Origin of Regioselectivity in the Copper-Catalyzed Borylation Reactions of Internal Aryl Alkynes with Bis(pinacolato)diboron

The detailed reaction mechanism for the borylation of internal aryl alkynes catalyzed by copper­(I) boryl complexes was studied by experiments and density functional theory (DFT) calculations. The calculated results indicate that the Cu­(I)-catalyzed borylation occurs through Ph–CC–R insertion into the Cu–B bond to give the α- and β-borylalkyl intermediates. Among the various substituent groups (Me, Et, <i>i</i>-Pr, <i>t</i>-Bu, 1,1-Et₂Pr, and Cum) at the R position, internal aryl alkynes having substituent groups less bulky than an aryl group converted to β-borylated products, whereas those having substituent groups bulkier than an aryl group converted to α-borylated products with high regio- and stereoselectivity