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 -Pr, t -Bu, 1,1-Et 2 Pr, 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.