Browsing by Subject "Reaction mechanism"

Now showing 1 - 3 of 3
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    5-Alkenylthiazoles as In-Out Dienes in Polar [4+2] Cycloaddition Reactions
    (Wiley, 2013-09-25) Alajarín, Mateo; Cabrera, Jose; Sanchez-Andrada, Pilar; Bautista, Delia; Pastor Vivero, Aurelia; Química Orgánica
    5-Alkenyl-2-aminothiazoles react as in-out dienes with a wide range of electron-poor dienophiles leading to the corresponding cycloaddition products in good to excellent yields. The [4+2] cycloadditions of 5-alkenyl-2-aminothiazoles can be classified as site-selective since only the diene moiety including the formal C-C double bond of the heterocycle and that of the side-chain is involved. Calculations of the HOMO energy values of representative 5-alkenyl-2-aminothiazoles are disclosed The cycloadditions are endo-selective with N-phenylmaleimide or maleic anhydride and regioselective when the reactions are conducted with non symmetrical dienophiles. Completely oxidized cycloadducts are obtained in the reactions of 5-alkenyl-2-aminothiazoles with naphthoquinone or DMAD. Unexpectedly, the reactions with PTAD are not stereospecific. A mechanism placed at the concerted/stepwise boundary is proposed.
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    Nuances of the voltammetry of homogeneous multi-electron molecular catalysts: an analytical theory for two-electron catalysis
    (Elsevier, 2022-03) Laborda, Eduardo; González Sánchez, Joaquín; Molina, Angela; Química Física
    A wide range of cases of homogeneous two-electron molecular catalysts of electrochemical reactions, as a paradigm of multi-electron catalysis, are investigated in voltammetry through a novel and rigorous analytical theory. Simple expressions are deduced for the current-potential response at static and rotating macroelectrodes in all possible situations in terms of catalytically active pathway(s), rate constant(s) and formal potentials of the electron transfers. Criteria and procedures have been proposed to identify the true catalytic states and pathways. The results reveal fundamental differences in the voltammetry of multi-electron catalysts with respect to that of mono-electron ones (EC’ mechanism), specifically with regard to the impact of mass transport on the features of the signal and on the accessibility to pure kinetic conditions.
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    Square Wave Voltcoulommetry of two-electron molecular electrocatalytic processes with adsorbed species. Application to the surface O2 reduction in acetonitrile at anthraquinone-modified glassy carbon electrodes
    (Elsevier, 2023-03-10) Hernández Tovar, José Víctor; López Tenés, Manuela; Gonzalez, Joaquín; Química Física
    An adequate analysis of the performance and efficiency of a given multi-redox molecular electrocatalyst versus a particular substrate of interest requires the knowledge of the overall reaction pathway and the determination of the rate constants of the catalytic steps taking place. For this purpose, a theoretical model is presented for the Square Wave Voltcoulommetry response of surface-confined two-electron catalysts, that is, molecular species with two different redox states that can act in different electrocatalytic forms as a function of the applied potential. The rigorous theoretical model incorporates convective mass transport effects and non-idealities in the electrochemical behaviour of the immobilized redox probes. The results obtained have been applied to the study of the surface oxygen reduction in an aprotic medium (acetonitrile) at an anthraquinone-modified glassy carbon electrode. The charge-potential responses indicate that the reaction pathway involves two different catalytic routes due to the two reduction products of the anthraquinone moiety, that is, the semiquinone anion radical and the dianion. A Koutecky-Levich approach, combined with the fittings of the experimental responses to the theoretical expressions deduced here, has allowed us to obtain the values of the different thermodynamic (formal potential, interaction parameters, surface coverage) and kinetic (rate constant) parameters of the process.