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

Abstract

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.

A General Palladium–Phosphine Complex To Explore Aryl Tosylates in the N‐Arylation of Amines: Scope and Limitations

“…N ‐Methyl‐ N ‐phenylquinolin‐6‐amine (Table , entry 5): R f =0.16 (hexane/EtOAc=9:1); 1 H NMR (400 MHz, CDCl 3 ): δ =3.32 (s, 3 H), 7.05–7.06 (m, 2 H), 7.10–7.12 (m, 2 H), 7.18–7.20 (m, 1 H), 7.27–7.35 (m, 3 H), 7.85–7.90 (m, 2 H), 8.65–8.66 ppm (m, 1 H); 13 C NMR (100 MHz, CDCl 3 ): δ =40.2, 110.6, 121.0, 123.0, 123.2, 123.4, 129.2, 129.4, 134.0, 143.6, 146.5, 147.1, 148.1 ppm.…”

Well-defined nickel and palladium precatalysts for cross-coupling

“…The groups of Shaughnessy 68 and Colacot 69 have utilized the η 3 -allyl motif to produce air-stable phosphine complexes that show excellent activity for various cross-coupling reactions. More recently, a team from Johnson Matthey led by Colacot 70 has synthesized precatalysts containing highly active Buchwald phosphines, including XPhos, RuPhos and BrettPhos. For the bulkier dialkylbiarylphosphines, the traditional chloro ligand was replaced with a weakly coordinating triflato ligand, allowing precatalysts with t BuXPhos, t BuBrettPhos and AdBrettPhos to be synthesized.…”

“…For the bulkier dialkylbiarylphosphines, the traditional chloro ligand was replaced with a weakly coordinating triflato ligand, allowing precatalysts with t BuXPhos, t BuBrettPhos and AdBrettPhos to be synthesized. These phosphine-ligated precatalysts were active for a range of cross-coupling reactions, including Buchwald–Hartwig, Suzuki–Miyaura and α-arylation reactions 70 . Using these sterically demanding phosphine ligands curbed the formation of deleterious Pd(i) species such that the catalysts that formed were more active and longer lived.…”

Synthesis of N‐Substituted Condensed Tetrahydropyridine‐Based Enaminones via Palladium‐Catalyzed Intramolecular C–N Cross‐coupling

“…To extend the field of their application, we tested some in the cyclization of enaminones 7 – 11 . First, we used π‐allylpalladium RuPhos complex allylPd L1 introduced by Colacot's group . The first results were inferior to those obtained by means of Pd 2 (dba) 3 (Table , entries 4 and 5).…”