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

[Image: see text] The light-driven activation of halophosphines R(2)PX (R = alkyl- or aryl, X = Cl, Br) by an Ir(III)-based photocatalyst is described. It is shown that initially formed secondary phosphines R(2)PH react readily with the remaining R(2)PX in a parent–child reaction to form diphosphines R(2)P–PR(2). Aryl-containing diphosphines can be further reduced to secondary phosphines R(Ar)(2)PH under identical photoredox conditions. Dihalophosphines RPX(2) 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 (N,N-di-iso-propylethylamine) electron donor. Electron transfer to R(2)PX 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(II) population escapes recombination, providing the reductant for the observed transformations.