Heavyweight Champion: Caesium Diorganophosphides Outperform Lighter Congeners in the Catalytic Hydrophosphination of Alkenes and Alkynes

We present a study of the ability of our recently reported well‐defined crown ether‐coordinated alkali metal phosphides 1 AM to catalyze hydrophosphination (HP) of alkynes and alkenes. In a comparative study including reaction monitoring by 1 H NMR spectroscopy, we show that the activity of caesium compound 1 Cs greatly exceeds that of its lighter congeners, enabling us to solve some reported challenges of catalytic hydrophosphination. Through the rarely used application of dialkyl phosphines, we were able to produce trialkyl phosphines from HP of styrene derivatives and activated as well as non‐activated alkynes by catalytic HP with n Bu 2 PH and t Bu 2 PH. Using t BuPhPH, P‐chiral products were obtained, still in racemic mixtures showcasing this system's potential. We also proved the method is suited for preparing unsymmetrical ethylene‐bridged bisphosphines by HP of Ph 2 P(vinyl) isolable in high yields. These advances hint that well‐defined organocaesium compounds could make a long‐term impact in chemistry.