Triphenylphosphine dibromide: a simple one-pot esterification reagent

We report a one-pot, expedient protocol for the conversion of carboxylic acids to their esters using excess triphenylphosphine dibromide, base, and the alcohol. The reaction gave the esterified product in moderate-to-high yields (30-95%). For chiral acids, the reaction proceeded with little or no racemization. Use of a chiral alcohol in this transformation gave the ester with retention of configuration of the stereogenic center. Information is presented indicating that esterification proceeds through the intermediate generation of an acyloxyalkoxyphosphorane and where steric interactions play an important role in the energetics of the reaction.     

Deposition of Polyacrylic Acid Films by Means of an Atmospheric Pressure Dielectric Barrier Discharge

The present work describes the plasma polymerisation of acrylic acid at atmospheric pressure. The influence of two operating parameters (monomer concentration and discharge power) on the properties of the deposited films is investigated. Results show that at a monomer concentration of 2.5 ppm and a discharge power of 9.5 W, the monomer is only slightly fragmented leading to a high amount of carboxylic acid groups on the deposited films. In contrast, when monomer concentration is decreased or discharge power increased, the incidence of monomer fragmentation processes is higher, leading to a lower amount of carboxylic acid groups on the films. This behaviour can be explained by a higher energy amount available per monomer molecule at low monomer concentrations and high discharge powers and a higher flux of positive ions attacking the surface at high discharge powers. Taking into account these results, it can be concluded that the deposition parameters should be carefully selected in order to preserve the stability of the monomer and thus obtain coatings with high carboxylic acid densities.     

Laser-cluster interaction: x-ray production by short laser pulses

We investigate the heating of the quasifree electrons in large rare-gas clusters (N exceeding 10(5) atoms) by short laser pulses at moderate intensities (I approximately = 10(15) W cm(-2)). We identify elastic large-angle backscattering of electrons at ionic cores in the presence of a laser field as an efficient heating mechanism. Its efficiency as well as the effect of collective electron motion, electron-impact ionization, and cluster charging are studied employing a mean-field classical transport simulation. Results for the absolute x-ray yields are in surprisingly good quantitative agreement with recent experimental results.     

Piecewise affine models of chaotic attractors: the Rossler and Lorenz systems

This paper proposes a procedure by which it is possible to synthesize Rossler [Phys. Lett. A 57, 397-398 (1976)] and Lorenz [J. Atmos. Sci. 20, 130-141 (1963)] dynamics by means of only two affine linear systems and an abrupt switching law. Comparison of different (valid) switching laws suggests that parameters of such a law behave as codimension one bifurcation parameters that can be changed to produce various dynamical regimes equivalent to those observed with the original systems. Topological analysis is used to characterize the resulting attractors and to compare them with the original attractors. The paper provides guidelines that are helpful to synthesize other chaotic dynamics by means of switching affine linear systems.     

Late‐Stage 18F‐Difluoromethyl Labeling of N‐Heteroaromatics with High Molar Activity for PET Imaging

Despite a growing interest in CHF₂ in medicinal chemistry, there is a lack of efficient methods for the insertion of CHF¹⁸F into druglike compounds. Herein described is a photoredox flow reaction for ¹⁸F‐difluoromethylation of N‐heteroaromatics that are widely used in medicinal chemistry. Following the two‐step synthesis for a new ¹⁸F‐difluoromethylation reagent, the photoredox reaction is completed within two minutes and proceeds by C?H activation, circumventing the need for pre‐functionalization of the substrate. The method is operationally simple and affords straightforward access to radiolabeled N‐heteroaromatics with high molar activity suitable for biological in vivo studies and clinical application.     

Supported Catalytically Active Supramolecular Hydrogels for Continuous Flow Chemistry

Inspired by biology, one current goal in supramolecular chemistry is to control the emergence of new functionalities arising from the self‐assembly of molecules. In particular, some peptides can self‐assemble and generate exceptionally catalytically active fibrous networks able to underpin hydrogels. Unfortunately, the mechanical fragility of these materials is incompatible with process developments, relaying this exciting field to academic curiosity. Here, we show that this drawback can be circumvented by enzyme‐assisted self‐assembly of peptides initiated at the walls of a supporting porous material. We applied this strategy to grow an esterase‐like catalytically active supramolecular hydrogel (CASH) in an open‐cell polymer foam, filling the whole interior space. Our supported CASH material is highly efficient towards inactivated esters and enables the kinetic resolution of racemates. This hybrid material is robust enough to be used in continuous flow reactors, and is reusable and stable over months.     

Chemospecific Cyclizations of α‐Carbonyl Sulfoxonium Ylides on Aryls and Heteroaryls

The functionalization of aryl and heteroaryls using α‐carbonyl sulfoxonium ylides without the help of a directing group has remained so far a neglected area, despite the advantageous safety profile of sulfoxonium ylides. Described herein are the cyclizations of α‐carbonyl sulfoxonium ylides onto benzenes, benzofurans and N‐p‐toluenesulfonyl indoles in the presence of a base in HFIP, whereas pyrroles and N‐methyl indoles undergo cyclization in the presence of an iridium catalyst. Significantly, these two sets of conditions are chemospecific for each groups of substrates.     

The Truncated Burr X-G Family of Distributions: Properties and Applications to Actuarial and Financial Data

In this article, the “truncated-composed” scheme was applied to the Burr X distribution to motivate a new family of univariate continuous-type distributions, called the truncated Burr X generated family. It is mathematically simple and provides more modeling freedom for any parental distribution. Additional functionality is conferred on the probability density and hazard rate functions, improving their peak, asymmetry, tail, and flatness levels. These characteristics are represented analytically and graphically with three special distributions of the family derived from the exponential, Rayleigh, and Lindley distributions. Subsequently, we conducted asymptotic, first-order stochastic dominance, series expansion, Tsallis entropy, and moment studies. Useful risk measures were also investigated. The remainder of the study was devoted to the statistical use of the associated models. In particular, we developed an adapted maximum likelihood methodology aiming to efficiently estimate the model parameters. The special distribution extending the exponential distribution was applied as a statistical model to fit two sets of actuarial and financial data. It performed better than a wide variety of selected competing non-nested models. Numerical applications for risk measures are also given.