Photoelectrochemical\nSynthesis of Benzo[<i>b</i>]phosphole Oxides via Sequential\nP–H/C–H Bond Functionalizations

Benzo[<i>b</i>]phosphole oxides are important <i>P</i>-heterocycles that find applications in optoelectronics\ndue to their inherent photophysical properties. Traditional routes\nfor the synthesis of such molecules from readily available precursors\nrequire stoichiometric amounts of transition metal salts, bases, oxidants,\nand additives, thereby lacking efficiency. Photochemical pathways\nstill need a terminal oxidant to complement the photocatalytic cycle,\nwhereas electricity may be a viable oxidant. Hence, photoelectrochemistry\n(PEC), combining photocatalysis and synthetic organic electrochemistry,\nwas used to simplify the synthetic protocols. We use the potency of\n4CzIPN for the consecutive P–H/C–H bond functionalizations\nfor preparing benzo[<i>b</i>]phosphole oxides from secondary\nphosphine oxides and nonactivated internal alkynes with up to 93%\nyields and with good functional group tolerance. Detailed mechanistic\ninvestigations confirm an intermolecular electron transfer between\n4CzIPN and aryl secondary phosphine oxides upon photoexcitation. The\nphotocatalyst is regenerated by anodic oxidation.