Allenylzincs and tert-butylsulfinylimines: a fruitful marriage for synthesis

The stereoselective synthesis of alkynyl 1,2-amino alcohols by the addition of 3-chloro- and 3-methoxymethoxy- allenylzincs to chiral tert-butylsulfinylimines is described. The methodology is applicable to the preparation of alkynyl 2-amino-1,3-diols (O,N,O stereotriads) using α-alkoxy tert-butylsulfinylimines as chiral starting materials. The scope and limitations of the methodology along with recent applications to the efficient asymmetric syntheses of natural and/or bioactive alkaloids and polyhydroxylated alkaloids are presented.     

Skew-product decompositions of Brownian motions on manifolds: A probabilistic aspect of the Lichnerowicz-Szabo theorem

We show that the only compact simply connected manifolds for which the radial part of Brownian motion enjoys the Markov property are compact two points homogeneous spaces, i.e. rank one symmetric spaces.     

Test d'indépendance nonparametrique

We consider the minimax problem of testing the independence of the components of a d-dimensional random vector against a set of alternatives defined by L₂-norm. We are interested in finding the minimax rate of testing and a test that attains this rate. The bound of the error of the first kind is a positive sequence which can decrease to zero as the number of observations increases. To cite this article: A.F. Yode, C. R. Acad. Sci. Paris, Ser. I 336 (2003).     

A Bistable Microelectromechanical System Actuated by Spin-Crossover Molecules

We report on a bistable MEMS device actuated by spin-crossover molecules. The device consists of a freestanding silicon microcantilever with an integrated piezoresistive detection system, which was coated with a 140 nm thick film of the [Fe(HB(tz)₃)₂] (tz=1,2,4-triazol-1-yl) molecular spin-crossover complex. Switching from the low-spin to the high-spin state of the ferrous ions at 338 K led to a reversible upward bending of the cantilever in agreement with the change in the lattice parameters of the complex. The strong mechanical coupling was also evidenced by the decrease of approximately 66 Hz in the resonance frequency in the high-spin state as well as by the drop in the quality factor around the spin transition.     

Evaluation of temperature gradients within combusting droplets in linear stream using two colors laser-induced fluorescence

The scope of this paper concerns the heating process of fuel droplets injected in a hot gaseous environment. The two colors laser-induced fluorescence technique allows measuring the temperature distribution within a droplet by scanning the droplet volume by a sufficiently small probe volume compared to the droplet volume itself. The temperature field is reconstructed using two approaches which have been compared. One is based on a geometrical optics model and the other is based on the 3D calculation of the internal excitation field within the droplet, using the generalized Lorenz-Mie theory. Experimental results have been obtained on a combusting monodisperse ethanol droplet stream (diameter around 200 m).     

Efficiency of random search procedures along the silicon cluster series: Si(n) (n=5-10, 15, and 20)

The efficiency of the simplest isomeric search procedure consisting in random generation of sets of atomic coordinates followed by density functional theory geometry optimization is tested on the silicon cluster series (Si(5-10, 15, 20)). Criteria such as yield, isomer distributions and recurrences are used to clearly establish the performance of the approach with respect to increasing cluster size. The elimination of unphysical candidate structures and the use of distinct box shapes and theoretical levels are also investigated. For the smaller Si(n) (n=5-10) clusters, the generation of random coordinates within a spherical box is found to offer a reasonable alternative to more complex algorithms by allowing straightforward identification of every known low-lying local minima. The simple stochastic search of larger clusters (i.e. Si(15) and Si(20)) is however complicated by the exponentially increasing number of both low- and high-lying minima leading to rather arbitrary and non-comprehensive results.     

Self-templating polythiophene derivatives: electronic decoupling of conjugated strands through staggered packing

Whereas molecular electronics needs well-controlled 3D geometries for decoupling or interconnecting individual molecules, conjugated polymers form disordered structures when deposited on a substrate. We show that this trend can be overcome in polythiophene derivatives designed so as to exploit weak sulfur-bromine interactions. A self-template effect follows, leading to staggered organizations of well-aligned electronically decoupled conjugated strands, as observed in situ by scanning tunneling microscopy and spectroscopy on graphite.     

Regioselective synthesis of novel substituted indazole-5,6-diamine derivatives

The synthesis of a series of novel indazole-5,6-diamine derivatives is described. This indazole ring system was incorporated in an octahydropyrrolo[3,4-b]phenazine scaffold and was diversely and regioselectively substituted on the nitrogen atoms at the 5- and 10-positions. Thus, the nitrogen atom at the 5-position was found to be more reactive toward electrophiles than the one at the 10-position. This difference of reactivity could be attributed to the electronic effect of the pyrazole moiety. Moreover, an unexpected tetrahydropyran protecting group migration was observed from the N-1 atom to the C-11 position of the octahydropyrrolo[3,4-b]phenazine scaffold.     

How linker's modification controls swelling properties of highly flexible iron(III) dicarboxylates MIL-88

A series of organically modified iron(III) terephthalate MIL-88B and iron(III) 4,4'-biphenyl dicarboxylate MIL-88D flexible solids have been synthesized and characterized through a combination of X-ray diffraction, IR spectroscopy, and thermal analysis (MIL stands for Material from Institut Lavoisier). The swelling amplitude of the highly flexible MOFs tuned by introducing functional groups onto the phenyl rings shows a clear dependence on the steric hindrance and on the number of groups per aromatic ring. For instance, while the introduction of four methyl groups per spacer in dried MIL-88B results in a large permanent porosity, introducing two or four methyl groups in MIL-88D allows an easier pore opening in the presence of liquids without drastically decreasing the swelling magnitude. The influence of the degree of saturation of the metal center and the nature of the solvent on the swelling is also discussed. Finally, a computationally assisted structure determination has led to a proposal of plausible structures for the closed (dried) and open forms of modified MIL-88B and MIL-88D and to evaluation of their framework energies subject to the nature of the functional groups.