Sterically Induced Acceleration of Aryl Halide Activation by Pd(0): A Radical Alternative to 2-Electron Oxidative Addition

The first elementary step of Pd⁽⁰⁾-catalyzed cross-coupling involves the activation of an aryl halide by a Pd⁽⁰⁾ catalyst, which is widely assumed to proceed in a formal 2-electron process, involving concerted cleavage of the aryl halide bond and formation of an aryl-Pd bond as to generate a Pd^(II) complex. Contrary to common reactivity assumptions under this mechanistic manifold, we observed that severe steric hindrance in the aryl halide and catalyst did not inhibit the thermal activation of the aryl halide but instead greatly accelerated it, giving full conversion of an ortho,ortho-di-tert-butyl-substituted aryl bromide in 1 min at room temperature on gram scale with a bulky Pd⁽⁰⁾ catalyst. Our mechanistic data revealed that a 1-electron-based halogen abstraction by Pd⁽⁰⁾ is operative in such sterically demanding settings.