Abstract

Abstract

The intramolecular hydroamination of carbon-carbon π-bonds is an effective method for the rapid construction of nitrogen heterocycles. The rapid evaluation of suitable ligands using virtual screening will aid in the development of a stereoselective analogue of the reaction by predicting effective substrate/ligand combinations. Electronic structure calculations of the relevant transition structures for different nucleophiles and alkynes at the B3LYP/LACVP* level of theory are presented. They provide the basis for the development of a transition state force field by the Q2MM method, which is discussed in detail. The parameter development and performance for the transition state force field is presented.

Electron transfer is the simplest reaction possible, yet it has a profound impact on the structure and reactivity of organic compounds. These changes allow a new look at some of the fundamental concepts that are used to explain organic chemistry, such as symmetry, aromaticity, and bonding. The results from high-level electronic structure calculations are used to analyze the mechanistic differences in the pericyclic reactions of simple hydrocarbons and their radical cation counterparts. The importance of state symmetry correlation, Jahn–Teller distortions, delocalization, and fractional bonding for the reaction pathways of hydrocarbon radical cations is discussed.

The effect of substituents on the rearrangement of vinylcyclopropane radical cation to cyclopentene was explored by density functional theory. Correlations have been established between the activation energy of the system and the nature, as well as the position, of the substituents. It was found that radical- or cation-stabilizing substituents in select positions reduce the activation energy for the rearrangement substantially, while activation energies for competing hydrogen shifts were largely left unchanged. We have identified positions in the vinylcyclopropane where substitution is necessary, and others that can be substituted less discriminately. The mechanism of the rearrangement changes from stepwise to concerted when a strongly cation-stabilizing substituent is used, because the first step in the stepwise reaction becomes a barrierless ring opening of a strained cyclopropane. Steric effects were also explored and have been found to be of crucial importance for the success of a rearrangement. Finally, a simple set of rules has been devised to predict the outcome of oxidation of a vinylcyclopropane without requiring detailed mechanistic investigations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Oxidative transformations by the hydroxyl radical are significant in advanced oxidation processes for the breakdown of organic pollutants, yet mechanistic details of the reactions are lacking. A combination of experimental and computational methods has been employed in this study to elucidate the reactivity of the hydroxyl radical with the widely used herbicide 2,4-D (2,4-dichlorophenoxyacetic acid). The experimental data on the reactivity of the hydroxyl radical in the degradation of the herbicide 2,4-D were obtained from γ-radiolysis experiments with both ¹⁸O-labeled and unlabeled water. These were complemented by computational studies of the ^.OH attack on 2,4-D and 2,4-DCP (2,4-dichlorophenol) in the gas phase and in solution. These studies firmly established the kinetically controlled attack ipso to the ether functionality as the main reaction pathway of ^.OH and 2,4-D, followed by homolytic elimination of the ether side chain. In addition, the majority of the early intermediates in the reaction between the hydroxyl radical and 2,4-DCP, the major intermediate, were identified experimentally. While the hydroxyl radical attacks 2,4-D by ^.OH-addition/elimination on the aromatic ring, the oxidative breakdown of 2,4-DCP occurs through ^.OH addition followed by either elimination of chlorine or formation of the ensuing dichlorophenoxyl radical.

Warum ist sie so selektiv, die asymmetrische Jacobsen-Katsuki-Epoxidierung? Dichtefunktionalrechnungen brachten hier neue Erkenntnisse: Axiale Koordination am Metallzentrum erhöht nicht nur die Reaktionsgeschwindigkeit, sondern führt überdies zu hochgradig nichtplanaren Konformationen des aktiven Katalysators (siehe Strukturbild).

The ion chemistry of anti-o,o′-dibenzene (1) was examined in the gaseous and the condensed phase. From a series of comparative ion cyclotron resonance (ICR) mass spectrometry experiments which involved the interaction of Cu⁺ with 1, benzene, or mixtures of both, it was demonstrated that 1 can be brought into the gas phase as an intact molecule under the experimental conditions employed. The molecular ions, formally 1^.+ and 1^.−, were investigated with a four-sector mass spectrometer in metastable-ion decay, collisional activation, charge reversal, and neutralization–reionization experiments. Surprisingly, the expected retrocyclization to yield two benzene molecules was not dominant for the long-lived molecular ions; however, other fragmentations, such as methyl and hydrogen losses, prevailed. In contrast, matrix ionization of 1 in freon (77 K) by γ-radiation or in argon (12 K) by X-irradiation leads to quantitative retrocyclization to the cationic dimer of benzene, 2^.+. Theoretical modeling of the potential-energy surface for the retrocyclization shows that only a small, if any, activation barrier is to be expected for this process. In another series of experiments, metal complexes of 1 were investigated. 1/Cr⁺ was formed in the ion source and examined by metastable ion decay and collisional activation experiments, which revealed predominant losses of neutral benzene. Nevertheless, comparison with the bis-ligated [(C₆H₆)₂Cr]⁺ complex provided evidence for the existence of an intact 1/Cr⁺ under these experimental conditions. No evidence for the existence of 1/Fe⁺ was obtained, which suggests that iron mediates the rapid retrocyclization of 1/Fe⁺ into the bis-ligated benzene complex [(C₆H₆)₂Fe]⁺.

Die Ionenchemie von anti-o,o′-Dibenzol 1 wurde sowohl in der Gasphase als auch in kondensierter Phase untersucht. Ionen-Cyclotron-Resonanz-Massenspektrometrie mit Hilfe von Cu⁺-Ionen zeigten, dass 1 unter den experimentellen Bedingungen als intaktes Molekül in der Gasphase existiert. Die Molekülionen, formal 1^.+ und 1^.−, wurden mit einem Viersektor-Massenspektrometer untersucht und verschiedenen Stoßexperimenten unterworfen. Erstaunlicherweise war die erwartete Retrocyclisierung, die zu zwei Benzolmolekülen führen sollte, nicht der Hauptprozess, sondern andere Fragmentierungen, wie Methyl- und Wasserstoffverluste überwogen. Im Gegensatz dazu führte die Matrixionisierung von 1 durch γ-Strahlung in freon (77 K) oder durch Röntgenstrahlung in Argon (12 K) zu quantitativer Retrocyclisierung unter Bildung des kationischen Benzoldimers, 2^.+. Die theoretische Modellierung der Retrocyclisierungs-Potentialenergiefläche zeigt, dass eine nur geringe Barriere für diesen Prozess erwartet werden kann. In einer weiteren Reihe von Experimenten wurden kationische Metallkomplexe von 1 untersucht. Gasphasenexperimente ergaben, dass im Falle des Chroms der Komplex 1/Cr⁺ erzeugbar war, während für Eisen eine schnelle Retrocyclisierung zum bisligierten Benzolkomplex [(C₆H₆)₂Fe]⁺ vorherscht.