Synthetic and Mechanistic Interrogation of Pd/Isocyanide-Catalyzed Cross-Coupling: π-Acidic Ligands Enable Self-Aggregating Monoligated Pd(0) Intermediates

Article abstract of DOI:10.1021/acs.organomet.7b00035

Despite the large number of judiciously designed ligands that have been exploited in palladium-catalyzed cross-coupling protocols, the incorporation of ligands bearing appreciable π-acidic properties has remained significantly underexplored. Herein, we demonstrate that well-defined and low-coordinate Pd⁰ complexes supported by m-terphenyl isocyanides function as competent catalysts for the Suzuki-Miyaura cross-coupling of aryl bromides and arylboronic acids. Two-coordinate Pd(CNAr^Dipp2)₂ was active for the coupling of unhindered aryl bromides at room temperature in 2-propanol, while increasing the temperature to 60 °C allowed for the use of mono- or di-ortho-substituted aryl bromides. Oxidative addition of the aryl bromide was shown to proceed via a dissociative mechanism, implicating monoligated Pd(CNAr^Dipp2) as the catalytically active intermediate. Attempts to access this fleeting species via activation of the Pd^II monoisocyanide PdCl(η³-C₃H₅)(CNAr^Dipp2) with alkoxide base yielded the dinuclear Pd^I species (μ-C₃H₅)(μ-OⁱPr)[Pd(CNAr^Dipp2)]₂. Although dinuclear Pd^I complexes are often produced as off-cycle species when using complexes of the type PdCl(η³-allyl)L as precatalysts, this represents the first time that the comproportionation product (μ-allyl)(μ-Cl)[PdL]₂ has been observed to undergo nucleophilic substitution with alkoxide, despite the fact that activating conditions for these precatalysts typically employ alkoxide bases. Remarkably, this alkoxide complex can undergo β-hydride elimination with expulsion of acetone and propene to produce two equivalents of catalytically active Pd(CNAr^Dipp2), which can self-aggregate to yield the isolable tripalladium cluster Pd₃(η²-Dipp-μ-CNAr^Dipp2)₃. This cluster is catalytically competent for the Suzuki-Miyaura reaction and functions as a formal source of monoligated Pd(CNAr^Dipp2) in solution.