Angewandte Chemie International Edition

Tuning catalysts: varying the halide moiety represents a valuable method of tuning the reactivity and selectivity of halide-containing metal complexes. The differences observed are a product of the steric and electronic properties of the halide ligand which has the ability to donate electron density to the metal center in a predictable manner (see picture). This review discusses the physical properties of various halide–metal complexes and the effect of varying the halide in various transition metal promoted processes. An understanding of the changes that can arise leads to the notion that the metal–halide moiety can be used as a tunable functionality in the design of better transition metal catalyzed reactions.

A difficult dividing line: The hydrogen bond is the most important of all the directional intermolecular interactions (see picture). It is understood from modern bonding concepts as a very broad phenomenon with open borders to other interactions, such as covalent bonding, ionic, cation–π, and van der Waals interactions. There are dozens of different types of X−H⋅⋅⋅A hydrogen bonds (A=acceptor) with dissociation energies that span more than two orders of magnitude. This review brings together the research of the last ten years on hydrogen bonds, particularly in the solid state.

Putting nitrogen under pressure: The application of high-pressure techniques (for example the depicted diamond anvil cell) allows the synthesis of numerous new materials with useful properties. Recent examples include diazenides, that is, compounds with N₂²⁻ ions (autoclave techniques), γ-P₃N_5, the first compound with tetragonal PN₅ pyramids (multianvil press), as well as spinel-Si₃N₄, a promising hard ceramic material (shock-wave methods).

Chemistry—recipes for success in the kitchen: Molecular gastronomy is the part of food science that investigates culinary processes, such as the formation of the two cheese soufflés shown. It is a multidisciplinary approach involving mainly chemistry and physics. Its five main aims are discussed and recent results are given.

Nucleic acids direct chemical reactions. Recent examples of DNA-templated synthesis suggest that DNA can direct various chemical reactions sequence-specifically and independent of distance in a surprisingly general manner.

A nucleophilicity scale has been developed that covers 30 orders of magnitude from simple arenes to stabilized carbanions (see picture). Amazingly, the addition of an electrophilicity parameter E and a nucleophilicity parameter N is sufficient to predict semiquantitatively the rates of a large variety of organic reactions.

Incarceration strongly increases the rate of the thermal decomposition of diazirine 1 to yield carbene 2. The increase of the inner phase rate is rationalized by a stabilization of the transition state through dispersion interactions between the breaking C−N bonds and one of the highly polarizable arene rings of the surrounding host 3.

At the interface of homogeneous, heterogeneous, and enzymatic catalysis is the catalytic hydrogenation of benzene to give cyclohexane by the triruthenium cluster 1. Experimental evidence and molecular modeling studies strongly support a catalytic mechanism in which the aromatic substrate is hydrogenated in the hydrophobic pocket spanned by the three η⁶-bound arene ligands without being coordinated to a Ru center.

Mesoporous silver with a three-dimensional periodic structure has been prepared electrochemically (see picture) starting from colloidal crystals immobilized in a hydrogel. In this new process it is possible to tune both the size of the enclosures and the thickness of the walls between the enclosed particles in the mesoscale ordered material.

Lock-and-key binding is an inadequate description for the molecular-recognition interaction of guanidinium cations and carboxylates in acetonitrile (see picture) as shown by direct calorimetric determinations in a tailored series of host–guest pairs.

A new tool for the investigation of reaction intermediates: By using DOSY NMR spectroscopy, the presence of a binuclear complex 1 as the key intermediate on the way to 2 (see spectrum) was detected for the first time in the title reaction.

The right choice of sugar auxiliary led to a breakthrough in the first asymmetric α-alkylations of sulfonic acid esters (see scheme). The high asymmetric inductions were reached with 1,2:5,6-di-O-isopropyliden-α-D-allofuranose as the auxiliary group, which can be cleaved off again under mild conditions. Enantiopure α-substituted methyl sulfonates are important building blocks and precursors of bioactive compounds.

The complete protonation of sulfuric acid, one of the strongest acids, can be achieved with the superacid HF/SbF₅. The salt H₃SO₄⁺SbF₆⁻ is isolated at −78 °C from the reaction of the superacid with bis(trimethylsilyl) sulfate (the structure of the D₃SO₄⁺ ion in the crystal of the analogous deuterium compound is depicted).

Bound and clamped: A novel heterocyclic base modification, the guanidino G-clamp, is designed to allow two Hoogsteen-type hydrogen bonds to form between the amino and imino nitrogen atoms of a tethered guanidinium group to O6 and N7 of guanosine, which results in a total of five hydrogen bonds (broken lines, see picture). Details of a crystal structure at 1.0-Å resolution of a modified DNA decamer containing this guanidino G-clamp analogue demonstrate its mechanism of binding.

The relative position of the Asp and Trp residues in a peptide chain is important for recognizing a tetraguanidinium receptor through hydrogen bonding and cation–π interactions. The molecule not only binds with high affinity (K_a=1.1×10⁸ M⁻¹), it also stabilizes the helical conformation of the peptide (see schematic representation) as demonstrated by NMR and CD spectroscopy.

Treating CsAu with liquid NH₃ yields the deep blue ammoniate CsAu⋅NH₃ (see picture and structure). The unusual electronic structure, as well as the properties, indicate that this unique compound, which behaves like an alkali metal when dissolved in ammonia, may serve as a guide to a more thorough understanding of solvated electrons.

More than 1000 yields and ee determinations are possible per day using an immunoassay. This highly efficient screening method is based on the remarkable binding specificity of antibodies. It has been employed in the development of straightforward procedures for the direct conversion of α-keto acids into chiral α-hydroxy acids, for which the enantioselective reduction of benzoyl formic acid to mandelic acid serves as a model reaction (see scheme).

Peptide length affects the size of the ridges observed in the atomic force microscopy (AFM) images of the Langmuir–Blodgett films of amphiphilic peptides: Well-ordered LB films can be prepared from a 14-residue amphiphilic peptide (left), while ordered LB films with a wider lattice (right) are obtained from an 18-residue peptide. The 100 nm×100 nm images pictured were obtained by AFM using carbon nanotube tips.

Coexistence of two kinds of ligand-based mixed-valence (LBMV) isomers is observed for the first time in the title compound (see scheme). Together with its mixed-charged state, the compound shows semiconducting behavior with an intermolecular ferromagnetic interaction, which is attributable to the association of five paramagnetic components in the crystal phase.

A hysteretic adsorption and desorption profile (right) accompanied by a transformation of the crystal structure is observed for the title coordination polymer, which possesses a pillared-layer structure (left), on exposure to H₂O or MeOH vapor under pressure.

Rings with platinum: The first platinum disulfur and diselenium complexes have been synthesized by taking advantage of new phosphane ligands bearing an extremely bulky substituent (see structure). The molecular structures of the disulfur and diselenium complexes are very similar, and these complexes, as well as an analogous dioxygen complex [(PPh₃)₂PtO₂], have a square-planar geometry and a three-membered PtE₂ (E=S, Se) ring.

The exceedingly different size of p orbitals is overcome in the novel doubly bonded system between phosphorus and bismuth atoms that occurs in the first stable phosphabismuthene 1. Compound 1 was obtained from the condensation reaction of Mes*PH₂ with an overcrowded dibromobismuthane by using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base (see scheme).

Conducting films and pellets formed from β-substituted polypyrroles (see picture; R=H, CH₃) erode under aqueous conditions in a pH-dependent manner, which allows erosion rates to be tailored through differences in polymer side-chain structure. These conducting materials support the attachment, proliferation, and differentiation of primary human cells, and could lead to the use of conducting polymers in a variety of new biomedical applications.

Methylidyne, vinylidene, and ethylidyne on a surface: Inelastic neutron scattering (INS) vibrational spectroscopy is used to investigate the surface intermediate species formed during the decomposition of methane on Ru/Al₂O₃ and Ni/SiO₂ catalysts. The findings are compared to previous work on idealized single-crystal model catalysts and begin to bridge the gap between surface science and real-world catalysts in terms of the type of material and pressure used.

Ethylene on a surface: C₂D_4−xH_x (x=0–4) species that result from H/D exchange reactions on a Pt(111) surface are quantitatively identified by using a Cs⁺ reactive ion scattering method (RIS). Two temperature-dependent pathways are identified for the exchange process (see scheme; a: T<265 K; b: T>265 K).

The structural influences of the supporting carboxylate ligand on biologically relevant Fe/O₂ chemistry are emphasized by the unusual Raman and Mössbauer spectroscopic properties of the intermediate derived from 1 relative to those of known (peroxo)diferric species. These (peroxo)diferric intermediates were generated from paddlewheel complexes (L=py, MeIm, and thf) of the new carboxylate ArCO₂⁻ ligand on reaction with O₂. MeIm=1-methylimidazole, py=pyridine.

In only one to four steps, complex structures containing multiple rings, stereocenters, and unsaturated units have been prepared by employing a new annulation reaction of unsaturated boronic esters with allylic and propargylic alcohols (see scheme).

Micropatterned thin films of tailorable molecular materials with uniform, nanoscale cavities, such as 1, were prepared by soft lithographic methods. In this form the materials readily diffract visible light, and because the efficiency of diffraction is enhanced by sorption of guest molecules, the diffraction response can be used for signal transduction in chemical-sensing applications.

One dimensional channels are formed in the three-dimensional open-framework iron selenite [C₄N₂H₁₂]_0.5[Fe₂F₃(SeO₃)₂] (I), which was synthesized hydrothermally from FeCl₃⋅6 H₂O, HF, and SeO₂ in the presence of the organic amine piperazine. The same inorganic framework was also obtained with the amines diethylenetetramine, 1,3-diaminopropane, and ethylenediamine. Compound I displays unusual magnetic frustration behavior that is attributed to the tetrahedral geometry.

Useful tetrahydropyran units such as 3 can be prepared with high diastereoselectivity (d.r.>30:1). A key step is the catalytic asymmetric allyl-transfer reaction from 1 to achiral aldehydes catalyzed by [{(R)-binol}Ti^IV{OCH(CF₃)₂}₂] to give 2 (90–97 % ee). A second allyl-transfer reaction from 2 to a carbonyl compound leads to 3. binol=2,2′-binaphthol.

Facile and efficient: a palladium-catalyzed intramolecular oxidative amination reaction [Eq. (1)] that uses molecular oxygen as a stoichiometric oxidant. These reactions require no cocatalyst for efficient reoxidation of the palladium, they operate in solvents ranging from heptane to dimethylsulfoxide, and achieve up to 250 turnovers (TO) and rates of 70 TO h⁻¹.

Spectroscopic characterization of cationic amides: ZEKE photoelectron spectroscopy has been successfully applied to isomers of the model peptide formanilide. The spectra indicate that significant charge delocalization occurs from the aromatic ring to the side chain in the ion. The picture shows the spectrum of the trans isomer together with the side-chain in-plane bending and amide stretching modes that are associated with the B′ and Σ′ bands. IE=ionization energy.

Aryl boronic acids can undergo a Heck-type reaction catalyzed by Ru^II in the presence of Cu^II, which serves as a reoxidant in each cycle (see scheme, step 1). Compatibility with halide substituents offers attractive synthetic potential (step 2).

The “commercially available chirality” of the binaphthyl units and the efficiency of the coupling reactions were crucial to the synthesis of double-helical alkynyl cyclophanes 1 in enantiopure form. The unique properties of these molecules have been unambiguously confirmed by single-crystal X-ray structure analysis and CD spectroscopy.

The substituent on boron allows the electronic nature of the heterocycle of B-heteroatom-substituted (R=H, O, S, N, P, and F) 1,2-azaborolyl complexes to be modulated. Given that 1,2-azaborolyl is isoelectronic with cyclopentadienyl (see scheme), one of the most widely used ligands in organometallic chemistry, it seems likely that the diverse array of compounds synthesized in this study will stimulate the development of applications of η⁵-(1,2-azaborolyl) ligands in metal-catalyzed processes.

Highly stereoselective radical cyclization and olefin metathesis reactions for the construction of oxacyclic building blocks were key steps in the convergent total synthesis of the orally active antifungal agent ambruticin (1).

Refluxing water is a good solvent for the coupling reactions of aryl and heteroaryl chlorides with phenylboronic acid (see scheme; FG=functional group). The reactions use oxime-derived palladacycles 1 as thermally stable catalysts, potassium carbonate as base, and tetrabutylammonium bromide (TBAB), and can be carried out in the presence of air.

Electroluminescence from 400 to 700 nm (see right picture) is exhibited by an organic electroluminescent device based on (mdppy)BF (left picture) as emitter and a 1,1′-biphenyl-4,4′-diamine derivative as a hole-transporting material. The device emits white light with high efficiency and good color purity.