The Role of Phosphorus Trichloride and Phosphorous Acid in the Formation of α-Hydroxymethylenebisphosphonic Acids from the Corresponding Carboxylic Acids - A Mechanistic Overview

Background: Although the synthesis of α-hydroxymethylenebisphosphonic acids (dronic acids) and derivatives by reaction of the corresponding carboxylic acids (or its derivatives) with P-reagents, such as phosphorus trichloride, phosphorous acid and phosphoryl chloride formed the subject of many studies, in most cases, the selection of the P-reagents, their ratio, and the conditions (solvent and temperature) were not optimized. Method: Selection of the appropriate P-reagents and their ratios, along with the conditions is possible, only if the reaction protocol (sequence of the steps) or the mechanism is known. For this purpose, the relevant information was extracted from the literature, critically discussed and the conclusions were drawn. Our own experiences are also summarized on the possible mechanisms, and on the related optimum set of the reaction parameters depending mainly on the nature of the solvent used. Conclusion: This review gives a better understanding of the dronate chemistry, and promotes “greener” and more practical syntheses.