Reactivity of nucleophiles

Efforts to establish a theoretical explanation of the reactivity of nucleophilic reagents have centered on correlations with intrinsic electron-donor properties which are the fundamental basis of nucleophilicity. According to Edwards and Pearson, in general, such properties include basicity, polarizability, and the presence of unshared electron pairs on the atom adjacent to the nucleophilic atom of the reagent. When only the first two of these properties are operative, Eq. (8), which was proposed by Edwards, has proved successful in... 

This increased reactivity of nucleophile is a distinct advantage => Reactions that might have taken many hours or days are often over in a matter of minutes. 

The reactivity of nucleophilic carbenes toward 2-bromo-2,3-dihydro-lff-l,3,2-diazaborole has also been described (37). The reaction of... 

Reactions and reactivity of nucleophiles with thiolsulfonates 137 Nucleophilic substitutions of sulfenyl derivatives general considerations 139 Bimolecular substitution at sulfenyl sulfur stepwise or concerted 140 Reversibility in reactions of nucleophiles with cyclic thiolsulfonates 145 Other reactions of thiolsulfonates 147... 

Ritchie (1975) has found that the reactivity of nucleophiles toward a number of different types of electrophilic centers can be correlated remarkably well by a very simple equation (192), where kNu is the rate constant for reaction of a... 

One of the products was the expected C-substituted compound. The other was an unstable species, which decomposed into 4-nitrocumyl alcohol during workup and was ascribed to 0-substitution. Kornblum (1975) had obtained the same products. He considered the C- and 0-substitution as Sj l and S 2 reactions, respectively. Dual reactivity of nucleophiles is well known. Since steric hindrance at the reacting carbon prevents S 2 reaction with 4-nitrocumyl chloride, Costentin et al. (1999) concluded that both the C- and 0-substitution products result from an Sr I reaction. 

Scheme 17 Reactivity of nucleophiles (ylides, PR3) towards unsaturated substrates...

This review will focus on the use of chiral nucleophilic A-heterocyclic carbenes, commonly termed NHCs, as catalysts in organic transformations. Although other examples are known, by far the most common NHCs are thiazolylidene, imida-zolinylidene, imidazolylidene and triazolylidene, I-IV. Rather than simply presenting a laundry list of results, the focus of the current review will be to summarize and place in context the key advances made, with particular attention paid to recent and conceptual breakthroughs. These aspects, by definition, will include a heavy emphasis on mechanism. In a number of instances, the asymmetric version of the reaction has yet to be reported in those cases, we include the state-of-the-art in order to further illustrate the broad utility and reactivity of nucleophilic carbenes. 

For cationic SIP, limitations when applied to surfaces are also evident. Like anionic polymerization, SIP on particles has been abundantly reported by several groups, most notably on work by Tsubokawa et al. The use of nanoparticles has also been widely reported. The same assumptions were made for cationic polymerization based on the grafting of electrophiles on surfaces and the reactivity of nucleophilic monomers for cationic polymerization. 

The relative reactivities with respect to nucleophilic SAE displacement increase in the order Cl < Br < I < F. The relative reactivities of nucleophiles are illustrated by the reactions of 2-bromopyridine replacement by the following groups occurs under the conditions given (an indicates that the product spontaneously tautomerizes) ... 

The most common deviation is the exceptionally high reactivity of nucleophiles, such as hydroperoxide, hypochlorite and hydroxamate ions, with atoms bearing lone-pair electrons next to the nucleophilic centre. This phenomenon, known as the alpha-effect287, is found for aminolysis reactions of esters also285, and is commonly observed for attack at electrophilic centres where reactivity depends fairly strongly on the basicity of the nucleophile. Negative deviations may be evidence of steric hindrance, or in a few cases, in particular that of hydroxide ion, may reflect special solvation effects on the pKa or the nucleophilicity (or both) of the nucleophile. 

The relative reactivities of nucleophiles toward sulfur differ a great deal depending on whether the sulfur is di-, tri-, or tetracoordinated, This is to be expected from the theory of hard and soft acids and bases. As the oxidation state... 

With the exception of F , the relative reactivities of nucleophiles toward sulfonyl sulfur are very similar to their relative reactivities in the same solvent toward carbonyl carbon. J. L. Kice and E. Legan, J. Amer. Chem. Soc., 95, 3912 (1974). 

A similar picture holds for other nucleophiles. As a consequence, there might seem little hope for a nucleophile-based reactivity relationship. Indeed this has been implicitly recognized in the popularity of Pearson s concept of hard and soft acids and bases, which provides a qualitative rationalization of, for example, the similar orders of reactivities of halide ions as both nucleophiles and leaving groups in (Sn2) substitution reactions, without attempting a quantitative analysis. Surprisingly, however, despite the failure of rate-equilibrium relationships, correlations between reactivities of nucleophiles, that is, comparisons of rates of reactions for one carbocation with those of another, are strikingly successful. In other words, correlations exist between rate constants and rate constants where correlations between rate and equilibrium constants fail. 

The significance of these results for differences in reactivities of nucleophiles is that, despite the unfavorable relative equilibrium constants, Me2S is more reactive toward the quinone methide than chloride ion by a factor of nearly 3000. This mismatch of rate and equilibrium effects is summarized in Scheme 35. It must imply (a) that there is a relatively long partial bond between sulfur and carbon in the transition state so that the unfavorable steric and electrostatic effects are not developed and (b) that the favorable carbon-sulfur bonding interaction is well developed despite the long bonding distance. 

Figure 1.6 Schematic representation of location and reactivity of nucleophilic and electrophilic oxygen. Vertical lines denote active...

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