Mechanism and Catalyst Design in Ru-Catalyzed Alkene Hydrophosphination

A thorough experimental examination of a series of half-sandwich Ru indenyl complexes [Ru­(η5-indenyl)­(PPh2)­(L)­(PPh3) (L = PPh2H, CO, NCPh)] in the catalytic hydrophosphination of tert-butyl acrylate by diphenylphosphine provides valuable lessons for the design of active and robust catalysts for this important P–C bond-forming reaction. Evidence for each fundamental step in the relevant catalytic cycles was gathered from reaction monitoring (1H and 31P NMR), kinetic analyses, stoichiometric control reactions, and the isolation and spectroscopic identification of key intermediates, catalyst deactivation products, and off-cycle byproducts. For L = PPh2H, two distinct catalytic cycles each rely on the outer-sphere, conjugate addition of the Ru–PPh2 ligand at the electron-deficient alkene. The cycles differ in their P–H activation steps (intra- vs intermolecular) but are connected by a common resting state [Ru­(η5-indenyl)­(PPh2)P2, where P is the hydrophosphination product Ph2PCH2CH2CO2But]. The complex with L = CO is inert to substitution by PPh2H, which precludes one of the two conjugate addition catalytic cycles. This catalyst provides critical evidence for the conjugate addition step in the form of a spectroscopically identified phospha-enolate intermediate, a long-lived species that participates in competing, off-cycle alkene oligomerization. Nitrile lability allows the complex with L = NCPh to access the same two conjugate addition cycles observed for the complex with L = PPh2H. However, the “free” benzonitrile both inhibits catalysis and participates in the formation of a deactivation product containing the 1-azaallyl fragment, which has been isolated and crystallographically characterized. Collectively, these results indicate a surprising complexity that can arise from a simple mechanistic premise for metal-mediated hydrophosphination, and demonstrate a variety of impacts of ancillary ligands on catalysis. They highlight design features that allowed us to develop a half-sandwich Ru Cp* catalyst [Ru­(η5-Cp*)­(PPh2)­(PPh2H)2] that exhibits a 30-fold increase in hydrophosphination activity.