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 inaccessible. All compounds exhibit enhanced catalysis under irradiation by light emitting diode (LED)-generated blue light. Nacnac-supported compounds generate radicals as evidenced by Electron Paramagnetic Resonance (EPR) spectroscopy and radical trapping reactions, whereas unsupported calcium compounds are EPR silent and appear to undergo hydrophosphination akin to thermal reactions with these compounds. These results buttress the notion that photoactivation of π-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.