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)­Pd2(L)2 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.