Angewandte Chemie International Edition

A spin-crossover cluster with the {Fe^II₄O₄} core structure is presented by D. Y. Wu, O. Sato et al. in their Communication on page 1475 ff. The cluster is synthesized by self-assembly and shows an abrupt spin transition, giving two high-spin and two low-spin states. It exhibits complete light-induced excited spin-state trapping effects. Importantly, synergy effects between the magnetic interaction and spin transition operate in the cluster.

Hexagonal tubes and rings with unusual shapes were generated through a unique self-template-directed growth mechanism. As described by M. Oh and co-workers in the Communication on page 1459 ff., initially formed solid coordination polymer particles (CPPs) act as templates for the growth of the final hexagonal tubes and rings. Simple calcination of these unusually shaped CPPs results in the spontaneous formation of ZnO particles while the unique shape is maintained.

Following the light: Photoredox catalysis along with aminocatalysis have proved to be the right combination for one of the most challenging asymmetric transformation in organic synthesis: the direct intermolecular α-alkylation of aldehydes.

Sleeping beauty? Is the scientific community about to kiss awake iron catalysis, a topic that has been dormant for decades? A recent study on a user-friendly direct cross-coupling reaction of alkyl and aryl halides constitutes an important step toward this end through the integration of two different iron-catalyzed elementary steps into a practical one-pot procedure.

Lawesson reagent for selenium fans: Phosphorus selenium heterocycles and, in particular, [PhPSe₂]₂ (known as Woollins' reagent) are proving valuable in the synthesis of new heterocycles and for the insertion of selenium into organic substrates. Incorporation of phosphorus and selenium atoms into the products leads to a surprising variety of structures.

An efficient mechanism for the sequence-specific inhibition of gene expression is RNA interference. In this process, double-stranded RNA molecules induce cleavage of a selected target RNA (see picture). This technique has in recent years developed into a standard method of molecular biology. Successful applications in animal models have already led to the initiation of RNAi-based clinical trials as a new therapeutic option.

Throw your hat in the ring: A highly diastereoselective synthesis of the ABC rings of (−)-norzoanthamine has been achieved starting from the (−)-Hajos–Parrish ketone (see scheme). Three asymmetric quaternary carbon centers on the C ring were constructed by a 1,4-addition, and an intramolecular Diels–Alder reaction provided a trans-decalin scaffold on the AB rings.

No bones about it: (−)-Norzoanthamine, a promising candidate for an anti-osteoporotic drug, was the target of a total synthesis (see scheme). The final bisaminal formation with AcOH/H₂O gave the DEFG ring, while the cyclization precursor was prepared by installing the remaining bisaminal unit after oxidative cleavage of the cyclopentanol moiety.

Bonded or not bonded? An ab initio valence bond study of [1.1.1]propellane shows that the two bridgehead carbons are linked by a strong and direct σ bond that is neither classically covalent nor classically ionic, but rather a charge-shift bond, in which the covalent–ionic resonance energy plays the major role. As such, the central bond of [1.1.1]propellane closely resembles the single bond of difluorine.

Ge, whiz! A detailed study of the synthesis, structure, redox chemistry, and bonding properties of pentagerma[1.1.1]propellane (1, see picture) examines fundamental aspects of the interactions between the bridgehead germanium atoms. DFT and CASSCF calculations unravel the biradicaloid characteristics of 1, and preliminary reactivity studies indicate that 1 features some radical-type behavior.

A copper-catalyzed transformation of peptidic thiol esters and boronic acids gives peptidyl ketones and takes place in DMF or DMF/H₂O at room temperature in air (see scheme). This aerobic reaction only occurs at a thiol ester group capable of coordinating to Cu through its appendage on the sulfur center and is not hampered by racemization of the reactants or products.

Tackling blocks: The isoprene-assisted radical coupling (I-ARC) of polymers prepared by cobalt-mediated radical polymerization (see picture) is the first efficient radical coupling method that is not restricted to short chains. When applied to AB diblock copolymers, I-ARC constitutes a straightforward approach to the preparation of novel symmetrical ABA triblock copolymers.

Size doesn't matter: Metallic nitride endohedral fullerenes (MNEFs) exhibit redox properties that are not dependent on the size of the carbon cage, but on its electronic properties. These findings support the ionic model of MNEFs.

Colliding double deckers: Addition of zinc to a reaction mixture of [{Cp*RuCl}₄]/pentamethyl-2,3-dihydro-1,3-diborole (C₃B₂Me₅H) in THF leads to three known double- and triple-decker complexes of [C₃B₂Me₅]⁻, and unexpectedly to the slipped triple-decker (see picture) with two fused diborole rings. The endo CH bonds of two MeCH groups donate two additional electrons to achieve the stable 34 VE configuration.

Noble molecule: [Mg(XeF₂)(XeF₄)](AsF₆)₂ is the first coordination compound in which XeF₄ acts as a ligand to a metal center. It is also the first known compound, in which XeF₂ and XeF₄ are simultaneously coordinated to the same metal center (see structure; purple Xe, green F, gray Mg, yellow As).

Metal lite: High-resolution neutron powder diffraction data reveals that the body-centered cubic crystal structure of lithium(0)tetraamine transforms to a simple cubic structure below 22 K. The detailed structure determinations will allow new insights into the coupled structural and electronic properties of the lightest metal.

Cross-coupling carbenes: The coupling of a propargylic ester with a diazoalkane in the presence of [RuCl(cod)Cp*] catalyst leads to the formation of functionalized conjugated dienes with high stereoselectivity. The reaction involves the cross-coupling of a vinylcarbene fragment, arising from a ruthenium-catalyzed propargylic ester rearrangement, with a diazoalkane carbene.

Formic acid adsorption on ruthenium nanoparticles of different sizes allows differentiation of differently bound formate species by solution ¹³C NMR spectroscopy (see picture). The chemical shifts are comparable to those of organometallic analogues, thus indicating that formate can act as a probe to distinguish surface features of metallic nanoparticles in solution with good quantification and resolution.

Shall we dance? Within the proposed mechanism for the palladium-catalyzed title reaction, the strained alkene norbornene (or norbornadiene) enters and exits the catalytic cycle in a catalytic “square dance”, acting as both a promoter and a coupling partner in the formation of four carbon–carbon bonds, two of them by challenging CH activation processes.

Putting the squeeze on: Hydrostatic pressure causes a shortening of the charge-transfer bond in the binary complex of morphinone reductase and NADH₄ (see diagram). Molecular dynamics simulations suggest that pressure reduces the average reaction barrier width by restricting the conformational space available to the flavin mononucleotide and NADH within the active site. The apparent rate of catalysis increases with pressure.

The design and elaboration of a series of macrocyclic templates that exhibit a propensity to adopt a β-strand-like peptide-backbone conformation led to potent and selective inhibitors of calpain 2. Macrocycle 1 retarded calcium-induced opacification in an ovine-lens culture assay and is a lead compound for the development of a drug for cataract treatment. Cbz=carbobenzyloxy.

This template will self-destruct: A unique particle-growth mechanism involves growth of new coordination polymers on the surface of initially formed hexagonal blocks and concomitant dissolution of the blocks to form hexagonal tubes (see scheme and scanning electron, optical, and fluorescence microscopy images). Calcination of the tubes yields ZnO particles of the same shape.

Chemical scrabble: Mechanistic studies of complex organocatalytic cascade reactions are challenging owing to the presence of many species involved. Electrospray ionization mass spectrometry provides details about a one-pot quadruple organocatalytic cascade reaction, and allows detailed characterization of the reaction and its key intermediates.

Epoxidation made easy: Subnanometer gold clusters immobilized on amorphous alumina result in a highly active and selective catalyst for propene epoxidation. The highest selectivity is found for gas mixtures involving oxygen and water, thus avoiding the use of hydrogen. Ab initio DFT calculations are used to identify key reaction intermediates and reaction pathways. The results confirm the high catalyst activity owing to the formation of propene oxide metallacycles. Al green, Au yellow, O red, and C gray.

SwitchPhos: Rhodium complexes formed from PPh₃ ligands functionalized with weakly basic amidine groups are highly active catalysts for the hydroformylation of alkenes. On bubbling with CO₂ in the presence of water, the yellow rhodium complexes move into the water phase, whereas bubbling with N₂ at 60 °C causes them to switch back into the organic phase. The catalysts can be used for reactions in water or an organic phase.

How low can you go? An Fe^II₄ square was prepared by self-assembly and exhibits both thermally induced and photoinduced spin crossover from a system with four high-spin (HS) centers to one with two high-spin and two low-spin (LS) centers. The spin-crossover sites are located on the same side of the square, and the spin transition and magnetic interactions (see picture) are synergistically coupled.

A new acquisition: Based on “phase-shifted mirrored sampling” (PMS) of indirect evolution periods of multi-dimensional experiments, new acquisition schemes eliminate, without application of a phase correction, dispersive signal components that exacerbate peak identification and shift peak maxima. The resulting enhanced resolution is of particular value for systems with high chemical shift degeneracy.

Longer switching wavelengths and good photochemical yields and stabilities of the cis isomers in reducing aqueous environments are achieved by introducing 2,2′-aminoalkyl substituents into 4,4′-diamido-substituted azobenzenes. The products are thus suitable for photocontrol of biomolecular structures in intracellular environments, such as switching between two peptide configurations (see picture).

Small with control: For miniaturization of protein aggregation experiments the interfacial chemistry must be controlled to avoid protein aggregation caused by interfacial adsorption. Plug-based microfluidics with defined surface chemistry (see schematic picture) can then be used to perform hundreds of aggregation experiments with volume-limited samples, such as cerebrospinal fluid from mice.

Film studies: Epitaxial films of BaZrN₂ (see TEM image) and BaHfN₂ are grown by polymer-assisted deposition on SrTiO₃ (STO) substrates. The films are phase-pure, allowing the intrinsic physical properties of the ternary nitrides to be studied. From 5 to 300 K, the films exhibit metallic-like resistivity–temperature behavior, with large residual resistivity ratios.

A bicycle built for tubulin: The total synthesis of (+)-chamaecypanone C has been achieved by using a tandem retro-Diels–Alder/Diels–Alder cascade reaction (see scheme). Initial biological studies demonstrate that (+)-chamaecypanone C is an inhibitor of tubulin assembly and binds at the colchicine site.

Click chemistry in vivo: Three phosphatidic acid derivatives with alkyne groups in their fatty acid chains were synthesized and incorporated into mammalian cell membranes. Copper(I)-catalyzed and strain-promoted azide–alkyne cycloaddition reactions were used for their visualization (see schematic representation and fluorescence microscopic image).

Selective CH activation with a series of neutral aluminum trisamide bases led to a wide range of polyfunctional aryl and heteroaryl aluminum reagents. Ester and cyano groups are stable under the reaction conditions for this direct alumination, and donor oxygen substituents are efficient directing groups. High metalation regioselectivities were observed with O,S and N,S heterocycles (see example).