Time filter

Source Type

Scheuermann C.J.,Institute For Organische Chemie
Chemistry - An Asian Journal | Year: 2010

The development of new and environmentally compatible methods for the synthesis of C-C and C-heteroatom bonds is of great significance to researchers. The cross dehydrogenative coupling (CDC) is emerging as an important tool for chemists with a variety of, often inexpensive, metal catalysts able to perform these reactions both regio- and enantioselectively. Perhaps more importantly CDC methodology has been shown to be extremely environmentally friendly and facile being able to be carried out in aqueous environments under an atmosphere of air. This Focus Review intends to give an overview of the most contemporary developments in this exciting and emerging area of cross-coupling chemistry. © 2010 Wiley-VCH Verlag GmbH&Co. KGaA. Source

Wink C.,Institute For Organische Chemie | Detert H.,Institute For Organische Chemie
Journal of Physical Organic Chemistry | Year: 2013

Di(p-aminostyryl)pyrazines with voluminous substituents on the nitrogen and in the adjacent positions were prepared via twofold aldol condensation. Absorption and emission spectra are influenced by increasing steric hindrance because the orbital overlap between nitrogen and π-system is modulated by voluminous groups. Strong solvatochromism of the fluorescence and huge Stokes shifts result from amplified donor-acceptor interaction in the excited state. Protonation occurs at the terminal amino groups first, followed by protonation of the central pyrazine. With increasing strength of acid, absorption and emission spectra are first shifted to the blue followed by a redshift. Copyright © 2012 John Wiley & Sons, Ltd. Source

Braida B.,CNRS Theoretical Chemistry Laboratory | Walter C.,Institute For Organische Chemie | Engels B.,Institute For Organische Chemie | Hiberty P.C.,University Paris - Sud
Journal of the American Chemical Society | Year: 2010

A series of nine 1,3-dipoles, belonging to the families of diazonium betaines, nitrilium betaines, and azomethine betaines, has been studied by means of the breathing-orbital valence bond ab initio method. Each 1,3-dipole is described as a linear combination of three valence bond structures, two zwitterions and one diradical, for which the weights in the total wave function can be quantitatively estimated. In agreement with an early proposition of Harcourt, the diradical character of 1,3-dipoles is shown to be a critical feature to favor 1,3-dipolar cycloaddition. Within each family, a linear relationship is evidenced between the weight of the diradical structure in the 1,3-dipole and the barrier to cycloaddition to ethylene or acetylene, with correlation coefficients of 0.98-1.00. The barrier heights also correlate very well with the transition energies from ground state to pure diradical states of the 1,3-dipoles at equilibrium geometry. Moreover, the weight of the diradical structure is shown to increase significantly in all 1,3-dipoles from their equilibrium geometries to their distorted geometries in the transition states. A mechanism for 1,3-dipolar cycloaddition is proposed, in which the 1,3-dipole first distorts so as to reach a reactive state that possesses some critical diradical character and then adds to the dipolarophile with little or no barrier. This mechanism is in line with the recently proposed distortion/interaction energy model of Ess and Houk and their finding that the barrier heights for the cycloaddition of a given 1,3-dipole to ethylene and acetylene are nearly the same, despite the exothermicity difference (Ess, D. H. and Houk, K. N. J. Am. Chem. Soc. 2008, 130, 10187). © 2010 American Chemical Society. Source

Bassan D.M.,Institute For Organische Chemie | Erdmann F.,Institute For Rechtsmedizin | Krull R.,Institute For Organische Chemie | Krull R.,Institute For Rechtsmedizin
Analytical and Bioanalytical Chemistry | Year: 2011

An analytical procedure for the simultaneous determination in human serum of 43 common drugs of abuse and their metabolites belonging to the different chemical and toxicological classes of amphetamines, benzodiazepines, dibenzazepines, cocaine, lysergic acid diethylamide, opioids, phencyclidine, tricyclic antidepressants, and zolpidem, using 33 deuterated standards, is presented. The sample treatment was developed to be a very simple protein precipitation and filtration. All analyses were performed with a high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry in positive ionization mode. All analytes were calibrated up to 550 μg/L. The limit of detection ranged from 0.6 ng/mL (EDDP) to 13.7 ng/mL (flunitrazepam). The method has been validated according to the guidelines of the Gesellschaft für Toxikologische und Forensische Chemie, using three multiple reaction mode (MRM) transitions and retention time for positive compound identification, instead of two MRMs, in anticipation of the new guidelines for January 2011. © 2011 Springer-Verlag. Source

Haiss P.,Institute For Organische Chemie | Zeller K.-P.,Institute For Organische Chemie
Organic and Biomolecular Chemistry | Year: 2011

As shown by deuterium labelling experiments, the deprotonation of the trimethylsulfonium ion (1) by the dimsyl anion (8) is accompanied by extensive hydrogen exchange. This cannot be explained by an acid-base equilibrium between the trimethylsulfonium ion (1) and the dimsyl anion (8) on one side and dimethylsulfonium methylide (2) and DMSO on the other side, because for thermodynamic reasons this process is irreversible due to the limited life-time of 2. Therefore, the isotopic exchange that accompanies the deprotonation is an indicator of a more complex deprotonation process. It is suggested that in a kinetically controlled reaction, a proton of 1 is transferred to the O-atom of 8 rather than to the carbanionic centre. This means that instead of DMSO, its tautomer, hydroxy-methylsulfonium methylide (10), is obtained in the deprotonation process. Similarly, in the acid-base interaction between DMSO and its conjugate base 8, the formation of the DMSO tautomer 10 is kinetically favoured. The intermediate 10 produced in this way transfers a DMSO-derived proton to 1 when it intervenes in the back reaction 10 + 2 → 8 + 1. An alternative mechanism based on methyl group exchange between 1 and 8 could be excluded by a 13C-labelling experiment. The hydrogen exchange according to the suggested scenario is taking place in competition with the reaction of dimethylsulfonium methylide (2) with electrophilic substrates. This explains the different degrees of isotopic exchange when compounds of different electrophilicities are used to scavenge 2 from the deprotonation-hydrogen distribution equilibria. © 2011 The Royal Society of Chemistry. Source

Discover hidden collaborations