An inner-sphere synthetic cycle for the hydrophosphination of alkenes is proposed, based on observed [2 + 2] cycloaddition of a wide range of alkenes at a coordinatively unsaturated RuPR 2 complex. Key intermediates in the cycle were prepared, and their reactions with various organic acid/ base pairs were examined to identify both new ruthenium precursors and base cocatalysts that allow turnover of the proposed cycle. Two new cationic ruthenium indenyl phosphine complexes were isolated and structurally characterized. Although preliminary screening studies show the moderate activity of these and related neutral phosphido complexes for catalytic hydrophosphination of acrylonitrile by both HPPh 2 and HPCy 2 , and comparable activity for the hydrophosphination of tert-butyl acrylate by HPPh 2 , no activity was observed for the analogous hydrophosphination of 1-hexene. This is attributed to strong binding of the substrate phosphine to the unsaturated, planar RuPR 2 fragment generated in situ, which inhibits the innersphere, alkene cycloaddition mechanism. An alternative, outer-sphere Michael addition process, involving a saturated complex with a strongly nucleophilic pyramidal Ru−PR 2 ligand, is proposed to rationalize the observed selectivity for catalytic hydrophosphination of activated, but not simple, alkenes. Implications for further catalyst development are discussed.

Kinetic profiles obtained from monitoring the solution phase substitution chemistry of [Ru(η 5 -indenyl)(NCPh)(PPh 3 ) 2 ] + (1) by both ESI-MS and 31 P{ 1 H} NMR are essentially identical, despite an

Alkyl-alkyl cross-coupling is a powerful C-C bond forming transformation typically catalyzed by late transition metals. Herein we report a mechanistic investigation into an early transition metal-catalyzed variant of this reaction. Through this mechanistic understanding, an ideal Y catalyst was determined through tuning of the metal in order to optimize for oxidation potential, the rate limiting step in this reaction. A wide substrate scope is revealed that includes a variety of functional groups as well as unactivated substrates.

Alkyl–alkyl cross-coupling through well-defined mechanisms that allow for controlled oxidative addition, prevent β-hydride elimination, and tolerate hindered electrophiles is still challenging. Described herein is a redox-active ligand-enabled alkyl–alkyl cross-coupling using a d0 metal. This tris­(amido) ScIII complex as well as the oxidized variant have been thoroughly characterized (NMR, X-ray, EPR, CV, UV–vis, DFT). Insight into the likely radical nature of the mechanism is disclosed. Additionally, a substrate scope that includes functional groups incompatible with late-transition-metal catalysis and both coupling partners bearing β-hydrogens is reported.

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)P 2, where P is the hydrophosphination product Ph2PCH2CH2CO2Bu t ]. 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.