Nucleophiles compared with leaving group

On the other hand, cleavage of the S-C bond is facile in nucleophilic substitutions involving sulfonium salts92 and the leaving ability of the dial-kylsulfide group compares with that of a bromide ion. As a result, there are several examples of type B substitutions in the literature. 

For substitutions proceeding by mechanism SE2(cyclic), it might be expected that substituent effects would reflect, at least in part, the relative importance of electrophilic attack at the carbon atom undergoing substitution compared with nucleophilic attack at the metal atom in the leaving group, i.e. the position of... 

Having established the three-dimentional structure of carbocations as planar, we can now study the stereochemical progression of Sk 1 reactions as compared to Sk2 reactions. As shown in Scheme 5.6, the stereochemical course of an Sk2 reaction is well defined because nucleophilic displacement of a leaving group proceeds with inversion of stereochemistry. Thus, the stereochemical outcome is defined by the stereochemistry of the starting material. As for SnI reactions, since the step required for initiation of these reactions involves formation of a planar species, incoming nucleophiles have equal access to both sides of the reactive carbocation. As shown in Scheme 5.7, this results in complete elimination of... 

Craig et al. demonstrated that the mechanical activation of a leaving group accelerates the nucleophilic substitution of dimethyl sulfoxide for substituted pyridines at square-planar pincer Pd(ll) metal centers (Figure 14). For the first time, they compared the dynamic SMFS behavior and stress-free kinetic data for bimolecular reactions. The stress-free dissociation rate constants of 1 x (2a)2 and 1 x (2b)2 extrapolated from the corresponding DFS are 0.7 0.4 and 20 3 s , respectively, which are in excellent agreement with the values determined by dynamic NMR (1 and 17 s , respectively). ... 

To derive the maximum amount of information about intranuclear and intemuclear activation for nucleophilic substitution of bicyclo-aromatics, the kinetic studies on quinolines and isoquinolines are related herein to those on halo-1- and -2-nitro-naphthalenes, and data on polyazanaphthalenes are compared with those on poly-nitronaphthalenes. The reactivity rules thereby deduced are based on such limited data, however, that they should be regarded as tentative and subject to confirmation or modification on the basis of further experimental study. In many cases, only a single reaction has been investigated. From the data in Tables IX to XVI, one can derive certain conclusions about the effects of the nucleophile, leaving group, other substituents, solvent, and comparison temperature, all of which are summarized at the end of this section. 

In the discussion of electrophilic aromatic substitution (Chapter 11) equal attention was paid to the effect of substrate structure on reactivity (activation or deactivation) and on orientation. The question of orientation was important because in a typical substitution there are four or five hydrogens that could serve as leaving groups. This type of question is much less important for aromatic nucleophilic substitution, since in most cases there is only one potential leaving group in a molecule. Therefore attention is largely focused on the reactivity of one molecule compared with another and not on the comparison of the reactivity of different positions within the same molecule. 

Although the rates were greatly different (as expected with such different leaving groups), the product ratios were the same, within 1 %. If this had taken place by a second-order mechanism, the nucleophile would not be expected to have the same ratio of preference for attack at the P hydrogen compared to... 

---