Hydrogen-Mediated Reductive Cross-Coupling of Aryl Iodides with Activated Aryl and Heteroaryl Bromides

Hydrogen-mediated reductive cross-couplings of aryl iodides with activated aryl and heteroaryl bromides are described, along with related homocouplings (Ullmann reactions) and vinylic reductive couplings that occur with <i>cine</i>-substitution. To corroborate key events in the catalytic cycle, Pd-to-Pd transmetalation and cross-selective reductive elimination, dianionic diarylpalladate complexes [Pd(Aryl)(μ-Br)Br]₂[NPr₄]₂ bearing <i>p</i>-fluorophenyl, <i>p</i>-methoxyphenyl and <i>p</i>-nitrophenyl moieties were prepared. Stoichiometric reactions of these complexes in the presence of iodide demonstrate that Pd-to-Pd transmetalation by way of monomeric arylpalladates occurs at a greater rate than reductive elimination and that cross-selective C-C reductive elimination is favored for electronically distinct aryl partners. Hydrogen-mediated reductive cross-coupling of aryl iodides with α-bromostyrene occurs with <i>cine</i>-substitution (as observed in related formate-mediated processes), consistent with carbopalladation of α-bromostyrene by monomeric arylpalladates Pd(Aryl)I₂[NBu₄] that arise upon dissociation of [Pd(Aryl)(μ-I)I]₂[NBu₄]₂.