Divergent Mechanistic Pathways for Copper(I) Hydrophosphination Catalysis: Understanding That Allows for Diastereoselective Hydrophosphination of a Tri-substituted Styrene

Readily available and bench-stable Cu(acac)2 (1) addresses many challenges in exploratory hydrophosphination
catalysis. Mechanistic investigations were performed to answer questions
that remain about the reactivity of 1, the role of light
in catalysis, and to provide direction for further study. A divergent
Hammett plot indicates different mechanisms based on electron density
at the alkene substrate. A radical process was eliminated based on
trapping reactions and in situ electron paramagnetic resonance experiments.
Isotopic labeling experiments, a zwitterionic trapping experiment,
stoichiometric model reactions, and catalytic reactions using proxy
intermediates indicate that both the conjugate addition and insertion-based
mechanistic pathways occur with this system, depending on the unsaturated
substrate. Computational analysis indicates that the lowest energy
transition is a ligand-to-metal charge transfer from the phosphido
ligand where the LUMO has significant Cu–P antibonding character,
suggesting that a weakened Cu–P bond accelerates insertion
under photocatalytic conditions. This hypothesis explains the greater
activity of 1 compared to prior copper-catalyzed hydrophosphination
reports and appears to be a general phenomenon for copper(I) catalysts.
These results have been leveraged to achieve heretofore unknown catalytic
hydrophosphination reactivity, namely the diastereoselective hydrophosphination
of a tri-substituted styrene substrate.