Nucleophilicity in aprotic solvents

Nitrite ion, an ainbident nucleophile in aprotic solvents, favors nitrogen atom attack on the double bond of various fluoro(halo)olefins. 2-Monohydroperfluoro-nitroalkanes can thus be produced [5] (equation 5)... 

Hie electrochemical characteristics of overoxidation vary widely among polymers, solvents, and nucleophiles.129 Its rate depends on the degree of oxidation of the polymer (and therefore on the potential applied), and the concentration127 and reactivity of the nucleophile. Polypyrroles usually become overoxidized at lower potentials than polythiophenes because of their lower formal potentials for p-doping. In acetonitrile, the reactivity of the halides follows their nucleophilicity in aprotic solvents,... 

Although superoxide ion is a powerM nucleophile in aprotic solvents, it does not exhibit such reactivity in water, presumably because of its strong solvation by that medium (A//hydration, lOOkcalmol" ) and its rapid hydrolysis and disproportionation. The reactivity of 02 - with aUcyl halides via nucleophilic substitution was first reported in 1970. These and subsequent kinetic studies - confirm that the reaction is first order in substrate, that the rates follow the order primary > secondary > tertiary for alkyl halides and tosylates, and that the attack by 02 - results in inversion of configuration (Sn2). 

The fluoride ion is only a weak nucleophile in water, due to high solvation. Aprotic solvents, however, do not solvate fluoride as readily. Accordingly fluoride is a strong nucleophile in aprotic solvents in the... 

Anions are generally more nucleophilic in aprotic solvents (these contain polar groups but no O-H or N-H bonds), such as dimethyl sulfoxide (Me2SO), than in protic solvents. 

Although superoxide ion is a powerful nucleophile in aprotic solvents, in water it has less reactivity, presumably because of its strong hydration. Hence, within water, superoxide anions are rapidly converted to dioxygen and peroxide ... 

Various sources of fluoride ion have been investigated, of which highly nucleophilic tetraalkylammonium fluorides ate the most effective Thuf, fluoro alkyl halides and N (fluoroalkyl)amines are efficiently synthesized by treatment of the corresponding trifluoromethanesulfonic esters with tetrabutylammonium fluoride trihydrate in aprotic solvents [5fl] (equation 34) The displacement reactions proceed quantitatively at room temperature within seconds, but tail with hydrogen fluoride-pyridine and give reasonable yields only with hydrogen fluo ride-alkylamine reagents... 

The reaction of perfluoroalkyl iodides with electron donor nucleophiles such as sodium arene and alkane sulfinates in aprotic solvents results in radical addition to alkenes initiated by an electron-transfer process The additions can be carried out at room temperature, with high yields obtained for strained olefins [4 (equations 3-5)... 

A hydrogen-bonded cyclic transition state can be postulated for a nucleophile like ethanolamine or ethylene glycol anion whose hydrogen bonding to an azine-nitrogen in aprotic solvents can facilitate reaction via a cyclic transition state such as 78, cf. Section II, F. Ethanolamine is uniquely reactive with 2-chloronitrobenzene by virtue of a cyclic solvate (17) of the leaving group, a postulate in line with kinetic evidence. 

In contrast with protic solvents, which decrease the rates of SN2 reactions by lowering the ground-state energy of the nucleophile, polar aprotic solvents increase the rates of Sn2 reactions by raising the ground-state energy of the nucleophile. Acetonitrile (CH3CN), dimethylformamide ((Chy NCHO,... 

However, even in aprotic solvents, the transition state is less solvated than the charged nucleophile Magnera, T.F. Caldwell, G. Sunner, J. Ikuta, S. Kebarle, P. J. Am. Chem. Soc., 1984, 106, 6140. 

Synthesis of 63 and 64 supports the olefin oxidation mechanisms in Fig. 16. These mechanisms have several important and noteworthy points about Ptm chemistry (1) olefins coordinate to Ptm at the axial position, which is contrasted to the -coordination of olefins perpendicular to the square-planar coordination plane of Ptn. Olefin coordination to Pt(III) should also be contrasted to the fact that olefins do not coordinate to Pt(IV). (2) Platinum111 is strongly electron-withdrawing, and the coordinated olefins receive nucleophilic attack. (3) The alkyl ce-carbon on the Ptm undergoes nucleophilic attack in aqueous solution, whereas in aprotic solvent, aldhyde (and possibly also ketone in other cases) is produced by reductive elimination. 

Nucleophilic substitution reactions of halide anions in aprotic solvents are often accompanied by elimination reactions. For instance, reactions of secondary alkyl halides with potassium fluoride solubilized in acetonitrile with the aid of 18-crown-6 [3] give olefins as the main reaction product (Liotta and Harris, 1974). Similarly, the dicyclohexyl-18-crown-6 complex of potassium iodide acted exclusively as a base in its reaction with 2-bromo-octane in DMF (Sam and Simmons, 1974). The strongly basic character of weakly solvated fluoride has been exploited in peptide synthesis (Klausner and Chorev, 1977 Chorev and Klausner, 1976). It was shown that potassium fluoride solubilized... 

Product 34 predominates in the polar aprotic solvent (acetonitrile), while in the polar protic solvent (methanol) products 35 are formed preferentially. The different products are caused by the relative rate of deprotonation against desilylation of the aminium radical, that is in turn governed by the action of enone anion radical in acetonitrile as opposed to that of nucleophilic attack by methanol. In an aprotic, less silophilic solvent (acetonitrile), where the enone anion radical should be a strong base, the proton transfer is favoured and leads to the formation of product 34. In aprotic solvents or when a lithium cation is present, the enone anion radical basicity is reduced by hydrogen bonding or coordination by lithium cation, and the major product is the desilylated 35 (Scheme 4). 

A new assumption to be discussed in this section is that the fourth-order kinetics in SatAr by amines in aprotic solvents is due to the formation of the substrate-catalyst molecular complex. Since 1982, Forlani and coworkers149 have advocated a model in which the third order in amine is an effect of the substrate-nucleophile interaction on a rapidly established equilibrium preceding the substitution process, as is shown in Scheme 15 for the reaction of 4-fluoro-2,4-dinitrobenzene (FDNB) with aniline (An), where K measures the equilibrium constant for ... 

The dichotomy of amine effects in aromatic nucleophilic substitution (ANS) in aprotic solvents... 

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