Photoelectrochemical Synthesis of Benzo[b]phosphole Oxides via Sequential P–H/C–H Bond Functionalizations

Benzo[b]phosphole oxides are important P-heterocycles that find applications in optoelectronics due to their inherent photophysical properties. Traditional routes for the synthesis of such molecules from readily available precursors require stoichiometric amounts of transition metal salts, bases, oxidants, and additives, thereby lacking efficiency. Photochemical pathways still need a terminal oxidant to complement the photocatalytic cycle, whereas electricity may be a viable oxidant. Hence, photoelectrochemistry (PEC), combining photocatalysis and synthetic organic electrochemistry, was used to simplify the synthetic protocols. We use the potency of 4CzIPN for the consecutive P–H/C–H bond functionalizations for preparing benzo[b]phosphole oxides from secondary phosphine oxides and nonactivated internal alkynes with up to 93% yields and with good functional group tolerance. Detailed mechanistic investigations confirm an intermolecular electron transfer between 4CzIPN and aryl secondary phosphine oxides upon photoexcitation. The photocatalyst is regenerated by anodic oxidation.