Chemistry - A European Journal

The rotation of individual rotors can be driven electrically using electrons from a scanning tunneling microscope tip, as discussed by E. C. H. Sykes et al. in their Communication on page 9678 ff. Above the threshold energy of the CH stretch excitation, electrons selectively drive rotation, but not translation of the thioether rotors. The ability to rotate individual molecules on command will permit the interrogation of many important effects like rotational energy transfer along 1D molecular chains and rotational orientation switching in ordered arrays.

No detours! Recent breakthroughs have shown that direct palladium(II)-catalyzed allylic CH alkylation can be achieved. This new procedure shows that the inherent requirement for a leaving group in the Tsuji–Trost palladium-catalyzed allylic alkylation can be lifted. These initial reports hold great promise for the development of allylic CH alkylation into a widely applicable methodology, thus providing a means to enhance synthetic efficiency in these reactions.

Assemble and build: The strategy depicted above merges different organocatalyzed conjugate addition reactions with the Seyferth–Gilmann homologation by which diverse optically active propargylic, homopropargylic and allenic products can be assembled in high yields and enantioselectivities.

Getting cagey: Dimerisation and coordination to lithium ions was observed after the reaction of the cage [P₂{(SiMe₂)₂O}₃] with Li[Al{OC(CF₃)₃}₄]. The obtained complex ion [Li§P₄{(SiMe₂)₂O}₆]⁺ can be decomposed by addition of THF yielding the free dimeric ligand [P₄{(SiMe₂)₂O}₆].

Avoid racemization! Short lipophilic oligopeptides utilizing nucleophilic N-π-methyl histidine residues catalyze the desymmetrization of meso-1,2-diols with enantiomeric ratios of up to 94:6. Direct one-pot oxidation, which avoids the well-known racemization of the monoacylated product, directly leads to α-acetoxy ketones with enantiomeric ratios of up to 97:3 and 97 % yield.

A novel polymeric platform for catalyst immobilization has been developed, featuring inverted solubility pattern: soluble in a nonpolar and insoluble in a polar medium. Asymmetric reduction of imines with Cl₃SiH (see scheme), catalyzed by chiral formamide anchored to this soluble polymer, proceeds under homogeneous conditions to give chiral amines in high yields and enantioselectivities (≤91 % ee).

Fucntionalizing BINOL: Bis-NHCCu complexes can be used for the oxidative coupling of 2-naphthols. Product yields are highest when using electron-deficient 2-naphthols. Mixed couplings between electron-rich and electron-poor 2-naphthols can be conducted to give good yields of the mixed products.

I see UC UV! Gd³⁺/Tm³⁺/Yb³⁺- and Ce³⁺/Tm³⁺/Yb³⁺-doped LiYF₄ nanocrystals are capable of generating UV, visible, and NIR upconversion (UC) emissions following λ=980 nm excitation (see figure). Gd³⁺ and Ce³⁺ can be sensitized by the Tm³⁺ ion, which results in UV upconversion in the region of λ=280–340 nm, making them attractive as biological probes or novel scintillating materials.

Harvesting the sunshine: Two new pyrrole-containing organic dyes were rationally designed and easily prepared. Thanks to the special electronic properties of pyrrole moieties and the introduction of the non-coplanar aromatic rings through a single bond (blue part in the above structure), a solar-cell device based on the sensitizer LI-1 yields an overall conversion efficiency as high as 91 % of the standard cell from N719.

Solvent-regulated structural transition: 1-Octanol controls the structure of BIC assembly from lamellar, quadrangular to hexagonal under the direction of the transformation of hydrogen-bond patterns (see STM images).

A real ghost: The [TIPS-box/Cu(OTf)₂] complex (box = isopropylidene-2,2-bis(oxazoline)) is a catalyst of the Diels–Alder and the 1,3-dipolar cycloaddition between acryloyl-1,3-oxazolidin-2-one with cyclopentadiene and diphenylnitrone with ee values of 75 and 85 %, respectively. The reacting complex (see figure) of the above reactions was, for the first time, isolated and its structure determined by X-ray spectroscopy.

Batteries not included: The rotation of individual molecular rotors can be driven electrically using electrons from a scanning tunneling microscope tip (see picture). Above the threshold energy of the CH stretch excitation, tunneling electrons are effective at selectively driving rotation, but not translation of the thioether rotors. The ability to rotate individual molecules on command will permit the interrogation of many important effects like rotational energy transfer along 1D molecular chains and rotational orientation switching in ordered arrays.

One-way atropisomerism: meso-(4-Butoxypyrid-3-yl)-substituted Zn^II porphyrins self-assemble to form zigzag-shaped cyclic hexamers, whereas the meso–meso-linked Zn^II diporphyrins 1 undergo self-sorting self-assembly to form different assemblies. Upon heating, entropically driven one-way atropisomerism to 1_in−in has been achieved through fragmentary reconstitution of supramolecular aggregates.

Lighting up the f elements: When excited with UV light, monometallic complexes of lanthanide and uranyl ions with the 2,2′-bipyrimidine ligand were found to be luminescent in the solid state from the visible to the near-IR (see picture). Particularly intense emission was obtained from Tb^III and Eu^III.

Multitasking: C-Silylation of the lateral ester constitutes the key strategic element of a total synthesis of all known ipomoeassin resin glycosides and a small collection of analogues. This uncommon protecting group manoeuvre precludes the lateral cinnamate from interfering with the RCM closure of the macrocycle and protects it against hydrogenation during the saturation of the cycloalkene.

Metal–cage and intracluster bonding in endohedral metallofullerenes (EMFs), including mono- and dimetallofullerenes; metal carbide, nitride, and oxide clusterfullerenes; and Sc₃CH§C₈₀, was studied by quantum theory of atoms in molecules. The picture shows for La₂§C₇₈ a molecular graph in the vicinity of the La atoms (bond critical point (CP) red, ring CP yellow, cage CP green).

The existence of σ aromaticity in cyclopropane (see picture) has been challenged by recent magnetic analysis. Ab initio valence bond computations reveal directly that the σ-aromatic stabilization energy of cyclopropane is at most 3.5 kcal mol⁻¹ relative to propane. This small energy difference raises the question whether there is any need to invoke the concept of σ aromaticity for cyclopropane.

Hierarchical aggregation: The self-assembly process of a set of fluorene co-oligomers with emission colours covering the entire visible spectrum has been studied in dilute and concentrated solution and in the solid state to reveal a hierarchical aggregation mechanism involving one-dimensional fibres and lyotropic gels (see figure).

Explore the outer limits! The outer core (OC) of Yersinia enterocolitica O:3 lipopolysaccharide is important in early phases of intestinal infection. 2-Acetamido-2,6-dideoxy-D-xylo-hex-4-ulopyranose (Sugp; see figure) was identified as its proximal residue. Its 4-keto group changed to the 4-dihydroxy form in aqueous environments, which was crucial for the biological role of the OC as a bacteriophage and enterocoliticin receptor, and in the OC epitope of a monoclonal antibody.

Fine-tuning the acidity! The acidic character of the C_cH (C_c= cluster carbon atoms) in o-carborane derivatives can be fine-tuned by selectively incorporating a defined number of iodine or methyl substituents into the framework boron vertices (see image).

The weak versus the strong: The iodinated ortho-carborane derivatives presented here (see graphic; white=H, pink=B, gray=C, purple=I) provide a ranking of the relative structural importance and influence of competing weak/strong hydrogen bonds and weak/strong halogen bonds that will assist in developing versatile strategies for the future homomeric and heteromeric assembly of molecular structures.

Behind the cubebol: A direct, stereoselective synthesis of (−)-cubebol based on a Pt-, Au-, or Cu-catalyzed cycloisomerization in which control of the configuration of the propargylic center is essential for the facial selectivity is described (see scheme).

Solid insight from²⁵Mg NMR: Magnesium-25 solid-state NMR spectra of a range of organic and inorganic magnesium oxyanion systems obtained at 11.7, 14.1 and 18.8 T are reported (see figure). The NMR parameters are shown to be very sensitive to the local structure. Pseudopotential plane-wave DFT calculations reproduce the experimental trends.

Key cationic intermediates involved in the three-component Biginelli reaction (see scheme) have been intercepted and further characterized by tandem MS experiments (ESI-MS/MS). The experimental and theoretical results were found to corroborate the iminium mechanism proposed by Folkers and Johnson, whereas no intermediates directly associated with either the more energy demanding Knoevenagel or enamine mechanisms could be intercepted.

Organozinc reagents: The tBu-P4 base was found to be an excellent catalyst for the activation of organozinc reagents; the base was used to promote the S_N2′ reaction of α,β-unsaturated esters bearing a γ-chloride in the reaction with various organozinc reagents in the presence of LiCl (see scheme). The reactions proceeded in high yield with excellent chemo- and regioselectivity.

Simplicity is best! Although sulfamidates have been widely used as excellent building blocks, their reactivity towards alcohols has remained unexplored due to their poor nucleophilicity and the generation of many secondary products. This paper reports the ring-opening reaction of chiral hindered sulfamidates with primary and secondary alcohols with excellent yields and with inversion of configuration at the quaternary carbon atom (see scheme).

Packing model conversion! A glutamine derivative gelator can gelatinize some solvents and the packing model and photophysical properties in the gel phase can be controlled by the solvent and speed of gelation (see figure). In general, a low speed of gelation can induce the gelator to adopt a packing model with strong π–π interactions between the aromatic units. Moreover, the gels emit strong green fluorescence when excited at 800 nm, due to two-photon absorption of the gelator.

Chiral Al catalysts: The geometries of the two diastereoisomers A and B of a series of chiral salan aluminium ethyl complexes (see figure) have been elucidated. The catalytic behavior and stereoselectivity of these complexes with regard to rac- and meso-lactide polymerization in the presence of propan-2-ol were studied.

Less gives more: A dramatic enhancement in the activity of glutathione peroxidase (GPx) mimics was observed when the tert-amino groups in benzylamine-based diselenides were replaced by sec-amino moieties (see scheme). This enhancement is due to the absence of any significant thiol-exchange reactions in the selenenyl sulfides derived from sec-amine-based diselenides.

Going with the flow: Simple mixing of multitopic MonoPhos-based ligands with [Rh(cod)₂]BF₄ (cod=cycloocta-1,5-diene) afforded well-designed, recyclable, self-supported catalysts that could be used for asymmetric hydrogenation by using a continuous flow system based on a self-supported catalyst as the stationary-phase material (see graphic).

Competitive inhibition: Folic acid (FA) as a suitable protecting agent facilitated the uptake of FA-Au nanoparticles (NPs) by HeLa cells through the specific recognition of the folic acid receptor (FR) on the cell surface. The free FA competed for FR binding (see graphic); this reduced the availability of FR to FA-Au NPs and inhibited the intracellular uptake of FA-Au NPs to some degree.

Water-soluble gold glyconanoparticles: In order to mimic high-mannose clustering on HIV gp120, multivalent manno-glyconanoparticles have been prepared by either direct glycosylation, peptide linkage, or thiourea linkage. The resultant systems, especially Manα1-2Manα-containing gold glyconanoparticles, have been shown to inhibit DC-SIGN binding to gp120 at nanomolar concentrations (see picture).

Liquids as ligands: A new type of task-specific ionic liquid, amino acid ionic liquids (AAILs), has been used as a chiral ligand in chiral ligand exchange HPLC and capillary electrophoresis (see image). This work demonstrates that AAILs are good alternatives to conventional amino acid ligands for ligand-exchange-based chiral separation.

Boom! New (1,2,3-triazolyl)tetrazoles. NC bonded (non-bridged) 5-(1,2,3-triazol-1-yl)tetrazoles were synthesized by click reactions using 1. The click reaction of 1 with 2 gave rise to 3, in 66 % yield. Heats of formation, detonation pressures, detonation velocities, and impact sensitivities are reported for these new substituted 5-(1,2,3-triazol-1-yl)tetrazoles.

Protein–carbon nanotubes: The debundled state of SWNTs (see figure) is metastable because of electrostatic repulsion between protein molecules adsorbed onto sidewalls of SWNTs. In the presence of alcohols, SWNTs are stabilized by reduction of hydrophobic interaction between the sidewalls, leading to greater dispersion.

Asymmetric rearrangement: Highly enantioselective [2,3]-Wittig rearrangement of oxygenated benzylic ethers was accomplished in the presence of di-tBu-bis(oxazoline) systems as external chiral ligands. The high enantioselectivity was maintained even with catalytic amounts of chiral ligand.

Gold! The highly stable anionic gold(I) complexes [1-(Ph₃PAu)-closo-1-CB₁₁H₁₁]⁻ (1), [1-(Ph₃PAu)-closo-1-CB₉H₉]⁻ (2), and [2-(Ph₃PAu)-closo-2-CB₉H₉]⁻ (3) were synthesized (see figure). The isomerization of Cs-3 to Cs-2 was studied in both the solid state and in solution. The isomerization and activation enthalpies of Cs-2/Cs-3 are very close to the values found for the parent Cs[closo-1-CB₉H₁₀]/Cs[closo-2-CB₉H₁₀].

A sterochemical puzzle: Natural bispyrrolidinoindoline diketetopiperazine alkaloids as well as synthetic analogues have been efficiently prepared by using a highly flexible methodology that has facilitated the structural revision of (+)-asperdimin. This revision reinforces the vital role of total synthesis in determining the structures of natural products.

Plugged in security: Several molecular double plug–socket species can be formed through modulating the disassembly and reassembly of a bis(crown ether) host and a secondary dialkylammonium guest. These species cannot only be regarded as switchable “molecular shields” for the protection of the anthracene group from photooxidation, but they can also perform the functions of molecular keypad locks (see figure; B=base, L=light).