Leaving groups internal

The leaving group also affects the amount of internal versus terminal alkene that is formed. The poorer the leaving group, the more El cb-like is the transition state. This trend is illustrated for the case of the 2-butyl system by the data in Table 6.6. Positively charged leaving groups, such as in dimethylsulfonium and trimethylammonium salts, may favor a more El cb-like transition state because their inductive and field effects increase the acidity of the p protons. 

MeCO20 + R0S02C6H4Me-p -> MeC02R + p-MeC6H4SO30 and internal leaving groups (cf. p. 94) ... 

Protonation of the leaving group anion, which prevents internal return of the ion pair to reactant, and has the effect of making substrate ionization irreversible. 

Rapid internal triflate displacements by neighboring acetyl or benzoyl groups have been mentioned above when the ester group and the leaving group have tranx-diaxial relationships. This leads to products where the configuration is retained, thus excluding... 

A less frequent case is a reaction described as Sn , short for substitution, nucleophilic (internal). This reaction type involves a unimolecular process in which the substrate dissociates to form an intimate ion pair however, no inversion of configuration occurs, presumably because steric hindrance forces the nucleophile to enter at the same side from which the leaving group departed. 

The ET photochemistry of (IR, 35)-(+)-c/i-chrysanthemol (c/i-127) proceeds via nucleophilic attack of the internal alcohol function on the vinyl group with simultaneous or rapid replacement of an isopropyl radical as an intramolecular leaving group, forming 128. This reaction is a mechanistic equivalent of an Sn2 reaction the mode of attack underscores the major role of strain relief in governing nucleophilic capture in radical cations. 

The synthesis of other anhydro sugar derivatives is usually accomplished by an internal displacement of a leaving group with inversion, as has already been emphasized for those anhydrides in which the anhydride bridge engages the anomeric position. Three such compounds that have been used in polymerizations deserve specific mention 5,6-anhydro-l,2-0-isopropylidene-a-D-glucofuranose,12 13 its 3-methyl ether,11 and 3,5-anhydro-l,2-0-isopropylidene-a-D-xylo-furanose,14 all of which were synthesized by p-toluenesulfonylation of the primary hydroxyl group of the parent, isopropylidene derivative, followed by treatment with base. 

In a few reactions, nucleophilic substitution proceeds with retention of configuration, even where there is no possibility of a neighboring-group effect. In the SNi mechanism (substitution nucleophilic internal) part of the leaving group must be able to attack the substrate, detaching... 

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