Synthesis of Unsymmetrically Substituted Phosphane Oxides (R1R2P(O)H) and Phosphinous Acids (R1R2POH)

This paper describes the synthesis of unsymmetrically substituted phosphinous acids and phosphane oxides featuring at least one electron‐withdrawing pentafluoroethyl group. The presence of a diethylamino function as a protecting group allows a selective reaction of RClPNEt2 (R=CF3, C6F5, C6H5) with LiC2F5. On treatment with para‐toluenesulfonic acid the isolated aminophosphanes R(C2F5)PNEt2 are readily converted into the corresponding phosphinous acids or phosphane oxides, respectively. Investigation of the tautomeric equilibrium between oxide and acid tautomer revealed (CF3)(C2F5)POH as a stable phosphinous acid, whereas the pentafluorophenyl and phenyl derivatives constitute a solvent dependent equilibrium between the acid and the oxide tautomer. A tautomeric equilibrium between phosphane oxide and phosphinous acid is invoked to explain the role of secondary phosphane oxides (SPOs) as preligands in coordination chemistry. Whereas CF3(C2F5)POH exists in solution and the gas phase solely as the acid tautomer, the less electron‐withdrawing C6F5(C2F5)POH and C6F5(C2F5)P(O)H are involved in equilibria in solution as well as in the neat liquid phase (see figure).