Angewandte Chemie International Edition

Low-spin to high-spin transition is observed for an iron(II) porphyrinate complex with an axial cyanide ligand, as reported by W. R. Scheidt and co-workers in their Communication on page 10144 ff. As demonstrated by temperature-dependent magnetic measurements, Mössbauer spectroscopy, and X-ray crystallographic analysis, coordination of a single axial cyanide ligand does not generate a sufficiently strong ligand field to ensure a low-spin complex under all conditions.

An exceptionally versatile antibody–magnetic-particle conjugate platform combines the properties of magnetic powders with those of antibody recognition for the detection of drugs and drug metabolites within latent fingerprints. In their Communication on page 10167 ff., D. A. Russell and co-workers report how high-definition fluorescence images of fingerprint patterns and chemical information regarding drug usage for a range of drugs or drug metabolites can be obtained by using these particles.

No loss, no gain: In the synthesis of biaryls either the electrophilic or nucleophilic partner can be replaced by a substrate with a carbon-based leaving group (see scheme). Whereas replacing the electrophilic unit is not worthwhile, the use of nucleophiles with a C-based leaving group has decisive advantages.

Chemical concepts and quantum mechanics: Heuristic concepts are indispensable tools to give the almost boundless body of knowledge in chemistry an ordered structure and to present it didactically. Many of these concepts were developed without a full understanding of their physical backgrounds. Pulling together results from quantum chemical calculations and conceptual quantities is not always straightforward and requires close consideration of the definition of the quantity in question.

Selectivity is the key! Chemoselective ligation and modification strategies are important methods for linking synthetic or recombinant unprotected peptides to generate large polypeptides (see scheme). Recent developments in this research area have led to numerous applications in the semisynthesis and total synthesis of proteins.

Before and after: Crystal structures of the DNA (6-4) photolyase from D. melanogaster—one structure in complex with DNA containing a (6-4) lesion (see picture) and one in which the lesion has been repaired—provide new insight into lesion recognition and repair. The proposed mechanism for light-induced, electron-transfer-based repair of the (6-4) lesion does not proceed via an oxetane intermediate.

First bloom: The total synthesis of the cytotoxic marine natural product blumiolide C (1) features a ring-closing metathesis (RCM) for the closure of a nine-membered ring to give the trans-bicyclo[7.4.0]oxatridecene core (see scheme; PMB=para-methoxybenzyl, TBS=tert-butyldimethylsilyl). Attachment of the side chain at C4 of the bicyclic core was achieved in a highly stereoselective manner and excellent yield through an aldol reaction/elimination sequence.

Fit for industry: Stereochemically pure 2-O-(α-D-glucopyranosyl)-sn-glycerol (αGG) was obtained in high yield from an efficient and selective biocatalytic process (see schematic outline). The sucrose phosphorylase catalyzed transfer of a glucosyl group from sucrose to glycerol unites the main advantages of transglycosidases, glycosyltransferases, and glycosynthases for glycoside synthesis and provides access to αGG as an industrial chemical.

A condensation–rearrangement sequence forms the basis of a high-yielding route to chiral homoallylic amines from readily accessible aldehydes (see scheme). This transformation is both the first enantioselective Brønsted acid catalyzed sigmatropic rearrangement and the first example of a catalytic asymmetric aza-Cope rearrangement.

Crystallization seeds in supersonic jets? T-shaped structures dominate the isomer-selective IR–UV double-resonance spectra of benzene–acetylene clusters in supersonic jets. The double T-shaped motif is also present in the 1:1 cocrystal (see picture).

Going loopy: A new iron(II) spin-crossover complex displays a 70 K wide thermal hysteresis loop, which can be explained by an intermolecular 2D network of hydrogen bonds. This is the first example of a spin-crossover complex with wide hysteresis arising from a hydrogen-bond network; it displays the widest hysteresis to date for a structurally characterized complex.

Long-distance RuN-ning: The depicted metal–metal-bonded diruthenium(III,IV) complex binds a terminal nitrido ligand with delocalized RuRuN multiple bonding. The RuN bond (1.76 Å) is remarkably long in comparison with mononuclear ruthenium nitrido complexes and has been shown by vibrational spectroscopy data to be exceptionally weak.

Stereoelectronic factors are apparently responsible for the high selectivity observed in the title S_N1-type reaction of acetates 1 in favor of the anti products 2. The preferred conformation of the intermediate propargylic cations would lead to the syn product; however, a second conformation exists, in which the tert-butyl group adopts an antiperiplanar orientation with respect to the incoming nucleophile. Tf=trifluoromethanesulfonyl, TMS=trimethylsilyl.

Nonclassical chirality transfer? Depending on the substitution pattern of propargyl acetates, a gold-catalyzed homologous Rautenstrauch reaction generates either 5- or 6-membered ring systems (see scheme). The stabilization of cationic intermediates is crucial for this reaction to succeed. The underlying principle for the good chirality transfer observed could be gold-stabilized nonclassical carbocations having configurational stability.

The elusive congener: Selenoacetic acid is prepared for the first time by warming a mixture of acetic acid and Woollins' reagent (Ph₂P₂Se₄) under moisture- and air-free conditions. In the crystal structure of CH₃C(O)SeH, amongst the possible syn (top) and anti (bottom) conformations, the anti is preferred (see picture; C gray, H white, O red, Se magenta).

Changing under pressure: Compressed CO₂ can switch the surfactant sodium bis-2-ethylhexylsulfosuccinate/water system between a lamellar liquid crystal (L_α) phase and a micellar solution (L₁) reversibly at ambient temperature. Applications of this simple, clean, and energy-efficient process in material synthesis are explored, along with a possible mechanism for the phase-transition phenomenon.

Finding the right partner: The catalyst system [NiCl₂(PCy₃)₂] mediates the title transformation with high efficiency and allows aryl–aryl and vinyl–aryl coupling to proceed. The first example of catalytic cross-coupling using the pivalates and organozinc reagents is described (see scheme; Cy=cyclohexyl, piv=pivalate).

Acid trip: A general scale that compares the acidities of metal oxides and sulfides quantitatively is presented, along with Sanderson's electronegativity values of lanthanides.

A fine selection: The installation of chiral centers at the α position of ketones is achieved with excellent enantioselectivity by means of iridium-catalyzed asymmetric hydrogenation of α,β-unsaturated ketones under mild conditions. The catalyst incorporates a phosphinooxazoline (PHOX)-based ligand, which exhibits high levels of asymmetric induction (see scheme; BAr_F⁻=tetrakis[3,5-bis(trifluoromethyl)phenyl]borate; cod=cyclooctadiene).

A short, enantioselective total synthesis of the title compound 1 is described. The 14-step synthesis features a highly stereoselective Brown-type pentenylation and a one-pot hydrosilylation/ring-closing metathesis (RCM)/protodesilylation sequence as the key steps. PG=protecting group.

Making F-ases: One-pot fluorination/base-swap biotransformations were developed by coupling the fluorinase enzyme to nucleoside phosphorylases to generate 5′-deoxy-5′-fluoronucleosides (FDAs). These biotransformations are amenable to radiolabeling syntheses starting from [¹⁸F]fluoride ion, as exemplified by the synthesis of [¹⁸F]-5′-deoxy-5′-fluorouridines (see scheme), and demonstrate a new application of fluorinase as a catalyst for ¹⁸FC bond formation.

A CN weakling? The cyanide ligand in the five-coordinate iron(II) porphyrinate complex [Fe(tpp)(CN)]⁻ (tpp=tetraphenylporphinato) is not a sufficiently strong-field ligand to cause the complex to be in the low-spin state under all conditions. Rather, the complex displays a reversible low-spin (LS) to high-spin (HS) crossover with no hysteresis.

Zinc for a minute: The reaction of the alkylzinc bromide [Zn{C(SiMe₃)₂(SiMe₂hpp)}]Br (see structure) with an excess of sodium hydride gives good yields of an unusual zinc hydride cluster, in which five zinc atoms are linked by (μ-H) bridges, to form a spirocyclic [Zn{(μ-H)Zn}₄(μ-H)₂]⁴⁺ array, in which two six-membered {Zn(μ-H)}₃-rings are fused at one zinc center.

Particle-larly good! Thermal annealing and etching of physical composite butyl-capped Si gels and SiO₂ nanoparticles at 900 °C under an Ar atmosphere is a versatile method for the formation of 3D porous bulk Si particles (see picture). Complete etching of the SiO₂ from the SiO₂/carbon-coated Si (c-Si) composite results in the retention of the remaining c-Si as a highly porous but interconnected structure, which preserves the starting morphology.

Highly stereoselective: A highly enantioselective (up to 97 % ee) and diastereoselective (>30:1 d.r.) Rh^II-catalyzed cyclopropanation of alkenes using a diazo reagent bearing two carboxy groups is described. This new methodology exploits the powerful trans-directing ability of amides to improve enantiocontrol. Mono- and disubstituted olefins are cyclopropanated in good yields. nttl=N-1,8-naphthoyl-tert-leucine.

Unequal partners: Reactions of complexes of the form [Cp*M(μ-H)₄M′Cp*] (1: M=M′=Ru; 2: M=Ru, M′=Os; 3: M= M′=Os) with P(OMe)₃ provide evidence of kinetic and thermodynamic heterometallic effects. Heterometallic complex 2 induces selective addition of P(OMe)₃ to the Ru center in an accelerated reaction. Moreover, the incorporated P(OMe)₃ migrates unidirectionally from Ru to Os owing to the formation of a strong OsP bond.

Cheap and stable: A PtBi catalyst was fabricated in three consecutive electrochemical steps (see picture): electrochemical oxidation of carbon paper to form an adequate catalyst support (1), Pt electrodeposition (2), and underpotential deposition of Bi onto the as-prepared Pt (3). This process resulted in a well-dispersed and thin catalyst layer as well as a significantly enhanced power performance with a Pt loading of only 0.5 mg cm⁻².

Dusting for prints: Antibody–magnetic-particle conjugates comprise protein-coated magnetic particles labeled with dye-tagged fragments (see picture). These particles bind to drugs and drug metabolites such as methadone and benzoylecgonine that are deposited within a latent fingerprint. The conjugates provide a means to image the fingerprint and enable the identification of an individual whilst simultaneously providing evidence of drug usage.

Two faces: Starting from mixed shell micelles, amphiphilic Janus particles with opposite hydrophobic and hydrophilic complex sides were obtained by intramicellar complexation between the mixed shell and the core at pH 3.1 (see picture). Hierarchical self-assembly of these Janus particles in water resulted in a nanoscale tubular superstructure, which was then converted into regular nanosheets upon ultrasonication.

Branching out: Stiff, aromatic dendrimers made up of five and seventeen pyrene units, respectively, are synthesized, characterized, and compared with two model compounds. The rigid and strongly twisted structure allows a precise spatial arrangement in which each unit is a chromophore. The high extinction coefficients and fluorescence quantum yields make these dendrimers attractive candidates for use as fluorescence labels.

Getting together and breaking up: Circular arrays of recombinant antibodies can be produced reversibly by chemically controlled protein macrocyclization. In the presence of bismethotrexate (bis-MTX), dimeric dihydrofolate reductase fusion protein (DHFR) with an anti-CD3 single-chain variable region (scFv) self-assembled spontaneously into diabodies, which were disassembled into monomers with an excess of the antibiotic trimethoprim (see picture).

Stuck on an sp²carbon: A new route to geometrically defined α-alkoxycarbonyl-substituted vinylboronates consists of the chemo- and stereoselective 1,2-addition of copper hydride to acetylenic esters followed by stereoretentive transmetalation with pinacolborane. This strategy is applied to the generation of an aryl acrylate intermediate in the synthesis of the antiinflammatory drug naproxen (see scheme).

Catalyst for THOught: The title reaction using acyclic nitroalkenals for the asymmetric installation of both CO and CN bonds led to functionalized tetrahydro-1,2-oxazines (THOs) having up to three stereogenic centers; the products were isolated in up to 90 % yield with excellent enantioselectivity and complete diastereoselectivity (see scheme). DFT calculations reveal an unprecedented transition state containing a molecule of water, which assists in the CN bond-forming step by participating in two hydrogen bonds.

Constant embrace: Distance measurements obtained from pulsed electron spin resonance spectra show that the arms of EcoRI endonuclease occupy the same positions to enfold the DNA in specific, non-specific, and miscognate complexes.

In the on-deck circle: A μ-oxo-μ-η²:η²-peroxo titanium(salan) complex (see structure; Ti purple, O red, N blue, C gray) was synthesized from the corresponding di-μ-oxo titanium(salan) complex using 30 % hydrogen peroxide. The μ-oxo-μ-η²:η²-peroxo complex is slowly converted into an active species involved in the catalytic cycle of asymmetric epoxidation.