Free Radical Coupling Research Articles

On the problem of potential shifts at glassy carbon electrodes: Part II. Voltammetric analysis of the surface interactions involved in the electrode processes of nitrobenzenes at freshly polished and electrochemically pretreated electrodes

The contribution of the heterogeneous (involving non-adsorbed solution species) and surface path (involving adsorbed species) to the electrode processes of aromatic nitro compounds has been studied as a function of the surface conditions of glassy carbon (GC-20) by linear sweep voltammetry (LSV) and cyclic voltammetry (CV). The influence of adsorption was negligible at the freshly polished electrodes. Oxidative electrochemical pretreatment led to the formation of active sites at which the adsorption of the reactant resulted in enhanced electron-transfer rates. The nature of the adsorption process and the characteristics of the electrode reaction in the adsorbed state were studied in detail for p-nitrotoluene. The adsorption followed the Frumkin isotherm with an interaction parameter of 1.2, indicating moderate attractive interactions between the adsorbed molecules. The rate constant for the adsorbed species was of the order of 10 5, higher than the apparent heterogeneous rate constant of p-nitrotoluene in the solution phase. The magnitude of the potential shift and the strength of adsorption varied with the relative position and electron-withdrawing power of the substituent in the aromatic ring of nitrobenzene. Free radical coupling was selectively catalysed at electrochemically pretreated electrodes.

The formation of an enantiomerically pure product of free radical coupling. The chemistry of diphenylcarbene in polycrystalline S-(+)-2-butanol

The formation of an enantiomerically pure product of free radical coupling. The chemistry of diphenylcarbene in polycrystalline S-(+)-2-butanol

Base catalysed rearrangements involving ylide intermediates. Part 15. The mechanism of the Stevens [1,2] rearrangement

The Stevens [1,2] rearrangement of acyl-stabilised ammonium ylides has been investigated with regard to stereoselectivity, intramolecularity and the formation of products in addition to the [1,2] rearrangement product. A detailed study of the effects of reaction conditions upon the rearrangement of the ylide derived from the salt (13) has shown that the stereoselectivity (retention of the configuration of the chiral migrating group) and intramolecularity decrease as solvent viscosity decreases. The rearrangement of the salt (13) in water at 0 °C is essentially intramolecular with virtually complete retention of the configuration of the migrating group. These results, together with the isolation of products that can be rationalised on the basis of random free-radical coupling, indicate that the [1,2] rearrangement of acyl-stabilised ammonium ylides normally involves a radical pair mechanism.

Benzodioxans by oxidative phenol coupling. Synthesis of silybin

Oxidative coupling of substituted catechols with isoeugenol or coniferyl alcohol in the presence of silver oxide affords 2,3-trans-1,4-benzodioxans in good yield. The reaction is highly regioselective when the catechol bears an alkyl substituent, much less so in the case of an electrophilic one. A free-radical coupling mechanism is proposed. A one-step biomimetic synthesis in high yield of the natural flavanolignans silybin and isosilybin from 2R,3R-dihydroquercetin and coniferyl alcohol is reported.

General aspects in polymer synthesis.

The term, synthetic polymer, denotes a macromolecule composed of simple structural units and usually derived from monomers by a process of polymerization. This process is possible only if certain chemical, thermodynamical, and mechanistic conditions are satisfied. The monomer must have a functionality of two or higher. The polymerization process must be characterized by a negative free energy change. The monomer must have activable or reactive functions. Activable structures are polarizable multiple bonds, and bonds entailing heteroatoms in ring compounds. Suitable reactive functions are those that participate in: carbonyl addition, substitution, multiple bond addition, and free-radical coupling. According to their growth mechanisms, polymerization reactions are divided into two major groups: chain-growth polymerization and step-growth polymerization. Polyaddition and polycondensation occur in either group, but polyinsertion is a chain-growth process exclusively. Radical, ionic, and heterogeneous initiations are entailed in chain growth, whereas both nucleophilic and electrophilic initiation prevails in step-growth. The chemical structure of polymers is determined mainly by constitutional and configuration parameters. Principal constitutional ones are related to interlinking of chains and structural units, composition, substituents, endgroups, and the molecular weight. Configurational ones refer particularly to the position of substituents on a central chain atom relative to the neighboring structural unit and thus to the tacticity of the polymer.

Free-radical coupling, cleavage, and redox reactions in cobalt-60 .gamma. radiolysis of aqueous methyl acetate. Effects of additives

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTFree-radical coupling, cleavage, and redox reactions in cobalt-60 .gamma. radiolysis of aqueous methyl acetate. Effects of additivesTibor Bernath, George H. Parsons Jr., and Saul G. CohenCite this: J. Am. Chem. Soc. 1975, 97, 9, 2413–2419Publication Date (Print):April 1, 1975Publication History Published online1 May 2002Published inissue 1 April 1975https://doi.org/10.1021/ja00842a018RIGHTS & PERMISSIONSArticle Views127Altmetric-Citations5LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (842 KB) Get e-Alerts Get e-Alerts

Phenol oxidation mechanisms. A search for phenoxylium intermediates

Results which point to the occurrence of an ionic intramolecular coupling process in the oxidation of certain diphenyl ether and diphenylmethane derivatives are described. This process is in contrast to the usual free-radical coupling route encountered under different conditions. An attempt to induce a related heterolytic intramolecular coupling during the oxidation of some bis-hydroxynaphthylmethane monoethers was unsuccessful.

Dimer acid structures: Cyclic structures of clay catalyzed dimers of normal linoleic acid, 9‐cis, 12‐cis‐Octadecadienoic acid

AbstractThe clay catalyzed dimer of linoleic acid has been examined by mass spectrometry of the unhydrogenated, the partially hydrogenated and completely hydrogenated dimer. The results show that monocyclic, bicyclic and tricyclic structures are present. Monocyclic structures predominate, bicyclic structures are also prominent, and tricyclic structures are relatively minor. The monocyclic structure is believed to arise from a Diels‐Alder type addition reaction. The bicyclic structure may result from a free radical coupling followed by intramolecular ring closure. The monocyclic structure in the unhydrogenated dimer appears to be mostly a benzene ring with saturated and unsaturated side‐chains. It probably is formed by hydrogen transfer from the Diels‐Alder cyclohexene structure first formed. Little, if any, of the Diels‐Alder dimer structure as such is present. The catalytic linoleate dimer has a higher ratio of monocyclic to bicyclic dimer than does the noncatalytic (thermal) dimer made from normal (nonconjugated) linoleate, while the thermal dimer of a conjugatedtrans‐trans linoleate is exclusively monocyclic. It is suggested that the clay catalyzes conjugation and hence favors the Diels‐Alder reaction, and then catalyzes hydrogen transfer to aromatize the cyclohexene ring.

Dimer acid structures. The thermal dimer of normal linoleate, methyl 9‐cis, 12‐cis octadecadienoate

AbstractThe thermal dimer (290C) of normal methyl linoleate and its hydrogenated form have been examined by mass spectrometry. Parent mass peaks of the hydrogenated dimer show the presence of monocyclic, bicyclic, and tricyclic structures.The monocyclic structure is formed via the conjugation‐Diels‐Alder mechanism.The bicyclic structure is best explained by an extension of the hydrogen transfer free radical coupling mechanism. The noncyclic dehydrodimer resulting from free radical coupling undergoes a relatively rapid intramolecular cyclization to a bicyclic structure, probably by an interval Diels‐Alder reaction. A model noncyclic dehydro‐linoleate dimer was shown to give a bicyclic dimer as the predominant structure under thermal dimerization conditions.The tricyclic dimer may result from intramolecular alkylation of the bicyclic structures.