Chemistry - A European Journal

Electrostatic nanoassemblies of cationic conjugated oligoelectrolytes and fluorescein-tagged ssDNA were used to characterize the growth conditions of Escherichia coli; this is described in the Communication by G. Hanrahan, C. M. Mello, G. C. Bazan et al. on page 756 ff. The nanoassemblies interacted with the bacteria and yielded a modified photoluminescence response. The collective response of five nanoassemblies produced unique signature patterns for Escherichia coli cultured under different growth conditions.

Switch over: Different approaches are presented that allow external switching of self-assembled nanostructures between different aggregation states and morphologies. There are multiple stimuli that can be applied to supramolecular oligomers, and first examples for useful applications can already be found in materials science.

Electrostatic nanoassemblies were employed to identify bacterial growth conditions. They comprise a cationic conjugated oligoelectrolyte and fluorescein-tagged ssDNA and were optimized with a hybrid, computational neural network model. The photoluminescence spectra contained the oligomer and sensitized fluorescein emission. The spectra changed depending on the growth history of the bacteria introduced (see figure).

Underwater exploration: The ring-closing metathesis of N,N-diallyltosylamine (DATs) and diallyldiethyl malonate has been studied in aqueous micellar medium, at room temperature, in the presence of four different gemini cationic surfactants and various ruthenium catalysts (see figure). For the first time, the adsorption mechanisms and the reaction steps involved in this heterogeneous catalytic process were elucidated.

GO MOFs! Azobenzoic acid functionalized graphene (A-GO) can act as a structure-directing template that influences hydrogel formation together with metal–organic frameworks (MOFs). Zn²⁺ MOFs of pyridine derivatives work as framework linkers between the A-GO sheets (MOF–A-GO, see figure). MOF–A-GO exhibits a strong fluorescence enhancement upon gel formation. In addition, MOF–A-GO selectively recognizes trinitrotoluene.

Pd^IIcaught in the act: The diaryl Pd^II intermediate of a Pd^II-catalyzed oxidative biaryl bond formation proceeding via a double CH bond activation has been isolated and fully characterized, including an X-ray crystal structure analysis (see figure). Stabilization due to chelation by adjacent pivaloyloxy and acetyl groups has allowed the isolation of this long-sought crucial intermediate. On gentle warming, the complex is transformed into a carbazole product, and the catalytically active Pd^II species is regenerated by oxidation with Cu^II.

PtCu nanostructures: PtCu nanocubes (NCs), concave nanocubes (CNCs), and PtPdCu CNCs with high-index facets (HIFs) were prepared through progressive galvanic replacements in a one-pot hydrothermal approach. The HIF-enclosed CNCs showed superior activities to (100)-enclosed NC catalysts for methanol oxidations owing to the modification of both the surface electronic structures and the surface atomic arrangements (see figure).

The pathway for the cleavage of an aromatic CC bond in quinoxaline by a tungsten(II) complex [W(PMe₃)₄(η²-CH₂PMe₂)H] is explored by performing detailed DFT calculations. The real active complex was found to be [W(PMe₃)₂(η²-CH₂PMe₂)H] rather than [W(PMe₃)₄]. The key step in the whole reaction is the reductive elimination of two hydrides that are located originally on quinoxaline (see scheme).

Catalytic surfaces have been prepared by complexation of palladium on self-assembled terpyridine monolayers on silicon. A reaction-based fluorogenic probe was developed to allow facile visualization of the catalytic potential of the surface (see figure). Superior activity of the immobilized catalyst compared with the homogeneous control reactions is demonstrated.

To be or not to bifluoride: Two synthetic pathways to unprecedented N-heterocyclic carbene copper(I) bifluoride complexes have been developed (see scheme). Catalytic tests demonstrated that copper(I) bifluorides are very efficient catalysts, which do not require any additional activating agent. The first Cu-catalyzed diastereoselective allylation of (R)-N-tert-butanesulfinyl aldimines was also established. The method enables efficient, simple and general synthesis of enantiomerically enriched homoallylic amines at room temperature in high yields.

Remote F effect: Unprecedented hetero-Diels–Alder reactions of mono- and bis-substituted cyclopentadienes have been realized by an asymmetric binary-acid catalyst that synergistically combines a chiral phosphoric acid 1 h/InBr₃ with good periselectivity, high regioselectivity, and excellent stereoselectivity. Substituent mapping of the chiral phosphoric acid indicates a dramatic remote ortho-fluoro effect on the stereocontrol (see scheme).

This base is just right! A new base, potassium 2,2,5,7,8-pentamethylchroman-6-oxide, for Pd-catalysed amination has been rationally designed that has sufficient basicity to efficiently deprotonate the intermediate aryl palladium ammonium complex, leading to far better rates than obtained with carbonate, which is known to be very mild. At the same time, this new base possesses minimal nucleophilicity to mitigate the degradation of base-sensitive functional groups in the starting materials and products, such as is commonly seen with potassium or sodium tert-butoxide.

Game of two electrophiles: Two partially positively charged sp³ carbon atoms can be connected by using a catalytic Ni species in the presence of an environmentally benign Zn reductant, delivering allylated alkanes (see scheme). This unprecedented approach allowed a variety of unactivated alkyl halides and substituted allylic carbonates to regioselectively afford E-alkenes in good to excellent yields.

Switchable fluorescent silica nanoparticles have been prepared by incorporating a fluorophore and a photoswitch within the particle core. The fluorescence can be reversibly switched between an on- and off-state via energy transfer. Particles have been obtained that contain different amounts of the photoswitch and fluorophore, and have been characterized in terms of size, switching properties, and fluorescence efficiency (see figure).

A COMBO please! The preparation of carboxymethylmonobenzocyclooctyne (COMBO) was carried out in four synthetic steps. This simple, non-fluorinated, copper-free click reagent showed excellent stability and reaction kinetics (see scheme). A COMBO-modified fluorescent label was employed in glycan imaging of metabolically modified HeLa cells.

Cutting the guest: The system formed from [Cp*Fe(η⁵-P₅)] and CuBr completely encapsulates the triple-decker complex guest [(CpCr)₂(μ,η⁵-As₅)] to form a spherical fullerene-like supramolecule containing 90 inorganic core atoms (see figure; left). When CuCl is used, however, an 80-vertex C₈₀ topological host molecule is formed, in which fragmentation of the template is now observed, with incorporation of the unprecedented complex [CpCr(η⁵-As₅)] within the spherical supramolecule (see figure, right).

Amorphous versus crystalline: Shape-persistent organic cage compounds (see scheme) with equal defined cavities, but different peripheral substituents, show permanent porosities in the solid state. If the materials are amorphous, the peripheral substituents do not influence the sorption of nitrogen, resulting in BET surface areas of approximately 700 m²g⁻¹, independent of the substituent bulkiness. In the crystalline state, the less bulky the substituents the higher the specific BET surface area, ranging from 22 up to 2071 m² g⁻¹!

Never-ending story: Multinuclear NMR spectroscopic characterization of the chain-propagating species in the o-F-substituted bis(enolatoimine) titanium catalyst system for living ethene polymerization and its non-fluorinated (non-living) counterpart is reported. For the living polymerization catalyst, weak coordination of aryl o-F substituents to Ti prevents the metal center from coming into contact with the β-CH₂ group of the growing polymer chain and thus hampers the chain termination.

Touching (nucleo)base: A dual intramolecular catalytic influence is demonstrated by the nucleobase and carboxyl groups in the chemical hydrolysis of amino acid nucleoside phosphoramidate prodrugs (see scheme). The replacement of the adenine N1 or N7 atoms instead of the N3 atom is shown to have a conformational role in which the protonated N3 is crucial in regulating the kinetics and mechanism of nucleotide (PN pathway) versus nucleoside (PO pathway) formation.

The acid test: The structural and biochemical aspects of xylose-DNA (dXNA; see scheme) have been explored, with the aim of using it as an orthogonal nucleic acid system to transfer genetic information without interfering with the natural system. The study identifies the properties for developing dXNA as an alternate information system, although it also finds the reasons why dXNA was not selected as the natural genetic system.

Synthetic power: Four novel folate–ansamitocin conjugates were prepared by a combined muta-/semisynthetic approach. They exert strong selectivity between cell lines that have expressed and that are devoid of folate receptors (FR). The study demonstrates both the power of target-specific chemotherapy with ansamitocins and reveals the diverse options mutasynthesis combined with semisynthesis provides.

Aim for the STAs: The metal–organic framework catalyst STA-12(Co) showed remarkable activity in the aerobic epoxidation of E-stilbene in DMF (see figure) and could be efficiently recycled. High selectivity (80–90 %) for stilbene oxide was observed. The induction period of the reaction could be reduced by styrene/stilbene co-epoxidation. Detailed parameter studies combined with spectroscopic investigations were performed to obtain an initial insight into the reaction mechanism.

One pot for the lot: Alkoxy-1,3,5-triazapentadien(e/ato)–Cu^II complexes have been synthesised by one-pot template synthesis, used for metal-mediated synthesis of pyrimidines and triazapentadienes, and shown to be effective catalysts in the oxidation of alcohols to carbonyl products (see scheme).

Supramolecular magnets: Manganese(III) 5,10,15-tris(pentafluorophenyl)corrole [Mn^III(tpfc)] forms chains or dimers depending on whether it was crystallized from methanol, ethyl acetate, or ethanol. Due to intermolecular π–π interactions, the depicted methanol solvate [Mn(tpfc)(MeOH)]⋅H₂O forms a new type of supramolecular spin-canted single-chain magnet without any bridging ligands.

Surprising hydrolysis: A new type of ferrocene derivative is reported, which surprisingly can hydrolyze under mild conditions. Upon reaction with H₂O under the irradiation of natural light at room temperature, these derivatives display a dramatic color change and fluorescence retrievement. The present mild dissociation reaction may give a new insight into the stability of ferrocene and its derivatives.

Esprit de core: Meso-/microporous core–shell composite molecular sieves with hierarchical pore structures (see figure), from opened mesopore channels (2.4–3.0 nm) to accessible micropores (≈0.51 nm), and graded acidity distributions, from weakly acidic mesopores to accessible strongly acidic ZSM-5 cores, have been synthesized.

Fast cyclization: Silyl radicals were generated through intramolecular homolytic substitution (S_Hi) in acyclic systems equipped with a catecholborane (CatBH) and a (Me₃Si)₃Si moiety (see scheme). Cyclization proceeded at unprecedented rates to give the corresponding silacycles and the desired Me₃Si radical, which was subsequently trapped.

Bond directionality: The directionality of halogen bonds is higher than that of conventional hydrogen bonds. The electrostatic interactions of the atomic quadrupole moments of the halogen atoms are the origin of the attraction and directionality of the halogen bond (see figure).

Concerted effort: The reactions of Y-substituted phenyl diphenylphosphinates (1 a–g; see scheme) and diphenylphosphinothioates (2 a–g) proceed through a concerted mechanism with TS₁ as a transition state. The nature of X and M in TS₁ determines whether metal-ion catalysis or inhibition occurs.

Gas-phase studies revealed that the introduction of the hydroxyl substituent to the 2,4,6-tridehydropyridinium cation (1) significantly influences its behavior toward neutral molecules. This is rationalized by the ability of the hydroxyl group to stabilize the cyclopropenium moiety of an ionic resonance structure, which results in a reduced dehydrocarbon atom separation for these two radical sites. The stronger coupling between these two radical sites drastically reduces their reactivity.

Pattern through a lens: A one-step method for achieving aligned organic nanowire arrays that can be patterned on a surface with well-controlled spatial arrangement has been developed (see figure); this is highly desirable in the fabrication of high-performance organic electronic and optoelectronic devices, as well as in device integration.

Odd coupling: For the first time, J coupling was observed between ¹H,¹H atoms in several synthetic molecules (see figure). DFT computations revealed an unexpected geometry dependence of the coupling, which can be potentially used in structural studies of van der Waals complexes.

High energy density materials: Hydroxylammonium (P: 37.42 GPa; vD: 9013 ms⁻¹) and hydrazinium (P: 33.84 GPa; vD: 8917 ms⁻¹) 4-nitramino-3,5-dinitropyrazolates show detonation properties comparable to RDX (P: 35.17 GPa; vD: 8977 ms⁻¹). The hydroxylammonium salt has an impact sensitivity approaching that of triaminotrinitrobenzene.

The asymmetric hydroformylation of styrene by using the Rh(R,S)-BINAPHOS catalyst was theoretically investigated. The electronic distortion and the steric effects explained the exceptional ligand coordination with the phosphane and phosphite moieties at the equatorial and apical sites, respectively. The stereochemical model derived from a new descriptor, the distance-weighted volume (V_W), indicates that the combination of the two chiral centres, each discriminating either an equatorial–apical path or the enantiomer, is crucial.