key: cord-0041434-7cqft8vw
authors: nan
title: Graphical Abstract: Angew. Chem. Int. Ed. 34/2007
date: 2007-08-20
journal: Angew Chem Int Ed Engl
DOI: 10.1002/anie.200790168
sha: 57893ef3f322e74bbe6f42ba7007dcf8d60dc593
doc_id: 41434
cord_uid: 7cqft8vw

nan

Ziegler-Natta revival: Enantiomerically pure, isotactic oligo/polystyrenes have been prepared by titanium-based singlesite catalysis. The optical activity of these polystyrenes was shown to be measurable up to a degree of polymerization of 45, providing a discrete border for the appearance of "cryptochirality" for larger macromolecules. MAO = methylaluminoxane.

How apt these sensors are! Electronic aptamer-based sensors (aptasensors) hold great promise as analytical devices for the detection of low-molecular-weight substrates or proteins. The Minireview highlights the advantages of electronic transduction of aptasensor systems and discusses the latest developments in the design of electrochemical, field-effect transistor, and piezoelectric aptasensor systems. Darwinian chemistry was first practiced by Spiegelman and colleagues in 1967 when they carried out the Darwinian evolution of RNA molecules in a test tube. In the following 40 years, the directed evolution of RNA has become a highly fruitful area of research and has deepened our understanding of evolutionary processes at the molecular level.

Complexes under pressure! The complexes [{NiCl 2 (PN)} 4 ] (PN = 2-diphenylphosphinomethyl-2-oxazoline or -2-thiazoline; see picture; Ni green, Cl pink, P orange, S yellow, N blue, C gray) have an unprecedented, centrosymmetric Ni 4 Cl 8 core with 20-electron metal centers and undergo pressure-induced fragmentation into the mononuclear, squareplanar 16-electron complexes [NiCl 2 (PN)], which are in equilibrium with the respective tetramer in solution.

A slippery character: Cyclooligomerization of resorcinol with a highly fluorinated aldehyde afforded a "teflon-footed" resorcinarene 1 F , which is soluble in wet fluorous solvents through formation of a hexameric capsule. The supramolecular capsules in fluorous solvents exhibited more selective and/or enhanced properties as a result of fluorophobic effects on molecular encapsulation (such as encapsulating eight molecules of benzene, see picture). Spread them out: A hierarchically ordered material in which gold nanoparticles are positioned along carbon nanotubes with a spacing of 2.6 nm is described. The organization is achieved by self-assembly and relies on the functionality of a surfactant protein called HFBI (a hydrophobin).

Solvent or solute dissymmetry? The dissymmetric ordering of solvent molecules around the chiral solute (see picture) contributes to the chiroptical signature. Indeed, the solvent can dominate the chiroptical response, as shown for (S)-methyloxirane in benzene.

Gaze into the crystal: A crystallographic approach for determining and analyzing free energy not only provides a way of measuring binding energy, it also provides a detailed picture identifying, at the atomic level, which interactions are responsible for changes in binding affinity (picture: superposition of dipeptidecyclophilin 3 structures; W e-h are conserved water molecules e-h).

That's discrimination! An array of boronic acid appended bipyridinium salts (BBVs) as receptor units is able to distinguish twelve saccharides in aqueous solution and at neutral pH values by a fluorescentindicator displacement assay. The picture shows the fluorescence increase of a fluorescent dye with BBV receptors after adding saccharides (d-ribose (Rib), dglucose (Glc), d-fructose (Fru), melibiose (Mel), and lactulose (Lal)).

Make up and break up: A pH-responsive system that displays switchable adhesion in situ is described. Polyacid gels and polybases grafted to a substrate (brushes) strongly adhere in water, but in an acidic environment, the gel and brush dissociate. Adhesion can be controlled by environmental acidity and the process can be repeated several times. Such adhesion may be used in actuators, microfluidics, drug delivery, or personal-care products. Majority rules: "Chiral amplification" in asymmetric transformations can be realized if a helical supramolecular polymer, derived from the substrate (S), obeys the "majority rule" in determining its handedness. This concept works properly for the asymmetric transformation of a chirality-memorizing D 2 -symmetric saddleshaped porphyrin.

Finding its true identity: The complete stereostructure of the marine pentacyclic triterpene polyether (þ)-enshuol is shown. Asymmetric total synthesis confirmed the configuration predicted on the basis of NMR spectroscopic data of the previously synthesized natural products aurilol and glabrescol, substructures of which are present in enshuol, and disproved an earlier prediction based on biogenetic considerations.

Irradiation of well-designed crystalline ketones can result in the solvent-free generation of compounds having adjacent quaternary stereogenic centers, as illustrated for the enantiospecific synthesis of the natural products (þ)-and (À)-(a)-cuparenone (see picture).

High enantioselectivity was achieved in the Ru-catalyzed propargylation of furans and N,N-dimethylaniline derivatives to afford the propargylated aromatic compounds. This first asymmetric propargy-lation of aromatic compounds provides a novel protocol for the asymmetric Friedel-Crafts alkylation using propargylic alcohols as a new type of electrophile. Cp* = C 5 Me 5 Four's definitely a crowd: The chemoselective activation of ArÀH bonds in methylbenzenes by treatment with photogenerated phenyl cations allowed the synthesis of sterically crowded biphenyl compounds, including tetra-ortho-substi-tuted biphenyls, by intermolecular crosscoupling (see scheme). This method is an appealing metal-free alternative to widely used transition-metal catalysis. TFE = 2,2,2-trifluoroethanol.

Phasing up to complex problems: A thermodynamic approach based on solution data has been proposed for the determination of the stability of gas complexes and elucidation of the selectivity of gas binding. Stability constants, reaction enthalpies, and entropies for the complexation of gaseous guests (n-alkylamines) by solid macrocyclic hosts (bcyclodextrin, cucurbit[6]uril) were calculated by using the Born-Haber type cycle (see picture).

Blisteringly fast: The potent cytotoxic blistering agent pederin has been synthesized (see scheme). The synthesis is diastereoselective and concise (just 12 steps for the longest linear sequence), and features a formal hetero-Diels-Alder reaction of a hindered diene, a Mukaiyama-Michael reaction to set two additional stereocenters, and a Curtius rearrangement to stereospecifically introduce the aminal functionality.

Cheap and safe: An iron-catalyzed crosscoupling reaction between alkyl halides and alkenyl Grignard reagents is described. This CÀC bond coupling reaction is promoted by the cheap and nontoxic FeCl 3 and displays good tolerance against various functional groups.

Two ways about it: The first metal Nheterocyclic carbene complexes derived from a cyclic tetraimidazolium salt show a remarkable versatility of ligand conformation and coordination geometry. With Pd II , a mononuclear square-planar complex is obtained, but with Cu I and Ag I , an unprecedented dinuclear motif with a short metal-metal interaction is observed (see structure; N blue, C white ellipsoids, H white circles).

Fluorine makes the difference: FIBX (see structure), the tetrafluoro derivative of the hypervalent iodine reagent, is more soluble and has higher reactivity than its nonfluorinated counterpart. An efficient synthesis of FIBX and initial reactions are presented. Some of these reactions can be conducted in standard organic solvents.

Owing to the increased reactivity, new transformations and catalytic reactions may be possible.

A transition-metal-free radical phosphonation using Me 3 SnPPh 2 and the less toxic Me 3 SiPPh 2 is reported. These readily available reagents react highly efficiently with primary and secondary alkyl radicals. Moreover, aryl radicals and tertiary alkyl radicals are phosphonated with Me 3 SnPPh 2 (see scheme; R = aryl, alkyl, vinyl; X = I, Br, OC(S)imidazolyl). DFT calculations provide insights into the mechanism of the reaction.

Please, after you… b-Ketoesters in mixtures underwent Noyori reduction one by one at room temperature under 4 bar of hydrogen pressure in the presence of a catalyst formed from Ru II and (S)-binap (see example). The rate of the asymmetric hydrogenation and hence the selectivity for a particular b-ketoester was found to depend on the Lewis basicity of the ester group. Binap = 2,2'-bis(diphenylphosphanyl)-1,1'-binaphthyl.

Two products out of eight possible isomers are obtained from the [CuPF 6 -(MeCN) 4 ]-catalyzed, highly enantioselective, and regiodivergent nitroso Diels-Alder reactions of 6-substituted 1,3-cyclohexadienes (see scheme; R = phenyl, alkyl; Ar = 2-pyridyl). These divergent reactions of racemic cyclohexadienes deliver valuable compounds for the synthesis of biologically interesting carbasugars. In a first application peracetylated 2-epi-validamine was synthesized.

In a twist: 1,1'-Linked oligocyclopropanols have been obtained by an iterated reaction of a cyclopropaneboronate with in situ generated lithium bromocyclopropylidenoid and subsequent oxidation. As predicted by DFT computations for sexicyclopropane (shown in blue) in the gas phase, the quinquecyclopropane unit in the 3,5-dinitrobenzoate of quinquecyclopropanol (yellow, dinitrobenzoyl group omitted for clarity) adopts a helical conformation in the crystal.

Cluster on cluster: [Ga 12 Ga 12 -(Br 18 Se 2 )]·12 THF units with a platonic polyhedral substructure are ordered through the crystal in straight lines by SeÀ Se contacts in an arrangement resembling superatoms (see picture). According to topological, spectroscopic, and energetic findings, these chains of clusters can be interpreted as a model for the lattice structure of photoconducting GaSe.

Discrimination of b-Ketoesters by Ruthenium(II)-Binap-Catalyzed

Divergent Reactions for Racemates: Catalytic, Enantioselective, and Regiodivergent Nitroso Diels-Alder Reactions

Ga 24 Br 18 Se 2 : A Highly Symmetrical Metalloid Cluster and Its One-Dimensional Arrangement in the Crystalline State as a Model for the Photoconductivity of Solid GaSe

A video clip is available as Supporting Information on the WWW (see article for access details)

Any old iron: Readily available and nontoxic FeCl 2 effectively catalyzes C À C bond formation by oxidative activation of benzylic CÀH bonds in the presence of tert-butyl peroxide as a stoichiometric oxidant (see scheme). The mild reaction conditions, good yields, low catalyst cost, and easy operation are the major advantages of this cross-dehydrogenation coupling reaction.On the right path: A chlorination-alkylation sequence involving the reaction of preformed mixed amido/chloro complexes (see structure) with trimethylaluminum causes precipitation of [{Ln a Al b Me c Cl d } n ], which for Ln = Nd polymerizes isoprene in greater than 99 % cis stereospecificity (M n /M w = 1.76); the catalyst performance is comparable to that observed following a "detour" alkylation-chlorination sequence employing [Nd(AlMe 4 ) 3 ]/Et 2 AlCl.Feeling the pinch: Aryl bromides can be coupled with phenylboronic acid quantitatively within a few minutes by using pincer-type catalysts bearing aminophosphine substituents. [Pd(Cl) 2 P(NR 2 ) 3 ] has been used as a template for the pincer core directly on the metal center (see scheme, NR 2 = piperidinyl, X = NH or O), which makes the independent synthesis and purification of the air-and moisturesensitive ligand systems unnecessary.Mn gets in the game: In the presence of a manganese catalyst and a stoichiometric amount of hydrosilane, aldehydes insert into C À H bonds of aromatic rings of compounds with directing groups (see scheme). This first example of a manga-nese-catalyzed chemical transformation through C À H bond activation gives silyl ethers in good to excellent yields and can also be applied to asymmetric transformation.

A. GuØrinot, S. Reymond, J. Cossy* 6521 -6524