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Radicals, Mechanisms and Synthesis: A Themed Issue in Honor of the Many Contributions of Prof. Dr. John C. Walton

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A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Organic Chemistry".

Deadline for manuscript submissions: 30 June 2024 | Viewed by 8469

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Special Issue Editors

Prof. Dr. John A. Murphy

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Guest Editor

Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, UK
Interests: radical chemistry and electron transfer reactions – their mechanisms and applications

Dr. Eoin Martin Scanlan

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Guest Editor

School of Chemistry, CRANN and AMBER Research Centres, Trinity College Dublin, Ireland
Interests: radical methodology; bioconjugation; drug discovery; peptide chemistry and glycoscience

Special Issue Information

Dear Colleagues,

John C. Walton joined the faculty of Dundee University in 1967 and moved to St. Andrews University in 1970, rising to full professor in 1997 and becoming Research Professor in 2007. He is renowned for the application of physical organic methods to organic mechanisms and for EPR spectroscopic studies of many kinds of radicals. He has developed radical-mediated synthetic protocols including those based around: “pro-aromatic” cyclohexadienyl reagents, oxime derivatives, photoredox methods employing titanium dioxide and for research on radical enhancement of heterolysis. He has published over 300 articles in learned journals as well as 3 books. He was awarded the Royal Society of Chemistry Silver Medal for Organic Reaction Mechanisms in 1994 and was elected Fellow of the Royal Society of Edinburgh in 1995.

Prof. Dr. John A. Murphy
Dr. Eoin Martin Scanlan
Guest Editors

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Keywords

radicals
radical ions
redox
mechanism
EPR spectroscopy
computational methods
physical organic chemistry

Published Papers (5 papers)

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Research

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12 pages, 1228 KiB

Open AccessArticle

Radical Mediated Decarboxylation of Amino Acids via Photochemical Carbonyl Sulfide (COS) Elimination

by Alby Benny, Lorenzo Di Simo, Lorenzo Guazzelli and Eoin M. Scanlan

Molecules 2024, 29(7), 1465; https://doi.org/10.3390/molecules29071465 - 25 Mar 2024

Viewed by 715

Abstract

Herein, we present the first examples of amino acid decarboxylation via photochemically activated carbonyl sulfide (COS) elimination of the corresponding thioacids. This method offers a mild approach for the decarboxylation of amino acids, furnishing N-alkyl amino derivatives. The methodology was compatible with amino acids displaying both polar and hydrophobic sidechains and was tolerant towards widely used amino acid-protecting groups. The compatibility of the reaction with continuous-flow conditions demonstrates the scalability of the process. Full article

(This article belongs to the Special Issue Radicals, Mechanisms and Synthesis: A Themed Issue in Honor of the Many Contributions of Prof. Dr. John C. Walton)

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15 pages, 2784 KiB

Open AccessArticle

A Continuous-Wave EPR Investigation into the Photochemical Transformations of the Chromium(I) Carbonyl Complex [Cr(CO)₄bis(diphenylphosphino)]⁺ and Reactivity with 1-hexene

by David Fioco, Andrea Folli, James Platts, Mario Chiesa and Damien M. Murphy

Molecules 2024, 29(2), 392; https://doi.org/10.3390/molecules29020392 - 12 Jan 2024

Viewed by 654

Abstract

Chromium complexes containing a bis(diphenylphosphino) ligand have attracted significant interest over many years due to their potential as active catalysts for ethylene oligomerisation when combined with suitable co-catalysts such as triethylaluminium (TEA) or methylaluminoxane (MAO). While there has been considerable attention devoted to the possible reaction intermediates and the nature of the Cr oxidation states involved, the potential UV photoactivity of the Cr(I) complexes has so far been overlooked. Therefore, to explore the photoinduced transformations of bis(diphenylphosphino) stabilized Cr(I) complexes, we used continuous-wave (CW) EPR to study the effects of UV radiation on a cationic [Cr(CO)₄(dppp)]⁺[Al(OC(CF₃)₃)₄]⁻ complex (1), where dppp represents the 1,3 bis-(diphenylphosphino)propane ligand, Ph₂P(C₃H₆)PPh₂. Our preliminary investigations into the photochemistry of this complex revealed that [Cr(CO)₄(dppp)]⁺ (1) can be readily photo-converted into an intermediate mer-[Cr(CO)₃(κ¹-dppp)(κ²-dppp)]⁺ complex (2) and eventually into a trans-[Cr(CO)₂(dppp)₂]⁺ complex (3) in solution at room temperature under UV-A light. Here, we show that the intermediate species (2) involved in this transformation can be identified by EPR at much lower temperature (140 K) and at a specific wavelength (highlighting the wavelength dependency of the reaction). In addition, small amounts of a ‘piano-stool’-type complex, namely [Cr(CO)₂(dppp-η⁶-arene)]⁺ (4), can also be formed during the photoconversion of [Cr(CO)₄(dppp)]⁺ using UV-A light. There was no evidence for the formation of the [Cr(L-bis-η⁶-arene)]⁺ complex (5) in these UV irradiation experiments. For the first time, we also evidence the formation of a 1-hexene coordinated [Cr(CO)₃(dppp)(1-hexene)]⁺ complex (6) following UV irradiation of [Cr(CO)₄(dppp)]⁺ in the presence of 1-hexene; this result demonstrates the unprecedented opportunity for exploiting light activation during Cr-driven olefin oligomerisation catalysis, instead of expensive, difficult-to-handle, and hazardous chemical activators. Full article

(This article belongs to the Special Issue Radicals, Mechanisms and Synthesis: A Themed Issue in Honor of the Many Contributions of Prof. Dr. John C. Walton)

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Review

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51 pages, 9904 KiB

Open AccessFeature PaperReview

Radical Reactions in Organic Synthesis: Exploring in-, on-, and with-Water Methods

by Chryssostomos Chatgilialoglu, Sebastian Barata-Vallejo and Thanasis Gimisis

Molecules 2024, 29(3), 569; https://doi.org/10.3390/molecules29030569 - 23 Jan 2024

Cited by 1 | Viewed by 1543

Abstract

Radical reactions in water or aqueous media are important for organic synthesis, realizing high-yielding processes under non-toxic and environmentally friendly conditions. This overview includes (i) a general introduction to organic chemistry in water and aqueous media, (ii) synthetic approaches in, on, and with water as well as in heterogeneous phases, (iii) reactions of carbon-centered radicals with water (or deuterium oxide) activated through coordination with various Lewis acids, (iv) photocatalysis in water and aqueous media, and (v) synthetic applications bioinspired by naturally occurring processes. A wide range of chemical processes and synthetic strategies under different experimental conditions have been reviewed that lead to important functional group translocation and transformation reactions, leading to the preparation of complex molecules. These results reveal how water as a solvent/medium/reagent in radical chemistry has matured over the last two decades, with further discoveries anticipated in the near future. Full article

(This article belongs to the Special Issue Radicals, Mechanisms and Synthesis: A Themed Issue in Honor of the Many Contributions of Prof. Dr. John C. Walton)

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22 pages, 4152 KiB

Open AccessReview

Some Aspects of α-(Acyloxy)alkyl Radicals in Organic Synthesis

by Béatrice Quiclet-Sire and Samir Z. Zard

Molecules 2023, 28(22), 7561; https://doi.org/10.3390/molecules28227561 - 13 Nov 2023

Viewed by 904

Abstract

The preparation and use of α-(acyloxy)alkyl xanthates to generate and capture α-(acyloxy)alkyl radicals is briefly reviewed. Their inter- and intramolecular additions to both activated and unactivated, electronically unbiased, alkenes, and to (hetero)aromatic rings, as well as their radical allylation and vinylation reactions are [...] Read more.

(This article belongs to the Special Issue Radicals, Mechanisms and Synthesis: A Themed Issue in Honor of the Many Contributions of Prof. Dr. John C. Walton)

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78 pages, 17647 KiB

Open AccessReview

Recent Advances in C–H Functionalisation through Indirect Hydrogen Atom Transfer

by Filip S. Meger and John A. Murphy

Molecules 2023, 28(16), 6127; https://doi.org/10.3390/molecules28166127 - 18 Aug 2023

Cited by 5 | Viewed by 4168

Abstract

The functionalisation of C–H bonds has been an enormous achievement in synthetic methodology, enabling new retrosynthetic disconnections and affording simple synthetic equivalents for synthons. Hydrogen atom transfer (HAT) is a key method for forming alkyl radicals from C–H substrates. Classic reactions, including the Barton nitrite ester reaction and Hofmann–Löffler–Freytag reaction, among others, provided early examples of HAT. However, recent developments in photoredox catalysis and electrochemistry have made HAT a powerful synthetic tool capable of introducing a wide range of functional groups into C–H bonds. Moreover, greater mechanistic insights into HAT have stimulated the development of increasingly site-selective protocols. Site-selectivity can be achieved through the tuning of electron density at certain C–H bonds using additives, a judicious choice of HAT reagent, and a solvent system. Herein, we describe the latest methods for functionalizing C–H/Si–H/Ge–H bonds using indirect HAT between 2018–2023, as well as a critical discussion of new HAT reagents, mechanistic aspects, substrate scopes, and background contexts of the protocols. Full article

(This article belongs to the Special Issue Radicals, Mechanisms and Synthesis: A Themed Issue in Honor of the Many Contributions of Prof. Dr. John C. Walton)

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