Electrophilic reactions nucleophilic substitution

Substitution Reactions. Aromatic heterocycHc A/-oxides undergo both electrophilic and nucleophilic substitution because the dipolar N-oxide group is both an electron donor and an electron acceptor, giving rise to the resonance stmctures ... 

Pyrazine and quinoxaline fV-oxides generally undergo similar reactions to their monoazine counterparts. In the case of pyridine fV-oxide the ring is activated both towards electrophilic and nucleophilic substitution reactions however, pyrazine fV-oxides are generally less susceptible to electrophilic attack and little work has been reported in this area. Nucleophilic activation generally appears to be more useful and a variety of nucleophilic substitution reactions have been exploited in the pyrazine, quinoxaline and phenazine series. 

In those reactions where the fV-oxide group assists electrophilic or nucleophilic substitution reactions, and is not lost during the reaction, it is readily removed by a variety of reductive procedures and thus facilitates the synthesis of substituted derivatives of pyrazine, quinoxaline and phenazine. 

All reactions of benzotriazole derivatives of the type Bt-CR RbS discussed above are based on electrophilic or nucleophilic substitutions at the ot-carbon, but radical reactions are also possible. Thus, the first report on unsubstituted carbon-centered (benzotriazol-l-yl)methyl radical 841 involves derivatives of (benzotriazol-l-yl)methyl mercaptan. 3 -(Benzotriazol-l-yl)methyl-0-ethyl xanthate 840 is readily prepared in a reaction of l-(chloromethyl)-benzotriazole with commercially available potassium 0-ethyl xanthate. Upon treatment with radical initiators (lauroyl peroxide), the C-S bond is cleaved to generate radical 841 that can be trapped by alkenes to generate new radicals 842. By taking the xanthate moiety from the starting material, radicals 842 are converted to final products 843 with regeneration of radicals 841 allowing repetition of the process (Scheme 134). Maleinimides are also satisfactorily used as radical traps in these reactions <2001H(54)301>. 

Oxidation of unfunctionalized alkanes is notoriously difficult to perform selectively, because breaking of a C-H bond is required. Although oxidation is thermodynamically favourable, there are limited kinetic pathways for reaction to occur. For most alkanes, the hydrogens are not labile, and, as the carbon atom cannot expand its valence electron shell beyond eight electrons, there is no mechanism for electrophilic or nucleophilic substitution short of using extreme (superacid or superbase) conditions. Alkane oxidations are therefore normally radical processes, and thus difficult to control in terms of selectivity. Nonetheless, some oxidations of alkanes have been performed under supercritical conditions, although it is probable that these actually proceed via radical mechanisms. 

This problem covers a reaction sequence and a variety of different reactions, some easier than others. This one includes enolate anions, electrophilic cyclization, nucleophilic substitution, and simple carboxylic acid chemistry. 

In Part 11 we concentrate on aromatic systems, starting with the basics of structure and properties of benzene and then moving on to related ciromatic compounds. We even throw in a section of spectroscopy of aromatic compounds. Chapters 7 and 8 finish up this pcirt by going into detail about substitution reactions of aromatic compounds. You find out all you ever wanted to know (and maybe more) about electrophilic and nucleophilic substitutions, along with a little about elimination reactions. 

Radicals react with aromatic rings by substitution in a manner superficially resembling electrophilic or nucleophilic substitution. The reaction proceeds in... 

The production and uses of PCNs have been reviewed previously [5-7, 12,33,34] and thus are only briefly described here. PCNs were produced commercially as complex technical mixtures with trade names which included Halowaxes (USA), Seekay Waxes (UK), Nibren Waxes (Germany), and Clonacire Waxes (France). The synthesis involved the chlorination of molten naphthalene using chlorine gas and metal chlorides (iron(III) or antimony(V)) as catalysts. Reaction temperatures ranged from 80-200 °C depending on the degree of chlorination required, and proceeded as nucleophilic and electrophilic reactions favoring substitution in the a-(l,4,5,8) positions on the naphthalene molecule (Fig. 1) [6]. 

These conclusions have been confirmed experimentally,36 for both electrophilic and nucleophilic substitution. There seems no doubt that structures such as IH are stable entities rather than transition states. Moreover, as Melander and others have shown,36 the absence of deuterium isotope effects in most electrophilic substitutions indicates that in such cases the transition state must be VII rather than VIII. The rate-determining step in the reaction is the formation of the intermediate (III). 

Wide synthetic possibilities for modification of coordinated ligands are opened up by the classic reactions of electrophilic and nucleophilic substitution in complexes of aliphatic, aromatic, and heterocyclic compounds [314,359,418 422]. For example, the transformations (3.196) were known long ago [419] ... 

These hetero cycles are especially deactivated toward electrophilic and nucleophilic substitution under standard conditions and most of the reactions of QDO and PDO involve N-oxide modifications or ring-substituent deriva-tizations. The present section outlines the well-known reactions described particularly in the last decade. 

Electrophilic and nucleophilic substitution and addition reactions of enamines... 

Thus, the kinetic results show that selenophene undergoes both electrophilic and nucleophilic substitution reactions somewhat more readily than does thiophene. Hence, the reactivity of the heterocycle increases when sulfur is replaced by selenium a possible explanation might be that the selenium atom is larger and more polarizable, and therefore more willing both to release its p electrons and to accept electrons into its free d orbitals. 

Nucleophiles Nu attack the N-Cl bond ((371) and (378)), formally an electrophilic or nucleophilic substitution of chlorine. In the reaction of electrophiles E+ with chloramine NH2CI, a nitrogen bound proton is substituted. This results in the transfer of the NHCC to E+. 

An example of a quantitative SMR study correlating electronic properties and catalytic parameters is provided by the glutathione conjugation of para-substituted l-chloro-2-nitro-benzene derivatives (183). The values of log/j2 (second order rate constant of the nonenzy-matic reaction) and log (enzymatic reaction catalyzed by various glutathione transferase preparations) were correlated with the Hammett resonance cr value of the substrates, a measure of their electrophilicity. Regression equations with positive slopes and values in the range 0.88-0.98 were obtained. These results quantitate the influence of substrate electrophilicity on nucleophilic substitutions mediated by glutathione, be they enzymatic or nonenzymatic. 

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