Ligand spheres in asymmetric hetero Diels-Alder reactions catalyzed by Cu(II) box complexes: experiment and modeling

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The stereoselective hetero Diels–Alder reaction between ethyl glyoxylate and cyclohexadiene catalyzed by [Cu(II)t-Bu-(box)](OTf)2 was investigated. The reaction was performed step-by-step and the geometry of the Cu(II) complexes formed in the course of the catalysis was analysed by EPR spectroscopy, advanced pulsed EPR methods (ENDOR, and HYSCORE) and DFT calculations. Our results show that one triflate counterion is directly coordinated to Cu(II) during the catalytic process (axial position). This leads to penta-coordinated Cu(II) complexes. Solvent molecules are able to alter the geometry of the Cu(II) complexes although their coordination is weak. These findings provide an explanation for the solvent and counterion effects observed in many catalytic reactions

ESR and ENDOR study on the radical ions of two non—alternant hydrocarbons: 1,3,5,7-tetra-tert-butyl-s-indacene and 2,7-di-tert-butyldicyclopenta [a,e] cyclooctene

The radical anions and radical cations of two alkyl-substituted non-alternant hydrocarbons, 1,3,5,7-tetra-tert-butyl-s-indacene and 2,7-di-tert-butyldicyclopenta [a,e] cyclooctene, were characterized by their proton coupling constants with the use of ESR and, in part, ENDOR spectroscopy. Considering the unusual electronic structures of the π-systems in question, these values agree fairly well with those predicted by simple MO theory. Also reported are the proton hyperfine data for the radical ions of the likewise alkyl-substituted non-alternant 8,16-diisopropyl-s-indaceno [1,2,3-cd:5,6,7-c′ d′] diphenalene

Radical Ions in the Pentalene Series. Part III. Three Paramagnetic Redox Stages of a Dicyclopenta[a,e]pentalene

Five redox stages have been observed for the recently synthesized 1,3,5,7-tetra(tert-butyl) derivative 1 of the dicyclopenta[a,e]pentalene, a novel non-alternant hydrocarbon: the radical cation 1 , the neutral compound 1, the radical anion Is, the dianion 1²-, and the radical trianion 1³. Information about the electronic structure of the three paramagnetic stages, 1, 1⁻, and 1³⁻ is provided by the use of ESR, ENDOR, and TRIPLE resonance spectroscopy. The unpaired electron in the trianion resides mainly on the ‘inner’ butadiene-π-system, whereas in the cation and the anion, it is largely localized on the two ‘outer’ five-membered rings

Biologically Relevant Small Radicals

Biologically relevant small radicals are at the focus of the working group 4 (WG4) of the COST Action CM0603 (Free Radicals in Chemical Biology, CHEMBIORADICAL). This article surveys the areas of research being undertaken by the partners in WG4. The character of the radicals is described together with experimental techniques utilized to follow their structure and reactivity. Specifically, C-, S-, N- and O-centered radicals of small size, and their interaction with different biomolecules are described. Processes at the molecular level exemplifying important biological signaling and damaging pathways are introduced

Hydrogen Abstraction from the C15 Position of the Cholesterol Skeleton

[EN] Cholesterol (Ch) is an integral part of cell membrane, where it is prone to oxidation. In humans, oxidation of Ch is commonly linked to various pathologies like Alzheimer's disease, atherosclerosis, and even cancer, which proceed via mechanisms involving enzymatic and free radical pathways. The latter begin with hydrogen abstraction (HA) from Ch by a reactive free radical. It has been established that the most efficient HA from Ch occurs at C7, although HA from C4 by peroxyl radicals has recently been observed. Conversely, HA from Ch positions other than the thermodynamically preferred C7 or C4 has never been reported. We have designed a Ch derivative where a benzophenone moiety is linked to C7 by a covalent bond. This mirrors a specific orientation of Ch within a confined environment. Product analysis and time-resolved spectroscopic studies reveal an unprecedented HA from C15, which is a thermodynamically unfavorable position. This indicates that a specific topology of reactants is crucial for the reactivity of Ch. The relative orientation of the reactants can also be relevant in biological membranes, where Ch, polyunsaturated fatty acids, and numerous oxidizing species are confined in highly restricted and anisotropic environments.This work was supported by the Carlos III Institute of Health (Grants No. PII6/01877, "Miguel Servet fellowship" CPII16/00052 to I.A.), and by the Generalitat Valenciana (Prometeo 2017/075). We would like to thank Dr Fedora Grande for sending an exchange student (M.B.). D.N. and G.G. thank NAWI Graz for support.Palumbo, F.; Andreu Ros, MI.; Brunetti, M.; Schmallegger, M.; Gescheidt, G.; Neshchadin, D.; Miranda Alonso, MÁ. (2019). Hydrogen Abstraction from the C15 Position of the Cholesterol Skeleton. The Journal of Organic Chemistry. 84(23):15184-15191. https://doi.org/10.1021/acs.joc.9b02181S15184151918423Zerbinati, C., & Iuliano, L. (2017). 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Bis(acyl)phosphine Oxides as Stoichiometric Photo-Reductants for Copper Nanoparticle Synthesis: Efficiency and Kinetics

Bis(acyl)phospine oxide (BAPO) photo-initiators show excellent properties in photo-induced radical polymerization. Furthermore, in aqueous or alcoholic solutions, the primary phosphorus-centred radical forms a solvent adduct, which acts as a reductant for metal cations. Here, we present the mechanism, stoichiometry and kinetics of this reduction process. Moreover, we address the controlled production of copper nanoparticles. This is accomplished using optical spectroscopy in conjunction with in-depth kinetic analysis. This study opens the door for the application of BAPOs as a light-activated reducing agents, allowing a facile, mild and controlled generation of metal nanoparticles and of metal/polymer nanocomposites.ISSN:2367-093

Holistic approach to chemical degradation of Nafion membranes in fuel cells

The state of health of polyfluorinated sulfonic-acid ionomer membranes (e.g. Nafion) in low-temperature proton exchange membrane fuel cells (LT-PEMFCs) is negatively influenced by degradation phenomena occurring during their operation. As a consequence, the performance and durability of the membrane are decreased. In this article, we focus on simulating and predicting chemical membrane degradation phenomena using a holistic zero-dimensional kinetic framework. The knowledge of chemical degradation mechanisms is widely spread. We have collected and evaluated an extensive set of chemical mechanisms to achieve a holistic approach. This yields a set of 23 coupled chemical equations, which provide the whole cause and effect chain of chemical degradation in LT-PEMFCs (based on the Fenton reaction between Fe [sup] 2+ and H [sub] 2O [sub] 2 via the attack of hydroxyl radicals on the membrane, loss of ionomer moieties and emission of fluoride). Our kinetic framework allows the reproduction of experimentally accessible data such as fluoride emission rates and concentrations of ionomer moieties (from both in situ and ex situ tests). We present an approach, which allows estimations of the membrane lifetime based on fluoride emission rates. In addition, we outline the demetallation of Fe-N-C catalysts as a source of additional harmful iron species, which accelerate chemical membrane degradation. To demonstrate the expandability and versatility of the kinetic framework, a set of five chemical equations describing the radical scavenging properties of cerium agents is coupled to the main framework and its influence on membrane degradation is analysed. An automated solving routine for the system of coupled chemical equations on the basis of the chemical kinetic simulation tool COPASI has been developed and is freely accessible online (http://ptc-pc-139.tugraz.at/ cgi-bin/Membrane_Degradation/)

Topological Dynamics of a Radical Ion Pair: Experimental and Computational Assessment at the Relevant Nanosecond Timescale

International audienceChemical processes mostly happen in fluid environments where reaction partners encounter via diffusion. The bimolecular encounters take place at a nanosecond time scale. The chemical environment (e.g., solvent molecules, (counter)ions) has a decisive influence on the reactivity as it determines the contact time between two molecules and affects the energetics. For understanding reactivity at an atomic level and at the appropriate dynamic time scale, it is crucial to combine matching experimental and theoretical data. Here, we have utilized all-atom molecular-dynamics simulations for accessing the key time scale (nanoseconds) using a QM/MM-Hamiltonian. Ion pairs consisting of a radical ion and its counterion are ideal systems to assess the theoretical predictions because they reflect dynamics at an appropriate time scale when studied by temperature-dependent EPR spectroscopy. We have investigated a diketone radical anion with its tetra-ethylammonium counterion. We have established a funnel-like transition path connecting two (equivalent) complexation sites. The agreement between the molecular-dynamics simulation and the experimental data presents a new paradigm for ion–ion interactions. This study exemplarily demonstrates the impact of the molecular environment on the topological states of reaction intermediates and how these states can be consistently elucidated through the combination of theory and experiment. We anticipate that our findings will contribute to the prediction of bimolecular transformations in the condensed phase with relevance to chemical synthesis, polymers, and biological activit