Photocatalytic Hydrophosphination Using Calcium Precatalysts

Hydrophosphination using calcium compounds as catalysts under irradiation is described as a foray into s-block photocatalysis. Transition-metal compounds have been highly successful hydrophosphination catalysts under photochemical conditions, utilizing substrates previously considered inaccessible. A calcium hydrophosphination precatalyst, Ca(nacnac)(THF)(N(SiMe3)2) (1, nacnac = HC[(C(Me)N-2,6-iPr2C6H3)]2), reported by Barrett and Hill, as well as the presumed intermediate, Ca(nacnac)(THF)(PPh2) (2), and the Schlenk equilibrium product, Ca[N(SiMe3)2]2(THF)2 (3) were screened under photochemical conditions with a range of unsaturated substrates including styrenic alkenes, Michael acceptors, and dienes with modest to excellent conversions, though unactivated alkenes were in accessible. All compounds exhibit enhanced catalysis under irradiation by LED-generated blue light. Nacnac-supported compounds generate radicals as evidenced by EPR spectroscopy and radical trapping reactions, whereas unsupported calcium compounds are EPR silent and appear to undergo insertion-based hydrophosphination akin to thermal reactions based. These results buttress the notion that photoactivation of -basic ligands is a broad phenomenon, extending beyond the d-block, but like d-block metals, consideration of ancillary ligands is essential to avoid radical reactivity.