Neighbouring group participation assistance

Neighbouring group participation is occasionally called anchlmeric assistance (Greek anchi = neighbouring merpart). 

You ve already met the most important ones—sulfides, esters, carboxylates. Ethers and amines (you will see some of these shortly) can also assist substitution reactions through neighbouring group participation. The important thing that they have in common is an electron-rich heteroatom with a lone pair that can be used to form the cyclic intermediate. Sulfides are rather better than ethers—this sulfide reacts with water much faster than rc-PrCl but the ether reacts with acetic acid four times more slowly than rc-PrOSC Ar. 

This influence of the nitrogen ion is known as neighbouring group participation or anchimeric assistance. 

Depending on such factors as the relative nucleophilicity of heteroatoms in the monomer and in the polymer, the flexibility of the chain (leading to the larger or smaller conformational assistance-neighbouring group participation) the relative rates of both processes may be different. If the rate of cyclization is comparable to the rate of propagation (like in oxetane polymerization) considerable amounts of cyclic oligomers (compared to final equilibrium concentration) are formed within the time needed to attain complete monomer conversion (cf. Sect. 5.3.5). If, however, the rate of cyclization is low (e.g. THF), macrocycles will still form slowly after the monomer-polymer equilibrium has been established. 

Figure 8.46 Schematic illustration to show the principle of intramolecular catalysis. Rate of reaction between the two substrates is enabled by binding and close proximity in the active site region (a). Neighbouring group participation involving neighbouring functional group assistance provides additional rate enhancement (anchimeric assistance) (b) for bond construction (c).

Summary This is an example of anchimeric assistance (also called neighbouring group participation) in nucleophilic substitution reactions... 

Keywoids Anchimeric assistance. Carbohydrates, Deamination, Fragmentation, Group migration, Neighbouring group participation. Nucleophilic substitution. Rearrangement, Ring contraction, Sulfonate displacement... 

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