Elucidating Reactivity Differences in Palladium-Catalyzed Coupling Processes: The Chemistry of Palladium Hydrides

Abstract

This communication describes a series of studies directed at obtaining a better understanding of the Heck reaction. For the first time, the postulated palladium-hydride intermediate (L₂PdHX) in the catalytic cycle of the Heck arylation has been identified. In addition, this study establishes that the base-mediated Pd(0)-regeneration step (L₂PdHX → PdL₂) of the cycle can be kinetically slow and thermodynamically unfavorable and that the process is remarkably sensitive to the structure of L (PCy₃ vs P(t-Bu)₃). Finally, this investigation demonstrates that, for certain catalyst systems, slow rates of Heck arylation can be correlated with reluctant reductive elimination of L₂PdHX, furnishing a possible rationalization for Brønsted-base (Cs₂CO₃ vs Cy₂NMe) and ligand (PCy₃ vs P(t-Bu)₃) effects that have been observed.