Esterification of Phosphonic Acids with Diazomethane

Abstract

Diazomethane (1) has long been recognized as a useful reagent for small scale esterification of carboxylic acids¹ (pK_a 4–5) and related compounds such as sulfonic acids² (pK_a ∼ 0). In connection with other work³ we needed an efficient method for converting allenic phosphonic acids and related 2-hydroxyoxaphospholenes to their corresponding esters, but a search of Chemical Abstracts revealed no reports on the esterification of such compounds with 1.⁴ Because the pK_a of typical phosphonic acids is around 2.5⁶ it seemed reasonable that they should be candidates for methylation with 1. This expectation is now confirmed.