Unraveling the Mechanism of Palladium-Catalyzed Base-Free Cross-Coupling of Vinyl Carboxylates: Dual Role of Arylboronic Acids as a Reducing Agent and a Coupling Partner

Investigating Arylazoformamide Ligands in Palladium(II) Precatalysts through the Suzuki-Miyaura Cross-Coupling Reaction

Herein, is the reported synthesis and utilization of redox-active arylazoformamide (AAF) ligands in palladium(II) precatalysts for the Suzuki-Miyaura cross-coupling reaction. Complexes were formed from 2 equiv of an AAF ligand with Pd(II)Cl2 in an appropriate solvent to create the square planar (AAF)2PdCl2 precatalyst. A thorough investigation of aryl bromides and arylboronic acids found that 1.0 mol % precatalyst with cesium carbonate (Cs2CO3) as base, 1,4-dioxane as solvent at 90 °C for 24 h allowed for excellent conversions to the biphenyl products (over 20 examples). To highlight the AAF ligand class, a set of comparison reactions were performed with redox-active arylazothioformamide ligands, i.e., an (ATF)2PdCl2 complex, other commercial palladium(II) complexes, and a Ni(II)Cl2 arylazoformamide coordination complex2. The (AAF)2PdCl2 complexes outperformed all of others tested. Mechanistically, it is proposed that the AAF ligand singly reduces to antiferromagnetically couple to the palladium(I) complex as a transmetalation intermediate. Ni-based precatalysts were found to be inactive for the studied Suzuki-Miyaura reaction. Overall, these ligand systems offer a unique look into redox-active palladium cross-coupling reactions as well as being phosphine-free and high yielding.

DFT Mechanistic Insights into the Ni(II)-Catalyzed Enantioselective Arylative Cyclization of Tethered Allene-Ketones

Density functional theory (DFT) has provided a detailed mechanistic picture for the redox neutral nickel(II)-catalyzed arylative cyclization reactions of a tethered allene-ketone with arylboronic acids. A mechanistic rationale for the high diastereo- and enantioselectivity achieved experimentally at high reaction temperature was uncovered through modeling the reaction with a chiral ligand and the predicted stereochemical outcome corroborates with experimental results. An unprecedented mechanism for the base-free organoboron transmetalation was revealed and the regioselectivity of migratory insertion of tethered allene-ketones as well as the stability of the possible allylnickel isomers (σ-allyl vs π-allyl) were clarified. The multifaceted nature of the reaction is revealed with certain elementary steps preferring cationic compared to the neutral state.