Cleavage of benzyltrimethylstannanes by aqueous methanolic perchloric acid

The rates of cleavage of some XC₆H₄CH₂SnMe₃ bonds by aqueous-methanolic perchloric acid have been measured spectrophotometrically, and the rate of cleavage of the MeSn bonds of PhCH₂SnMe₃ and Me₄Sn by monitoring the methane evolution. The results indicate that for X = H, p-Me, o-Me, p-Bu^t, o-, m- and p-F and -Cl, and o-Br, the cleavage of the CH₂SnMe₃ bond involves attack of the acid at the benzylic atom, and is not much faster than that of the SnMe bonds, but that a mechanism involving ring-protonation is important for X = m-Me, and greatly predominant for X = m-OMe.     

Der quadratfreie Kern der Eulerschen φ-Funktion

It is shown by a simple induction that any square-free natural number is the square-free kernel of Euler's -function of infinitely many natural numbers. Furthermore every square-free natural number >1 is the square-free kernel of Euler's -function of infinitely many square-numbers.     

Longitudinal (1)H relaxation optimization in TROSY NMR spectroscopy

A general method to enhance the sensitivity of the multidimensional NMR experiments performed at high-polarizing magnetic field via the significant reduction of the longitudinal proton relaxation times is described. The method is based on the use of two vast pools of "thermal bath" 1H spins residing on hydrogens covalently attached to carbon and oxygen atoms in 13C,15N labeled and fully protonated or fractionally deuterated proteins to uniformly enhance longitudinal relaxation of the 1HN spins and concomitantly the sensitivity of multipulse NMR experiments. The proposed longitudinal relaxation optimization is implemented in the 2D [15N,1H]-LTROSY, 2D [15N,1H]-LHSQC and 3D LTROSY-HNCA experiments yielding the factor 2-2.5 increase of the maximal signal-to-noise ratio per unit time at 600 MHz. At 900 MHz, the predicted decrease of the 1HN longitudinal relaxation times can be as large as one order of magnitude, making the proposed method an important tool for protein NMR at high magnetic fields.     

Selective Transport of Lead(II) and Strontium(II) Through a Crown Ether-Based Polymer Inclusion Membrane Containing Dialkylnaphthalenesulfonic Acid

New polymer inclusion membranes (PIMs) containing 18-membered crownethers and dialkylnaphthalenesulfonic acid are proposed for Sr2+ and Pb2+removal from nitric acid solutions. The influence of source phasecomposition and stripping agents was characterized and permeabilitycoefficients were calculated. The PIMs are easy to prepare and may be usefulin separation and concentration procedures for these cations from complexmixtures such as nuclear waste. Long-term stability was obtained for atleast several weeks of constant use during which no significant change ofpermeability was observed.     

Preparation of polar phase coated open tubular columns for steroid analysis

Summary Open tubular columns made of soda glass may be successfully coated with polar stationary phases after etching with dry HCl. Such columns are useful in difficult separations of steroids and their subsequent identification.     

Reaction of Tetracopper(I)-Phosphonitocavitand with PhSeSiMe3: Synthesis and Structure of a Polyanionic Cluster [pyH]6[(CuCl)9(μ3-SePh)5(μ4-SePh)]

Treatment of tetracopper(I)-phosphonitocavitand [1·Cu₄(μ-Cl)₄(μ₄-Cl)] (2) (1 = tetraphosphonitocavitand [rccc-2,8,14,20-tetrakis-(iso-butyl)-phosphonitocavitand (C₄₄H₄₈O₈P₄Ph₄)]) with PhSeSiMe₃ in THF at low temperature afforded a novel polyanionic cluster [pyH]₆[(CuCl)₉(μ₃-SePh)₅(μ₄-SePh)] (4) as a major product along with a new tetracopper(I)-phosphonitocavitand (3) with a centered μ₃-Cl. Molecular structure of anionic cluster in 4 consists of six PhSe⁻ bridging ligands containing five μ₃-SePh and one exceptional μ₄-SePh bridging nine copper atoms, of which eight copper atoms have trigonal coordination geometry and the other has distorted tetrahedral geometry. Dedicated to Professor Han-Qin Liu on the occasion of his 70th birthday.     

Model complexes for metallated polythiophenes: gold(I) and palladium(II) complexes of bis(diphenylphosphino)oligothiophenes

The preparations of two new phosphinothiophene ligands, 3,3'-bis(diphenylphosphino)-2,2'-bithiophene (dppbt; 1) and 3,3' "-dihexyl-3',3' '-bis(diphenylphosphino)-2,5':2',2' ':5' ',2' "-quaterthiophene (hdppqt; 2) are reported. Oxidation of 1 gives 3,3'-bis(diphenylphosphine oxide)-2,2'-bithiophene (3), and the crystal structure of this compound was determined. Pd(II) and Au(I) complexes of these ligands have been synthesized and characterized. Crystal structures of [(dppbt)PdCl(2)] (1-Pd), [(hdppqt)PdCl(2)] (2-Pd), [(dppbt)(AuCl)(2)] (1-Au), and [(hdppqt)(AuCl)(2)] (2-Au) were obtained. [(dppbt)(AuCl)(2)] crystallized in two solid-state forms; crystals grown from CH(2)Cl(2)/Et(2)O show a gold-gold interaction of 3.3221(4) A, but from CH(2)Cl(2)/toluene, the molecule crystallizes as a toluene adduct (1-Au-tol) and does not show any gold-gold interaction. All the complexes were characterized via UV-vis spectroscopy and cyclic voltammetry, and the effect of the metal on the energy of the pi-pi transition and oxidation potential was determined. These data are correlated to the interannular torsion angles in the oligothienyl groups from the crystal structure studies.     

Röntgenkleinwinkeluntersuchungen an Lösungen eines Polystyrolpräparates mit enger Molekulargewichtsverteilung

Zusammenfassung Es wurde das Streuverhalten von Polystyrol im thermodynamisch schlechten Lösungsmittel Cyclohexan zwischen Entmischungstemperatur und-Temperatur untersucht. Beim -Punkt und nahe dabei liegt das Verhalten eines statistisch verknäuelten Fadenmoleküls vor. Unterhalb der-Temperatur beginnt eine Aggregation. Knapp oberhalb der Entmischungstemperatur entspricht das Molekulargewicht etwa einer Verhängung zweier Fadenmoleküle. Ein Auftreten von kompakten Polymerknäueln ist bei dieser Temperatur nicht festzustellen. Der Einfluß des Fadenquerschnitts auf die Streukurve ist beträchtlich, da der Querschnitt nicht viel kleiner als die Persistenzlänge ist. Trotzdem läßt sich die Persistenzlänge (a=12,2 Å) aus dem Übergangspunkt zwischen den etwa 1/(2)² und 1/(2) proportionalen Bereichen bestimmen, da der Knick im Übergangsbereich infolge der großen Anzahl von Persistenzlängen pro Molekül sehr ausgeprägt ist.

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X-Ray small-angle investigations on solutions of a polystyrene sample with a narrow molecular weight distribution

Investigations were made of the scattering behaviour of polystyrene in the thermodynamically bad solvent cyclohexane between precipitation-temperature and-temperature. At the-point and close to it the behaviour of a statistically coiled chain molecule is found. Below-temperature an aggregation is beginning. Just above precipitation-temperature the molecular weight corresponds to an entanglement of two chain molecules. Appearance of compact polymer coils could not be noted at this temperature. The influence of the chain crosssection upon the scattering curve is considerable as the crosssection is not much smaller than the persistence length. Nevertheless, the persistence length (a=12.2 Å) may be determined from the transition point between the ranges roughly proportional to 1/(2)² and 1/(2) as the break within the transition range is very distinct on account of the great number of persistence lengths per molecule.

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Mit 3 Abbildungen

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Herrn Prof. Dr.E. Broda zum 60. Geburtstag gewidmet.     

Biomimetic cycloaddition approach to tropolone natural products via a tropolone ortho-quinone methide

[reaction: see text] A study toward a possible biomimetic hetero Diels-Alder reaction is reported between humulene and a novel tropolone ortho-quinone methide. A suitable tropolone ortho-quinone methide precursor has been prepared from 3-methyl-2-furoate. Heating the ortho-quinone methide precursor gave a tropolone ortho-quinone methide, which in the presence of humulene underwent a hetero Diels-Alder reaction to give a deoxy analogue of epolone B.     

Evidence for facile atropisomerism and simple (non-nucleophilic) biradical-forming cycloaromatization within kedarcidin chromophore aglycon

The kinetics and thermodynamics of atropisomerism within the protected kedarcidin chromophore aglycon 8, as well as a series of ansa-bridged synthetic intermediates leading to 8, were determined by 1H NMR spectroscopy. The data show that the ratio of atropisomeric forms of chloropyridine-bridged ansa intermediates is subject to wide variation with seemingly subtle structural variation. The vinyl bromide 4, for example, in the first X-ray structure determination of a kedarcidin ansa-bridged system, was found to exist as a single atropisomer in the solid state, but a nearly equal mixture (K = 0.70) of isomers in solution (t1/2 for isomer interconversion approximately 0.2 s at 20 degrees C). The aglycon 8, a 2.2:1 mixture of atropisomers, was found to undergo direct unimolecular biradical-forming cycloaromatization at ambient temperature in a mixture of 1,4-cyclohexadiene-benzene, without nucleophilic activation. The product 9 was formed as a single atropisomer (k = 2 x 10-4 s-1, t1/2 = 58 min, 81% yield), suggesting that the rate of atropisomerism within 8 is rapid with respect to cycloaromatization. The rate of cycloaromatization of 8 was found to be highly solvent-dependent, being more rapid in the presence of a good hydrogen-atom donor, consistent with the earlier model studies of Hirama et al. that showed that certain nine-membered cyclic (Z)-enediynes may equilibrate with their biradical cycloaromatization products. Incubation of 8 with beta-mercaptoethanol, under conditions mimicking experiments leading to DNA cleavage with kedarcidin, showed no evidence for nucleophilic activation. The product of direct cycloaromatization (9) was isolated instead. The evidence suggests that kedarcidin, like the enediyne agent C-1027, is capable of spontaneous thermal biradical formation without prior chemical activation.