Visible-Light-Mediated Regioselective Organophotocatalytic C3–H Phosphonylation of Pyrazolo[1,5- <i>a</i> ]pyrimidines

In this work, we have developed an approach for direct regioselective phosphonylation of pyrazolo[1,5-<i>a</i>]pyrimidines in the presence of organophotocatalyst 4CzIPN under visible light irradiation to obtain a series of (pyrazolo[1,5-<i>a</i>]pyrimidin-3-yl)phosphonates. The discovered reaction represents the first example of direct phosphonylation of pyrazolo[1,<i>5-a</i>]pyrimidines. Various (pyrazolo[1,<i>5-a</i>]pyrimidine-3-yl)phosphonates containing both alkyl and aryl substituents at different positions were obtained in yields ranging from 39 to 95%. The use of commercially available 4CzIPN as a photocatalyst along with simple and mild reaction conditions, a wide range of substrates, the absence of heavy metals, as well as the use of clean energy, available oxidant, and a common solvent are all attractive features of this method.