Carboxylate, neighbouring group participation

Sulphoxides with -carboxylic acid or amide groups are converted, in nearly quantitative yields, to the sulphone whilst other acids and amides did not show oxidation. In the cases where oxidation did occur, the acid group was converted to the acid chloride (equation 31) whilst the amide was converted to the nitrile (equation 32). These results indicate that neighbouring-group participation in the case of carboxylic acids and amides occurs only when a five-membered intermediate is formed. In the case of hydroxyl groups, four-, five-or six-membered intermediates are favourable. 

Both the reactions are essentially the additions of iodine carboxylate (formed in situ) to an alkene, i.e., the reaction of an alkene with iodine and silver salt. The Prevost procedure employs iodine and silver carboxylate under dry conditions. The initially formed transiodocarboxylate (b) from a cyclic iodonium ion (a) undergoes internal displacement to a common intermediate acylium ion (c). The formation of the diester (d) with retention of configuration provides an example of neighbouring group participation. The diester on subsequent hydrolysis gives a trans-glycol. 

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. 

The amino group needs to be converted to a hydroxyl group with retention of configuration dia-zotization followed by hydrolysis does just this because of neighbouring group participation from the carboxylic acid. 

By a similar process, amino acids have been converted to a-fluorocarboxylic acids by fluorodediazotisation processes [234] and, generally, retention of configuration is observed due to neighbouring group participation of the adjacent carboxyl group (Figure 3.47 and Table 3.8). 

---