Sterically Encumbered Tetraarylimidazolium Carbene Pd-PEPPSI Complexes: Highly Efficient Direct Arylation of Imidazoles with Aryl Bromides under Aerobic Conditions

A series of sterically encumbered tetraarylimidazolium carbene Pd-PEPPSI complexes were conveniently prepared and fully characterized. These sterically encumbered Pd-PEPPSI complexes act as active precatalysts for the direct arylation of imidazoles with aryl bromides under aerobic conditions. The catalytic performance of Pd-PEPPSI complexes in cross-coupling processes is investigated. Under the optimal protocols, the cross-coupling reactions regioselectively produced C5-arylation products in moderate to excellent yields, which could tolerate a wide range of functional aryl bromides

Pd-PEPPSI Complexes Bearing Bulky [(1,2-Di‑(<i>tert</i>-butyl)­acenaphthyl] (DtBu-An) on <i>N</i>‑Heterocarbene Backbones: Highly Efficient for Suzuki–Miyaura Cross-Coupling under Aerobic Conditions

With the goal of achieving highly efficient palladium-catalyzed cross-coupling reactions under mild reaction conditions, the Pd-PEPPSI complexes <b>C1</b> and <b>C2</b> bearing 1,2-di­(<i>tert</i>-butyl)­acenaphthyl (DtBu-An) backbones were synthesized and characterized, and their use in Suzuki–Miyaura cross-coupling was investigated. The effects of catalyst structure and reaction conditions on the cross-coupling efficiency were evaluated in detail. The significant differences in catalytic activity compared with classical PEPPSI-IPr and PEPPSI-IPr^An precatalysts are discussed, where the axial sterics on the backbone play an important role. At low palladium loadings of 0.05–0.1 mol % and upon the addition of the relatively weak base K₃PO₄, the palladium complex <b>C1</b> was found to be highly efficient for the coupling of (hetero)­aryl chlorides with arylboronic acids under aerobic conditions, affording the corresponding biaryls in excellent yields

Pd-PEPPSI Complexes Bearing Bulky [(1,2-Di‑(<i>tert</i>-butyl)­acenaphthyl] (DtBu-An) on <i>N</i>‑Heterocarbene Backbones: Highly Efficient for Suzuki–Miyaura Cross-Coupling under Aerobic Conditions

With the goal of achieving highly efficient palladium-catalyzed cross-coupling reactions under mild reaction conditions, the Pd-PEPPSI complexes <b>C1</b> and <b>C2</b> bearing 1,2-di­(<i>tert</i>-butyl)­acenaphthyl (DtBu-An) backbones were synthesized and characterized, and their use in Suzuki–Miyaura cross-coupling was investigated. The effects of catalyst structure and reaction conditions on the cross-coupling efficiency were evaluated in detail. The significant differences in catalytic activity compared with classical PEPPSI-IPr and PEPPSI-IPr^An precatalysts are discussed, where the axial sterics on the backbone play an important role. At low palladium loadings of 0.05–0.1 mol % and upon the addition of the relatively weak base K₃PO₄, the palladium complex <b>C1</b> was found to be highly efficient for the coupling of (hetero)­aryl chlorides with arylboronic acids under aerobic conditions, affording the corresponding biaryls in excellent yields

Flexible Steric Bulky Bis(Imino)acenaphthene (BIAN)-Supported N‑Heterocyclic Carbene Palladium Precatalysts: Catalytic Application in Buchwald–Hartwig Amination in Air

To achieve efficient palladium-catalyzed cross-coupling reaction under mild reaction conditions with the flexible steric bulk strategy, a series of Pd-PEPPSI (PEPPSI: pyridine-enhanced precatalyst preparation, stabilization, and initiation) complexes <b>C1</b>–<b>C6</b> were synthesized and characterized, in which unsymmetric flexible steric bulk was introduced on the <i>N</i>-aryl of ancenaphthyl skeleton. These well-defined palladium complexes were found to be excellent precatalysts for Buchwald–Hartwig amination of aryl chlorides with amines in air. The electronic effect of the Pd-PEPPSI complexes and the effect of ancillary pyridine ligands were evaluated, among which complex <b>C3</b> exhibited the most efficiency. It was demonstrated that the cross-coupling products were obtained in excellent yields in the presence of 0.5–0.1 mol % palladium loading. A wide range of aryl- and heteroaryl chlorides as well as various amines were compatible. The oxidative addition of aryl chlorides is revealed to be the rate-determining step in the catalytic cycle. The catalytic activity can be enhanced by introducing electron-donating groups to the Pd-PEPPSI complexes. This type of Pd-PEPPSI precatalyst showed the most efficiency reported to date for the challenging C–N cross-coupling reactions requiring no anhydrous and inert atmosphere protections, suggesting flexible steric bulk as a promising catalyst design strategy

Pd-PEPPSI-IPent^An Promoted Deactivated Amination of Aryl Chlorides with Amines under Aerobic Conditions

We report herein a highly efficient Pd-catalyzed amination by “bulky-yet-flexible” Pd-PEPPSI-IPent^An complexes. The relationship between the <i>N</i>-heterocyclic carbenes (NHCs) structure and catalytic properties was discussed. Sterically hindered (hetero)­aryl chlorides and a variety of aliphatic and aromatic amines can be applied in this cross-coupling, which smoothly proceeded to provide desired products. The operationally simple protocol highlights the rapid access to C_Ar–N bond formation under mild conditions without the exclusion of air and moisture

Pd-PEPPSI-IPent^An Promoted Deactivated Amination of Aryl Chlorides with Amines under Aerobic Conditions

We report herein a highly efficient Pd-catalyzed amination by “bulky-yet-flexible” Pd-PEPPSI-IPent^An complexes. The relationship between the <i>N</i>-heterocyclic carbenes (NHCs) structure and catalytic properties was discussed. Sterically hindered (hetero)­aryl chlorides and a variety of aliphatic and aromatic amines can be applied in this cross-coupling, which smoothly proceeded to provide desired products. The operationally simple protocol highlights the rapid access to C_Ar–N bond formation under mild conditions without the exclusion of air and moisture

Aerobic and Efficient Direct Arylation of Five-Membered Heteroarenes and Their Benzocondensed Derivatives with Aryl Bromides by Bulky α‑Hydroxyimine Palladium Complexes

In the present work, a series of α-hydroxyimine palladium complexes with bulky substituents (i.e., {[Ar-NC­(R)–C­(R)₂–OH]­PdCl₂} (<b>C1</b>, R = Me, Ar = 2-diphenylmethyl-4,6-dimethylphenyl; <b>C2</b>, R = Me, Ar = 2,6-bis­(diphenylmethyl)-4-methylphenyl; <b>C3</b>, R = Me, Ar = 2,6-bis­(diphenylmethyl)-4-methyoxylphenyl; <b>C4</b>, R = Me, Ar = 2,6-bis­(diphenylmethyl)-4-chlorophenyl; <b>C5</b>, R = Ph, Ar = 2,6-dimethylphenyl; <b>C6</b>, R = Ph, Ar = 2,6-diisopropylphenyl)) were synthesized and characterized. The structures of palladium complexes <b>C1</b> and <b>C2</b> were determined by X-ray diffraction. These bidentate N,O-palladium complexes were applied for direct arylation under aerobic conditions. The effects of the reaction conditions and ligand substitution on the catalytic activity were evaluated. Upon a low palladium loading of 0.5 mol %, the bulky palladium complex <b>C6</b> was successfully used to catalyze the cross-coupling of a variety of five-membered heteroarenes and their benzo-condensed derivatives with (hetero)­aryl bromides. The mechanistic investigation on the direct arylation supported the involvement of a Pd(0)/Pd­(II) CMD process

Aerobic and Efficient Direct Arylation of Five-Membered Heteroarenes and Their Benzocondensed Derivatives with Aryl Bromides by Bulky α‑Hydroxyimine Palladium Complexes

In the present work, a series of α-hydroxyimine palladium complexes with bulky substituents (i.e., {[Ar-NC­(R)–C­(R)₂–OH]­PdCl₂} (<b>C1</b>, R = Me, Ar = 2-diphenylmethyl-4,6-dimethylphenyl; <b>C2</b>, R = Me, Ar = 2,6-bis­(diphenylmethyl)-4-methylphenyl; <b>C3</b>, R = Me, Ar = 2,6-bis­(diphenylmethyl)-4-methyoxylphenyl; <b>C4</b>, R = Me, Ar = 2,6-bis­(diphenylmethyl)-4-chlorophenyl; <b>C5</b>, R = Ph, Ar = 2,6-dimethylphenyl; <b>C6</b>, R = Ph, Ar = 2,6-diisopropylphenyl)) were synthesized and characterized. The structures of palladium complexes <b>C1</b> and <b>C2</b> were determined by X-ray diffraction. These bidentate N,O-palladium complexes were applied for direct arylation under aerobic conditions. The effects of the reaction conditions and ligand substitution on the catalytic activity were evaluated. Upon a low palladium loading of 0.5 mol %, the bulky palladium complex <b>C6</b> was successfully used to catalyze the cross-coupling of a variety of five-membered heteroarenes and their benzo-condensed derivatives with (hetero)­aryl bromides. The mechanistic investigation on the direct arylation supported the involvement of a Pd(0)/Pd­(II) CMD process