Stereochimie d'heterocycles du groupe IVB—I : Syntheses stereoselectives d'alcoxy-1 et d'amino-1 sila-1 cyclobutanes

Stereoselective synthéses (60 to 90%) of some silacyclobutanes were performed by alcoholysis or aminolysis of 2 (or 3)-methyl-1-chloro-1-silacyclobutanes. Relative configurations of 2 (or 3)- methyl-1-alkoxy (or 1-amino)-1-silacyclobutanes thus prepared have been assigned from their NMR spectra. The mechanism of these stereoselective reactions, based on the rapid stereomutation of 1-chloro-1-silacyclobutanes, is discussed.     

Stereochimie d'heterocycles du groupe IVB—II : Syntheses stereoselectives d'hydrogeno-1 et d'alkyl-1 sila-1 cyclobutanes

The reaction between n-butyllithium and 1,2 (or 1,3)-dimethyl-1-t-butoxy-1-silacyclobutanes proceeds with retention of configuration at silicon, and yields 10/90 (or 80/20) Z/E mixture of 1,2 (or 1,3)-dimethyl-1-n-butyl-1-silacyclobutanes. Methylation of 2 (or 3)-methyl-1-t-butoxy-1-silacyclobutanes (Z/E = 15/85 or 65/35) by methylmagnesium iodide gives 15/85 (or 65/35) Z/E mixture of 1,2 (or 1,3)-dimethyl-1-silacyclobutanes. The proposed configurations for substituted silacyclobutanes are supported by spectroscopic data (RMN) and chemical results (correlation of configuration and stereomutation of hydrogenosilacyclobutanes).     

Fast reproducible identification and large-scale databasing of individual functional cognitive networks

Background  

Although cognitive processes such as reading and calculation are associated with reproducible cerebral networks, inter-individual variability is considerable. Understanding the origins of this variability will require the elaboration of large multimodal databases compiling behavioral, anatomical, genetic and functional neuroimaging data over hundreds of subjects. With this goal in mind, we designed a simple and fast acquisition procedure based on a 5-minute functional magnetic resonance imaging (fMRI) sequence that can be run as easily and as systematically as an anatomical scan, and is therefore used in every subject undergoing fMRI in our laboratory. This protocol captures the cerebral bases of auditory and visual perception, motor actions, reading, language comprehension and mental calculation at an individual level.     

On curvature and feedback classification of two-dimensional optimal control systems

The goal of this paper is to extend the classical notion of Gaussian curvature of a two-dimensional Riemannian surface to two-dimensional optimal control systems with scalar input using Cartan’s moving frame method. This notion was already introduced by A. A. Agrachev and R. V. Gamkrelidze for more general control systems using a purely variational approach. Further, we will see that the “control” analogue of Gaussian curvature reflects similar intrinsic properties of the extremal flow. In particular, if the curvature is negative, arbitrarily long segments of extremals are locally optimal. Finally, we will define and characterize flat control systems. __________ Translated from Sovremennaya Matematika i Ee Prilozheniya (Contemporary Mathematics and Its Applications), Vol. 33, Suzdal Conference-2004, Part 1, 2005.     

On the curvature of two-dimensional optimal control systems and Zermelo’s navigation problem

The goal of this paper is to extend the notion of Gaussian curvature of two-dimensional surfaces to nonlinear time-optimal control systems in the plane by applying the moving frame method. This notion of curvature was introduced earlier by A. A. Agrachev and R. V. Gamkrelidze by means of Jacobi curves. Here we give a self-contained presentation of its two-dimensional version and apply the results to the well-known Zermelo navigation problem. __________ Translated from Sovremennaya Matematika i Ee Prilozheniya (Contemporary Mathematics and Its Applications), Vol. 21, Geometric Problems in Control Theory, 2004.     

Structure and dynamics of surfactant interfaces in organically modified clays

Organic modification of clays with surfactants is required for the preparation of polymer-clay nanocomposites for a variety of applications. We have studied the structure and dynamics of interfaces in synthetic clays modified with phosphonium surfactants. The chemical shifts, line widths, and relaxation times measured by 31P, 13C, and 1H NMR and the relaxation times measured by impedance spectroscopy allow us to monitor the dynamics over a wide range of time scales. The results show that the phosphonium headgroup is most restricted and that the mobility increases with increasing separation from the clay surface. The carbon chemical shifts show that the 16-carbon and 12-carbon surfactant tails of hexadecyltributyl phosphonium and dodecytriphenyl phosphonium are disordered at the interface and experience mobility over a range of time scales. The dynamics depend most strongly on the structure of the surfactant headgroup, and tributylphosphoniums are more mobile than the triphenylphosphoniums. Two dimensional chemical shift anisotropy spin exchange experiments show that the phosphorus atoms in the triphenylphosphonium surfactant are immobile on the clay surface on a 1 s time scale. The dynamics measured by impedance spectroscopy show a similar dependence on headgroup structure, even though the processes occur on very different time scales and length scales. The relationship between the structure and dynamics of the interface and the properties of composites are considered.     

Highly Reactive Nonheme Iron(III) Iodosylarene Complexes in Alkane Hydroxylation and Sulfoxidation Reactions

High‐spin iron(III) iodosylarene complexes bearing an N‐methylated cyclam ligand are synthesized and characterized using various spectroscopic methods. The nonheme high‐spin iron(III) iodosylarene intermediates are highly reactive oxidants capable of activating strong C? H bonds of alkanes; the reactivity of the iron(III) iodosylarene intermediates is much greater than that of the corresponding iron(IV) oxo complex. The electrophilic character of the iron(III) iodosylarene complexes is demonstrated in sulfoxidation reactions.