Mechanism and Catalyst Design in Ru-Catalyzed Alkene Hydrophosphination

A thorough experimental examination of a series of half-sandwich Ru indenyl complexes [Ru(eta(5)-indenyl)(PPh2)(L)(PPh3) (L = PPh2H, CO, NCPh)] in the catalytic hydrophosphination of ter[ 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 (H-1 and P-31 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(eta(5)-indenyl)(PPh2)P-2, 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(eta(5)-Cp*)(PPh2)(PPh2H)(2)] that exhibits a 30-fold increase in hydrophosphination activity.