Mechanistic Elucidation of the Stepwise Formation of a Tetranuclear Manganese Pinned Butterfly Cluster via N–N Bond Cleavage, Hydrogen Atom Transfer, and Cluster Rearrangement

2014 ◽  
Vol 136 (52) ◽  
pp. 17974-17986 ◽  
Author(s):  
Clifton R. Hamilton ◽  
Michael R. Gau ◽  
Regina A. Baglia ◽  
Sean F. McWilliams ◽  
Michael J. Zdilla
Keyword(s):  
Hydrogen Atom ◽  
Bond Cleavage ◽  
Hydrogen Atom Transfer ◽  
Atom Transfer ◽  
Butterfly Cluster ◽  
Stepwise Formation

scholarly journals Breaking C–C Bonds via Electrochemically Mediated Hydrogen Atom Transfer Reactions

2021 ◽  
Author(s):  
Bing Yan ◽  
Changxia Shi ◽  
Gregg T. Beckham ◽  
Eugene Y.-X. Chen ◽  
Yuriy Román-Leshkov
Keyword(s):  
Hydrogen Atom ◽  
Reaction Pathway ◽  
Bond Cleavage ◽  
Ambient Pressure ◽  
Hydrogen Atom Transfer ◽  
Oxidation Potential ◽  
Side Reactions ◽  
Atom Transfer ◽  
Direct Oxidation ◽  
Mediated Oxidation

Cleaving inert sp<sup>3</sup>-sp<sup>3</sup> carbon-carbon (C-C) bonds selectively remains a major challenge in organic chemistry and a main bottleneck in the chemical upcycling of recalcitrant polyolefin waste. Here, we present an electrochemical strategy using redox mediators to activate and break C-C bonds at room temperature and ambient pressure. Specifically, we use <i>N</i>-hydroxyphthalimide (NHPI) as a redox mediator that undergoes electrochemical oxidation to form the <a>phthalimide-<i>N</i>-oxyl (PINO) radical </a>to initiate hydrogen atom transfer (HAT) reactions with benzylic C-H bonds. The resulting benzylic carbon radical is readily captured by molecular oxygen to form a peroxy radical that decomposes into oxygenated C-C bond-scission fragments. This indirect, mediated approach for C<sub>sp3</sub>-C<sub>sp3</sub> bond cleavage reduces the oxidation potential by > 1.2 V compared to the direct oxidation of the substrate, thereby eliminating deleterious side reactions, such as solvent oxidation, that may occur at high potentials. Studies with a bibenzyl model compound revealed a bifurcated reaction pathway following the initial HAT step. At a bibenzyl conversion of 61.0%, the C-C bond cleavage pathway generates benzaldehyde and benzoic acid products at 38.4% selectivity, and the C-H bond oxygenation pathway leads to 1,2-diphenylethanone and benzil products at 39.2% selectivity. Changes in reaction selectivity were investigated with various model compounds, including bibenzyl, 1,3-diphenylpropane, 1,4-diphenylbutane, and their derivatives. Product selectivity is correlated with the C-C bond strength of the reactant, with weaker C-C bonds favoring the C-C bond cleavage pathway. We also evaluated the mediated oxidation of oligomeric styrene (<i>M</i><sub>n</sub> = 510 Da, OS<sub>510</sub>) which were converted into oxygenated products. Lastly, proof-of-concept depolymerization of polystyrene (PS, ~10,000 Da) into oxygenated monomers, dimers, and oligomers was demonstrated using NHPI-mediated oxidation.

scholarly journals Reactivity of hydropersulfides toward the hydroxyl radical unraveled: disulfide bond cleavage, hydrogen atom transfer, and proton-coupled electron transfer

2018 ◽  
Vol 20 (7) ◽  
pp. 4793-4804 ◽  
Author(s):  
Josep M. Anglada ◽  
Ramon Crehuet ◽  
Sarju Adhikari ◽  
Joseph S. Francisco ◽  
Yu Xia
Keyword(s):  
Electron Transfer ◽  
Hydrogen Atom ◽  
Hydroxyl Radical ◽  
Disulfide Bond ◽  
Bond Cleavage ◽  
Hydrogen Abstraction ◽  
Hydrogen Atom Transfer ◽  
Atom Transfer ◽  
Oh Radical ◽  
Proton Coupled Electron Transfer

Hydropersulfides (RSSH) are highly reactive towards OH radical, and depending on the nature of R substitute, a selective OH substitution with S–S bond cleavage competes with the hydrogen abstraction by the radical.

scholarly journals Breaking C–C Bonds via Electrochemically Mediated Hydrogen Atom Transfer Reactions

2021 ◽  
Author(s):  
Bing Yan ◽  
Changxia Shi ◽  
Gregg T. Beckham ◽  
Eugene Y.-X. Chen ◽  
Yuriy Román-Leshkov
Keyword(s):  
Hydrogen Atom ◽  
Reaction Pathway ◽  
Bond Cleavage ◽  
Ambient Pressure ◽  
Hydrogen Atom Transfer ◽  
Oxidation Potential ◽  
Side Reactions ◽  
Atom Transfer ◽  
Direct Oxidation ◽  
Mediated Oxidation

Cleaving inert sp<sup>3</sup>-sp<sup>3</sup> carbon-carbon (C-C) bonds selectively remains a major challenge in organic chemistry and a main bottleneck in the chemical upcycling of recalcitrant polyolefin waste. Here, we present an electrochemical strategy using redox mediators to activate and break C-C bonds at room temperature and ambient pressure. Specifically, we use <i>N</i>-hydroxyphthalimide (NHPI) as a redox mediator that undergoes electrochemical oxidation to form the <a>phthalimide-<i>N</i>-oxyl (PINO) radical </a>to initiate hydrogen atom transfer (HAT) reactions with benzylic C-H bonds. The resulting benzylic carbon radical is readily captured by molecular oxygen to form a peroxy radical that decomposes into oxygenated C-C bond-scission fragments. This indirect, mediated approach for C<sub>sp3</sub>-C<sub>sp3</sub> bond cleavage reduces the oxidation potential by > 1.2 V compared to the direct oxidation of the substrate, thereby eliminating deleterious side reactions, such as solvent oxidation, that may occur at high potentials. Studies with a bibenzyl model compound revealed a bifurcated reaction pathway following the initial HAT step. At a bibenzyl conversion of 61.0%, the C-C bond cleavage pathway generates benzaldehyde and benzoic acid products at 38.4% selectivity, and the C-H bond oxygenation pathway leads to 1,2-diphenylethanone and benzil products at 39.2% selectivity. Changes in reaction selectivity were investigated with various model compounds, including bibenzyl, 1,3-diphenylpropane, 1,4-diphenylbutane, and their derivatives. Product selectivity is correlated with the C-C bond strength of the reactant, with weaker C-C bonds favoring the C-C bond cleavage pathway. We also evaluated the mediated oxidation of oligomeric styrene (<i>M</i><sub>n</sub> = 510 Da, OS<sub>510</sub>) which were converted into oxygenated products. Lastly, proof-of-concept depolymerization of polystyrene (PS, ~10,000 Da) into oxygenated monomers, dimers, and oligomers was demonstrated using NHPI-mediated oxidation.

Direct Cyclization of Alkenyl Thioester via Transition Metal‐hydrogen Atom Transfer/radical‐polar Crossover Mechanism

2019 ◽  
Author(s):  
Shiori Date ◽  
Kensei Hamasaki ◽  
Karen Sunagawa ◽  
Hiroki Koyama ◽  
Chikayoshi Sebe ◽  
...  
Keyword(s):  
Natural Products ◽  
Hydrogen Atom ◽  
Transition Metal ◽  
Hydrogen Atom Transfer ◽  
Synthetic Method ◽  
Atom Transfer ◽  
Reaction Conditions ◽  
Hydride Hydrogen ◽  
Sulfur Heterocycles ◽  
One Step

<div>We report here a catalytic, Markovnikov selective, and scalable synthetic method for the synthesis of saturated sulfur heterocycles, which are found in the structures of pharmaceuticals and natural products, in one step from an alkenyl thioester. Unlike a potentially labile alkenyl thiol, an alkenyl thioester is stable and easy to prepare. The powerful Co catalysis via a cobalt hydride hydrogen atom transfer and radical-polar crossover mechanism enabled simultaneous cyclization and deprotection. The substrate scope was expanded by the extensive optimization of the reaction conditions and tuning of the thioester unit.</div>

Catalytic Synthesis of Cyclic Guanidines via Hydrogen Atom Transfer and Radical-Polar Crossover

2020 ◽  
Author(s):  
Shunya Ohuchi ◽  
Hiroki Koyama ◽  
Hiroki Shigehisa
Keyword(s):  
Hydrogen Atom ◽  
Functional Group ◽  
Hydrogen Atom Transfer ◽  
Catalytic Synthesis ◽  
Membered Ring ◽  
Biologically Active ◽  
Atom Transfer ◽  
Powerful Method ◽  
Protective Groups ◽  
The Common

A catalytic synthesis of cyclic guanidines, which are found in many biologically active compounds and natu-ral products, was developed, wherein transition-metal hydrogen atom transfer and radical-polar crossover were employed. This mild and functional-group tolerant process enabled the cyclization of alkenyl guanidines bearing common protective groups, such as Cbz and Boc. This powerful method not only provided the common 5- and 6-membered rings but also an unusual 7-membered ring. The derivatization of the products afforded various heterocycles. We also investigated the se-lective cyclization of mono-protected or hetero-protected (TFA and Boc) alkenyl guanidines and their further derivatiza-tions.

Sulfamides Direct Radical-Mediated Chlorination of Aliphatic C–H Bonds

2019 ◽  
Author(s):  
Melanie Short ◽  
Mina Shehata ◽  
Matthew Sanders ◽  
Jennifer Roizen
Keyword(s):  
Hydrogen Atom ◽  
Transfer Reaction ◽  
Hydrogen Atom Transfer ◽  
Chain Propagation ◽  
Propagation Mechanism ◽  
Atom Transfer ◽  
Radical Chain ◽  
Radical Intermediates ◽  
Transfer Processes ◽  
Unusual Position

Sulfamides guide intermolecular chlorine transfer to gamma-C(sp<sup>3</sup>) centers. This unusual position-selectivity arises because accessed sulfamidyl radical intermediates engage in otherwise rare 1,6-hydrogen-atom transfer processes. The disclosed chlorine-transfer reaction relies on a light-initiated radical chain-propagation mechanism to oxidize C(sp<sup>3</sup>)-H bonds.

Sulfamides Direct Radical-Mediated Chlorination of Aliphatic C–H Bonds

2019 ◽  
Author(s):  
Melanie Short ◽  
Mina Shehata ◽  
Matthew Sanders ◽  
Jennifer Roizen
Keyword(s):  
Hydrogen Atom ◽  
Transfer Reaction ◽  
Hydrogen Atom Transfer ◽  
Chain Propagation ◽  
Propagation Mechanism ◽  
Atom Transfer ◽  
Radical Chain ◽  
Radical Intermediates ◽  
Transfer Processes ◽  
Unusual Position

Sulfamides guide intermolecular chlorine transfer to gamma-C(sp<sup>3</sup>) centers. This unusual position-selectivity arises because accessed sulfamidyl radical intermediates engage in otherwise rare 1,6-hydrogen-atom transfer processes. The disclosed chlorine-transfer reaction relies on a light-initiated radical chain-propagation mechanism to oxidize C(sp<sup>3</sup>)-H bonds.

Hydrogen Atom Transfer Reaction Free Energy as a Predictor of Abiotic Nitroaromatic Reduction Rate Constants: A Comprehensive Analysis

2019 ◽  
Author(s):  
Dominic Di Toro ◽  
Kevin P. Hickey ◽  
Herbert E. Allen ◽  
Richard F. Carbonaro ◽  
Pei C. Chiu
Keyword(s):  
Free Energy ◽  
Hydrogen Atom ◽  
Hydrogen Atom Transfer ◽  
Energy Model ◽  
Second Order ◽  
Transfer Model ◽  
Atom Transfer ◽  
Order Rate ◽  
Linear Free Energy ◽  
Free Energy Model

<div>A linear free energy model is presented that predicts the second order rate constant for the abiotic reduction of nitroaromatic compounds (NACs). For this situation previously presented models use the one electron reduction potential of the NAC reaction. If such value is not available, it has been has been proposed that it could be computed directly or estimated from the electron affinity (EA). The model proposed herein uses the Gibbs free energy of the hydrogen atom transfer (HAT) as the parameter in the linear free energy model. Both models employ quantum chemical computations for the required thermodynamic parameters. The available and proposed models are compared using second order rate constants obtained from five investigations reported in the literature in which a variety of NACs were exposed to a variety of reductants. A comprehensive analysis utilizing all the NACs and reductants demonstrate that the computed hydrogen atom transfer model and the experimental one electron reduction potential model have similar root mean square errors and residual error probability distributions. In contrast, the model using the computed electron affinity has a more variable residual error distribution with a significant number of outliers. The results suggest that a linear free energy model utilizing computed hydrogen transfer reaction free energy produces a more reliable prediction of the NAC abiotic reduction second order rate constant than previously available methods. The advantages of the proposed hydrogen atom transfer model and its mechanistic implications are discussed as well.</div>

Catalyst- and Silane- Controlled Enantioselective Hydrofunctionalization of Alkenes by Cobalt-Catalyzed Hydrogen Atom Transfer and Radical-Polar Crossover

2020 ◽  
Author(s):  
Kousuke Ebisawa ◽  
Kana Izumi ◽  
Yuka Ooka ◽  
Hiroaki Kato ◽  
Sayori Kanazawa ◽  
...  
Keyword(s):  
Hydrogen Atom ◽  
Absolute Configuration ◽  
Kinetic Resolution ◽  
Hydrogen Atom Transfer ◽  
Biologically Active ◽  
Atom Transfer ◽  
High Concentration ◽  
Chain Reaction ◽  
Radical Chain ◽  
Radical Chain Reaction

Catalytic enantioselective synthesis of tetrahydrofurans, which are found in the structures of many biologically active natural products, via a transition-metal catalyzed-hydrogen atom transfer (TM-HAT) and radical-polar crossover (RPC) mechanism is described herein. Hydroalkoxylation of non-conjugated alkenes proceeded efficiently with excellent enantioselectivity (up to 94% ee) using a suitable chiral cobalt catalyst, <i>N</i>-fluoro-2,4,6-collidinium tetrafluoroborate, and diethylsilane. Surprisingly, absolute configuration of the product was highly dependent on the steric hindrance of the silane. Slow addition of the silane, the dioxygen effect in the solvent, thermal dependency, and DFT calculation results supported the unprecedented scenario of two competing selective mechanisms. For the less-hindered diethylsilane, a high concentration of diffused carbon-centered radicals invoked diastereoenrichment of an alkylcobalt(III) intermediate by a radical chain reaction, which eventually determined the absolute configuration of the product. On the other hand, a more hindered silane resulted in less opportunity for radical chain reaction, instead facilitating enantioselective kinetic resolution during the late-stage nucleophilic displacement of the alkylcobalt(IV) intermediate.

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