Anchimeric assistance group participation

All mechanistic pathways that generate product (H) represent examples of neighboring group participation, but anchimeric assistance is observed only if the rate-limiting step is that of ring closured Since each of the steps may be readily reversible under the conditions of the reaction, it is not always clear whether a given reaction was conducted under conditions of kinetic or thermodynamic control, which step was rate-limiting, or which step controlled the stereochemistry. 

As shown by the relative rates of methanolysis of thioethers PhS(CH2)nCl (Scheme 4.50), the same substrate can react with or without neighboring-group participation, depending on the nucleophile and on the reaction conditions. Under conditions which favor bimolecular substitution, anchimeric assistance by neighboring groups is observed only rarely. 

In connection with the methoxy participation, the gas-phase pyrolytic elimination of 4-chloro-1 -butanol was investigated177. The products are tetrahydrofuran, propene, formaldehyde and HCl. It is implied that the OH group provides anchimeric assistance from the fact that, besides formation of the normal unstable dehydrochlorinated intermediate 3-buten-l-ol, a ring-closed product, tetrahydrofuran, was also obtained. The higher rate of chlorobutanol pyrolysis with respect to chlorethanol and ethyl chloride (Table 27) confirmed the participation of the OH group through a five-membered ring in the transition state. 

The rate of nucleophilic substitution at positions ft to selenium is greatly enhanced. This anchimeric assistance has been ascribed to neighboring group participation with formation of the corresponding seleniranium ion in the ratedetermining step as shown in Equation (11). 

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