CARSA,a computer-assisted system for synthesis research in industry

Computer applications in chemical industry are outlined and the CARSA system (computer-assisted research in synthesis and application) is described. This modular system consists of the three main parts: WIFODATA (data base; substance and finding documentation), QSAR (programs for quantitative structure/activity relationships) and RDSS (reaction design by synthon substitution; program package for planning of syntheses). The vital RDSS component is discussed in more detail; the underlying philosophy is explained, and examples are given from practical work. Some future steps towards a real molecular-design system are outlined.     

Mechanisms for the solvolytic decompositions of nucleoside analogues—I: Acidic hydrolysis of 2-substituted 1-(1-ethoxyethyl)benzimidazoles

A few 2-substituted 1-(1-ethoxyethyl)benzimidazoles have been prepared and the rate constants for their hydrolysis measured at various temperatures and oxonium ion concentrations. The formal kinetics followed and the effect of varying the 2-substitution on the hydrolysis rate suggest that the acid-catalyzed cleavage of these compounds involves a rapid initial protonation of the benzimidazole ring and a subsequent rate-limiting heterolysis of the protonated substrate to form a free nitrogen base and an ocarbenium ion derived from the ethoxyethyl group. The values obtained for the entropy of activation are consistent with the assumed unimolecular nature of the rate-limiting step. The effects of 2-substituents on the acidities of the protonated substrates and their heterolysis rates have been compared. The latter partial reaction has been suggested to be the subject of steric acceleration.     

Ein Synergetischer Effekt bei Chlorierungen von Silicagel bzw. Metakaolin mit gasförmigen,Aluminiumchlorid enthaltenden Chlorierungsmitteln

A Synergetic Effect in Chlorination of Silicagel and Metakaolin with Aluminium Chloride Containing Chlorinating Agents The reactivity of silicagel with CCl₄, C₂Cl₆, C₂Cl₄, COCl₂, S₂Cl₂/Cl₂, and Cl₂/carbon is low up to 1000 K. Aluminium chloride under these conditions is a little more reactive above 600 K. But gaseous mixtures of aluminium chloride with CCl₄, C₂Cl₆, and S₂Cl₂/Cl₂ resp., show a synergetic effect: They attain conversion grades much higher than the sum of those obtained with each of the single components. The reactivity of aluminium chloride/CCl₄ mixtures with silicagel in dependence on temperature and partial pressure has been followed. Metakaolin is reacted by such a mixture synergetically and with increased silicate conversion. Practical consequences of this effect are demonstrated and essential phenomena involved in metakaolin chlorination are explained. A mechanistic model of the synergetic effect has been developed.     

Electrostatic lattice coefficients and binding energy of orthorhombic La2-x Sr x CuO4

Three valency models for orthorhombic La_2-x Sr _x CuO₄ were investigated for increasing Sr concentrationsx (0x0.21): 1. Cu²⁺Cu³⁺, 2. apex O^2–O^– and 3. in-plane O^2–O^–. All calculations were done by using structural parameters valid for the temperature range from 10 to 22 K. We thereby calculated the electrostatic interaction energy which, next to ionization potentials and electron affinities, comprises a major of the binding energyE _B of crystals. Second-order effects were accounted for by calculating the strength of ionic dipole moments induced by crystal electric fields at relevant lattice sites. Their largest strengths are comparable to the dipole moment of the water molecule. Three out of five dipoles in La_2-x Sr _x CuO₄ vanish during the transition from the orthorhombic to the tetragonal phase. The binding energy differences between the different models suggest that the system is in a state of model 1. However, the differences are very small, being in the order of 0.3 to 0.76 eV atx=0.13.     

On noise reduction methods for chaotic data

Recently proposed noise reduction methods for nonlinear chaotic time sequences with additive noise are analyzed and generalized. All these methods have in common that they work iteratively, and that in each step of the iteration the noise is suppressed by requiring locally linear relations among the delay coordinates, i.e., by moving the delay vectors towards some smooth manifold. The different methods can be compared unambiguously in the case of strictly hyperbolic systems corrupted by measurement noise of infinitesimally low level. It was found that all proposed methods converge in this ideal case, but not equally fast. Different problems arise if the system is not hyperbolic, and at higher noise levels. A new scheme which seems to avoid most of these problems is proposed and tested, and seems to give the best noise reduction so far. Moreover, large improvements are possible within the new scheme and the previous schemes if their parameters are not kept fixed during the iteration, and if corrections are included which take into account the curvature of the attracting manifold. Finally, the fact that comparison with simple low-pass filters tends to overestimate the relative achievements of these nonlinear noise reduction schemes is stressed, and it is suggested that they should be compared to Wiener-type filters.     

Thermische Beständigkeit von linearen Polyisoprenen unterschiedlicher Kettenlänge

Zusammenfassung An einer polymerhomologen Reihe von linearen Polyisoprenen mit Molekulargewichten zwischen 40000 und > 10⁷ wurde im Festzustand und in L?sung die Abh?ngigkeit der thermischen Stabilit?t von der jeweiligen Kettenl?nge untersucht. Es zeigte sich, da? die zum Zerbrechen der Makromoleküle notwendigen Temperaturen mit steigendem Molekulargewicht deutlich abnehmen und für Polyisoprene mit Molekulargewichten > 10⁷ bei Raumtemperatur liegen.

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Summary The dependence of thermal stability from the molecular weight of linear macromolecules was tested using polyisoprenes with m. w. between 40000 and > 10⁷. We could demonstrate that the temperatures at which molecular fractures occur, decrease with increasing chain length and approach room temperature at molecular weights of about 10⁷.

     

Chemistry of λ-2H-azaphosphirene metal complexes

In this review, important aspects of λ³-2H-azaphosphirene metal complexes are discussed in relation to synthesis, physical properties and synthetic applications; ab-initio calculations on relative energies of CH₂NP isomers and of λ³-2H-azaphosphirene metal complexes (Cr, Mo, W) are also presented. Currently, there are three routes to this unsaturated three-membered ring system, which are discussed in detail: two of them use a rearrangement of metal carbene complexes, whereas the other relies on [2+1] cycloaddition reactions of electrophilic terminal phosphanediyl complexes and carbonitriles. The structural data show characteristics of a very strained heterocyclic ring system, which partially helps to understand the reactivity of this heterocycle complex in solution. The synthetic potential of λ³-2H-azaphosphirene metal complexes is illustrated by selected examples, which demonstrate their ability to serve, under very mild conditions as precursor for various new building blocks, i.e. nitrilium phosphanylide complexes, electrophilic terminal phosphanediyl complexes and phosphavinyl-nitrene complexes.     

Two unusual, competitive mechanisms for (2-ethynylphenyl)triazene cyclization: pseudocoarctate versus pericyclic reactivity

The cyclization of (2-ethynylphenyl)triazenes in ODCB at 200 degrees C gives exclusively cinnolines, whereas addition of CuCl to 1,2-dichloroethane solutions of the triazenes at 50 degrees C results in the sole formation of isoindazoles. DFT calculations and deuterium labeling studies suggest the intermediacy of a 3-dehydrocinnolinium ion, produced through a pericyclic transition state. Calculations and trapping studies strongly implicate a carbene intermediate in isoindazole formation, which proceeds through a pseudocoarctate transition state.