Chemistry - A European Journal

Introducing crown ethers with carboxylic acid functionalities into the struts of metal–organic frameworks (MOFs) has been achieved by J. F. Stoddart et al. and is described in their Full Paper on page 13356 ff.. These novel crown ethers not only retain the characteristics of their parent crown ether, but can also coordinate to inorganic secondary building units to build MOFs that are capable of docking electron-accepting substrates precisely at their active sites. (Cover picture created by Pin Fang, 2KGames China.)

Fine substitution α-Alkoxycarbonyl and α-aminocarbonyl compounds are good substrates for the recently developed tosylhydrazide-promoted Pd-catalyzed cross-coupling of carbonyls with aryl halides. For details see the Full Paper by J. Barluenga and C. Valdés et al. on page 13291 ff.

Deuteration leads to stronger binding: Deuterated guests bind within the confined space of the molecular capsule more preferentially than protiated guests (see scheme). The van 't Hoff analysis of the competitive association reveals that the preferential binding of the deuterated guests is driven by enthalpy and opposed by entropy.

α-Substitution is fine: α-Alkoxycarbonyl and α-aminocarbonyl compounds are good substrates for the recently developed tosylhydrazide-promoted Pd-catalyzed cross-coupling of carbonyls with aryl halides. The reaction gives rise to enol ethers and enamines, respectively, which are useful synthetic intermediates in heterocyclic synthesis. Otherwise, they can be hydrolyzed to give α,α-disubstituted aldehydes.

A highly enantioselective organocatalytic tandem Michael–aldol–dehydration reaction that provides an expedited access to highly functionalized chiral cyclohexenone derivatives (such as that depicted) by using a simple diamine catalyst is described.

A bifunctional quinidine derivative, containing a trifluoromethyl group, was found to catalyze a tandem intramolecular oxa-Michael addition/electrophilic fluorination reaction of activated α,β-unsaturated ketones (see scheme). A series of chiral fluorinated flavanone derivatives were obtained in excellent yields and with high enantioselectivities.

Relaxed carbocations yield stressed isomers: The chief mechanistic principles governing the synthesis of positional isomers of DiSpiroFluorene–IndenoFluorene (DSF-IF) have been unveiled. The present work explains the apparent paradox that increased bulkiness of the substituents borne by the fluorene moieties leads to the more encumbered isomer (see figure). These molecules also display different and tunable emission properties of interest for organic electronic applications.

Halogen-free CC coupling: Catalytic amounts of [Pd₂(dba)₃] (dba=dibenzylideneacetone) and a sterically encumbered phosphite (L1) promote the addition of CH bonds of terminal alkynes (2) to alkylidenecyclopropanes (1). The reaction gives rise to 1,4-enynes (3) in good yields. The catalytic cycle probably consists of a Pd-catalyzed CH activation of the terminal alkyne, a regioselective hydropalladation followed by distal bond cleavage of the alkylidenecyclopropane, and a final reductive elimination.

Oxidative CS coupling: The oxidative coupling reaction of 1,2-diaryldisulfides by using MoCl₅ or mixtures thereof with TiCl₄ gives fast and efficient access to dibenzo[b,e] [1,4]-dithiins such as that depicted (thianthrenes).

Selecting one out of two enantiotopic alkynes: A furan–diyne substrate with enantiotopic alkynyl groups was prepared and the gold-catalyzed cycloisomerization by a series of different chiral cationic gold(I) complexes was investigated (see scheme).

endo-Rule for gold: A highly endo-selective gold-catalyzed cycloisomerization of 1,4-diynes was developed. By employing electron-rich phosphines and N-heterocyclic carbenes as ligands, a number of 1,4-diynes could be cyclized in a desymmetrizing fashion to form dihydrodioxepines and enantiomerically enriched tetrahydrooxazepines (see scheme).

Recognition of disubstituted benzenes: We have achieved selective formation of two diastereomeric cage-like boronic esters (homo-[3+2] and hetero-[3+2]) depending on the substitution pattern of added xylenes. Furthermore, it was found that selective formation of hetero-[3+2] in the presence of o- or m-xylene was induced during the precipitation process.

Forcing the issue: By exploring force-transformed potential energy surfaces of both cis and trans 1,2-disubstituted benzocyclobutene, we unravel the mechanism of force-induced ring-opening. The expected conrotatory process of the cis reactant leading to E,Z product is only observed at very small forces, whereas disrotatory ring-opening yields the E,E dienes at larger forces, which is shown to be a concerted process.

What's it got in its pockets? Symmetry carry-over crystallization of hexadehydrotribenzo[12]annulene derivatives was provided by specific complexation between the annulene and the chloroform molecule through mainly dispersion forces (see picture). In the complex, a hydrogen atom of the chloroform fits into the annulene's “pocket” surrounded by π electrons.

Gold nanoprobe: Heparin–DOPA-coated gold nanoparticles (HEPA–AuNPs) showed low toxicity, prolonged stability, and an enhanced X-ray absorption coefficient in vitro. In vivo microCT images showed that HEPA–AuNPs produced enhanced liver-specific CT images compared with iodine-based contrast agents, thus highlighting its potential as a novel, liver-specific CT imaging agent (see figure).

Dispersive interaction in frustrated Lewis pair (FLP) formation is investigated. It is peculiarly large in FLPs with the help of perfluorophenyl groups, which can directly attract alkylphosphines. A direct phosphorus–boron interaction also exists to a non-negligible extent. As a result, the FLP shows a superior entropic stabilization over classical Lewis pairs.

Crown incorporation: General approaches have been developed for the syntheses of rigid-strut-containing crown ethers. These novel crown ethers not only retain the characteristics of their parent crown ethers, since they can bind cationic guests and serve as templates for making mechanically interlocked molecules, such as [2]catenanes, but they also present coordination sites to connect with secondary building units in the construction of metal–organic frameworks (see scheme).

Opening of the black box: Anisotropic nanocrystalline boehmite (AlOOH) is formed within minutes from a simple metal oxide precursor in sub- and supercritical water. Particle growth was monitored in situ by WAXS and SAXS by using high-energy synchrotron radiation (see figure). At elevated temperatures AlOOH dehydrates to aluminium oxide.

One way or the other: Intermediates on the way from Al(PR₂) molecules to thermodynamically favored bulk AlP point to a novel channel for the decomposition of aluminum(I) compounds (see scheme).

At a Lossen: It is shown at the molecular level that acetohydroxamic acid, upon coordination to zinc(II) acetate, transfers a proton to the acetato ligand. The hydroxamic acid and its anion are strongly bound in zinc complexes (see figure) and upon activation their complexes undergo a Lossen rearrangement rather than dissociation.

It all hinges on this: Molecular hinges based on bipyridine units that are fixed to chiral cyclopeptidic clamps have been prepared (see figure). These molecular machines exhibit a metal-ion-driven open–close mechanism that is strictly unidirectional and the amplitude can be adjusted to different values depending on metal ions, solvents, and chiral clamps.

Aryl benzyl sulfide oxidation: A reaction mechanism is proposed for the enantioselective oxidation of aryl benzyl sulfides by tert-butyl hydroperoxide in the presence of a titanium/hydrobenzoin catalyst. The mechanistic scheme is confirmed by DFT computations based on the MPWB1K functional. The computed reaction profile (see figure) explains the experimentally observed enantioselectivity, which is determined by the thermodynamics of the first phase of the reaction.

Selective hydration: The acetamido motif provides a hydration focus that locates bound water molecules on the “axial” faces of the pyranose ring—a striking and unprecedented role that may, in part, explain the differing and widespread role of GlcNAc in nature.

Compound interest: Much work has been done on P450 model complex reactions, yet this is the first report to cover the generation and stabilization of the three most discussed reactive intermediates (Cpds 0, I, and II) and to study their reactivity towards a broad variety of substrates (see scheme).

Three new routes are reported for the synthesis of perchlorate salts of imidazolium and phosphonium compounds from ammonium perchlorate (see scheme). The syntheses proceed in a single step from the chosen precursors with high yield and purity. The electrochemical remediation of the perchlorate ion from an ionic liquid was also achieved suggesting the potential for a continuous perchlorate removal/remediation process for converting perchlorate to chloride using an ionic liquid as both reagent and electrolyte.

Holey host: A facile method has been developed for the fabrication of porous silicon carbide (SiC) by sintering a mixture of SiC powder and carbon pellets at relatively lower temperature (see graphic). The pore density and total pore volume of the resulting porous SiC could be tuned, and the porous SiC can be used as a host for the growth of the ZSM-5 zeolite.

Supramolecular aggregates: The solid-state aggregation of a series of glucose- and galactose-appended metal–salphen complexes was studied. While the carbohydrate leads the metal complexes (such as Fe, see figure) to form micron-scale clusters, the zinc complexes organize into helical nanofibers that are microns in length.

Surprising formation of F₂PNO: Photolysis of F₂P(O)N₃ in solid argon matrix generates difluorophosphoryl nitrene F₂P(O)N. Instead of a 1,3-fluorine shift, the latter undergoes an unexpected Curtius-type rearrangement to difluoronitrosophosphine F₂PNO on UV irradiation (see scheme).

Under the sun: A new family of fullerene-based compounds, namely, [60]- and [70]fullerene homo- and heterodimers linked through 2-pyrazolino–pyrrolidino bridges (1–3), has been synthesised. Stronger absorption in [70]fullerene-containing dimers relative to the [60]homodimer is observed in the visible range (see picture). The compounds are suitable as absorbers for organic photovoltaic devices.

A rational synthetic route to metallaborane clusters (V, Nb, Ta) involving the reaction of Group 5 Cp complexes with monoborane reagents has been developed. Cluster expansion of [(Cp*Ta)₂(B₂H₆)₂] with [BH₃⋅thf] leads to the isolation of the first example of a bimetallic Group 5 tetrahydroborate complex, [(Cp*Ta)₂B₄H₈(μ-BH₄)] (see scheme).

Spectroscopy with a twist: The positions, intensities, and band shapes in Raman optical activity (ROA) spectra of helical polypeptides (see figure) can be explained by quantum chemical calculations.

Thermolytic transformation: The synthesis of new gold or silver, cyclic and polymeric, metallophosphazenes is reported. Pyrolysis of these new metallophosphazenes was conducted and yielded nanostructured materials in which the composition depended on the nature of the metal. Pyrolysis of the gold derivatives gave macroporous metallic gold sponges (see figure) without the requirement of either an external reducing agent or a porous support, which makes it a promising material for catalytic applications.

Keep it in the family: A new family of dendritic oligothiophenes, up to the 3rd generation, comprising 42 thiophene units and 12 ethynylene units has been effectively synthesized by fourfold Sonogashira-type cross-coupling reactions (see scheme). Due to their promising optoelectronic properties, these ethynylene-functionalized materials show a potential for academic and fundamental applications.