Chemistry - A European Journal

The cover picture shows the frontispieces of the Concept, Review, and Minireview articles published in this issue, highlighting the diverse range of chemistry that will be covered at the 4th EuCheMS Chemistry Congress. In the top row from left to right: B. Maes et al. (p. 10092) from the symposium Medicinal Chemistry, G. Pacchioni (p. 10144) from the symposium Computational Chemistry, and M. Mastalerz (p. 10082) from the symposium Molecular and Hybrid Porous Crystals. In the middle row from left to right: U. Scherf and A. Patra (p. 10074) from the symposium Polymer Chemistry, M. Kiskinova et al. (p. 10196) from the symposium Structural Research for Tomorrow, and M. T. Reetz et al. (p. 10160, Plenary Lecture). In the bottom row from left to right: D. Enders and C. C. J. Loh (p. 10212) from the symposium General Synthetic Methods, M. Miyata, I. Hisaki et al. (p. 10066) from the symposium Supramolecular Chemistry and Nanostructured Materials, and V. Zanotti et al. (p. 10174) from the symposium New trends in Organometallic Chemistry.

GaGa dancers welcome cationic Ga catalysts in the family of π-Lewis acids. While most members of this family are complexes of noble metals, the carbophilicity of this main-group element could be exploited in tandem reactions involving activation of alkynes and alkenes toward nucleophilic attack. The cationic species are stabilized by a strongly coordinating N-heterocyclic carbene and a labile nitrile. For more details see the Communication by V. Gandon et al. on page 10239 ff.

A definition in hand: A twofold helix (2₁ helix) is an essential motif in approximately 70 % of organic crystals. Although handedness of the 2₁ helix has not been discussed from a mathematical viewpoint, it can be defined by considering the molecular shape and manner of assembly. Herein, the supramolecular-tilt-chirality method is proposed to define the handedness, and is illustrated by way of several examples.

Porous fluorescence! Fluorescent microporous organic polymers (MOPs) and related nanomaterials have attracted significant attention in recent years due to their unique and versatile characteristics and promising application potential in light-harvesting and sensing. Conceptual strategies for the fabrication of fluorescent MOPs (as bulk solid or as nanoparticles) and possible utilization of these materials in various optical applications are highlighted.

From molecule to material: The development of porous materials on the basis of discrete molecules show that it is possible to construct assemblies that can be desolvated, without collapsing, to obtain permanent porous structures. Such structures can have very high surface areas and good gas-sorption properties. Here, existing systems are analyzed in the context of whether it will become possible to achieve materials with ultra-high surface areas.

On the bumpy road to direct success: With a growing number of convenient synthetic methodologies at hand, the direct α-functionalization of cyclic amines has become more straightforward than ever before. This Review covers the recent methodological advances in this challenging field (from the end of 2006) and classifies them according to their associated in situ formed reactive intermediates.

Oxide ultrathin films: Currently, it is possible to prepare oxide films with a thickness of a few nanometers. By playing around with the low-dimensionality of the oxide layer (see figure), one can exploit new properties and functionalities that are not present in the corresponding bulk materials. The applications range from catalysis, fuel cells, gas sensors, corrosion protection, and biocompatible materials to microelectronic and spintronic devices, ferroelectrics, thermoelectrics, and solar energy materials.

Evolution in a test-tube: Directed evolution is a protein engineering method for manipulating the stereoselectivity and thermostability of enzymes as catalysts in organic chemistry (see figure). This Minireview summarizes the recent progress in the application of this method in the quest to create stereoselective mutants of Baeyer–Villiger monooxygenases.

Iron can do: Diiron complexes might provide promising alternatives to precious metals in the challenging field of metal-promoted CC bond formation. This Review is aimed at evidencing distinctive aspects and advantages associated with the presence of two adjacent iron centres and focuses on CC-bond-forming reactions occurring at bridging ligands in diiron complexes (see figure).

Fuel for thought! The recent advances of synchrotron-based X-ray microscopy methods for in situ monitoring of the redox reactions and of evolving structure and chemical composition of key fuel-cell components are illustrated by selected results obtained with model proton-exchange fuel-cells (PEFC) and solid-oxide fuel-cells (SOFC). This type of information is prerequisite for understanding and controlling the performance and durability of such devices (see figure).

A ‘golden’ handshake: While organocatalysis and gold catalysis have evolved as pivotal areas in modern organic synthesis, a novel niche area, namely, the merger of both catalytic systems in the same reaction flask, has not been put under the spotlight until the last four years. Moreover, the synthetic utility of this young emerging concept has permeated major areas of organocatalysis, and therefore, a systematic Minireview of this rapidly growing area is presented.

Quadruple domino/Wittig–one pot: An organocatalytic, asymmetric, three-component quadruple cascade, which leads to tetracyclic double-annulated indole derivatives, is described. The domino products were further derivatized by aldehyde olefination under one-pot conditions to obtain complex isoindolo[2, 1-a]indole derivatives bearing six stereogenic centers in a highly stereoselective fashion.

Set the ace(tone): A palladium catalyst derived from the bidentate XantPhos ligand and Pd(OAc)₂ has enabled broadly applicable mono-α-arylations of acetone to be performed with air- and moisture-stable aryl imidazolylsulfonates as most user-friendly electrophiles (see scheme).

Polyfunctional SF₅-substituted cycles: Several organometallic sequences using zinc, copper, and magnesium intermediates were developed to prepare a broad range of SF₅-substituted aromatic and heterocyclic compounds of potential interest for pharmaceutical applications.

Ga-neration X: Well-defined neutral and cationic gallium(III) halide complexes have been synthesized and evaluated in catalysis. Both the (NHC)GaX₃/AgSbF₆ catalytic mixture and isolated (NHC)GaX₂⁺ species function as exquisite π-Lewis acids in typical GaX₃-catalyzed reactions (see scheme). The cationic complexes are more active than GaX₃ and yet more resistant to hydrolysis, which allows lower catalytic loadings and faster reactions.

Reacting rationally: During the 1e oxidation of ferrocyanide by the catalytic TAML activator/H₂O₂, four Fe^IV tetra-amido macrocyclic ligand (TAML) intermediates were detected that are involved in a fast acid–base equilibrium. The counterintuitive reactivity pattern is explained by the overall free-energy change during the reduction of Fe^IV to Fe^III TAML complexes, with competing contributions from electronic and solvation energy changes (see figure).

Thrive on a polymer: A polymer-supported method is developed and used to prepare well-intergrown and free-standing metal–organic framework (MOF) membranes. By using this method, a stainless steel net/PMMA–PMAA-supported HKUST-1 membrane was fabricated and applied in gas separation. Results indicated that such a membrane has a high separation factor to H₂/N₂, H₂/CH₄, H₂/CO₂ gas mixtures (see figure).

Polymer modification: This study describes a new route for the mono-addition synthesis of polystyrene–fullerene conjugates by means of the very recently popularized thiol–ene chemistry. All instrumental analyses revealed the successfull and mono-addition nature of the conjugation process (see scheme).

Tubes versus spheres: A new inclusion complex of alkylpyrogallol[4]arene and ferrocene is described. Unlike the previously reported dimer, the tubular motif shown is not stable in methanolic solution and dissociates into a dimer. Its formation is thus likely due to a high alkylpyrogallol[4]arene/ferrocene ratio coupled with shifting solvent conditions during evaporation.

The fantastic four: Cystoliths are mineralized objects that are mainly composed of amorphous calcium carbonate (ACC), which is found in the leaves of several plants. They have a unique composition and architecture of four distinct amorphous phases. A Mg-rich silica phase is essential for the formation of two distinct ACC phases. The inner ACC phase has inherently higher stability, presumably required by the sequential formation mechanism.

Supramolecular photochemistry: A RuS coordination bond, which spontaneously forms in the dark and is cleaved by visible-light irradiation, is used to hop ruthenium complexes at a water-bilayer interface at room temperature (see figure).

Active yet tamed: Molybdenum benzylidyne complexes endowed with triarylsilanolates as ancillary ligands are superbly active and exquisitely selective alkyne metathesis catalysts, but can be rendered air-stable by reversible complexation with 1,10-phenanthroline or 2,2′-bipyridine (see, e.g., figure). A systematic molecular editing of their basic structural motif furnished a “library” of such catalysts and gave detailed insights into the parameters determining their favorable application profile.

Trapped in a cage: From gas adsorption in situ X-ray diffraction analysis, a self-consistent modeling scheme was established dedicated to the pore wall structure of ordered large cagelike mesopores surrounded by a silica matrix that contains the intrawall random porosity. From limited diffraction information from SBA-16, nitrogen fluid evolution in mesopores and the intrawall pore filling in the matrix are systematically traced, and eventually essential pore structure parameters are successfully quantified.

The use of topological concepts and the application of molecular symmetry arguments are crucial when it comes to identifying the structure and dynamics of higher-order topologies, such as those exemplified by molecular figures-of-eight. Using topological concepts, the ability to design and synthesize this particular molecular representation of a mathematical topology has been demonstrated.

Continuous carboxylation of ethanol to diethylcarbonate has been obtained by using Nb^V-loaded-CeO₂ mixed oxides under water removal by pervaporation (see figure). These catalysts showed a higher activity compared with other CeO₂-based catalysts. XRD analyses have demonstrated that Nb^V substitutes Ce^IV with up to 11 % molar ratio in the CeO₂ lattice, causing a severe change of the acid–base properties.

Controlled charge injection! Biphenylthiols with different substituents between phenyl groups in the bridge position, once chemisorbed onto gold source-drain electrodes, can be used to vary the charge injection and electrical performances of organic thin-film transistors. By varying the torsion angle between the phenyl rings, one can introduce changes in packing densities and tilt angles of the molecules self-assembled on the electrode that play a direct role in influencing the charge transport through the molecular monolayer (see figure).

New asymmetric, catalytic protocols for the synthesis of biologically relevant α-methylene-δ-lactones and δ-lactams are described (see scheme). The multi-bond-forming strategies (catalyzed by (S)-(−)-α,α-diphenyl-2-pyrrolidinemethanol trimethylsilyl ether) involve a Michael addition of trimethyl phosphonoacetate to α,β-unsaturated aldehydes as the key step. The strategy benefits from the diversity of the final products obtained, practicality and broad substrate scope.

Mild laccase-catalysed oxidation of ergot alkaloids allowed, for the first time, the functionalisation of the C-4 position of these molecules. Accordingly, C-4 hydroxy derivatives were isolated and characterised (see scheme).

Biotransformation: The asymmetric bioreduction of α,β-unsaturated carboxylic esters by ene-reductases could be tuned by varying the degree of CC bond activation (see scheme). An additional α-halogenated substituent proved to be beneficial for enzymatic activity, whereas β-alkyl or β-aryl substituents were detrimental for the reactivity of nonhalogenated substrates.

As simple as selective: BenzaPhos ligands, benzamide-containing chiral monophosphites of modular structure and trivial synthesis, have been screened in the Rh-catalyzed hydrogenation of olefins (see scheme), giving excellent enantioselectivities with three benchmark and two industrially relevant substrates. Control experiments and computational studies suggest an important role of ligand–substrate hydrogen bonding in the stereodiscriminating step of the catalytic cycle.

Convergency in the synthesis of the geldanamycin skeleton has been achieved by employing a magnesium carbenoid as a two-carbon building block, connecting two aldehyde fragments in an efficient and high yielding manner. The preparation of α-chloroethyl ketones from aldehydes has been investigated (11 examples, 64–95 % yield) and the title compounds have been prepared from advanced precursor 1.

Acid makes the difference! A directed Ru-catalyzed C(sp³)H α-alkylation protocol for cyclic amines has been developed (see scheme). Kinetic studies revealed that the carboxylic acid aids catalyst activation, increases catalyst longevity, and reverses the reaction selectivity. The alcohol selected as the solvent plays an active role in the catalysis.

Photoswitchable fluorophores can be entrapped within the hydrophobic interior of an amphiphilic polymer. The fluorescence of the resulting supramolecular assemblies can be switched for hundreds of cycles on a microsecond timescale in aqueous environments (see scheme).

Point of order: Different mechanisms for the nucleophilic addition step of the Pd-catalyzed allylic alkylation reaction have been investigated. The preferred pathway proceeds through a highly ordered transition state (see figure). The stereochemical outcome of the reaction is governed by a delicate balance between steric and electronic factors that stabilize or destabilize the possible diastereomeric transition states.

Re-turn to sender: Molecular turnstiles, which were based on strapped-type tetra-aryl X₂Sn^IV porphyrins (X=Cl or OH) that contained both H-bond-donor and -acceptor sites, were prepared; switching between their open and closed states was studied in solution by 1D and 2D NMR spectroscopy.

Caged porphyrins: Photophysical and photochemical characterization of a series of porphyrin–fullerene conjugates have been performed. The conjugates are intended to act as mimics of the photosynthetic center and emphasis was placed on the dependence of microenvironmental effects on electron-transfer dynamics (see figure).

Head on: Design, synthesis and evaluation of dynamic amphiphiles with singly or doubly charged peptide dendrons as head-groups; hydrazones, oximes and disulfides as bridges; and with fragrant tails are reported. Amphiphiles with two charges and four tails (see figure) are identified as the most powerful activators of DNA as ion transporters and thus as the most promising family to screen for gene and siRNA delivery.

Useful correlations: Seven different synthetic nonheme Fe^IVO species were investigated with both theoretical and experimental methods in order to obtain a statistical base from which useful predictions and insights can be made (see figure).