Cobalt-Catalyzed Regioselective Deoxygenative Triborylation of α,β-Unsaturated Aldehydes

We have developed a cobalt-catalyzed selective deoxygenative 1,1,3-triborylation reaction of α,β-unsaturated aldehydes with pinacolborane (HBpin) by employing a catalyst formed in situ from bench-stable and commercially available Co(acac)2 and Xantphos. A range of cinnamaldehydes undergo this deoxygenative 1,1,3-triborylation reaction, providing the corresponding 1,1,3-triborylalkane products in good isolated yields with high regioselectivity. Mechanistic studies, such as control experiments, deuterium labeling reactions, and identification of potential organic intermediates, suggest that this 1,1,3-triborylation of α,β-unsaturated aldehydes occurs through a sequence combining the cobalt-catalyzed carbonyl hydroboration of α,β-unsaturated aldehydes, deoxygenative borylation of allyloxy boronates, and gem-diborylation of allylic boronates.