Neighboring group participation decreasing

Electron-withdrawing substituents at C(7) in cephalosporins decrease the rate of acid hydrolysis just as substituents at C(6) in penicillins do. However, in terms of acid-catalyzed hydrolysis cephalosporins are much less reactive than penicillins, even when the substituents on the side chains are similar [76], because cephalosporins do not show neighboring-group participation. The absence of neighboring-group participation in cephalosporins can be explained by the lower basicity of the N-atom of the dihydrothiazine ring compared to that of the thiazolidine ring of penicillins. 

The electrophile initially formed is further complexed by a Lewis acid, thus decreasing neighboring group participation into the electrophilic center, generating the superelectrophile. 

Like other superelectrophiles, protonation of the acyl cations leads to decrease of neighboring group participation and a lowering of the LUMO energy level, which facilitates reactions with even weak nucleophiles. 

Hydrolysis of the ester does not change the configuration. The Woodward modification of the Prevost reaction is similar, but results in overall syn hydroxylation. The alkene is treated with iodine and silver acetate in a 1 1 molar ratio in acetic acid containing water. Here again, the initial product is a (3-halo ester the addition is anti and a nucleophilic replacement of the iodine occurs. However, in the presence of water, neighboring-group participation is prevented or greatly decreased by solvation of the ester function, and the mechanism is the normal Sn2 process, ... 

Winstein and coworkers14 have introduced a variable termed the driving force, L, attributable to participation of a neighboring group in the rate-determining step, which is a measure of the decrease in free energy attended by the conversion of an open carbonium ion (VIII) to the cyclic carbonium ion (VII). The driving force was defined as... 

Furthermore, the electrical double layer of the micelle is compressed by the enhanced ionic strength, and the repulsion between the anionic head groups of neighboring AOT molecules decreases in the presence of metal cations [119]. Therefore, more AOT molecules participate in micelle formation, and the shape transition originates from the driving force that tends to expose the maximum number of free water molecules to the anionic head groups of the AOT molecules Under these... 

Without protonation of the carbonyl group, weak nucleophiles (TV,/V-dime-thylaniline and phenol) would only react slowly or not at all with the carbonyl groups. Similarly, complexation with Lewis acids can enhance the electrophilic reactivities of carbonyl compounds. This occurs by decreasing participation (using Winstein s concept) of the neighboring oxygen... 

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