Toward a Clean Alternative to Friedel−Crafts Acylation:  In Situ Formation, Observation, and Reaction of an Acyl Bis(trifluoroacetyl)phosphate and Related Structures

Reaction of acyl trifluoroacetates with phosphoric acid in the presence of trifluoroacetic anhydride (TFAA) leads to the ready formation of acyl bis(trifluoroacetyl)phosphates, which are powerful acylating agents. Formation of these species and the subsequent acylation reaction are carried out, without added solvent, in a single in situ reaction process. In this reaction system, anisole is rapidly acylated at ambient temperature using a variety of carboxylic acids giving the para isomer exclusively. TFAA acts as an activating agent and can be recovered from the reaction system as trifluoroacetic acid (TFA) and converted back to TFAA using a dehydrating agent, while phosphoric acid behaves as a covalent catalyst in the process. This reaction system has many features which are required elements of a clean alternative to the Friedel−Crafts process.