Nucleofugality acidity

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. 

In the case above, 100 is protonated in the last step by the acid HA, but if the acid is omitted and a suitable nucleofuge is present, it may leave, resulting in a cyclo-pentene. In these cases the reagent is an allylic anion, but similar 3 + 2 cycloadditions involving allylic cations have also been reported. 

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... 

If we now consider a 1,3-diol (3), accepting a similar mechanism to the abovementioned for 1,2-diols, in acid conditions the fragmentation of the molecule will take place according to a process known as "Grob fragmentation" [2] to give water as the "nucleofuge", an alkene (4) and a carbonyl compound (5) as the... 

Kinetic studies of the alkaline hydrolyses (pH 11-14) of a series of pentachlorophenyl esters of >-(/7-hydroxyphenyl)alkanoic acids (19 m = 1-4) have been reported. The reasonably high nucleofugality of the pentachlorophenoxide (pK ... 

Several polymeric acyl-transfer reactants have been used to give am-ide/ester products in the solution phase. The excess polymer-bound acyl-transfer reactants and polymer-bound nucleofuge byproducts are easily removed after completion of the reactions. One such application involved the activated nitrophenyl esters 25 (reaction 8).40 A mixture of 10 acid chlorides was converted to an equimolar mixture of 10 amide products a potent preemergent herbicide was discovered using this parallel synthetic approach.41... 

The interpretation is based on the balance of competition between nucleophile-led reactions (via contact ion pairs or concerted and tending towards ElcB) and electrophile-led reactions (the El extreme, via solvated ion pairs). The solvated ion pairs are favoured with better nucleofuges, and/or greater solvating power, and collapse via loss of the more hydride-like /1-hydrogen collapse of the contact ion pairs involves preferential loss of the most acidic /1-hydrogen, and demands interaction with the nucleophile. 

It is of interest to examine the general case when a nucleofuge is displaced from a carbonyl group by a nucleophile with a labile hydrogen, HNu, which becomes much more acidic upon formation of the tetrahedral intermediate (Scheme 11.10). Catalysis by the general base B will be observed when the intermediate 4 breaks down to reactants faster than it transfers a proton to water. The rate constant for formation of 5, which may or may not represent the overall rate constant of reaction, is given by kn [B] K, where K is the equilibrium constant for formation of 4 and kK is the rate constant of proton transfer from 4 to the catalyst B. 

Another possibility is to use [bis(trifluoroacetoxy)iodo]benzene (BTI) and the sodium salt of a carboxylic acid. This method does not require heating because of the good nucleofugality of the trifluoroacetoxy group. When stoichiometric quantities of reagents are used, mixed [bis(acyloxy)iodo] arenes may be obtained [26]. In order to prepare [bis(trifluoroacetoxy)iodo]benzene itself,... 

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