-efficient domino sequences (see scheme) to generate complex polycyclic frameworks in an operationally simple manner.

Capacitive chemical sensors, coated with modified cyclodextrins, provide antipode signals upon exposure to the enantiomers of methyl propionate compounds, thus allowing an unambiguous determination of the enantiomer identity (see graphic). The dielectric coefficients of the enantiomer–receptor complexes significantly differ, so that this simple sensor technique can be used to detect the subtle effects of different molecular orientations.

Self-oscillating microgels have been obtained by introducing a Ru catalyst for the Belousov–Zhabotinsky (BZ) reaction into a cross-linked copolymer. In this microgel system, the chemical energy of the BZ reaction is converted to rhythmic swelling/deswelling with self-flocculating/self-dispersing oscillation of the microgel around its phase-transition temperature (see scheme; Ru(bpy)₃ denotes a Ru tris-bipyridine complex bearing a vinyl group).

Slide rules: A new class of membrane-active rigid-rod molecules comprising π-acidic, n-semiconducting oligoperylenediimides is introduced (see picture). They are able to combine passive anion transport across lipid bilayers with photoactive electron transport in the other direction across intact vesicle membranes. Thus, the compounds demonstrate artificial photosynthetic activity.

Ring around the amine: Reactions which give heterometallic Cr_xCu_y rings can be controlled through choice of reaction times, and the mixed products can be separated and crystallized. These new compounds include Cr₇Cu, Cr₈Cu₂, Cr₁₂Cu₂, and Cr₁₁Cu₂ rings (see structure; Cu orange, Cr green, F yellow, O red, N blue, C black). The last compound is the largest ring known with an odd number of metal atoms.

A borate in the hand is a convenient reagent for palladium- and copper-catalyzed CC and CN bond-forming reactions, especially when, like the title bench-stable complexes, it demonstrates high transmetalation efficiency. Cyclic triolborates can be synthesized readily from organoboronic acids (see scheme), can be handled and stored without special precautions, and are relatively soluble in organic solvents.

Taking on water: A novel catalytic asymmetric metal–carbenoid insertion reaction with water has been developed with copper complexes of chiral spiro bisoxazoline ligands as catalysts. This reaction provides an efficient and practical procedure for preparing chiral α-hydroxyesters and acids starting from readily available materials in high yields and enantioselectivities. BAr_F⁻=[B{3,5-(CF₃)₂C₆H₃)}₄]⁻.

Divided we stand, together we fall: Incompatible reagents, such as water and LiAlH₄, Grignard, or cuprate reagents, can be site-isolated on either side of a polydimethylsiloxane (PDMS) thimble (see scheme; SDS=sodium docecyl sulfate) for cascade reactions. The hydrophobic nature of the PDMS membrane prohibits polar molecules, such as water, from diffusing across the membrane.

Iron competes with pricey ruthenium: The first catalyst systems using iron complexes for asymmetric H₂ hydrogenation at 50 °C and asymmetric transfer hydrogenation at room temperature has been discovered. The transfer hydrogenation activity is similar to that of the best ruthenium catalyst. Surprisingly, the precatalysts have a tetradentate diiminodiphosphine ligand which has no NH functionality.

Extensive pharming: An expedient entry into the tetracycle 2, a late-stage synthetic intermediate en route to the broad-spectrum antibiotic (−)-platensimycin (1) has been accomplished. The strategy involves a series of cyclizations starting from the inexpensive chiral pool starting material (R)-(−)-carvone.

A twist in the Ugi four-component reaction: The reaction of an aldehyde, an isocyanide, N-methylhydroxylamine, and acetic acid in the presence of 4-Å molecular sieves afforded α-ketoamides in moderate to good yields (see scheme). In contrast, 3-Å molecular sieves did not promote the desired multicomponent reaction. R¹=alkyl; R²=alkyl, aryl.

Crossing the seven C's: The three carbon atoms of several allenyl fragments can be incorporated into seven-membered carbocycles by means of a Pt-catalyzed intramolecular [4C+3C] cycloaddition with dienes (see scheme). The transformation provides a straightforward and atom-economical entry to a variety of cycloheptane-containing polycycles from readily available acyclic precursors.

The metal does the trick! Atomic absorption spectroscopy (AAS) of cobalt atoms is used as an accurate method to determine the cellular uptake and nuclear localization of metal bioconjugates. Surprisingly, the PNA conjugates show highest uptake efficiency, and an accumulation 150 % higher than in the culture medium is achieved.

Separated and reunited: The photochemical magnetic field effect on the unbound fluorophore observed for 9,10-dimethylanthracene/N,N-dimethylaniline in mixtures of propylacetate/butyronitrile results from the radical pair mechanism in combination with extensive reversibility in the excited state (see picture; HFI: hyperfine interaction). This observation leads to a deeper understanding of the processes involving transient, spin-correlated radical ion pairs.

Aiming high: The coordination of the tin ligand stanna-closo-dodecaborate at Ni, Pd, and Pt results in the formation of compounds with high negative charges. These complexes contain metal centers with high coordination numbers and in high formal oxidation states. In the case of the platinum derivative, a far upfield chemical shift in the ¹⁹⁵Pt NMR spectrum (δ=−7724 ppm) was observed.

Doing the rounds: A highly straightforward approach has been developed that provides access to DNA minicircles containing a 21-mer single-stranded gap region of defined sequence (see picture). The single-stranded domain within a small circular DNA nanoobject provides the exciting opportunity to further functionalize a minicircle at defined positions in almost unlimited fashion, simply by hybridization with a synthetic 21-mer oligonucleotide.

Kissing complexes formed on the basis of highly specific noncanonical interactions of RNA hairpins can be harnessed for the controlled assembly of DNA nanoobjects. Two DNA minicircles, each equipped with a different RNA hairpin motif (see picture), mediate a tight and specific binding of the two circular DNA nanoobjects. These interactions may offer the possibility to construct DNA nanoobjects with increased complexity.

The right kind of upbringing: The size of DNA nanoparticles can be adjusted by an enzymatic reaction. By varying the incubation time of spherical nucleic acid block copolymer micelles (see picture) with a non-template-dependent DNA polymerase, the size of the nanoobjects can be gradually increased.

A denatured membrane protein in 8 M urea was characterized. Two hydrophobic clusters, separated by 50 amino acids in the polypeptide chain, are shown to bind independently to detergent micelles (see picture). Long-range interactions between the two clusters are not observed. These observations provide new insights into protein folding mechanisms.