Phosphorylating power of red phosphorus towards aldehydes in basic and in acidic media

The reactivity of red phosphorus towards aldehydes was investigated under b asic and acidic media. It was demonstrated that the real phosphorylating ag ent involved in the reaction was phosphane (PH3) in basic media, and hypoph osphorous acid (H3PO2) in acidic media. A convenient one-pot synthesis of ( alpha-hydroxyalkyl)phosphinic acids from red phosphorus and aldehydes in ba sic media was realized under sonication. The same reaction under acidic med ia in the presence of hydriodic acid led to the corresponding phosphonic ac ids. The (alpha-hydroxyalkyl)phosphinic acids were readily prepared under s onication from hypophosphorous acid and aldehydes in the presence of cataly tic amounts of hydrochloric acid. The mechanism of the addition reaction of PH3 to benzaldehyde was elucidated and shows the complexity of the reactio n as a function of the experimental conditions.