Chemistry - A European Journal

Migrating birds N-heterocyclic carbenes (NHCs) are known to form strong bonds with transition-metal complexes. In their Full paper on page 7063 ff., A. T. Normand, M. S. Nechaev, and K. J. Cavell describe a family of palladium(II) NHC complexes that undergo an autocatalytic reaction with aryl electrophiles in which no less than three PdNHC bonds are sequentially broken. The key step of this process is the concerted transfer of NHC and iodide ligands between two palladium complexes (see cover). This behavior is reminiscent of that of migrating birds, which have strong ties with their breeding grounds, but can travel away from them. In the case of NHCs, traveling too far can lead to a fatal issue if they “land” on a palladium complex from which reductive elimination can occur. Photography by Tim Holte, design by Joëlle Charpy.

Programming porosity: This review highlights recent strategies to prepare nanopatterned functional surfaces through self-assembly of organic molecular modules. Use of this method allows easy design and programming at the molecular level of 2D architectures onto surfaces (see scheme) to give periodic cavities capable of hosting functional molecules with potential applications, such as nanomedicine, molecular-based memory storage and sensors.

Torquoselective: Enantiomers of axially chiral α,β-unsaturated amidines were prepared by catalytic cascade reactions consisting of [2+2] cycloaddition and thermal cycloreversion. The cascade reaction proceeds in a stereoselective (torquoselective) manner, confirmed by the intermediate depicted.

The three-component formal [4+2] cycloaddition reaction of azlactones, cinnamaldehydes, and primary amines using phosphoric acid as the catalyst provides access to nitrogenous structural motifs (see scheme), which are found in numerous natural and unnatural products that exhibit important biological activities, such as 3-amino-3,4-dihydropyridinones, benzo[a]quinolizidines, 3-amino-δ-lactams, and 3-aminopiperidinones.

Symmetric fluoromethylation made easy: A highly enantioselective fluoromethyl addition to α,β-unsaturated aldehydes is described (see scheme). The synthesis of chiral fluorinated blocks, including δ-fluoro-β-aminoalcohol derivatives is disclosed.

A new mode of activation: The asymmetric diarylprolinol-catalyzed α-sulfenylation of β-ketoesters provides the products in good-to-excellent enantioselectivities and yields. Studies suggest that the sterically bulky secondary amines activate β-ketoesters through hydrogen bonding. Lg=leaving group.

EASCy: Imidazolones have been synthesized from small peptides and an amino aldehyde building block. Under strongly acidic conditions the imidazolones acted as electrophiles or as nucleophiles in diastereoselective electrophilic aromatic substitution cyclization (EASCy) reactions yielding crude products with purities of 76–99 %. The imidazolones also underwent Cu^II-catalysed N^2/5-arylation generating highly pure amino acid derived heterocyclic scaffolds.

A single electrode: We describe an electrochemical single-nucleotide polymorphism genotyping protocol using a single electrode that is modified with a ferrocene/isoquinoline conjugate connected to DNA probes, which possess different redox potentials. Three types of zygotes (a wild-type homozygote, a mutant homozygote, and a heterozygote) were identified by a single square wave voltammetric measurement.

Work together: Acid–base bifunctional mesoporous silica materials were prepared by co-condensation of tetraethoxysilane and silanes possessing amino and sulfonic acid groups. The samples showed increased catalytic activity for various one-step reaction cascades compared with the homogeneous catalysts. An acid–base cooperative activation mechanism is proposed (see scheme).

Strong bonds easily broken: The reaction between a family of Pd^II–NHC complexes and aryl halides yields 2-arylimidazolium salts with complete selectivity. The mechanism of this intriguing reaction was investigated by kinetic experiments and DFT calculations. An autocatalytic ligand transfer mechanism is shown to operate and strong evidence was found for an unusual NHC exchange between two palladium complexes.

Metal cooperation: The direct zincation of various 4-substituted pyridines at the 2-position by potassium–dialkyl–TMP–zincate bases is manifested in novel dimeric dipotassium-capped anthracene-like structures with coordinatively saturated zinc centers (see figure).

Second-sphere relaxation! Owing to its two inner-sphere water molecules, the Gd³⁺ complex of the cyclodecapeptide c(DREPGEWDPG) induces particularly large high-field relaxivities of the water protons. Interpretation of the high-field contributions to relaxivity with independently determined molecular parameters evidences a strong second-sphere mechanism caused by interactions of water molecules with the hydrophilic peptide scaffold (see figure).

An ionogel protected catalyst: An ionic-liquid-modified sol–gel method for the synthesis of catalytic materials from homogeneous catalysts is described. The resultant materials (pictured here) are highly active, selective and leach resistant and exhibit excellent recycling. The catalyst is protected within the nano-sized pores of the ionogel matrix.

How large are they? The size of ions depends on the solvent. What's their size when solvated in liquid helium (see picture)? Mass spectra of halogen anions, cations, and molecular cations complexed with helium atoms exhibit conspicuous abundance drops when the number of helium atoms reaches a value characteristic for each ion, indicating closure of a solvation shell. Ionic radii are deduced from these values.

Off the beaten tracks: A modular approach for the design of micromolar tRNA^Lys₃ ligands from millimolar fragments has been established (see graphic). The use of NMR spectroscopy has proved to be very useful in this approach, not only to detect low-affinity complexes between small compounds and RNA, but also to provide information for linker-length estimation and binding-site localisation.

A modular strategy for the efficient assembly of amphotericin B analogues bearing modifications in the macrolactone ring is presented. The strategy relies on the efficient gram-scale preparation of all the subunits of amphotericin B. Synthesis and biophysical studies on 35-deoxy ampphotericin B provide new clues to the mode of action of amphotericin B.

Weak-bonding coordination compounds: Adducts of TaCl₅ and NbCl₅ with various P₄S_n cage molecules were prepared and characterized. Ab initio calculations reveal that the coordinative bond is very weak and the binding energy is governed mainly by long-range van der Waals dispersion.

Self-immolation! Fréchet dendrons disassemble in the gas-phase. The process is triggered by detachment of the focal amine. Tandem MS experiments reveal an intriguing cascade fragmentation mechanism.

Building bridges: Photochemically induced borylene transfer to cobalt and nickel carbonyl complexes, [CpCo(CO)₂] and [Cp₂Ni₂(CO)₂] from [(OC)₅MBN(SiMe₃)₂] (M=Cr, W) generates the first instances of homodinuclear bridged cobalt and nickel (see figure) borylene complexes featuring bridging CO ligands.

Two different endotoxins: Burkholderia multivorans is an opportunistic human pathogen isolated in a cystic fibrosis center and causative of an outbreak with lethal outcome. The three-dimensional structures of the two O-antigens (one of which is shown in the figure) produced by a highly virulent clinical isolate have been determined.

A combined effort: More than 25 AZARYPHOS (aza–aryl–phosphane) ligands for ambifunctional catalysis by a metal and a nitrogen lone pair have been obtained by general and scalable syntheses (see graphic). The simple access to a family of bifunctional ligands opens a door to ligand screening approaches.

Heterometallic nitrido complexes containing single-cube [MTi₃N₄], corner-shared double-cube [MTi₆N₈], or the unprecedented (see picture) corner-linked double-cube [Ga₂Ti₆N₈] cores were obtained by reaction of [{Ti(η⁵-C₅Me₅)(μ-NH)}₃(μ₃-N)] with amido [M(NR₂)_x] Group 13–15 derivatives.

Metal-driven: The preparation of functionalized zinc and magnesium reagents is reported. LiCl catalyzes the insertion of Mg and Zn into various aromatic and heterocyclic bromides (see scheme). Several functional groups, such as -OBoc, -OTs, -Cl, -F, -CF₃, -OMe, -OPiv, -OAc, -NMe₂, and -N₂NR₂, are well tolerated under these new mild conditions. Zn/LiCl and Mg/LiCl lead to orthogonal regioselectivities for insertions into polybrominated arenes. Large-scale reactions have been performed.

Organize yourself! A variety of 2 D polygons (rectangles, triangles, and rhomboids) and 3 D cages (triangular prisms and distorted and nondistorted trigonal bipyramids) can be simultaneously and selectively formed in one pot via self-organization upon mixing various organoplatinum acceptors and pyridyl donors of differing geometry (see figure).

Aromatic azido reduction! Aluminium and gadolinium triflates catalyze the chemoselective reduction of aromatic azides to the corresponding amines in combination with sodium iodide by an intramolecular azido reductive tandem cyclization reaction (see scheme). This methodology selectively reduces aryl azides with enhanced yields and proceeds in a shorter reaction time. The mechanistic aspects have been investigated by online monitoring of the reaction employing ESI-MS.

Don't flip out, emit! DFT calculations can explain the different phosphorescence efficiencies of a family of five structurally similar cyclometalated Pt^II complexes by considering spin–orbit coupling and electronic structures in the S₀ ground and T₁ excited states. For example, on going from the S₀ to the T₁ state, the lateral aryl rings of strongly emitting [Pt(NCN)Cl] (NCN=1,3-bis(2-pyridyl)phenyl⁻) remain coplanar, whereas those of its weakly emissive isomer [Pt(CNN)Cl] (CNN=6-phenyl-2,2′-bipyridyl⁻) flip up due to Jahn–Teller distortion (see picture), which results in a large nonradiative decay rate

Enjoy the simplicity! One-pot sequential Heck reduction–cyclization (HRC) reactions leading to variously substituted 2-quinolones have been developed that use a multitask palladium catalyst under ligand- and base-free conditions at moderate temperature (see scheme).