A Three-Step Conversion of Phenyl-1H-phosphinic Acid to Dialkyl Phenylphosphonates Including Two Microwave-Assisted Direct Esterification Steps

Alkyl phenyl-H-phosphinates were synthesized by microwave(MW)-assisted direct esterification of phenyl-H-phosphinic acid. The phosphinates were then oxidized by m-chloroperbenzoic acid to the corresponding phenylphosphonic ester-acids that were esterified further under MW conditions to afford dialkyl phenylphosphonates. This is the first case that a mixed phosphonic derivative was esterified directly that is possible only under MW conditions. The mechanism and energetics of the esterifications investigated were evaluated by high level B3LYP calculations in solution applying the solvent model. The relatively high enthalpy of activation values of 175-186 kJ mol-1 prevents the direct esterifications on conventional heating, but may be overcome on MW irradiation.