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

The light-driven activation of halophosphines R2PX (R = alkyl- or aryl, X = Cl, Br) by an Ir-III-based photocatalyst is described. It is shown that initially formed secondary phosphines R2PH react readily with the remaining R2PX in a parent-child reaction to form diphosphines R2P-PR2. Aryl-containing diphosphines can be further reduced to secondary phosphines (R2PH)-P-Ar under identical photoredox conditions. Dihalophosphines RPX2 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 R2PX is however unexpectedly slow and cannot compete with recombination with the oxidized donor DIPEA(center dot+). 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.