Angewandte Chemie International Edition

The dirhodium complex formed via a low-coordinate rhodium(I) intermediate has a structure with three different amine–borane activation modes: bridging σ borane, “agostic” BH⋅⋅⋅Rh interaction, and a base-stabilized boryl ligand. This BH activation of an amine–borane ligand, described by A. Weller and A. B. Chaplin in their Communication on page 581 ff., has relevance to the catalytic synthesis of new Group 13/15 materials.

Membranes derived from metal–organic frameworks present a new category of molecular sieves. As demonstrated by Y. Li., J. Caro et al. in their Communication on page 548 ff., a supported zeolitic imidazolate framework (ZIF-7) membrane exhibits high H₂ selectivity by serving as a molecular sieve. The picture shows the synthesis of the ZIF-7 membrane which includes surface seeding and microwave-assisted solvothermal synthesis.

“The secret of being a successful scientist is pertinacity, high frustration tolerance, and ingenuousness. If I could be anyone for a day, I would be Leonardo da Vinci …” This and more about Christian Limberg can be found on page 484.

John Wiley & Sons, Hoboken 2009. 238 pp., hardcover € 84.90.—ISBN 978-0470373170

Beating ring strain: Two novel cyclopropanation reactions (see scheme; LA=Lewis acid, Nu=nucleophile) achieved a great level of selectivity using surprisingly mild conditions. The first used epoxides as a source of methylene in the synthesis of enantioenriched cyclopropanes. The second pushed the limit of the well-established allylic alkylation chemistry to the synthesis of three-membered rings with excellent ee values.

Climate change: Atmospheric models predict that climate change will lead to an accelerated recovery of the ozone layer (see plot; orange/red: satellite data, blue: simulations). However, reliable predictions are complicated by the ozone-depleting effect of N₂O. If emissions of this greenhouse gas remain at current levels, by 2050 they could account for 30 % of the ozone-destroying effects of chlorofluorocarbons at their peak.

Obscure mechanism: [12]Annulyne 1 and three of its isomers were reported in two recent publications as products from the reaction of potassium tert-butoxide (KOtBu) with 1,5-hexadiyne (2), although a plausible mechanism was not proposed. A careful examination of the NMR spectra has now proven that only the two possible 1,3-hexadien-5-ynes (3) were produced. This result had already been obtained by Sondheimer et al. in 1961.

Balancing act: Important advances in ancillary ligand design enable the construction of platinum group metal zwitterions. The reactivity of such zwitterions and their more traditionally employed cationic relatives in σ-bond activation chemistry are compared and contrasted.

A manifold of hydrogen-bonding options on a highly active, functionalized capsule surface exhibiting 20 crown ether type pores allows the fixation and recognition of 20 ammonium cations (see picture; W green, Fe orange, O red, N blue, H light gray), which are partially released in solution, thereby leading to related equilibria.

Redox reactions were utilized to deposit periodically arranged Ag and Au nanostructures within the solvent channels of cross-linked lysozyme crystals (see picture for Ag-doped lysozyme crystals). The ability to crystallize lysozyme in various polymorphic forms provides an opportunity to tailor the architecture of the metalized nanostructures through judicious choice of the protein lattice.

A bubbly personality: Coalescence and fusion of carbon dioxide nanobubbles in a tumor leads to microbubbles that can be imaged by ultrasound (see picture). The bubbles are released from gas-generating polymeric nanoparticles that are coated with hydrophobically modified chitosan. Hydrolysis of the polymer poly(BL-PO), which consists of a biodegradable main backbone and a carbonate side chain, results in the release of carbon dioxide.

TEMPO-bound metalladithiolenes, [M(tempodt)₂]ⁿ⁻ (M=Au³⁺, n=1; M=Ni²⁺, n=2), show a SOMO–HOMO converted unique electronic structure. The one-electron oxidation of [M(tempodt)₂]ⁿ⁻ produces a π radical on the π-conjugated skeleton, which causes an intramolecular cyclization reaction (see picture).

Straining at the leash: The main features of electron-accepting materials with a 9,9′-bifluorenylidene backbone are strain relief and a gain in aromaticity. These dimers (see picture) exhibit absorption near the red spectral region (ca. 600 nm) and HOMO (5.58–5.06 eV) and LUMO (3.37–3.09 eV) energy levels, which, together with high solubility and thermal stability render these materials attractive acceptors for bulk heterojunction (BHJ) solar cells.

Superbugs stalled! Two newly designed synthetic dicationic peptoids (see one example; red O, blue N, green Cl) show promising in vitro bactericidal activity against a range of Gram-positive pathogens, including organisms resistant to vancomycin, methicillin, and linezolid, with only slow development of resistance. Moreover their potency is maintained in vivo.

Whitewater RAFTing: High-molecular-weight (HMW) (co)polymers can be formed using ARGET ATRP with alkyl pseudohalides as initiators and chain-transfer agents. This system works with parts-per-million quantities of copper(I) catalyst whilst retaining excellent control over molecular weight, molecular weight distribution, and chain-end functionality. Copper wire (see picture) can be used as reducing agent several times without additional treatment.

What makes a good cavity? A molecular cavity enabled the stabilization of a selenenyl iodide (RSeI) intermediate formed in 5′-deiodination of a thyroxine derivative by an organoselenol (see scheme). The chemical processes proposed for the iodothyronine deiodinase catalytic cycle were experimentally established.

Molecular strainer: An ultramicroporous zeolitic imidazolate framework (ZIF-7) was tested for its gas-separation properties in membrane applications. ZIF-7 was appended to an asymmetric alumina support using microwave-assisted seeded growth, and the resulting ZIF-7 membrane exhibits high H₂ selectivity as a molecular sieve and has excellent thermal stability.

Back and forth: Reversible structural change and changes in the physical properties of a quasi-1D dinuclear Pt complex occurred with dehydration and rehydration. Upon dehydration, the electronic state changed from an alternating charge-polarization and charge-density-wave (ACP+CDW) state to a narrow-gapped CDW state with an increase in the electrical conductivity and molar spin susceptibility at room temperature.

The α-hemolysin nanopore is a promising sensor for ultra-rapid sequencing of DNA strands within nanopores. By using immobilized synthetic oligonucleotides, it is shown that additional sequence information can be gained when two recognition sites, rather than one, are employed within a single nanopore (see picture).

Breaking the mirror: The reaction sequence described in the title was applied to the synthesis of a valuable chiral drug precursor and the rapid stereoselective assembly of complex compounds with multiple chiral centers (see scheme).

One for the Rh(oad): The first rhodium-catalyzed asymmetric intramolecular hydroamination of unactivated olefins was developed by using dialkylbiaryl phosphine ligands (see scheme; cod=1,5-cyclooctadiene, Cy=cyclohexyl). A variety of 2-methylpyrrolidines have been synthesized with high enantioselectivities.

Open for business: The enantio- and diastereoselective nucleophilic ring opening of five-membered and six-membered cyclic sulfamidates under asymmetric phase-transfer catalysis is presented. A range of pro-nucleophiles have been successfully alkylated in good yields and in good to excellent enantioselectivites.

King of the ring: S-(trifluoromethyl)benzo[b]thiophenium salts 1, as analogues of Yagupolskii–Umemoto type reagents, were synthesized by novel triflic acid catalyzed intramolecular cyclization of ortho-ethynylaryltrifluoromethylsulfanes 2. 1 j is especially useful for the electrophilic trifluoromethylation of β-ketoesters and dicyanoalkylidenes.

Surprisingly useful are thioimidate N-oxides in the preparation of aryl- and vinyl-substituted cyclic ketonitrones through the Liebeskind–Srogl reaction. This novel synthetic approach appears to be efficient and have a broad scope (see scheme).

The missing link: A bimetallic complex that is relevant to the mechanism of transition-metal-catalyzed amine–borane dehydrocoupling has been isolated. The structure (see picture) contains three different amine–borane activation modes within the same molecule.

Ask the locals: Dynamic deformation of the dicopper tetracarboxylate paddlewheel unit within a metal–organic framework from square-prismatic (see picture, left) to distorted (right) occurs at very low energies. This deformation, which contributes strongly to the negative thermal expansion of this system, is a local vibration induced by a redistribution of electron density at the CuO junctions.

Resolving complex systems: The formation of a dynamic hemithioacetal system and its application toward the discovery of β-galactosidase inhibitors were successfully investigated. The designed dynamic system, which has a virtual character in neutral aqueous media, was subjected to a direct in situ identification of the best inhibitors by ¹H STD NMR spectroscopy (ONPG: o-nitrophenyl-β-galactopyranoside).

Dragon Ball La: The unprecedented addition of NO₂ and two benzene moieties to metallofullerene La§C₈₂ is described. The X-ray structure (see picture, right) shows an NO₂ group that is singly bound to a cage carbon that has a high degree of radical character, and two benzene moieties that are linked to highly pyramidalized cage carbon atoms through [5,6]-bond junctions.

Au-ff on a trip: Chiral ligands (L*) and chiral anions [(S)-TriPAg] are employed in the gold(I)-catalyzed enantioselective intramolecular additions of hydrazines and hydroxylamines to allenes. These complementary methods allow access to chiral vinyl isoxazolidines, oxazines, and differentially protected pyrazolidines. PNB=para-nitrobenzoyl.

The shining: Readily accessible BINOL amino alcohol (S)-1 is a highly enantioselective fluorescent sensor for structurally diverse α-hydroxycarboxylic acids, and is the first highly enantioselective fluorescent sensor for the recognition of linear aliphatic α-hydroxycarboxylic acids and α-tertiary-hydroxycarboxylic acids.

Particularly enriching: Core–shell–shell Fe₃O₄§SiO₂§PMMA composite microspheres (see picture; PMMA=poly(methyl methacrylate)) were synthesized readily by combining a sol-gel approach with a seeded aqueous-phase radical polymerization method. The microspheres were successfully applied toward the effective, rapid, and convenient enrichment of low-concentration peptides and proteins for mass spectrometric analysis.

A manganese-based contrast agent for magnetic resonance imaging has been obtained by partial reduction/dissolution of solid β-MnOOH inside the cavity of apoferritin (see scheme). The resulting nanocarriers contain up to 300–400 Mn^II aqua ions and have a remarkable relaxivity of 4000–7000 mM⁻¹ s⁻¹ per apoferritin unit.

Sticking around in the heat: Self-selective, programmable fasteners, based on hybrid core/multishell nanowire forests with an outer shell of poly(N-isopropylacrylamide) hydrogel, reversibly change their wet adhesion properties in response to a change in temperature. The shear adhesion strength of the nanowire fasteners can be modulated by a factor of 170 in under 5 min in response to a temperature change of 5 °C.

Enlarge! CH bond activation and β-carbon elimination are combined in a net intramolecular hydroacylation of alkylidenecyclobutanes and alkylideneazetidines in the presence of rhodium catalysts, affording eight-membered-ring compounds in high yield (see scheme). This study demonstrates the possibility of exploiting the strain energy of azetidines through β-carbon elimination in new transition-metal-catalyzed reactions.

Caught by the octopus: The first structurally characterized metallasilatranes exhibit strong, formally dative Pd→Si and Pt→Si bonds (see picture; C gray, N blue, Si red, S yellow, Cl green, Pt black). The Si atoms are hexacoordinate, and the transition-metal (TM) atom is a lone-pair donor. Short TMSi separations, pronounced upfield ²⁹Si NMR signals, and strong Si–Pt coupling show the interaction between the TM atom and a main-group element with an octet shell.

Paired up: A highly active and enantioselective ion-pair epoxidation catalyst, consisting of an achiral Mn^III–salen complex and a chiral phosphate counteranion, mediates the epoxidization of a wide range of alkenes with high yields and enantioselectivities (see scheme). The unique role of the counteranion is to stabilize an enantiomorphic conformation of the cationic Mn catalyst.

Nanoparticles with luminescent vitamins: ZrO(FMN) nanoparticles containing flavin mononucleotide (FMN) as a luminescent dye are cheap, easy-to-prepare, and biocompatible luminescent markers (see picture). Based on dye-modified zirconium phosphates, moreover, different colors as well as luminescent switching can be realized.