Reactions nucleofuges

In this reaction I" is the nucleophile, and Br" is called the leaving group (or nucleofuge). Beyond this, the classification symbolism may include a designation of the molecularity of the reaction. Molecularity is the number of reactant molecules included in the transition state. The above reaction is an 8 2 reaction, because both reactants are present in the transition state. On the other hand, this substitution... 

Sulfonic acids themselves are unfit for electrophilic transfer of sulfonyl groups because of the poor nucleofugality of the hydroxide anion. However, the high acidity obviously leads to an equilibrium between the acids and their anhydrides and water, from which water can be removed either by special reaction conditions (i.e., azeotropic distillation with appropriate solvents) or chemically with anhydride forming agents316 (equation 63). sulfonic acid anhydride sulfonylations are compiled in Table 10. 

However, in some circumstances, an arylsulphonyl group is a sufficiently good nucleofuge that an elimination reaction is possible. This has actually been known for some 60 years,... 

Another approach to an oxidative desulphonylation reaction is to oxidize an a-sulphonyl carbanion with an oxidizing agent that is also nucleofugal. An example of this was presented by Little and Sun Ok Myong203 who used a molybdenum peroxide complex (M0O5.Pyridine.HMPA) as the oxidant. However, this reagent is expensive and... 

When a molecule has in an allylic position a nucleofuge capable of giving the SnI reaction, it is possible for the nucleophile to attack at the y position instead of the a position. This is called the SnI mechanism and has been demonstrated on 2-buten-l-ol and 3-buten-2-ol, both of which gave 100% allylic rearrangement when treated... 

In many cases it is not certain whether the nucleofuge X is lost first, creating an intermediate nitrene or nitrenium ion, or whether migration and loss of the nucleofuge are simultaneous, as shown above.It is likely that both possibilities can exist, depending on the substrate and reaction conditions. 

Reactions of P-halogen-NHPs with Lewis acids leading to halide abstraction, or metathetic replacement of the halide by a nucleofugic, noncoordinating anion form by far the most important routes to access phosphenium ions in general [51, 52] and also 1,3,2-diazaphospholenium cations in particular. As Lewis acid assisted P-X... 

The proportion of C-alkylation increases in the order OTs < Br < I, a sequence which is often associated with the balance of hardness between nucleophile and nucleofuge (Smith and Hanson, 1971). The work of Kurts et al. (1974) indicates that the overall reaction rate of the crown ether-assisted alkylation increases in the order Na+ < K+ < Rb+ < Cs+, which, according to these authors, reflects the increasing distance between cation and anion in the ion pairs. The high reactivity of the tetraphenylarsenate also fits in with this picture. The decrease of the kc/k0 ratio is only small in good cation-solvating solvents such as dimethyl sulfoxide (DMSO). Alkylation of the sodium derivative of [103] with ethyl iodide in DMSO gave kc/kQ = 15.7 addition of... 

Section III will show the importance of amine aggregates in SjvAr reactions in aprotic solvents when the second step is rate-determining it is obvious that branching of the amine will diminish the formation of the aggregates that help the proton transfer and the nucleofuge departure from the cr-complex. 

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... 

Here K = [B B]/[B]02 stands for the amine auto-association constant. Usually, in the reactions with amines with poor nucleofuges the second step is rate-determining, the inequality... 

L. Strekowski, M. Lipowska and G. Patonay, Substitution reactions of a nucleofugal group in heptamethine cyanine dyes. Synthesis of an isothiocyanato derivative for labeling of proteins with a near-infrared chromophore, J. Org. Chem. 57.4578-4580(1992). 

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