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

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.