X-ray crystallographic and NMR structural studies of trans-2,3-dichloro-5-ethyl-2,3-dihydrothieno[2 3-b]pyridine syn-1-oxide reactions of thiophene rings with hypochlorite reagents

X-ray analysis of a crystalline product obtained by treatment of 5-ethylthieno[2,3-b]pyridine with excess acidified hypochlorite establishes its stereochemistry as trans-2,3-dichloro-5-ethyl-2,3-dihydrothieno-[2,3-b]pyridine syn-1-oxide (5), wherein the pyridine ring is planar and the dihydrothiophene ring is non-planar with a C2-S-C7a angle of 86.6°. The trans geometry is corroborated by a proton-proton coupling constant J_2,3 of 6.8 Hz. Comparison of ¹H and ¹³C nmr data for 5 with analogous crystalline 2,3-dichloro-1-oxide addenda isolated in the isosteric benzo[b]thiophene and thieno[2,3-b]pyridine parent systems indicates that some proposed stereochemical assignments are questionable.     

Addition of Vindoline to p-Benzoquinone: Regiochemistry,Stereochemistry and Symmetry Considerations

Vindoline and catharanthine are the major alkaloids of Catharanthus roseus and are extracted in large quantities to prepare the pharmaceutically important Vinca type alkaloids vincaleukoblastine, vincristine and navelbine. The higher yield of vindoline relative to catharanthine makes it an attractive substrate for developing new chemistry and adding value to the plant. In this context, we have reacted vindoline with a selection of electrophiles among which benzoquinone. Conditions were developed to optimize the synthesis of a mono-adduct, of five bis-adducts, and of tri-adducts and tetra-adducts, several of these adducts being mixtures of conformational isomers. Copper(II) was added to the reactions to promote reoxidation of the intermediate hydroquinones and simplify the reaction products. The structures were solved by spectroscopic means and by symmetry considerations. Among the bis-isomers, the 2,3-diadduct consists of three unseparable species, two major ones with an axis of symmetry, thus giving a single set of signals and existing as two different species with indistinguishable NMR spectra. The third and minor isomer has no symmetry and therefore exhibits nonequivalence in the signals of the two vindoline moieties. These isomers are designated as syn (minor) and anti (major) and there exists a high energy barrier between them making their interconversion difficult. DFT calculations on simplified model compounds demonstrate that the syn-anti interconversion is not possible at room temperature on the NMR chemical shift time scale. These molecules are not rigid and calculations showed a back-and-forth conrotatory motion of the two vindolines. This “windshield wiper” effect is responsible for the observation of exchange correlations in the NOESY spectra. The same phenomenon is observed with the higher molecular weight adducts, which are also mixtures of rotational isomers. The same lack of rotations between syn and anti isomers is responsible for the formation of four tri-adducts and of seven tetra-adducts. On a biological standpoint, the mono adduct displayed anti-inflammatory properties at the 5 μM level while the di-adducts and tri-adducts showed moderate cytotoxicity against Au565, and HeLa cancer cell lines.     

Statistical inference for a Wiener-based degradation model with imperfect maintenance actions under different observation schemes

This paper studies the statistical inference in a degradation model with imperfect maintenance. Technological or industrial devices subject to degradation undergo maintenance actions that reduce their degradation level. The underlying degradation process is a Wiener process with drift. Maintenance effects are assumed to be imperfect, described by an Arithmetic Reduction of Degradation ($AR{D}_1$) model. The system is regularly inspected and the degradation levels are measured. Four different observation schemes are considered so that degradation levels can be observed between maintenance actions as well as just before or just after maintenance times. The paper studies the estimation of the model parameters under the four observation schemes. Maximum likelihood estimators are derived for each scheme. The quality of the estimations is assessed and the observation schemes are compared through an extensive simulation and performance study.     

Ab initio quantum-mechanical calculations of the propagation enthalpies for the radical and anionic polymerizations of formaldehyde and methanimine

The gas phase enthalpies of formation for oligomeric radicals and anions H(CH₂NH)_n^* and H(CH₂O)_n^* were theoretically determined by ab initio quantum-mechanical calculations with n in the range 1 to 6. From these results, the reaction enthalpies for each of the first five propagation steps of the polymerization were estimated for methanimine (H₂C = NH) and formaldehyde (H₂C = O). At the same step of oligomerization, enthalpies associated with anionic polymerizations are always more negative than enthalpies corresponding to radical polymerizations, but the difference between them decreases with increasing n. Both Delta;H (propagation) vs. n curves tend rapidly, particularly for radical polymerizations, towards an asymptotic value independent of the mode of polymerization and equal to - 12 kcal/mol for formaldehyde and - 14 kcal/mol for methanimine. Experimental data for the gas phase polymerization of formaldehyde are in good agreement with our theoretical value. These results demonstrate that heats of polymerization can be reasonably estimated by intensive calculation methods if a careful choice of the reaction mimicking the propagation step is done.     

Water-soluble Self-assembled {Pd84}Ac Polyoxopalladate Nano-wheel as a Supramolecular Host

Polyoxopalladates (POPs) are a class of self-assembling palladium-oxide clusters that span a variety of sizes, shapes and compositions. The largest of this family, {Pd₈₄}^Ac, is constructed from 14 building units of {Pd₆} and lined on the inner and outer torus by 28 acetate ligands. Due to its high water solubility, large hydrophobic cavity and distinct ¹H NMR fingerprint {Pd₈₄}^Ac is an ideal molecule for exploring supramolecular behaviour with small organic molecules in aqueous media. Molecular visualisation studies highlighted potential binding sites between {Pd₈₄}^Ac and these species. Nuclear Magnetic Resonance (NMR) techniques, including ¹H NMR, ¹H Diffusion Ordered Spectroscopy (DOSY) and Nuclear Overhauser Spectroscopy (NOESY), were employed to study the supramolecular chemistry of this system. Here, we provide conclusive evidence that {Pd₈₄}^Ac forms a 1 : 7 host-guest complex with benzyl viologen (BV²⁺) in aqueous solution.     

Invariant values of kinetic parameters — Evaluation of fire retardancy application to the PP-APP/PER system

A method allowing the determination of the formulation leading to the best fire retardancy properties is presented in the case of intumescent systems. The method consists in the determination of invariant physicochemical parameters deduced from TG analysis: invariant activation energy and invariant preexponentiel factor. The relation between the values given by an evaluation test (LOI) and the invariant activation energy is then discussed.