Studies in the mechanism of acylation by acyl trifluoroacetates

Abstract

Previous work on unsymmetrical anhydrides in general and acyl trifluoroacetates in particular is reviewed with special reference to the mechanisms of reactions in which they act as acylating agents. The reaction of acetyl trifluoroacetate with isopropanol and eleven phenols in carbon tetrachloride solution at 25° has been studied using infrared spectroscopy to estimate the enters produced. Isopropanol reacts with acetyl trifluoroacetate plus trifluoroacetic acid to give a mixture of the two possible esters. Most of the work with phenols was carried out using p-chlorophenol. This reacts with acetyl trifluoroacetate and with acetyl trifluoroacetate plus trifluoroacetic acid to give only the acetate. Reaction with acetyl trifluoroacetato in the presence of pyridine gives a mixture of the two possible esters. The rate of reaction of acetyl trifluoroacotate with isopropanol and all the phenols has been measured and the dependence of the rate on the concentration of the hydroxy-compound found. For isopropanol and p-chiorophenol the affect of added trifluoroacetic acid on the rate has been measured and for p-chlorophenol the dependence of the rate on the anhydride concentration found. Study has also been made of the reactions occurring when a carbon tetrachloride solution of acetyl trifluoroacetate and a phenol is shaken with an alkaline aqueous solution. Usually such treat lentresults in the formation of the phenyl trifluoroacetate. Possible mechanisms of the above reactions are discussed with particular attention being paid to associations occurring through the formation of hydrogen bonds.