Neighboring group effects anchimeric assistance

There are three clues that point to the operation of a neighboring group effect. The first is an unusual, often backward stereochemical result. Generally, this involves retention of configuration when inversion might have been anticipated. The second is formation of an unexpected product that appears to be the result of a rearrangement. The unexpected product arises because an intermediate is formed that can react further in more than one way. The third clue is an unexpected increase in the rate of a reaction. The anchimeric assistance provided by an intramolecular nucleophile often reveals itself in the form of a rate increase. 

Anchimeric assistance may also explain slight changes in mechanism resulting from the presence of a neighboring group. One likely case is the acid-catalyzed hydrolysis of phenylglycosides. Raftery s group" found that the secondary kinetic isotope effect (/tr/ d) was 1.13 for acid hydrolysis of phenyl-4-0-(2-acetamido-2-deoxy-j8-D-glu-... 

There is much additional evidence to support the postulate that the effect of neighboring sulfur is due to anchimeric assistance. Cyclohexyl chloride undergoes solvolysis in ethanol-water to yield a mixture of alcohol and ether. As usual for secondary alkyl substrates, reaction is SnI with nucleophilic assistance from the solvent (see Sec. 14.17). A C5H5S— group on the adjacent carbon can speed... 

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