scholarly journals Light-Driven Depolymerization of Native Lignin Enabled by Proton- Coupled Electron Transfer

2019 ◽  
Author(s):  
Suong Nguyen ◽  
Phillip Murray ◽  
Robert Knowles
Keyword(s):  
Electron Transfer ◽  
Visible Light ◽  
Bond Cleavage ◽  
Direct Conversion ◽  
Alkoxy Radical ◽  
Radical Intermediate ◽  
Polymer Backbone ◽  
Good Efficiency ◽  
Chemical Reagents ◽  
Proton Coupled Electron Transfer

<div><p>Here we report a catalytic, light-driven method for the redox-neutral depolymerization of native lignin biomass at ambient temperature. This transformation proceeds via a proton-coupled electron-transfer (PCET) activation of an alcohol O–H bond to generate a key alkoxy radical intermediate, which then drives the <i>β</i>-scission of a vicinal C–C bond. Notably, this depolymerization is driven solely by visible light irradiation, requiring no stoichiometric chemical reagents and producing no stoichiometric waste. This method exhibits good efficiency and excellent selectivity for the activation and fragmentation of <i>β</i>-O-4 linkages in the polymer backbone, even in the presence of numerous other PCET-active functional groups. DFT analysis suggests that the key C–C bond cleavage reactions produce non-equilibrium product distributions, driven by excited-state redox events. These results provide further evidence that visible-light photocatalysis can serve as a viable method for the direct conversion of lignin biomass into valuable arene feedstocks.</p></div>

scholarly journals Light-Driven Depolymerization of Native Lignin Enabled by Proton- Coupled Electron Transfer

2019 ◽  
Author(s):  
Suong Nguyen ◽  
Phillip Murray ◽  
Robert Knowles
Keyword(s):  
Electron Transfer ◽  
Visible Light ◽  
Bond Cleavage ◽  
Direct Conversion ◽  
Alkoxy Radical ◽  
Radical Intermediate ◽  
Polymer Backbone ◽  
Good Efficiency ◽  
Chemical Reagents ◽  
Proton Coupled Electron Transfer

<div><p>Here we report a catalytic, light-driven method for the redox-neutral depolymerization of native lignin biomass at ambient temperature. This transformation proceeds via a proton-coupled electron-transfer (PCET) activation of an alcohol O–H bond to generate a key alkoxy radical intermediate, which then drives the <i>β</i>-scission of a vicinal C–C bond. Notably, this depolymerization is driven solely by visible light irradiation, requiring no stoichiometric chemical reagents and producing no stoichiometric waste. This method exhibits good efficiency and excellent selectivity for the activation and fragmentation of <i>β</i>-O-4 linkages in the polymer backbone, even in the presence of numerous other PCET-active functional groups. DFT analysis suggests that the key C–C bond cleavage reactions produce non-equilibrium product distributions, driven by excited-state redox events. These results provide further evidence that visible-light photocatalysis can serve as a viable method for the direct conversion of lignin biomass into valuable arene feedstocks.</p></div>

scholarly journals Theoretical Study of C–H Bond Cleavage via Concerted Proton-Coupled Electron Transfer in Fluorenyl-Benzoates

2018 ◽  
Vol 140 (46) ◽  
pp. 15641-15645 ◽  
Author(s):  
Elvira R. Sayfutyarova ◽  
Zachary K. Goldsmith ◽  
Sharon Hammes-Schiffer
Keyword(s):  
Electron Transfer ◽  
Theoretical Study ◽  
Bond Cleavage ◽  
Proton Coupled Electron Transfer

Synthesis and Photocleavage of Quinoline Methyl Ethers: A Mild and Efficient Method for the Selective Protection and Deprotection of the Alcohol Functionality

2016 ◽  
Vol 69 (7) ◽  
pp. 763
Author(s):  
Anthony A. Provatas ◽  
Gary A. Epling ◽  
James D. Stuart
Keyword(s):  
Electron Transfer ◽  
Visible Light ◽  
Methyl Ether ◽  
Efficient Method ◽  
Substitution Reaction ◽  
Radical Scavenger ◽  
Single Electron Transfer ◽  
Radical Intermediate ◽  
Strong Base ◽  
Selective Protection

The synthesis and photocleavage of quinolinyl methyl ether-protected alcohols is reported in this study. A variety of quinoline methyl chlorides were synthesized, and protection of the various alcohols was performed via a substitution reaction in the presence of a strong base. Photocleavage of the quinolinyl methyl ether moiety proceeded under visible light with the formation of the charged quinolinyl radical intermediate through a single-electron transfer in the presence of a photosensitizer dye leading to the deprotected alcohol in excellent yields. The utility of triethylamine as a sacrificial reductant and d-sorbitol as a radical scavenger were also investigated in this study.

Asymmetric alkylation of remote C(sp3)–H bonds by combining proton-coupled electron transfer with chiral Lewis acid catalysis

2017 ◽  
Vol 53 (64) ◽  
pp. 8964-8967 ◽  
Author(s):  
Wei Yuan ◽  
Zijun Zhou ◽  
Lei Gong ◽  
Eric Meggers
Keyword(s):  
Electron Transfer ◽  
Hydrogen Atom ◽  
Visible Light ◽  
Lewis Acid ◽  
Acid Catalysis ◽  
Hydrogen Atom Transfer ◽  
Lewis Acid Catalysis ◽  
Asymmetric Alkylation ◽  
Proton Coupled Electron Transfer ◽  
Chiral Lewis Acid

The catalytic asymmetric alkylation of the remote, unactivated δ-position of N-alkyl amides was enabled by the combination of visible-light-induced proton-coupled electron transfer, 1,5-hydrogen atom transfer, and chiral Lewis acid catalysis.

scholarly journals Proton-Coupled Electron Transfer in the Reaction of 3,4-Dihydroxyphenylpyruvic Acid with Reactive Species in Various Media

2015 ◽  
Vol 2015 ◽  
pp. 1-13 ◽  
Author(s):  
J. J. Fifen ◽  
Z. Dhaouadi ◽  
M. Nsangou ◽  
O. Holtomo ◽  
N. Jaidane
Keyword(s):  
Electron Transfer ◽  
Proton Transfer ◽  
Activation Barrier ◽  
Bond Cleavage ◽  
Hydrogen Atom Transfer ◽  
Free Energies ◽  
Solvent Interactions ◽  
Proton Coupled Electron Transfer ◽  
Nonpolar Solvents ◽  
One Step

The distinction of concerted proton-coupled electron transfer (CPCET) from sequential one as well as proton transfer-electron transfer (PT-ET) from electron transfer-proton transfer (ET-PT) in the O–H bond cleavage reactions in various media has always been a difficult task. In this work, the activation barrier of the CPCET mechanism, its rate constants, and reaction free energies related to ET-PT and PT-ET involving coreactive species were presented as good parameters to attempt the problem. DFT calculations were carried out studying the described pathways subsequent to the scavenging of OH• and OBr- by the 3,4-DHPPA in various media. The solvation was described in a hybrid manner using IEF-PCM model conjointly with a model that takes into account some solute-solvent interactions. As a result, we found that the scavenging of hydroxyl radical by 3,4-DHPPA is thermodynamically governed by a one-step hydrogen atom transfer (CPCET) from the acid to the radical in all media. In kinetic viewpoint, CPCET still dominates in the vacuum and in nonpolar solvents, but in polar solvents it could compete strongly with the ET-PT mechanism so that the latter could slightly dominate.

scholarly journals Chemistry With N-Centered Radicals Generated by Single-Electron Transfer-Oxidation Using Photoredox Catalysis

CCS Chemistry ◽  
2019 ◽  
pp. 38-49 ◽  
Author(s):  
Heng Jiang ◽  
Armido Studer
Keyword(s):  
Electron Transfer ◽  
Recent Literature ◽  
Bond Cleavage ◽  
Photoredox Catalysis ◽  
Proton Coupled Electron Transfer ◽  
Cascade Processes ◽  
Reactive N ◽  
Radical Generation ◽  
Cleavage Reactions ◽  
Oxidative Generation

This review covers the recent literature on oxidative generation of N-centered radicals using photoredox catalysis. The concept of proton-coupled electron transfer is briefly discussed. Applications of such reactive N-centered radicals in cascade processes comprising arene amidation, alkene amidation, C—C bond cleavage reactions, and remote C—H functionalization are addressed. In addition, novel reagents allowing for clean oxidative N-radical generation are discussed.

scholarly journals Semiempirical method for examining asynchronicity in metal–oxido-mediated C–H bond activation

2021 ◽  
Vol 118 (36) ◽  
pp. e2108648118
Author(s):  
Suman K. Barman ◽  
Meng-Yin Yang ◽  
Trenton H. Parsell ◽  
Michael T. Green ◽  
A. S. Borovik
Keyword(s):  
Electron Transfer ◽  
Bond Activation ◽  
Dominant Role ◽  
Bond Cleavage ◽  
Semiempirical Method ◽  
Proton Coupled Electron Transfer ◽  
Enzymatic Function ◽  
Synthetic Metal ◽  
High Valent ◽  
Free Energy Analysis

The oxidation of substrates via the cleavage of thermodynamically strong C–H bonds is an essential part of mammalian metabolism. These reactions are predominantly carried out by enzymes that produce high-valent metal–oxido species, which are directly responsible for cleaving the C–H bonds. While much is known about the identity of these transient intermediates, the mechanistic factors that enable metal–oxido species to accomplish such difficult reactions are still incomplete. For synthetic metal–oxido species, C–H bond cleavage is often mechanistically described as synchronous, proton-coupled electron transfer (PCET). However, data have emerged that suggest that the basicity of the M–oxido unit is the key determinant in achieving enzymatic function, thus requiring alternative mechanisms whereby proton transfer (PT) has a more dominant role than electron transfer (ET). To bridge this knowledge gap, the reactivity of a monomeric MnIV–oxido complex with a series of external substrates was studied, resulting in a spread of over 104 in their second-order rate constants that tracked with the acidity of the C–H bonds. Mechanisms that included either synchronous PCET or rate-limiting PT, followed by ET, did not explain our results, which led to a proposed PCET mechanism with asynchronous transition states that are dominated by PT. To support this premise, we report a semiempirical free energy analysis that can predict the relative contributions of PT and ET for a given set of substrates. These findings underscore why the basicity of M–oxido units needs to be considered in C–H functionalization.

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.

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