Acyloxyphosphonium versus Aminophosphonium Intermediates: Application to the Synthesis of N-Acylbenzotriazoles

In attempts to convert carboxylic acids directly into N-acylbenzotriazoles by using Ph3P/I-2 as an acid-activating system, the outcome of the reaction is reversed from no reaction to almost quantitative yield of the expected product simply by switching the order of the addition of the reagents to the presumed acyloxyphosphonium intermediate. If triethylamine was present before treatment with 1H-benzotriazole, anhydride was always exclusively generated without a detect-able amount of the expected product. However, if the base was applied after the addition of 1H-benzotriazole, the reaction proceeded smoothly to afford N-acylbenzotriazoles in good to excellent yields within short reaction times. P-31 NMR spectroscopy revealed the presence of a benzotriazophosphonium species in preventing the formation of the anhydride by attack of the carboxylate anion at the acyl function of the acyloxyphosphonium salt.