Origins of Internal Regioselectivity in Copper-Catalyzed Borylation of Terminal Alkynes

Installation
of a boron functionality into a more substituted carbon
of terminal alkynes has been a challenging issue in chemical synthesis,
since inherently Lewis acidic boron moieties, in principle, favor
their attachment to a terminal carbon. Herein, we report on the highly
internal-selective borylation of terminal alkynes under copper catalysis,
wherein diminishment of boron-Lewis acidity and ligand-derived steric
bulk around a copper center are the key to the success. In particular,
the use of an anthranilamide-substituted boron moiety [B­(aam)] is
of high synthetic significance, because its properly diminished Lewis
acidity enabled the internal regioselectivity and the Suzuki–Miyaura
cross-coupling activity to be compatibly achieved. This method provided
a direct and universal approach to variously substituted branched
alkenylboron compounds, regardless of electronic and steric properties
of a substituent on terminal alkynes.