On the mechanism of reductive cleavage of aryl phosphates

Evidence is presented which strongly indicates that cleavage of aryl phosphates with electron donors may occur either by a one-electron or two-electron pathway. Thus, high concentrations and greater reducing power of the electron donor favor production of arene (C-0 cleavage product) while low concentrations and/or lower reducing power of the electron donor favor production of phenol (P-0 cleavage product) from aryl phosphates. A rationale based on the intermediacy of a trigonal bipyramidal phosphate ester anion radical which either undergoes P-0 (a) scission or is reduced further and undergoes C - 0 (0) scission, is presented. Comparison is also made with the electron transfer chemistry of sulfonate esters.