Formate-Mediated Aryl Halide Reductive Cross-Coupling Catalyzed by Iodide-Bridged Palladium(I) Dimers: Experi-mental and Computational Studies

The first efficient reductive cross-couplings of aryl halides mediated by an abundant feedstock, sodium formate, are de-scribed. These processes, which exploit air-stable Pd(I) iodide dimers, are especially effective for challenging 2-pyridyl systems due to intervention of chelated intermediates. Furthermore, orthogonality with respect to Suzuki and Buchwald-Hartwig coupling processes is displayed, as pinacol boronates and anilines are tolerated. Although palladium-catalyzed transfer hydrogenolyses of aryl halides mediated by formate are longstanding, it is not a major competing pathway under the present conditions. Experimental and computational studies corroborate a novel catalytic cycle for cross-coupling where the Pd(I) precatalyst, [Pd(I)(PtBu3)]2, is converted to the active dianionic catalyst, [Pd2I4][NBu4]2, from which aryl halide oxi-dative addition is more facile. Rapid, reversible Pd-to-Pd transmetalation delivers iodide-bridged diarylpalladium dimers. The hetero-diarylpalladium dimers are more stable than the homodimers and have lower barriers to reductive elimination, resulting in high cross-selectivity.