Nucleofuges nitro

For Ar reactions with amines, the presence of a nitro group in a position ortho to the nucleofuge plays an important role. In spite of the steric effects which will tend to decrease the reactivity of o-nitroaromatics, and the fact that the rate-enhancing effect of the resonance stabilization of the transition state will be more important from the para position, a k°/kp ratio greater than unity is usually found in the reactions of nitroaromatics... 

Solvation of thiolates is similarly low in both protic and dipolar aprotic solvents because of the size and polarisability of the large weakly basic sulfur atom, so is unlikely to contribute appreciably to the observed solvent effect. The intermediate nitro radical anion is stabilised by H-bonding in a manner which retards its dissociation in the SrnI mechanism (upper equation in Scheme 10.35). In contrast, the electron flow in the direct substitution at X (lower equation in Scheme 10.35) is such that solvation by methanol promotes the departure of the nucleofuge. In summary, protic solvation lowers the rate of the radical/radical anion reactions, but increases the rate of the polar abstraction yielding disulfide. 

Als aktiviert sind in diesem Zusammenhang auch Phosphorsaure-ester zu verstehen, die leicht nucleophil substituierbare (nucleofuge) Reste als Alkohol-Komponente enthalten. Ein interessantes Beispiel hierfiir sind Phosphorsaure-(2-chlormethyl-4-nitro-phenyl-ester), aus welchen mit Pyridin besonders reaktive Intermediate entstehen, die mit Alkoholen oder Phenolen glatt unter Bildung entsprechender Diester (55—97%) reagieren83,84 ... 

The high nucleofugality of the nitro group is noteworthy, due in part to its ability to stabilize the cyclohexadienyl intermediate (Table 6.1). Its high polarizability makes it partieularly suseeptible to replacement by polarizable nucleophiles, such as thiolate ions. Positively charged substituents, NMCj and SMe, are also readily replaced [53], as are protonated heteroarenes such as imidazo-... 

Conventional nucleophilic substitution of halogens and other nucleofugal groups X in electron-deficient arenes, particularly nitroarenes, proceeds via addition of nucleophiles at positions occupied by X to form o -adducts. The addition is coimected with dearomatization—thus, the adducts, which are in fact nitronate anions of nitro cyclohexadienes, undergo rapid rearomatization via spontaneous departure of X" to form products of nucleophilic aromatic substitution (Sj Ar) reaction. Detailed discussion of these processes is presented in Chapter 6. 

Besides the three major ways of conversion of the a -adducts into products of Sj,jArH presented in Sections 11.2-11.4, there are a few other processes, less general, but nevertheless of substantial interest. There are many cases of conversion of the a -adducts of nucleophiles to nitroarenes that proceed by departure of nucleofugal groups located in vicinity of the addition sites. This type of reactions is termed cme-substitution. Most frequently, the cme-substitution proceeds via departure of the nitro group in the form of nitrite anion from the a -adducts in positions ortho to the nitro group (Scheme 11.47) [73]. 

Photochemical nucleophihc displacements have been studied extensively, and the early work is documented in the exhaustive review by CorneHsse and Havinga. The most detailed studies have been made of methoxynitroarenes rather than halides, and it is well estabhshed that nucleophihc displacement occurs most readily meta to NOj. Of the various hahde nucleofuges, only fluoride appears to behave analogously to methoxy. Thus, m-fluoronitrobenzene reacts photochemicaUy with OH to give m-nitrophenol, but the reaction is not observed for m-chloro- or m-bromonitrobenzenes. With oxidizable nucleophiles such as secondary amines, photoreduction of the nitro group takes place in preference to substitution... 

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