Mechanistic Insights and Synthetic Explorations of the Photoredox-Catalyzed Activation of Halophosphines.

The light-driven activation of halophosphines RPX (R = alkyl- or aryl, X = Cl, Br) by an Ir-based photocatalyst is described. It is shown that initially formed secondary phosphines RPH react readily with the remaining RPX in a parent-child reaction to form diphosphines RP-PR. Aryl-containing diphosphines can be further reduced to secondary phosphines RPH under identical photoredox conditions. Dihalophosphines RPX are also activated by the photoredox protocol, giving rise to unusual 3-, 4-, and 5-membered cyclophosphines. Transient absorption studies show that the excited state of the Ir photocatalyst is reductively quenched by the DIPEA (,-di-iso-propylethylamine) electron donor. Electron transfer to RPX is however unexpectedly slow and cannot compete with recombination with the oxidized donor DIPEA. As DIPEA is not a perfectly reversible donor, a small proportion of the total Ir population escapes recombination, providing the reductant for the observed transformations.