Regiocontrolled CuI-Catalyzed Borylation of Propargylic-Functionalized Internal Alkynes

Good to excellent reactivity and regiocontrol have been achieved in the Cu-I-catalyzed borylation of dialkyl internal alkynes with bis(pinacolato)diboron. The presence of a propargylic. polar group (OH, OR, SAr, SO2Ar, or NHTs), in combination with PCy3 as ligand, allowed maximizing the reactivity and site-selectivity (beta to the propargylic function). DFT calculations suggest a subtle orbitalic influence from the propargylic group, matched with ligand and substrate size effects, as key factors involved in the high beta-selectivity. The vinylboronates allowed the stereoselective synthesis of trisubstituted olefins, while allylic substitution of the SO2Py group without affecting the boronate group provided access to formal hydroboration products of unbiased dialkylalkynes.