Showing papers in "ChemInform in 1980"

Ziegler-Natta Catalysis

Abstract: Publisher Summary This chapter discusses the scope of Ziegler catalysis; stereoselectivity, kinetics, and mechanism of Ziegler catalysis; homogeneous Ziegler–Natta catalysts; and side reactions in homogeneous catalysts. Ziegler catalysis involves rapid polymerization of ethylene and α-ole-fins with the aid of catalysts based on transition-element compounds, normally formed by the reaction of a transition-element halide or alkoxide or alkyl or aryl derivative with a main-group element alkyl or alkyl halide. There are thousands of patents involving every combination of pure or mixed main-group alkyls with transition-element compounds, each claiming advantages. The result of the early work led to the development of “second-generation” Ziegler catalysts. Polymers produced with unmodified Ziegler catalysts showed extremely high molecular weight and broad distribution, and in some cases, there was evidence for “living polymer.” All homogeneous catalyst systems for ethylene polymerization become heterogeneous when polyethylene is formed. On using vanadium-based homogeneous catalysts, polymers consisting of syndiotactic stereo blocks and stereo-irregular blocks are obtained. Very high stereoselectivity is observed for racemic 4-methyl- 1-hexene and racemic 3,7-dimethyl-1-octene, where the asymmetric carbon atom is in the α-position relative to the double bond. Stereoselectivity is caused by the chirality of the catalytically active center, and not by chiral atoms in the growing chain. It must be concluded from the results that reactions take place, which change the number of active sites present, due to the different behavior of the polymers in solution. Study of these new catalysts is intensive. After a short induction period, the activity of polymerization increases as a function of the monomer concentration.

Electrolytes: from dilute solutions to fused salts

Abstract: Solutions with composition extending continuously from molecular liquids such as water to fused salts are relatively unusual but of considerable interest. Conductance and thermodynamic properties are considered for several examples. New equations for the activities of the respective components represent the data more accurately than previous treatments and delineate the similarities and differences between such systems and nonelectrolyte solutions.

Rate constants and equilibrium constants for thiol-disulfide interchange reactions involving oxidized glutathione

Abstract: The rate of reduction of oxidized glutathione (GSSG) to glutathione (GSH) by thiolate anions (RS-) follows a Br/nsted relation in pK"s of the conjugate thiols (RSH):pnu" - 0.5. This value is similar to that for reduction of Ellman's re- agent: 0nu" = 0.4-0.5. Analysis of a number of rate and equilibrium data, taken both from this work and from the literature. indicates that rate constants, k, for a range of thiolate-disulfide interchange reactions are correlated well by equations of the formlogk=C*0nu.p Kunu"*0"pK""*0re pKule(nuc=nucleo phile.c=central, andlg=leavinggro upsulfur):eq36-3 8 give representative values of the Br/nsted coefficients. The values of these Br/nsted coefficients are not sharply defined by the available experimental data. although eq 36-38 provide useful kinetic models for rates of thiolate-disulfide interchange reac- tions. The uncertainty in these parameters is such that their detailed mechanistic interpretation is not worthwhile. but their qualitative interpretation-that all three sulfur atoms experience a significant effective negative charge in the transition state, but that the charge is concentrated on the terminal sulfurs-is justified. Equilibrium constants for reduction oiCSSG using a,cr-dithiols have been measured. The reducing potential of the dithiol is strongly influenced by the size of the cyclic disulfide formed on its oxidation: the most strongly reducing dithiols are those which can form six-membered cyclic disulfides. Separate equilibrium constants for thiolateanion-di sulfide interchange (Ks-) and for thiol-disulfide interchange (KsH) have been esti- mated from literature data: Ks- is roughly proportionalto 2ApK. is the difference between the pKus of the two thiols involved in the interchange. The contributions of thiol pKu values to the observed equilibrium constants for reduction of GSSG with a,co-dithiols appear to be much smaller than those ascribable to the influence of structure on intramolecular ring forma- tion. These equilibrium and rate constants are helpful in choosing dithiols for use as antioxidants in solutions containing pro- teins: dithiothreitol (DTT), 1.3-dimercapto-2-propanol (DMP), and 2-mercaptoethanol have especially useful properties. 201 I

Model studies for the synthesis of quassinoids. 1. Construction of the BCE ring system

Abstract: Bei der Dien-Addition des Diens (I) mit dem Hydrochinon (II) in Gegenwart von Silbero xid entsteht das Addukt (III), das mit Zink-Eisessig zu (IV) reduziert und dann in verschiedene polycyclische Naturstoffe wie (V), (VI) und (VII) ubergefuhrt wird.

Sulfur Adsorption and Poisoning of Metallic Catalysts

Abstract: The catalytic activity of most transition metals is drastically reduced by the presence of hydrogen sulfide or other sulfur-containing compounds at extremely low concentration in the reagents. This poisoning effect is a major problem in many catalytic reactions, especially hydrogen reaction such as methanation of coal synthesis gas or reforming of naphthas. On the other hand, beneficial effects on the selectivity can be obtained by a partial and well-controlled poisoning of catalysts by sulfur. For example, an enhancement of the selectivity for heavier hydrocarbons in the CO + H2 reaction has been reported [1–3]. Increases of the selectivity for partial hydrogenation of diolefins to the corresponding monoolefins were also observed [4, 5]. In the catalytic reforming of naphthas, partial poisoning by sulfur minimizes excessive hydrocracking.

Investigation of exchange processes by two-dimensional nmr spectroscopy

Abstract: A new general technique for the investigation of exchange processes in molecular systems is proposed and demonstrated. Applications comprise the study of chemical exchange, of magnetization transfer by inter‐ and intramolecular relaxation in liquids, and of spin diffusion and cross‐relaxation processes in solids.

Relationship of octanol/water partition coefficient and molecular weight to rat brain capillary permeability

Abstract: The rat brain capillary permeability coefficient was determined for 27 compounds. The relationship of permeability to octanol/water partition coefficient and molecular weight was found to be predictable for drugs with molecular weights less than 400.

Chiral Metal Atoms in Optically Active Organo-Transition-Metal Compounds

Abstract: Publisher Summary This chapter discusses chiral metal atoms in optically active organo-transition-metal compounds. An important approach to stereochemical problems is to make use of the concept of chirality. The chiral center most frequently encountered is the asymmetric carbon atom––a tetrahedral C atom––bonded to four different substituents. Chiral molecules may be studied by a great many techniques. Occasionally, optically active ligands have been employed in organo-transition-metal chemistry to demonstrate the stereospecificity of reactions at metal–ligand bonds with respect to the α-carbon atom of the ligand. The chapter focuses on organo-transition-metal compounds having chiral metal atoms whose optical activity have been demonstrated. The principle of introducing a diastereoisomer relationship into a pair of mirror-image isomers is the basis for each optical resolution. After the preparation of diastereoisomers by the introduction of an optically active resolving agent, the next problem in an optical resolution is to separate the diastereoisomers. The optical purity of enantiomers has been determined by nuclear magnetic resonance (NMR) spectroscopy with the help of optically active shift-reagents. Thus, for racemization, the steric effect seems to be most important. Metal–alkyl bonds can be carbonylated, and metal–acyl bonds can be decarbonylated. Optically active organo-transition-metal compounds exhibit extremely large specific rotations, usually exceeding the specific rotations encountered in organic chemistry by a factor of one or two powers of ten.

A particularly convenient preparation of benzohydroximinoyl chlorides (nitrile oxide precursors)

Abstract: Aus den entsprechenden Aldehyden werden die Oxirne (I) hergestellt, die mit N-Chlorsuccinimid (NCS) zu den Benzohydroximinoyl-chloriden (II) chloriert werden.

Ultraviolet photoelectron studies of the ground-state electronic structure and gas-phase tautomerism of purine and adenine

Abstract: Ultraviolet photoelectron spectroscopy has been employed to study electronic structure and tautomerism in adenine and purine and in various methyl-substituted derivatives of these molecules. The assignment of the photoelectron spectra has been aided by results from HAM^ molecular orbital calculations which are empirically parameterized to provide orbital ionization potentials. The calculations predict that the lone-pair orbitals in purine and adenine are similar in spacial distribution and have similar ionization potentials. This conclusion is supported by the spectroscopic data which indicate that in purine the three highest occupied lone-pair orbitals have ionization potentials of -9.6, - 10.6, and - 11.7 eV. In adenine these orbitals have ionization potentials of -9.6, - 10.5, and 11.39 eV. The planar exocyclic amino group in adenine causes the A structure of this molecule to be significantly different than that in purine. In adenine the ionization potentials of the five highest occupied A orbitals are 8.48, -9.6, -10.5, 12.10, and 13.21 eV. In purine the ionization potentials of the four highest occupied a orbitals are -9.6, -10.2, -11.9, and 13.10 eV. The band arising from the fifth highest occupied A orbital in purine has an ionization potential greater than 13.5 eV and is not resolved. A comparison of the photoelectron spectra of purine, adenine, and their 7- and 9-methyl-substituted derivatives indicates that the spectra of purine and adenine are much more similar to the spectra of their 9-methyl derivatives than to the spectra of their 7-methyl derivatives. This observation supports the conclusion that in an isolated environment the N(9)H tautomers of both purine and adenine are more stable than the N(7)H tautomers.

Chance factors in studies of quantitative structure-activity relationships

Abstract: Multiple regression analysis is a basic statistical tool used for QSAR studies in drug design. However, there is a risk or arriving at fortuitous correlations when too many variables are screened relative to the number of available observations. In this regard, a critical distinction must be made between the number of variables screened for possible correlation and the number which actually appear in the regression equation. Using a modified Fortran stepwise multiple-regression analysis program, simulated QSAR studies employing random numbers were run for many different combinations of screened variables and observations. Under certain conditions, a substantial incidence of correlations with high r2 values were found, although the overall degree of chance correlation noted was less than that reported in a previous study. Analysis of the results has provided a basis for making judgements concerning the level of risk of encountering chance correlations for a wide range of combinations of observations and screened variables in QSAR studies using multiple-regression analysis. For illustrative purposes, some examples involving published QSAR studies have been considered and the reported correlations shown to be less significant than originally presented through the influence of unrecognized chance factors.

Vapor phase association in acetic and trifluoroacetic acids. Thermal conductivity measurements and molecular orbital calculations

Abstract: Thermal conductivity measurements are reported for the vapors of acetic acid and trifluoroacetic acid in the temperature range 350 to 415 K over the pressure range 100 to 2000 Torr. Very large enhancements of the thermal conductivity were found and analysis of the data is consistent with a monomer-dimer equilibrium for both vapors. The standard thermodynamic parameters for the dimerization reactions are -..delta..H/sub 2/ = 14.64 kcal mol/sup -1/ and -..delta..S/sub 2/ = 35.50 cal mol-/sup 1/ K/sup -1/ for acetic acid, and -..delta..H/sub 2/ = 13.66 kcal mol-/sup 1/ and -..delta..S/sub 2/ = 36.15 cal mol-/sup 1/ K/sup -1/ for trifluoroacetic acid. No evidence was found for the presence of significant amounts of associated species larger than the dimer. Ab initio molecular orbital calculations were performed on the monomers and dimers of both acids to determine the lowest energy structures.

Boron Heterocycles as Ligands in Transition-Metal Chemistry

Abstract: Publisher Summary This chapter discusses the ligand properties of Lewis acid boron heterocycles and the newest developments in the expanding field of triple- and tetra-decker sandwich compounds Replacement of carbon atoms by boron atoms in π-carbocyclic and heterocyclic systems leads to a wide variety of planar Lewis acid boron heterocycles Six-membered Lewis acid ligands can be derived from benzene by substituting boron, nitrogen, and sulfur for carbon As an alternative to the aromaticity formalism commonly used, compounds containing electron-deficient elements in addition to metal atoms can be regarded as clusters In clusters composed solely of main-group elements, the following electronic relationship among closo, nido, and arachno structures has been found In the paramagnetic electron-rich bis(borinato)cobalt complexes, the labilized metal-ligand interaction allows ligand exchange and transfer to other metals Cyclic voltammetry of (borinato)cyclopentadienyliron and of bis(borinato) complexes of iron, chromium, and vanadium revealed oxidation of the iron complexes with varying degrees of reversibility, depending on the solvent Between benzene and borazine, several isomeric boron-carbon-nitrogen heterocycles would be expected to exist as potential η6-ligands In comparison with nitrogen, a sulfur atom is a “soft” electron donor, and therefore sulfur-containing ligands should readily form transition-metal complexes The complexes decompose slowly in C6H6 and CH2CI2 solution but are stable in dioxane The most interesting feature of electron-poor boron heterocycles in complex chemistry is their use as bridging ligands in di- and trinuclear complexes