Die Reaktion des tert.Butyl-lithiums mit Aluminiumtribromid; Molekülstrukturen von [HAl(CMe3)2]3 und [LiHAl(CMe3)3]2

Reaction of tert.Butyl-lithium with Aluminiumtribromide; Molecular Structures of [HAl(CMe₃)₂]₃ and [LiHAl(CMe₃)₃]₂ The reaction of tert.butyl-lithium with aluminiumtrihalide at low temperatures was reinvestigated. Four compounds were isolated: trimeric di(tert.butyl)alane(III) 1 , monomeric (solution in benzene)/dimeric (solid state) lithium-tri(tert.butyl)alanate(III) 2 , tri(tert.butyl)alane 3 and lithium-tetra(tert.butyl)alanate 4 . X-ray structure analyses gave a planar sixmembered Al? H heterocycle for 1 and a Li? H-bridged dimer showing intramolecular interactions of lithium with C? H σ-bonds for 2 .     

Variation of mean SiO bond lengths in silicon-oxygen octahedra

The mean bond length SiO in silicon-oxygen octahedra is a function of the mean coordination number of the oxygen atoms (CN) in the octahedron: (SiO)_mean = 1.729 + 0.013CN. The radius of Si in six coordination against oxygen is 0.407 Å.     

Stereochemistry of planarchiral compounds,part X

2,7-Dibromo-1,6-methano[10]anulene (3) and 2,9-Dibromo-syn-1,6:8,13-diimino[14]anulene (9) were quantitatively separated into their enantiomers by chromatography on triacetylcellulose (TAC) in ethanol. X-ray structure analysis (Bijvoet technique) established the chiralities (+)(R)-3 and (+)(S)-9 for the dextrorotatory enantiomers.Comparison of the CD spectra allowed the configurational assignment to further optically active [10] and [14] anulenes which were also accessible by chromatography onTAC. Conversion of (+)(R)-2-bromo-1,6-methano[10]anulene (2) into the corresponding methylester (–)-4 confirmed its previously proposed chirality (–)(R).2,7-Dibromo-1,6-oxido[10]anulene (7) and 2,9-dibromo-syn-diimino[14]anulene (9) are in contrast to the 2,9-dibromo-syn-dioxido[14]anulene (10) optically stable until 250°C. Consequently their inversion barriers are higher than 42 kcal (176 kJ) mol^–1.The CD spectra of mono and disubstituted anulenes (with C₁ and C₂ symmetry, resp.) are compared: For the [10]anulenes theCotton effect around 330 nm seems to be specific for their configuration with a positive effect indicating (S)-chirality and vice versa. Some regularities concerning the chromatographic resolutions are discussed.

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Stereochemie planar chialer Verbindungen, 10. Mitt.: Röntgenkristallstruktur und absolute Chiralität überbrückter [10]- und [14] Anulene

Zusammenfassung 2,7-Dibrom-1,6-methano[10]anulen (3) und 2,9-Dibrom-syn-1,6:8,13-diimino[14]anulen (9) wurden durch Chromatographie an Triacetylcellulose (TAC) in Ethanol quantitativ in ihre Enantiomeren getrennt. Röntgenstruktur-analyse (Bijvoet-Technik) bewies für die rechtsdrehenden Enantiomeren die Chiralität (+)(R)-3 bzw. (+)(S)-9.Ein Vergleich der CD-Spektren ermöglichte die Konfigurationszuordnung weiterer optisch aktiver [10]- und [14]Anulene, die gleichfalls durch Chromato-graphie anTAC erhalten worden waren. Umwandlung von (+)(R)-2-Brom-1,6-methano[10]anulen (2) in den entsprechenden Methylester (–)-4 bestätigte dessen schon früher vorgeschlagene Chiralität (–)(R).Dibrom-1,6-oxido[10]anulen (7) und Dibrom-diimino[14]anulen (9) sind im Gegensatz zum Dioxido[14]anulen (10) bis 250°C optisch stabil. Ihre Inver-sionsbarrieren liegen somit über 42 kcal (176kJ) mol^–1.Die CD-Spektren von mono- und disubstituierten Anulenen (mit C₁ bzw. C₂-Symmetrie) werden verglichen: Für die [10]Anulene scheint derCottoneffekt um 330 nm konfigurationsspezifisch zu sein, wobei ein positiver Effekt (S)-Chiralität anzeigt — und vice versa. Einige Regelmäßigkeiten bezüglich der chromatographischen Enantiomerentrennung werden diskutiert.

     

PHOTOLYSIS OF PHOSPHODIESTER BONDS IN PLASMID DNA BY HIGH INTENSITY UV LASER IRRADIATION

Abstract— The cleavage of phosphodiester bonds in DNA exposed to high intensity UV laser pulses in aerated aqueous solution has been investigated using a krypton fluoride excimer laser (248 nm) and bacterial plasmid DNA. The dependence of strand breakage on fluence and intensity has been studied in detail and shows that the process is non-linear with respect to intensity. The relationship between the quantum yield for strand breakage and intensity shows that the strand breakage reaction involves two-photon excitation of DNA bases. The quantum yield rises with intensity from a lower value of 7 times 10^-5 until a maximum value of 4.5 times 10^-4 is attained at intensities of 10¹¹ W m^-2 and above. This value is approximately fifty-fold higher than the quantum yield for strand breakage induced by exposure to low density UV irradiation (254 nm, 12 W m^-2). DNA sequencing experiments have shown that strand breakage occurs by the specific cleavage of the phosphodiester bond which lies immediately 3' to guanine residues in the DNA, leaving some alkali-labile remnant attached to the terminal phosphate. A mechanism for DNA strand breakage which involves the generation of guanine radical cations is proposed.     

Persistent bissilylated arenium ions

A series of bissilylated arenium ions 1 with different substitution patterns on the aryl ring have been synthesized by hydride abstraction from 2-aryl-substituted 2,6-dimethyl-2,6-disilaheptanes (2) via transient silylium ions. The arenium ions have been identified by their characteristic NMR chemical shifts, (delta(29)Si=19.1-25.6, delta(13)C(ipso) =89.0-102.4, delta(13)C(ortho)=160.9-182.0, delta(13)C(meta)=132.5-146.9, delta(13)C(para)=150.2-169.9) supported by quantum mechanical calculations of structures, energies, and magnetic properties at the B3LYP/6-311G(d,p)//B3LYP/6-31G(d) + DeltaZPVE level of theory. The calculations clearly reveal the charge dispersing and stabilizing effect of the silyl substituents in arenium ions 1. The bissilylated benzenium ion 1a is more stable than the parent benzenium ion (C(6)H(7)(+)) by 37.6 kcalmol(-1). The synthesized arenium ions 1 are stable in solution at room temperature for periods ranging from a few hours to days. This unusual stability is attributed to: 1) the thermodynamic stabilization of the arenium ion by two beta-silyl substituents and 2) the essentially non-nucleophilic reaction conditions (the use of the weakly coordinating [B(C(6)F(5))(4)](-) anion and aromatic hydrocarbons as solvents). Addition of stronger nucleophiles than aromatic hydrocarbons (for example, acetonitrile) results in desilylation of the arenium ion 1 and recovery of the 2-aryl-2,6-disilaheptane moiety.     

Partially concurrent eluent evaporation with an early vapor exit; detection of food irradiation through coupled LC-GC analysis of the fat

This paper reports two subjects. It describes LC-GC transfer by partially concurrent eluent evaporation at a greatly accelerated rate, as required for optimal compatibility with 2–3 mm i.d. LC columns and LC flow rates up to some 500 μl/min. Evaporation rates around 200 μl/min were obtained using a 0.53 mm. i.d. uncoated pre-column and an early vapor exit. A stationary-phasecoated “retaining” pre-column was used for preventing escape of volatile solutes through the vapor exit. The technique was used for the detection of food irradiation by analyzing selected radiolysis products of triglycerides, namely alkanes/alkanes and aldehydes. Extracted fat of chicken, hazel-nuts, and soup mixes was injected in LO and the relevant fractions were transferred on-line to GC. For chicken and nuts, detection of irradiation was possible down to doses below 0.5 kGy. Detection limits were higher for soup mixes due to interfering peaks.     

An unprecedented type of migratory insertion reactions of unsaturated C3 units into Rh-O and Rh-C bonds

A series of iodo- and hydroxorhodium(I) complexes of the general composition trans-[RhX(=C=C=CRR')(PiPr3)2] (X = I: 5-7; X = OH: 8-11) was prepared from the related chlororhodium(I) precursors. The hydroxo compounds behave as organometallic Br?nsted bases and react with acids like MeCO2H, PhCO2H, PhOH, or TsOH by elimination of water to give the substitution products trans-[RhX'(=C=C=CRR')(PiPr3)2] (X' = MeCO2: 12, 13; X' = PhCO2: 14; X' = PhO: 15, 16; X' = TsO: 17, 18) in good to excellent yields. In contrast to the tosylates 17, 18, which react with CO by cleavage of the allenylidene-metal bond to give trans-[Rh(OTs)(CO)(PiPr3)2] (19), treatment of the acetato and phenolato derivatives 12, 13 and 15, 16 with CO affords by migratory insertion of the allenylidene unit into the Rh-O bond the alkynyl complexes trans-[Rh[C(triple bond)CCR(R')X'](CO)(PiPr3)2] (X' = MeCO2: 20, 21; X' = OPh: 22, 23). Similarly, the reactions of the hydroxo compounds 8, 10, and 11 with CH2(CN)2 and either CO or CNMe yield the carbonyl and the isocyanide complexes trans-[Rh[C(triple bond)CCR(R')CH(CN)2](L')(PiPr3)2] (L' = CO: 25-27; L' = CNMe: 28-30), respectively. By protolytic cleavage of the Rh-C sigma bond the gamma-functionalized alkynes HC(triple bond)CCR(R')CH(CN)2 (31, 32) are generated from 25, 26 and HCl in benzene. The molecular structure of 22 was determined by X-ray crystallography.     

New derivation and a k-particle generalization of SCF-type theories

A hierarchy of necessary conditions that an exact density matrix of a pure state or an ensemble has to satisfy is derived, namely the hermiticity of certain operators F^(k). For k = 1 this reduces to the well-known Hartree-Fock condition. It is then shown that the kth set of conditions is equivalent to stationarity of the energy with respect to unitary k-particle transformations. k-Particle generalizations of Hartree-Fock theory are then discussed both in the spirit of k-particle pseudoeigenvalue equations and in the framework of a Newton–Raphson-type constructive scheme.