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 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 precatalyst, [Pd(I)(PBu)], is converted to the dianionic species, [PdI][NBu], 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.