Acyl trifluoroacetates

Studies of Trifluoroacetic Acid. Part XII. Acyl Trifluoroacetates and their Reactions, E. J. Bourne, M. Stacey, J. C. Tatlow, and R. Worrall, J. Chem. Soc., (1954) 2006-2012. 

Roberts and Wells354 were the first to use triflic acid in the acylation of phospha-ferrocenes with acetic anhydride and benzoic anhydride to afford ketophosphaferro-cenes in good yields (62-78%). Recently, acyl trifluoroacetates in excess triflic acid have been shown to be even more effective reagents355 [Eq. (5.141)]. 

When carboxylic acids are treated with trifluoroacetic anhydride, mixed anhydrides (acyl trifluoroacetates) are formed. These compounds are useful acylating agents. What is the product of the reaction of an alcohol with acyl trifluoroacetate ... 

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]. 

The Reaction of Acyl Trifluoroacetates with Hydroxy Compounds. 81... 

The role of trifluoroacetic anhydride in these acylations was indicated by the diminished yield of acylation product obtained when the ratio of trifluoroacetic anhydride to hydroxy compound lay below unity, optimum conditions requiring a slight excess of this reagent. A catalytic function was, therefore, excluded, and the view was advanced that trifluoroacetic anhydride serves the purpose of converting the added carboxylic acid into the corresponding acyl trifluoroacetate. Later work on the nature of the equilibria between acyl anhydrides and acids in the presence of trifluoroacetic anhydride showed that the acylating capacity of a mixture of a carboxylic acid and trifluoroacetic anhydride is enhanced by the trifluoroacetic acid liberated when the unsymmetrical anhydride is formed. Further, cryo-scopic studies on solutions in acetic acid of the pure, unsymmetrical anhydride, acetyl trifluoroacetate, have shown that, contrary to an earlier conclusion that acetic anhydride is not formed to any appreciable extent, acetyl trifluoroacetate is, in fact, almost completely converted into acetic anhydride in excess acetic acid. In carrying out an acylation with trifluoroacetic anhydride and a carboxylic acid, it is, therefore, important to avoid an excess of the acid, so that the maximum concentration of the unsymmetrical anhydride is present in the equilibrium system. Extensive studies made on the action of acyl trifluoroacetates on hydroxy compounds under different conditions, with the simultaneous formation of the acyl and trifluoroacetyl derivatives, will be discussed later. 

The rate of acetylation of 0-(hydrox3Tnethyl)cellulose (and other hydroxy compounds) by mixtures of carboxylic acids and their anhydrides has been found to increase greatly in the presence of trifluoroacetic acid. The acceleration is very much less with mono- and tri-chloroacetic acids, presumably because they form unsymmetrical anhydrides which are less effective acylating agents than acyl trifluoroacetates. The exceptional acylating power of the latter anhydrides is shown by their use in the synthesis of alkyl aryl ketones from polyalkylbenzenes, phenyl ethers, furan, and thiophene under mild conditions. The principle has been extended to include acids... 

The preparation of pure acetyl trifluoroacetate for the infrared studies referred to above was achieved by (a) fractional distillation of an equimolecular mixture of acetic anhydride and trifluoroacetic anhydride, or (b) the addition of pyridine to a mixture of acetic acid and trifluoroacetic anhydride, and fractional distillation of the filtrate. Acetyl trifluoroacetate is a colorless liquid, b.p. 95°, which gradually becomes colored on standing. In solution in carbon tetrachloride or other inert solvent, no coloration occurs and the solutions are stable. Similar methods of preparation have been used by other workers, and several different acyl trifluoroacetates have been reported. A different procedure for the preparation of acyl trifluoroacetates is the addition of the appropriate acyl chloride to a solution of silver trifluoroacetate in ether. The unsymmetrical anhydrides were found to be stable during the subsequent distillation, but disproportionation occurs in the presence of silver trifluoroacetate. 

Galli, C. 1979. Acylation of arenes and heteroarenes with in situ generated acyl trifluoroacetates. Synthesis 303-304. 

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