An experimental and theoretical study of stereoselectivity of furan-maleic anhydride and furan-maleimide diels-alder reactions

The stereoselectivity of the reaction of furan (1) with maleic anhydride (2) and maleimide (3) was studied experimentally and theoretically. Although the two reactions are highly similar with regard to their preference for endo and exo steroisomers, notable differences were experimentally observed and explained on the basis of calculated reaction-free energies and transition-state barriers. The experimental values of rate constants (k(1+2endo) = (1.75 +/- 0.48) x 10(-5); mol(-1) l s(-1); k(1+2exo) = (3.10 +/- 0.55) x 10(-5); mol(-1) l s(-1); k(1+3endo) = (1.93 +/- 0.082) x 10(-5); mol(-1) l s(-1), k(1+3exo) = (1.38 +/- 0.055) x 10(-5); mol(-1) l s(-1) all at 300 K) and the observed reaction course clearly confirm that neither of these reactions are prototypical examples of Diels-Alder [4 + 2] cycloadditions, whose dominant preference is for endo isomers. However, only by comparing their energetics-calculated at the CCSD(T) level of theory-with the analogous reactions involving cyclopentadiene (8) as a diene can these observations be understood. The low thermodynamic stability of furan [4 + 2] adducts opens retro-Diels-Alder reaction channels and overrules the very small kinetic preference (calculated and measured here) of initial formation for endo stereoisomers. On a macroscopic scale "an irregular"-thermodynamically more stable-exo stereoisomer was consequently observed as a dominant species.     

Laser Ablation Synthesis of Gold Selenides by using a Mass Spectrometer as a Synthesizer: Laser Desorption Ionization Time‐of‐Flight Mass Spectrometry

Methods for the rapid construction of new chemical motifs have the potential to accelerate the development of nanoscience. The synthesis of new chemical entities by laser ablation has been systematically demonstrated by using mixtures of gold and selenium. The compounds generated are detected by time‐of‐flight mass spectrometry and, for selected compounds, the structure is investigated by using density functional theory optimization. In total, 67 new gold selenide clusters have been synthesized, demonstrating an unsuspected richness in gold chemistry. Chemical species generated in the gas phase might inspire new routes for the synthesis of novel compounds in the solid state.     

On the Possibility of Uphill Intramolecular Electron Transfer in Multicopper Oxidases: Electrochemical and Quantum Chemical Study of Bilirubin Oxidase

The catalytic cycle of multicopper oxidases (MCOs) involves intramolecular electron transfer (IET) from the Cu‐T1 copper ion, which is the primary site of the one‐electron oxidations of the substrate, to the trinuclear copper cluster (TNC), which is the site of the four‐electron reduction of dioxygen to water. In this study we report a detailed characterization of the kinetic and electrochemical properties of bilirubin oxidase (BOx) – a member of the MCO family. The experimental results strongly indicate that under certain conditions, e.g. in alkaline solutions, the IET can be the rate‐limiting step in the BOx catalytic cycle. The data also suggest that one of the catalytically relevant intermediates (most likely characterized by an intermediate oxidation state of the TNC) formed during the catalytic cycle of BOx has a redox potential close to 0.4 V, indicating an uphill IET process from the T1 copper site (0.7 V) to the Cu‐T23. These suggestions are supported by calculations of the IET rate, based on the experimentally observed Gibbs free energy change and theoretical estimates of reorganization energy obtained by combined quantum and molecular mechanical (QM/MM) calculations.     

Tetrathiafulvalene–Oligo(para‐phenyleneethynylene) Conjugates: Formation of Multiple Mixed‐Valence Complexes upon Electrochemical Oxidation

Short monodisperse oligo‐ (para‐phenyleneethynylene) (pOPE) units bearing laterally attached tetrathio‐substituted tetrathiofulvalene (TTF) units have been synthesised from functionalised aromatic building blocks by using the Sonogashira cross‐coupling methodology. The unusual redox properties of these TTF–pOPE conjugates were observed by employing electrochemical methods, such as cyclic voltammetry and exhaustive electrolysis. We found that formally one half of the TTF units in the pOPE monomer 1 , dimer 2 , and trimer 3 (with 2, 4, and 6 TTF units, respectively) are electrochemically silent during the first‐step oxidation at 0.49 V. We propose the formation of persistent mixed‐valence complexes from the TTF and TTF^+. units present in an equal ratio. Such mixed‐valence dyads (single or multiple in the partially oxidised 1 – 3 ) exhibit an unusual stability towards oxidation until the potential of the second oxidation at 0.84 V is achieved. This finding suggests that below this potential the oxidation of the respective mix‐valence complexes is extremely slow.     

Linquats: Synthesis,Characterization, and Properties of Linear Extended Diquats

We report an innovative synthetic route to linear extended diquats (linquats). Our approach is short and efficient and features a highly modular reaction sequence based on two‐fold quaternization followed by the key intramolecular [2+2+2] alkyne cycloaddition. The physico‐chemical properties of four new linquats were characterized by spectroscopic methods, X‐ray crystallography, and electrochemistry complemented by information obtained from DFT calculations. Electron deficient N‐heteroaromatic cations with linear extended diquat motif with high electron affinities have been recently recognized as attractive n‐type semiconductors for chemical and biological sensing. Their advantageous redox properties such as very fast reversible electron transfers make the title compounds interesting for applications.     

Spectroelectrochemical determination of the electron consumption

Measurements of UV-vis spectra as a function of the consumed charge in an optically transparent thin layer cell (OTTLE) offer a fast, small-volume and reliable method for the determination of the number of electrons n exchanged in the redox reaction. The determination of n is based on the calibration procedure using the ferrocene oxidation and the methyl viologen reduction. Virtues of this method are demonstrated on the determination of n of two oligomers of 'extended viologens' with multiple redox centers and correspondingly large n. The described method eliminates problems of side reactions of reactive intermediates with the parent oxidized form, which often impede a reliable estimation of n by the established coulometric method.     

Sono-extraction as a pretreatment approach for the screening evaluation of element mobility of sediment samples

Application of economically important and time saving pretreatment for the screening element mobility evaluation of contaminated sediments is presented. Ultrasonically-assisted single-step extraction (USAE) was carried out by EDTA solution. The extraction time of USAE was optimized and obtained results were compared with results estimated by conventional (EDTA extraction) and by sequential extraction (modified BCR protocol). The original three step BCR protocol was modified by addition of the first step (water leaching) and the fifth step, total digestion of sediment residue (acid mixture with HF). Zn, Cu and Pb have been determined in extracts by ICP-OES. Good conformity of the ultrasonically-extracted element contents and sum of contents, extracted during first three steps (water-soluble, acid-extractable, reducible — i.e., the most mobile fractions) of sequential extraction, was found. The sono-extraction reduced operating time of the first three steps of sequential extraction from 48 h to 15 min. Thus, USAE can serve as a rapid screening assessment of the mobile and potentially mobile element portions in sediments and other similar solid state environmental media. Analytical quality control was realized by comparison of the sums of element contents obtained at individual (five) extraction steps. Total element contents were also determined by an independent method (XRF).     

The Use of Cobalt‐Mediated Cycloisomerisation of Ynedinitriles in the Synthesis of Pyridazinohelicenes

A cobalt‐mediated [2+2+2] cycloisomerisation of ynedinitriles to helical pyridazines in good to high yields was developed. The construction of the pyridazine nucleus from one alkyne and two nitrile units is proposed to follow either a conventional organometallic mechanism or to be triggered by a single‐electron transfer from a Co^II species. Various [5]‐, [6]‐ and [7]helicene pyridazines were prepared.