Showing papers in "ChemInform in 1984"

Apparent molal volumes of amino acids, N-acetylamino acids, and peptides in aqueous solutions

Abstract: Volumes molaires apparents de 24 α-aminoacides, 6 N-acetylaminoacides et 9 peptides en solutions aqueuses a 298,15 K

The composition, structure and hydrogen bonding of the β-diketones

Abstract: Proton transfer and hydrogen bonding are two aspects of the chemistry of hydrogen that respectively govern the behaviour and structure of many molecules, both simple and complex, from water to DNA. The β-dicarbonyls exhibit both of these features, and in ways which have singled them out for detailed study for many years. They provide the best known examples of keto ⇆ enol tautomerism, with the advantage of slow proton transfer and high concentrations of the enol tautomers in most cases. These enols are stabilized by intramolecular OHO hydrogen bonds which at various times have been thought of as being centred, linear hydrogen bonds that are somehow incorporated into the delocalised π system to give “aromatic” systems. Research involving structural, spectroscopic and computational techniques has deepened our understanding of these compounds and changed our picture of them. The hydrogen bonding is surprisingly strong, surprising since it is neither centred, nor linear, nor involved in the ring’s delocalized bonding, although it is certainly coupled to it. This review deals with controversies that have surrounded the β-dicarbonyls and discusses the current view that the enol tautomers are a double-minimum potential well with a low energy barrier. The review ends with a brief look at the β-thioxoketones, which provide an analogous system based on a heteronuclear SHO hydrogen bond in the enol or enethiol tautomer.

Prostaglandin endoperoxides. 14. Solvent-induced fragmentation of prostaglandin endoperoxides. New aldehyde products from PGH2 and a novel intramolecular 1,2-hydride shift during endoperoxide fragmentation in aqueous solution

Abstract: Im Gegensatz zum Endoperoxid (I) wird das Dioxabicycloheptan (IIa) in H2O mit einer Geschwindigkeit zersetzt, die um 2 Grosenordnungen hoher ist als die von Reaktionen in Losungsmitteln vergleichbarer Polaritat. Fur die Aldehydbildung aus (IIa) und (IIb) ergibt sich ein kinetischer Isotopieeffekt (KIE) von l.5 aber kein D-Verlust und keine pH-Abhangigkeit der Reaktionsgeschwindigkeit.

Reactions of the .pi.-thiophene ligand in tricarbonyl(.eta.-thiophene)manganese[1+) ion ((.eta.-C4H4S)Mn(CO)3+]. Mechanistic possibilities for catalytic hydrodesulfurization

Abstract: Reaction of Mn(CO)/sub 5/OSO/sub 2/CF/sub 3/ with refluxing thiophene gives a high yield of (Mn(CO)/sub 3/(thiophene))SO/sub 3/CF/sub 3/ (1). This complex reacts with CN/sup -/ to give Mn(CO)/sub 3/(thiophene CN) (2a) and with BH/sub 4//sup -/, HFe(CO)/sub 4//sup -/, and HW(CO)/sub 5//sup -/ to give MN(CO)/sub 3/(thiophene H) (2c). A single-crystal X-ray diffraction study of 2a shows that it crystallizes in the monoclinic cell P2/sub 1//c with a = 12.008 (2) A, b = 6.6935 (8) A, c = 12.172 (2) A, ..beta.. = 95.63 (2)/sup 0/, and four molecules per unit cell. The structure demonstrates that cyanide addition occurs at a carbon atom adjacent to sulfur causing this now-saturated carbon to be 0.59 A out of the plane of the thiophene ring. Compound 1 reversibly adds P(n-Bu)/sub 3/ to form Mn(CO)/sub 3/(thiophene PBu/sub 3/)/sup +/. Protonation of 2c by HCl, HSO/sub 3/CF/sub 3/, HI, and CF/sub 3/CO/sub 2/H yields an unstable complex of 2,3-dihydrothiophene. The dihydrothiophene is readily displaced by MeCN or CO. Hydride addition to the ..pi..-bonded thiophene in 2c, followed by protonation, is suggested as a possible model for the first steps in hydrodesulfurization of thiophene.

Acyclic stereoselection. 22. Diastereofacial selectivity in the Lewis acid mediated reactions of allylsilanes with chiral aldehydes and enones

Abstract: Reactions d'aldehydes (phenyl-2 ou benzyloxy-3 propanal par exemple) et d'enones (phenyl-5 hexene-3 one-2) avec des allylsilones (allyl trimethyl- ou methyl-2 allyl trimethyl silane)

Spectroscopy of the intermediate states of the solid state polymerization reaction in diacetylene crystals

Abstract: This contribution gives a review of recent spectroscopic investigations concerning the photophysical and photochemical primary and secondary processes of the solid state polymerization reaction in diacetylene single crystals. It will be shown, that diacetylenes are an unique model system for the study of the reaction mechanism of a solid state chemical reaction which is characterized by a variety of reaction intermediates. The polymerization reaction in these crystals is of special importance, due to the resulting polymer single crystals, which exhibit extraordinary anisotropic physical properties.

Lithiation Reaction of 1,5-Dibromothiophene. 13C-NMR Spectra of 3-Substituted Derivatives.

Abstract: Selon les conditions et le compose de depart, la lithiation se fait en -3, en -2,5, en -2 ou en -2,4; reactions des produits lithies avec des electrophiles

Biology and chemistry of the carbonic anhydrases

Abstract: This book contains 10 sections, each consisting of several papers. Some of the paper titles are: The Early Days of Research on Carbonic Anhydrase; Primary Structures and Genetic Changes in Mammalian Carbonic Anhydrase Isozymes; Organization of the Mouse and Human Carbonic Anhydrase II Genes; Isolation of the Chicken Carbonic Anhydrase II Gene; and Origins and Molecular Evolution of the Carbonic Anhydrase Isozymes.

Polymer-supported phase transfer catalysts: Reaction mechanisms

Abstract: Polymer-supported phase transfer catalysts are insoluble polymers having covalently bound functional groups active as catalysts for reactions between anions and neutral organic substrates. The active functional groups may be quaternary ammonium or phosphonium ions, crown ethers, cryptands, grafted poly(ethylene glycols), or analogues of dipolar aprotic solvents. The polymer most often used is polystyrene, but other synthetic polymers, silica gel, and alumina have been used also. The reactions are carried out in triphase mixtures of organic liquid, solid or aqueous inorganic salt; and solid catalyst. The chemical reactions take place within the polymer matrix or on the polymer surface. With heterogeneous catalysts the reaction rates may be limited by mass transfer of reactants to the catalyst surface, intraparticle diffusion of reactants to the active sites, or intrinsic reactivity at the active sites. An analysis of the experimental parameters which affect catalytic activity and their mechanisms of action is presented.

Structure and bonding in metal-pentadienyl and related compounds

Abstract: Metal Complexes of the allyl (C3H5) and cyclopentadienyl (C5H5) ligands have played a major role in the development of organometallic chemistry. Metal allyl complexes are well known for the many intricate organic transformations which they may mediate, yet often possess limited stability. Metal cyclopentadienyl complexes on the other hand possess very high stabilities but only very limited catalytic chemistry. The pentadienyl ligand (C5H7 as well as various methylated derivatives) has similarities to both the allyl and cyclopentadienyl ligands, and it has now been demonstrated that metal pentadienyl complexes possess not only thermal stability but also chemical and catalytic reactivities. This favorable combination then will allow for detailed correlations to be made between chemical reactivities and the physical natures (structural, spectroscopic, etc.) of these compounds. In this article the presently available information on metal-pentadienyl compounds is discussed in order to gain some understanding of their nature compared to the related allyl and cyclopentadienyl systems, and to anticipate future applications of metal-pentadienyls. Particular emphasis is given to the bis(pentadienyl)metal complexes, which have been referred to as the “open metallocenes”.

Isoelectronic molecules with triple bonds to metal atoms (m = mo, w): crystal and molecular structures of tri-tert-butoxytungsten ethylidyne and nitride

Abstract: Le complexe (t-BuO) 3 W≡CMe cristallise dans le groupe P1 et le complexe (t-BuO) 3 W≡N dans le groupe P6 3 cm. Coordination bipyromidale rhomboedrique autour des atomes de tungstene. Le compose ethylidyne a une structure centrosymetrique mettant en jeu deux unites bipyramidale rhomboedrique possedant une arete commune. Le compose nitruro est lineaire et polymere

Organotin-mediated selective desulfurization: tri-n-butyltin hydride reduction of unsymmetric sulfides

Abstract: Reduction de (alkyl methyl)-, (alkyl benzyl)- et (alcene-2yl alkyl) sulfures et de cetosulfures, cyanosulfures et alkylthioacetates d'alkyle a l'aide du tributylstannane

Visible light induced hydrogen production from in situ generated colloidal rhodium-coated cadmium sulfide in surfactant vesicles

Abstract: Production de H 2 par irradiation sous rayonnement visible de Cds colloidal recouvert de Rh, retenu dans un vesicule, en presence de PhSH10 −3 M. Modele idealise de photoreduction de H 2 O par PhSH sensibilisee par CdS