Angewandte Chemie International Edition

Polyolefins are ubiquitous materials in everyday life: Although these polymers are traditionally synthesized by heterogeneous transition-metal catalysts, recent advances in single-site catalysts have given birth to a wide array of new materials of precise stereochemistry (see picture). Although olefin-polymerization techniques are superior to their ionic and radical counterparts regarding stereochemical control, they have been inferior in the category of living polymerization. Recent advances in alkene-polymerization catalysts are rapidly eliminating this deficit.

The manipulation of iron uptake in plants is close to being realized. Proteins participating in iron transport have been identified and characterized in the model plant Arabidopsis thaliana. From this result it will be possible to engineer crop plants that take up more iron for nutritional improvement or plants that clean up toxic minerals from contaminated environments.

Improved protecting-group strategies are used in two successful methods for the synthesis of oligoribonucleotides. The effective production of these compounds is of growing importance in the context of RNA interference, a phenomenon that is exploited for investigations into protein function.

The structures of AuH and AuI are already known, now the long-sought-after molecules AuH₃ (=(H₂)AuH) and AuH₅ (=H₃Au(H₂)) have been detected by matrix isolation spectroscopy. Based on new calculations, the global minimum for AuI₃, a missing member of the gold halides, has been elucidated to be that of an L-shaped moiety (C_s symmetry; charge transfer from the I₂ unit to the AuI).

Stray light is a possible source of interference in the electroluminescence spectrum of ITO/NPB/(mdppy)BF/LiF/Al reported by Wang and co-workers (see picture, trace a). Chou et al. observed that four major peaks that correspond to mercury overlap with the reported peaks (trace b).

Stray light is a possible source of interference in the electroluminescence spectrum of ITO/NPB/(mdppy)BF/LiF/Al reported by Wang and co-workers (see picture, trace a). Chou et al. observed that four major peaks that correspond to mercury overlap with the reported peaks (trace b).

Not C₅Me₅⁺(I): C₅Me₅⁺ remains triplet! The reaction of triphenylmethyl tetrakis(pentafluorophenyl)borate with pentamethylcyclopentadiene does not lead to a stable antiaromatic singlet pentamethylcyclopentadiene cation, but to the unexpected pentamethylcyclopentenyl cation (see scheme).

Not C₅Me₅⁺(II): The search for the elusive singlet cyclopentadienyl cation must continue. Quantum-mechanical calculations of both structure and NMR chemical shifts suggest that the recently reported X-ray structure is that of a cyclopentenyl cation (see picture).

Not C₅Me₅⁺(III): Because of the evidence presented in the above Correspondence the authors are retracting the conclusions of their publication “The Stable Pentamethylcyclopentadienyl Cation” which were entirely those of the main author and imply no reflection on the part of his co-workers (whose experimental and theoretical work is valid).

Efficient guest exchange: The organic zeolite analogue TPP⋅x(THF) (x=0.35–0.65) takes up I₂ quickly when exposed to iodine vapor. The previously colorless crystals (a) color at the ends (b), and after 1–2 days the iodine has permeated all the way through the crystal (c). The conductivity values of the TPP⋅y(I₂) crystals are of the same order as those of elemental I₂. TPP=tris(o-phenylenedioxy)cyclotriphosphazene.

Stopcock fluorophores at the ends of zeolite L channels can trap electronic excitation energy from pyronine⁺ molecules inside the crystal (see scheme, top). The reverse process, that is, the injection of electronic excitation energy through such stopcocks (bottom), was achieved with oxonine⁺ molecules inside the zeolite channels.

The reaction of molecular nitrogen with the layered subnitride Sr₂N leads to intercalation and formation of single-phase Sr₂N[N₂]_0.25 (≙Sr₄N₃; see scheme) under mild conditions (T=650°C, p=9 bar). N₂ is reduced to the diazenide stage [N₂²⁻], while half of the strontium is oxidized to Sr²⁺. The intercalation is reversible.

Regioselective synthetic approaches to tetrasubstituted imidazoles (see picture) are reported. These highly substituted heterocycles are potent inhibitors of cytokine release and therefore interesting candidates for anti-inflammatory drugs.

Two generally applicable reagents for photoaffinity probes have been developed. They contain an m-nitrophenyl ether function with a trifluoromethyldiazirine side chain (photophore group), as well as a biotin tag for the identification of labeled proteins or peptides and either a free hydroxy or a squaramide group for the attachment of any suitably functionalized ligand which directs the reagent to the binding site of the target molecule (see picture).

The simultaneous formation of E and Z double bonds results from the syn elimination of H and/or D atoms from different conformations of 4-hydroxysphinganine [D₄]1. Δ⁸-Sphingolipid desaturase from Helianthus annuus is heterologously expressed in yeast and catalyzes the transformation to E olefin 2 (88%) and Z olefin 3 (12%).

A tertiary arsane ligand in a bridging position occurs for the first time in the structurally characterized transition-metal complex 2, which was prepared from 1 by stepwise ligand substitution of both the trialkylstibane and acetylacetonate ligands. The analogous chainlike phosphane-bridged dimer [ClRh(μ-PMe₃)(μ-CPh₂)₂Rh(μ-Cl )₂Rh(μ-PMe₃)(μ-CPh₂)₂RhCl] has also been isolated and characterized by X-ray crystallography.

Fast electron transfer between the Mn centers in the mixed-valent complex 1, a dinuclear analogue of complexes with Cp′-N-ligands (Cp=C₅H₅), supports the occurrence of cooperative effects in such highly preorganized bimetallic systems. An unsual η¹:η¹:η⁵ coordination of the pyrazolate group is observed for K⁺ 1⁻ in the solid state.

Racemization-free coupling of Fmoc-N-methyl amino acids on a solid support is quantitative with a new triphosgene-activation method. With this method, the total synthesis of the nematicidal cyclododecapeptide omphalotin A (see picture) was accomplished, which because of its high content of N-methyl amino acids had not yet been accessible.

The wings of the butterfly structure are widely extended in 2 (see picture, R=Me₃SiCH₂), a complex with a dibismuthene ligand, which coordinates as a “side-on”-bridging, four-electron donor to two tungsten atoms. Complex 2 is formed from the reaction of [W(CO)₅(thf)] with alkylbismuth five- and three-membered rings of cyclobismuthanes 1, which exist in equilibrium.

What's new about complexes 1 and 2, textbook examples of coordination compounds? Quantum-chemical simulations reveal an exceptionally strong sensitivity of their ⁵⁷Fe NMR spectroscopy chemical shifts to the FeC bond length, which, in turn, changes noticeably on going from the gas phase to aqueous solution.

Sublimable without decomposition is the first barium triple-decker sandwich complex [(⁴Cp)Ba(cot)Ba(⁴Cp)] (⁴Cp=C₅HiPr₄; cot=cyclooctatetraene; see picture). Short distances (significantly below the sum of Van der Waals radii) between two of the methyl groups of each ⁴Cp ligand indicate a Ba⋅⋅⋅CH₃ interaction.

Configurational stability is conferred on the complex 1 (R=4-MeOC₆H₄) by the carefully designed tetradentate bis(1,10-phenanthroline) ligand. The resolution and asymmetric synthesis of 1 were readily achieved by using tris(tetrachlorobenzenediolato)phosphate(V) anions (2) as chiral auxiliaries. The picture shows the separation of the enantiomers of 1 by preparative ion-pair thin-layer chromatography.

High-resolution arrays of antibodies can be prepared in a highly parallel manner by a combination of affinity purification and microcontact printing. Arrays with lateral dimensions of between 100 and 3 μm were prepared by using planar, affinity stamps that were patterned by using various soft lithographic techniques. The fluorescence microscopy image shown demonstrates the placement of anti-chicken and anti-goat antibodies on a glass substrate from a stamp.

A crop of gold circles and lines: Psoralen-functionalized Au nanoparticles incorporated into DNA, and Au-nanoparticle-functionalized poly-L-lysine, yield linear and circular nanowires, respectively, on mica surfaces (see atomic force microscopy images).

A method for in vivo affinity chromatography, the yeast three-hybrid assay simplifies protein identification and amplification at the end of affinity panning. The bacterial system described should increase, by several orders of magnitude, the number of protein variants that can be assayed (see picture; SLF = synthetic analogue of FK506; Mtx = methotrexate; FKBP12 = FK506-binding protein 12; DHFR = dihydrofolate reductase; λcI = λ-repressor; αNTD = N-terminal domain of the α-subunit of RNA Pol; RNA Pol = RNA polymerase).

Liberating oxygen: In a novel example of O₂ evolution/activation based on a ligand, in this case, one electronically activated by the OsN multiple bond, compound 2 (tpy=2,2′:6′,2″-terpyridine) is converted reversibly into 1⁺ on addition of H⁺ ions. These reactions are remarkable both for their occurrence and for the rates at which they occur.

Binding oxygen: A four-coordinate chromium(II) complex binds O₂ to yield the first structurally characterized chromium(III) superoxo complex. The superoxide ligand is coordinated in a “side-on” fashion (see structure; BARF=tetrakis(3,5-bis(trifluoromethyl)phenyl)borate). This bonding mode seems to be more widespread than commonly appreciated.

Two fluoride ions in a single azacryptand: The two fluoride ions are bridged by a water molecule (see structure) to give an anion-based cascade complex, which mimics the analogous transition-metal cascade complexes with these ligands.

High diastereoselectivity is achieved in the alkylation of α,α-disubstituted amide enolates to form quaternary carbon products (see scheme; LiDBB=di-tert-butyldiphenyllithium). No chelating groups are necessary for stereocontrol in either the enolate-formation or alkylation steps. In many cases, amplification of the alkylation diastereoselectivity above the isomeric ratio of the intermediate enolates is observed.

The rate-determining step of the ethylene oxychlorination reaction on CuCl₂/γ-Al₂O₃ catalyst could be identified by an in situ, time-resolved XANES study. According to these data (see figure; E=photon energy) and the simultaneously determined catalyst activity, the rate-determining step is the oxidation of CuCl. The dopant potassium chloride, added to the industrial catalysts, decreases the rate of the reduction step, which becomes the rate-determining step for the KCl/CuCl₂/γ-Al₂O₃ catalyst.

Do electrons prefer to visit planes or spheres? Reorganization energies for intramolecular electron transfer involving a 3D acceptor (C₆₀) and a 2D acceptor (NIm; see scheme) have been determined. Comparison of reorganization energies for the intra- versus intermolecular electron transfer has provided, for the first time, valuable insight into the intrinsic reorganization energies relating to different molecular shapes.

Monodentate phosphorus ligands, too, can be effective: Chiral amine derivatives can be obtained in high enantioselectivities (up to 99.7 % ee) by the asymmetric hydrogenation of enamides in the presence of rhodium complexes of the chiral spiro phosphorus ligands (S)-1, the first monodentate P ligands that are effective in this class of reactions.

A versatile atomic boron versus hydrogen exchange led to the formation of perdeuterated benzoborirene in the gas phase according to Equation (1). It might well be that this concept could also be adapted to form more complex heteroaromatic boron-bearing molecules in the gas phase.

Scribing in the presence of reactive species enables silicon to be chemomechanically patterned and simultaneously functionalized with monolayers. Arrays of patches of monolayers are created by scribing different regions of silicon in the presence of different reagents, as shown for the homologous series of 1-alkenes from 1-pentene (A1) to 1-octadecene (B4).

With double jets to nanobelts: Well-defined very thin CdWO₄ nanorods/nanobelts can be easily synthesized by double-jet crystallization in aqueous solution at room temperature in the absence of a polymeric crystallization modifier (see picture, left). Further hydrothermal ripening leads to the self-assembly of the nanorods/nanobelts into raftlike bundles, whereas very thin 2D sheetlike nanocrystals (see picture, right) and 1D nanorods with diameter 2.5 nm are obtained in the presence of double-hydrophilic block copolymers. Both the 1D and 2D polymer-modified species show highly increased fluorescence efficiency.

A highly regioselective benzyl and allyl protection of D-hexopyranosides allows their application in the synthesis of biologically potent α1→2 linked disaccharide derivatives by means of a regioselective one-pot protection–glycosylation (see scheme).

The interaction of an electrophilic carbene carbon atom with the π-electron density of an aryl group favors an intramolecular migratory insertion reaction (see scheme; Pf=C₆F₅). This reaction gives alkyl compounds (2 or 3) which can be identified prior to their eventual hydrolysis to 4.

The chiral organocatalyst bisflavin 1 catalyzes the asymmetric Baeyer–Villiger reaction of cyclobutanones with H₂O₂ (see scheme). The corresponding lactones are obtained with up to 74 % ee.

Controllable reactivity of Cd precursors in the growth of CdS nanocrystals was achieved by varying the concentration of stabilizing ligand with a noncoordinating solvent. Such tunable reactivity is critical for developing the synthesis of semiconductor nanocrystals to the level of that of CdSe nanocrystals. The high quality of the CdS nanocrystals is indicated by the sharpness of the UV/Vis and photoluminescence spectra (see diagram, A=absorbance, I_PL=photoluminescence intensity).

Ligand formed by CN bond cleavage: Transition-metal-promoted heterolytic cleavage of the CN bond in 1 results in the formation of five new complexes of cis hyponitrite with Group 10 transition metals. The new complexes include [Ni(η²-O₂N₂)(dppf)] (dppf=1,1′-bis(diphenylphosphanyl)ferrocene) which is structurally characterized and the thermal decomposition of which follows unimolecular kinetics.

Ruthenium–enzyme tandem catalysis: The novel racemization catalyst 1 improves the dynamic kinetic resolution (DKR) of secondary alcohols dramatically. The DKR proceeds at room temperature with isopropenyl acetate as an acyl donor. In addition, the DKR is faster even with much less lipase than in previous DKRs.

Simple meso-epoxides can be asymmetrically functionalized: Ligand-assisted direct hydrogen–lithium exchange allows the generation of destabilized oxiranyl lithium species and their subsequent trapping by a wide array of electrophiles (see scheme; E=group formed by addition of electrophile). When carried out in the presence of (−)-sparteine as ligand, this reaction provides a new enantioselective route to epoxides.

A combination of Pd- and Cu-catalyzed couplings make the half-wheels (see left and middle picture; Cp=C₅H₅) and the seco-wheel (right) shown here accessible. Single-crystal structures of all three target molecules have been obtained.

Introducing THF to synergic sodium–magnesium amide mixtures (3 tmpH:1 nBuNa:1 Bu₂Mg in a hydrocarbon solution; tmpH=2,2,6,6-tetramethylpiperidine) has a surprising outcome: it leads to the cleavage of the ether and the formation of S₆-symmetric molecules with (Mg₆O₆⋅(NaN)₆) cores (see picture).

A unique zigzag ⋅⋅⋅-Bi-Fe-⋅⋅⋅ chain forms the basis of the structure of [nBuBiFe(CO)₄]_∞ (1; see picture), which was characterzized by single-crystal X-ray diffraction. Compound 1 was isolated from the ultrasonication of the dimeric product [{nBuBiFe(CO)₄}₂] from the reaction of [Et₄N]₃[Bi{Fe(CO)₄}₄] with nBuBr followed by acidification with HOAc.

New wheels: The 1:2 reaction of ferric nitrate with benzilic acid (Ph₂C(OH)COOH) in methanol at pH≈4.0 results in the formation of a new member of the molecular ferric-wheels family with the carboxylatobis(alkoxo) bridging unit. The molecular structure of [{Fe(OMe)₂[Ph₂C(OH)COO]}₁₂] (see picture) consists of a centrosymmetric ring of 12 Fe^III atoms held together by 24 μ₂-methoxide ligands and 12 1,3-bridging carboxylate ligands. The 12 metal ions are nearly coplanar and the ring size is ≈11.4 Å. The Mössbauer spectrum and magnetic susceptibility measurements are presented.

An unprecedented yellow polymer with low-coordinate phosphorus atoms in the backbone has been prepared. The material is soluble in polar organic solvents, and moderate molecular weights (M_n=2900–10 500 g mol⁻¹) were estimated from ³¹P NMR spectroscopic end-group analysis. The possible π-conjugation was investigated by UV/Vis spectroscopy, which revealed a red shift in λ_max for the polymer when compared with colorless molecular-model systems (see picture; left: model system, right: new polymer, in THF).

Intermediate isolated? The reaction of [Fe(CO)₅] with Me₂NCCNMe₂ follows an unprecedented associative pathway to give the ferrabicyclobutenone 1. Complexes such as 1 could be key intermediates in the iron-mediated cyclization of alkynes to cyclopentadienones. Compound 1 undergoes a variety of CC coupling and CC-bond-cleavage reactions to afford a multitude of new organoiron compounds including 2 and 3.

The biologically potent antitubercular and anticancer otteliones A and B (see scheme) have been synthesized from readily available starting materials by using a short, simple, efficient, and flexible strategy. The structural ambiguity of the natural products has been resolved.

Six Mo dimers arranged around a central PO₄ tetrahedron form the core of the novel ε-Keggin polyoxocation [ε-PMo₁₂O₃₆(OH)₄{La(H₂O)₄}₄]⁵⁺ (see structure) capped with four {La(H₂O)₄}³⁺ units. A ³¹P NMR spectroscopy study shows that the ε-Keggin ion is unstable in aqueous solution, and leads to the formation of a polyanion/polycation salt, which is crystallographically characterized.

Olefin metathesis even at 0°C! A phenyl substituent in the ruthenium catalyst 2 leads to greatly increased initiation rates in different metathesis reactions, for example, the cyclization of 1 to form 3.

Good candidates for miniaturized, ultrasensitive gas sensors in many applications are individual single-crystalline SnO₂ nanoribbons. Here it is shown that they can be used to detect ppm-level concentrations of NO₂ at room temperature under UV illumination. The picture illustrates that they work reliably even near their resolution limit under 365-nm light.

In conventional fluids the molecular dipole moments of the individual molecules cancel out, which leads to a macroscopic apolar structure. Directed molecular design using microsegregation and tailoring the molecular shape of compounds such as 1, can lead to fluid layer structures with a macroscopic polar order.