Electrophilicity of 5-benzylidene-1,3-dimethylbarbituric and -thiobarbituric acids

The kinetics of reactions of acceptor-stabilized carbanions 2a-m with benzylidenebarbituric and -thiobarbituric acids 1a-e has been determined in a dimethyl sulfoxide solution at 20 degrees C. Second-order rate constants were employed to determine the electrophilicity parameters E of the benzylidenebarbituric and -thiobarbituric acids 1a-e according to the correlation equation log k(20 degrees C) = s(N + E). With E parameters in the range of -10.4 to -13.9, the electrophilicities of 1a-e are comparable to those of analogously substituted benzylidenemalononitriles.     

How Does Electrostatic Activation Control Iminium‐Catalyzed Cyclopropanations?

Prediction of lg k = s ( N + E ) verified : In contrast to previous statements, ordinary α,β‐unsaturated iminium ions do react with sulfur ylides. Electrostatic interactions accelerate the reactions by a factor of more than 10⁵ and are responsible for the high stereoselectivity.

     

Kinetics of Hydride Abstractions from 2‐Arylbenzimidazolines

The rates of the hydride abstractions from the 2‐aryl‐1,3‐dimethyl‐benzimidazolines 1a – f by the benzhydrylium tetrafluoroborates 3a – e were determined photometrically by the stopped‐flow method in acetonitrile at 20 °C. The reactions follow second‐order kinetics, and the corresponding rate constants k₂ obey the linear free energy relationship log k₂(20 °C)= s(N+E), from which the nucleophile‐specific parameters N and s of the 2‐arylbenzimidazolines 1a – c have been derived. With nucleophilicity parameters N around 10, they are among the most reactive neutral C? H hydride donors which have so far been parameterized. The poor correlation between the rates of the hydride transfer reactions and the corresponding hydricities (ΔH⁰) indicates variable intrinsic barriers.     

Applicability of tandem mass spectrometry to the automated comparative sequencing of long-chain oligonucleotides

An algorithm for the comparative sequencing (COMPAS) of oligonucleotides is shown to be suitable for the sequence verification of nucleic acids ranging in length from a few to 80 nucleotides. The algorithm is based on the matching of a fragment ion spectrum generated by collision-induced dissociation to m/z values predicted from a known reference sequence employing established fragmentation pathways. Prior to mass spectrometric investigation, the oligonucleotides were on-line purified by ion-pair reversed-phase high-performance liquid chromatography using monolithic separation columns. This study evaluated the potential and the limits of COMPAS regarding the length and the charge state of oligonucleotides, the selected collision energy, and the analyzed amount of sample using a quadrupole ion trap mass spectrometer. The results revealed that oligonucleotides could be very reliably re-sequenced up to 60-mers, although the algorithm was successfully used to even verify sequences up to 80-mers. The relative collision energy was typically in the range between 13 and 20%, which allowed in most cases a verification of the reference sequence in a window of at least three consecutive collision energies. To select a proper charge state for fragmentation, a compromise had to be found between high signal intensity and low charge state. Furthermore, by reducing the eluent flow rate during elution of the oligonucleotide, the sequence of a 50-mer was successfully verified from the analysis of 295 fmol of the raw product. COMPAS was proven to be reproducible and was applied to the genotyping of the polymorphic, Y-chromosomal locus M9 contained in a 62-base pair polymerase chain reaction product.     

Photolytic generation of benzhydryl cations and radicals from quaternary phosphonium salts: how highly reactive carbocations survive their first nanoseconds

UV irradiation (266 or 280 nm) of benzhydryl triarylphosphonium salts Ar(2)CH-PAr(3)(+)X(-) yields benzhydryl cations Ar(2)CH(+) and/or benzhydryl radicals Ar(2)CH(?). The efficiency and mechanism of the photo-cleavage were studied by nanosecond laser flash photolysis and by ultrafast spectroscopy with a state-of-the-art femtosecond transient spectrometer. The influences of the photo-electrofuge (Ar(2)CH(+)), the photo-nucleofuge (PPh(3) or P(p-Cl-C(6)H(4))(3)), the counterion (X(-) = BF(4)(-), SbF(6)(-), Cl(-), or Br(-)), and the solvent (CH(2)Cl(2) or CH(3)CN) were investigated. Photogeneration of carbocations from Ar(2)CH-PAr(3)(+)BF(4)(-) or -SbF(6)(-) is considerably more efficient than from typical neutral precursors (e.g., benzhydryl chlorides or bromides). The photochemistry of phosphonium salts is controlled by the degree of ion pairing, which depends on the solvent and the concentration of the phosphonium salts. High yields of carbocations are obtained by photolyses of phosphonium salts with complex counterions (X(-) = BF(4)(-) or SbF(6)(-)), while photolyses of phosphonium halides Ar(2)CH-PPh(3)(+)X(-) (X(-) = Cl(-) or Br(-)) in CH(2)Cl(2) yield benzhydryl radicals Ar(2)CH(?) due to photo-electron transfer in the excited phosphonium halide ion pair. At low concentrations in CH(3)CN, the precursor salts are mostly unpaired, and the photo-cleavage mechanism is independent of the nature of the counter-anions. Dichloromethane is better suited for generating the more reactive benzhydryl cations than the more polar and more nucleophilic solvents CH(3)CN or CF(3)CH(2)OH. Efficient photo-generation of the most reactive benzhydryl cations (3,5-F(2)-C(6)H(3))(2)CH(+) and (4-(CF(3))-C(6)H(4))(2)CH(+) was only achieved using the photo-leaving group P(p-Cl-C(6)H(4))(3) and the counter-anion SbF(6)(-) in CH(2)Cl(2). The lifetimes of the photogenerated benzhydryl cations depend greatly on the decay mechanisms, which can be reactions with the solvent, with the photo-leaving group PAr(3), or with the counter-anion X(-) of the precursor salt. However, the nature of the photo-leaving group and the counterion of the precursor phosphonium salt do not affect the rates of the reactions of the obtained benzhydryl cations toward added nucleophiles. The method presented in this work allows us to generate a wide range of donor- and acceptor-substituted benzhydryl cations Ar(2)CH(+) for the purpose of studying their electrophilic reactivities.     

Kinetics and mechanism of organocatalytic aza-Michael additions: direct observation of enamine intermediates

The imidazoles 1a-g add to the CC-double bond of the iminium ion 2 with rate constants as predicted by the equation log k = s(N)(N + E). Unfavourable proton shifts from the imidazolium unit to the enamine fragment in the adduct 3 account for the failure of imidazoles to take part in iminium-activated aza-Michael additions to enals.     

Chemical Composition and Microstructure of Plasma-Nitrided Aluminium

 Detailed examinations were made by AES depth profiling, SEM, TEM and electron diffraction to get information about the relation between treatment conditions and the state of plasma-nitrided aluminium. The chemical composition and the elemental depth distribution were proofed to be depending on gas phase mixture, pressure and temperature during plasma treatment. The admixture of hydrogen during presputtering for surface cleaning and during nitriding results both in an improved nitriding behaviour and in a reduction of the formation of conical-shaped particles at the surface. The microstructure of the nitride layer isn’t depending on tested process conditions significantly. Surface and interface between layer and substrate are roughly in a scale of a few ten nanometers owing to sputtering effects. The main phase inside the layer is nanocrystalline AlN of the known hexagonal modification. In addition, some crystallites of remaining aluminium are present as a second phase. In contrast to nitrogen-implanted aluminium no preferred lattice orientation of the AlN phase was evident.