Sulfur groups carboxylic acid acylations

Carboxylic acids react both with fluoroaminosulfiiranes such as DAST and with sulfur tetrafluonde. Whereas DAST converts the acids to acyl fluorides only, sulfur tetrafluonde further fluorinates the primarily formed acyl fluorides and ultimately converts the carboxyl group to a trifluoromethyl group. 

Q Thionyl chloride reacts like an acyl chloride. In this step, its sulfur-oxygen double bond plays the role of the carbonyl group of an acyl chloride. The nucleophile, the oxygen of the carboxylic acid, attacks the sulfur and displaces the pi electrons onto the oxygen. 

First, we will discuss reactions in which hydrogen or a metallic ion (or in one case phosphorus or sulfur) adds to the heteroatom and second reactions in which carbon adds to the heteroatom. Within each group, the reactions are classified by the nature of the nucleophile. Additions to isocyanides, which are different in character, follow. Acyl substitution reactions that proceed by the tetrahedral mechanism, which mostly involve derivatives of carboxylic acids, are treated at the end. 

Sulfonamides are hydrolyzed more slowly than amides of carboxylic acids examination of the structures involved shows us what probably underlies this difference. Nucleophilic attack on a trigonal acyl carbon (Sec. 20.4) is relatively unhindered it involves the temporary attachment of a fourth group, the nucleophilic reagent. Nucleophilic attack on tetrahedral sulfonyl sulfur is relatively hindered it involves the temporary attachment of a /// group. The tetrahedral... 

When acid derivative 2 reacts with sulfuric acid, the oxygen atom is the base and the conjugate acid product of this acid-base reaction is oxocarbenium ion 3, which is resonance stabilized. When 2 is an acid chloride, anhydride, ester, or amide, a heteroatom is attached to the positive carbon in 3. As in Chapter 18 (Section 18.1), the acid-base reaction of the carbonyl unit in 2 to give 3 facilitates reactions with nucleophiles. The reaction of intermediate 3 with a nucleophile ( Y) gives tetrahedral intermediate 4 contrary to acyl addition, reaction 4 contains an X group that can function as a leaving group. Loss of X leads to the final product of this reaction 5. If the nucleophile ( Y) is hydroxide, compormd 5 is the carboxylic acid (X = OH). If the nucleophile Y is an alcohol, the product 5 is an ester, and if Y is an amine, the product 5 is an amide. This first reaction is therefore the acid-catalyzed acyl substitution reaction of acid derivatives. 

The base-induced reaction of the symmetrical dipropanoic anhydride (15) illustrates the mechanism of base hydrolysis with anhydrides, in a reaction that gives two equivalents of propanoic acid. Acyl addition of the nucleophilic hydroxide gives tetrahedral intermediate 16. In this case, the best leaving group is the carboxylate anion 18, which gives propanoic acid, 17. Because 17 is formed in a basic solution, reaction with hydroxide gives 18. A second step is required to convert 17 to 18, using aqueous acid (aqueous sulfuric acid or aqueous HCl). 

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