High facial diastereoselectivity in intra- and intermolecular reactions of chiral benzylic cations

Chiral carbenium ions can be attacked by arene nucleophiles with high facial diastereoselectivity (dr >/= 94/6). Benzylic cations, such as 2, were generated under acidic conditions and reacted with arenes in intra- and intermolecular Friedel-Crafts alkylation reaction. The depicted reaction 1 --> 3 represents one example for the unprecedented, highly diastereoselective intermolecular Friedel-Crafts alkylation reactions which were observed in this study.     

Intramolecular metal—double bond interactions : VII. Intramolecular cyclization of alkenyl derivatives of lithium,aluminum, gallium and indium

The intramolecular cyclization reactions of tri-1-hex-5-enyl Group III derivatives and of 1-hex-5-enyllithium have been investigated and it is proposed that the reactions may be depicted as internal addition across the terminal double bonds of the alkenyl species. The cyclization reaction proceeds more readily for the 1-hex-5-enyl derivatives than for any other chain length and is dependent on temperature, concentration and solvent. ¹H NMR data and cryoscopic molecular weight data are presented for several diisobutyialkenyl- and diisobutylalkyl-aluminum derivatives. These data have been interpreted in terms of a direct intramolecular interaction between the metal center and the π-electron system of the olefinic site. The intramolecular cyclization reactions for the 1-hept-6-enyl, 1-oct-7-enyl- and 1-undec-10-enylaluminum derivatives also were investigated and it was found that cyclization occurred only for the 1-hept-6-enyl derivative.     

Concise total syntheses of the bioactive mesotricyclic diterpenoids jatrophatrione and citlalitrione

The highly functionalized [5.9.5] tricyclic framework resident in jatrophatrione (1) and citlalitrione (2) has been synthesized. The route begins with the tandem anionic oxy-Cope rearrangement/methylation/transannular ene cyclization of 21 and subsequent introduction of a conjugated enone double bond. Hydroxyl-directed 1,4-reduction of this functionality in 25 with LiAlH(4)/CuI/hexamethylphosphoramide/tetrahydrofuran sets the stage for the implementation of a Grob fragmentation and expedited generation of 27. Stereocontrolled intramolecular hydrosilylation allows for the subsequent introduction of a cyclic carbonate as in 53. This intermediate undergoes remarkably efficient, fully regiocontrolled Treibs reaction to generate 54, with this maneuver serving as a pivotal step for making 1 available five steps later. Treatment of 1 with m-chloroperbenzoic acid leads to 2, with attack occurring preferentially on a alpha-face of the double bond more remote to the carbonyl.     

A new class of C1-symmetric monosulfoximine ligands for enantioselective hetero Diels-Alder reactions

Quinoline-based C1-symmetric sulfoximines have been used as chiral ligands in copper-catalyzed asymmetric hetero Diels-Alder reactions leading to cycloadducts with up to 96% ee.     

Influence of initial oxygen coverage and magnetic moment on the NO decomposition on rhodium (111)

In this study, density functional theory calculations were performed to investigate the influence of oxygen preoccupation on the nitrogen oxide decomposition on rhodium. Besides gauging the coverage dependence of the adsorption energy of NO on the (111) rhodium facet, the influence of the initial oxygen coverage on the kinetics and thermodynamics of the nitrogen oxide decomposition reaction was also studied. The results are discussed with respect to a novel NOx decomposition catalyst. Furthermore, the influence of spin effects on the adsorption geometry as well as the adsorption energy is examined. It will be addressed why spin effects only have a minor influence on the behavior of NO on a rhodium surface.     

Polynuclear manganese grids and clusters—A magnetic perspective

High nuclearity paramagnetic, spin-coupled transition metal clusters and grids are fascinating chemists and physicists partly because of their structural beauty, and the challenge of creating them, but also because of their novel physical properties. Magnetic interactions between the spin centers are a primary focus. This review will examine a selection of Mn(II) polynuclear grids and clusters, with nuclearities in the range Mn₄ to Mn₉. Theoretical treatments of the magnetic properties are discussed, and approaches to solving the exchange problem for ‘large’ spin systems related to computational difficulties. A freely available software package (MAGMUN4.1) is presented as a means of dealing simply with spin-coupled clusters in general, and symmetry reduction methods are discussed briefly as a means of dealing with ‘large’ spin systems.     

Charge-transfer phenomena in novel,dual-component,sugar-based organogels

The synthesis of a new saccharide-based gelator (2) containing a donor moiety has been described. Gelation experiments of a dual-component gel consisting of a saccharide-based gelator bearing an acceptor group (1) and of 2 have been performed in a variety of organic solvents and water. Moreover, gelation tests at different molar ratios of 1 and 2 have been performed in water, octanol, and diphenyl ether. In these last two solvents a gel color change was observed, from colorless to yellow, upon cooling of the sample to room temperature. This phenomenon was further investigated by UV-visible spectroscopy, which revealed the presence of charge-transfer interactions in the gel, in octanol. Temperature-dependence UV spectroscopy confirmed that such interactions occur in the gel but not in the corresponding solution sample. Furthermore, T(gel) measurements show that dual-component gels of 1 and 2 present increased thermal stability at a 50:50 ratio of the two gelators, in dependence of the solvent. Transmission electron microscopy (TEM) images of the single-component gels in diphenyl ether revealed that they consist of a fibrous network, while the dual-component gel presents a novel, helical, fibrous-bundle structure.     

Optimized intermolecular potential for aromatic hydrocarbons based on anisotropic united atoms. III. Polyaromatic and naphthenoaromatic hydrocarbons

In this third article of the series, a new anisotropic united atoms (AUA) intermolecular potential parameter set has been proposed for the carbon force centers connecting the aromatic rings of polyaromatic hydrocarbons to predict thermodynamic properties using both the Gibbs ensemble and NPT Monte Carlo simulations. The model uses the same parameters as previous AUA models used for the aromatic CH force centers. The optimization procedure is based on the minimization of a dimensionless error criterion incorporating various thermodynamic data of naphthalene at 400 and 550 K. The new model has been evaluated on a series of polyaromatic and naphthenoaromatic hydrocarbons over a wide range of temperatures up to near-critical conditions. Vaporization enthalpy, liquid density, and normal boiling temperature are reproduced with good accuracy. The new potential parameters have also been tested successfully on toluene, 1,3,5-trimethylbenzene, styrene, m-xylene, n-hexylbenzene, and n-dodecylbenzene to demonstrate their transferability to alkylbenzenes.     

Coloring linear hypergraphs: the Erdős–Faber–Lovász conjecture and the Combinatorial Nullstellensatz

Designs, Codes and Cryptography - The long-standing Erdős–Faber–Lovász conjecture states that every n-uniform linear hypergaph with n edges has a proper vertex-coloring using...