Cobalt-Catalyzed 1,1,3-Triborylation of Terminal Alkynes

We have developed a cobalt-catalyzed regioselective 1,1,3-triborylation reaction of terminal alkynes with pinacolborane (HBpin) with a catalyst generated in situ from readily available and bench-stable Co(acac)2 and xantphos. A variety of terminal alkynes undergo this triborylation reaction, affording the corresponding 1,1,3-triborylalkanes in good yields with high selectivity. The synthetic utility of this catalytic protocol has been demonstrated by developing selective stepwise functionalization of 1,1,3-triborylalkane products. The results of mechanistic studies, such as conducting control experiments and deuterium-labeling reactions, monitoring the reaction process, and identifying reaction intermediates, suggest that this 1,1,3-triborylation reaction proceeds through 1,3-diborylation of alken-1-ylboronates formed by cobalt-catalyzed hydroboration of terminal alkynes.