Here, we report straightforward and selective synthetic procedures for mono- and diesterification of phosphonic acids. A series of alkoxy group donors were studied and triethyl orthoacetate was found to be the best reagent as well as a solvent for the performed transformations. An important temperature effect on the reaction course was discovered. Depending on the reaction temperature, mono- or diethyl esters of phosphonic acid were obtained exclusively with decent yields. The substrate scope of the proposed methodology was verified on aromatic as well as aliphatic phosphonic acids. The designed method can be successfully applied for small- and large-scale experiments without significant loss of selectivity or reaction yield. Several devoted experiments were performed to give insight into the reaction mechanism. At 30 °C, monoesters are formed via an intermediate (1,1-diethoxyethyl ester of phosphonic acid). At higher temperatures, similar intermediate forms give diesters or stable and detectable pyrophosphonates which were also consumed to give diesters. 31P NMR spectroscopy was used to assign the structure of pyrophosphonate as well as to monitor the reaction course. No need for additional reagents and good accessibility and straightforward purification are the important aspects of the developed protocols.
Keywords: alkyl orthoesters; phosphonic acids; selective esterification; temperature effect.