Angewandte Chemie International Edition

Like a revolving door that can easily open, close, and return to the start, the first framework material that can undergo reversible electron-transfer reactions is described by L. Cronin et al. in their Communication on page 6881 ff. The solid contains large pockets in which the redox reagents can reside; the manganese(II)/(III)-containing framework is able to move between these oxidation states and is the first in a new class of redox solids.

Chiral metal nanoparticles and their nanointerface have received little attention, but their properties are of great interest. In their Communication on page 6917 ff., H. Fujihara and co-workers describe that palladium nanoparticles [(1.6±0.2) nm] stabilized with chiral 2,2′-bis(diphenylphosphino)-1,1′-binaphthene (binap) ligands can efficiently catalyze asymmetric Suzuki–Miyaura cross-coupling reactions of naphthyl halides with naphthylboronic acid at room temperature with short reaction times.

The preparation of nanoparticles with sufficient chemical and thermal stability for bio-related applications has long been a major issue in nanobiotechnology. New types of highly stable Au, CdSe/ZnS, and F₃O₄ nanoparticles have been stabilized with Tween derivatives through coordination to the particle surface as well as van der Waals interaction with the primary ligands on the nanoparticle surface. The Au nanoparticles are more stable than the corresponding Au nanoparticles functionalized with thiolated DNA.

What is the nature of the gold–carbon bond—double, single? The structure of a crucial intermediate of many gold-catalyzed reactions has become a matter of debate. A current study demonstrates that the carbocationic form determines the outcome of many cycloisomerizations.

A new vision for ET: Electron-transfer (ET) processes in biological and synthetic materials (such as proteins and semiconductors, respectively) are of immense importance. A unified approach to describe these processes is useful in understanding, and controlling the various facets of electron transfer in condensed matter. The diagram shows a plot of the decay constant for the electron-tunneling process against the effective tunneling barrier in different materials.

Getting a grip on hydrogen: Hydrogen typically adsorbs very weakly in physisorptive systems. In microporous materials, however, coordinatively unsaturated metal centers can contribute to an increase in the enthalpy of H₂ adsorption. Strategies to incorporate such metal sites in metal–organic frameworks have led to remarkable results within a short period, thus promising to deliver new materials with improved H₂ affinity in the near future.

Multi-spinning: The use of modified triphosphates and DNA polymerases has resulted in the multiple spin-labeling of DNA (see picture). A DNA helix entirely labeled with paramagnetic centers has been generated in this way and investigated by EPR spectroscopy.

Smart writing by electrons: A new technique enables the fabrication of full-coverage polymer brush patterns of variable shape on a nanometer length scale. The key to this technique is electron-beam activation of amino groups, which serve as the nucleation centers for surface-initiated polymerization. The technique untangles chemistry and morphology, and it is possible to monitor only morphology effects on chemically homogeneous surfaces and interfaces.

Under pressure: The new modification Ge(hR8) is obtained upon pressurization of clathrate-type Ge(cF136) or controlled decompression of a Ge(tI4)-type high-pressure phase. The atomic arrangement comprises four-bonded germanium atoms with a topological organization bearing remarkable similarity to high-pressure host–guest assemblies of other main-group elements.

It takes two to tango: When ligand H₄-1 (see scheme) reacts with Mo^IV and Ti^IV, the different binding preferences of the O–O and S–S donor groups lead to the formation of the helicate [TiMo(1)₃]⁴⁻ with a parallel orientation of the ligand strands. With different cations, the supramolecular complexes {Λ,Λ-[TiMo(1)₃]-(PNP)-Δ,Δ-[TiMo(1)₃]}⁷⁻ and {Δ,Δ-[TiMo(1)₃]-Na-Δ,Δ-[TiMo(1)₃]}⁷⁻ can be crystallized. PNP=bis(triphenylphosphoranylidene)ammonium.

You can count on the counterion in ion-pair intermediates to induce high levels of asymmetry in the title reaction. The efficient transformation proceeds under mild reaction conditions in the presence of an air-stable N-triflylphosphoramide in a low catalyst loading of just 1 mol % to give substituted α-hydroxyesters in good yields and with excellent ee values (see scheme; R=aryl).

In the footsteps of Victor Grignard: The simple LiCl-mediated insertion of magnesium into aryl chlorides and bromides at moderate temperatures leads to functionalized organomagnesium reagents (see scheme). An unprecedented range of functional groups may be present in the substrates (e.g. CN, CO₂R, OTs, OBoc; Ts=p-toluenesulfonyl, Boc=tert-butylcarbonyloxy).

Peptide ligation at hydrophobic sites is possible with a ligation–desulfurization strategy in which penicillamine serves as a precursor of valine. The β,β-dimethylcysteine peptides reacted surprisingly fast in native chemical ligation reactions. Even the sterically crowded and unpolar Leu–Val bond can be formed in high yield.

Three's a charm, but four's not half bad either: A simple cobalt–diphosphine complex facilitates the synthesis of cyclobutene derivatives through the chemoselective transformation of strained five-membered unsaturated rings with internal alkynes (see scheme). This atom-efficient, intermolecular reaction generates polycyclic products in excellent yields and with excellent exo selectivity. dppp=1,3-bis(diphenylphosphanyl)propane.

Steric and electronic characteristics of the bulky, strongly electron-withdrawing 2,4-(CF₃)₂C₆H₃ group are ideally suited to stabilize the unusual form of a diphosphoxane, R₂POPR₂, the anhydride of a phosphinous acid. The high-yield synthesis and first structural characterization of a tetraorganyl diphosphoxane are described (see structure; C gray, H white, O red, F yellow, P green).

Microdroplets for nanoparticles: An extremely reliable method to create droplet pairs, based on hydrodynamic coupling of two spatially separated nozzles, has been developed. Droplets containing the reagents for the precipitation of iron oxide are electrocoalesced to synthesize iron oxide nanoparticles in a very fast (millisecond-scale) and reproducible reaction (see picture).

Core knowledge: The synthesis of the functionalized cores of both the originally proposed and revised structures of palau′amine have been achieved (see structures). A Diels–Alder/[3,3]-sigmatropic rearrangement sequence and a Beckmann rearrangement enable the stereocontrolled formation of the highly congested cores.

The whole “pybox” and dice: Highly enantioselective amination reactions of both allylic and benzylic CH bonds are catalyzed by cationic ruthenium(II)–pybox complexes (see structure). A novel mode of stereocontrol, which is induced by the versatile pybox ligand, is proposed to account for the excellent enantioselectivity in these reactions. Boc=tert-butoxycarbonyl, pybox=pyridine bisoxazoline.

Coping with tandem reactions: The total synthesis of marine fungal metabolite, trichodermamide B, is accomplished using a tandem nitrosation/oxaza-Cope rearrangement as the key step to establish the characteristic heterocyclic ring system of the natural product. Preparation of the putative biosynthetic precursor of aspergillazine A provides further insight into the possible biosynthetic pathway to aspergillazines.

Soai, so good: The Soai reaction is the sole reported case of amplifying asymmetric autocatalysis. A mechanism is proposed for this reaction, supported by first-principle calculations, clarifying not only the reasons for the detected chiral induction and amplification, but also the roles performed by isopropyl groups, γ-pyridinic nitrogen atoms, and the δ-substituent.

Lean, mean, chiral allene: Optically pure complexes 2 react with propargyl bromides and mesylates 1 to afford the title products (see scheme; LG=leaving group). The regioselectivity and the configuration of the chiral axis are completely controlled, and the stereoselectivity at the benzylic position is high. Kinetic resolution of racemic propargyl mesylates can also be achieved with these reagents.

Mastering unusual amino acids: The first total synthesis of the potent protease inhibitor cyclotheonamide C (see structure representation) has been achieved. The key features of the synthesis are a three-component tandem procedure to create a masked α-keto-β-arginine within a peptide chain, and the control of the extended conjugation of a vinylogous α,β-dehydrotyrosine.

A triflic reaction: The role of triflic anhydride is not limited to the transfer of a triflyl group to the nitrogen atom during the interaction of pyridine with bis(trimethylsilyl)ketene acetals: it can also provide an electrophilic CF₃ group able to transform the formed dihydropyridines substituted with trimethylsilyl ester groups into α-trifluoromethyllactones (see scheme).

Not so Mannich now: A heterobimetallic La/Li/pybox complex was key in direct catalytic asymmetric Mannich-type reactions, using α-keto anilides as synthetic homoenolate equivalents to afford γ-amino amide products in up to >99 % yield, 95 % ee, and >97:3 syn-selectivity. Stereoselective reduction of the α-keto moiety afforded the β-alkyl-γ-amino-α-hydroxy amide with three contiguous stereocenters (PG=protecting group).

Intramolecular activation of C-terminal peptides: Mildly activated N-acylurea peptides are readily formed on the solid support following chain assembly, avoiding over-activation of the C-terminus. The utility of these peptides is demonstrated by the native chemical ligation of unprotected peptides (see scheme; R=amino acid side chain).

The special guest: In the host–guest chemistry of dinuclear complexes that contain the 9H-fluorene-9,9′-dimethanediphenylphosphinite ligand the synergy between the phenyl groups and the axial fluorene moiety leads to the formation of molecular cavities that have selective steric affinity for the anions BF₄⁻ or PF₆⁻ (see picture). An unprecedented M⋅⋅⋅F(PF₅)⋅⋅⋅M bridging coordination mode of PF₆⁻ has now been established.

Diene for conjugate addition: Iron(II) chloride switches the reaction course of the three-component coupling of a Grignard reagent, an alkyl halide, and a dienamide, thus making the amide a simple yet versatile chiral template.

Crossroads: Selective sulfoxide/lithium exchange reactions lead to the unique preparation of diastereo- and enantiomerically pure polyalkylated cyclopropanes (see scheme). This reaction allows the independent formation of both diastereoisomers from a common starting material.

Going back to nature: Electron-deficient oxadienes and electron-poor acetylene carboxylates react in the presence of an organocatalyst to give natural product inspired tricyclic benzopyrones efficiently (up to 99 % yield) and stereoselectively (up to 87 % ee; see scheme). This efficient and operationally simple asymmetric annulation involves the generation of zwitterions from acetylene carboxylates.

Adaptive Alkylation: Palladium-catalyzed asymmetric alkylation enables access to fully substituted enantioenriched oxygenated stereocenters, which can be transformed easily to α-hydroxyketones, esters, and acids, providing a catalytic, enantioselective synthesis for natural products.

A dynamic approach: The key step in the total synthesis of hypocrellin A involves a potentially biomimetic 1,8-diketone aldol reaction, which constructs the seven-membered ring. The helical configuration is established first from an axially chiral biaryl unit and is then used in a dynamic stereochemical transfer process to form the remaining stereocenters in the intramolecular aldol reaction (see scheme).

Back and forth: A concerted reversible redox reaction occurs in a pure metal oxide extended polyoxometalate framework when the accessible pockets are filled with a suitable redox agent (see picture). Direct control over the framework properties is demonstrated by repeated reversible switching between an expanded and a contracted structure. Successive recrystallizations from hot water repeatedly destroys and regenerates the framework.

A spin-crossover from Fe^II(S=2)-Nb^IV(S=1/2)-Fe^II(S=2) to Fe^II(S=0)-Nb^IV(S=1/2)-Fe^II(S=2) occurs in Fe₂[Nb(CN)₈]⋅(3-pyCH₂OH)₈⋅4.6 H₂O (3-py=3-pyridyl) with decreasing temperature (see picture; red Fe, blue Nb). The low-temperature phase shows ferrimagnetism with a Curie temperature of 12 K owing to an antiferromagnetic interaction between the remaining Fe^II (S=2) and the Nb^IV (S=1/2) centers.

A near-perfect model: The design of short peptides with cyclic and linear components allows mimicking of the structure and reactivity of protein tetracysteinate zinc sites, as illustrated by the superposition of the Zn(Cys)₄ site of Hsp33 (blue) and its 20 amino acid model (green).

Back flip: Field-induced transformations between the two homogeneously chiral enantiomers of a bent-core mesogen (see picture) take place by two different mechanisms (A and B). Their combination leads to an unexpected field-induced chirality flipping (C) between the oppositely tilted structures without polarity reversal.

Beyond the classics: An exchange-coupled three-spin cluster of copper(II) with nitroxides can be converted by light from a strongly to a weakly coupled spin state (see picture), the latter being metastable at low temperatures. This kind of magnetic photoswitching is very similar to the LIESST effect in spin-crossover complexes of iron. Such compounds form a new class of systems capable of optical switching of their magnetic properties.

Titinic strength: The mechanical resistance of titin domain I27 (see picture, left) was increased by substituting residues from a stronger homologue in regions of the protein important for mechanical strength (red). The unfolding profile of I27st (see picture, right) shows that the mechanical strength is significantly increased compared to wild-type I27 (black arrow).

Functionalization of polyoxovanadate clusters using phosphonate and arsonate ligands results in molecular capsules (see structure; green V, purple P, red O, black C). Through the use of extended homologous ligands, these hybrid organic-inorganic capsules can be successively elongated into tubular molecules.

Synthesis in deep water: For the title reaction, which proceeds with high selectivities, acid–base interactions between the Sc catalyst and HCHO are important. The reaction was used in the synthesis of an odorant to demonstrate its utility (see scheme; ligand DS not shown; PI-Pd=polymer-incarcerated palladium; BTF=benzotrifluoride).

Click together and gel: Non-gelating AB-type heterobifunctional macromonomers “click” together to form polytriazole-containing dendronized polymers. These polymers form interchain hydrogen bonds, the extent of which is influenced by the size of the dendritic appendage.

Similar but different: Chiral mono- and bisphosphine ligands have been used to prepare chiral stabilized palladium nanoparticles (Pd NPs). Despite the chiral Pd NPs having similar diameters of 1.2–1.7 nm, they show very different catalytic activities and enantioselectivities in the room-temperature asymmetric Suzuki–Miyaura cross-coupling reactions of aryl halides with aryl boronic acids (see picture).