On the direct reduction of arsonic acids to arsenoso compounds: mechanisms and preparations†

By using a variety of reducing agents, either alone or in the presence of iodide as a catalyst, the reduction of arsonic acids to arsenoso compounds and/or arsonous acids has been studied. The solvent and temperature are important for a clean reduction. The mechanism of the reduction involves prior protonation of the  AsO₃H₂ group, and can be rationalized in the framework of the ‘hard and soft acids and bases’ principle, which is used to predict other reducing systems and to explain other literature data. For preparative purposes, triphenylphosphine/iodine, hexamethylphosphorous triamide/iodine and ascorbic acid/iodine give flexibility of choice depending on the substrate. The first two of these systems decompose arsonic acids with a weak C–As bond but the last system is sufficiently mild towards the same arsonic acids. Copyright © 2000 John Wiley & Sons, Ltd.