Angewandte Chemie International Edition

The most important and interesting substance on our planet is undoubtedly water. The recent success in quantum chemical calculations and spectroscopic investigation of isolated water clusters has significantly increased our understanding of the bulk phase. The picture shows three experimentally determined water hexamers which play an important role in various phases.

The advantages of homogenous and heterogeneous catalysts can be combined by using transition metal functionalized dendrimers. Periphery-functionalized dendimers (left) will promote cooperative effects between the metal centers, whereas core-functionalized dendrimers (right) can profit from site-isolation effects and be used as enzyme mimics. Both systems are generally much larger than the substrates and products, thus providing an intrinsic solution to the problem of separating homogeneous catalysts. In the last decade considerable progress has been made in the development of novel dendritic catalysts. In this review the different concepts and progress in transition metal dendritic catalysis will be discussed.

Specific fluorescence with high quantum yields is exhibited by (E)-stilbene haptens that are complexed with monoclonal antibodies. The temperature-dependent dynamics of these host–guest systems can lead to the generation of fluorescent exciplexes of the (E)-stilbene (St) chromophore and an indole (In) building block in the H chain of the antibody (see diagram). Fine-tuning of the photophysics and photochemistry through protein–ligand interactions promises interesting applications.

“Refitting” existing protein scaffolds to generate new enzymes with designed properties is a powerful tool in molecular evolution. Two recently published reports demonstrate the engineering of the ubiquitous (βα)₈-barrel (see picture) scaffold to obtain new enzymatic catalysts with altered binding and catalytic properties. These are the first examples of the conversion of one enzyme's activity into that of another by using directed evolution.

Supramolecular interaction of fullerenes with cyclic dimers of metalloporphyrins (1-M, M=central metal ions) showed a pronounced dependence on the nature of the metal ions, whereby 1-RhMe/fullerene systems displayed extremely high association constants (ca. 10⁸ M⁻¹) and low dissociation activities. Crystal-structure analysis and NMR data suggested a charge-transfer interaction and van der Waals interactions between the host and guest.

Photochemical detection of a nucleic acid was achieved by means of a DNA-cross-linked CdS nanoparticle array attached to an electrode (see schematic diagram). The array was constructed from the target DNA (blue) and CdS nanoparticles bearing oligonucleotides complementary to its 3′- (green) and 5′-ends (red). The photocurrent generated on irradiation of the array increases with increasing concentration of the target.

Four P₄chains link eight aluminum centers in the [P₁₆Al₈] macrocycle [{(PCH₂CH₂PAlMe₂)₂}₄]⋅4 [AlMe₃] (1), which has a unique macrocyclic structure comprised solely of main group elements. Compound 1 is produced by the reaction of [AlMe₃] (see scheme) with the catalytically produced organotetraphosphane (PCH₂CH₂PH)₂ (2).

The concept of aromaticity is extended to a series of bimetallic heterocyclic species of general formula XAl₃⁻ (X = Si, Ge, Sn, Pb). The delocalization of the π electrons is important for the stability of the cyclic aromatic structure (see picture).

A dumbbell-shaped dialkylammonium ion (see scheme) templates the formation, about its NH₂⁺ center, of a crown ether like macrocycle under thermodynamic control from dialdehyde and diamine precursors. The imine-containing [2]rotaxane that ensues was converted by reduction into a kinetically stable interlocked molecule, which has been characterized in the solid state by X-ray crystallography in both free base and salt forms.

Imagine—after a homogeneous reaction is complete, the chemist adds a catalyst or exposes the reaction mixture to soft UV light and pure product precipitates from the reaction mixture. This can be achieved with “precipitons”—protecting groups that have controllable solubility states. The first advances toward that goal are described (see scheme).

The highly symmetric structure of a cagelike host enables the inclusion geometry of guest molecules in the host to be predicted in a dynamic fashion by analyzing not the guest but the host framework. For example, an S₄ orientation of two 1,2-diketone guests in the clathrate complex was predicted by NMR spectroscopy and subsequently confirmed by X-ray analysis (see picture).

Chiral aluminum complexes 3 have been employed as catalysts for the stereoselective synthesis of cis-5-substituted 2-oxazoline-4-carboxylates from the aldol reaction of oxazole 1 and aldehydes. cis-Product diastereoselection ranges from 3:1–50:1, while enantioselectivities vary from 92–99 % for reactions with the 25 aromatic aldehydes investigated.

Through a reporter molecule containing a carbohydrate conjugate such as 1, low-affinity molecular interactions with lectins can be characterized (landscaped) by measuring both the cross-sectional excitation and emission modifications of the reporter molecule (see picture). The three-dimensional recognition pattern (fingerprint) and not the affinity is the selection criterion for molecular recognition.

Such a self-assembly on a surface has not before been considered: Atomic force microscopy (AFM) was used as a “molecular ruler” to observe the direct assembly of coordination cages on self-assembled monolayers (SAMs). A simple process was employed using microcontact-printed (μCP) modified substrates. Metal-induced self-assembly of cavitand-based cages on SAMs (see picture) was proven by means of electrochemistry, contact-angle, and X-ray photoelectron spectroscopy measurements. AFM measurements on individual molecules has enabled the observation of the coordination process on a single-molecular level.

A new catalytic enantioselective method has been developed for the allylation of imines 1 by substituted allylstannanes 2 with chiral zirconium catalysts prepared from zirconium alkoxides and 1,1′-bi-2-naphthol (BINOL) derivatives. The allylated products 3 were obtained in high yields and with excellent enantiomeric excesses. The basic concept of the reaction provides a convenient entry to various chiral building blocks.

A new class of noncoordinating anions are the carbosilane dendrimers with perfluoroarylborate end groups (see picture). They activate and stabilize metallocenes to give catalytic systems that polymerize olefins, such as ethene, propene, and 1-hexene, with high activity.

Short Ba−C bonds and a BaHB₂ four-center bond characterize the baracarborane 1, which was obtained by metalation of 2,3-bis(trimethylsilyl)-2,3-dicarba-nido-hexaborane with [(thf)₄Ba{Zn(CH₂SiMe₃)₃}₂]. A dialkylzinc side product reacts neither with the carborane nor with 1.

The “naked” ternary thiazyl dichloride anion NSCl₂⁻ (see picture) is surprisingly stable and can be isolated on a macroscopic scale when a large countercation is present. The structure was determined by X-ray diffraction and compared with theoretically obtained data. The unequivocal identification was supported by Raman, IR, and ¹⁴N/¹⁵N NMR spectroscopic studies. The structure and bonding are discussed on the basis of MO and NBO analysis.

Rarity value is something that main group complexes of N-heterocyclic carbenes still have. With a triscarbene chelating carbon analogue of Trofimenko's tris(pyrazolyl)borate the solvent free, dinuclear lithium complex 1 was successfully synthesized, in which the lithium atoms are only bound to carbene carbon atoms.

With higher yields than obtained by the stepwise method, the title domino reaction starts with acyclic olefin substrates and forms two new C−C single bonds with excellent regio- and stereocontrol. A synthetically useful β-dicarbonyl functionality is also introduced (see scheme, EWG=electron-withdrawing group).

The deoxygenation of the C=O group of aldehydes and ketones with dialkenyllithium compounds leads to the formation of highly substituted cyclopentadiene derivatives with a new C(sp³)−C(sp²) bond [Eq. (1)].

The C−H and M−M bond-cleavage and -formation steps in the interconversion of 1 a and 1 b are part of a novel organometallic photochemical system—a functional thermo-optical switch. Thermodynamic and mechanistic aspects of this system are presented.

Corner-sharing Co₃(μ₃-OH) triangles make up the framework of two novel compounds obtained by hydrothermal synthesis from CoCl₂, NaOH, and benzene-1,2,3-tricarboxylic acid. In the structure (see picture) of one of these—[Co₂(OH){1,2,3-(O₂C)₃C₆H₃}(H₂O)]⋅H₂O—octahedral (crosshatched) and tetrahedral (dotted) Co sites alternate, and trigonal-bipyramidal sites (striped) are attached at bridging OH groups. The compounds are rare examples of the Δ-chain topology, and both have unusual magnetic behavior.

Oxidation of elemental sulfur (S₈) with dimethyldioxirane (or CF₃CO₃H) generates a disulfur monoxide equivalent (“S₂O”). Yields of “S₂O” of 30–40 % are obtained from equimolar amounts of S₈ and an oxidizing agent. “S₂O” reacts with diazoalkanes to give dithiirane 1-oxides [Eq. (1)].

Diastereomeric ratios as high as 99:1 have been obtained by the solid-state isomerization of mixtures of atropodiastereomers such as trans-1 a and cis-1 a, whereas their solution equilibration gave ratios close to 50:50.

Insights into the mechanism of the rhodium-catalyzed hydrophosphorylation of alkynes with the cyclic hydrogen phosphonate 1 are provided by isolable Rh^III intermediates 2 and 3. The reactions proceed at room temperature and give high yields of exclusively (E)-alkenyl phosphonates 4, which are not readily prepared by other routes. X=halide, L=phosphane or hydrogenphosphonic acid.

The reactions of transient olefin–L_nPd⁰complexes must be controlled for successful allylic-substitution polymerization. These reactions reveal a novel aspect of C−C bond-forming polycondensation (see scheme).

Why do higher order cyanocuprates often show higher reactivity than the classical Gilman reagent? Computational theoretical studies suggest that the key lies in the properties of the bridging [LiCNLi]⁺ moiety in the cuprate (see scheme).

Lightning and corona discharges associated with thunderstorms, long recognized as important sources of atmospheric NO, cause ionization of air containing a high local concentration of O₃. Under such conditions, the title reaction represents a likely source of tropospheric N₂O, whose important role in stratospheric O₃ depletion and as a greenhouse gas is well established.

It pays to be organized: Steric repulsion between fucose and the methyl substituent of the adjacent tetrahydropyran reduces the distance between galactose and fucose in the E-selectin antagonist 1 compared to the distance in closely related compounds that lack the methyl group. This effect causes a solution conformation of 1 that already resembles its conformation when bound to E-selectin and, thus, leads to a significantly improved bioactivity.

Heating shikimic acid in near-critical water leads to the formation of phenol. Since shikimic acid can now be obtained by the microbial conversion of glucose, a benzene-free route to phenol could become an alternative to the industrial Hock oxidation of cumene derived from benzene (see scheme).

A facile and fast enantioselective synthesis of α-amino acids with high ee values was achieved by the asymmetric alkylation of the glycine derivative 1 under phase-transfer conditions with (R)- or (S)- 2-amino-2′-hydroxy-1,1′-binaphthyl (NOBIN; see scheme). The ee value of the catalyst can be as little as 40 % without decreasing the ee value of the amino acid products. This occurs as a result of a significant positive nonlinear effect in the alkylation reaction.

Although many metallabenzenes are known, complex 1 appears to be the first well-characterized example of a metallabenzyne. It has been synthesized by treatment of [OsCl₂(PPh₃)₃] with an excess of HC≡CSiMe₃ in wet benzene. The structure of the novel complex 1 was confirmed by single-crystal X-ray diffraction, which showed the six-membered ring to be essentially planar with a maximum deviation from the least-squares plane of 0.047 Å.

The carbon-centered radical 1 of the antimalarial drug artemisinin is generated after activation by reduced heme 2 (only scaffold shown), and is able to alkylate the heme macrocycle at the meso positions. Alkylation occurs preferentially at the α, β, or δ positions.

The proligand citric acid has been used to assemble two new anionic Ni^II clusters, [Ni₇(cit)₆(H₂O)₂]¹⁰⁻ and [Ni₂₁(cit)₁₂(OH)₁₀(H₂O)₁₀]¹⁶⁻ (see picture; Ni=black spheres), from basic aqueous solution. The former possesses a high-spin S=7 ground state, while the latter shows predominantly antiferromagnetic exchange.

The replacement of the double bond of ceramide by a cyclopropene (for example, erythro-1) has resulted in the first potent inhibitor of dihydroceramide desaturase, which is a key enzyme in the biosynthesis of sphingolipids.

Hydrolytic removal of the core unit and cross-linking of their wedges enables dendrimers to be “cored” (see schematic representation). By internalizing the alkenes, the ring-closing metathesis reaction will “stitch” the dendritic wedges together intramolecularly, even at dendrimer concentrations of 10⁻³ M. In contrast, peripheral alkenes react intermolecularly at this concentration.

An ordered assembly of four building blocks (2–5) forms the basis of the synthesis of 1, which contains the core structure of TMC-95A and B—two potent proteasome inhibitors (see scheme; TIPS=triisopropylsilyl, Cbz=benzoxycarbonyl, Boc=tert-butoxycarbonyl).