Generating Active “L-Pd(0)” via Neutral or Cationic π‑Allylpalladium Complexes Featuring Biaryl/Bipyrazolylphosphines: Synthetic, Mechanistic, and Structure–Activity Studies in Challenging Cross-Coupling Reactions

Two new classes of highly active yet air- and moisture-stable π-R-allylpalladium complexes containing bulky biaryl- and bipyrazolylphosphines with extremely broad ligand scope have been developed. Neutral π-allylpalladium complexes incorporated a range of biaryl/bipyrazolylphosphine ligands, while extremely bulky ligands were accommodated by a cationic scaffold. These complexes are easily activated under mild conditions and are efficient for a wide array of challenging C–C and C–X (X = heteroatom) cross-coupling reactions. Their high activity is correlated to their facile activation to a 12-electron-based “L-Pd(0)” catalyst under commonly employed conditions for cross-coupling reactions, noninhibitory byproduct release upon activation, and suppression of the off-cycle pathway to form dinuclear (μ-allyl)­(μ-Cl)­Pd<sub>2</sub>(L)<sub>2</sub> species, supported by structural (single crystal X-ray) and kinetic studies. A broad scope of C–C and C–X coupling reactions with low catalyst loadings and short reaction times highlight the versatility and practicality of these catalysts in organic synthesis.