Carboxylate, neighbouring group

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. 

As a increases, the average distance between ionized groups decreases so that these neighbouring groups begin to have an effect. When a exceeds 0-3, individual water spheres begin to overlap and eventually coalesce into a cylindrical form. With further increases in a, a second outer cylindrical sheath of water appears in which water molecules are oriented by the cooperative effect of two or more carboxyl groups. 

Just as intramolecular catalysis has been observed in the hydrolysis of carboxylate esters, a variety of neighbouring groups can participate in the hydrolysis of phosphates. The alkaline hydrolysis of dimethyl phosphoacetoin [47] has found to be ca. 2 x 106 times faster in water at 25°C than that of... 

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. 

Volume 10 is devoted to formation and solvolysis of esters and related reactions, with discussion of the effect of neighbouring groups and biological implications, e.g. enzyme action, where appropriate. The first chapter deals mainly with esters of the inorganic acids of phosphorus and sulphur, Chapter 2 with the formation and solvolysis of esters of organic acids and the final chapter with the solvolysis of related derivatives of carboxylic acids, e.g. halides, amides, anhydrides, cyanides, carbamic acid derivatives. 

With more complicated structures such as lactones the identification problems become more complex as the reaction of lactones in water result in the formation of a carboxylic acid group and a neighbouring phenol... 

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). 

At low concentration, the reaction proceeds via an a-lactone i.e. the carboxylate anion, which is generated by the hydroxide anion, acts as a neighbouring group and reacts internally. This lactone is then attacked by another hydroxide anion that opens up the ring to form the a-hydroxyl-carboxylate anion, with overall retention of configuration at the a-carbon. At higher concentrations of hydroxide anions, the reaction proceeds directly by a normal intermolecular SN2 reaction with overall inversion at the a-carbon. 

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