Angewandte Chemie International Edition

A supramolecular metal–ligand assembly… catalyzes the [3,3]-sigmatropic rearrangement of enammonium guests in aqueous solution. The cover picture shows the proposed catalytic cycle. The space-restrictive host cavity forces the substrates to bind in a reactive chairlike conformation and thus accelerates the rate of rearrangement. Release and hydrolysis of the rearranged product enable catalytic turnover. For more information, see the Communication by R. G. Bergman, K. N. Raymond, and D. Fiedler on p. 6748 ff.

The well-known Woodward–Hoffmann rules explain how the feasibility and stereochemical outcome of pericyclic reactions are governed by the symmetry properties of the molecular orbitals of the reactants and products. Triggered by a recent claim by E. J. Corey, Roald Hoffmann recalls the first steps in the development of this theory.

In his bookThe Periodic Table, Primo Levi, with touching thoughtfulness, sketched the magic of the elements that make up this world. As with all enduring works of art, The Periodic Table is open to numerous interpretations. Another visit, a quarter of a century after its publication, proves to be most rewarding.

The strength lies within: This review proposes that receptors and enzymes (shown in blue in the scheme) derive an important contribution to binding their ligands and transition states (respectively) by decreasing their dynamic behavior. Conversely, ligand binding energy is reduced where the binding process increases the dynamic behavior of the receptor protein.

Given the non-renewable nature of synthetic polymers from petroleum feedstocks, there is increasing interest in developing routes to biodegradable polymeric materials from renewable resources. Polycarbonates made from CO₂ and epoxides (see scheme) have the potential to meet these important goals. Reviewed here are well-defined catalysts for epoxide–CO₂ copolymerization and related reactions.

Half-dissociated: Experimental and computational findings conclude that water forms a highly ordered superstructure on defect-free surfaces of zinc oxide, in which every second water molecule is dissociated (see picture). The results are of general relevance for heterogeneous catalysis.

Light switches: The photon capture area of single fluorescent molecules has been directly measured by switching them “on and off” with light. Stimulated emission of single molecules (see picture) enables the precise control of the excited state population probability of an individual molecule at room temperature.

Giving cells some stick: Osteoblast adhesion to titanium, a common material for implants, is stimulated by coating with an optimized and highly specific nonpeptidic ligand for the αvβ3 integrin (see picture). This new technology is advantageous to conventional coating by peptides or proteins in many practical aspects (selectivity and activity, stability against enzymatic degradation, sterilization, costs).

Different octahedral units form the basis of the structures of the very strong Lewis acids aluminum bromide fluoride (ABF) and aluminum chloride fluoride (ACF) (see picture). A structure model for these phases is established in which the octahedra are linked through μ-bridging fluorine atoms and μ₃-bridging X atoms (X=Cl, Br). The bridging of three octahedra by a heavy halogen atom explains the results of NMR and EXAFS analysis.

Markers by mass: The effect of terpenoids on organisms can be investigated by using DNA adducts as markers. High-resolution MS was used to characterize DNA/α-pinene oxide adducts after enzymatic digestion. The reaction of α-pinene oxide with the N7 position of guanosine leads to cleavage of the N-glycosidic bond and formation of the N7 guanine adducts (see picture).

Impressive tolerance is displayed in the efficient and chemoselective organocatalytic transfer hydrogenation of α,β-unsaturated aldehydes in the presence of a Hantzsch dihydropyridine and a catalytic amount of dibenzylammonium trifluoroacetate (see scheme). Various sensitive functional groups such as the nitro, cyano, alkenyl, and benzyloxy groups survive these reaction conditions.

A choice selection: A dynamic combinatorial library of oligonuclear coordination compounds is formed from a flexible tris(catechol) ligand with titanium(IV) ions in the presence of lithium carbonate or potassium carbonate. A trinuclear complex (1) can be isolated by crystallization from DMF. In contrast, well-defined mononuclear (2) compounds are obtained by addition of Na⁺ as a template, and tetranuclear compounds (3) are isolated by changing the solvent to DMSO.

Small rings: Only a few methods are known for the synthesis of cyclopropane carbaldehydes and cyclopropyl ketones 2. In a new, simple route to highly enantioenriched trisubstituted three-membered rings, 4-hydroxy-1-alkenyl N,N-diisopropyl carbamates 1 are treated with NaH, resulting in migration of the carbamoyl group and intramolecular enolate alkylation.

An efficient method for the enantioselective preparation of structurally diverse cyclopropanes has been developed. Sequential homoaldol coupling and activation steps result in a three-carbon homologation of an aldehyde to give a nonracemic, stereochemically rich cyclopropylcarboxaldehyde (see scheme).

The complex macrolide apoptolidinone (1) was synthesized in 19 steps (longest linear sequence) from (S)-malic acid. Key reactions include A) two stereoselective aldol reactions, B) a late-stage Grubbs cross-metathesis reaction to install a trisubstituted vinyl boronate, and C) an intramolecular Suzuki–Miyaura reaction.

After all, neat As(N₃)₅can be isolated: By analogy with AsCl₅, neat As(N₃)₅ was predicted to be a highly unstable compound, and previous attempts at its synthesis had resulted in intense explosions. The successful syntheses and characterization of neat As(N₃)₅ and Sb(N₃)₅ and the crystal structure of the [Sb(N₃)₆]⁻ ion (see picture) are now described.

A rearrangement strategy was used for the synthesis of α- and β-gem-difluorocarba-D-glucose, which are close congeners of α- and β-D-glucose, in which the endocyclic oxygen atom has been replaced by a gem-difluoromethylene group (see scheme). The two anomers α or β were obtained stereoselectively by the use of steric or electronic control, respectively. TIBAL=triisobutylaluminum.

Hypervalency and secondary bonding are the driving forces behind the rapid absorption of gaseous carbon dioxide by two organotellurium and organotin oxides and the unexpected formation of a unique tellurastannoxane cluster (see structure; dark red Te, black Sn, gray C, light red O). The absorption is reversible with the liberation of carbon dioxide being observed at temperatures between 90 and 145 °C.

Three terminal SnE bonds are present in the imidotin clusters [(thf)LiSn₃E₃(μ₃-NtBu)₄]⁻ (E=Se, Te; see picture: N blue; Sn gray; Te orange; Li purple; O red) that are readily obtained under mild conditions, as solvent-separated ion pairs with [Li(thf)₄]⁺ (E=Se, Te) or [(thf)Li([12]crown-4)]⁺ counterions (E=Se), by the reactions of the anionic cluster [(thf)LiSn₃(μ₃-NtBu)₄]⁻ with the appropriate chalcogen.

Hold the metal: Upon exposure of acetates 1 to substoichiometric quantities of triphenylphosphane in the presence of 2, regioselective metal-free allylic substitution occurs through a formal tandem S_N2′–S_N2′ substitution mechanism to provide γ-butenolides 3 with high levels of regio- and diastereocontrol (see scheme).

Template-directed processes involving polysaccharide, dextran, and preformed zeolite crystals/nanoparticles are used to prepare elaborate porous frameworks of interconnected filaments of NaY zeolite/silica nanoparticles, as well as macroscopic fibers of crystallographically aligned silicalite nanocrystals (see picture; bar is 5 μm).

Catch a falling star: Peptides from chiral, C^α-methylated α-amino acids (see scheme) found in L enantiomeric excess in meteorites show diastereoselectivity when reacting with racemic proteinogenic amino acids. Accordingly, the prebiotic soup of proteinogenic amino acids may have evolved into a chirally unbalanced system, eventually seeding the homochirality of life.

Removal service: A deletion mutant lacking the Gly600 residue of a prokaryotic squalene–hopene cyclase was prepared. Surprisingly, the mutant cyclase has no enzyme activity for 1 and 3, but shows a high conversion ratio for 2 (see scheme). Deleting Gly600 alters the specificity from prokaryotic into that of eukaryotic-type cyclases.

Carbon copies: Alkaline-earth metal (Mg, Ca, and Sr) derivatives 2 of a silagermabicyclo[1.1.0]butane-2,4-diide are obtained by the treatment of 1 with magnesium or calcium in THF or by the reaction of the dianion 3 with MgBr₂, CaI₂, or SrI₂ in THF. Such stable derivatives of charged bicyclo[1.1.0]butanes have been elusive species until now.

Sifting through the surface: Permselective core/shell microgels have been prepared by using a labile cross-linker, which can be cleaved stoichiometrically to control the porosity of the shell. Proteins that are smaller than the pore size are allowed to permeate through the shell to bind with the core-bound ligand (arrows; see picture).

During the biosynthesis of glycopeptide antibiotics of the vancomycin family, several oxidative phenol coupling reactions take place. An oxygenase (OxyB) from the vancomycin producer catalyzes the first of these coupling reactions to a significant extent only when the putative hexapeptide substrate is linked as a thioester to a peptide carrier domain (PCD) derived from the non-ribosomal peptide synthetase (see picture).

Multiple CC bonds are formed in an unprecedented reaction of an ylide with 2 equivalents of a rhodium-stabilized carbene. The cascade cyclization of the intermediate formed from the three components gives bicyclic pyrrolidine products with excellent diastereoselectivity (see scheme).

O₂for you: The trans-μ-1,2-peroxo Ni₂ dimer 3, a structural motif unknown previously in nickel coordination chemistry, was produced by reaction of the (tmc)nickel(I) precursor 1 with O₂. Thermal decomposition of 3 in CH₃CN leads to 2, in which the atoms of the hydroxide ligand derive from O₂ and the hydrocarbon solvent.

Independent of the ring size in the aliphatic side chains, peptides made up of β^2,3-cyclic aminoxy acids, novel chiral building blocks of foldamers, adopt uniform secondary structures consisting of rigid β NO turns and 1.8₉-helix structures (see figure). This contrasts previous results from other groups with cyclic ring-constrained β-peptides.

Central to the design of a new organocatalyst system for aldehyde–aldehyde aldol reactions is the necessity of iminium geometry control during the enamine addition step (see scheme). Significant structural variation in both the aldol donor and aldol acceptor are possible while maintaining high reaction efficiency and enantioselectivity. TFA=trifluoroacetic acid.

Spring-loaded supramolecular chemistry: Pyridine-2-carboxaldehyde reacts with the m-phenylenediamine shown in aqueous solution in the presence of Cu^I ions to give a tetracopper(I) grid complex (see structure). This structure is stable in water, but not in any of the organic solvents tried. The strain incorporated into the structure is compensated by a diffuse pressure applied by the hydrophobic effect. R=2-hydroxyethylcarbamoyl.

The long and the short of it: N⋅⋅⋅O distances less than 2.8 Å (see picture) have been implicated in determining the 220 nm circular dichroism band intensity in helical peptides. An analysis of helical peptide crystal structures, however, reveals the average experimentally determined hydrogen-bond lengths are 2.978–3.113 Å (α helix) and 2.907–3.211 Å (3₁₀ helix).

Water-soluble chiral Ru complexes with a β-cyclodextrin unit have been shown to catalyze the reduction of aliphatic ketones (see scheme) with up to 97 % ee and in good to excellent yields in the presence of sodium formate. The β-cyclodextrin unit is an essential component of the catalyst. It contributes to the unprecedented levels of enantioselectivity observed through the preorganization of the substrates in the hydrophobic cavity.

A new twist of RNA fate: Site-selective chemical functionalization at the C2′′OH group of paromomycin (1) afforded a novel analogue with potent inhibitory activity against several bacterial strains, including a multidrug-resistant S. aureus (MRSA) strain. X-ray cocrystal-structure determination of the complex with the A site of E. coli RNA revealed a new mode of binding in which significant conformational and positional changes had taken place in rings III and IV.

Protecting group chemistry was used to functionalize the channel entrances of zeolite L crystals. This approach is broadly applicable and can be used to obtain a wide range of new zeolite-based materials. The picture shows two microcrystals modified with amino groups at the channel entrances, which reacted further with a red-luminescent head molecule. FG=functionalizing group, prot.=protector.

Get into shape under redox: Rearrangement of [Ru₄S₃(arene)₄] accompanies redox processes and occurs in its reduction of H⁺ to H₂ (see scheme; Ru=Ru(cymene)). The [Ru₄S₃(arene)₄]²⁺ ion is so crowded that arene rotation is hindered, which allows the detection of atropisomers.

Visualizing 3D structures (especially of supported catalysts) by the nondestructive method of electron tomography is best accomplished by high-angle annular dark-field (HAADF) rather than by bright-field (BF) imaging; bimetallic clusters of [Ru₁₀Pt₂] (diameter ca. 0.4 nm), supported on nanoporous silica, are invisible in BF but readily visible in HAADF images (see pictures).

Catalytic containers: A supramolecular metal–ligand assembly [M₄L₆] is utilized as a catalytic host for the unimolecular carbon–carbon bond-forming rearrangement of enammonium cations (see scheme). The restricted reaction space of the supramolecular structure forces the substrate to adopt a reactive conformation upon binding to the interior. The assembly achieves up to 850-fold rate acceleration of the rearrangement.