Aryl halide cross-coupling via formate-mediated transfer hydrogenation

Transfer hydrogenation is widely practised across all segments of chemical industry, yet its application to aryl halide reductive cross-coupling is undeveloped because of competing hydrogenolysis. Here, exploiting the distinct reactivity of Pd^I species, an efficient catalytic system for the reductive cross-coupling of activated aryl bromides with aryl iodides via formate-mediated hydrogen transfer is described. These processes display orthogonality with respect to Suzuki and Buchwald–Hartwig couplings, as pinacol boronates and anilines are tolerated and, owing to the intervention of chelated intermediates, are effective for challenging 2-pyridyl systems. Experimental and computational studies corroborate a unique catalytic cycle for reductive cross-coupling where the Pd^I precatalyst, [Pd(I)(P^tBu₃)]₂, is converted to the dianionic species, [Pd₂I₄][NBu₄]₂, from which aryl halide oxidative addition is more facile. Rapid, reversible Pd-to-Pd transmetallation delivers mixtures of iodide-bridged homo- and hetero-diarylpalladium dimers. The hetero-diarylpalladium dimers are more stable and have lower barriers to reductive elimination, promoting high levels of cross-selectivity.