Organic reactions abstracted

Most of the free-radical mechanisms discussed thus far have involved some combination of homolytic bond dissociation, atom abstraction, and addition steps. In this section, we will discuss reactions that include discrete electron-transfer steps. Addition to or removal of one electron fi om a diamagnetic organic molecule generates a radical. Organic reactions that involve electron-transfer steps are often mediated by transition-metal ions. Many transition-metal ions have two or more relatively stable oxidation states differing by one electron. Transition-metal ions therefore firequently participate in electron-transfer processes. 

T. Kawaaaki, A. Kodama, Y. Eukui, K. Kobayashi, T. Ohta, and M. Some , Book of Abstracts, 17th Symposium on Progress in Organic Reactions and Syntheses, Eukuoka, Nov. 1991, p. 206. 

We soon found out that the material we had to deal with was much more extensive than we had anticipated. It was necessary to evaluate several thousands of papers dealing with azolide reactions in recent years. Chemical Abstracts lists more than 1500 references to CDI alone from 1967 to the present. Thus, what was originally planned as a progress review chapter in one of the existing series on organic reactions grew up into a real book, which we hope to be of value to organic chemists and biochemists interested in synthetic methods. 

The fewer factors that lower ion-radical stability, the more easily ion-radical organic reactions proceed. Because ion-radicals are charged species with unpaired electrons, solvents for the ion-radical reactions have to be polar too, incapable of expelling cationic or anionic groups that the ion-radical bears as well as chipping off radicals from it (especially to abstract the hydrogen atom). Static solvent effects can be subdivided on general and specific ones. 

Experimental observations indicate that the oxidation of cobalt (II) to cobalt (III) and the formation of ethylenediamine from N-hydroxyethylethylene-diamine occur simultaneously. This is quite the opposite to what is usually assumed in other instances of transition metal catalysis of organic reactions—for example, the catalytic effect of manganese in the oxidation of oxalic acid (7, 8), of iron in the oxidation of cysteine to cystine (22) and of thioglycolic acid to dithioglycolic acid (5, 23), of copper in the oxidation of pyrocatechol to quinone and in the oxidation of ascorbic acid (29, 30), and of cobalt in the oxidation of aldehydes and unsaturated hydrocarbons (4). In all these reactions the oxidation of the organic molecule occurs by the abstraction of an electron by the oxidized form of the metal ion. 

It is possible to subscribe to CA Selects, which provides copies of all abstracts within various narrow hclds. such as organofluorinc chemistry, organic reaction mechanisms, organic stereochemistry, etc. 

For preliminary communications and lecture, see G. A. Olah, Conference Lecture at the 9th Reaction Mechanism Conference, Brookhaven, New York, August 1962 Abstracts, 142nd National Meeting of the American Chemical Society (Atlantic City, NJ, 1962), p. 45 W. S. Tolgyesi, J. S. McIntyre, J. J. Bastien, and M. W. Meyer, p. 121 G. A. Olah, Angew. Chem. 75, 800 (1963) Rev. Chim. Rep. Populaire Roumaine 7, 1139 (1962) Preprints, Div. Petr. Chem., ACS 9(7), 21 (1064) Organic Reaction Mechanism Conference, Cork, Ireland, June 1964, Special Publications No. 19, Chemical Society, London, 1965. 

In trapping experiments, nitroxides will only trap carbon-centred radicals, and not oxygen-centred ones. This is particularly important since oxygen-centred radicals are often used as initiators (Section 10.2). The nitroxide should also not undergo other reactions, such as addition to double bonds or H-abstraction this increases the probability that it will trap selectively carbon-centred radicals which act as chain carriers in many synthetically useful organic reactions, as propagating species in polymerisations and as reactive intermediates in biological pathways. 

Nitrations of aromatic compounds represent one of the most important classes or organic reactions. However, although a lot of data has been collected in relation to simple substituted benzenes, and the reactions have been subjected to quantitative and theoretical studies, the nitration of phenyl heterocycles has been much less studied in a systematic way. This chapter has attempted to bring together as much information as possible on the topic through a manual search of Chemical Abstracts and recent literature. 

Atkinson (1987) developed very reliable fragment additivity SARs for estimating kHO(air) from molecular structure using more than 400 compounds in the database (Chapter 14 describes procedures for using these SARs). SARs for HO are based on the premise that rate constants for each of the several different classes of reactions of HO with organic compounds — abstraction of H- atom (kH), addition to double, triple or aromatic bonds (kE), and reaction with S or N atoms (kA) — can be estimated separately and then summed to give the total molecular rate constant, kHO ... 

Sajiki, H. Hattori, K. Hirota, K. Abstracts, 24th Symp. Progress in Organic Reactions and Syntheses, IP-17, Nov. 1998, Chiba, Japan. 

The known coenzyme Bi2-dependent enzymes all perform chemical transformations in enzymatic radical reactions that are difficult to achieve by typical organic reactions. Homolytic cleavage of the Co bond of the protein-bound coenzyme B12 (3) to a 5 -deoxy-5 -adenosyl radical (9) and cob(n)alamin (5) is the entry to reversible H-abstraction reactions involving the 5 -position of the radical (9). Indeed, homolysis of the Co bond is the thermally most easily achieved transformation of coenzyme B12 (3) in neutral aqueous solution (with a homolytic (Co-C)-BDE of about 30 kcal mol ). However, to be relevant for the observed rates of catalysis by the coenzyme B12-dependent enzymes, the homolysis of the Co-C bond of the protein-bound coenzyme (3) needs to be accelerated by a factor of about 10 , in the presence of a substrate. Coenzyme B12 might then be considered, first of aU, to be a structurally sophisticated, reversible source for an alkyl radical, whose Co bond is labihzed in the protein-bound state (Figure 8), and the first major task of the... 

CH Activation is sometimes used rather too loosely to cover a wide variety of situations in which CH bonds are broken. As Sames has most recently pointed out, the term was first adopted to make a distinction between organic reactions in which CH bonds are broken by classical mechanistic pathways, and the class of reactions involving transition metals that avoid these pathways and their consequences in terms of reaction selectivity. For example, radicals such as RO- and -OH readily abstract an H atom from alkanes, RH, to give the alkyl radical R. Also in this class, are some of the metal catalyzed oxidations, such as the Gif reaction and Fenton chemistry see Oxidation Catalysis by Transition Metal Complexes). Since this reaction tends to occur at the weakest CH bond, the most highly substituted R tends to be formed, for example, iPr-and not nPn from propane. Likewise, electrophilic reagents such as superacids see Superacid), readily abstract a H ion from an alkane. The selectivity is even more strongly in favor of the more substituted carbonium ion product such as iPr+ and not nPr+ from propane. The result is that in any subsequent fimctionalization, the branched product is obtained, for example, iPrX and not nPrX (Scheme 1). 

Copper(I) complexes catalyse a variety of organic reactions which are of synthetic and industrial importance.305 In such processes that involve halide abstraction from aryl or alkyl halides, the abstraction step by a Cu(I) catalyst is believed to be the rate-determining step. In order to circumvent the property of facile disproportionation of Cu, various methods of stabilising Cu(I) and influencing reaction rates were considered.306 A kinetics study of ligand (L) effects on the reactivity of Cu(I)L complexes towards C13CC02 was undertaken. The results indicated that the rate of the chlorine abstraction reaction was affected by several factors. These were the redox potential of the Cu(II/I)L couple, the hybridisation on Cu(I) in the Cu(I)L complex, steric hindrance, and electron density on the central Cu(I) cation at the binding site of the chlorine atom to be abstracted. The volume of activation,... 

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