Characterizing individual scattering events by measuring the amplitude and phase of the electric field diffusing through a random medium

We describe observations of the amplitude and phase of an electric field diffusing through a three-dimensional random medium, using terahertz time-domain spectroscopy. These measurements are spatially resolved with a resolution smaller than the speckle spot size and temporally resolved with a resolution better than one optical cycle. By computing correlation functions between fields measured at different positions and with different temporal delays, it is possible to obtain information about individual scattering events experienced by the diffusing field. This represents a new method for characterizing a multiply scattered wave.     

Dynamic combinatorial libraries of metalloporphyrins: templated amplification of disulfide-linked oligomers

Disulfide-linked cyclic porphyrin oligomers from dimer to tetramer can be selected and amplified virtually quantitatively from a dynamic combinatorial library using bis-thiol substituted zinc(II) porphyrin units with appropriate amine donor templates.     

Lewis acid mediated cyclisations of silylated methylenecyclopropyl hydrazones

Silylated methylenecyclopropyl hydrazones on treatment with BF3 x Et2O cyclise to give heterocyclic products involving a novel sequence of hydride and silyl shifts via a series of increasingly stable cationic intermediates.     

The role of the counteranion in the cation-pi interaction

Chemical double mutant cycles have been used to quantify cation-pi interactions in chloroform as a function of the nature of the counteranion. The cation-pi interaction is -2.5 +/- 0.4 kJ mol(-1) and independent of the anion, even though the overall stability of the complexes varies by an order of magnitude due to competition of the anion for alternative binding sites.     

Rhodium-catalyzed enantioselective diboration of simple alkenes

Enantioselective catalytic reactions that operate directly on inexpensive unactivated alkenes are extraordinarily useful for the preparation of chiral organic building blocks and new materials. While a number of such processes have been developed, our ability to meet the intensifying demand for inexpensive stereochemically complex materials will require a significant expansion of practical catalytic asymmetric reaction methodology. In this regard, the rhodium-catalyzed enantioselective diboration reaction has been developed in order to address a number of extant problems in catalytic alkene transformation simultaneously. This process provides an enantiomerically enriched reactive dimetalated intermediate which can be converted to a variety of difunctional reaction products.     

Platinum-catalyzed tandem diboration/asymmetric allylboration: access to nonracemic functionalized 1,3-diols

[reaction: see text] A single-pot tandem catalytic diene diboration/carbonyl allylation reaction is described that uses a commercially available chiral diboron reagent. The chirality of the intermediate diboration adduct is transferred to the product in the carbonyl allylation reaction, thereby providing access to enantioenriched chiral products. Notably, the reaction allows for construction of a quaternary stereocenter and furnishes a synthetically versatile C-B bond in the reaction product.     

Addition,elimination, exchange,and epimerization in nitro sulfones

Three reactions were studied in the diastereomers of 1‐(benzenesulfonyl)‐2‐nitro‐1‐phenylpropane ( 1A and 1B ) and briefly in related compounds: elimination of the benzenesulfonyl group, epimerization of one diastereomer to the other, and deuterium/hydrogen exchange at the methine group next to nitro in starting material. The two diastereomers showed quite different reactivity. The high melting diasteromer showed rapid elimination and some exchange. The low melting diastereomer (at approximately a half‐life) showed extensive epimerization, and elimination to the alkene, but little exchange. There is little effect of aromatic substituents on reaction course. The situation is complicated by re‐addition of benzenesulfinate to the alkene. The addition reaction was similar to elimination in agreement with the Principle of Microscopic Reversibility expectations. An electron transfer mechanism for addition is calculated to be comparatively favorable. Copyright © 2007 John Wiley & Sons, Ltd.     

Use of cyclodextrin‐modified gold nanoparticles for enantioseparations of drugs and amino acids based on pseudostationary phase‐capillary electrochromatography

The application of chemical‐modified gold nanoparticles (GNPs) as chiral selector for the enantioseparation based on pseudostationary phase‐CEC (PSP‐CEC) is presented. GNPs modified by thiolated β‐CD were characterized by NMR and FT‐IR. The nanoparticle size was determined to be of 9.5 nm (+2.5 nm) by Transmission Electron Microscopy (TEM) and UV spectra. Four pairs of dinitrophenyl‐labeled amino acid enantiomers (DL‐Val, Leu, Glu and Asp) and three pairs of drug enantiomers (RS‐chlorpheniramine, zopiclone and carvedilol) were analyzed by using modified GNPs as the chiral selector in PSP‐CEC. Good theoretical plate number (up to 2.4×10⁵ per meter) and separation resolution (up to 4.7) were obtained even with low concentration of modified GNPs (0.8–1.4 mg/mL). The corresponding concentration of β‐CD in the buffer was only 0.30?0.53 mM, which was much lower than the optimum concentration of 15 mM if pure β‐CD was used as chiral selector. Our results showed that thiolated β‐CD modified GNPs have more sufficient interaction with the analytes, resulting in significant enhancement of enantioseparation. The study shed light on potential usage of chemical modified GNPs as chiral selector for enantioseparation based on PSP‐CEC.