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-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.