Chemistry - A European Journal

The sudden variation of the magnetism of the β-p-NCC₆F₄CNSSN^. crystal at 36 K is experimentally known to arise through spin canting. A detailed analysis of the magnetic susceptibility χ(T) reveals a turning point in the residual susceptibility below T_N. In their Full Paper on page 2741 ff., M. Deumal, J. J. Novoa et al. analyze this change in χ(T) by performing a first-principles bottom-up study by using crystal data above (160 K) and below (12 K) the magnetic transition temperature.

The borrowing hydrogen methodology is active in the transition-metal-catalyzed C-alkylation of indoles with easily available aliphatic amines. This novel atom efficient and selective method for indole alkylation proceeds in the presence of the Shvo catalyst under basic conditions. It constitutes a complementary and valuable method for alkylations and benzylations. Hence, various substrates, including primary, secondary, and even tertiary amines, react in good to excellent yield with the indoles. For more details see the Communication by M. Beller et al. on page 2705 ff.

Breaking strong bonds: This Minireview highlights recent strategies to functionalize alkyl CH bonds by transition-metal-catalyzed CH activations that involve inner-sphere-type mechanisms. These methods allow the introduction of carbon- or heteroatom-based functional groups and show great promise in the synthesis of complex organic molecules.

All change! Comparison of the gas-phase decomposition reactions of [CH₃CuCH₃]⁻ and [CH₃AgCH₃]⁻ reveals that [CH₃CuCH₃]⁻ undergoes a competition between a dyotropic rearrangement and bond homolysis, whilst [CH₃AgCH₃]⁻ only undergoes bond homolysis. Ab initio calculations reveal that the different behavior in [CH₃AgCH₃]⁻ stems from both the lowering of the homolytic bond dissociation energy and an increase in dyotropic rearrangement activation energy.

Clarity through confusion: The photooxidation of an N-confused porphyrin dianion provides a unique N-confused biliverdin analogue with two types of coordination surroundings: (NNNO) and (CNOO), as demonstrated by a palladium(II) complex (see figure).

Blossoming promises fruit! A new class of shelf stable organoboron reagents has been obtained from ortho- and laterally lithiated aziridines and benzylamines. Their usefulness as Suzuki–Miyaura reagents has been investigated, proving that they could be fruitful reagents for the preparation of arylated aziridines and benzylamines, even in enantioenriched form, and promising new bifunctional Lewis pairs for catalysis or small-molecule binding.

Simple yet efficient: A series of diverse substituted tetrahydroquinoxalines and dihydroquinoxalinones have been synthesized for the first time in a highly enantioselective fashion (see scheme). Using this metal-free hydrogenation it is possible to isolate these products, which are of high pharmaceutical interest, in good yields and with excellent enantioselectivities in a minimal number of reaction steps.

Arrays of stilbenes have been prepared in high purity by using Heck reactions on planar cellulose support, and demonstrate the compatibility of the small-molecule macroarray platform (see figure) with metal-mediated reactions for the first time. Biological screening of the stilbene library revealed several compounds with the ability to inhibit quorum sensing responses in bacteria, expanding the activity of this compound class.

A highly stereoselective AuCl₃-catalyzed hydrative cyclization of 1-epoxy-1-alkynylcyclopropanes was observed for cis-epoxides. Since this cyclization produced 1-oxyallyl cations efficiently, we accomplished a two-step [4+2] annulation of epoxyalkynes with dienes and enones, to provide complex oxacyclic compounds with excellent diastereoselectivity (see scheme).

Single-layer graphene stabilization: An efficient methodology to make stable aqueous solutions of single-layer graphene has been demonstrated by exploiting charge-transfer interactions with a coronene tetracarboxylate acceptor molecule (see figure). Microscopic studies reveal exfoliation of few-layer graphene and selective stabilization of single-layer graphene in large quantities.

Lend me your hydrogen! The first transition-metal-catalyzed alkylations of indoles with aliphatic amines proceeding under transfer hydrogenation conditions are developed (see scheme). This reaction is based on the borrowing hydrogen methodology.

Oxidation is the key: A redox-switch based on a water-soluble Re^II complex is presented. Upon binding to biological ligands such as the side chains in peptides or proteins, Re^II in, for example, [Re^II(tacn)(L)(CO)₂]²⁺ (in wich L=target site, TACN = 1,4,7-triazacyclononane), becomes strongly oxidizing and is reduced to the robust Re^I complex [Re^I(tacn)(L)(CO)₂]⁺. This reduction leads to essentially irreversible metalation of the biomolecule.

Twist-and-shout: An anion receptor showing a turn-on fluorescence response to chloride ions due to an induced conformational change upon anion binding is reported (see picture).

Does the complex contain an SS bond? This question has been recently debated by Alvarez, who claims that there is no SS bond, and Mealli and Hoffmann, who claim there is an SS bond. Here, these two opposing viewpoints are reconciled by an in-between alternative for the electronic structure that is supported by DFT calculations and the available experimental data. The compound (see picture) is described as the first that has been recognized to contain the unusual S₂³⁻ ion, having a three-electron sulfur–sulfur σ bond and an SS bond order of 0.5.

Is there a bond or isn′t there? In a further effort to bring light to the debate on the existence of an SS bond in Tolman′s cluster compound (shown here), this correspondence is a reply to the preceeding communication.

Rhodium and eggs anyone? Dynamic behavior of amino acid residues during the process of accumulation of metal ions (see picture) on a protein surface is directly observed by using X-ray crystal analysis of hen egg white lysozyme porous crystals containing Rh^III ions.

Spin transitions: The transition from antiferromagnetism to spin canting in the β-p-NCC₆F₄CNSSN^. molecular magnet is investigated by using a first-principles theoretical procedure. Spin canting below 36 K is due to the presence of two ferromagnetic sublattices that are antiferromagnetically coupled (see figure). The evolution of the ferromagnetic J_AB contributions explains the change in the slope of the residual χ in the low-temperature region.

Acceptable suitors: An improved catalytic system for the dehydrogenative coupling of primary alcohols has been developed by using inactivated alkenes as suitable hydrogen acceptors. The synergic action of homogeneous Rh^I amide complexes and supported palladium nanoparticles (see picture) as the heterogeneous component of the catalytic system is indicated by a number of experiments.

Potential chemotherapeutic agents: A series of resorcylic acid macrolactone analogues of the natural product radicicol (see scheme) has been synthesized; these compounds are inhibitors of heat shock protein 90 (Hsp90), an emerging attractive target for novel cancer therapeutic agents.

Fit nanoparticles: Nanoparticles employed in the synthesis of silicalite-1 zeolite undergo differentiation of the silicate framework connectivity and size during the induction stage. Larger nanoparticles (6 nm) with a highly connected silicate framework (see picture; dark-gray particles) are units for nucleation and aggregative crystal growth, whereas smaller nanoparticles (2 nm; see picture; light-gray particles) do not participate in the crystallization.

Size matters: Titanium(IV)salan (ONNO-type) complexes show a surprising size-dependent cytotoxicity in human cancer cells (see figure). A systematic study of more than 40 complexes with various salan and alkoxide ligands revealed the size interdependency of the different ligands. A possible mode of action is proposed based on data from cytotoxicity assays and time-resolved ¹H NMR spectroscopy studies.

Sweeter than sweet: An artificial glycocalix self-assembles when carbohydrate ligands decorate a carbohydrate vesicle (see figure).

Tricyclic eight-membered ring diquats were prepared and shown to be the first configurationally stable derivatives of their kind. Resolution was achieved by using chiral BINPHAT and TRISPHAT anions. Their absolute configuration was unambiguously determined by X-ray analysis and circular dichroism (CD). These derivatives are the first stable eight-membered-ring derivatives with only three ring constraints (see figure).

A good Mannich is hard to find: The first catalytic, enantioselective, vinylogous Mannich reaction of acyclic silyl dienolates 1 has been developed. It furnishes δ-amino α,β-unsaturated esters 3 in excellent yields and up to 97 % ee (see scheme). Phosphoric acid 2 with a 2,2′-dihydroxy-1,1′-binaphthyl (BINOL) backbone was established as the optimal chiral Brønsted acid.

Nitrogen to neutrality: The substitution of HNMe₂ in [(HNMe₂)B(azolyl)₃] (azolyl=pyrazolyl, methimazolyl) by a range of N-donor ligands provides a high-yielding route to neutral tripodal ligands (see scheme). The IR spectra of Mn^I–tricarbonyl complexes of the ligands allow comparison of their donor properties with the anionic parent ligands.

Defining directions: The octahedral bis-2,6-di(quinolin-8-yl)pyridine ruthenium(II) complex shows an excited-state lifetime of 3 μs. Its potential for light-harvesting applications is exemplified in rodlike donor–photosensitizer–acceptor (D-P-A) assemblies with this motif (see picture). The D₂-symmetric bis-tridentate photosensitizer unit gives the triads as single, linear isomers that display vectorial electron transfer.

Nanocage superacids: Here we demonstrate for the first time the preparation and characterization of a highly stable, superacid-functionalized, well-ordered 3D-nanocage material with tunable pore diameters that shows superior performance in the synthesis of coumarin derivatives over zeolites and metal-substituted mesoporous catalysts (see figure).

Construction of complexity: An efficient one-pot, three-component tandem reaction for the asymmetric synthesis of six-membered spirocyclic oxindoles with a formal [2+2+2] annulation was developed. The Michael addition of aliphatic aldehydes to electron-deficient olefinic oxindoles gave chiral C3 components (see scheme; Boc: tert-butoxycarbonyl; TMS: trimethylsilyl), which combined with electrophiles to afford substituted spirocyclic oxindoles.

Contact ion pairs: The molecular structures of diazaphospholenium triphospholides are best described in terms of contact ion pairs with a polarized P⁽⁺⁾P⁽⁻⁾ arrangement, rather than as molecular diphosphines with a covalent PP bond (see figure). The polarization can be traced to a combination of electronic and steric factors, so that these compounds could be considered as special cases of frustrated Lewis pairs.

The first monomeric arylsilicon(II) chlorides, SiArCl(Im-Me₄), are reported in which Ar is C₆H₃-2,6-Mes₂ (Mes=C₆H₂-2,4,6-Me₃) or C₆H₃-2,6-Trip₂ (Trip=C₆H₂-2,4,6-iPr₃)] and Im-Me₄ is the N-heterocyclic carbene 1,3,4,5-tetramethylimidazol-2-ylidene. Their molecular (see picture) and electronic structures are compared with those of the homologous germanium compounds.

Get triplet 1,n-biradicals! A new strategy for photocyclizations and for generating triplet 1,n-biradicals (see scheme) by intermolecular photoreactions of N-(ω-hydroxyalkyl)tetrachlorophthalimides with alkenes provides direct verification to possible reaction pathways of the 1,n-biradicals and gives a new insight into the factors deciding reaction-pathway competition and reaction stereoselectivity.

Straightjacketing BODIPYs: Forcing the boron–dipyrromethene (BDP, BODIPY) chromophore into a sterically highly constrained conformation by attachment of rigid dihydronaphthalene extensions to the dipyrrin core (see figure) allows specific adjustment of the optical properties of the dyes. This paper sketches the possibilities of BODIPY tuning.

Desymmetrization versus deactivation: The Pd⁰/Trost ligand complex and allylic 1,4-biscarbonates undergo a facile redox reaction with the formation of a diamidato Pd^II complex and 1,3-dienes. The catalytic desymmetrization of the meso-biscarbonate with the Pd⁰/Trost ligand complex and HCO₃⁻ delivers, with high enantioselectivity, the corresponding alcohol and/or bicyclic carbonate (see scheme). The Pd⁰/Trost ligand complex is stable in the absence of dioxygen.

Seeing is believing: Cl⁻ binding to the interlocked [2]rotaxane induces macrocycle translocation away from the squaraine station and a reversible increase in red fluorescence emission (such as that depicted). Immobilization of the rotaxane sensor creates a prototype dipstick system for fluorometric and naked-eye colorimetric detection of Cl⁻ in aqueous solution.

Lipid A structure: The chemical structure of lipid A isolated from Bradyrhizobium elkanii has been established (see scheme). The backbone is composed of 2,3-dideoxy-2,3-diamino-D-glucopyranose disaccharide substituted by three mannose units. It contains 12:0(3-OH) and 14:0(3-OH) as amide-linked fatty acids and two long (ω-1)-ester-linked hydroxylated fatty acids with 25–33 carbon atoms. One of the acyloxyacyl residues can be further acylated by 3-hydroxybutyric acid.

Micropancakes: Highly uniform and well-dispersed YBO₃ hierarchical architectures with a pancake-like shape have been successfully prepared by a hydrothermal conversion method (left image). The luminescence colors of the YBO₃ samples co-doped with Eu³⁺ and Tb³⁺ can be tuned from red, through yellow and green-yellow, to green by simply adjusting the relative doping concentrations of the activator ions (right image).

A simple and selective amine protecting group is described using 2,2-bis(ethoxycarbonyl)vinyl- (BECV). The method provides a simple way for the conversion of amino acids in to amino esters under very mild conditions, and is also compatible with several functional-group transformations on anilines and aliphatic amines.