Ru‐Catalyzed Alkene Hydrophosphination: Correlating Substrate Scope with An Outer‐Sphere Mechanism

Abstract A detailed examination of substrate scope is described for alkene hydrophosphination catalyzed by a series of Cp*Ru complexes. An outer‐sphere mechanism is proposed that involves nucleophilic addition of a Ru phosphido ligand to the alkene and subsequent proton transfer to the resulting phospha‐carbanion from Ru‐bound substrate phosphine. Selected kinetic studies and the observation of catalyst resting state complexes that inevitably contain both a reactive phosphido ligand and substrate phosphine suggest that the conjugate addition is turnover limiting. However, analysis of catalyst activities as a function of the alkene activating group, and observation of off‐cycle alkene oligomerization initiated by the phospha‐carbanion intermediate for some alkene and phosphine substrates point to a more complex situation. These results suggest that alkenes that are “too” activated or substrate phosphines that are not sufficiently P─H acidic cause the intramolecular proton transfer to become turnover‐limiting, with rates sensitive to the conjugate addition “pre‐equilibrium”. This unusual compilation of experimental evidence for the impact of substrate variation on the activity and selectivity of a hydrophosphination catalyst provides insights for guiding future catalyst (re)design, including the challenge of providing a coordination environment that enhances phosphine P─H acidity to achieve a broader substrate phosphine scope.