Angewandte Chemie International Edition

Membrane proteins can now be studied in their native environments by using high-resolution solid-state NMR spectroscopy. Techniques that probe rigid, mobile, and water-exposed protein segments provide insight into the membrane-embedded structure of sensory rhodopsin II from Natronomonas pharaonis. The secondary structure, dynamics, and membrane topology of this seven-helix receptor are discussed by M. Engelhard, M. Baldus, and co-workers in their Communication on page 459 ff.

A “π-conjugated catenane” or more accurately a catenane formed from two intertwined conjugated macrocycles has been formed for the first time by a double metal template strategy. Interestingly, a through-space electronic interaction between the donor unit (oligothiophene, metallic color) and the acceptor component of the other ring (phenanthroline, blue) is evident. Further details can be found in the Communication by P. Bäuerle et al. on page 363ff.

An up-and-coming sibling: Unlike the cyanide ion (CN⁻), which is one of the fundamental ions in inorganic chemistry, cyaphide (CP⁻) is not stable unless it is coordinated to a metal center, either as a bridging ligand or, as just recently demonstrated, as a terminal ligand (see scheme; L=monodentate phosphane, X=halide).

Shuttle chain in operation: “Chain-shuttling” polymerization is a promising strategy for the synthesis of high-performance polyolefins. Polymerization takes place block by block using two different catalysts to generate new block copolymers with special properties.

Filled pockets: The most important interactions involved in biological phosphate recognition have been characterized. From 3003 considered structures, 2456 feature phosphate binding sites (as shown in the picture; ligand skeleton green, C gray, O red, N blue, P orange) without metal ions. The search reveals similarities to phosphate binding by synthetic receptors and points to new approaches for filling phosphate sites in drug-discovery research.

Aminations with amidates: The first C₂-symmetric amidate complexes of zirconium were used for catalytic asymmetric hydroaminations of amino alkenes to prepare chiral gem-disubstituted pyrrolidines with up to 93 % ee (see scheme). The modular nature of the chiral complexes facilitates their preparation and screening as catalysts.

Use your borane: A stereocontrolled synthesis of allyl boranes from chiral sulfur ylides and their subsequent reaction with aldehydes to give homoallylic alcohols is described. The allyl boranes also could undergo a stereospecific 1,3-borotropic rearrangement to give another set of homoallylic alcohols (see scheme). The reaction has been applied to an efficient synthesis of iso-agatharesinol.

Make it a double: A double metal template strategy has been used to form a “π-conjugated catenane” consisting of intertwined macrocycles with oligothienyl and phenanthroline units (see calculated structure). The optical and redox properties as well as the structural and conformational analyses give clear evidence that the two macrocycles in the catenane influence each other by through-space donor–acceptor interactions.

A New Class of Metal–Imido Complex: Octahedral hexatantalum clusters [(ArN)₁₄Ta₆O] (see picture; C gray, N blue, O red, Ta orange, Ar=Ph) were formed in good yields by treatment of [Bn₃TaNtBu] (Bn=benzyl) or [Ta(NMe₂)₅] with an excess of the appropriate aniline and an equivalent of water. Crystallographic analysis revealed the presence of a central oxygen atom, which was further identified by electrospray mass spectrometry using ¹⁷O/¹⁸O-enriched material.

Pushing and pulling by three tert-butyl and two carboxylic acid groups, respectively, results in a sensitizer (see picture) that exhibits an incident photon-to-current conversion efficiency of 75 %. A cell sensitized with this compound gives a short-circuit photocurrent density of (6.50±0.20) mA cm⁻², an open-circuit voltage of (635±30) mV, and a fill factor of 0.74±0.03, which corresponds to an overall conversion efficiency of 3.05 %.

On the side: The crystal structure of a peroxido Mn^III complex bearing a non-heme ligand shows that the peroxide ligand is bound to a manganese ion symmetrically in a side-on η² fashion. The complex is capable of deformylating aldehydes by a nucleophilic reaction but not capable of oxygenating substrates by an electrophilic reaction. tmc=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane.

The beauty of symmetry: The Si₂(CH₃)₇⁺ ion can be observed under the low pressures used in Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The optimized geometry of the ion, as determined from theoretical calculations and FT-ICR experimental results, reveals a C_3h-symmetric structure, in which a planar CH₃ group is symmetrically bonded to two eclipsed Si(CH₃)₃ moieties (see picture; Si blue, C dark gray, H pale gray).

A new dimension has been given to the Horner–Wadsworth–Emmons reaction almost 50 years after its inception. In an unprecedented vinylogous version of the reaction, aldehydes and cyano-substituted vinyl phosphonates serve as substrates (see scheme). The novel transformations to form 1,3-dienes with a trisubstituted double bond exhibit very high regio- and stereoselectivities.

Michael–Michael–retro-Michael cascade reactions are promoted by the highly efficient organocatalyst 9-amino-9-deoxyepiquinine (1). The asymmetric direct vinylogous Michael addition of α,α-dicyanoalkenes to α,β-unsaturated ketones may be followed by an intramolecular Michael addition and a retro-Michael reaction to afford polysubstituted 2-cyclohexen-1-one derivatives with high enantioselectivity (see example).

When product and catalyst are the same: The chiral product 3 of an asymmetric Mannich reaction (see scheme), investigated under various experimental conditions and by calculations, is shown to act as a chiral catalyst for its own formation (autocatalysis). A generally applicable catalytic cycle is proposed to involve equilibria between hydrogen-bonded substrate–product complexes.

Razzle-dazzle… and allenyl boranes 1 are obtained in enantiomerically pure form from 2 through a novel CHTMS insertion followed by a sterically driven suprafacial 1,3-borotropic rearrangement. Asymmetric allenylboration with 1 gives syn β-trimethylsilyl (TMS) homopropargylic alcohols 3 in 80–96 % yield with d.r. 99:1 and 94–99 % ee. When R=isopropenyl, allylboration occurs to provide the 1,2,3-trienyl alcohol 4 with >99 % ee.

Nanozymes with a heart of gold: A functional artificial protein has been prepared by grafting a dodecapeptide onto the surface of gold nanoparticles (see picture). The system catalyzes the hydrolysis of carboxylate esters and features enzyme-like properties.

Shapes changed by solvent: An ab initio method for calculating the absorption spectra of large molecules including solvent effects and molecular vibrations shows how the solvent can shift the spectra and modulate their shapes (see picture; black lines: stick representation of absorption spectrum). The computed spectra of coumarin C153 in various solvents agree with the experimental ones.

No preactivation of the alcohol substrates is required in allylic, propargylic, and benzylic aminations catalyzed by a combination of commercially available Bi(OTf)₃ and KPF₆, mostly at room temperature (see scheme). The substitution reactions of readily accessible allylic, propargylic, and benzylic alcohols with sulfonamides, carbamates, and carboxamides give the desired amine products in up to 99 % yield.

Designing and testing biomimetic switches: Multireference perturbation theory is used to model a light-driven molecular switch featuring the same photoisomerization mechanism as the chromophore of the visual pigment rhodopsin (see picture; QM/MM: quantum mechanics/molecular mechanics). By exploiting a synthetic route based on nitrilium-cation cyclization, it was shown that the designed system can indeed be prepared and characterized.

Electric light orchestra: The electrochemiluminescence (ECL) intensity of the [Ru(bpy)₃]²⁺/2-(dibutylamino)ethanol (DBAE) system at Au and Pt electrodes are about 10 and 100 times, respectively, greater than that of the commonly used [Ru(bpy)₃]²⁺/tripropylamine (TPA) system (see picture). Thus, DBAE is a promising co-reactant for [Ru(bpy)₃]²⁺ ECL immunoassays and DNA probe assays.

Four thermal-rearrangement pathways and a domino reaction leading to quinones arise from the thermolysis of cyclobutenones. The course of vinylcyclobutenone rearrangements is dictated by the nature of the substituent, R (see scheme): a cyclopentenone arises when R is an electron-rich alkene. In other cases thermolysis gives a cyclohexadienone, which may collapse with elimination to form a quinone, or tautomerize into a hydroquinone or cyclohexenedione.

Got to get up to get down: Oxidation of [(1,2,4-C₅H₂tBu₃)Mo(PMe₃)₂H₃] induces H₂ reductive elimination and yields the first stable 15-electron molybdenum hydride complex, [(1,2,4-C₅H₂tBu₃)Mo(PMe₃)₂H]⁺ (see picture). The stability of this complex can be attributed to steric protection, as no additional electron density is available on the ligands for a π-stabilization mechanism.

An efficient and highly convergent enantioselective synthesis of (+)-homochelidonine has been achieved (see scheme; Cbz=benzyloxycarbonyl, MOM=methoxymethyl) and relied on a new and powerful desymmetrizing ring-opening reaction of a meso-azabicycle with an aryl boronic acid. The route should allow access to other hexahydrobenzo[c]phenanthridine alkaloids.

From the deep: Vigulariol has been efficiently synthesized in 20 steps and 4.0 % overall yield from commercially available starting materials. Key to the synthesis was the use of copper(II) hexafluoroacetylacetate [Cu(hfacac)₂] to form the oxabicyclo[6.2.1]undecene system (see scheme; TBS=tert-butyldimethylsilyl). The strategy should provide access to other members of the cladiellin family.

Meeting point: Carbon nanotubes (CNTs) are produced by an equimolar reaction between C₂H₂ and CO₂ (see scheme). CO₂ can be introduced as a gas or a carbonate support of the metal particles. When CaCO₃ is used, the growth of a nanotube stems from the triple-point junction, which corresponds to the area surrounding the metal support interface where CO₂, C₂H_2, and the support meet.

Back in the saddle: Copper corroles do not contain a Cu^III ion, but rather a well-hidden Cu^II ion. A comparative crystallographic and theoretical study (see spin-density representation) of isostructural corroloid complexes with copper ions in different formal oxidation states suggests that the divalent state is stabilized through a saddling distortion of the corrole ligand.

The Y-shaped π systems in trinuclear cyanines can, through a formal two-electron transfer, form a core methylenecyclopropane unit with three shorter, isolated π systems (see picture). Studies of the spectroelectrochemistry verify the reversibility of this redox reaction, which has an unusually pronounced electrochromism.

The type and the size of the saccharide side chain can affect the conformation of the peptide portion of glycopeptides. This was shown by conformational analysis in solution for a series of glycoundecapeptides with systematically varied tumor-associated carbohydrate antigens of type A from the recognition region of the cell adhesion glycoprotein LI-cadherin (gray C, red O, blue N; the saccharide side chain is more intensely colored).

In the loop: High-resolution solid-state NMR spectroscopy is used to study the secondary structure, dynamics, and membrane topology of an entire seven-helix receptor, sensory rhodopsin II from Natronomonas pharaonis (see picture). The experiments are carried out on a single isotope-labeled sample in a native membrane environment.

Doing the locomotion: When benzene molecules diffuse through the metal organic framework MOF-5, they accumulate in pockets (see picture). Molecular dynamics simulations reveal that the correlated lattice motion increases this binding. Only when the the lattice dynamics are considered in the simulation, can the experimental diffusion constants be reproduced.

Hitting the teens! Five CC bonds and eight stereocenters were formed in a one-pot asymmetric multicomponent domino reaction, which led to the functionalized tricyclic diterpenoid-type core structures (see scheme, IMDA=intramolecular Diels–Alder reaction).