Acylations with Mixed Anhydrides of Trifluoroacetic Acid

In mixed trifluoroacetic anhydrides, the strong electron-withdrawing tri-fluoroacetyl group polarizes the bonds to the anhydride oxygen in such a way that it tends to split to acylium cation and trifluoroacetate anion [/0/. Generally, the products of the reaction of hydroxy compounds with acyl trifluoroacetates are esters of carboxylic acids and trifluoroacetic acid, because the acyl carbonyl is better fit for a nucleophilic attack than the carbonyl next to the trifluoromethyl group [707]. 

However, the products may also be trifluoroacetates of the hydroxy compounds. Trifluoroacetates are favored with hydroxy compounds such as nonpolar alcohols, which are better nucleophiles than the more acidic alcohols containing polar groups such as trifluoromethyl, or phenols whose acidity is higher than that of nonpolar alcohols by as much as six or seven orders of magnitude. In addition, formation of trifluoroacetates is enhanced by addition of carbon tetrachloride, trifluoroacetic acid, or both. The presence of trifluoracetic acid may affect the reaction equilibrium between the trifluoroacetate of the hydroxy compound and the carboxylic acid by reacting with the byproduct of the reaction, carboxylic acid, and thus increase the formation of the trifluoroacetate (Guldberg-Waage s law). 

The following table shows results of treatment of hydroxy compounds with benzoyl trifluoroacetate (1.3 mmol) at 20°C for 24 hours [702], 

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Acylation with Mixed Anhydrides of Trifluoroacetic Acid... 

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