Microwave-Assisted Ionic Liquid-Catalyzed Selective Monoesterification of Alkylphosphonic Acids—An Experimental and a Theoretical Study

It is well-known that the P-acids including phosphonic acids resist undergoing direct esterification. However, it was found that a series of alkylphoshonic acids could be involved in monoesterification with C(2)–C(4) alcohols under microwave (MW) irradiation in the presence of [bmim][BF(4)] as an additive. The selectivity amounted to 80–98%, while the isolated yields fell in the range of 61–79%. The method developed is a green method for P-acid esterification. DFT calculations at the M062X/6–311+G (d,p) level of theory (performed considering the solvent effect of the corresponding alcohol) explored the three-step mechanism, and justified a higher enthalpy of activation (160.6–194.1 kJ·mol(−1)) that may be overcome only by MW irradiation. The major role of the [bmim][BF(4)] additive is to increase the absorption of MW energy. The specific chemical role of the [BF(4)] anion of the ionic liquid in an alternative mechanism was also raised by the computations.