10 research outputs found

    Proton-Coupled Electron Transfer in the Reduction of Arenes by SmI<sub>2</sub>–Water Complexes

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    The presence of water has a significant impact on the reduction of substrates by SmI<sub>2</sub>. The reactivity of the Sm­(II)-water reducing system and the relationship between sequential or concerted electron-transfer, proton-transfer is not well understood. In this work, we demonstrate that the reduction of an arene by SmI<sub>2</sub>-water proceeds through an initial proton-coupled electron transfer. The use of thermochemical data available in the literature shows that upon coordination of water to Sm­(II) in THF, significant weakening of the O–H bond occurs. The derived value of nearly 73 kcal/mol for the decrease in the bond dissociation energy of the O–H bond in the Sm­(II)–water complex is the largest reported to date for low-valent reductants containing bound water

    Correction to “Uncovering the Mechanism of the Ag(I)/Persulfate-Catalyzed Cross-Coupling Reaction of Arylboronic Acids and Heteroarenes”

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    Correction to “Uncovering the Mechanism of the Ag(I)/Persulfate-Catalyzed Cross-Coupling Reaction of Arylboronic Acids and Heteroarenes

    Mechanistic Study of Silver-Catalyzed Decarboxylative Fluorination

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    The silver-catalyzed fluorination of aliphatic carboxylic acids by Selectfluor in acetone/water provides access to fluorinated compounds under mild and straightforward reaction conditions. Although this reaction provides efficient access to fluorinated alkanes from a pool of starting materials that are ubiquitous in nature, little is known about the details of the reaction mechanism. We report spectroscopic and kinetic studies on the role of the individual reaction components in decarboxylative fluorination. The studies presented herein provide evidence that Ag­(II) is the intermediate oxidant in the reaction. In the rate-limiting step of the reaction, Ag­(I)-carboxylate is oxidized to Ag­(II) by Selectfluor. Substrate inhibition of the process occurs through the formation of a silver-carboxylate. Water is critical for solubilizing reaction components and ligates to Ag­(I) under the reaction conditions. The use of donor ligands on Ag­(I) provides evidence of oxidation to Ag­(II) by Selectfluor. The use of sodium persulfate as an additive in the reaction as well as NFSI as a fluorine source further supports the generation of a Ag­(II) intermediate; this data will enable the development of a more efficient set of reaction conditions for the fluorination

    Uncovering the Mechanism of the Ag(I)/Persulfate-Catalyzed Cross-Coupling Reaction of Arylboronic Acids and Heteroarenes

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    The catalytic cross-coupling of arylboronic acids with pyridines through single-electron oxidation provides efficient access to substituted heterocycles. Despite the importance of this reaction, very little is known about its mechanism, and as a consequence, it is unclear whether the full scope of the transformation has been realized. Here we present kinetic and spectroscopic evidence showing a high degree of complexity in the reaction system. The mechanism derived from these studies shows the activation of Ag­(I) for reduction of persulfate and an off-cycle protodeboronation by the pyridine substrate. These results provide key mechanistic insights that enable control of the off-cycle process, thus providing higher efficiency and yield

    Solvent-Dependent Substrate Reduction by {Sm[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub>(THF)<sub>2</sub>}. An Alternative Approach for Accelerating the Rate of Substrate Reduction by Sm(II)

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    The impact of solvent on electron transfer from Sm­(II) to substrates was measured by determining the rate of reduction of 1-bromo-, 1-chlorododecane, and 3-pentanone in THF and hexanes using the highly soluble reductant {Sm­[N­(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub>(THF)<sub>2</sub>}. Rates were found to be 3 orders of magnitude faster in hexanes than THF, and reductions of alkyl halides were inverse first order in THF. These findings show the solvent milieu significantly impacts the rate of substrate reduction, a consideration that may prove useful in synthesis

    Proton-Coupled Electron Transfer in the Reduction of Carbonyls by Samarium Diiodide–Water Complexes

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    Reduction of carbonyls by SmI<sub>2</sub> is significantly impacted by the presence of water, but the fundamental step(s) of initial transfer of a formal hydrogen atom from the SmI<sub>2</sub>–water reagent system to produce an intermediate radical is not fully understood. In this work, we provide evidence consistent with the reduction of carbonyls by SmI<sub>2</sub>–water proceeding through proton-coupled electron transfer (PCET). Combined rate and computational studies show that a model aldehyde and ketone are likely reduced through an asynchronous PCET, whereas reduction of a representative lactone occurs through a concerted PCET. In the latter case, concerted PCET is likely a consequence of significantly endergonic initial electron transfer

    Reversibility of Ketone Reduction by SmI<sub>2</sub>–Water and Formation of Organosamarium Intermediates

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    The reduction of ketones by SmI<sub>2</sub>–water has long been thought to proceed through a reversible initial electron transfer with the formation of organosamarium intermediates in a follow-up step. Kinetic experiments on the reduction of two model ketones and structurally similar ketones with a pendant alkene are shown to be consistent with a rate-limiting reduction by SmI<sub>2</sub>–water through a proton-coupled electron-transfer (PCET). Literature values for the rates of radical cyclizations and reduction of radicals by SmI<sub>2</sub> and thermochemical data for radical reduction by SmI<sub>2</sub>–water further support a rate-limiting initial step for ketone reductions. These data suggest that discrete organosamarium species may not be intermediates in ketone reductions by SmI<sub>2</sub>–water

    Effect of Crown Ethers on the Ground and Excited State Reactivity of Samarium Diiodide in Acetonitrile

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    Electron transfer from the ground and excited states of Sm­[15-crown-5]<sub>2</sub>I<sub>2</sub> complex to a series of electron acceptors (benzaldehyde, acetophenone, benzophenone, nitrobenzene, benzyl bromide, benzyl chloride, 1-iodohexane, and 1,4-dinitrobenzene) was investigated in acetonitrile. Electron transfer from the ground state of the Sm­(II)-crown system to aldehydes and ketones has a significant inner sphere component indicating that the oxophilic nature of Sm­(II) prevails in the system even in the presence of bulky ligands such as 15-crown-5 ether. Activation parameters for the ground state electron transfer were determined, and the values were consistent with the proposed mechanistic models. Since crown ethers stabilize the photoexcited states of Sm­(II), the photochemistry of Sm­[15-crown-5]<sub>2</sub>I<sub>2</sub> system in solution state has been investigated in detail. The results suggest that photoinduced electron transfer from Sm­(II)-crown systems to a wide variety of substrates is feasible with rate constant values as high as 10<sup>7</sup> M<sup>–1</sup> s<sup>–1</sup>. The results described herein imply that the present difficulty of manipulating the extremely reactive excited state of Sm­(II) in solution phase can be overcome through stabilizing the excited state of the divalent metal ion by careful design of the ligand systems

    Secondary Amides as Hydrogen Atom Transfer Promoters for Reactions of Samarium Diiodide

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    Two secondary amides (<i>N</i>-methylacetamide and 2-pyrrolidinone) were used as additives with SmI<sub>2</sub> in THF to estimate the extent of N–H bond weakening upon coordination. Mechanistic and synthetic studies demonstrate significant bond-weakening, providing a reagent system capable of reducing a range of substrates through formal hydrogen atom transfer

    Substituent Effects and Supramolecular Interactions of Titanocene(III) Chloride: Implications for Catalysis in Single Electron Steps

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    The electrochemical properties of titanocene­(III) complexes and their stability in THF in the presence and absence of chloride additives were studied by cyclic voltammetry (CV) and computational methods. The anodic peak potentials of the titanocenes can be decreased by as much as 0.47 V through the addition of an electron-withdrawing substituent (CO<sub>2</sub>Me or CN) to the cyclopentadienyl ring when compared with Cp<sub>2</sub>TiCl. For the first time, it is demonstrated that under the conditions of catalytic applications low-valent titanocenes can decompose by loss of the substituted ligand. The recently discovered effect of stabilizing titanocene­(III) catalysts by chloride additives was analyzed by CV, kinetic, and computational studies. An unprecedented supramolecular interaction between [(C<sub>5</sub>H<sub>4</sub>R)<sub>2</sub>TiCl<sub>2</sub>]<sup>−</sup> and hydrochloride cations through reversible hydrogen bonding is proposed as a mechanism for the action of the additives. This study provides the critical information required for the rational design of titanocene-catalyzed reactions in single electron steps
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