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][BF4] 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 center dot mol(-1)) that may be overcome only by MW irradiation. The major role of the [bmim][BF4] additive is to increase the absorption of MW energy. The specific chemical role of the [BF4] anion of the ionic liquid in an alternative mechanism was also raised by the computations.

Automated summary

Investigations have shown that a series of alkylphoshonic acids can undergo monoesterification with C-2-C-4 alcohols under microwave irradiation in the presence of [bmim][BF4], showing high selectivity and reasonable isolated yields. DFT calculations explored a three-step mechanism, indicating that the high enthalpy of activation can only be overcome by MW irradiation, with [bmim][BF4] enhancing the absorption of MW energy.