Sustainable Production of Reduced Phosphorus Compounds: Mechanochemical Hydride Phosphorylation Using Condensed Phosphates as a Route to Phosphite.

In pursuit of a more sustainable production of phosphorous acid (H 3 PO 3 ), a versatile chemical with phosphorus in the +3 oxidation state, we herein report that condensed phosphates can be employed to phosphorylate hydride reagents under solvent-free mechanochemical conditions to furnish phosphite (HPO 3 2- ). Using potassium hydride as the hydride source, sodium trimetaphosphate (Na 3 P 3 O 9 ), triphosphate (Na 5 P 3 O 10 ), pyrophosphate (Na 4 P 2 O 7 ), fluorophosphate (Na 2 PO 3 F), and polyphosphate ("(NaPO 3 ) n ") engendered phosphite in optimized yields of 44, 58, 44, 84, and 55% based on total P content, respectively. Formation of overreduced products including hypophosphite (H 2 PO 2 - ) was identified as a competing process, and mechanistic investigations revealed that hydride attack on in-situ-generated phosphorylated phosphite species is a potent pathway for overreduction. The phosphite generated from our method was easily isolated in the form of barium phosphite, a useful intermediate for production of phosphorous acid. This method circumvents the need to pass through white phosphorus (P 4 ) as a high-energy intermediate and mitigates involvement of environmentally hazardous chemicals. A bioproduced polyphosphate was found to be a viable starting material for the production of phosphite. These results demonstrate the possibility of accessing reduced phosphorus compounds in a more sustainable manner and, more importantly, a means to close the modern phosphorus cycle.