Die Cope-Umlagerung als Prinzip einer repetierbaren Ringerweiterung

The Cope Rearrangement, a Reaction for Repeatable Ring Expansions Starting with the unsaturated β-ketoesters of type I, a vinyl group is introduced into the β-position by 1,4-addition of vinyl magnesium chloride to give II (Scheme 3). Treatment of the β-ketoester II with phenyl vinyl sulfoxide in the presence of sodium hydride yields the sulfoxides III, which on thermolysis lead to the α, β-divinyl ketoesters of type IV via elimination of sulfinic acid (Scheme 3). The Cope-System IV undergoes rearrangement to V, which is again an unsaturated β-ketoester. The latter is suitable for a further ring expansion sequence. These reaction steps were carried out with the nine-, twelve- and fifteen-membered ketoesters 32, 33 and 34 , as well as with the open-chain compound 35 (Table 1). With the cyclohexane derivative 31 , ring expansion could not be achieved with the described sequence.     

Zum Mechanismus massenspektrometrischer Fragmentierungsreaktionen: XII-Einfluß einer sterischen Hinderung der Mesomerie bei der Fragmentierung von N,N-Dimethyl-N′-4-Biphenylformamidinen

The formation of cyclic [M ? H]⁺ ions (a) in the mass spectra of N,N-dimethyl-N′-arylformamidines is suppressed by electron donating groups in the aryl group para to the formamidine group, due to the participation of resonance structures of type c. This effect is also observed in the mass spectra of N,N-dimethyl-N′-4-biphenylformamidines (III) substituted in the 4′-position. By substitution in the 2-, 2′-, 6- and 6′- positions of III, the formation of resonance structure c′ in the molecular ions is so difficult that these derivatives show the normal fragmentations. This is observed with two methyl groups in the 2- and 2′- positions of III.     

Connected and alternating vectors: Polyhedra and algorithms

Given a graphG = (V, E), leta ^S, S L, be the edge set incidence vectors of its nontrivial connected subgraphs.The extreme points of = {x R ^E: a^sx |V(S)| - |S|, S L} are shown to be integer 0/± 1 and characterized. They are the alternating vectorsb ^k, k K, ofG. WhenG is a tree, the extreme points ofB 0,b ^kx 1,k K} are shown to be the connected vectors ofG together with the origin. For the four LP's associated with andA, good algorithms are given and total dual integrality of andA proven.On leave from Swiss Federal Institute of Technology, Zurich.     

Zur katalytischen Bestimmung von Kupferspuren

Zusammenfassung Das Verfahren beruht auf der Mitfällung der Kupferspuren an Quecksilbersulfid mit Natriumthiosulfat, Entfernung des Hg und katalytischer Bestimmung des Cu mit Hilfe der rhodanidindizierten Eisen(III)-Thiosulfatreaktion nach der Simultankomparationsmethode [2]. Es gestattet die Bestimmung von Kupferspuren bis 5 · 10^–6% (gegebenenfalls 0,01 ppm) in beispielsweise Alkali-, Mg-, Ca-, Sr-, Zn-, Mn-, Co-, Ni-, Al-, Cr-, Cd-Salzen, Borsäure, Salzsäure und anderen Substanzen. Die Verbindungen dürfen nicht als Nitrate vorliegen oder andere Oxydationsmittel sowie stark komplexbildende Anionen enthalten. Mitgeteilte Modelluntersuchungen an AlCl₃ und ZnCl₂ (Cu-Gehalt 0,1 ppm) ergaben in letzterem Falle eine Varianz S von rund 3%.

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On the catalytic determination of traces of copperII. Isolation of traces of copper by coprecipitation with mercury sulphide and their catalytic determination according to the simultaneous-comparison method

The procedure is based on the coprecipitation of traces of copper with mercury sulphide by means of sodium thiosulphate, removal of the mercury and catalytic copper determination by means of the simultaneous-comparison method using the reaction between iron(III) thiocyanate and thiosulphate [2]. Traces of copper down to 5×10^–6% (0.01 ppm) can be determined in alkali, Mg, Ca, Sr, Zn, Mn, Co, Ni, Al, Cr and Cd salts, in boric acid, hydrochloric acid and other substances. Nitrates and other oxidizing agents and also strongly complexing anions must be absent. Results with AlCl₃ and ZnCl₂ (Cu content 0.1 ppm) are given. In the latter case the variation was about 3%.

     

Die konformationsenergie des ferroc̄enylrestes

From equilibration experiments with ferrocenyl cyclohexanes substituted at position 4, an average conformational free enthalpy of 2.9±0.1 kcal/mole was established for the ferrocenyl group. The spatial requirement of ferrocenyl at the equilibrium equatorial ? axial is approximately equivalent to that of the phenyl residue (3.0±0.1 kcal/mole).     

On the correlation of implantation defects and implanted species

Using high-sensitive deep level transient spectroscopy (DLTS), we have determined ion-related defects in monocrystalline silicon in the asimplanted state. In comparison with secondary ion mass spectrometry (SIMS) data and neutron depth profiling (NDP) results it is demonstrated that the defect profiles and the chemical distributions have nearly identical shape. From these experimental facts it can be concluded that this electrical spectroscopy can be applied for the detection of very low concentrations (down to 10¹⁰ cm^–3) of implanted ion species.     

Photochemical Z-->E isomerization of a hemithioindigo/hemistilbene omega-amino acid.

The molecule HTI, which combines hemithioindigo and hemistilbene molecular parts, allows reversible switching between two isomeric states. Photochromic behaviour of the HTI molecule is observed by irradiation with UV/Vis light. The photochemical reaction, a Z/E isomerization around the central double bond connecting the two molecular parts, is investigated by transient absorption and emission spectroscopy. For a special HTI molecule, namely, an omega-amino acid, the Z-->E isomerization process occurs on a timescale of 30 ps. In the course of the reaction fast processes on the 1-10 ps timescale are observed which point to motions of the molecule on the potential-energy surface of the excited state. The combination of transient absorption experiments in the visible spectral range with time-resolved fluorescence and infrared measurements reveal a photochemical pathway with three intermediate states. Together with a theoretical modelling procedure the experiments point to a sequential reaction scheme and give indications of the nature of the involved intermediates.     

Additionsreaktionen von 3-Dimethylamino-2,2-dimethyl-2H-azirin an Isothiocyanate

Addition Reactions of 3-Dimethylamino-2, 2-dimethyl- 2 H-azirine and Isothiocyanates. The title azirine readily reacts with two molecules of benzyl- or methylisothiocyanate to form the zwitterionic 1:2 addition compounds 4 and 13 , respectively (Scheme 2). The presumed 1:1 addition products, which are intermediates in the formation of 4 and 13 , cannot be detected. The structure of 4 and 13 follows from their spectroscopic and chemical properties. With water they give the thiourea derivates 5 and 14 , respectively; treatment with aqueous acid leads to the Δ²-1, 3-thiazolin-5-on-derivates 7 and 15 , respectively. With sodium borohydride compounds 8 and 16 , respectively, are obtained (Scheme 2). The zwitterionic compounds 4 and 13 are able to react further with one molecule of the isothiocyanates to give, in high-yield, triazines 9 and 18 , respectively (Scheme 3). The structure of these compounds was again derived from their spectroscopic data. The mechanism for the formation of 9 and 18 is given in Scheme 3. Acid catalysed hydrolysis of 9 and 18 lead to the trithiocyanuric acid derivates 12 and 20 , and to the spiro compounds 11 and 19 , respectively (Sceme 6). Reaction of 4 with one molecule of phenylisocyanate gives triazine 10 (Scheme 5). According to the X-ray analysis of the methyl compound 18 , there are strong steric interactions in this molecule which are due to the side chain. This is demonstrated by the small distances between C(2) … C(13), N(7) … C(11), and C(8) … C(11) (Table 4). These steric interactions, in addition, cause widening of the bond angles N(1)? C(2)? N(7) and C(9)? N(10)? C(11) (Fig.2). Furthermore, the triazine ring is no longer planar. This deformation of the ring diminishes repulsion between the methyl groups C(13) and C(15).     

Sequence Analysis of Polypeptides and Proteins by Combined Gas Chromatography-Mass Spectrometry

A new method for determining the amino acid sequence of polypeptides consists in initial partial hydrolysis to yield a complex mixture of oligopeptides. After derivatization to enhance its volatility, the mixture is analyzed by combined gas chromatography and mass spectrometry. The sequence of the polypeptide is established by a computer from the identified oligopeptides. So far polypeptides having up to 40 amino acids have been analyzed by this method. The advantages and disadvantages of the new method compared with the stepwise procedure of the Edman degradation are considered. Since the two methods are based on fundamentally different principles they may prove to be complementary.     

Gas phase molecular structure of bis(trifluoromethyl). mercury (CF3)2 Hg

The molecular structure of bis(trifluoromethyl)mercury has been determined by electron diffraction of gases. The best agreement between experiment and model was obtained for freely rotating CF₃ groups and the following geometric parameters (r°_α values): C-F = 1.345(3) Å, Hg-C = 2.101(5) Å and <FCF = 106.8°(0.2). The effect of CH₃/CF₃ substitution on the Hg-C bond length is discussed.