Dimethylsulfoxide anion, nucleophilic

The second critical test of this conjugate base mechanism is based on the fact that this five-coordinated intermediate, if indeed it exists, would not always have to react with the solvent, though the solvent would be what it would react with under most circumstances. We have run this type of base hydrolysis in the presence of many anions of high concentration, and the only thing that we can find is the hydroxo complex so at least in water solution, water seems to be what this five-coordinated intermediate picks up. But in dimethylsulfoxide it certainly is possible to throw in various anions, and since dimethylsulfoxide is not as good as water in coordination, other nucleophiles may react. We do find in dimethylsulfoxide that a base, such as hydroxide ion, speeds up the rate of base hydrolysis but the product, instead of being a hydroxo compound, is the complex corresponding to whatever anion we have added, such as nitrite ion, azide ion, and thiocyanate ion. 

The cyanide anion is a powerful nucleophile. When methyl 2-oxobicyclo[3.1.0]hexane-l-carboxylate (1) was treated with sodium cyanide in dimethylsulfoxide, the 3-cyanomethylpen-tanone derivative 2 was obtained in 69% yield.With 2-hydroxy-2-methylpropanenitrile in methanol, dimethyl 3-cyanomethylhexane-l,4-dioate (3) was isolated as the sole product. Its formation involved nucleophilic opening of both rings by a homo-Michael addition and the reversal of an ester condensation. 

The first Halex experiments have been carried out with neat chloroaromatics at high temperatures (400 - 500 °C) but the introduction of dipolar aprotic solvents in the late 50 s brought a dramatic improvement for the use of this process on a large scale under realistic conditions (0° < 200 - 250 °C) (ref. 4). It can be noticed that protic solvents, which decrease the nucleophilicity of the fluoride anion by strong hydrogen-bonding, are less adapted than aprotic ones. Commonly used dipolar aprotic solvents are dimethylsulfoxide (DMSO), tetramethylenesulfone (or... 

Ihe azide Ig was prepared from the methanesulfonic acid ester of the initiator Ib. Nucleophilic displacement of the mesylate group by the azido anion in dimethylsulfoxide leads to the formation of the compound Ig obtained as a yellow oil after purification by column flash chromatography. The structures of the phenones Id-g are in correspondence with the results of the elemental analysis and the spectral data. 

Much faster alkylation of enolate anions can often be achieved in dimethylfor-mamide (DMF), dimethylsulfoxide (DMSO) or 1,2-dimethoxyethane (DME) than in the usual protic solvents. The presence of hexamethylphosphoramide (HMPA) or a triamine or tetramine can also enhance the rate of alkylation. This is thought to be because of the fact that these solvents or additives solvate the cation, but not the enolate, thereby separating the cation-enolate ion pair. This leaves a relatively free enolate ion, which would be expected to be a more reactive nucleophile than the ion pair. Reactions with aqueous alkali as base are often improved in the presence of a phase-transfer catalyst such as a tetra-alkylammonium salt. ... 

The superoxide anion is a highly reactive species because of its nucleophilic namre. For this reason non-aqueous aprotic solvents such as acetonitrile (ACN), dimethyl-formamide (DMF), dimethylsulfoxide (DMSO) and pyridine have been selected for studies involving the 02/02 redox process due to the stabilisation of superoxide in such media [4]. 

Protic solvents are especially effective at solvating anions. If we use a less effective solvent such as dimethylsulfoxide (DMSO) or dimethylformamide (DMF), the contribution of desolvation should become less. Indeed, the nucleophilicity of F" and HO is enormously increased in these solvents, the ratio of the rates of their Sj 2 reactions to those of the large iodide ion being greater by several powers of ten than it is in a protic solvent such as water or methanol. 

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