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

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journal contribution
posted on 13.02.2017, 20:46 by Brandon R. Barnett, Liezel A. Labios, Julia M. Stauber, Curtis E. Moore, Arnold L. Rheingold, Joshua S. Figueroa
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(CNArDipp2)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(CNArDipp2) as the catalytically active intermediate. Attempts to access this fleeting species via activation of the PdII monoisocyanide PdCl­(η3-C3H5)­(CNArDipp2) with alkoxide base yielded the dinuclear PdI species (μ-C3H5)­(μ-OiPr)­[Pd­(CNArDipp2)]2. Although dinuclear PdI 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(CNArDipp2), which can self-aggregate to yield the isolable tripalladium cluster Pd32-Dipp-μ-CNArDipp2)3. This cluster is catalytically competent for the Suzuki–Miyaura reaction and functions as a formal source of monoligated Pd­(CNArDipp2) in solution.