Direct esterification of phosphinic acids under microwave conditions: extension to the synthesis of thiophosphinates and new mechanistic insights

The direct esterification of phosphinic acids has been extended to the preparation of thiophosphinates using thiols, but the conversions are only ca. 50%. The outcome is in agreement with the unexpectedly high enthalpy of activation and endothermicity suggested by quantum chemical calculations. At the same time, formation of the thiophosphinates confirms the AAC2 (phosphinylation) mechanism and excludes the SN reaction paths. Formation of an olefinic intermediate under the reaction conditions is also excluded experimentally.