Regenerative role of the red phosphorus in the couple 'HIaq/Pred'

Article abstract of DOI:10.1016/S0022-328X(96)06558-8

The synthesis of ortho-substituted benzoic acid 3 from benzalphthalide 1 in the presence of 'hydriodic acid-red phosphorus' (HI_aq/P_red) occurs via the formation of benzylphthalide 2 as reaction intermediate. Its preparation is optimized by use of the 'supported reaction in dry media' process (Silica/HI_aq) under microwave irradiation for 8 min. The role of the red phosphorus in the reduction reaction is elucidated: it intervenes in a catalytic cycle by an oxido-reductive disproportionation with the liberated iodine, affording either hypophosphorous acid in aqueous media, or P₂I₄ in anhydrous media with concomitant regeneration of HI. Thus, the presence of P_red in the couple 'HI_aq/P_red' allows recycling of the hydriodic acid and enhances its reducing efficiency.