Efficient synthetic approach to heterocycles possessing the 3,3-disubstituted-2,3-dihydrobenzofuran skeleton via diverse palladium-catalyzed tandem reactions

Various palladium-catalyzed cascade reactions of O-alkylated 2-iodophenol were explored in order to synthesize novel dihydrobenzofurans. An efficient tandem cyclization/anion capture reaction was developed to yield 3,3-disubstituted-2,3-dihydrobenzofurans. A small library of these compounds was prepared with a parallel organic synthesizer. A multi-component version of this reaction involving 2-iodophenol, an alkylating agent and a nucleophile, provided the same products. The methoxycarbonyl-substituted heterocyclic ring was hydrolyzed to a free acid, which could be used for further transformations.     

Specular reflectivity of 13.5-nm light from Sn islands deposited on grazing incidence mirror surfaces

We have investigated the effect of exposure to thermal Sn atoms on the 13.5-nm specular reflectivity of single-layer Pd mirror surfaces. The mirror surfaces were characterized using low-energy ion scattering spectroscopy (LEISS) and the relative 13.5-nm specular reflectivity measured in situ during exposure to doses up to several 10¹⁶ cm^-2 Sn particles. The relative reflectivity loss at 13.5 nm is found to deviate from the theoretical Fresnel reflectivity. Losses are < 20% for fluence levels > 10¹⁶ cm^-2. The difference is due to deposited Sn nucleating at early stages of exposure and coalescing to nanoscale islands. Sn atomic surface fraction from LEISS reaches 50 a/o (atomic percent) at a fluence of 0.25×10¹⁶ cm^-2 and ultimately levels off at 93–95% for fluences greater than 2.0×10¹⁶ cm^-2. The specular reflectivity approaches the Fresnel theoretical value of 38.7% for thermal Sn deposition at large doses and equilibrates to 36.4%. The difference is mainly due to surface roughness of the deposited Sn film. PACS 61.18.Bn; 79.20.Rf; 78.70.Ck; 78.67.Bf     

Analysis of the depth resolution limit of luminescence diffuse optical imaging

We introduce a methodology to determine quantitatively the depth resolution limit in luminescence diffuse optical imaging. The approach is based on a Cramer-Rao statistical analysis, a noise model, and calculations of photon transport in tissues. We illustrate the method in the case of luminescence imaging in a brain-skull model, showing its potential applications in molecular imaging on small animals.     

Stereoselective synthesis and conformational analysis of aromatic C‐thionucleosides

Tetrahydrothiophene derivatives 2a–2d , which are useful intermediates in the synthesis of C‐thionucleosides, were obtained by a tandem strategy that involves the base‐catalyzed conjugate addition of ethyl 2‐mercaptoacetate to trans‐cinnamaldehyde followed by cyclization. The solvent and the nature of the amine used as Lewis base influence the stereo‐ selectivity of the reaction. The reduction of each isolated derivative 2a–2d was performed with LiAlH₄ to afford C‐thionucleosides 3a–3d . The configuration and conformation of each diastereomer 2a–2d and 3a–3d was assigned by means of the analysis of vicinal proton–proton coupling constants. The X‐ray structure of 2d, 3a , and 3b confirmed the configuration assigned to each isomeric tetrahydrothiophenes. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:289–298, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20205     

Echo tracer dispersion in flows of polymer solutions through porous media: A tool for detecting weak permeability heterogeneities?

Tracer dispersion in Newtonian and shear-thinning fluids (scleroglucan-water polymer solutions) flowing through single and double porosity grain packings has been studied experimentally using both classical transmission dispersion and echo dispersion (in the latter, the concentration variation front is pumped back through a detector at the inlet after penetrating for a chosen distance into the sample). Transmission dispersion increases markedly in both types of samples with the shear thinning index of the fluid at all Péclet numbers (except when molecular diffusion is dominant). Echo and transmission experiments give nearly identical dispersivity values for Newtonian fluids while echo dispersivity is lower than transmission for shear thinning ones. The normalized dispersivity difference has same order of magnitude for single and double porosity samples and increases with the shear thinning exponent α (by a factor of 2 between α = 0.35 and α = 0.60). This difference may be due to heterogeneities inducing permeability variations of small amplitude over distances of the order of the sample section : their influence on tracer dispersion is partly reversible with respect to a change of the flow direction and is only detectable if it is amplified by the shear-thinning properties of the fluid. Received 19 September 2001     

Design of extended surface-supported chiral metal-organic arrays comprising mononuclear iron centers

A design strategy for fabricating a surface-supported chiral metal-organic system comprising a regular arrangement of mononuclear iron centers and nanocavities is presented. By sequential deposition of 1,2,4-benzenetricarboxylic acid (tmla) molecules and iron atoms on a Cu(100) surface under ultrahigh vacuum conditions, chiral square-planar Fe(tmla)4 metal-organic complexes are generated, which order in extended homochiral arrays. Structure formation and envisioned functionality of such metal-organic architectures are discussed.     

Structural properties of a calcium aluminosilicate glass from molecular-dynamics simulations: a finite size effects study

We study a calcium aluminosilicate glass of composition (SiO(2))(0.67)-(Al(2)O(3))(0.12)-(CaO)(0.21) by means of molecular-dynamics simulations, using a potential made of two-body and three-body interactions. In order to prepare small samples that can subsequently be studied by first principles, the finite size effects on the liquid dynamics and on the glass structural properties are investigated. We find that finite size effects affect the Si-O-Si and Si-O-Al angular distributions, the first peaks of the Si-O, Al-O, and Ca-O pair correlation functions, the Ca coordination, and the oxygen atoms' environment in the smallest system (100 atoms). We give evidence that these finite size effects can be directly attributed to the use of three-body interactions.     

Crown-annelated oligothiophenes as model compounds for molecular actuation

Crown-annelated quater- (4T) and sexithiophenes (6T) with oligooxyethylene chains of various lengths attached at the 3-positions of the terminal thiophene rings have been synthesized. Analysis of the cation-binding properties of the macrocycles by (1)H NMR and UV-vis spectroscopy reveals the formation of a 1:1 complex with Ba(2+), Sr(2+), or Pb(2+) and shows that cation complexation results in a conformational transition in the pi-conjugated system. Theoretical analysis of this process by density functional methods predicts that this conformational transition results in a narrowing of the highest occupied-lowest unoccupied molecular orbital gap of the conjugated system with a decrease of the redox potentials (E(0)(1) and E(0)(2)) associated with the formation of the 4T cation radical and dication. Cyclic voltammetry shows that, depending on the binding constant, the presence of metal cation produces a negative or a positive shift of E(0)(1) while E(0)(2) always shifts negatively. This unusual behavior is discussed in terms of interplay between electrostatic interactions and conformational changes associated with cation binding.     

Tracer dispersion in power law fluids flow through porous media: Evidence of a cross-over from a logarithmic to a power law behaviour

An analytical model is presented to describe the dispersion of tracers in a power-law fluid flowing through a statistically homogeneous and isotropic porous medium. The model is an extension of Saffman's approach to the case of non-Newtonian fluids. It is shown that an effective viscosity depending on the pressure gradient and on the characteristics of the fluid, must be introduced to satisfy Darcy's law. An analytical expression of the longitudinal dispersivity is given as a function of the Peclet number Pe and of the power-law index n that characterizes the dependence of the viscosity on the shear rate . As the flow velocity increases the dispersivity obeys an asymptotic power law: . This asymptotic regime is achieved at moderate Peclet numbers with strongly non-Newtonian fluids and on the contrary at very large values when n goes to 1 ( for n=0.9). This reflects the cross-over from a scaling behaviour for towards a logarithmic behaviour predicted for Newtonian fluids (n=1). Received: 22 July 1997 / Revised and Accepted: 2 July 1998